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Oxidation Kinetics and Mechanisms in HT-9 Ferritic/Martensitic Stainless Steel


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OXIDATION KINETICS AND MECHANISMS IN HT-9 FERRITIC/MARTENSITI C STAINLESS STEEL By SORAYA BENTEZ VLEZ A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Soraya Bentez Vlez

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This document is dedicated to the loving me mory of my maternal grandmother, Sara Vlez Muiz. Mama Lala, I finally made it.

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iv ACKNOWLEDGMENTS I would like to thank Dr. Darryl P. Butt fo r the opportunity to be part of his research group and for providi ng the opportunity to obtain a summer internship at Los Alamos National Laboratory. I would like to thank Dr. Scott Lillard, Dr. Ning Li, and Dan Rustoi from Los Alamos National La boratory (LANL) and the Department of Energy (DOE) for making it possible for the summer internship at LANL to become my graduate research. I would lik e to thank and acknowledge the assistance of Dr. Gary Was and Dr. Jeremy Busby for f acilitating the use of the Mi chigan Ion Beam Laboratory located at the University of Michigan and for providing the material for this research. In addition, I would like to acknowledge the as sistance provided by the Major Analytical and Instrumentation Center (MAIC) located at the University of Florida. I would especially like to express my gratitude to Wayne Acree, Kerry Siebein, Gerald Bourne, Dr. Valentin Craciun, Andrew Gerger, and J uyun Woo for their qu ick assistance during critical times. On a more personal note, I would like to thank Edgardo Pabit, Jairaj Payapilly, Jonsang Lee, and Abby Queale for being such wonderful colleagues. I appreciate the technical discussions, the inte llectual conversations, and mo st of all, the times of laughter. I would also like to extend my gr atitude to Dr. Luisa A. Dempere and Dr. Mary Fukuyama for their support and words of wi sdom. Finally, I would like to thank my parents and Brian DeCarlo. They believed in me even when I did not, and their support, love and patience are what helped me complete this work.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT.....................................................................................................................xiii 1 INTRODUCTION............................................................................................................1 2 BACKGROUND INFORMATION.................................................................................3 Fundamentals of Oxidation of Metals and Alloys........................................................3 Thermodynamics of Oxidation..............................................................................6 Oxidation Kinetics Rate Equations.......................................................................9 Logarithmic reaction rate.............................................................................10 Parabolic reaction rate..................................................................................10 Linear reaction rate.......................................................................................11 Other kinetics equations...............................................................................11 Determination of Oxidation Kinetics from Rate Data.........................................14 Oxide Films Developed During Hi gh-Temperature Application........................15 Common Oxides..................................................................................................17 General Oxidation Fe-Cr Alloys.........................................................................20 Literature Review on the Oxidati on of Stainless Steel in Air....................................23 Factors Affecting Oxidation Behavior................................................................24 Effect of alloy composition..........................................................................24 Effect of surface treatment...........................................................................26 Effect of alloy microstructure......................................................................28 Effect of water vapor....................................................................................29 General Oxidation of Stainless Steels.................................................................30 HT-9 Ferritic/Martensitic Stainless Steel...................................................................31 General Applications of HT-9 Stainless Steel.....................................................34 ADS Applications of HT-9 Stainless Steel.........................................................34 3 EXPERIMENTAL PROCEDURE.................................................................................37 Sample Preparation.....................................................................................................37 Oxidation Scans..........................................................................................................40

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vi Equipment............................................................................................................40 Non-Isothermal Scan Procedure..........................................................................45 Isothermal Scan Procedure..................................................................................47 Weight Change Calculations...............................................................................49 Oxide Characterization...............................................................................................50 Surface Characterization.....................................................................................50 Cross-Section Characterization...........................................................................50 Oxide Compound Characterization.....................................................................51 Platinum Marker Experiment.....................................................................................52 4 RESULTS.......................................................................................................................54 Non-Isothermal Scan Results.....................................................................................54 Isothermal Scan Results..............................................................................................55 SEM Surface Characterization Results.......................................................................67 600C to 825C Temperature Range...................................................................67 850C to 863C Temperature Range...................................................................70 875C to 950C Temperature Range...................................................................74 SEM Cross-Section Characterization Results............................................................82 600C to 825C Temperature Range...................................................................83 850C to 863C Temperature Range...................................................................85 875C to 950C Temperature Range...................................................................91 Platinum Marker Experiment Results.......................................................................107 XRD Characterization Results..................................................................................109 5 DISCUSSION OF THE RESULT................................................................................112 Oxidation Scan..........................................................................................................112 Morphology, Composition and Structure of the Oxide Film....................................116 600C to 825C Temperature Range.................................................................117 850C to 863C Temperature Range.................................................................117 875C to 950C Temperature Range.................................................................119 Oxide Compounds....................................................................................................124 6 CONCLUSIONS...........................................................................................................126 LIST OF REFERENCES.................................................................................................128 BIOGRAPHICAL SKETCH...........................................................................................134

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vii LIST OF TABLES Table page 2-1. Principal binary oxides of iron, ch romium, manganese, and vanadium..................20 2-2. Example of ternary and comple x oxides of stainless steels.....................................20 2-3. Composition of HT-9 steel in weight pe rcent according to different references.....32 3-1. Composition of HT-9 steel in weight percent (wt%)...............................................38 3-2. Example of a non-isothermal program.....................................................................46 3-3. Example of an isothermal program..........................................................................47 3-4. Isothermal scan temperatures and holding time combinations................................47 4-1. Surface area and weight change for HT-9 samples oxidized under non-isothermal conditions........................................................................................54 4-2. Surface area and weight change for HT-9 samples oxidized under isothermal conditions.................................................................................................................56 4-3. Calculated m values based on linear regressions of the segments of double logarithmic plots.......................................................................................................61 4-4. Linear (kL), parabolic (kP), cubic (kC), logarithmic (kLog), and inverse logarithmic (kInv) rate constants for each isothermal plot segment..........................65 4-5. Oxide compounds that best match the XRD profiles.............................................109

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viii LIST OF FIGURES Figure page 2-1. Schematic illustration of the of the oxidation reaction between a metallic substrate and oxygen..................................................................................................5 2-2. Ellingham-Richardson diagram of the oxides of metals and alloys commonly used in high temperature application.........................................................................7 2-3. Fe-O phase diagram...................................................................................................8 2-4. Graphical illustration comparing the di fferent oxidation reaction rates equations....9 2-5. Graphic illustration of paralinear kinetics (A) and break away oxidation (B)..........13 2-6. Variation in oxide structure with ch romium content of a Fe-Cr alloy based on data obtained from isothermal (constant temperature) scans at 1000C in 0.13 atm of oxygen...........................................................................................................16 2-7. Schematic illustration of the oxide film structure for Fe-Cr alloys of low chromium content.....................................................................................................22 2-8. Development of a 12% Cr ferritic /martensitic stainless steel..................................32 2-9. Constitutional diagram for Fe-Cr alloys (A) and Shaeffler diagram [58a] (B)........33 3-1. Secondary electron (SE) image of the microstructure of the as-received HT-9 stainless steel alloy after etching the surface with Marbles Reagent......................39 3-2. SE image and line scan of a partic le located along the boundary between the martensite phase and the ferrite phase......................................................................40 3-3. Schematic of the equipment set-up..........................................................................41 3-4. Photograph of the actual equipmen t used during the oxidation scans.....................42 3-5. Cross-section view of the TGA 2050 balance assembly and furnace assembly......43 3-6. Example of a non-isothermal temperature profile....................................................46 3-7. Example of an isothermal scan temperature profile.................................................48

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ix 3-10. Oxide scrapped from the surface mounted for XRD characterization.....................52 3-11. HT-9 sample with the Pt marker afte r drying for 24 hours in a dessicator cabinet and prior to the isothermal scan...............................................................................53 4-1. Calculated weight chan ge versus temperature plots for HT-9 samples oxidized under non-isothermal conditions..............................................................................55 4-2. Calculated weight chan ge versus time plots for HT-9 samples oxidized under isothermal conditions...............................................................................................57 4-3. Isothermal scan plots of HT-9 samp les oxidized at 900C in dry air for 30 minutes, 90 minutes, and 24 hours...........................................................................58 4-4. Calculated double logarithm pl ots of the isothermal data........................................59 4-5. Example of identifying the oxidation ki netics based on the results of the linear regressions applied to each segm ent of a double logarithmic plot...........................60 4-6. Example of validating a kinetics rate equation (parabolic rate equation) for a sample oxidized at 850C for 24 hours....................................................................62 4-7. Possible oxidation kinetics based on the results of the linear regressions performed on a double logarithm plot of the isothermal data..................................63 4-8. Possible oxidation kinetics based on valid ating the simple kine tics rate equations to the isothermal data...............................................................................................64 4-9. Calculation of the activation energies for the linear (A) and parabolic (B) oxidation models......................................................................................................66 4-10. SE images and corresponding EDS spectra for the sample oxidized in dry air at 600C for 48 hours...................................................................................................68 4-11. SE images and corresponding EDS spectra for the sample oxidized in dry air at 700C for 48 hours...................................................................................................68 4-12. SE images and corresponding EDS spectra for the sample oxidized in dry air at 800C for 48 hours...................................................................................................69 4-13. SE images and corresponding EDS spectra for the sample oxidized in dry air at 825C for 24 hours...................................................................................................69 4-14. SE images and corresponding EDS spectra for the sample oxidized in dry air at 850C for 24 hours...................................................................................................70 4-15. SE images and corresponding EDS spectra for the sample oxidized in dry air at 863C for 24 hours...................................................................................................71

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x 4-16. SE images and corresponding EDS spectra of the various features of the Fe-rich oxide structures observed on the sample oxidized in dry air at 850C for 24 hours. ..................................................................................................................72 4-17. SE images and corresponding EDS spectra of the various features of the Fe-rich oxide surface of the sample oxidized in dry air at 863C for 24 hours....................73 4-18. Image of the cracks on the surface of the Fe-rich oxide film developed on the sample oxidized in dry air at 950C for 6 hours......................................................74 4-19. SE images and corresponding EDS spectra for the sample oxidized in dry air at 875C for 24 hours...................................................................................................75 4-20. SE images and corresponding EDS spectra for the sample oxidized in dry air at 900C for 24 hours...................................................................................................76 4-21. SE images and respective EDS spectra of the various features of the outermost oxide layer of the sample oxidized in dry air at 875C for 24 hours.......................77 4-22. SE images and respective EDS spectra of the various features of the outermost oxide layer of the sample oxidized in dry air at 900C for 24 hours.......................78 4-23. SE images and corresponding EDS spectra for the sample oxidized in dry air at 950C for 6 hours.....................................................................................................79 4-24. SE images and respective EDS spectra of the various features of the outermost oxide layer of the sample oxidized in dry air at 950C for 6 hours.........................80 4-25. SE images and corresponding EDS spectra for the sample oxidized in dry air at 900C for 30 minutes...............................................................................................81 Figure 4-26. SE images and corresponding EDS spectra for the sample oxidized in dry air at 900C for 90 minutes................................................................................81 4-27. SE images and respective EDS spectra of the various features on the surface of the Fe-rich oxide structures of the sa mple oxidized in dry air at 900C for 90 minutes................................................................................................................82 4-28. HT-9 sample on alumina pan after isot hermal oxidation in dry air at 600C for 48 hours..............................................................................................................83 4-29. Cross-section SE image for the sa mples oxidized at 600C, 700C, and 800C for 48 hours and the sample oxidized at 825C for 24 hours...................................84 4-30. Cross-section SE image, elementa l mapping, and line scan profiles for the sample oxidized in dry air at 850C for 24 hours....................................................86

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xi 4-31. Cross-section SE image, elementa l mapping, and line scan profiles for the sample oxidized in dry air at 863C for 24 hours....................................................87 4-32. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide at spot 1 of 4-31........................................................................................................88 4-33. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide at spot 2 of 4-31........................................................................................................89 4-34. Cross-section SE image and elemental mapping of the oxide at spot 3 of 4-33......90 4-35. Cross-section SE image, elementa l mapping, and line scan profiles for the sample oxidized in dry air at 875C for 24 hours....................................................93 4-36. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide in spot 1 of 4-35.......................................................................................................94 4-37. Cross-section SE image, elemental mappi ng, and line scan profile of the oxide at spot 2 of 4-35............................................................................................................95 4-38. Cross-section SE image, elemental mappi ng, and line scan profile of the oxide at spot 3 of 4-37............................................................................................................96 4-39. Cross-section SE image, elementa l mapping, and line scan profiles for the sample oxidized in dry air at 900C for 24 hours....................................................97 4-40. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide at spot 1 in 4-39........................................................................................................98 4-41. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide at spot 2 in 4-39........................................................................................................99 4-42. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide at spot 3 in 4-41......................................................................................................100 4-43. Cross-section SE image, elementa l mapping, and line scan profiles for the sample oxidized in dry air at 950C for 6 hours....................................................101 4-44. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide at location 1 in 4-43................................................................................................102 4-45. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide at location 2 in 4-43................................................................................................103 4-46. Cross-section SE image, elemental ma pping, and line scan profiles of the oxide at location 3 in 4-43................................................................................................104

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xii 4-47. EPMA results in weight percent fr om two different locations on the oxide developed on the 850C/24 hrs sample (A) and the thin oxide developed on the 863C/24 hrs sample (B)........................................................................................105 4-48. EPMA results in weight percent fr om different locations across the oxide developed on the sample oxidized in dry air at 863C for 24 hours......................105 4-49. EPMA results in weight percent fr om different locations across the oxide developed on the sample oxidized in dry air at 875C for 24 hours......................106 4-50. EPMA results in weight percent fr om different locations across the oxide developed on the sample oxidized in dry air at 900C for 24 hours......................106 4-51. EPMA results in weight percent fr om different locations across the oxide developed on the sample oxidized in dry air at 950C for 6 hours........................107 4-52. Pt marker sample after isothermal scan in dry air at 900C for 24 hours..............107 4-53. Cross-section SE image, elemental mapping, and line scan profiles of the Pt marker sample........................................................................................................108 4-54. XRD profiles of the surface of non-ox idized (baseline) and oxidized HT-9 samples.........................................................................................................110 4-55. XRD profiles of the oxide scrapped fr om the surface of non-oxidized (baseline) and oxidized HT-9 samples....................................................................................111 5-1. Classification of isothermal plots (A ) and plot of the overall weight change versus the temperature for the isothermal scans (B)..............................................113 5-2. Isothermal plots of samples that seem to exhibit breakaway oxidation kinetics....115 5-3. SE images of the changes in surface morphology, thickness, and structure of the oxide film with increase d oxidation temperature...................................................116 5-4. SE image of the partial dissolution of grain structures at the internal oxidation zone. 121 5-5. Evolution of the Fe-rich oxide as a function of holding time for samples oxidized in dry air at 900C...................................................................................123

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xiii Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy OXIDATION KINETICS AND MECHANISMS IN HT-9 FERRITIC/MARTENSITI C STAINLESS STEEL Soraya Bentez Vlez December 2005 Chair: Darryl P. Butt Major Department: Materials Science and Engineering Lead and lead alloys, such as lead bismut h eutectic (LBE), have gained worldwide recognition as potential candidates for coolant and target material in accelerator-driven systems (ADS) due to their excellent ch emical, physical, and nuclear properties; however, they are corrosive to stainless stee ls, which are candidate materials for piping and molten metal containers in ADS applications. The Russians have used LBE as a coolant in their nuclear submarines for more than several decades and their experience has given way to the hypothesis that oxide s protect steels from LBE corrosion, thus renewing interest in further understanding the oxidation kinetics and mechanisms of candidate stainless steel alloys. The proposed work will contribute to the ongoi ng research effort by elucidating the oxidation kinetics and mechanisms, and charac terizing the oxides of HT-9, a candidate stainless steel for ADS application. HT-9 (DIN X20CrMoWV12 or Fe-12Cr-MoVW) is a 12 wt% Cr martensitic/ferritic stainless steel alloy developed by Sandvik (Sweden).

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xiv The approach is to perform thermogravimetri c analysis (TGA) to evaluate the oxidation kinetics and assess oxidation rates. Flat bar samples polished to one micron were subjected to non-isothermal and isothermal oxi dation scans. Non-isothermal scans were done in dry air from room temperature to 950C with ramping rates of 2C/minute and 5C/minute to obtain preliminary oxidation beha vior information. Isothermal scans were done in dry air from 600C to 800C for 48 hours, from 800C to 900C for 24 hours, and at 950C for 6 hours. The structure a nd composition of the oxide film developed during the isothermal scans were characteri zed using scanning electron microscopy with energy dispersive spectrosc opy capabilities (SEM/EDS) and X-ray diffraction (XRD). Oxidation of HT-9 exhibited a complex behavior that does not follow a simple oxidation model and changes as a function of exposure time. At low temperatures (T 825C), the weight change ( W) was minimal and a protec tive, single-layer, Cr-rich spinel and/or corundum oxide film formed. At high temperatures (T 875C), the W was faster and the oxidation behavior pre dominantly followed linear kinetics. The developed oxide film is a double layer structur e, consisting of an outer Fe-rich corundum oxide layer and an inner Fe/Cr-rich spinel oxid e layer. This oxide film grows by outward diffusion of metallic ions from the substrate. An internal oxidation zone with intergranular cracking was also observed at temperatures of 863C and above. Breakaway oxidation occurred within the 850 C to 863C range, in which the oxidation behavior shifts from parabolic kinetics (protec tive) to linear kinetics (non-protective) as initially foreseen in preliminary non-isothermal oxidation scans.

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1 CHAPTER 1 INTRODUCTION The Fe-12Cr ferritic/martensitic stainless steel HT-9 (DIN X20CrMoWV12 or also known as Fe-12Cr-MoVW) may have appli cation in the nuclear power industry. Potential uses for this alloy are in the fa brication of in-core and out-of-core nuclear reactors components. An example of an out-o f core application is the experimental use of HT-9 as cladding and ducts in the Fast Flux Test Facility (FFTF), a liquid-metal cooled nuclear test reactor located at Ri chmond, WA. Other potential applications include first wall and tritium breeding blanket components in fusion reactors and as pipes and vessels in accelerator-driven systems (ADS). The Advanced Fuel Cycle Initiative (AFC I) is a Department of Energy sponsored ADS project for the handling of nuclear wast e. The concept is to enclose radiotoxic waste, such as minor actinid es and long-lived fission produc ts, inside a stainless steel vessel that contains a molten lead alloy. The vessel is bombarded with high-energy protons generating neutrons. The neutrons are absorbed by the radiotoxic waste, triggering nuclear transformations (transmutation) to occur within the waste material. Thus, the waste is converted to a less hazardous or non-radioac tive material that is easier to handle and dispose and the energy liberated during the tr ansmutation process can be used to generate electricity. However, liq uid metals, such as molten lead alloys, are corrosive to stainless steel reducing the life expectancy of the components and leading to potential failure. The Russians have used molte n lead alloys as a coolant in their nuclear submarines for several decades. Their experience has shown that an oxide layer can

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2 protect the steel from corrosion by molten l ead alloys. This has given way to the hypothesis that oxides of passi vated stainless steels may protect the alloy from the corrosive environment of molten metals, prompting interest in gaining a better understanding of the oxidation behavior of HT-9, for which information on its oxidation behavior is nonexistent. Background information on the oxidation be havior and common oxidation products for the iron-chromium alloy family are presented in Chapter 2. This chapter also includes a literature review on the oxida tion of stainless steels in ai r and information on the alloy used during this research. Chapter 3 provide s information on the experimental procedure and materials characterization techniques. E xperimental and characterization results are presented in Chapter 4 and discussed in Ch apter 5. Conclusion s are presented in Chapter 6.

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3 CHAPTER 2 BACKGROUND INFORMATION This chapter presents background informa tion on the kinetics and mechanisms of oxidation, the general oxidation processes, and most common reaction products for the iron-chromium alloy family. The information is mostly based on Hauffe [1], Kofstad [2, 3], Birks and Meier [4], Khanna [5], and the ASM Handbook Volume 13a [6] unless specified otherwise. Literature review on the oxidation of stainless steels in air with emphasis on ferritic/martensitic stainless steels and inform ation on the stainless steel alloy used during this research is also presented. Fundamentals of Oxidation of Metals and Alloys Oxidation is a chemical reaction that o ccurs between a metalli c substrate and the reacting gases present in the environment due to the thermodynamic instability of the substrate when exposed to these gases. The most common reacting gas is oxygen and the by-products are oxides. The reaction rate and oxidation be havior depend on many factors that can result in a complex reaction beha vior in which several mechanisms are simultaneously active. In general, these factors are Composition, pretreatment, and surface finish of the substrate. Gas composition and partial pres sure of the environment. Temperature. Time. Depending on the combination of these fact ors, the reaction can be slow to the point that the substrate is vi rtually unattacked, or it can be fast leading to eventual

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4 component failure. The last case is esp ecially detrimental for high temperature applications for oxidation rates increase with temperature. The general equation that describes the chemical reaction between the metallic substrate and oxygen to form an oxide is written as b aO M O b aM 22 Equation 2-1 M and O is the metal and oxygen, respectively. a is the number of moles of the metal and b is the number of moles of oxygen. The onset of oxidation occurs with th e adsorption of oxygen on the substrate surface, which dissolves and diffuses into the substrate as the reaction proceeds. This leads to the formation of an oxide compound on the surface as a layer or as separate nuclei. The parameters that influence th e process of adsorpti on and initial oxide formation are crystal defects and orientation at the surface, surface finish, and impurities in the substrate and gaseous environment. Once a continuous oxide film has covered the surface, the reaction continues via solid-state diffusion of th e reacting species through the oxide film. One of the reacti ng species is metallic ions, which are termed cations and have a negative charge. The other reac ting specie is oxygen ions, which are termed anions and have a positive charge. The driv ing force for the transport of the reacting species depends on the thickness of the oxide fi lm. For thin film, the driving force may be an electric field in or acr oss the film with the addition of electron transport. If the layer is thick, the driving force may be a chemical potential gradient across the film. The oxidation process is schemati cally shown in Figure 2-1. Depending on the oxidizing environment and the substrate, the oxide film may consist of a single oxide layer or of multiple oxide layers, in which the layers may be

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5 porous or compact. A compact oxide film tends to have a protective nature, which serves as a diffusion barrier between the reacting speci es and thus limits the reaction process. At higher temperatures, the oxide film may volatize and/or melt, which may lead to catastrophic failure. Step 5: Development of macrocracks with possible molten oxide phase formation and/or oxide sublimation Step 4: Development of microcracks, voids, and cavities within the oxide film. The microcracks can serve as shortcircuit diffusion paths. Step 3: Oxide film growth with diffusion of the reacting species through the oxide film. Internal oxidation may occur below the substrate surface. Step 2: Oxide nucleation and growth with dissolution and inward diffusion of oxygen. Step 1: Adsorption of dissociated oxygen on the surface of the metallic substrate. Step 5: Development of macrocracks with possible molten oxide phase formation and/or oxide sublimation Step 4: Development of microcracks, voids, and cavities within the oxide film. The microcracks can serve as shortcircuit diffusion paths. Step 3: Oxide film growth with diffusion of the reacting species through the oxide film. Internal oxidation may occur below the substrate surface. Step 2: Oxide nucleation and growth with dissolution and inward diffusion of oxygen. Step 1: Adsorption of dissociated oxygen on the surface of the metallic substrate. Oxide O2(g) O Oxide O2(g) O O2(g) O O-2e-M+nInternal Oxidation Particles O2(g) O O-2e-M+nInternal Oxidation Particles O2(g) M+ne-O-2 Cavity with microcrack O Void O2(g) M+ne-O-2 Cavity with microcrack O Void O2(g) Macrocrack MOv(g) O-2e-M+nO O2(g) Macrocrack MOv(g) O-2e-M+nO O2(g) Metal Substrate O O O2(g) Metal Substrate O O Figure 2-1. Schematic illustration of the of the oxidation reaction between a metallic substrate and oxygen. Figure based on Kofstad [2, 3] and the ASM Handbook Volume 13a [6].

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6 Thermodynamics of Oxidation The overall driving force for the oxida tion reaction between oxygen and a metallic substrate is the standard Gi bbs free energy of formation ( G) of the oxide compounds formed during the chemical reaction between oxygen and the metallic substrate. From the thermodynamic perspective, the oxide will only form if its dissociation pressure in equilibrium with the metallic substrate is less than the environments oxygen partial pressure (pO2) at a given temperature. The Ellin gham-Richardson diagram, also known as the Richardson-Jeffes diagram, allows the graphical determination of the G of the oxides for the data is presented as a func tion of temperature a nd corresponding oxide dissociation pressure. (Figure 2-2. [6]) The G of the oxides is plotted along the Y-axis. The larger the negative value the more stable is the oxide. The temperature is plotted along the X-axis. Auxiliary outer scales are included on the top, right, and bottom of the diagram that represent the pO2 in oxygen-containing environm ent or consisting of mixed gases (CO/CO2, H2/H2O). As an example, the G for chromia (Cr2O3), an oxide of chromium (Cr), at 600C is approximately -600 kJ for 2/3 mole of chromia (red line) and the corresponding oxide dissoci ation pressure in an oxygen containing environment is approximately 10-35 atmospheres (atm) (blue line). Thus, Cr will not oxidize to chromia at 600C if the pO2 in the environment is less than 10-35 atm. If the metallic substrate forms more than one type of oxide, the use of a phase diagram will aid in understanding the oxidation process. The phase diagram presents the different type of oxides that can form as a function of temperature and composition of the reacting species (Figure 2-3. [7]) This allows the prediction of a sequence of oxides that can develop in the oxide f ilm, with the most oxygen de ficient oxide next to the

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7 oxide/substrate interface and the most oxygen rich oxide next to the gas/oxide interface. The temperature is plotted on the Y-axis wh ile the composition is plotted on the X-axis. Figure 2-2. Ellingham-Richardson diagram of the oxides of metals and alloys commonly used in high temperature application. The G for Cr2O3 at 600C is approximately -600 kJ/mol O2, represented by the red arrow, and the corresponding dissociation pressure is approximately 10-35 atm, represented by the blue arrow. [6]

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8 The iron-oxygen (Fe-O) phase diagram pres ents three oxides of iron: wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3 or -Fe2O3). If iron is oxidized in an oxygen-containing environment at 400C, the oxide film will consists of two layers according to the Fe-O phase diagram: Fe3O4 next to the substrate and Fe2O3 next to the oxygen phase. FeO would not form for it is unstable below 570C, where it decomposes to iron and magnetite. If the oxidation occurred at 800C, the oxide film would consist of three layers in the following sequence starti ng with the one next to the substrate and moving outwards: FeO, Fe3O4, and Fe2O3. Figure 2-3. Fe-O phase diagram. [7] For metallic substrates that form a vari ety of oxides, knowledge of thermodynamics in conjunction with phase diagrams may only serve as a guid e to understanding the oxidation behavior. As previous ly mentioned, the overall driving force is the change in

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9 the G associated with the oxidation reaction, but it is not related to the rate of the reaction. The oxidation reaction rate is a ki netics problem that depends on the reaction mechanism and the rate-limiting process. Oxidation Kinetics Rate Equations To identify the reaction mechanisms and de fine the rate-limiting process occurring during the oxidation reaction, it is important to have knowledge of reaction rates and corresponding equations. These rates and equations are func tion of several factors: Substrate pretreatment and su rface finish, environments pO2 and temperature, and elapsed time. The simplest oxidation reactions obey loga rithmic, parabolic, or linear models, which represent limiting and ideal cases. It is common to encounter deviations from these ideal models under real life applications, in which the rate data can only be fitted by use of intermediate rate equations and/or combination of the ideal models. The various simple reaction rates are shown in Figure 2-4, where X represents the change in the parameter of interest, such as the thickness of the oxide or the we ight of the sample. X Parabolic Linear Inverse Logarithmic Logarithmic Time Figure 2-4. Graphical illustration compar ing the different oxidation reaction rates equations. Figure based on the ASM Handbook Volume 13a [6]. X represents the variation in the oxides thickness or mass, oxygen consumed per surface area, or quantity of metal that transforms into oxide

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10 Logarithmic reaction rate Logarithmic kinetics typically occurs at temperatures below 400C, and for oxide film thickness of 100 nm or less. Initially, the reaction rate is very fast with the rate dropping to a low or negligible value with time. Ionic and/or electron trans port processes through the oxide film are the rate-limiting mechanism, and the driving force is an electric field in or across the film. Logari thmic behavior is repr esented by two different equations that are difficult to distinguish, which are Logarithmic, also known as direct logarithmic. A t t k Xo ) log(log Equation 2-2a Inverse logarithmic. ) log( 1 t k B Xinv Equation 2-2b X is the oxides thickness or mass, oxygen consumed per surface area, etc. klog and kinv are the logarithmic and inverse logarithmic rate constants, respectively. t and to are time. A and B are constants. Parabolic reaction rate The oxidation behavior of the majority of metals and metallic alloys follow parabolic kinetics during high temperature oxi dation. The rate-limiting step is the thermal diffusion of the ionic species through a compact oxide film and the driving force is the chemical potential gradient that deve lops across the film. The diffusion process may involve outward diffusion of cations, inwa rd diffusion of anions, or both. It may also involve the transport of electrons acr oss the oxide film. As the oxide thickness increases with time, the react ion rate decreased due to the increase in the diffusion distance. The equation that de scribes parabolic kinetics is

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11 C t k Xp 2 Equation 2-3 X is the oxides thickness or mass, oxygen consumed per surface area, etc. kp is the parabolic rate constant. t is time and C is a constant. Linear reaction rate In linear kinetics, the oxidation rate is c onstant with time and independent of the amount of gas or metallic substrate consumed Surface and/or phase boundary processes are the rate-limiting mechanism. This behavior is observed in the following situations: The oxide is porous and non-protective. The oxide vaporizes. The oxide film cracks and spalls off due to internal stress. The oxide melts and forms eutec tic phases with the substrate. The equation for linear kinetics is as follows: C t k Xl Equations 2-4 X is the oxides thickness or mass, oxygen consumed per surface area, etc. kl is the linear rate constant. t is time and C is a constant. Other kinetics equations A number of intermediate reaction rates ha ve been developed that better represent the oxidation behavior in real life application and/or of complex metallic alloy systems. Initially, the oxidation reaction may follow one type of kinetics and gradually change to another type, thus resulting in a complex oxida tion behavior in which the kinetics change with time. These intermediate equations ar e combinations of the ideal rate laws and sometimes may not be able to fully explain the complex oxidation behaviors. Some the intermediate kinetics rate equations are General parabolic equation. Cubic oxidation.

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12 Paralinear oxidation. Breakaway oxidation. General parabolic equation. During high temperature oxidation, the onset of oxidation follows linear kinetics, eventually becoming parabolic with time. The equation that describes this behavior may be expressed as [2, 3]: C t k X k Xp l 2 Equation 2-5a X is the oxides thickness or mass, oxygen consumed per surface area, etc. kl and kp the linear and parabolic rate constant, respectively. t is time. A and C are constants. Equation 2-5a is known as the general parabol ic equation, and if the constant C is neglected, the equation may be rewritten as [2, 3]: l pk X t k X Equation 2-5b Cubic oxidation. Cubic oxidation occurs when the reaction rate falls between logarithmic and parabolic kinetics. This is characterized by an initially fast logarithmic behavior followed by the slower parabo lic behavior. Cubic behavior may be approximated by the following equation: 3 3 3C t k X Equation 2-6 X is the oxides thickness or mass, oxygen consumed per surface area, etc. k3 is the cubic parabolic rate. t is time and Cm is a constant. Paralinear oxidation. In paralinear kinetics, the oxidation is initially parabolic and gradually becomes linear with time. (Figur e 2-5A.) This type of behavior occurs when a compact oxide film transforms as a whole or partially into a non-protective porous layer. An inner compact protective layer remains within the oxide film next to the

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13 substrate, while the outer laye r is non-protective due to th e development of pores, voids, cracks within the oxide film, and/or the oxide volatizes. The rate equation for paralinear kinetics has been derived assuming that th e specimens surface area remains constant with time and the equation may be expressed as [2, 3]: t k x k k k k k Xl l p p l pln Equation 2-7 X is the oxides thickness or mass, oxygen consumed per surface area, etc. kp and kl are the parabolic and linear ra te constants, respectively. t is time. Linear Paralinear Time Parabolic X Time Parabolic X Linear Breakaway A B Figure 2-5. Graphic illustration of paralinear kinetics (A) an d breakaway oxidation (B). X is the oxides thickness or mass, amo unt of oxygen consumed, etc. Figures based on the ASM Handbook Volume 13a [6]. Breakaway oxidation. Breakaway oxidation occurs when the protective oxide film reaches a certain thickness in which cont inuous cracking and spalling occurs, a point at which the oxide loses its protective nature The oxidation is initially parabolic, until breakaway occurs and the oxidation then follows a linear behavior. (Figure 2-5B.) This type of kinetics can also occur when one of the components in the metallic alloy is selectively oxidized. Selective oxidation can deplete the surface of the component that forms the protective oxide, thus the protective oxide cannot reform and/or heal after

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14 spalling. Breakaway oxidation may seem simila r to paralinear kinetics, but the difference is that while the metallic surface is left bare during breakaway oxidation, an inner protective oxide layer remains during paralinear oxidation. Determination of Oxidation Kinetics from Rate Data Several methods are helpful in identifyi ng the kinetics involved during an oxidation process. One of the methods is based on the results of a linear regression performed on a double logarithm plot of the data. This me thod takes the logarithm of Equation 2-6 to obtain the following equation [2, 3] C t m X log 1 log Equation 2-8 X is the oxides thickness or mass, oxygen consumed per surface area, etc. m is the parameter for kinetic information. t is time and C is a constant. The data is then plotted as the Log(X) vers us the Log (t). The slope of the plot is calculated and compared to the slope of Equa tion 2-8 to find the va lue of m, thus the calculated slope is equal to 1/m. The identi fication of the kinetic rate is provided by the parameter m, which has values of 1, 2, and 3, corresponding to linear, parabolic, and cubic oxidation, respectively. Another method for elucidating the kinetics behavior is validating the data to the various kinetics rate equations. For example, to test for parabolic kinetics, the data is plotted as X2 versus time, and a linear regr ession is performed. The R2 value is then evaluated to see how well the parabolic mode l fits the data. This process would be repeated for each of the kinetics rate equations.

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15 Oxide Films Developed During High-Temperature Application Stainless steel alloys used for high-temper ature applications ar e required to have the ability to form a protective oxide film and the oxides identified as meeting this requirement are chromia (Cr2O3), alumina (Al2O3), and potentially silica (SiO2). [6, 8, 9, 10] These oxides have a low density of point defect s, thus serve as prot ective barriers due to their low diffusivity for ionic transport. This has led to the development of chromia former and alumina former steel families such as Fe-Cr, Fe-Al, Fe-Mn, Fe-Cr-Ni, and FeCr-Al. [8] The amount of alloying elements re quired for developing these protective oxides and the potential development of non-protective oxides depends on the base composition of the stainless steel, intended application, and working environment. Figure 2-6 shows the variation in oxide struct ure with chromium content for a Fe-Cr alloy oxidized at 1000C in 0.13 atm of oxygen. [6] From Figure 2-6, the oxide film changes from a Cr-rich protective oxide to a Fe-rich non-protective oxide with a decrease in chromium content. The minimum amount of chromium required to form chromia is approximately 10 wt%. The breakdown of protective oxides may be caused by cracking and/or spalling due to growth or thermal cycles, er osion, abrasion, and/or wear. [6, 8] When this occurs, the rate at which the oxide heal s (i.e. restores) will depe nd on the amount of the oxide forming elements within the bulk. If the cont ent is too low, the pr otective oxide will not heal, leading to the development of non-pr otective oxides and potential component failure. If the amount is too high, healing of the protective oxide is ensured but it might adversely affect mechanical propertie s, fabrication, and/or welding. [11] For example, chromia will not heal when the chromium content in the near-surface region where the oxide initially formed drops below 10 wt% due to chromium depletion. On the other

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16 hand, if the chromium content is between 20-25 wt%, the protective oxide will heal but the high amount of chromia will induce the fo rmation of sigma phases, which increases the alloys brittleness. [4, 12] Therefore, other alloying elem ents are added to aid in the healing of the protective oxide film, improve adhesion of the film, and/or reduce the detrimental effects caused by increasing the amount of the protective oxide forming elements. Some of these alloying elements are nickel, manganese, molybdenum, vanadium, and silicon. [8, 11, 13] Figure 2-6. Variation in oxide structure with chromium cont ent of a Fe-Cr alloy based on data obtained from isothermal (constant temperature) scans at 1000C in 0.13 atm of oxygen. [6]

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17 Common Oxides Although the ultimate goal is the developm ent of a continuous protective singlelayer film of chromia when stainless steels are exposed to high-temperature oxidizing environments, this rarely occu rs during real life applications. Chromia reacts with the monoxides of the alloying elements to form new oxides with a spinel st ructure of the type MCr2O4 in which M represents an alloying element. An example is FeCr2O4 (chromite), which is a mixture of the iron monoxide FeO and Cr2O3. [14] If the oxide structure of the alloying elements are similar, they may form solid solutions, such as (Fe, Cr)2O3 which is solid solution of Fe2O3 and Cr2O3. Thus, a variety of oxides may develop, ranging from protective to non-protective, fr om simple binary oxides to complex mixed oxides, due to the constituents present in stainless steel. The oxides may be divided into three basic crystal or lattice structure Cubic: MO Corundum: M2O3 Spinel: M3O4 or AB2O4 where M is a metal, A and B are two different metallic elements, and O is oxygen. Some of the principle binary oxides of the main c onstituent of stainless steel are presented in Table 2-1, and examples of common ternar y and complex oxides are presented in Table 2-2. The structure, morphology, com position, and complexity of the resulting oxide film will depend on the alloy composition and the working environment. Iron oxides. The main oxides of iron according to the Fe-O phase diagram (Figure 2-3) are FeO, Fe3O4, and Fe2O3, which are non-stoichiometric compounds. Wustite is metal deficient oxide and its general formula is FexO with x values ranging from 0.836 at the Fe-FeO boundary to 0.954 at the FeO-Fe3O4 boundary. It is considered a p-type

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18 semiconductor, and thermodynamically stable above 575C and at reduced oxygen activity (i.e. low pO2). FeO is antiferromagnetic and grows by outward diffusion of iron ions. Magnetite is a mixed oxide of Fe+2 and Fe+3 ions of the form Fe+2Fe+3 2O4. It has a slight stoichiometry variati on at high temperatures with a metal-deficiency at pO2 higher than the required for stoichiometry, and a metal-excess at pO2 lower than this value. It is ferrimagnetic, and its electrical properties are complex. Fe3O4 is an inverse spinel at and below room temperature (RT) and the di stribution of the tw o cations becomes randomized at temperatures above RT. Magne tite is obtained by th e partial oxidation of FeO or by heating Fe2O3 above 1400C. Hematite has a small stoichiometric variation and a variety of modifications wi th the more important being the and the form. -Fe2O3 is obtained when Fe3O4 oxidizes above 400C and it grows by outward diffusion of iron ions. It is consider ed a p-type semiconductor, al though it behaves as an n-type from 650C to 800C. -Fe2O3 is a ferrimagnetic metastable compound obtained by careful oxidation of Fe3O4. Chromium oxides. From an engineering pers pective, the most important chromium oxide is Cr2O3 because of its protective nature and it is the only thermodynamically stable oxide, especially at high temperature. It has a high melting temperature of 2450C and vaporizes to CrO3 at high temperatures and high pO2 according to the following reaction ) ( 4 3 2 13 2 3g CrO O O Cr2 Equation 2-9 Thus, this reaction becomes important at temperatures above 1000C in one atm O2 environment. Chromia is an electronic semiconductor and its conductivity changes depending on the temperature and pO2. It behaves as a metal-deficient (p-type)

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19 semiconductor at low temperatur es (T<1250C) or at high pO2, and as a metal-excess (n-type) semiconductor at low pO2. At high temperatur es (T>1250C), it is an amphoteric semiconductor. Cr2O3 films grow by counter-current diffusion of chromium and oxygen along short-circuit diffusi on paths and the growth occurs within the film. The oxide grows normal and parallel to the surface of the substrate, thus resulting in large stresses and strains that can lead to oxide cracking and potential spalling. The other oxides of chromium have lower melting temperatures and/or are intermediate compounds. CrO2 has a melting point of 400C and is an intermediate product in the decomposition of CrO3 to Cr2O3. CrO3 is a volatile oxide with a low melting point of 197C and Cr3O4 is a metastable compound. Manganese oxides and vanadium oxides. MnO2 is the most important of the manganese oxides, although it is not the most stable for it decomposes to Mn2O3 at temperatures above 530C. This oxi de has various modifications being -MnO2 (pyrulosite) the only stoichiometric form which has a rutile crystal structure. Mn2O3 (bixbyite) is the only oxide of the transition metal with M+3 ions that does not have a corundum structure Its form is known as the mineral hausmannite. Mn3O4 is a mixed oxide of Mn+2 and Mn+3 ions of the form Mn+2Mn+3 2O4 and forms when any manganese oxide is heated above 1000C in air. Mn O (mangonosite) is a basi c oxide that forms when any oxide of manganese is reduced in hydr ogen. It is subjected to stoichiometric variation and is a classic example of an antiferroma gnetic compound. V2O5 (shcherbinaite) is the most important of the vanadium oxides. It has a low melting point temperature of 674C, thus can liquefy at high temperatures, adve rsely affecting the oxidation resistance of the a lloy, and can even lead to catastrophic failure.

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20 Table 2-1. Principal binary oxides of iron, chromium, manganese, and vanadium. [14, 15] Composition, wt% Oxygen Oxidation State Crystal Structure Oxides of Iron (Fe) FeO 23.1-25.6 +2 Cubic -Fe2O3 30.1 +3 Corundum -Fe2O3 (Maghemite) 30.1 +3 Cubic Fe3O4 27.6-28.4 +2/+3 Spinel Oxides of Chromium (Cr) CrO2 38.1 +4 Rutile CrO3 48 +6 Orthorhombic Cr2O3 31.6 +3 Corundum Cr3O4 29.1 +2/+3 Spinel Oxides of Manganese (Mn) MnO 20-25 +2 Cubic -MnO2 36.81 +4 Rutile Mn2O3 30.4 +3 C-type rare-earth M2O3 -Mn3O4 28 +2/+3 Spinel Oxides of Vanadium (V) VO 24-27 +2 Cubic VO2 38.6 +4 Rutile V2O3 32-33 +3 Corundum V2O5 43.9 +5 Orthorhombic Table 2-2. Example of ternary and complex oxides of stainless steels. [14, 15] Crystal Structure (Fe, Cr)2O3 Corundum (Mn, Fe)2O3 Corundum FeCr2O4 Spinel FeV2O4 Spinel MnCr2O4 Spinel (Mn, Cr)3O4 Spinel Mn(Fe, Mn)2O4 Spinel (Mn, Fe)(Cr, V)2O4 Spinel (Mn, Fe)(V, Cr)2O4 Spinel General Oxidation Fe-Cr Alloys When a clean surface of a binary Fe-Cr alloy is exposed to oxygen, both iron and chromium atoms oxidize, thus the initial oxide film is mainly wustite with possibly a thin outer layer of magnetite and some chromia. The oxide film thickens by solid-state diffusion and the main competing processes o ccurring at the oxide/subs trate interface are

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21 Iron from the bulk diffuses outward through the oxide and reacts with the oxygen at the oxide surface, forming iron oxides, mainly FeO. Chromium from the bulk diffuses to the oxi de/substrate interf ace reducing FeO to iron and oxidizes to Cr2O3. The reduction of FeO occurs for it is less thermodynamically stable than Cr2O3. Oxygen from the reduced FeO diffuses inwa rds oxidizing the chro mium within the bulk, thus Cr2O3 particles form at and below the oxide/substrate interface. This is termed internal oxidation. The diffusion cross-section of the alloy is partially blocked by the internal oxidation particles and a conti nuous protective chromia may or may not develop. If the chromium content is sufficiently high, chromium from the bulk diffuses towards the interface and a continuous Cr2O3 oxide film forms. Oxidation kinetics is then controlled by diffusion through the growing ch romia, thus considered to obey parabolic kinetics. The permanence of such a layer depends on se veral factors, being the more important: The concentration and diffusion of the chromium within the bulk. The diffusion of oxygen within the bulk. The chromia growth rate. If the chromium content is low, a con tinuous layer does not develop, and a steady state is reached in which iron oxides and chro mia continue to form. Simultaneously, a solid-state reaction between the FeO and Cr2O3 occur yielding FeCr2O4. Because iron oxides grow at a slower rate than Cr2O3, the resulting oxide film structure consists of several layers (Figure 2-7) which are An outer layer formed by sublayers of ir on oxides in the order starting with the one next to the substrate a nd moving outwards: FeO, Fe3O4, and Fe2O3. An internal layer consisting of FeO and FeCr2O3. An internal oxidation la yer consisting of Cr2O3 particles within the bulk. Thus, the main diffusion process involved in the overall oxidation of Fe-Cr alloys is considered the outward diffusion of iron ions through the inner and outer layers of the

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22 oxide film. Within the inner oxide layer, voi ds, cavities, and pores develop as iron ions diffuse outward. This aids the inner oxide la yer to grow into the bulk due to the inward dissociative diffusion of oxygen across these de fects. The iron w ithin the internal oxidation region reacts with the oxygen formi ng FeO, which in turns reacts with the chromia particles to form FeCr2O4. The result of this overall reaction process is that the transition boundary between the oxides inne r and outer layers is delimited by the substrates original surface. The amount of FeCr2O4 within the inner oxide layer increases with increase in chromium content until part of the diffu sion cross-section is blocked, resulting in a decr ease in oxidation rate. Outer Layer of Iron Oxides Inner Layer of FeO and FeCr2O3 Oxide Film Internal Oxidation Layer Cr2O3Particles Substrate Original Substrate Surface Voids, Pores, Cavities Outer Layer of Iron Oxides Inner Layer of FeO and FeCr2O3 Oxide Film Internal Oxidation Layer Cr2O3Particles Substrate Original Substrate Surface Voids, Pores, Cavities Figure 2-7. Schematic illustration of the oxide film structure for Fe-Cr alloys of low chromium content. The outer layer c onsists of iron oxides in the following sequence starting closest to the metal substrate and moving outwards: FeO, Fe3O4, and Fe2O3. The mode in which the internal oxida tion particles nucleate is temperature dependent. [3] At high temperatures, volume diffusi on predominates resulting in a more homogenous nucleation of the oxi de particles. Grain boundary diffusion predominates at low temperatures, thus the oxide particles will preferentially nucleate along grain boundaries. This can be detrimental to the m echanical properties of the alloy. On the other hand, it can be beneficial if the internal ox ides are connected to the oxide film.

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23 They can anchor the oxide film to the substrate, thus improving adherence and avoiding spallation. Literature Review on the Oxidat ion of Stainle ss Steel in Air The oxidation behavior of st ainless steel is considered to obey parabolic kinetics, but this may be an oversimplification. [16] Douglass and Rizzo-Assuncao studied the isothermal oxidation of a Fe-19.6Cr-14.Mn a lloy in air for 24 hours in the temperature range from 700C to 1000C and observed that the general oxidation curves were parabolic, but true parabolic behavior was not observed due to the simultaneous formation of several oxides as the protective oxide was forming. [17] Hoelzer et al. observed that the oxidation of ODS Fe-Cr stainless steel re sembled parabolic kinetics when investigating the oxide films formed in lab air for 10,000 hours at 700C, 800C, and 900C. [18] During the investigation on the oxid ation of stabilized ferritic Fe-Cr stainless steels with different chro mium content in water vapor, Henry et al. observed that parabolic kinetics were followed wh en the samples were oxidized in flowing Ar+15% O2 at 900C up to 400 hours. [19] Brylewski et al. investigated the oxidation kinetics of DIN 50049 in air from 750C to 900C from 70 to 480 hours, and observed parabolic oxidation over the studied temperature range. [20] Other researches have observed non-parabolic kinetics or even change in behavior during an oxidation scan. Saeki et al. studied the oxidation in of 430 stai nless steel with different manganese content in 0.165 atm O2-N2 at 1000C from 0 to 1800 seconds, and observed that the oxidation rates did not fo llow parabolic kinetics. [21] This was also observed by Vosson et al. when studying the limits of oxidation resist ance in dry lab air of different types of stainless steels at 650C for 30 to 3000 hours. [22] When investigating the composition, structure, and thickness of the oxide films formed on MANET I when exposed to various

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24 oxidizing condition for different exposure times, Iordanova et al. observed that a simple kinetics law was not followed during the enti re oxidation process for samples oxidized in air at 600C up to 2 hours and at RT up to 1450 hours. [23] Doilnitsyna stated that the oxidation kinetics might vary qualitative a nd quantitative due th e various oxidation conditions. [24] Possible causes for the deviation fr om parabolic kinetics according to some of the researchers are Different type of oxides simultaneously fo rming with the protective chromia, thus altering the properties and st ructure of the oxide film. [16, 17, 21, 23] Chromium content at the oxide/substrate in terface and/or vari ation in the cation diffusivity during the oxidation process. [22, 23] Local oxidation gradient is di fferent from the general oxidation gradient across the oxide film because of transfer in differe nt directions due to grain curvature and internal grain oxidation. [24] Factors Affecting Oxidation Behavior The oxidation behavior and re sulting oxide films of stainl ess steels is influences by several factors. Some of these factors are composition (i.e., chromium content and alloying additions), surface treatment, all oy microstructure, and oxidizing environment (i.e., presence of water vapor). Effect of alloy composition It has been demonstrated that oxidati on rates and oxide thic kness are chromium content dependent. [17, 25-27] For example, high-Cr alloys exhibit lower oxidation rates and thinner oxide films than low-Cr alloys [17, 25, 26] Although the general guideline is a minimum chromium content of 10 wt% to 12. 5 wt% for the development of a protective oxide (i.e. chromia) in dry air, the minimu m value varies with stainless steel family, temperature, and alloying elements. [8, 17, 19, 22, 28]

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25 Stainless steel type. For austenitic alloys to fo rm a continuous chromia oxide the chromium content should be within the 16 wt% to 20 wt% range. [28] In the case of martensitic alloys, the chromium content is generally between 9 wt% and 12 wt%. If the martensitic alloy contains more then 12 wt% Cr it is difficult to austenitize the steel and develop a fully martensitic structure during the heat treatment processes. [29] Temperature. The critical chromium content at which the transition from protective to non-protecti ve oxidation occurs is temperature dependent. [8, 17, 27] Douglass and Rizzo-Assuncao observed that the minimu m chromium content for the development of a protective chromia oxide increased with increasing temperature from 12 wt% Cr at 800C to 21 wt% at 1000C. [17] According to Colson and Larpin, high temperature oxidation resistance in the 900C to 1200C te mperature range depends on the chromium content. [8] If the content falls below the minimum value of 10 wt%, the protective oxide may be destroyed by the formation of non-protective oxides, such as Fe2O3 or (Fe, Cr)2O3, on top of Cr2O3 film. Alloying elements. Martensitic alloys and ferritic alloys with less than 12 wt% of chromium are considered marginal chromia formers, requiring addition of selectively oxidizing elements to aid in the development of protective oxides. [20, 29, 30] These elements are generally silicon and manganese, which also are considered to enhance oxide adhesion. [20, 29] Silicon is considered to enhance oxidation resistance [10, 18, 19, 22, 31] and delay breakaway oxidation [32] by forming a silica (SiO2) film, which acts as a diffusion barrier for iron and chromi um ions. Silica may form as a continuous or discontinuous layer, or as oxide precipitates at the oxide/substrate or spinel/corundum interface. [10, 18, 19, 32] Manganese is a common addition to ferritic and martensitic alloys

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26 in substitution of nickel. [33] Manganese monoxide (MnO) has a more negative G than Cr2O3, thus considered to influe nce the oxidation kinetics. [21, 23] It is still debatable whether manganese addition is beneficial or detrimental to the oxidation process. Manganese is benefici al for it reduces chromium va porization slowing the oxidation process, but it is detrimental for it forms a spinel with chromia, leading to higher oxidation rates than pure Fe-Cr alloys. [20, 21] Other alloying add itions are the reactive elements (i.e. cerium, yttrium, hafnium, a nd thorium) which improve high-temperature oxidation resistance by allowing inward diffusion of oxygen to predominate and hindering the outward diffusion of cations. [8, 34, 35] When alloying element are added, complex spinel oxides may develop unless the chromium content increases such that only a Cr2O3 film develops. [17] These complex spinels may serve as a protective oxide film, but its growth may differ from that of pur e chromia resulting in a complex non-ideal oxidation behavior. [17, 21-23] Effect of surface treatment Oxidation is influenced by the surface treatment. Different procedures induce variation in the morphology and structure of th e initial oxide, which defines the oxidation resistance. [34-36] Mechanical polishing, grinding, and milling results in preferential distribution of the alloying elements within the oxide film [36], and enhances chromium diffusion due to the introduction of fast diffusion paths such as dislocations and subgrain boundaries. [25, 36-38] Cold working also promotes faster chromia formation due to the numerous nucleation sites intr oduced during cold working. [25, 36] Ostwald and Grabke observed differen ces in the formed oxides and bulk diffusivities during their study on the effects of surface treatment on the oxidation of 9-20

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27 wt% Cr stainless steels in different oxidiz ing environment at 600C from 1 to 100 hours. [37] Samples were electropolished (EP), polishe d (P), grinded (G), and sandblasted with fine (SBF) or coarse (SBC) particles. The study demonstrated that for all environments, the oxide formation increased the with degree of surface deformation in the order EP < P < G< SBF < SBC with the thickest film forming on the most severely deformed surface (i.e., SBC finish). [37] Bulk diffusivity was also observed to follow the same trend as the oxide formation. Depletion of selectively oxidized elements beneath the oxide/substrate interface decreased with incr eased surface deformation, with sharpest depletion in the EP samples and shallowe st depletion in the SBF/SBC samples. [37] Guillamet et al. studied the effects of surface prep aration on the oxidation of AISI 304 and AISI 316 in air from 900C to 1100C, and observed differences in the structure and the composition of the oxide films formed on the mechanically polished surfaces and the etched surfaces. [39, 40] The oxide film of the mechanically polished samples mainly consisted of Cr2O3 and MnCr2O4 with small amount of -Fe2O3 for both type of stainless steels. For the etched samples, both alloys developed an oxide film consisting of Fe3O4 and -Fe2O3 at 900C, but at higher temperatur es, the oxide film of the AISI 316 consisted of Cr2O3 and MnCr2O4 and the oxide film of the AISI 304 consisted of -Fe2O3. Kuiry et al. also observed this when in vestigating the oxidation of mechanically polished and electropolished AI SI 316 under isothermal and non-isothermal conditions. [36] Mechanically polished sample exhibi ted a Cr-rich, uniform, fine-grained oxide film. The oxide film of the electr opolished sample was a Fe-rich, non-uniform, coarse-grained oxide film with a nodular app earance and an internal oxidation zone of Cr/Ni-rich particles. Graham et al. obser ved that Fe-26Cr alloys samples that were

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28 electropolished had a different oxide structur e than those that were electropolished and vacuum annealed, although the film thic kness for both conditions were similar. [34, 35] The former developed an amor phous oxide and the oxide of th e latter consisted of large cubic grains. Effect of alloy microstructure Alloys with a fine grain microstructure exhibit better oxidatio n resistance than those with a coarse grain microstructure. [25, 38] This is attributed to the higher number of grain boundaries in the fine-grain ed structures that act as short-circuit diffusion paths, thus allowing faster chromium diffusion from the bulk to the oxide film. Grain size effect was also observed to influence the i nner layer thickness of th e oxide films formed in alloys with less than 20 wt% Cr. [25] The inner oxide layer of fine-grained alloys consisted of an uniform layer with unif orm healing along the grain boundaries at the oxide/substrate interface. Two types of structures were observed for the coarse-grained alloys: a uniform layer with no healing, or a non-uniform layer due to the local healing along the grain boundaries at the oxide/substrate interface, which inhibited the growth of the inner oxide layer at these sites. Diffusivity of chromium is higher for ferri tic and martensitic st ainless steels than for corresponding austenitic all oys due to the higher grain boundary density in the first two types of stainless steels. [8, 30, 37, 38] During the oxidation study of X20 (11Cr1MoV) at 600C in various environment from 1 hour to 672 hours, Segerdahl et al. attributed the better performance of the alloy to its co mplex microstructure when compared to austenitic alloys. [30] X20 is a tempered martensitic steel with martensitic and/or ferritic microstructure, in which lathe-like martensitic grains of micron size form within the former austenitic grains, resul ting in a high density of low-a ngle grain boundaries that act

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29 as fast diffusion paths. Peraldi and Pint st udied the influence of chromium and nickel on the oxidation behavior of austen itic and ferritic alloys, and observed differences in the oxidation behavior. [38] Ferritic alloys and austenitic al loys with high nickel content did not show oxide spallation, alt hough it did occur for low-Ni aust enitic alloys. It was also noticed that ferritic alloys performed better than austenitic alloys at 800C, which was attributed to a higher chromium diffusivity within the bulk. [38] Effect of water vapor Most stainless steel form and maintain a ch romia layer in dry air, but the presence of water vapor can have a detrimental eff ect, inducing rapid and catastrophic oxidation such as breakaway oxidation. [19, 30, 32, 33, 38, 41-43] During this phenomenon, slow growing chromia is formed during the initial protectiv e stage, after which the oxidation suddenly increases due to the formation of thick Fe-rich oxides. [19, 32] It is still not well understood the sudden formation of the Fe-rich oxid es and there are various theories Development of cracks within the passive oxide. [19, 32] Vaporization of Cr2O3 as volatile chromium species. [19, 30, 32] Chromium depletion in substrate region beneath the oxide/sub strate interface. [19, 32] Change in the oxides ioni c conductivity due to hydr ogen dissolution in the oxide. [30] The onset of breakaway oxidation varies with alloy composition and water vapor partial pressure (pH2O). [19, 30, 32] The onset is delayed by increasing the content of chromium or silicon content, or by decreasing the pH2O. During the protective stage, the parabolic rate constant in the presence of water vapor is higher than in dry air. [19, 30] Henry et al. attributed this observation to an in creasing in the concentration of hydroxide species in the oxide film due the presen ce of the water vapor, which has a higher diffusivity because of it smaller ionic radius. [19]

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30 General Oxidation of Stainless Steels Stainless steel alloys develop a native passiv e oxide that is typically 1-2 nm thick. [43-45] The composition mainly consists of chromium oxides and oxyhydroxides [44] and it has a double layer structure in which the outer Fe-rich layer is thic ker than the inner Cr-rich layer [45]. Upon oxidation, stainless steels follow the general oxidation behavior of binary Fe-Cr alloys. The composition and structure of the resulting oxide film depends on the oxidizing environment and the composition alloy. It is known that stainless steels form protective films of mixed oxide s rather than pure chromia, although pure chromia films have formed under the proper conditions. [8, 10, 17, 19, 22, 23, 28, 30, 33, 38, 46] These alternate protective oxides have been identified as Chromia with impurities of iron, manganes e, or nickel. Example: (Fe, Cr)2O3. Mixed oxides of the main alloying elements, particularly the spinel type. Example: FeCr2O4, MnCr2O4. The oxide film is generally a double-layer structure with an in ner Cr-rich layer at the oxide/substrate interface, and a Fe-rich la yer at the gas/oxide interface. Example of some of the oxides identified with in the inner and outer layers are [10, 17-19, 23, 25, 27, 29, 36-38, 44, 4749] Inner Layer: Cr2O3, FeO, Fe2O3, Mn2O3, SiO2, (Fe, Cr, Ni)1+XO, (Mn, Fe)2O3, NiCr2O4, NiFe2O4, (Fe, Cr, Mn)3O4, (Fe, Cr, Ni)3O4, Outer Layer: FeO, Fe2O3, Fe3O4, -Mn2O3, (Mn, Fe)2O3, (Mn,Cr)2O3, MnCr2O4, (Fe, Cr)3O4, (Mn, Fe)Cr2O4 In some studies, a third middle layer wa s detected, which was identified as Fe2O3 [17], MnFe2O4 [23], or (Fe, Cr, Ni)1-xO [38]. Other studies observed a continuous or discontinuous thin silicon oxide layer at the oxide/substrate interface consisting of

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31 amorphous SiO2 [10, 19, 22, 44] or a Si-rich oxide [18, 28, 42] film. Internal oxidation was not observed in some of the oxidation studies. [36] When internal oxi dation was observed, it consisted of Cr2O3 and/or SiO2 precipitates within the substrate [17, 19, 25, 28], or a chromia film along the martensitic lathe or grain boundaries [25, 29]. HT-9 Ferritic/Martensitic Stainless Steel Fe-Cr stainless steels are currently used as structural material for gas turbine components [26], and for fossil fuel and nucle ar power plants components [13, 26, 41, 50-53]. This alloy is being considered for fusion power systems [26, 41, 54], advanced accelerator systems [55], and solid oxide fuel cell (SOFC) application [20]. For nuclear applications, ferritic and martensitic stainless steels are pr eferred over austenitic stainless steel due to their better mechanical and thermal properties. [13, 26, 50-52, 54] High resistance to radiati on induced void swelling, cr eep, and He embrittlement. High thermal conductivity and low coefficient of expansion reduce the development of thermal stress and fatigue. Excellent dimensional stability at high di splacement with less than 0.5 % swelling. HT-9 (DIN X20CrMoWV12 1 or Fe -12Cr-MoVW) is high-chromium heat-resistant alloy that may have a martens itic and/or ferritic microstructure depending on the austenitizing and tempering treatments. Sandvik (Sweden) first developed the stainless steel alloy for high temperature application that did not require the inherent corrosion resistance of aust enitic stainless steel. [41] HT-9 evolved for the alloying of AISI 410 with molybdenum (Mo), vanadium (V ), and tungsten (W) to obtain an alloy with improved creep rupture streng th as presented in Figure 2.8. [13, 26] These alloying elements are known to increase the mechanical strength by solid solution and

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32 precipitation strengthening. [13] The typical composition of HT -9 is presented in Table 2-3 along with the composition from different steel manufacturers. [11, 13, 26, 41, 50, 52] Fe-12Cr AISI 410 Fe-12Cr-0.5Mo Fe-12Cr-1MoV HT-91 (DIN X20CrMoV12) Fe-12Cr-1MoWV HT-9 (DIN X20CrMoWV12) Add MoAdd Mo & VAdd W 35 MPa60 MPa Fe-12Cr AISI 410 Fe-12Cr-0.5Mo Fe-12Cr-1MoV HT-91 (DIN X20CrMoV12) Fe-12Cr-1MoWV HT-9 (DIN X20CrMoWV12) Add MoAdd Mo & VAdd W 35 MPa60 MPa105hours Creep-Rupture Strength at 600C Figure 2-8. Development of a 12% Cr ferritic/m artensitic stainless st eel. Figure based on Viswanathan and Bakker [13], and Klueh and Harries [26]. Table 2-3. Composition of HT-9 steel in weight percent according to different references. Typical Composition [11] Sandvik [26, 41] Vallourec Mannesman [13] Carpenter Technology Corporation [50, 52] Cr 11.5 11.5-12 12 11.8 Ni 0.5 0.5-0.6 0.5 0.5-0.6 Mn 0.6 0.6 0.6 0.5 Si 0.4 0.4 0.4 0.2-0.3 C 0.2 0.2 0.2 0.2 Mo 1 1 1 1 P 0.030 max 0.001-0.012 S 0.020 max 0.002-0.003 Cu 0.04 W 0.5 0.5 0.5 0.4-0.5 V 0.3 0.3 0.25 0.33 Al 0-0.035 Fe Balance Balance Balance Balance The typical microstructure of HT-9 consists of a tempered martensite with predominantly M23C6 precipitates and possibly, small amount of -ferrite. [50-52] M23C6 is a Cr-rich carbide that in addition, may c ontain iron, tungsten, molybdenum, vanadium, and nickel. This carbide pr ecipitates along high-e nergy regions such as prior-austenite grain boundaries and mart ensitic lathe boundaries. [52, 56a, 57a] Based on the constitutional

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33 diagram for Fe-Cr alloys [29] and the Schaeffler diagram[58a] shown in Figure 2-9, the microstructure will generally consists of a martensitic phase and a ferritic phase. The amount of each phase will depend on the austenitizing and tempering treatments 015 10 5 200 600 800 1000 400 1400 1200 1600Cr content or Cr requirement, wt%Temperature, C Austenite Ferrite Ms, martensite start temperature 12 015 10 5 200 600 800 1000 400 1400 1200 1600Cr content or Cr requirement, wt%Temperature, C Austenite Ferrite Ms, martensite start temperature 12 SchaefflerDiagram Constitutional Diagram for Fe-Cr AlloysPredicted microstructure for a Fe-12Cr stainless steel alloy. A B Figure 2-9. Constitutional diagram for Fe-Cr all oys (A) and Shaeffler diagram [58a] (B). Figure 2-9A based on Ennis and Quaddakkers [29].

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34 General Applications of HT-9 Stainless Steel HT-9 is a well-established alloy widely used in Europe and South Africa for the fabrication of boiler and heat exchanger tubes, pipes, and header for steam temperatures of up to 540C in fossil fuel power generation industries. [13, 55] It has an extensive stressrupture database exceeding 100,000 hours of operating within the 500C to 600C temperature range with operating experience of over 20 years in Germany, Belgium, Holland, Scandinavia and South Africa. [13] Regardless, HT-9 ha s found little use in the US, England, and Japan due to fabrication difficulties, especially during welding and post-welding heat treatments. [13, 55] The difficulties arise due to the high carbon content and low martensitic transformation temperature (TM) of the alloy that could promote austenite phase retention after welding, high residual stresses, and cracking before and during the stress relief treatment. [13, 59] Even though the difficulties have been overcome by careful control of the heat treatment proc esses, it still has not found much application in the US. [13] HT-9 is considered as a substitute for aust enitic alloys in the fabrication of in-core and out-of-core components for nuclear power applications, particularly for liquid-metal cooled fast-breeder reactors (LMR). [26, 41, 50-54] HT-9 cladding and ducts are currently used in the Fast Flux Test Facility (FFTF), a LMR system located at Richmond, WA. [26, 41] It is a candidate alloy for first wa ll and tritium breeding blanket components in fusion reactor applications. [26, 41, 54], and for pipes and molten metal vessels in the accelerator-driven system (ADS) applications. [55] ADS Applications of HT-9 Stainless Steel The need to address the issues related to the management of nuclear waste has prompted the search for alternate options of handling nuclear waste. One option is the

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35 transformation of long-lived radiotoxic material into products that are short-lived or nonradioactive by means of a fission reacti on, a process known as transmutation. [60] Transmutation is the concept behind the deve lopment of advanced accelerator systems (ADS), which in the United States operates under the Department of Energys (DOE) Advanced Fuel Cycle Initiative (AFCI) progr am, an outgrowth of the DOEs Advanced Accelerator Applicat ions (AAA) program. [61] In ADS, a target material is bombarded by high-energy protons, generating high-energy neutrons called spallation neutrons. When the generated spallation neutrons are absorbed by the nuclear waste, fission reactions are induced, thus transmuting the nuclear waste. Some of the benefits of ADS are the minimization of nuclear waste, the reducti on of plutonium by-product in nuclear power plant, and the energy generated during the tran smutation process can be used to generate electrical energy. [60, 61] Unfortunately, the transmuta tion process imposes stringent material requirements. The materials select ed for ADS application should be able to withstand high neutron fluxes, elevat ed temperature, and corrosion. Lead and lead alloys, such as lead-bismuth eutectic (LBE), have been identified as candidate materials for ADS application of target material (generating spallation neutrons) and coolant (for removing heat dur ing transmutation). However, molten lead and molten LBE is corrosive to HT-9, one of the stainless steel candidates considered for piping and for molten metal containers. [55] Liquid metal corrosion can manifest itself as liquid penetration along grain boundaries, stainless steel di ssolution, and formation of undesirable compounds at the LBE/ steel interfaces that can re duce the life expectancy of the steel components and lead to catastrophic failure. [62-64]

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36 The Russians have used LBE as a coolan t in their nuclear submarines for over several decades. Their e xperience has shown that the steel is protected from LBE corrosion by an oxide layer and that cont rolling the oxygen concen tration within the molten metal is important for the development and maintenance of the protective oxide film. [65] Barbier et al. has studied the behavior of aust enitic and martensitic steels in flowing LBE under controlled oxygen c oncentration (levels within the 10-6 wt%) with exposure times of up to 3116 hours. [62, 66] Their results indicate that the development of an oxide layer on the steel surface can protect the metal from dissolution, an effect of liquid metal corrosion. The behavior of pa ssivated (pre-oxidized) austenitic steel and martensitic steel in static LBE under isothe rmal conditions have been investigated by Soler Crespo et al. and Lillard et al. [67, *] In both studies, the oxi de layer developed prior to LBE exposure protected the steel from liqui d metal corrosion. This has given way to the hypothesis that the oxides films of passi vated stainless steel may protect the alloy from LBE corrosion, thus prompting interest in understanding the oxidation kinetics and mechanisms of HT-9 stainless steel, and in elucidating the structure and composition of its oxide films. Lillard, R.S., C. Valot, M.A. Hill, P.D. Dickerso n, and R.J. Hanrahan, Personal Communication. 2003.

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37 CHAPTER 3 EXPERIMENTAL PROCEDURE This chapter discusses the thermogravim etric analysis (TGA) methodology used for the isothermal and non-isothermal oxidation sc ans, including the equipment used during the scan and experimental parameters. Sa mple preparation and oxide characterization techniques are also presented. Sample Preparation HT-9 (martensitic/ferritic, Fe-12Cr-1Mo VW or DIN X20CrMoWV12 1) stainless steel alloy used for this research was provided by the University of Michigan (UM). It is part of an ingot previously provided to UM by Oak Ridge National Laboratory and later heat treated at Argonne Nati onal Laboratory-West. The provided steel alloy piece was cut into 20 mm x 4 mm x 1.5 mm flat bars by Microcut, Inc. (York, PA) using electrical discharge machining (Wire-EDM). Samples for the various oxidation scans were obtained by cutting the flat bars in half on a low speed diamond wheel saw (S outh Bay Technology, Model 650) using a cubic boron nitride (CBN) wafer blade (Sout h Bay Technology, 4 inch x 0.012 inch coarse/low blade). The cut samples were wet polished from 320 gr it to 1200 grit on all the exposed surfaces using sili con carbide (SiC) grinding paper (LECO 8 inch wet/dry C weight grinding paper). The two main surfaces were further polished to a 1 m finish using an alumina (Al2O3) powder suspension (LECO alpha alumina powder) on a synthetic rayon polishing cloth (B uehler MicroCloth or equiva lent). After polishing, the samples were ultrasonically cleaned for 5 mi nutes in methanol followed by 5 minutes in

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38 acetone after which they were thoroughly drie d and stored in a dessicator cabinet until needed. Prior to an oxidation scan, a sample would be retrieved from the dessicator cabinet and its length, width, and thickness m easured with a caliper (Scienceware). The initial weight is obtained using a precisi on balance (Sartorius Research Semimicro Balance, model R 160 P). Wavelength Dispersive Spectroscopy (W DS) for compositional analysis was performed on a polished sample using an EPMA JEOL Superprobe 733 located at the Major Analytical Instrumentation Center (MAI C) at the University of Florida (UF). Measurements were taken on seven different locations on the samples surface. The detected elements and the average weight pe rcents (wt%) for each element are presented in Table 3-1. The wt% of each element is based on the average of the seven measurements. The results obtained by WDS are in good agreement with the composition provided by UM and the typi cal composition found in the literature [11]. Table 3-1. Composition of HT-9 steel in weight percent (wt%). Composition Provided by UM Typical Composition [11] Average of WDS Measurements Standard Deviation of WDS Measurements Cr 11.63 11.5 11.43 1.22 Ni 0.5 0.5 0.73 0.24 Mn 0.52 0.6 0.65 0.03 Si 0.22 0.4 0.10 0.02 C 0.2 0.2 0.11 0.27 Mo 1.0 1 0.95 0.29 P 0.02 S 0.006 Cu 0.04 W 0.52 0.5 0.30 0.28 V 0.3 0.3 0.27 0.42 Co 0.08 0.16 0.03 Fe Balance Balance Balance 1.79

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39 The HT-9 sample used for WDS analysis was further polished to a 0.25 m finish using an alumina (Al2O3) powder suspension (LECO alpha alumina powder) on a synthetic rayon polishing cloth (B uehler MicroCloth or equiva lent). After polishing, the sample was ultrasonically cleaned for 5 mi nutes first in methanol followed by acetone, and then thoroughly dried. The surface was et ched using Marbles Reagent to reveal the microstructure of the as-received material. The sample was mounted on an aluminum (Al) scanning electron microscope (SEM) st ub using copper tape and the etched surface characterized with a JEOL Field Emissi on Gun (FEG) SEM (Model JSM-6335F) located at MAIC-UF. The microstructu re consists of a mixture of ferritic and martensitic phases with carbide particles along the boundaries between the two phases, as shown in Figure 3-1. A line scan across a pa rticle reveals an enrichment of carbon and chromium, and depletion of iron, thus the particles seem to be Cr-r ich carbides. (Figure 3-2.) Ferrite Martensite Carbide Particles Figure 3-1. Secondary electron (SE) image of the microstructure of the as-received HT-9 stainless steel alloy after etching the surface with Marbles Reagent.

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40 SE Mn C V S Cr Si Fe SE Mn C V S Cr Si Fe 1 1 1 1 n m Figure 3-2. SE image and line scan of a pa rticle located along the boundary between the martensite phase and the ferrite phase. The line scan profile indicates that the particle is enriched in carbon (C) and chromium (Cr), and depleted in iron (Fe). Oxidation Scans Equipment The equipment used for the experimental scans can be grouped into three main categories: 1. Thermogravimetric analyzer (TGA) system and supporting accessories. 2. Data acquisition system. 3. Gas delivery system and supporting equipment. The schematic of the equipment set-up is shown in Figure 3-3 and images of the actual equipment are shown in Figure 3-4.

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41 Argon Air Gas Switching Accessory Thermal Gas Purifier Balance Purge Flow Meter Furnace Purge Flow Meter TGA 2050 Furnace Heat Exchanger GBIP Cable Data Acquisition System To Exhaust Interconnecting Cable Figure 3-3. Schematic of the equipment setup. The set-up is divided into the data acquisition system, the TGA system, and the gas delivery system.

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42 Thermal Gas Purifier Data Acquisition System Furnace Heat Exchanger TGA 2050 Argon and Air Gas Cylinders Gas Switching Accessory Double-Tube Flow Meter A B Figure 3-4. Photograph of th e actual equipment used duri ng the oxidation scans. The flow meter in A is a double-tube configur ation. The scaled tube on the left side of the flow meter regulates the fu rnace purge gas flow rate and the one on the right side regulates the balance purge gas flow rate. TA Instruments TGA 2050. The TGA consists of two main components, a desktop cabinet and a heat exchanger. Th e cabinet houses the ba lance assembly, the furnace assembly, the Platinel II thermocouple, the purge gas inlet for the furnace and for the balance, and the systems electronic and mechanical parts. The TGA is controlled by a computer to which it is connected via a GPIB cable. The balance assembly consists of

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43 a balance arm with two hang-down wires, one for the sample pan and one for the tare (counterbalance weight) pan, and a bala nce arm sensor unit. (Figure 3-5.) Balance Purge Gas In Furnace Purge Gas Inlet Electric Resistance Heater Furnace Base Quartz Tube Rubber Cap Quartz Liner Water Cooled Jacket Furnace Core Off-Gases Outlet Rubber Gasket Balance Arm Tare Pan Sample Pan Thermocouple Balance Purge Gas Inlet Hang-Down Wire Balance Electronics Cover TGA 2050 Balance Assembly Furnace Assembly Balance Purge Gas In Furnace Purge Gas Inlet Electric Resistance Heater Furnace Base Quartz Tube Rubber Cap Quartz Liner Water Cooled Jacket Furnace Core Off-Gases Outlet Rubber Gasket Balance Arm Tare Pan Sample Pan Thermocouple Balance Purge Gas Inlet Hang-Down Wire Balance Electronics Cover TGA 2050 Balance Assembly Furnace Assembly Figure 3-5. Cross-section view of the TGA 2050 balance assembly and furnace assembly. The sensor unit consists of a transducer which is coupled to the balance arm, an infrared LED light source, a pair of photodiodes, and a printed circuit board assembly. As the balance arm moves due to weight changes, th e amount of light that hits each diode is unequal. This causes a changes in the amount of current the transducer requires to keep the balance arm in the horizontal reference (null) position, thus the samples weight change is directly proportional to the variations in the tr ansducer current flow. The furnace assembly consists of a water-cooled j acket that houses the quartz sample tube, the

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44 quartz liner, the furnace core (r efractory ceramics), and the re sistance heating elements. (Figure 3-5.) The quartz liner protects the he ating elements and the furnace core from corrosive gases that could be present with in the sample tube. The furnace assembly moves vertically for sample loading and unl oading. Although the furnace has a vertical configuration, the purge gas fl ow is horizontal. The purge gas enters through the right side gas inlet, flowing direct ly across the sample placed on an open sample pan, and out the left side gas outlet. A spectrometer can be hooked up to the gas outlet to analyze the gases evolved during the oxidation process. The heat exchanger dissipates the heat generated by the furnace and consists of a fa n, a radiator, a water reservoir, a pump, and temperature and flow sensors. Data acquisition system. The data acquisition system consists of a Windows 2000 based computer running two TA Instruments software: Thermal Advantage and Universal Analysis. Thermal Advantage is the controller software through which the user inputs the experimental parameters th at controls the operation of the TGA, and receives and saves in a file the raw data. Universal Analysis is the data analysis software that reads that raw data file, allowing the us er to plot, manipulate and save the data in other file formats. Both programs are proprietary of TA Instruments. Gas delivery system. The gas delivery system consists of a TA Instruments Gas Switching Accessory (GSA), a Restek Corpor ation single-tube thermal gas purifier, a double-tube flow meter, and tw o gas cylinders with their regulators. The GSA is a solenoid-controlled gas manifold that allows for manual or automatic furnace purge gas switching during an experimental run. It is connected to the TGA via an interconnecting cable, which controls the switching when the GSA is in automatic mode. The thermal

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45 gas purifier consists of a sm all furnace with a zirconium-granule filled stainless steel converter tube. The purifier removes oxyge n, water, carbon monoxide, carbon dioxide, and hydrocarbons (except methane) to parts per billion (ppb) levels in the gas stream. The left-side tube of the doubletube flow meter controls the furnace purge gas flow rate and is connected to the GSA purge gas inlet. The right-side tube controls the balance purge gas flow rate and is connected to the TGA balance purge inlet. The gases used are from Praxair, which are Ultra High Purity Argon (Ar) identified as Gas 1 and Grade Zero Air identified as Gas 2. The Ar gas consta ntly flows through th e thermal gas purifier prior to flowing into the furnace and balan ce to minimize the presence of oxygen in the gas stream. Non-Isothermal Scan Procedure Prior to loading a sample into the TGA, the non-isothermal parameters are input into the controller software which will ramp the furnace from room temperature (RT) to 950C at 2C/minute or at 5C/ minute, and then cooled down to RT. The data sampling rate is set to 6 data points/mi nute (i.e. sampling rate of 10 s econds/point). An example of a non-isothermal program is shown in Tabl e 3-2 and an example of the temperature profile for this type of scan is shown in Fi gure 3-6. The furnace purge gas is switched to dry air prior to loading the sample for Ar is the default purge gas that constantly flows through the furnace and balance when the TGA is not in use. The furnace purge gas is dry air during the non-isothermal scan (i.e. ramp-up to selected temperature) and Ar during the cool down. During the experimental scan, the furnace purge gas flow rate is kept at a constant 90 cm3/minute and the balance purge gas flow rate at 10 cm3/minute.

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46 Table 3-2. Example of a non-isothermal program. Program Segment Command Meaning/Description 1. Data Storage: On Starts data recording. 2. Ramp 2.00C/minute to 950.00C The furnace is ramped from RT to 950C at 2C/minute. 3. Select Gas: 1 The gas is switched from dry air (Gas 2) to Ar (Gas 1) prior to th e cool down segment. 4. Equilibrate at 30.00C This is the cool down segment. The controller automatically sets the cooling rate. Cool Down Segment Non-Isothermal Scan (Ramp-up Segment) Cool Down Segment Non-Isothermal Scan (Ramp-up Segment) Figure 3-6. Example of a non-isothermal temper ature profile. This plot was generated by Universal Analysis using the raw data file for a sample oxidized in dry air from RT to 900C at 2C/minute. To initiate a non-isothermal scan, a sample is placed on the raised edge of an alumina pan and loaded into the TGA. The sample is held for an hour within the closed furnace in which dry air is flowing prior to initiating the scan, which allows for the removal of lab air that coul d have been introduced into the furnace when loading the sample. After an hour, the program is star ted, in which the furnace is ramped to the selected final temperature at the selected ra mp rate, and then cooled down to RT. The sample is then unloaded, its final weight recorded, and the surfaces visually inspected.

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47 Isothermal Scan Procedure Prior to loading a sample into the TGA, th e isothermal scan is programmed into the controller software and the data sampling rate is set to 6 data point s/minute (i.e. sampling rate of 10 seconds/point). An example of an isothermal program is shown in Table 3-3 and its corresponding temperature profile is sh own in Figures 3-7 and 3-8. The various combinations of isothermal temperature and holding time used during this research are shown in Table 3-4. Table 3-3. Example of an isothermal program. Program Segment Command Meaning/Description 1. Data Storage: On Starts data recording. 2. Equilibrate at 950.00C This is the ramp-up segment. The controller automatically sets the ramping rate. 3. Mark end of cycle 0 An indicator is inserted in the data file to identify the end of the equilibrate segment. 4. Isothermal for 5.00 minutes Holding tim e to allow temperat ure to stabilize. 5. Select Gas: 2 The gas is switc hed from Ar (Gas 1) to dry air (Gas 2) prior to the isothermal scan. 6. Isothermal for 5.00 minutes Holding time to allow gas flow to stabilize. 7. Mark end of cycle 0 A marker is inse rted in the date file to identify the end of the holding time. 8. Isothermal for 360.00 minutes Isothermal holding time of 6 hours. 9. Select Gas: 1 The gas is switched from dry air (Gas 2) to Ar (Gas 1) prior to th e cool down segment. 10. Equilibrate at 30.00C This is the cool down segment. The controller automatically sets the cooling rate. Table 3-4. Isothermal scan temperat ures and holding time combinations. Temperature Holding Time 600C, 700C, and 800C 48 hours 825C, 850C, and 875C 24 hours 863C 24 hours and 48 hours 900C 30 minutes, 90 minutes, and 24 hours 950C 6 hours

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48 Cool Down Segment Isothermal Scan Temperature Stabilization Segment Gas Flow Stabilization Segment Isothermal Hold Segment Ramp-Up Segment A B Figure 3-7. Example of an isothermal scan te mperature profile. The plots were generated by Universal Analysis using the ra w data files for the 700C/48hrs, 850C/24hrs, and 950C/6hrs samples. B is a closer view of the ramp-up segment of the scans. To start an isothermal scan, a sample is pl aced on the raised e dge of an alumina pan and loaded into the TGA. Once the sample is loaded, it is held for three hours within the

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49 To start an isothermal scan, a sample is pl aced on the raised e dge of an alumina pan and loaded into the TGA. Once the sample is loaded, it is held for three hours within the closed furnace with a constant Ar flow. Th is waiting period allows the removal of lab air that could have been introduced into the furnace when loading the sample. After the waiting period, the program is initiated a nd the furnace is ramped to the selected temperature by at a ramping rate selected by the controller, held for a period of time at the selected temperature, and then allowe d to cool down to RT before unloading the sample. During the isothermal scan, the furnace purge gas is automatically switched between dry air and Ar according to the progr ammed instructions. The purge gas is dry air during the isothermal segment, and Ar dur ing the ramp-up and cool down segments. The flow rate is kept at 90 cm3/minute at all times, regardle ss of the type of gas. The balance purge gas is Ar flowing at 10 cm3/minute. At the end of the scan, the sample is unloaded, its final weight recorded, a nd all the surfaces visually inspected. Weight Change Calculations The weight change ( W) is calculated using Equation 3-1 and the raw data associated to the non-isothermal or isothermal segment. The calculated data is then plotted as W versus temperature for non-isothermal scans and as W versus time for the isothermal scans. A W W Wo i Equation 3-1 W is the weight change, mg. Wi is the weight at time i, mg. Wo is the weight at the beginning of the scan (i.e., for the isothermal scans it is the time dry air is introduced into the furnace, and for the non-isothermal scan it is the start of the ramp-up), mg.

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50 A is the total surface ar ea of the samples, cm2. Oxide Characterization The morphology and composition of the oxides developed during the isothermal scans were characterized by scanning elec tron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) system. The pha ses and compounds present in the oxides were elucidated by X-ray diffraction (XRD). Surface Characterization Secondary electron (SE) imaging and elem ental composition of the surface of the oxidized HT-9 samples were characterized with a JEOL Field Em ission Gun (FEG) SEM (Model JSM-6335F) equipped with an Oxford Link ISIS system for EDS analysis (MAIC-UF). Samples were mounted on alumi num SEM sample holders with the aid of conductive copper tape. Oxide from the backsi de of the sample was removed prior to mounting to ensure good contact between the sample and the sample holder. Cross-Section Characterization After analyzing the oxidized surface, portion of the samples were cut off with South Bay Technology low speed diamond wheel saw (Model 650) equipped with a 4 inches x 0.012 inches CBN wafer blade. The pieces were hot compression mounted using a thermosetting mounting compound (Bue hler Epomet F mounting powered). Small flat bars of stainless steel were also mounted with the HT-9 samples to provide edge retention protection during polishing. After the mount had cooled down to room temperature, it was metallurgically polishe d from 320 grit to 1200 grit (LECO 8 inch wet/dry C weight SiC grinding paper) followed by a 1.0 m to 0.25 m finish (LECO alpha alumina powder suspension on a Buehler MicroCloth or equiva lent synthetic rayon polishing cloth). After polis hing, the mount was ultrasonically cleaned to remove

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51 polishing residue and then thor oughly dried. The ultrasonic cleaning was first done in a soapy distilled water bath followed by a distil led water bath with a distilled water rinse between baths. The mount was carbon coated at MAIC to provide sample conductivity during SEM observation. The thickness, co mposition, and structural layers for the developed oxide films were characterized with the JEOL JSM-6335F SEM coupled with an Oxford Link ISIS system at MAIC-UF. Oxide Compound Characterization A non-oxidized sample (i.e. reference sample) and samples oxidized from 825C to 950C were mounted on glass slides with the aid of double sided adhesive tape. A glass slide was inserted into the chamber of a Phillips APD 3720 diffractometer (PW 1710 Based system) connected to a Windows ba sed computer running Phillips PC-APD controller software located at MAIC-UF. The diffractometer was set to perform a continuous scan from 15 to 70 in 0.04 increments. After the scan was completed, a peak search was done using the PC-APD software and the results were run through Phillips PC-Identify software for peak matching. PC-Identify compared the provided data to its database of known co mpounds and generated a list of potential candidates. Once the list was generated, each candidate was evaluated for best fit to the obtained results. After characterizing the samples in bulk form, the oxide of the samples oxidized from 850C to 900C was scrapped off from the surface and mounted on glass slides. (Figure 3-10.) The oxide fragment that sp alled during handling of the 950C sample was also mounted on a glass slide as shown in Figure 3-10. The mounted oxide powders and

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52 oxide fragment were characterized following the same procedure as for the samples in bulk form. The 600C to 800C samples were characte rized on a Phillips XPert MRD System operated by MAIC staff. The diffractometer wa s set to perform a continuous scan from 20 to 80 in 0.02 increments at a 3 glancing angle. After the scans were completed, the data was evaluated following the same procedure used to evaluate the data obtained with the APD 3720 diffractometer Figure 3-10. Oxide scrapped from the surface mounted for XRD characterization. The fragment for the 950C/6hrs sample is an oxide fragment that spalled during handling. Platinum Marker Experiment A thin platinum (Pt) line is painted on the surface of a non-oxidized HT-9 sample prepared as previously mentioned using Pt ink from Heraeus Circuit Division. The sample is placed in a desiccator cabinet for 24 hours to allow the ink to thoroughly dry. Figure 3-11 shows the HT-9 sample with the dried Pt ink marker. After 24 hours, the sample is loaded into the TGA and oxidized in dry air at 900 C for 24 hours following the procedure for an isothermal oxidation scan. After the scan is completed, the sample is unloaded and the surfaces visually inspected. To find the location of the Pt marker, a cross-sectional sample is prepared by cu tting a portion of the oxidized sample and

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53 metallurgically polishing the cross-secti on from 320 grit to 1200 grit (LECO 8 inch wet/dry C weight SiC grinding paper) followed by 1.0 m to 0.25 m finish (LECO alpha alumina powder suspension on a Buehler MicroCloth or equiva lent synthetic rayon polishing cloth). After polishing, the cross-section sample was ultrasonically cleaned to remove polishing residue and then thoroughly dr ied. The ultrasonic cleaning was first done in soapy distilled water followed by a di stilled water rinse, th en in acetone. The sample was vertically mounted on an alumin um SEM sample holder to which a hole had been previously drilled for the purpose of hol ding the sample in a vertical position. To avoid charging during the SEM characteriza tion, conductive copper tape was used to ground the sample to the sample holder. The tape was attached to the backside of the sample, from which the oxide had been remove d prior to mounting. The location of the Pt marker was determined from the SE images, elemental mapping, and line scans obtained during the characterization process using the JEOL JSM-6335F SEM coupled with an Oxford Link ISIS unit (MAIC-UF). Pt ink HT-9 Sample Pt ink HT-9 Sample Figure 3-11. HT-9 sample with the Pt mark er after drying for 24 hours in a dessicator cabinet and prior to the isothermal scan.

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54 CHAPTER 4 RESULTS The results from the experimental oxidat ion scans and the various characterization techniques are presented in this chapter. The discussion of the results is presented in Chapter 5. Non-Isothermal Scan Results Obtaining kinetics information from non-isothe rmal scans is a controversial issue. One school of thought argues that it is possible, while others do not agree. The general consensus is that non-isothermal curves pr ovide on overview of the oxidation kinetics and mechanisms by indicating where changes in ox idation behavior occur. It is for this reason that non-isothermal scans were performed. HT-9 samples were prepared and subjected to non-isothermal scans according to the experimental procedure detailed in Chapte r 3. The ramping rate for samples 1 and 1a was 2C/minute, and 5C/minute for samples 2 and 2a. The weight change ( W) of each sample was calculated using Equation 3-1 and the raw data generated during their respective scans as detailed in Chapter 3. The resulting W versus temperature curves are shown in Figure 4-1. The overall W and surface area is presented in Table 4-1. Table 4-1. Surface area and weight ch ange for HT-9 samples oxidized under non-isothermal conditions. Average surface area is 0.99 cm2. Sample Surface Area, cm2 Weight Change, mg/cm2 1 0.94 0.730 1a 1.04 0.575 2 0.92 0.220 2a 1.05 0.163

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55 Non-Isothermal Scans of HT-9 in Dry Air from RT to 950CTemperature, C 01002003004005006007008009001000 W, mg/cm2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1 1a 2a Change in predominant oxidation kinetics/mechanisms. 2Ramping Rates Dashed Lines: 2C/minute Solid Lines: 5C/minute Figure 4-1. Calculated weight change versus temperature pl ots for HT-9 samples oxidized under non-isothermal conditions. Samples were oxidized dry air and heated to 950C at a ramping rate of 2C/minute (1 and 1a) or 5C/minute (2 and 2a). The non-isothermal plots show a change in profile between 800C and 900C, which is associated to changes in the oxi dation behavior. The significance of this observation is that two differe nt behaviors could be exp ected during the isothermal oxidation scans from 600C to 960C. Thus, the predominant oxidation kinetics and the oxide film of he samples oxidized above th e 800C to 900C transitional range could be different than the samples oxidized below this transitional range. Isothermal Scan Results Isothermal scans are useful in elucida ting the oxidation kinetics and mechanism. Reaction rate constants and activ ation energies can be derive d from an analysis of the resulting plots, aiding in de fining the oxidation behavior.

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56 HT-9 samples were prepared and subjected to isothermal scans according to the experimental procedure detailed in Chapter 3. The W of each sample was calculated using Equation 3-1 and the raw data generate d during their respectiv e scans as discussed in Chapter 3. The calculated W versus time curve for each sample is shown in Figure 4-2. The overall weight gain and su rface area is presented in Table 4-2. After evaluating the calculated isothermal plots, two additional samples were oxidized at 900C for 30 minutes and 90 minut es following the procedure for isothermal oxidation scans. The purpose of the two additi onal scans at shorter holding times is to aid in determining the initial oxidation behavi or of the samples oxidized at temperatures above 863C. The W versus time plots are shown in Figure 4-3, and the overall weight gain and surface area are included in Table 4-2. Table 4-2. Surface area and weight change for HT-9 samples oxidized under isothermal conditions. Average surface area is 1.00 cm2. Sample Surface Area, cm2 Weight Change, mg/cm2 600C/48hrs 1.00 0.027 600C/48hrs (a) 1.08 0.013 700C/48hrs 1.02 0.045 800C/24hrs 1.07 0.096 800C/48hrs 1.00 0.147 825C/24hrs 0.98 0.183 850C/24hrs 0.95 1.230 850C/24hrs (a) 0.97 1.975 850C/24hrs (b) 0.98 1.612 863C/24hrs 0.98 12.720 863C/48hrs 0.99 23.481 875C/24hrs 0.98 17.714 900C/30mins 0.99 0.196 900C/90mins 1.07 0.580 900C/24hrs 0.97 17.706 950C/6hrs 0.96 8.919

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57 Isothermal Scans of HT-9 in Dry AirTime, minutes 0250500750100012501500175020002250250027503000 W, mg/cm2 0 2 4 6 8 10 12 14 16 18 20 22 24 Time, minutes 0250500750100012501500175020002250250027503000 W, mg/cm2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 950C/6 hrs 900C/24 hrs 875C 24 hrs 863C/48 hrs 863C 24 hrs 850C/24 hrs (a) 850C/24 hrs 850C/24 hrs (b) 825C/24 hrs 600C/48 hrs 800C/48 hrs 700C/48 hrs 800C/24 hrs 600C/48 hrs (a) Zoom Figure 4-2. Calculated weight change versus time plots for HT-9 samples oxidized under isothermal conditions. Samples were oxidized in dry air at different temperatures for different holding times.

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58 Isothermal Scans of HT-9 in Dry Air at 900CTime, minutes 010203040506070809010050010001500 W, mg/cm2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 10.0 20.0 30 minutes 90 minutes 24 hours Figure 4-3. Isothermal scan plots of HT-9 samples oxidized at 900C in dry air for 30 minutes, 90 minutes, and 24 hours. Two methods were used to determinate wh ich kinetics rate equa tion best fits the isothermal data, as previously discussed in Chapter 2. In one method, identifying the possible oxidation behavior is based on the re sults of a linear regression performed on a double logarithm plot of the isothermal data. This method is only valid for identifying linear, parabolic, and cubic behavior. The second method consists in validating the experimental data to each kine tics rate equation. Thus, th is second method can identify linear, parabolic, cubic, logarithmic, and inverse logarithmic behaviors. For the first method, the logarithm of the W data was calculated and plotted as a function of the calculated logar ithm of time as shown in Figur e 4-4. Straight lines are drawn on the calculated double l ogarithm plot, dividing the pl ot into segments. The intersections of the drawn lines define th e approximate moment during the isothermal

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59 scan in which a change in the predomin ant oxidation behavior occurred. Linear regressions are applied to each segment to obta in the line equation, and the resulting line equation is then compared to Equation 2-8 to find the value of m. An example of the application of this method is shown in Fi gure 4-5 for a sample oxidized at 850C for 24 hours. Isothermal Scans of HT-9 in Dry Air Log(Weight Change) vs. Log(Time)Log(Time, minutes) 0.00.51.01.52.02.53.03.5Log( W, mg/cm2) -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 950C/6 hrs 900C/24 hrs 875C/24 hrs 863C/48 hrs 863C 24 hrs 850C/24 hrs 825C/24 hrs 600C/48 hrs 800C/48 hrs 700C/48 hrs 800C/24 hrs 600C/48 hrs (a) Figure 4-4. Calculated double logar ithm plots of the isothermal data. Identification of the kinetics rate equation is based on the parameter m, which is the inverse of the slope obtained during the linear regressions. This parameter has values of 1, 2, and 3, corresponding to linear, parabolic, and cubic kine tics, respectivel y. If the calculated m values are vastly different than the values of 1, 2 and 3, the kinetics for the oxidized sample in question ca nnot be determined by this me thod. Calculated m values for the isothermally oxidized HT-9 samples are presented in Table 4-3.

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60 Isothermal Scans of HT-9 in Dry Air at 850C for 24 hours (a) Log(Weight Change) vs. Log(Time)Log(Time, minutes) 0.00.51.01.52.02.53.03.5Log( W, mg/cm2) -1.75 -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 Segment 2 y = 2.06x + -6.26 R2= 0.9935 462 minutes Segment 1 y = 0.41x + -1.79 R2= 0.9957 Figure 4-5. Example of identify ing the oxidation kinetics base d on the results of the linear regressions applied to each segm ent of a double logarithmic plot. For the second method, the W data is plotted according to the rate equation being validated. For example, to validate the parabo lic rate equation the da ta is plotted as the W squared ( W2) versus time. The plot is divided into the segments previously defined in the double logarithmic method and linear regr essions are applied to each segment. The resulting coefficients of determination (R2 values) are evaluated to find which kinetics rate equation best fits the experimental data. As an additional step, W data is calculated based on the possible kinetics rate equations and the resulting plots compared with the experimental data. Thus, the possibl e oxidation kinetics is based on the R2 value and how well the calculated kinetics rate equatio ns plot follows the experimental data. Figure 4-6 illustrates an exampl e of validating a kinetics ra te equation (parabolic rate

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61 equation) for a sample oxidized at 850C fo r 24 hours. Figures 4-7 and 4-8 present the possible oxidation behaviors ac cording to the two methods. Table 4-3. Calculated m values based on lin ear regressions of the segments of double logarithmic plots. Segment Sample 1 2 3 600C/48hrs 5.23 -11.10 600C/48hrs (a) 7.03 201.81 -2.61 700C/48hrs 4.28 75.91 800C/24hrs 2.73 3 7.80 800C/48hrs 2.62 3 3.90 825C/24hrs 2.23 2 3.65 850C/24hrs 2.57 3 0.70 1 850C/24hrs (a) 2.44 2 0.48 850C/24hrs (b) 2.29 2 0.53 1 863C/24hrs 1.92 2 0.50 1 863C/48hrs 1.92 2 0.66 1 875C/24hrs 1.64 2 0.71 1 900C/30mins 1.25 1 900C/90mins 1.37 1 0.36 900C/24hrs 1.16 1 0.99 1 950C/6hrs 0.78 1 1.17 1 The reaction rate constants kL, kP, kC, kLog, and kInv are obtained from the slope of the linear regression performed during the kine tics rate equation validation. The obtained rate constants for each model are presented in Table 4-4. Activation energies (Ea) for the linear and parabolic models are calculated by plotting the natural logarithm of the rate constants (Ln(k)) versus the inverse of the temperature (1/T). Li near regressions are applied and the slopes are multiplied by the Universal Gas Constant (R=8.315 J/mol-K) to obtain the activation energy. Figure 4-9 presents how the activ ation energies were obtained from the linear regres sions of the Ln(k) versus 1/T plot. The data for the samples oxidized at 600C for 48 hours were not included in the linear regressions.

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62Isothermal Scans of HT-9 in Dry Air at 850C for 24 hours (a) (Weight Change)2 vs. TimeTime, minutes 01503004506007509001050120013501500( W, mg/cm2)2 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Segment 1 R2= 0.9914 Segment 2 R2= 0.8338Isothermal Scans of HT-9 in Dry Air at 850C for 24 hours (a) Weight Change vs. TimeTime, minutes 050100150200250300350400450500 W, mg/cm2 0.00 0.05 0.10 0.15 0.20 0.25 Cubic R2= 0.9845 Parabolic R2= 0. 9914 Segment 1 RegionAB Figure 4-6. Example of validating a kinetics rate equation (parabolic rate equation) for a sample oxidized at 850C for 24 hour s. A) The experimental data is plotted as the W2 versus time, and divided into segments previously defined in the double logarithmic method. R2 values from the linear regression are evaluated to determinate how well the parabolic model fits the experimental data. B) The possible kinetics rate equations for Segment 1 are plotted on the W versus time plot and the resulting curve profiles are compared with the experimental data. Thus, the oxidation behavior of Segment 1 could be described by parabolic kine tics or by cubic kinetics.

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63 Possible Oxidation BehaviorBased on the linear regression of the log(weight change) vs. log (time) of each section of an isothermal plot. 0.000.250.500.751.001.251.501.752.002.252.502.753.003.253.50 600/48h 600/48h(a) 700/48h 800/24h 800/48h 825/24h 850/24h 850/24h(a) 850/24h(b) 863/24h 863/48h 875/24h 900/30m 900/90m 900/24h 950/6hIsothermal Temperature, CLog(Time, minutes) ? ?? ? ? ? ? P ? ? C C ? P P C L L P L L P L L L L L ? L L L L Figure 4-7. Possible oxidation kinetics base d on the results of the linea r regressions performed on a double logarithm plot of the isothermal data. Legend: L: Linear P. Parabolic C. Cubic ?: Indeterminate

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64 Possible Oxidation BehaviorBased on applying each kinetics rate equation to each section of an isothermal plot. 0.000.250.500.751.001.251.501.752.002.252.502.753.003.253.50 600/48h 600/48h(a) 700/48h 800/24h 800/48h 825/24h 850/24h 850/24h(a) 850/24h(b) 863/24h 863/48h 875/24h 900/30m 900/90m 900/24h 950/6hIsothermal Temperature, CLog(Time, minutes) ? In v L/P L P/ L C/P Inv/P C Lo g P/C C P P L/P P/C C L L P L L L/P L L L L L L L P L P Figure 4-8. Possible oxidation kinetics ba sed on validating the simple kinetics rate equations to the isothermal data. Legend: L: Linear P. Parabolic C. Cubic Log: Logarithmic Inv: Inverse Logarithmic

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65Table 4-4. Linear (kL), parabolic (kP), cubic (kC), logarithmic (kLog), and inverse logarithmic (kInv) rate constants for each isothermal plot segment. Rate Constant Value per Segment, kL, g/cm2-s kP, g2/cm4-s kC, g3/cm6-s kLog, g/cm2-log(min) kInv, cm2/g-log(min) Sample 1 2 3 1 2 1 2 1 2 1 2 600C/48hrs -4.7e-11 -2.9e-15 -1.5573.5 600C/48hrs (a) 5.3e-9 -1.0e-10 2.7e-13 700C/48hrs 1.3e-15 8.9e-20 1.7e-5 800C/24hrs 5.2e-14 5.0e-17 6.6e-18 800C/48hrs 6.6e-13 9.8e-14 -5757.2 825C/24hrs 3.0e13 1.2e-16 850C/24hrs 1.6e-8 1.1e-16 850C/24hrs (a) 3.0e-8 1.3e-12 850C/24hrs (b) 2.4e-8 1.6e-12 863C/24hrs 1.3e-8 1.8e-7 3.2e-12 863C/48hrs 1.4e-8 1.5e-7 3.12e-12 875C/24hrs 2.6e-8 2.2e-7 900C/30mins 9.2e-8 900C/90mins 6.3e-8 2.4e-7 900C/24hrs 7.8e-8 4.0e-9 950C/6hrs 4.4e-7 4.0e-9

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66Isothermal Scans of HT-9 in Dry Air Linear Rate Constant kL vs. Temperature(Temperature, K)-1 8.00e-48.25e-48.50e-48.75e-41.60e-31.70e-3Ln(kL, g/cm2-s) -20 -19 -18 -17 -16 -15 -14 600C/48hrs 850C/24hrs 863C/24hrs 863C/48hrs 875C/24hrs 900C/30mins 900C/90mins 900C/24hrs 950C/6hrs Isothermal Scans of HT-9 in Dry Air Parabolic Rate Constant kP vs. Temperature(Temperature, K)-1 8.00e-48.50e-49.00e-49.50e-41.00e-31.05e-31.10e-31.15e-31.20e-3Ln(kP, g2/cm4-s) -36 -34 -32 -30 -28 -26 -24 -22 -20 -18 600C/48hrs 700C/48hrs 800C/24hrs 800C/48hrs 825C/24hrs 850C/24hrs 863C/24hrs 863C/48hrs 900C/24hrs 950C/6hrs Ea 629 kJ/mol R2=0.8485Isothermal Scans of HT-9 in Dry Air Parabolic Rate Constant kP vs. Temperature(Temperature, K)-1 8.00e-48.50e-49.00e-49.50e-41.00e-31.05e-31.10e-31.15e-31.20e-3Ln(kP, g2/cm4-s) -36 -34 -32 -30 -28 -26 -24 -22 -20 -18 600C/48hrs 700C/48hrs 800C/24hrs 800C/48hrs 825C/24hrs 850C/24hrs 863C/24hrs 863C/48hrs 900C/24hrs 950C/6hrs Ea 629 kJ/mol R2=0.8485 Ea 305 kJ/mol R2=0.9414AB Figure 4-9. Calculation of the ac tivation energies for the linear (A) and parabol ic (B) oxidation models. The rate constant (k ) values for samples with same isothermal parameters (i.e. same temp erature and same holding time) and/or with multiple k values are plotted as the average of k values with erro r bars representing the ma ximum and minimum values.

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67 SEM Surface Characterization Results The morphologies and compositions of th e surface oxides were elucidated with a JOEL FEG-SEM (model JSM-6335F) equipped with an Oxford Link ISIS System (MAIC-UF). The SEM was operated in s econdary electron (SE) image mode with smallest aperture setting for maximum fi eld-of-depth due to the tortuous surface topography. Presentation of th e results is grouped into thr ee temperature ranges based on the observed morphologies: 600C to 825C, 850C to 863C, and 875C to 950C. Energy dispersive spectroscopy (EDS) was performed at th e center of the images or at the center of the structure of interest. 600C to 825C Temperature Range The samples oxidized at 600C and 700C have similar surface structure consisting of small platelets of approximately 100 nm to 200 nm. The platelets of the 600C sample are rounded, having a fish-scale appearan ce while the platelets of the 700C are polygonal shaped. The composition of the oxide film for both samples is mainly oxygen, iron, chromium, and manganese, with small am ounts of silicon, sulfur, and vanadium. The 700C also presents small oxide particles about one m in size. The composition of these particles consists of vanadium, ma nganese, iron, chromium, and oxygen with a trace amount of silicon. The surface morphology of the oxi de film for the 800C and 825C samples consists of Cr-rich oxide crystals less than one m in size with dispersed clusters of large smooth oxide grains. These grains are rich in vanadium and manganese with some iron and chromium. SE images and the corresponding EDS spectra for the samples oxidized in this temperature ra nge are shown in Figure 4-10 to 4-13.

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68 A 1m 100nm Figure 4-10. SE images and corresponding ED S spectra for the sample oxidized in dry air at 600C for 48 hours. A is a closer view of the surface morphology of the oxide film. A B C 10m 1m 100nm 100nm Figure 4-11. SE images and corresponding ED S spectra for the sample oxidized in dry air at 700C for 48 hours. A and B are higher magnification images of surface areas that appear to have different morphologies when viewed at a low magnification. C is a higher magnificati on view of one of the dispersed small particles.

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69 B A 100m 1m 1m Figure 4-12. SE images and corresponding ED S spectra for the sample oxidized in dry air at 800C for 48 hours. A is a cluster of smooth oxide grains and B is a higher magnification image of the surface of the oxide film consisting of small oxide crystals. A C B 100m 1m 1m 100nm Figure 4-13. SE images and corresponding ED S spectra for the sample oxidized in dry air at 825C for 24 hours. A is a higher magnification image of the surface of the oxide film, consisting of small oxi de crystals as observed in the 800C sample. B is a higher magnification view of a cluster of smooth oxide grains and C is a closer view of one of the gr ains in the grain cluster shown in B.

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70 850C to 863C Temperature Range The samples oxidized at 850C and 863C have a more complex surface morphology consisting of a combination of f eatures observed in the samples oxidized below 850C and the samples oxidized above 863C. The surface of the 850C sample is covered with Cr-rich oxide crystals with disp ersed clusters of large smooth oxide grains and Fe-rich oxide nodules as presented in Fi gure 4-14. The compos ition and structure of the grain clusters are almost identical to those observed in the 800C and 825C samples. C B A D E 100m 1m 10m 1m 10m 10m Figure 4-14. SE images and corresponding ED S spectra for the sample oxidized in dry air at 850C for 24 hours. A and B ar e higher magnifications views of the oxide films surface and of an oxide grai n cluster, respectively. C and E are closer view of the Fe-rich oxide structures D is a closer view of the interior of the spalled Fe-rich oxide nodule shown in C.

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71 The surface of the 863C sample is cove red with a Fe-rich oxide except for a narrow perimeter band of Cr-rich oxide crystals separating the oxide developed at the center of the sample from the oxide developed at the edges of the sample. (Figure 4-15.) The morphology, size, and composition of the Cr -rich oxide crystals is similar to those observed on the 800C to 850C samples with the exception that a small amount of copper was detected during EDS analysis. B A C 1mm 100nm 100m 10m Oxide at the Center of the Sample Oxide at the Edge of the Sample Figure 4-15. SE images and corresponding ED S spectra for the sample oxidized in dry air at 863C for 24 hours. B and C is the Fe-rich oxide at the edge and center of the sample, respectively. D is a higher magnification view of the Cr-rich oxide crystals within the narrow perimeter band. The Fe-rich oxide observed at both 850C and 863C have several distinguishable features that could be different growth stages of the oxide. EDS analysis of these features indicate a composition of oxygen and iron with trace amounts of chromium and/or manganese. SE images and corresponding EDS spectra of the Fe-rich oxide structures developed on the 850C and 863C sample are presented in Figures 4-16 and 4-17 respectively.

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72 G F E D C B A 10m 1m 1m 10m 1m 10m 1m 1m Figure 4-16. SE images and corresponding EDS spectra of the various features of the Fe-rich oxide structures observed on th e sample oxidized in dry air at 850C for 24 hours. The image in the middle corresponds to image E of Figure 4-14.

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73 C B A F E D 100m 10m 1m 10m 1m 10m 1m Figure 4-17. SE images and corresponding EDS spectra of the various features of the Fe-rich oxide surface of the sample oxidized in dry air at 863C for 24 hours. The image in the middle corresponds to image B of Figure 4-15.

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74 875C to 950C Temperature Range Samples oxidized at and above 875C are entirely covered in iron oxide. The 950C sample was the only one to have visi ble cracks as shown in Figure 4-18. The surface morphology of the 875C to 950C samples is almost identical to the surface of the Fe-rich oxide observed on the 863C sample. No Cr-rich oxide crystals were observed on the surface of the samples oxi dized within this temperature range. HT-9 Sample 950C/6hrs Cracks Figure 4-18. Image of the cracks on the surf ace of the Fe-rich oxide film developed on the sample oxidized in dry air at 950C for 6 hours. During SEM sample preparation of the 875 C and 900C samples, part of the oxide spalled from one of the corner s revealing three distinct oxide layers as seen in Figures 4-19 and 4-20 respectively. The outermost layer (1) is a Fe-rich surface oxide that exhibits various features whic h could be different stages of oxide growth. (Figures 4-21 and 4-22 for the 875C and 900C samples respectively.) The composition of this layer mainly consists of iron and oxygen with a trace amount of manganese, and only one feature exhibited small amounts of copper. Th e structure of the innermost layer (3) is

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75 similar for both samples, consisting of small equi axed grains with what appear to be prior grain boundary structures. (Image D in Figur e 4-19 and image E in Figure 4-20.) The composition of this layer is oxygen, chromium, and iron, with small amounts of silicon, sulfur, manganese, and vanadium. F E A B C D 1 1 2 2 3 3 Legend 1: Outermost Layer 2: Middle Layer 3: Innermost Layer Prior Grain Boundary Structures 100m 100m 100m 100m 1m 10m 100nm Figure 4-19. SE images and corresponding ED S spectra for the sample oxidized in dry air at 875C for 24 hours. A is the outermost layer 1 and D is the inner most layer 3. B (in-plane view) and C (crosssection view) are cl oser views of the three distinctive layers observed after the oxide spalled during sample preparation. E and F are higher magnificat ion images of the structures of the innermost layer 3.

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76 B A E D C 1 2 3 Legend 1: Outermost Layer 2: Middle Layer 3: Innermost Layer Prior Grain Boundary Structures 100m 10m 10m 10m 1mm 10m Figure 4-20. SE images and corresponding ED S spectra for the sample oxidized in dry air at 900C for 24 hours. A is the ou termost layer 1, and B (cross-section view) is a closer view of the three di stinctive layers observed after the oxide spalled during sample preparation. C, D, and E are higher magnification images and corresponding EDS spectra of each layer.

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77 C B A E F G D H 100m 1m 1m 1m 1m 10m 1m 10m 1m Figure 4-21. SE images and respective EDS spectra of the various features of the outermost oxide layer of the sample oxidi zed in dry air at 875C for 24 hours The image in the middle corresponds to image A of Figure 4-19.

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78 C B A G F E 10m 1m 1m 1m 1m 1m 10m Figure 4-22. SE images and respective EDS spectra of the various features of the outermost oxide layer of the sample oxidi zed in dry air at 90 0C for 24 hours. The image in the middle corresponds to image A of Figure 4-20.

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79 The oxide of the 950C sample spalled along one of the cracks formed during the oxidation process (Figure 4-18) during sample preparation, revealing two different structure layers as seen in Figure 4-23. The composition and morphology of the bottom layer is similar to the innermost layer (3) of the 875C and 900C samples. (Figure 4-23.) The surface of the top layer of the oxide film exhibits the le ast number of surface features when compared to the surface of the Fe-ri ch oxide of the 863C to 900C samples. (Figure 4-24.) The composition of the su rface oxide is oxygen and iron with a trace amount of manganese according to the EDS spectra. C B A D E Top Layer Bottom Layer Bottom Layer Top Layer Crack Boundary 100m 1m 10m 1m 1m 10m Crack Figure 4-23. SE images and corresponding ED S spectra for the sample oxidized in dry air at 950C for 6 hours. A is the top layer and E is the bottom layer. B is a cross-section view of the spalled oxide, revealing oxide layers with different structures. C and D are higher magnifi cation images of the top and bottom layers respectively.

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80 B A D C 10m 1m 10m 1m 1m Figure 4-24. SE images and respective EDS spectra of the various features of the outermost oxide layer of the sample oxidi zed in dry air at 95 0C for 6 hours. The image in the middle corresponds to image A of Figure 4-23. The samples oxidized at 900C for 30 and 90 minutes have similar surface morphologies consisting of small oxide grains with dispersed Fe-rich oxide structures as shown in Figures 4-25 and 4-26 respectively. The composition of the small oxide grains mainly consists of chromium, iron, a nd oxygen, with some manganese and a trace amount of vanadium. Silicon was only detected in the 30 minutes sample. The composition of the Fe-rich oxide mainly consists of iron and oxygen with some chromium and manganese, and a trace amount of vanadium. The 90 minutes sample also exhibits large Fe-rich oxide structures, whose surface exhibits various features similar to those observed on Fe-rich oxide of the 850C to 950C samples. (Figure 4-27). These features are mainly composed of iron a nd oxygen, with some chromium and manganese.

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81 B A 10m 1m 1m Figure 4-25. SE images and corresponding ED S spectra for the sample oxidized in dry air at 900C for 30 minutes. A is a hi gher magnification of the surface of the oxide film consisting of small oxide crysta ls. B is closer view of one of the Fe-rich oxide nodules. A C D E B 10m 100m 1m 10m 1m 1m Figure 4-26. SE images and corresponding ED S spectra for the sample oxidized in dry air at 900C for 90 minutes. A, D, and E are closer views of the various Ferich oxide structures. C is a higher magnification view of the surface of the oxide consisting of sm all oxide crystals.

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82 B A D C 10m 1m 1m 1m 1m Figure 4-27. SE images and respective EDS spectra of the various features on the surface of the Fe-rich oxide structures of the sample oxidized in dry air at 900C for 90 minutes. The image in the middle corresponds to image B of Figure 4-26. SEM Cross-Section Characterization Results The thickness, composition, and layer struct ure of the oxide films were elucidated with a JOEL FEG-SEM (model JSM-6335F) equippe d with an Oxford Link ISIS System (MAIC-UF). The SEM was operated in SE image mode. The elements considered for the elemental mapping and line scan are based on the recurring elem ents observed during the EDS analysis. Carbon was not included in the list of elements. It was not possible to clearly define the oxide thickness of the sa mples oxidized from 600C to 825C using the FEG-SEM, thus a Strata Dual Beam-Focus ed Ion Beam (FIB) (model DB 235) operated by MAIC staff was used to obtain cross-section images of these samples. To prepare the

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83 samples for the FIB, portion of the samples were cut off, cleaned and dried. The pieces were mounted onto a FIB stub using carbon tape and then carbon coated at MAIC. Once the samples were placed inside the FIB, a th in strip of Pt is deposited on the surface of the sample to protect the surface from damage during the ion milling process. Trenches were ion milled at the edge of the Pt line, the samples tilted, and the thickness measured using the FIBs software which has a correc tion feature for sample tilting. The crosssection images are divided into temperatur e ranges based on the observed morphologies: 600C to 825C, 850C to 863C, and 875C to 950C. 600C to 825C Temperature Range The oxide film of the 600C sample coul d not be resolved, although a very thin oxide layer did develop as indi cated by the surfaces bluish cast as shown in Figure 4-28, which is associated to the low temperatur e oxidation process known as tarnishing. The oxide thickness of the 700C sample is less than half a micron, while the thickness for the 800C and 825C samples is approximately one micron. The cross-section images for the sample within this temperature range are shown in Figure 4-29. Figure 4-28. HT-9 sample on alumina pan afte r isothermal oxidation in dry air at 600C for 48 hours. The sample has a bluish cast which is associated with the process of tarnishing.

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84 600C/48hrs 700C/48hrs 800C/48hrs 825C/24hrs Substrate Pt Film C Coating Substrate Pt Film Oxide C Coating Substrate Pt Film Oxide C Coating C Coated Surface C Coated Surface Substrate Pt Film Oxide C Coating C Coated Surface C Coated Surface 0.31m 0.34m 0.91m 1.16m 1.01m 0.87m Figure 4-29. Cross-section SE image for th e samples oxidized at 600C, 700C, and 800 C for 48 hours and the sample oxidized at 825C for 24 hours.

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85 850C to 863C Temperature Range The 850C sample has a thin compact oxide film approximately 1.5 m thick as presented in Figure 4-30. The compositi on of the film is oxygen, chromium, and manganese based on the results of the elemen tal mapping and line scan. The oxide of the 863C sample is more complex for it appears to be two types of oxides, a thin oxide film and a thick oxide film which are identified as spot 1 and spot 2 respectively in the low magnification view of the cross-section. (Figur e 4-31.) The thin oxide (spot 1) is almost identical in thickness, layer structure, and composition to the oxide observed in the 850C sample. (Figure 4-32.) The thick oxide (spot 2) appears to be compact with cavities and voids along the center of the oxide film and at the oxide/substrate inte rface as observed in Figure 4-33. The outer region of the oxide also presents some cavities and voids with cracks along the width of the sample. No caviti es or cracks are seen in the inner region of the oxide. The approximate oxide thickness is 130 m and the composition varies across the thickness. According to the elem ental mapping and line scan results, the oxide film is divided into two compositional regi ons and the boundary appears to be along the center of the oxide film, where the cavities a nd voids are located. The outer region is mostly iron and oxygen, while the inner region is chromium, iron, and oxygen. A closer view at the oxide/substrate interface, identi fied as spot 3 in Figure 4-33, reveals the presence of internal oxidation. Oxid e is observed along the grain boundaries and particles or precipitates are obs erved within the grain structure of the substrate adjacent to the oxide/substrate interface. (Figure 4-34.) Based on the elemental mapping, oxygen is detected within the grain boundaries, which ar e enriched in chromium and depleted in iron.

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86 Line Scan 4787nm Elemental Mapping O Fe Mn Cr V S Si SE O Fe Mn Cr V S Si SE 850C/24hrs Substrate Oxide 1.21m 1.48m Figure 4-30. Cross-section SE image, elem ental mapping, and line scan profiles for th e sample oxidized in dry air at 850C for 24 hours.

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87 Elemental Mapping O Fe Mn Cr V S Si SE Line Scan 252 m Oxide O Fe Mn Cr V S Si SE 1 2 863C/24hrs Substrate Epomet Mount 123.59m 53.30m Figure 4-31. Cross-section SE image, elem ental mapping, and line scan profiles for th e sample oxidized in dry air at 863C for 24 hours.

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88 Elemental Mapping O Fe Mn Cr V S Si SE Line Scan 7012nm O Fe Mn Cr V S Si SE 863C/24hrs-Spot 1 Substrate Epomet Mount Oxide Figure 4-32. Cross-section SE image, elemental mapping, and line scan profiles of the oxid e at spot 1 of Figure 4-31.

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89 Line Scan 1 4 8 m Elemental Mapping O Fe Mn Cr V S Si SE O Fe Mn Cr V S Si SE 3 Voids, Cavities 863C/24hrs-Spot 2 Substrate Epomet Mount Oxide Figure 4-33. Cross-section SE image, elemental mapping, and line scan profiles of the oxid e at spot 2 of Figure 4-31.

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90 Elemental Mapping O Fe Mn Cr V S Si SE Internal Oxidation Oxide Particles or Precipitates within the Substrate Oxide along the Grain Boundary 863C/24hrs Spot 3 Substrate Figure 4-34. Cross-section SE image and elemental mapping of the oxide at spot 3 of Figure 4-33.

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91 875C to 950C Temperature Range The oxide film formed on the 875C to 950C samples has similar composition and layer structure as the thick oxide film of the 863C sample The average oxide thickness is 152 m for the 875C and 900C samples, and 81 m for the 950C sample. The oxide of the 875C sample exhi bits two different structur al and compositional regions divided by an interfacial structur e along the center line of the f ilm as seen in Figure 4-35. The outer region has a solid appearance with some dispersed cavities and voids that are also present along the interfacial structure. The composition the outer layer consists of iron and oxygen. The inner region has a grai ny texture with a composition of chromium, iron, and oxygen. Cracks are observed along th e full width of the oxide film. A closer look at the interfacial structur e, identified as spot 1 in Figure 4-35, reveals a narrow band of small oxide particles. (Figure 4-36.) From the line scan across this structure, chromium has a convex profile and iron a con cave profile, thus it is possible that the particles are Cr-rich oxides. Internal oxi dation is observed at the oxide/substrate interface, identified as spot 2 in Figure 4-35. The internal oxidation features and their composition are presented in Figure 4-37 and are similar to those observed in the 863 sample. A closer view of the internal oxi dation region (Figure 4-38) shows the presence of cracks along the grain boundary oxide and at a triple point junction. No cracks were observed within the prior grain structure, where the particles are located. The oxide film developed on the 900C samp le has essentially the same structural features and composition as the oxide film of the 875C sample. (Figures 4-39 to 4-42.) The interfacial structure along th e center line of the oxide film previously observed in the 875C sample is more pronounced and has incr eased its width as obs erved in Figures

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92 4-39 and 4-40. The iron and chromium line scan have the same profile as the ones obtained for the 875C, although they are more defined. The oxide film of the 950C sample is slight ly different from the oxide film of the other two samples within this temperature ra nge. It does not present the interfacial structure along the center line of the film and the oxide/substrate interface is wider, as observed in Figure 4-43. Cracks are observed along the oxides full width, and cavities and voids are present through out the oxide f ilm. (Figure 4-44.) No difference in the surface texture is observed (i.e., solid outer la yer versus grainy inner layer), although it follows the compositional variation observed in the other samples within this group. A closer view of the oxide/substrate interface (F igure 4-45) reveals a structure formed of small particles as seen in the other samples, with what appears to be dispersed fragments of internal oxidation structures (i.e., gr ain boundary oxide and substrate embedded with particles). The interf ace is enriched in chromium, and ch romium and iron is detected at the sites where the internal oxidation frag ments are located. The internal oxidation region has the same features a nd composition seen in other samp les that present this type of oxidation. (Figure 4-46.) After evaluation of the results from the cr oss-section analysis, the relative amounts of iron, chromium, silicon, manganese, a nd oxygen were corroborated by electron probe microanalysis (EPMA). A JOEL Superprobe 733 operated by a MAIC staff was used to perform EPMA analysis at the outer and inner regions of the oxide film, and at the oxide along the grain boundaries within the internal ox idation region. The results are presented in Figures 4-47a to 4-51.

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93 Line Scan 178 m Elemental Mapping O Fe Mn Cr V S Si SE O Fe Mn Cr V S Si SE Crack 2 1 Voids, Cavities 875C/24hrs Substrate Epomet Mount Oxide 151.16m Figure 4-35. Cross-section SE image, elem ental mapping, and line scan profiles for th e sample oxidized in dry air at 875C for 24 hours.

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94 O Fe Mn Cr V S Si SE O Fe Mn Cr V S Si SE Line Scan 9112nmInner Oxide Elemental Mapping 875C/24hrs-Spot 1 Outer Oxide Interface Figure 4-36. Cross-section SE image, elemental mapping, and line scan profiles of the oxide in spot 1 of Figure 4-35.

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95 Line Scan 30.8m Elemental Mapping O Fe Mn Cr V S Si SE O Fe Mn Cr V S Si SE 3 875C/24hrs-Spot 2 Substrate Internal Oxidation Oxide Figure 4-37. Cross-section SE image, elemental mapping, and line scan profile of the oxid e at spot 2 of Figure 4-35.

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96 Line Scan 11.9 m Elemental Mapping O Fe Mn Cr V S Si SE O Fe Mn Cr V S Si SE Crack Particles Oxide along the Grain Boundary 875C/24hrs-Spot 3 Substrate Internal Oxidation Figure 4-38. Cross-section SE image, elemental mapping, and line scan profile of the oxid e at spot 3 of Figure 4-37.

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97 Elemental Mapping O Fe Mn Cr V S Si SE Crack 2 Line Scan 354mSubstrate O Fe Mn Cr V S Si SE Epomet Mount Oxide 1 Voids, Cavities 900C/24hrs 184.69m 142.50m Figure 4-39. Cross-section SE image, elem ental mapping, and line scan profiles for th e sample oxidized in dry air at 900C for 24 hours.

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98 Line Scan 17.2 mO Fe Mn Cr V S Si SE Elemental Mapping O Fe Mn Cr V S Si SE Interface 900C/24hrs-Spot 1 Inner Oxide Outer Oxide Figure 4-40. Cross-section SE image, elemental mapping, and line scan profiles of the oxid e at spot 1 in Figure 4-39.

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99 Elemental Mapping O Fe Mn Cr V S Si SE 47.3 mOxide O Fe Mn Cr V S Si SE Internal Oxidation Substrate Line Scan 3 900C/24hrs-Spot 2 Figure 4-41. Cross-section SE image, elemental mapping, and line scan profiles of the oxid e at spot 2 in Figure 4-39.

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100 Elemental Mapping O Fe Mn Cr V S Si SE 16.8 mLine Scan O Fe Mn Cr V S Si SE Crack Particles Oxide along the Grain Boundary 900C/24hrs-Spot 3 Substrate Internal Oxidation Figure 4-42. Cross-section SE image, elemental mapping, and line scan profiles of the oxid e at spot 3 in Figure 4-41.

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101 Elemental Mapping O Fe Mn Cr V S Si SE 2 1Line Scan 1 0 6 mSubstrate O Fe Mn Cr V S Si SE 3 Oxide 950C/6hrs 5 4 7 7 m 1 4 5 0 m Figure 4-43. Cross-section SE image, elem ental mapping, and line scan profiles for th e sample oxidized in dry air at 950C for 6 hours.

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102 Elemental Mapping O Fe Mn Cr V S Si SE Line Scan O Fe Mn Cr V S Si SE Epomet Mount 67 m Void, Cavity Crack Oxide 950C/6hrs-Spot 1 Figure 4-44. Cross-section SE image, elem ental mapping, and line scan profiles of th e oxide at location 1 in Figure 4-43.

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103 Elemental Mapping O Fe Mn Cr V S Si SE 22.5 mOxide Internal Oxidation Line Scan O Fe Mn Cr V S Si SE Fragment of the Substrate Embedded with Particles Fragment of the Grain Boundary Oxide 960C/6hrs-Spot 2 Figure 4-45. Cross-section SE image, elem ental mapping, and line scan profiles of th e oxide at location 2 in Figure 4-43.

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104 Elemental Mapping O Fe Mn Cr V S Si SE Line Scan O Fe Mn Cr V S Si SE 1 4 9 m Cracks Oxide Particles Oxide at Grain Boundary Substrate Internal Oxidation 960C/6hrs-Spot 3 Figure 4-46. Cross-section SE image, elem ental mapping, and line scan profiles of the oxide at location 3 in Figure 4-43.

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105 2 1850C/24hrs 863C/24hrs 3AB 10m 1m Elements 1, wt% 2, wt% 3, wt% Fe 4.20 3.55 5.51 Cr 67.19 68.52 60.34 Si 1.59 0.87 2.54 Mn 2.96 1.64 5.40 O 24.06 25.42 26.21 Figure 4-47. EPMA results in weight percen t from two different locations on the oxide developed on the 850C/24 hrs sample (A ) and the thin oxide developed on the 863C/24 hrs sample (B). OR IR GBO 863C/24hrs Zoom 1m 10m Elements OR, wt% IR, wt% GBO, wt% Fe 69.40 60.65 31.29 Cr 0.33 12.72 42.48 Si 0.24 0.34 0.50 Mn 0.19 0.42 2.31 O 29.84 25.88 23.41 Figure 4-48. EPMA results in weight percen t from different locations across the oxide developed on the sample oxidized in dry air at 863C for 24 hours. Legend: OR: Oxide outer region. IR: Oxide inner region. GBO: Grain boundary oxide.

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106 OR IR GBO875C/24hrs Zoom 1m 10m Elements OR, wt% IR, wt% GBO, wt% Fe 71.30 60.12 25.88 Cr 0.05 12.57 44.14 Si 0.19 0.28 0.64 Mn 0.50 0.36 2.59 O 28.03 25.96 26.73 Figure 4-49. EPMA results in weight percen t from different locations across the oxide developed on the sample oxidized in dry air at 875C for 24 hours. OR IR GBO 900C/24hrs Zoom 1m 10m Elements OR, wt% IR, wt% GBO, wt% Fe 71.29 59.41 24.30 Cr 0.08 11.81 44.86 Si 0.00 0.00 0.46 Mn 0.43 0.67 3.47 O 28.20 28.11 26.91 Figure 4-50. EPMA results in weight percen t from different locations across the oxide developed on the sample oxidized in dry air at 900C for 24 hours. Legend: OR: Oxide outer region. IR: Oxide inner region. GBO: Grain boundary oxide. Legend: OR: Oxide outer region. IR: Oxide inner region. GBO: Grain boundary oxide.

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107 ORIR 950C/6hrs Zoom GBO1 GBO2 1m 10m Elements OR, wt% IR, wt% GBO1, wt% GBO2, wt% Fe 71.97 36.92 25.81 26.69 Cr 0.45 34.28 44.24 44.76 Si 0.06 0.52 0.61 0.24 Mn 0.00 0.00 2.67 2.65 O 27.52 28.28 26.68 25.67 Figure 4-51. EPMA results in weight percen t from different locations across the oxide developed on the sample oxidized in dry air at 950C for 6 hours. Platinum Marker Experiment Results After oxidizing the sample in dry air at 900C for 24 hours, the Pt marker was visible at the surface of the oxide as seen in Figure 4-52. A cross-sectional view of the sample shows the Pt ink at the surface of the oxide film. (Figure 4-53). HT-9 Sample Pt ink Alumina Sample Holder Figure 4-52. Pt marker sample after isothe rmal scan in dry air at 900C for 24 hours. Legend: OR: Oxide outer region. IR: Oxide inner region. GBO: Grain boundary oxide.

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108 Elemental MappingO Fe Cr V Pt SE MnLine Scan 270 m O Fe Mn Cr V Pt SE Oxide Layer Substrate Pt Ink Inner Region Outer Region 900C/24hrsPt Particles Figure 4-53. Cross-section SE image, elemental mapping, and line scan profiles of the Pt marker sample.

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109 Pt particles are embedded through out the oxides outer region. A higher concentration of particles are observes at the outer region/inner region interface, indicating the locations of the samples orig inal surface. Cavities, cracks and voids are observed within the outer regions at the interface. XRD Characterization Results The oxide compounds developed during the oxidation scans were identified using Phillips APD 3720 or XPert MRD (MAIC-UF). Peak matching was performed using Phillips PC-Identify software. The XRD prof iles are presented in Figures 4-54 to 4-55, and the oxide compounds that best match the obt ained profiles are presented in Table 4-5. Table 4-5. Oxide compounds that best match the XRD profiles. Sample Surface Scraped Oxide 700C/48hrs Mn1.5Cr1.5O4 (Mn, Fe)(Cr, V)2O4 800C/24hrs Mn1.5Cr1.5O4 (Cr1.9, V0.09, Fe0.01)O3 825C/24hrs Mn1.5Cr1.5O4 (Cr1.9, V0.09, Fe0.01)O3 3Cr2O3-Fe2O3 850C/24hrs (a) Cr2O3 Fe2O3 FeCr2O4 863C/24hrs Fe2O3 FeCr2O4 Fe3O4 875C/24hrs Fe2O3 FeCr2O4 Fe3O4 (Fe0.6, Cr0.4)2O3 900C/30mins Fe2O3 (Fe0.6, Cr0.4)2O3 900C/90mins Fe2O3 900C/24hrs Fe2O3 FeCr2O4 Fe3O4 Fe2O3 950C/6hrs Fe2O3 FeCr2O4 Fe3O4

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1102 Theta 1520253035404550556065707580 SS B A B BBB B B AAAAA A CC CCC A D D DDDDDDD 825C/24hrs 900C/24hrs 950C/6hrs 875C/24hrs 863C/24hrs 850C/24hrs (a) Non-oxidized HT-9 600C/48hrs 700C/48hrs 800C/48hrs 900C/30mins 900C/90mins D CC2 Theta 1520253035404550556065707580 SS B A B BBB B B AAAAA A CC CCC A D D DDDDDDD 825C/24hrs 900C/24hrs 950C/6hrs 875C/24hrs 863C/24hrs 850C/24hrs (a) Non-oxidized HT-9 600C/48hrs 700C/48hrs 800C/48hrs 900C/30mins 900C/90mins D CCXRD Profiles Figure 4-54. XRD profiles of the surface of nonoxidized (baseline) and oxidized HT-9 sa mples. Samples were characterized in bulk form, except for the profile of the 950C/6hrs sample which is from the surface of a spalled oxide fragment. Legend: S: Fe-Cr stainless steel substrate A: M2O3 type, Cr-rich B: M3O4 type [M=(Mn, Cr)] or AB2O4 [A=(Mn, Fe), B=(Cr, V)] C: M2O3 type [M=(Cr, Fe)] D: M2O3 type, Fe-rich

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1112 Theta 152025303540455055606570 900C/24hrs 950C/6hrs 875C/24hrs 863C/24hrs 850C/24hrs (a) Fe Cr B B A A A C C C Cr A A C A B FeXRD Profiles of the Scrapped Oxide B B C Figure 4-55. XRD profiles of the oxide scrapped from the surface of non-oxidized (baseline) and oxidized HT-9 samples. Samples were characterized in powder form, ex cept for the profile of the 950C/6hrs sample which is from the surface of a bulk sample in which part of the oxide spalled off. Legend: Fe: Iron Cr: Chromium A: M3O4 type [M=Fe] or AB2O4 [A=Fe, B=Cr] B: M2O3 type [M=(Cr, Fe)] C: M2O3 type, Fe-rich

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112 CHAPTER 5 DISCUSSION OF THE RESULT Oxidation Scan Oxidation of HT-9 in dry air form 600C to 950C exhibits a complex behavior in which several different kineti cs and mechanisms are simu ltaneously active, although it does follow the general oxidation behavior of Fe-Cr alloys. From the non-isothermal results, there are two different behaviors, one protective, and the other non-protective, with a behavior change in the 850C to 900 C temperature range. This observation is supported by the isothermal scan results. From the W versus time plots, the isothermal plots with similar profiles can be divided into two temper ature ranges, from 600C to 825C and from 900C to 950C. (Figure 5-1A .) The lower temperature range is associated to the formation of a protective type oxide film while the higher temperature range is associated to the de velopment of non-protective type oxide film. The isothermal plots from 850C to 875C have different prof iles, suggesting a transition from protective to non-protective (i.e. change in kinetics behavior) as predicted in the non-isothermal plot. Based on the overall weight change obtai ned during the isotherm al scans, the rate of W is slower for temperatures below 850 C and increases for temperatures above 850. This is presented in Figure 5-1B, in which the W is plotted as a function of temperature for samples oxidized from 600 C to 800C for 48 hours and from 825C to 900C for 24 hours. The data points for 600C and 800C is the average of the weight change for more than one sample was oxi dized under the same temperature and holding time condition.

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113Isothermal Scans of HT-9 in Dry AirTime, minutes 0250500750100012501500175020002250250027503000 W, mg/cm2 0 2 4 6 8 10 12 14 16 18 20 22 24 Transition 850C T 875C Non-Protective 875C < T Protective T < 850CIsothermal Scans of HT-9 in Dry Air Overall Weight Change vs. TemperatureTemperature, C 600650700750800850900950 W, mg/cm2 0 2 4 6 8 10 12 14 16 18 20 Slow W Rate Fast W RateAB Figure 5-1. Classification of isothermal pl ots (A) and plot of the overall weight chan ge versus the temperature for the isother mal scans (B). In A, the plots are grouped into protective, transition, and non-protectiv e temperature ranges based on the curve profiles. In B, the rate of weight ch ange is slower for T < 850C and fast for T > 850C. The data points for the 600C and 800C is the average of the weight change.

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114 None of the samples followed a particul ar simple oxidation model (i.e., linear, parabolic, cubic, logarithmic, inverse loga rithmic) through out the entire holding time according to the results of the two methods used to determinate the oxidation kinetics, although some of them seem to follow the mo re complex model of breakaway oxidation. Each plot was divided into two or three segm ents, indicating change in kinetics with time during the isothermal hold. For some segm ents, both methods agreed on the oxidation model, while disagreement occurred for other segments, especially for samples oxidized within the 600C to 825C temperature rang e. The kinetics rate constants were calculated according to the re sults obtained from the s econd method of validating the experimental data to each kine tics rate equations. The para bolic rate constants for the 800C/48 hrs, 825C/48 hrs, 850C/24 hrs, a nd 875C/24 hrs are in the order of 10-12 to 10-13 g2/cm4-s. These values are in th e lower end of the typical 10-9 to 10-13 g2/cm4-s range for chromia formers oxidized from 750C to 1100C. [2, 4, 20] The kP values for the samples oxidized at 900C and 950C is in the order of 10-9 g2/cm4-s, which is within the upper part of the range. In general, a si mple kinetics model could not describe the observed oxidation behavior, and the calculated rate cons tants and the calculated activation energies based on the parabolic or li near rate constants could not be directly compared to known values. As mentioned above, some of the samples a ppear to exhibit breakaway oxidation. The behavior of breakaway oxidation occurs when the initially developed protective oxide losses its protective natu re, point at which a non-protectiv e oxide starts to develop. In term of kinetics, the oxidation initially follows a parabolic behavior followed by a linear behavior after the breakdow n of the protective oxide. The isothermal plots for the

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115 850C to 900C samples seem similar to the profile of breakaway oxidation, as presented in Figure 5-2. The time at which the cha nge in kinetics (i.e., breakaway) occurs decreases as the oxidation te mperature increases, from approximately 700 minutes at 850C to approximately 80 minutes at 900C. Weight Change vs. Time Comparing 850C plot with the 863C to 900C plotsTime scale for 850C, min. 0250500750100012501500Weight Change/Area, mg/cm2 0.0 0.5 1.0 1.5 2.0 863C 050100150200250300350400450500 875C 0255075100125150175200225250 900C 0255075100125150175 850C/24hrs 850C/24hrs (a) 850C/24hrs (b) 863C/24hrs 863C/48hrs 875C/24hrs 900C/90mins 900C/24hrs Linear Regime Time Parabolic Regime X Graphic Illustration of Breakaway Oxidation Figure 5-2. Isothermal plots of samples that s eem to exhibit breakaway oxidation kinetics. A graphic illustration of a typical breakaway oxidati on plot is included for comparison. X represents the changing parameter of interest such as oxides thickness or mass, or amount of oxygen consumed. According to the results fr om the two methods used to determinate the oxidation kinetics, the 850C to 863C samples show an initial parabolic be havior followed by a

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116 linear behavior, consistent w ith breakaway oxidation. Fo r the 875C and 900C samples, the results indicate that the initial behavior is linear followed by linear behavior for the 875C sample, and linear or parabolic behavi or for the 900C. Although the plots for the 875C to 900C samples may initially seem to follow breakaway oxidation, this might not be the case. Morphology, Composition and Structure of the Oxide Film The surface morphology of the oxide films evolves from small platelets with dispersed particles and/or particle clusters to a complex and convol uted structure with increase in temperature as summarized in Figure 5-3. The film thickness and number of layers also changes from a single thin film to a thick multilayer film. Cross Section Surface Morphology800C/48hrs800C/48hrs 863C/24hrs 863C/24hrs 900C/24hrs900C/24hrsMorphology: small platelets/crystals with dispersed particles or particles clusters. Cross Section: Thin single-layer film. Morphology: convoluted surface with various structural features surrounded by small platelets/crystals along the outside perimeter. Cross Section: Combination of a thin single-layer film and thick multilayer film. Morphology: convoluted surface with various structural features. Cross Section: Thick multilayer film. Temperature Increase 10m 1mm 100m 1m 100m 100m Figure 5-3. SE images of the changes in surface morphology, thickne ss, and structure of the oxide film with increased oxidation temperature.

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117 600C to 825C Temperature Range The 600C and 700C samples share similar oxide morphology and composition, with the exception of the dispersed oxide pa rticles that are only present in the 700C sample. The surface is formed by small oxide platelets whose composition consists of the same elements and it is a single thin layer oxide film. As the oxidation temperature increases to 800C and 825C, the platelets be comes well defined oxide crystals and the size of the crystals increases. The oxide f ilm also increases from less than 400 nm to approximately 1 m and the composition varies from Fe/Cr-rich to Cr/Mn-rich as observed in the EDS spectrums presented in Ch apter 4. The dispersed oxide particles in the 700C sample could be the precursors fo r the oxide grain clus ters in the 800C and 825C. The composition of the oxide particles a nd the oxide clusters consists of the same elements, with the difference being in that the relative amount of iron and chromium decreases with increase in temperature. 850C to 863C Temperature Range For the 850C and 863C samples, the char acterization results indicate that the developing oxide goes through a transition from protective to non-protective, supporting the assumption of breakaway oxidation during th e evaluation of the isothermal plots. From Figure 5-3, the protective regime o ccurred during the initial 700 minutes of the isothermal scan for the 850C sample and the initial 230 minutes for the 863C sample. During the initial regime, a protective a Cr-ric h oxide film developed, thus the observed parabolic behavior. After the breakdown of the protective oxide, linear behavior follows as a non-protective Fe-rich oxide film forms. The protective oxide is approximately a 1 m thick Cr-rich oxide according to the EDS results and verified by EPMA analysis The surface morphology consists of small

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118 oxide crystals with V/Mnrich oxide grain clusters that are were only observed on the 850C sample. It is possible that these cluste rs vaporized with an increase in temperature for certain manganese oxides and vanadi um oxides liquefy and/or volatize at temperatures between 600C and 800C [1-3, 5]. This could explain why the clusters were observed from 800C to 850C, but not at temperature above 850C (i.e., at 863C). The non-protective oxide is first detected as small dispersed Fe-rich oxide nodules at 850C. As the temperature increases to 863C, the nodules grow and merge covering the majority of the surfac e area. The surface morphology of the Fe-rich oxide is complex, presenting different structural features that could be the oxide at different stages of development. The oxide film changes fr om a single-layer structure to a double-layer one that varies in composition across its thic kness. From the elemental mapping and line scan analysis, the outer layer is a Fe-rich ox ide and the inner layer is a Fe/Cr-rich oxide. Comparing EPMA results for the inner and outer layers, the relative amount of chromium is very low in the outer region than in th e inner region, although both layers contain similar relative amount of iron. Thus, the EP MA results support th e initial notion about the composition of the oxide derived from the cross-section char acterization results. When the oxide film becomes non-protect ive, an internal oxidation zone concurrently develops in the substrate adjacent to the oxide/substrate interface. This zone consists of particles embedded in the substrat e grain structure and an oxide film at the substrates grain boundaries. The presen ce of oxygen was detected at the grain boundaries during elemental mapping analysis which indicates inward diffusion of oxygen. This oxygen reacts with the iron and th e chromium within the substrate, forming oxide at the grain boundary a nd possibly the embedded particles which may be oxide

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119 precipitates. According to the EPMA results, relatively high amount of oxygen was detected in the grain boundary, thus supporting the notion that an oxide film developed in these location. The boundary between the outer and inner la yers of the non-protective oxide film presents voids and cavities. These structural features are believed to be located at the original surface of the sample based on wher e iron, chromium, and oxygen were detected during the elemental mapping. The Pt marker experiment confirms this statement for Pt particles was observed to be embedded w ithin the outer layer and along the inner layer/outer layer interface, but not at the inner layer or at the internal oxidation zone. 875C to 950C Temperature Range The surface oxide for the 875C to 950C sa mples share the same features and composition as the Fe-rich oxide developed on the 850C and 863C samples. The surface morphology is complex with different stru ctural features that could be different stages of oxide evolution. The variety of f eatures decreases as the isothermal temperature increases, with the 950C sample exhibiting th e least number of struct ural features. The 950C sample was the only one to exhibit thin white platelets vertically embedded in the oxides surface and large surface cracks. The oxide film for the 875C and 900C samples has a double layer structure which is visually distinctive: an outer smooth textur ed layer and an inner gr ainy textured layer. According to results from the cross-secti onal characterization a nd verified by EPMA analysis, the outer region is formed by a Fe -rich oxide while the inner region is formed by a Fe/Cr-rich oxide. The EPMA results also indicate a slightly hi gher relative amount of iron in the outer layer when compared to th e inner layer. Other vi sible structures that divide the oxide film into two layers are the presence of cracks, voids and cavities along

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120 the interface between the two layers. Clos er inspection of this interface reveals the presence of thin non-continuous bands of sma ll particles which is sl ightly wider in the 900C sample than in the 875C sample. Internal oxidation is observed within the substrate adjacent to the oxide/substrate interface that exhibit similar features as previously observed in the 863C sample, with the addition of visible cracks along the grai n boundary oxide film and intergranular cracks within the substrate adjacent to the in ternal oxidation zone. From the elemental mapping, the grain boundaries are enriched in oxygen and chromium and depleted in iron. EPMA analysis show the presence of iron, chromium, and oxygen at the grain boundary and that there is sligh tly more chromium than iron when comparing the relative amounts of both elements. Thus a Cr/Fe-rich oxide film developed along the substrates grain boundaries immediately adjacent to the oxide/substrate interface due to the inward diffusion of oxygen which reacts with the iron and chromium of the base metal. An interesting feature that was observed during closer inspection of the internal oxidation zone is the presence of areas and/or non-continuou s band of cavity-like formations which may or may not contain part icles. These areas are located within the oxide film adjacent to the internal oxidation zone (i.e., at the oxide/substrate interface) and are more pronounced in the 900C sample than in the 863C sample. It seems that the prior grain structures have dissolved, l eaving behind cavity-like formations. Some of these formations contain particles that wher e at one point embedded in the substrate as shown in Figure 5-4. A possible cause for this phenomenon is the outward diffusion of iron ions from the substrate for it is known th at iron tends to dissol ve and rapidly diffuse through Cr-rich oxide [4]. It is known that the presen ce of cavities and voids at the

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121 oxide/substrate interface is typically associ ated with outward diffusion of metal ions [3], also supporting the possible cause stated above. Oxide particles without the matrix Matrix embedded with oxide particles Outline of a prior grain structure 900C/24hrs Figure 5-4. SE image of the pa rtial dissolution of grain struct ures at the internal oxidation zone. Grain boundaries are outline and it can be seen that part of the grain matrix has dissolved, leaving behind the precipitated oxide particles. The oxide film for the 950C sample share the same compositional variation as the samples oxidized within the 863C to 900C temperature range, but the structural features are different. From the cross-secti on images, the oxide film is a smooth textured single layer structure followed by a broad band of particles at the oxide/substrate interface and an internal oxidation zone. El emental mapping and line scan reveal that there are actually two layers, an outer Fe-r ich oxide layer and an inner Fe/Cr-rich oxide layer. According to EPMA results, the i nner layer has a lower relative amount of iron when compared to the outer layer and the re lative amount of iron a nd chromium in this inner layer is similar. The internal oxidat ion zone of the 950C sample share the same

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122 features and composition as those of the 875 C and 900C samples, with the addition of intergranular crack within the substrat e immediately adjacent to this zone. At closer inspection of the broad band at the oxide/substrate interface, it is noticed that not only it contains partic les but also some of the inte rnal oxidation feature such as fragments of grain boundary oxide and/or substrate fragments. It is possible the broad band evolved from the areas and/or non-contin uous band of cavity-lik e formations that were observed in the 875C and 900C sample s, which were observed to increase in width as the temperature incr eased from 875C to 900C. At the higher temperature of 950C, the outward diffusion of iron ions woul d be faster, resulting in a widening of the band due to faster dissolution of the substr ate and causing the previously cracked grain boundary oxide to fragment. The oxide of the samples oxidized at 900 C for 30 minutes and 90 minutes share similar structural features with the samples oxidized at 850C and 863C and the composition of the oxide nodules is similar to these sample, although the composition of the surface oxide is different. The surface oxide for the 30 minutes and 90 minutes samples is formed by crystal oxides just li ke the 850C and 863C sa mples, but it is a Cr/Fe-rich oxide instead of a Cr-rich oxide. The Fe-rich oxide first appear as dispersed nodules in the 30 minutes sample which grow and merge covering la rger areas of the surface as the holding time increases to 90 minutes, eventually covering the whole surface of the sample as observed in the sample held for 24 hours. (Figure 5-5) This process is similar to the Fe-rich oxide evol ution observed in the sample oxidized from 850 to 875C. The Fe-rich oxide also exhibit di fferent structural features that have been previously observed in the Fe-rich oxide of the 850C to 950C samples.

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123 Isothermal Scans of HT-9 in Dry Air at 900C Holding Times: 30 minutes, 90 minutes, and 24 hoursTime, minutes 030609040080012001600 W, mg/cm2 0.0 0.3 0.6 3.0 6.0 9.0 12.0 15.0 18.0 900C/30mins 900C/90mins 900C/24hrs 10m 10m 10m Figure 5-5. Evolution of the Fe-rich oxide as a function of holding time for samples oxidized in dry air at 900C. According the Figure 5-5, the nucleation and growth of the Fe-rich oxide nodules occurs during initial 80 minut es of the isothermal scan in conjunction with the development of the Cr/Fe-rich surface oxide crystals. During this time period, iron and chromium oxide are developing, and even t hough chromium oxide ar e more stable than iron oxides, iron oxide predominates At this point, it is still follows a linear behavior but the W rate increases. This would lead to a change in the slope of the isothermal, thus the isothermal plot profile would appear to be similar to the profile of breakaway oxidation as previously presente d in Figure 5-2. This propos ed process agrees with the results of the two methods used to determinate the oxidation kinetics.

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124 Oxide Compounds A non-oxidized HT-9 sample was used as a baseline reference and its XRD profile presents two well define peaks at 2 44.5 (strongest peak) and 2 64.5. According the peak matching software, the best match is the XRD profile for Fe-Cr stainless steel alloys (JCPD #34-0396). These two peaks were also detected in the XRD profile for the samples oxidized within the 600C to 850C temperature range and the samples oxidized at 900C for 30 minutes and 90 minutes. The height of these peaks decreased as the isothermal temperature increased above 850C, and were not detected at 900C. This correlates with the change in the oxide film structure from a thin single layer film (thickness 1 m) for T 850C to a thick multilayer film (thickness > 100 m) for T 863C. The XRD results for the samples oxidized within the 700C to 825C temperature range indicate an oxide film formed by is formed by oxides with corundum (M2O3) and spinel (M3O4 or AB2O4) structures where M is a combin ation of some or all of the following elements: iron, chromium, manganese and vanadium. Identification of the oxide compound of iron, chromium, and manganese is complex because their oxides have similar crystal structure and lattice pa rameters making compound identification difficult [34, 35]. The most recurrent oxide compounds are a manganese chromium oxide [Mn1.5Cr1.5O4] and eskolaite [(Cr1.9, V0.09, Fe0.01)O3] which is a chromium oxide. These findings are in agreement with th e results from the EDS analysis. For the 850C sample, the XRD results i ndicate an oxide filmed formed by Cr2O3 and Fe2O3. After scrapping the oxide from the surface, only chromite (FeCr2O4) was detected during the XRD anal ysis. It is known that Fe2O3 and Cr2O3 are soluble in each

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125 other, thus forming a large ra nge of solid solution compounds [4] such as (Fe, Cr)2O3, 3Cr2O3!Fe2O3, and FeCr2O4, making the compound identification process difficult. Taking into consideration th at iron and/or chromium oxi de compounds were detected during the XRD analysis, it can be stated that oxide for the 850C sample consist of a Cr-rich oxide with Fe-rich oxide structures on the surface as previously thought during the EDS analysis. For samples oxidized within the 863C to 950C temperature range, the oxide is formed by corundum (M2O3) and spinel (M3O4 or AB2O4) structures where M = Fe or (Fe, Cr), A = Fe, and B = Cr. Fe2O3 was the only oxide detected during XRD for the oxide surface (bulk form), and FeCr2O4 and Fe3O4 were detected during the XRD analysis of the scrapped oxide (powder fo rm). This support the finding of the EDS analysis and cross-sectional characteriza tion of an outer Fe-rich oxide layer (Fe2O3) and an inner Fe/Cr-rich oxide layer (FeCr2O4 and Fe3O4). Fe3O4 has an AB2O4 type of structure (i.e., FeFe2O4), thus the iron in the B site can easily be substituted by chromium to form FeCr2O4.

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126 CHAPTER 6 CONCLUSIONS The oxidation of HT-9 marten sitic/ferritic stainless steel in dry air from 600C to 950 for different holding time exhibits a comp lex behavior that does not follow a simple oxidation model during the entire isothermal scan. A change in the oxidation behavior from protective (parabolic) to non-protective (linear) occurs within the 850C to 863C temperature range as predicted in the non-is othermal plot and ve rified by the findings during the characterization of the oxidized sample. Thus, br eakaway oxidation occurs at 850C and 863C, with the transition from parabolic behavior to linear behavior occurring earlier in the oxidation process as the temperature increases. Samples oxidized above at 875C and above have non-protective oxides, which are associated with linear kinetics. Initially, th e oxidation follows linear kinetics as nucleation and growth of Fe-rich and Cr-rich oxides simultaneously occurs. Although Cr-rich oxides are more stable than Fe-rich oxides, the latter predominat es for it has a faster reaction rate. At this point, the behavior continues to be linear but with a faster W rate than during the initial linear stage. It has been mo re challenging to define the oxi dation behavior of the samples oxidized from 600C to 825C and no defin ite conclusion can be drawn, although the 800C and 825C seem to follow a parabolic an d/or cubic behavior based on the obtained results. Further research within this temperature is recommended. The oxide film shows structural and compositional changes as the oxidation temperature increases from 600C to 950C. Within the 600C to 825C, the oxide film is a thin single layer structure (thickness 1 m) form by Cr-rich corundum and spinel

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127 oxides. The surface consists of small oxide platelets at 600C and 700C that grow in size and become well defined oxi de crystals at 800C and 825 C. Small oxide particles detected at 700C evolve into Mn/V-rich oxide grain clusters as the temperature increases. At 850C to 863C, the oxide film changes from a single layer Cr-rich corundum oxide film (thickness =1 m) 850C to a double layer Fe-rich film (thickness 100 m). Nodules of Fe-rich corundum oxide nucleate 850C, and grow and merge at 863C, covering the majority of the surface area. The outer layer of thick film is Fe2O3 layer, and the inner layer is FeCr2O4 and Fe3O4. At 863C, an internal oxidation zone develops at the oxide/substrate interface with intergranular cracking occurring in the internal oxidation zone and in the substrate. The oxide film developed within the 875C to 950C had the same structural and compos itional characteristics as the thick Fe-rich oxide film developed on the 863C. The inte rnal oxidation zone for the samples oxidized within this higher temperature range also share the same compositional and structural traits as 863C sample. The growth of th e thick oxide film is by outward diffusion of metallic ions from the substrate and the bounda ry between the Fe-rich outer layer and the Fe/Cr-rich inner layer is the location of the samples surface prior to oxidation.

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134 BIOGRAPHICAL SKETCH Soraya Bentez Vlez was born in Mayagez, Puerto Rico, on July 7, 1965. Her parents are Dr. Fernando L. Bentez and Sara N. Vlez. She graduated cum laude with a Bachelor of Science in Mechanical Engineer ing from the University of Puerto RicoMayagez Campus in June 1988. Upon gradua tion, she held a position as a Packaging and Materials Engineer at Hewlett-Packard Manufacturi ng Operation in Aguadilla, Puerto Rico, which sparked her interest in materials science and engineering. She enrolled in the Mechanical Engineering Depart ments graduate program at the University of Puerto Rico-Mayagez Campus as a part -time graduate student while working fulltime. During the summer of 1993, she comp leted an internship at Argonne National Laboratory in Idaho Falls, Idaho. In her fina l years of graduate work, she worked as an instructor for the same department from January 1996 to August 1997 where she taught upper level undergraduate courses. In June 1997, she received a Master in Science in mechanical engineering with a concentration in materials science. She enrolled in the Materials Science and Engineering Department s doctoral program at the University of Florida in August of 1997. Her graduate re search stems from an internship at Los Alamos National Laboratory in Los Alam os during the summer of 2002. After completing the internship, she worked at Santa Fe Community College as a College Prep instructor during the 2002 Fall term and later as an Adult Educations specialist from January 2003 to December 2004. She received a non-thesis masters degree in December of 2003 and is scheduled to graduate w ith a doctoral degree in December of 2005.


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Permanent Link: http://ufdc.ufl.edu/UFE0012151/00001

Material Information

Title: Oxidation Kinetics and Mechanisms in HT-9 Ferritic/Martensitic Stainless Steel
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012151:00001

Permanent Link: http://ufdc.ufl.edu/UFE0012151/00001

Material Information

Title: Oxidation Kinetics and Mechanisms in HT-9 Ferritic/Martensitic Stainless Steel
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012151:00001


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OXIDATION KINETICS AND MECHANISMS IN HT-9
FERRITIC/MARTENSITIC STAINLESS STEEL

















By

SORAYA BENITEZ VELEZ


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Soraya Benitez Velez

































This document is dedicated to the loving memory of my maternal grandmother, Sara
Velez Mufiiz. Mama Lala, I finally made it.















ACKNOWLEDGMENTS

I would like to thank Dr. Darryl P. Butt for the opportunity to be part of his

research group and for providing the opportunity to obtain a summer internship at Los

Alamos National Laboratory. I would like to thank Dr. Scott Lillard, Dr. Ning Li, and

Dan Rustoi from Los Alamos National Laboratory (LANL) and the Department of

Energy (DOE) for making it possible for the summer internship at LANL to become my

graduate research. I would like to thank and acknowledge the assistance of Dr. Gary Was

and Dr. Jeremy Busby for facilitating the use of the Michigan Ion Beam Laboratory

located at the University of Michigan and for providing the material for this research. In

addition, I would like to acknowledge the assistance provided by the Major Analytical

and Instrumentation Center (MAIC) located at the University of Florida. I would

especially like to express my gratitude to Wayne Acree, Kerry Siebein, Gerald Bourne,

Dr. Valentin Craciun, Andrew Gerger, and Juyun Woo for their quick assistance during

critical times.

On a more personal note, I would like to thank Edgardo Pabit, Jairaj Payapilly,

Jonsang Lee, and Abby Queale for being such wonderful colleagues. I appreciate the

technical discussions, the intellectual conversations, and most of all, the times of

laughter. I would also like to extend my gratitude to Dr. Luisa A. Dempere and Dr. Mary

Fukuyama for their support and words of wisdom. Finally, I would like to thank my

parents and Brian DeCarlo. They believed in me even when I did not, and their support,

love and patience are what helped me complete this work.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TA BLES .. ................................ ........ ................................ .. vii

LIST OF FIGURES ...................................................... ................... viii

A B S T R A C T .......................................... ..................................................x iii

1 IN TR OD U CTION ......................................................... ... ........ ........... .... .... 1

2 BACKGROUND INFORMATION ........................................ ........................... 3

Fundamentals of Oxidation of Metals and Alloys.................................................3
Thermodynamics of Oxidation.................... ........ ...............6
O xidation K inetics R ate E equations ............................................ .....................9
L ogarithm ic reaction rate ........................................ ......... ............... 10
P arab olic reaction rate ............................................................. .... ........... 10
L near reaction rate .................. ................. .... .... .. .... ... ............ ... 11
Other kinetics equations ............... ................... ....... ........................... 11
Determination of Oxidation Kinetics from Rate Data......................................14
Oxide Films Developed During High-Temperature Application........................15
Com m on Oxides ......... ....... ..... ... .................. .... ........ .......... .... 17
General Oxidation Fe-Cr Alloys ........................ ............ ........ .......20
Literature Review on the Oxidation of Stainless Steel in Air ..................................23
Factors Affecting Oxidation Behavior ..................................... ............... 24
Effect of alloy composition ........................................ ...............24
Effect of surface treatm ent ........................................ ........ ............... 26
Effect of alloy m icrostructure ........................................... ............... 28
Effect of w ater vapor ............................................................... ............... 29
General Oxidation of Stainless Steels ...................................... ............... 30
HT-9 Ferritic/M artensitic Stainless Steel ....................................... ............... 31
General Applications of HT-9 Stainless Steel.................... ................34
AD S Applications of HT-9 Stainless Steel ............................... ............... .34

3 EXPERIMENTAL PROCEDURE ................................................... .................. 37

Sam ple Preparation ........... ........................................................ ...... .... .. 37
O x id atio n S c an s ................................................................. .................................4 0


v









Equipment........................................... 40
Non-Isothermal Scan Procedure......... ......... .... ..................................45
Isotherm al Scan Procedure ........................... .......... ..................................47
W eight Change Calculations ........................................ .......................... 49
Oxide Characterization ................. ............ ......... .. ............ ...............50
Surface Characterization .................................. .....................................50
Cross-Section Characterization ........................................ ........ ............... 50
Oxide Com pound Characterization ........................... ........... .. ............... 51
Platinum M arker Experiment .................................. .....................................52

4 R E S U L T S ...............................................................................5 4

N on-Isotherm al Scan Results .................................. .....................................54
Isotherm al Scan R esults.............................................................. .. ... 55
SEM Surface Characterization Results.................................... ....................... 67
6000C to 8250C Temperature Range.......................... ................................67
8500C to 8630C Tem perature Range ............................. ............................... 70
8750C to 9500C Temperature Range.......................... ................................74
SEM Cross-Section Characterization Results .................................... ............... 82
6000C to 8250C Temperature Range............. ............. ................................ 83
8500C to 8630C Temperature Range............. ............. ................................ 85
8750C to 9500C Temperature Range............. ............. ................................ 91
Platinum Marker Experiment Results........................ ........... ............... 107
X R D C haracterization R esults............................................................. ............... 109

5 D ISCU SSION O F TH E RESU LT .................................................................... ...... 112

O x idation Scan .................... .............. ......... ..................................... 112
Morphology, Composition and Structure of the Oxide Film ................................... 116
6000C to 8250C Temperature Range ............... .................................117
8500C to 8630C Temperature Range............... .............................................. 117
8750C to 9500C Temperature Range............... .............................................. 119
Oxide Com pounds .................. ................................... .. ..... ...............124

6 C O N C L U SIO N S....... ............................................................................... ..............126

L IST O F R E FE R E N C E S ........................................................................ ................... 128

BIOGRAPHICAL SKETCH ............................................................. ............... 134
















LIST OF TABLES


Table p

2-1. Principal binary oxides of iron, chromium, manganese, and vanadium ................20

2-2. Example of ternary and complex oxides of stainless steels. ............. .................20

2-3. Composition of HT-9 steel in weight percent according to different references. ....32

3-1. Composition of HT-9 steel in weight percent (wt%). ................... ................38

3-2. Example of a non-isothermal program .......................................... ............... 46

3-3. Example of an isothermal program .............................................. ............... 47

3-4. Isothermal scan temperatures and holding time combinations. ............................47

4-1. Surface area and weight change for HT-9 samples oxidized under
non-isotherm al conditions. ...... ........................... .......................................54

4-2. Surface area and weight change for HT-9 samples oxidized under isothermal
conditions. ...........................................................................56

4-3. Calculated m values based on linear regressions of the segments of double
lo g arith m ic p lo ts ................................................................... ............... 6 1

4-4. Linear (kL), parabolic (kp), cubic (kc), logarithmic (kLog), and inverse
logarithmic (kinv) rate constants for each isothermal plot segment.......................65

4-5. Oxide compounds that best match the XRD profiles.........................................109
















LIST OF FIGURES


Figure pge

2-1. Schematic illustration of the of the oxidation reaction between a metallic
substrate and oxygen. .............................. .................... .. .............. ............. 5

2-2. Ellingham-Richardson diagram of the oxides of metals and alloys commonly
used in high tem perature application. ............................................. ............... 7

2-3. Fe-O phase diagram ........................................ .................. ............

2-4. Graphical illustration comparing the different oxidation reaction rates equations. ...9

2-5. Graphic illustration of paralinear kinetics (A) and breakaway oxidation (B)..........13

2-6. Variation in oxide structure with chromium content of a Fe-Cr alloy based on
data obtained from isothermal (constant temperature) scans at 10000C in 0.13
atm of ox y g en ......................................................................................... 16

2-7. Schematic illustration of the oxide film structure for Fe-Cr alloys of low
chrom ium content .................................................. ........ .. ........ .... 22

2-8. Development of a 12% Cr ferritic/martensitic stainless steel .................................32

2-9. Constitutional diagram for Fe-Cr alloys (A) and Shaeffler diagram [58a] (B)........33

3-1. Secondary electron (SE) image of the microstructure of the as-received HT-9
stainless steel alloy after etching the surface with Marble's Reagent.....................39

3-2. SE image and line scan of a particle located along the boundary between the
m artensite phase and the ferrite phase................................... ....................... 40

3-3. Schem atic of the equipm ent set-up. ............. ................................ ...................41

3-4. Photograph of the actual equipment used during the oxidation scans. ..................42

3-5. Cross-section view of the TGA 2050 balance assembly and furnace assembly. .....43

3-6. Example of a non-isothermal temperature profile................................................46

3-7. Example of an isothermal scan temperature profile..............................................48









3-10. Oxide scrapped from the surface mounted for XRD characterization ...................52

3-11. HT-9 sample with the Pt marker after drying for 24 hours in a dessicator cabinet
and prior to the isotherm al scan. ........................................ .......................... 53

4-1. Calculated weight change versus temperature plots for HT-9 samples oxidized
under non-isotherm al conditions ........................................ ......................... 55

4-2. Calculated weight change versus time plots for HT-9 samples oxidized under
isotherm al conditions. ...................... .. .... ................ ............................ 57

4-3. Isothermal scan plots of HT-9 samples oxidized at 9000C in dry air for 30
minutes, 90 minutes, and 24 hours. ........................................ ....................... 58

4-4. Calculated double logarithm plots of the isothermal data................... ............59

4-5. Example of identifying the oxidation kinetics based on the results of the linear
regressions applied to each segment of a double logarithmic plot...........................60

4-6. Example of validating a kinetics rate equation (parabolic rate equation) for a
sample oxidized at 850 C for 24 hours. ......................................... ...............62

4-7. Possible oxidation kinetics based on the results of the linear regressions
performed on a double logarithm plot of the isothermal data...............................63

4-8. Possible oxidation kinetics based on validating the simple kinetics rate equations
to the isotherm al data. ...................... .. ...................... .. .... .... ............... 64

4-9. Calculation of the activation energies for the linear (A) and parabolic (B)
oxidation m models. ................................................... ................. 66

4-10. SE images and corresponding EDS spectra for the sample oxidized in dry air at
6000C for 48 hours. ............................ .......... ......... ...... ...... ...... 68

4-11. SE images and corresponding EDS spectra for the sample oxidized in dry air at
7000C for 48 hours. ............................ .......... ......... ...... ...... ...... 68

4-12. SE images and corresponding EDS spectra for the sample oxidized in dry air at
800 C for 4 8 hours.. ............................. ......................... ......... .. .... ..... ...... 69

4-13. SE images and corresponding EDS spectra for the sample oxidized in dry air at
8250C for 24 hours. ............................................. .........................69

4-14. SE images and corresponding EDS spectra for the sample oxidized in dry air at
8500C for 24 hours. ............................................. .........................70

4-15. SE images and corresponding EDS spectra for the sample oxidized in dry air at
8630C for 24 hours. ............................................. .........................71









4-16. SE images and corresponding EDS spectra of the various features of the Fe-rich
oxide structures observed on the sample oxidized in dry air at 8500C for
24 hours. ............................................................................72

4-17. SE images and corresponding EDS spectra of the various features of the Fe-rich
oxide surface of the sample oxidized in dry air at 8630C for 24 hours ..................73

4-18. Image of the cracks on the surface of the Fe-rich oxide film developed on the
sample oxidized in dry air at 9500C for 6 hours. .............................................. 74

4-19. SE images and corresponding EDS spectra for the sample oxidized in dry air at
8750C for 24 hours. ............................ .......... ........ ...... ...... ...... 75

4-20. SE images and corresponding EDS spectra for the sample oxidized in dry air at
9000C for 24 hours. ............................ .......... ........ ...... ...... ...... 76

4-21. SE images and respective EDS spectra of the various features of the outermost
oxide layer of the sample oxidized in dry air at 8750C for 24 hours ....................77

4-22. SE images and respective EDS spectra of the various features of the outermost
oxide layer of the sample oxidized in dry air at 9000C for 24 hours ....................78

4-23. SE images and corresponding EDS spectra for the sample oxidized in dry air at
950C for 6 hours. ..................................... .. .. ........ ........ ...... 79

4-24. SE images and respective EDS spectra of the various features of the outermost
oxide layer of the sample oxidized in dry air at 9500C for 6 hours. ........................80

4-25. SE images and corresponding EDS spectra for the sample oxidized in dry air at
9000C for 30 m minutes. ...................... .. .................... .. .... .... .......... .....81

Figure 4-26. SE images and corresponding EDS spectra for the sample oxidized in
dry air at 9000C for 90 m minutes ........................................ .......................... 81

4-27. SE images and respective EDS spectra of the various features on the surface of
the Fe-rich oxide structures of the sample oxidized in dry air at 9000C for
90 m minutes. ........................................................ ................. 82

4-28. HT-9 sample on alumina pan after isothermal oxidation in dry air at 6000C
for 4 8 h ou rs. ....................................................... ................. 83

4-29. Cross-section SE image for the samples oxidized at 6000C, 7000C, and 800C
for 48 hours and the sample oxidized at 8250C for 24 hours.................................84

4-30. Cross-section SE image, elemental mapping, and line scan profiles for the
sample oxidized in dry air at 8500C for 24 hours. ................................................86









4-31. Cross-section SE image, elemental mapping, and line scan profiles for the
sample oxidized in dry air at 8630C for 24 hours. ................................................87

4-32. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
at spot 1 of 4-31 .................................................................. ....... 88

4-33. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
at spot 2 of 4-31 .................................................................. ....... 89

4-34. Cross-section SE image and elemental mapping of the oxide at spot 3 of 4-33......90

4-35. Cross-section SE image, elemental mapping, and line scan profiles for the
sample oxidized in dry air at 8750C for 24 hours. ................................................93

4-36. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
in spot 1 of 4-35. ............. ............... ............................... ....... .................. .......... 94

4-37. Cross-section SE image, elemental mapping, and line scan profile of the oxide at
spot 2 of 4-35 .................................... .................................. 95

4-38. Cross-section SE image, elemental mapping, and line scan profile of the oxide at
spot 3 of 4-37............. .... ................................................... ....... .... 96

4-39. Cross-section SE image, elemental mapping, and line scan profiles for the
sample oxidized in dry air at 9000C for 24 hours. ................................................97

4-40. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
at sp o t 1 in 4 -3 9 .................................................................... 9 8

4-41. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
at sp o t 2 in 4 -3 9 .................................................................... 9 9

4-42. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
at spot 3 in 4-41 .............. ................................. ..... ..... ......... 100

4-43. Cross-section SE image, elemental mapping, and line scan profiles for the
sample oxidized in dry air at 9500C for 6 hours. ............. ................................... 101

4-44. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
at lo cation 1 in 4 -4 3 ............ ................................................................... ..... 102

4-45. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
at lo cation 2 in 4 -4 3 ............ ................................................................... ..... 10 3

4-46. Cross-section SE image, elemental mapping, and line scan profiles of the oxide
at lo cation 3 in 4 -4 3 ............ ................................................................... ..... 104









4-47. EPMA results in weight percent from two different locations on the oxide
developed on the 8500C/24 hrs sample (A) and the thin oxide developed on the
863C/24 hrs sam ple (B). ..... ........................... .......................................105

4-48. EPMA results in weight percent from different locations across the oxide
developed on the sample oxidized in dry air at 8630C for 24 hours....................105

4-49. EPMA results in weight percent from different locations across the oxide
developed on the sample oxidized in dry air at 8750C for 24 hours....................106

4-50. EPMA results in weight percent from different locations across the oxide
developed on the sample oxidized in dry air at 9000C for 24 hours....................106

4-51. EPMA results in weight percent from different locations across the oxide
developed on the sample oxidized in dry air at 9500C for 6 hours......................107

4-52. Pt marker sample after isothermal scan in dry air at 9000C for 24 hours ............107

4-53. Cross-section SE image, elemental mapping, and line scan profiles of the Pt
m arker sam ple. ......................................................................108

4-54. XRD profiles of the surface of non-oxidized (baseline) and oxidized
HT-9 sam ples. .................................... ............................... ........110

4-55. XRD profiles of the oxide scrapped from the surface of non-oxidized (baseline)
and oxidized H T-9 sam ples .......................................................... ............ .111

5-1. Classification of isothermal plots (A) and plot of the overall weight change
versus the temperature for the isothermal scans (B). ............................................113

5-2. Isothermal plots of samples that seem to exhibit breakaway oxidation kinetics.... 115

5-3. SE images of the changes in surface morphology, thickness, and structure of the
oxide film with increased oxidation temperature..............................116

5-4. SE image of the partial dissolution of grain structures at the internal oxidation
zone. 121

5-5. Evolution of the Fe-rich oxide as a function of holding time for samples
oxidized in dry air at 900 C ........................................... ........................... 123















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

OXIDATION KINETICS AND MECHANISMS IN HT-9
FERRITIC/MARTENSITIC STAINLESS STEEL


Soraya Benitez Velez

December 2005

Chair: Darryl P. Butt
Major Department: Materials Science and Engineering

Lead and lead alloys, such as lead bismuth eutectic (LBE), have gained worldwide

recognition as potential candidates for coolant and target material in accelerator-driven

systems (ADS) due to their excellent chemical, physical, and nuclear properties;

however, they are corrosive to stainless steels, which are candidate materials for piping

and molten metal containers in ADS applications. The Russians have used LBE as a

coolant in their nuclear submarines for more than several decades and their experience

has given way to the hypothesis that oxides protect steels from LBE corrosion, thus

renewing interest in further understanding the oxidation kinetics and mechanisms of

candidate stainless steel alloys.

The proposed work will contribute to the ongoing research effort by elucidating the

oxidation kinetics and mechanisms, and characterizing the oxides of HT-9, a candidate

stainless steel for ADS application. HT-9 (DIN X20CrMoWV12 or Fe-12Cr-MoVW) is

a 12 wt% Cr martensitic/ferritic stainless steel alloy developed by Sandvik (Sweden).









The approach is to perform thermogravimetric analysis (TGA) to evaluate the oxidation

kinetics and assess oxidation rates. Flat bar samples polished to one micron were

subjected to non-isothermal and isothermal oxidation scans. Non-isothermal scans were

done in dry air from room temperature to 9500C with ramping rates of 20C/minute and

5C/minute to obtain preliminary oxidation behavior information. Isothermal scans were

done in dry air from 6000C to 8000C for 48 hours, from 8000C to 9000C for 24 hours,

and at 9500C for 6 hours. The structure and composition of the oxide film developed

during the isothermal scans were characterized using scanning electron microscopy with

energy dispersive spectroscopy capabilities (SEM/EDS) and X-ray diffraction (XRD).

Oxidation of HT-9 exhibited a complex behavior that does not follow a simple

oxidation model and changes as a function of exposure time. At low temperatures

(T < 8250C), the weight change (AW) was minimal and a protective, single-layer, Cr-rich

spinel and/or corundum oxide film formed. At high temperatures (T > 8750C), the AW

was faster and the oxidation behavior predominantly followed linear kinetics. The

developed oxide film is a double layer structure, consisting of an outer Fe-rich corundum

oxide layer and an inner Fe/Cr-rich spinel oxide layer. This oxide film grows by outward

diffusion of metallic ions from the substrate. An internal oxidation zone with

intergranular cracking was also observed at temperatures of 8630C and above.

Breakaway oxidation occurred within the 8500C to 8630C range, in which the oxidation

behavior shifts from parabolic kinetics (protective) to linear kinetics (non-protective) as

initially foreseen in preliminary non-isothermal oxidation scans.














CHAPTER 1
INTRODUCTION

The Fe-12Cr ferritic/martensitic stainless steel HT-9 (DIN X20CrMoWV12 or also

known as Fe-12Cr-MoVW) may have application in the nuclear power industry.

Potential uses for this alloy are in the fabrication of in-core and out-of-core nuclear

reactors components. An example of an out-of core application is the experimental use

of HT-9 as cladding and ducts in the Fast Flux Test Facility (FFTF), a liquid-metal

cooled nuclear test reactor located at Richmond, WA. Other potential applications

include first wall and tritium breeding blanket components in fusion reactors and as pipes

and vessels in accelerator-driven systems (ADS).

The Advanced Fuel Cycle Initiative (AFCI) is a Department of Energy sponsored

ADS project for the handling of nuclear waste. The concept is to enclose radiotoxic

waste, such as minor actinides and long-lived fission products, inside a stainless steel

vessel that contains a molten lead alloy. The vessel is bombarded with high-energy

protons generating neutrons. The neutrons are absorbed by the radiotoxic waste,

triggering nuclear transformations (transmutation) to occur within the waste material.

Thus, the waste is converted to a less hazardous or non-radioactive material that is easier

to handle and dispose and the energy liberated during the transmutation process can be

used to generate electricity. However, liquid metals, such as molten lead alloys, are

corrosive to stainless steel reducing the life expectancy of the components and leading to

potential failure. The Russians have used molten lead alloys as a coolant in their nuclear

submarines for several decades. Their experience has shown that an oxide layer can









protect the steel from corrosion by molten lead alloys. This has given way to the

hypothesis that oxides of passivated stainless steels may protect the alloy from the

corrosive environment of molten metals, prompting interest in gaining a better

understanding of the oxidation behavior of HT-9, for which information on its oxidation

behavior is nonexistent.

Background information on the oxidation behavior and common oxidation products

for the iron-chromium alloy family are presented in Chapter 2. This chapter also includes

a literature review on the oxidation of stainless steels in air and information on the alloy

used during this research. Chapter 3 provides information on the experimental procedure

and materials characterization techniques. Experimental and characterization results are

presented in Chapter 4 and discussed in Chapter 5. Conclusions are presented in

Chapter 6.














CHAPTER 2
BACKGROUND INFORMATION

This chapter presents background information on the kinetics and mechanisms of

oxidation, the general oxidation processes, and most common reaction products for the

iron-chromium alloy family. The information is mostly based on Hauffe [1], Kofstad [2, 3]

Birks and Meier [4], Khanna [5], and the ASM Handbook Volume 13a [6] unless specified

otherwise. Literature review on the oxidation of stainless steels in air with emphasis on

ferritic/martensitic stainless steels and information on the stainless steel alloy used during

this research is also presented.

Fundamentals of Oxidation of Metals and Alloys

Oxidation is a chemical reaction that occurs between a metallic substrate and the

reacting gases present in the environment due to the thermodynamic instability of the

substrate when exposed to these gases. The most common reacting gas is oxygen and the

by-products are oxides. The reaction rate and oxidation behavior depend on many factors

that can result in a complex reaction behavior in which several mechanisms are

simultaneously active. In general, these factors are

* Composition, pretreatment, and surface finish of the substrate.
* Gas composition and partial pressure of the environment.
* Temperature.
* Time.

Depending on the combination of these factors, the reaction can be slow to the

point that the substrate is virtually unattacked, or it can be fast leading to eventual









component failure. The last case is especially detrimental for high temperature

applications for oxidation rates increase with temperature.

The general equation that describes the chemical reaction between the metallic

substrate and oxygen to form an oxide is written as


aoM + O2 >MOOb Equation 2-1


* M and O is the metal and oxygen, respectively.
* a is the number of moles of the metal and b is the number of moles of oxygen.

The onset of oxidation occurs with the adsorption of oxygen on the substrate

surface, which dissolves and diffuses into the substrate as the reaction proceeds. This

leads to the formation of an oxide compound on the surface as a layer or as separate

nuclei. The parameters that influence the process of adsorption and initial oxide

formation are crystal defects and orientation at the surface, surface finish, and impurities

in the substrate and gaseous environment. Once a continuous oxide film has covered the

surface, the reaction continues via solid-state diffusion of the reacting species through the

oxide film. One of the reacting species is metallic ions, which are termed cations and

have a negative charge. The other reacting specie is oxygen ions, which are termed

anions and have a positive charge. The driving force for the transport of the reacting

species depends on the thickness of the oxide film. For thin film, the driving force may

be an electric field in or across the film with the addition of electron transport. If the

layer is thick, the driving force may be a chemical potential gradient across the film. The

oxidation process is schematically shown in Figure 2-1.

Depending on the oxidizing environment and the substrate, the oxide film may

consist of a single oxide layer or of multiple oxide layers, in which the layers may be










porous or compact. A compact oxide film tends to have a protective nature, which serves

as a diffusion barrier between the reacting species and thus limits the reaction process.

At higher temperatures, the oxide film may volatize and/or melt, which may lead to

catastrophic failure.


O2(g)
0


Oxide
4- e4


O,(g)


Mcla.l Subhr.lrale

Step 1: Adsorption of dissociated oxygen on
the surface of the metallic substrate.


0o-2


O2(g)

M+n Internal Oxidation
Particles
Q = 4-- i


Step 3: Oxide film growth with diffusion of
the reacting species through the oxide film.
Internal oxidation may occur below the
substrate surface.


MOv(g


Step 2: Oxide nucleation and growth
with dissolution and inward diffusion of
oxygen.


Cavity with
microcrack
I 02(g)


0..1 e .Mn Void
O.*:*.*:*%:*. :*.*.:*.*.


Step 4: Development of microcracks,
voids, and cavities within the oxide film.
The microcracks can serve as short-
circuit diffusion paths.


) Macrocrack



O-21 / e- IM^n


O2(g)


Step 5: Development of macrocracks with
possible molten oxide phase formation and/or
oxide sublimation


Figure 2-1. Schematic illustration of the of the oxidation reaction between a metallic
substrate and oxygen. Figure based on Kofstad [2, 3] and the ASM Handbook
Volume 13a [6].


1 0









Thermodynamics of Oxidation

The overall driving force for the oxidation reaction between oxygen and a metallic

substrate is the standard Gibbs free energy of formation (AG) of the oxide compounds

formed during the chemical reaction between oxygen and the metallic substrate. From

the thermodynamic perspective, the oxide will only form if its dissociation pressure in

equilibrium with the metallic substrate is less than the environment's oxygen partial

pressure (po2) at a given temperature. The Ellingham-Richardson diagram, also known

as the Richardson-Jeffes diagram, allows the graphical determination of the AG of the

oxides for the data is presented as a function of temperature and corresponding oxide

dissociation pressure. (Figure 2-2. [6]) The AG of the oxides is plotted along the Y-axis.

The larger the negative value the more stable is the oxide. The temperature is plotted

along the X-axis. Auxiliary outer scales are included on the top, right, and bottom of the

diagram that represent the po2 in oxygen-containing environment or consisting of mixed

gases (CO/C02, H2/H20). As an example, the AG for chromia (Cr203), an oxide of

chromium (Cr), at 6000C is approximately -600 kJ for 2/3 mole of chromia (red line) and

the corresponding oxide dissociation pressure in an oxygen containing environment is

approximately 10-35 atmospheres (atm) (blue line). Thus, Cr will not oxidize to chromia

at 6000C if the po2 in the environment is less than 10-35 atm.

If the metallic substrate forms more than one type of oxide, the use of a phase

diagram will aid in understanding the oxidation process. The phase diagram presents the

different type of oxides that can form as a function of temperature and composition of the

reacting species (Figure 2-3. [7]) This allows the prediction of a sequence of oxides that

can develop in the oxide film, with the most oxygen deficient oxide next to the









oxide/substrate interface and the most oxygen rich oxide next to the gas/oxide interface.

The temperature is plotted on the Y-axis while the composition is plotted on the X-axis.


Figure 2-2.Ellingham-Richardson diagram of the oxides of metals and alloys commonly
used in high temperature application. The AG for Cr203 at 6000C is
approximately -600 kJ/mol 02, represented by the red arrow, and the
corresponding dissociation pressure is approximately 10-35 atm, represented
by the blue arrow. [6]


11- 10-" 10-" 1 0 I0, 10-4 10-1 10-" 1D-" 10-" 10-"










The iron-oxygen (Fe-O) phase diagram presents three oxides of iron: wustite

(FeO), magnetite (Fe304), and hematite (Fe203 or a-Fe203). If iron is oxidized in an

oxygen-containing environment at 4000C, the oxide film will consists of two layers

according to the Fe-O phase diagram: Fe304 next to the substrate and Fe203 next to the

oxygen phase. FeO would not form for it is unstable below 570C, where it decomposes

to iron and magnetite. If the oxidation occurred at 8000C, the oxide film would consist of

three layers in the following sequence starting with the one next to the substrate and

moving outwards: FeO, Fe304, and Fe203.

ALomic Percent Oxygen
0 10 20 70 40 50 O


eoo- ,I"L I6n + L, L2s_ ..
iROO




itra)
L ___ ______ __________

,- --- s\




400S
100- ,I"







I0 20 40
Fe WeighL Percent Ovygen

Figure 2-3.Fe-O phase diagram. [7]

For metallic substrates that form a variety of oxides, knowledge of thermodynamics

in conjunction with phase diagrams may only serve as a guide to understanding the

oxidation behavior. As previously mentioned, the overall driving force is the change in









the AG associated with the oxidation reaction, but it is not related to the rate of the

reaction. The oxidation reaction rate is a kinetics problem that depends on the reaction

mechanism and the rate-limiting process.

Oxidation Kinetics Rate Equations

To identify the reaction mechanisms and define the rate-limiting process occurring

during the oxidation reaction, it is important to have knowledge of reaction rates and

corresponding equations. These rates and equations are function of several factors:

Substrate pretreatment and surface finish, environment's po2 and temperature, and

elapsed time.

The simplest oxidation reactions obey logarithmic, parabolic, or linear models,

which represent limiting and ideal cases. It is common to encounter deviations from

these ideal models under real life applications, in which the rate data can only be fitted by

use of intermediate rate equations and/or combination of the ideal models. The various

simple reaction rates are shown in Figure 2-4, where X represents the change in the

parameter of interest, such as the thickness of the oxide or the weight of the sample.

Linear
Inverse Parabolic
X LogaInversethmic Logarithmic Parabolic
X Logarithmic


X represents the variation in the
oxide's thickness or mass, oxygen
consumed per surface area, or quantity
of metal that transforms into oxide




Time

Figure 2-4. Graphical illustration comparing the different oxidation reaction rates
equations. Figure based on the ASM Handbook Volume 13a [6].









Logarithmic reaction rate

Logarithmic kinetics typically occurs at temperatures below 4000C, and for oxide

film thickness of 100 nm or less. Initially, the reaction rate is very fast with the rate

dropping to a low or negligible value with time. Ionic and/or electron transport processes

through the oxide film are the rate-limiting mechanism, and the driving force is an

electric field in or across the film. Logarithmic behavior is represented by two different

equations that are difficult to distinguish, which are

* Logarithmic, also known as direct logarithmic.

X = kog log(t + to) + A Equation 2-2a

* Inverse logarithmic.


1
= B k,,, log(t)
X


Equation 2-2b


* X is the oxide's thickness or mass, oxygen consumed per surface area, etc.
* klog and kinv are the logarithmic and inverse logarithmic rate constants, respectively.
* t and to are time.
* A and B are constants.

Parabolic reaction rate

The oxidation behavior of the majority of metals and metallic alloys follow

parabolic kinetics during high temperature oxidation. The rate-limiting step is the

thermal diffusion of the ionic species through a compact oxide film and the driving force

is the chemical potential gradient that develops across the film. The diffusion process

may involve outward diffusion of cations, inward diffusion of anions, or both. It may

also involve the transport of electrons across the oxide film. As the oxide thickness

increases with time, the reaction rate decreased due to the increase in the diffusion

distance. The equation that describes parabolic kinetics is









X2 =kpt + C


* X is the oxide's thickness or mass, oxygen consumed per surface area, etc.
* kp is the parabolic rate constant.
* t is time and C is a constant.

Linear reaction rate

In linear kinetics, the oxidation rate is constant with time and independent of the

amount of gas or metallic substrate consumed. Surface and/or phase boundary processes

are the rate-limiting mechanism. This behavior is observed in the following situations:

* The oxide is porous and non-protective.
* The oxide vaporizes.
* The oxide film cracks and spalls off due to internal stress.
* The oxide melts and forms eutectic phases with the substrate.

The equation for linear kinetics is as follows:

X = kt + C Equations 2-4

* X is the oxide's thickness or mass, oxygen consumed per surface area, etc.
* kl is the linear rate constant.
* t is time and C is a constant.

Other kinetics equations

A number of intermediate reaction rates have been developed that better represent

the oxidation behavior in real life application and/or of complex metallic alloy systems.

Initially, the oxidation reaction may follow one type of kinetics and gradually change to

another type, thus resulting in a complex oxidation behavior in which the kinetics change

with time. These intermediate equations are combinations of the ideal rate laws and

sometimes may not be able to fully explain the complex oxidation behaviors. Some the

intermediate kinetics rate equations are

* General parabolic equation.
* Cubic oxidation.


Equation 2-3









* Paralinear oxidation.
* Breakaway oxidation.

General parabolic equation. During high temperature oxidation, the onset of

oxidation follows linear kinetics, eventually becoming parabolic with time. The equation

that describes this behavior may be expressed as [2, 3]

X2+ kX =kpt + C Equation 2-5a

* X is the oxide's thickness or mass, oxygen consumed per surface area, etc.
* kl and kp the linear and parabolic rate constant, respectively.
* t is time.
* A and C are constants.

Equation 2-5a is known as the general parabolic equation, and if the constant C is

neglected, the equation may be rewritten as [2,3]:

t
X = kp k, Equation 2-5b
X

Cubic oxidation. Cubic oxidation occurs when the reaction rate falls between

logarithmic and parabolic kinetics. This is characterized by an initially fast logarithmic

behavior followed by the slower parabolic behavior. Cubic behavior may be

approximated by the following equation:

X3 = kt + C3 Equation 2-6

* X is the oxide's thickness or mass, oxygen consumed per surface area, etc.
* k3 is the cubic parabolic rate.
* t is time and Cm is a constant.

Paralinear oxidation. In paralinear kinetics, the oxidation is initially parabolic

and gradually becomes linear with time. (Figure 2-5A.) This type of behavior occurs

when a compact oxide film transforms as a whole or partially into a non-protective

porous layer. An inner compact protective layer remains within the oxide film next to the









substrate, while the outer layer is non-protective due to the development of pores, voids,

cracks within the oxide film, and/or the oxide volatizes. The rate equation for paralinear

kinetics has been derived assuming that the specimen's surface area remains constant

with time and the equation may be expressed as [2, 3]

k K k
X = k' In -, Equation 2-7
k k k (x k1t)

* X is the oxide's thickness or mass, oxygen consumed per surface area, etc.
* kp and kl are the parabolic and linear rate constants, respectively.
* t is time.



X -Paralinear X -Breakaway



n -a Parabolic -" Parabolic

earLinear
Linear

A -B
Time Time

Figure 2-5.Graphic illustration of paralinear kinetics (A) and breakaway oxidation (B). X
is the oxide's thickness or mass, amount of oxygen consumed, etc. Figures
based on the ASM Handbook Volume 13a [6].

Breakaway oxidation. Breakaway oxidation occurs when the protective oxide

film reaches a certain thickness in which continuous cracking and spelling occurs, a point

at which the oxide loses its protective nature. The oxidation is initially parabolic, until

breakaway occurs and the oxidation then follows a linear behavior. (Figure 2-5B.) This

type of kinetics can also occur when one of the components in the metallic alloy is

selectively oxidized. Selective oxidation can deplete the surface of the component that

forms the protective oxide, thus the protective oxide cannot reform and/or heal after









spelling. Breakaway oxidation may seem similar to paralinear kinetics, but the difference

is that while the metallic surface is left bare during breakaway oxidation, an inner

protective oxide layer remains during paralinear oxidation.

Determination of Oxidation Kinetics from Rate Data

Several methods are helpful in identifying the kinetics involved during an oxidation

process. One of the methods is based on the results of a linear regression performed on a

double logarithm plot of the data. This method takes the logarithm of Equation 2-6 to

obtain the following equation [2, 3]


logX = logt+C Equation 2-8
m

* X is the oxide's thickness or mass, oxygen consumed per surface area, etc.
* m is the parameter for kinetic information.
* t is time and C is a constant.

The data is then plotted as the Log(X) versus the Log (t). The slope of the plot is

calculated and compared to the slope of Equation 2-8 to find the value of m, thus the

calculated slope is equal to 1/m. The identification of the kinetic rate is provided by the

parameter m, which has values of 1, 2, and 3, corresponding to linear, parabolic, and

cubic oxidation, respectively.

Another method for elucidating the kinetics behavior is validating the data to the

various kinetics rate equations. For example, to test for parabolic kinetics, the data is

plotted as X2 versus time, and a linear regression is performed. The R2 value is then

evaluated to see how well the parabolic model fits the data. This process would be

repeated for each of the kinetics rate equations.









Oxide Films Developed During High-Temperature Application

Stainless steel alloys used for high-temperature applications are required to have

the ability to form a protective oxide film and the oxides identified as meeting this

requirement are chromia (Cr203), alumina (A1203), and potentially silica (SiO2). [6, 8, 9, 10]

These oxides have a low density of point defects, thus serve as protective barriers due to

their low diffusivity for ionic transport. This has led to the development of chromia

former and alumina former steel families such as Fe-Cr, Fe-Al, Fe-Mn, Fe-Cr-Ni, and Fe-

Cr-Al. [81 The amount of alloying elements required for developing these protective

oxides and the potential development of non-protective oxides depends on the base

composition of the stainless steel, intended application, and working environment.

Figure 2-6 shows the variation in oxide structure with chromium content for a Fe-Cr alloy

oxidized at 1000C in 0.13 atm of oxygen. [6] From Figure 2-6, the oxide film changes

from a Cr-rich protective oxide to a Fe-rich non-protective oxide with a decrease in

chromium content. The minimum amount of chromium required to form chromia is

approximately 10 wt%.

The breakdown of protective oxides may be caused by cracking and/or spelling due

to growth or thermal cycles, erosion, abrasion, and/or wear. [6, 8] When this occurs, the

rate at which the oxide "heals" (i.e. restores) will depend on the amount of the oxide

forming elements within the bulk. If the content is too low, the protective oxide will not

heal, leading to the development of non-protective oxides and potential component

failure. If the amount is too high, healing of the protective oxide is ensured but it might

adversely affect mechanical properties, fabrication, and/or welding. [11] For example,

chromia will not heal when the chromium content in the near-surface region where the

oxide initially formed drops below 10 wt% due to chromium depletion. On the other










hand, if the chromium content is between 20-25 wt%, the protective oxide will heal but

the high amount of chromia will induce the formation of sigma phases, which increases

the alloys brittleness. [4, 12] Therefore, other alloying elements are added to aid in the

healing of the protective oxide film, improve adhesion of the film, and/or reduce the

detrimental effects caused by increasing the amount of the protective oxide forming

elements. Some of these alloying elements are nickel, manganese, molybdenum,

vanadium, and silicon. [8, 11, 13]


Fe203


F O F


Fe



10 Fe 2Cr Fe/Cr oxide

Fe0

S0 FFeCr)203
SFe 9Cr


I \. Fe,04. _^__F.--- FeaO,
o0-9
Y Cr203

SFe 16Cr -FeFe,, ,.Cr,0,


Cr,0


Fe 28Cr

10-" 1 J L
0 10 20 30 40 50 60 70 80 90 100
Alloy chromium content, wti%

Figure 2-6.Variation in oxide structure with chromium content of a Fe-Cr alloy based on
data obtained from isothermal (constant temperature) scans at 10000C in
0. 13 atm of oxygen. [6]









Common Oxides

Although the ultimate goal is the development of a continuous protective single-

layer film of chromia when stainless steels are exposed to high-temperature oxidizing

environments, this rarely occurs during real life applications. Chromia reacts with the

monoxides of the alloying elements to form new oxides with a spinel structure of the type

MCr204 in which M represents an alloying element. An example is FeCr204 (chromite),

which is a mixture of the iron monoxide FeO and Cr203. [14] If the oxide structure of the

alloying elements are similar, they may form solid solutions, such as (Fe, Cr)203 which is

solid solution of Fe203 and Cr203. Thus, a variety of oxides may develop, ranging from

protective to non-protective, from simple binary oxides to complex mixed oxides, due to

the constituents present in stainless steel. The oxides may be divided into three basic

crystal or lattice structure

* Cubic: MO
* Corundum: M203
* Spinel: M304 or AB204

where M is a metal, A and B are two different metallic elements, and O is oxygen. Some

of the principle binary oxides of the main constituent of stainless steel are presented in

Table 2-1, and examples of common ternary and complex oxides are presented in

Table 2-2. The structure, morphology, composition, and complexity of the resulting

oxide film will depend on the alloy composition and the working environment.

Iron oxides. The main oxides of iron according to the Fe-O phase diagram (Figure

2-3) are FeO, Fe304, and Fe203, which are non-stoichiometric compounds. Wustite is

metal deficient oxide and its general formula is FexO with x values ranging from 0.836 at

the Fe-FeO boundary to 0.954 at the FeO-Fe3O4 boundary. It is considered a p-type









semiconductor, and thermodynamically stable above 5750C and at reduced oxygen

activity (i.e. low po2). FeO is antiferromagnetic and grows by outward diffusion of iron

ions. Magnetite is a mixed oxide of Fe+2 and Fe+3 ions of the form Fe+2Fe+3204. It has a

slight stoichiometry variation at high temperatures with a metal-deficiency at p02 higher

than the required for stoichiometry, and a metal-excess at p02 lower than this value. It is

ferrimagnetic, and its electrical properties are complex. Fe304 is an inverse spinel at and

below room temperature (RT) and the distribution of the two cations becomes

randomized at temperatures above RT. Magnetite is obtained by the partial oxidation of

FeO or by heating Fe203 above 14000C. Hematite has a small stoichiometric variation

and a variety of modifications with the more important being the a- and the y- form.

a-Fe203 is obtained when Fe304 oxidizes above 4000C and it grows by outward diffusion

of iron ions. It is considered a p-type semiconductor, although it behaves as an n-type

from 650C to 8000C. y-Fe203 is a ferrimagnetic metastable compound obtained by

careful oxidation of Fe304.

Chromium oxides. From an engineering perspective, the most important

chromium oxide is Cr203 because of its protective nature and it is the only

thermodynamically stable oxide, especially at high temperature. It has a high melting

temperature of 24500C and vaporizes to Cr03 at high temperatures and high po2

according to the following reaction

1 3
Cr2,0 + 02 CrO, (g) Equation 2-9
2 4

Thus, this reaction becomes important at temperatures above 10000C in one atm 02

environment. Chromia is an electronic semiconductor and its conductivity changes

depending on the temperature and po2. It behaves as a metal-deficient (p-type)









semiconductor at low temperatures (T<12500C) or at high po2, and as a

metal-excess (n-type) semiconductor at low po2. At high temperatures (T>12500C), it is

an amphoteric semiconductor. Cr203 films grow by counter-current diffusion of

chromium and oxygen along short-circuit diffusion paths and the growth occurs within

the film. The oxide grows normal and parallel to the surface of the substrate, thus

resulting in large stresses and strains that can lead to oxide cracking and potential

spelling. The other oxides of chromium have lower melting temperatures and/or are

intermediate compounds. Cr02 has a melting point of 4000C and is an intermediate

product in the decomposition of CrO3 to Cr203. CrO3 is a volatile oxide with a low

melting point of 197C and Cr304 is a metastable compound.

Manganese oxides and vanadium oxides. MnO2 is the most important of the

manganese oxides, although it is not the most stable for it decomposes to Mn203 at

temperatures above 5300C. This oxide has various modifications being 3-Mn02

(pyrulosite) the only stoichiometric form which has a rutile crystal structure. Mn203

(bixbyite) is the only oxide of the transition metal with M+3 ions that does not have a

corundum structure It's ca- form is known as the mineral hausmannite. Mn304 is a mixed

oxide of Mn+2 and Mn+3 ions of the form Mn+2Mn+3204 and forms when any manganese

oxide is heated above 10000C in air. MnO (mangonosite) is a basic oxide that forms

when any oxide of manganese is reduced in hydrogen. It is subjected to stoichiometric

variation and is a classic example of an antiferromagnetic compound. V205

(shcherbinaite) is the most important of the vanadium oxides. It has a low melting point

temperature of 6740C, thus can liquefy at high temperatures, adversely affecting the

oxidation resistance of the alloy, and can even lead to catastrophic failure.









Table 2-1. Principal binary oxides of iron, chromium, manganese, and vanadium. [14, 15]
Composition, Oxidation C
SCrystal Structure
wt% Oxygen State


Oxides of Iron (Fe)
FeO
a-Fe203
y-Fe203 (Maghemite)
Fe304
Oxides of Chromium (Cr)
Cr02
Cr03
Cr203
Cr304
Oxides of Manganese (Mn)
MnO
P-MnO2
Mn203
a-Mn304
Oxides of Vanadium (V)
VO
V02
V203
V205


23.1-25.6
30.1
30.1
27.6-28.4


38.1
48
31.6
29.1

20-25
36.81
30.4
28

24-27
38.6
32-33
43.9


+2
+3
+3
+2/+3

+4
+6
+3
+2/+3

+2
+4
+3
+2/+3

+2
+4
+3
+5


Cubic
Corundum
Cubic
Spinel

Rutile
Orthorhombic
Corundum
Spinel

Cubic
Rutile
C-type rare-earth M203
Spinel

Cubic
Rutile
Corundum
Orthorhombic


Table 2-2. Example of ternary and complex oxides of stainless steels. [14, 15]
Crystal Structure


(Fe, Cr)203
(Mn, Fe)203
FeCr204
FeV204
MnCr204
(Mn, Cr)304
Mn(Fe, Mn)204
(Mn, Fe)(Cr, V)204
(Mn, Fe)(V, Cr)204


Corundum
Corundum
Spinel
Spinel
Spinel
Spinel
Spinel
Spinel
Spinel


General Oxidation Fe-Cr Alloys

When a clean surface of a binary Fe-Cr alloy is exposed to oxygen, both iron and

chromium atoms oxidize, thus the initial oxide film is mainly wustite with possibly a thin

outer layer of magnetite and some chromia. The oxide film thickens by solid-state

diffusion and the main competing processes occurring at the oxide/substrate interface are









* Iron from the bulk diffuses outward through the oxide and reacts with the oxygen at
the oxide surface, forming iron oxides, mainly FeO.

* Chromium from the bulk diffuses to the oxide/substrate interface reducing FeO to
iron and oxidizes to Cr203. The reduction of FeO occurs for it is less
thermodynamically stable than Cr203.

* Oxygen from the reduced FeO diffuses inwards oxidizing the chromium within the
bulk, thus Cr203 particles form at and below the oxide/substrate interface. This is
termed internal oxidation.

The diffusion cross-section of the alloy is partially blocked by the internal

oxidation particles and a continuous protective chromia may or may not develop. If the

chromium content is sufficiently high, chromium from the bulk diffuses towards the

interface and a continuous Cr203 oxide film forms. Oxidation kinetics is then controlled

by diffusion through the growing chromia, thus considered to obey parabolic kinetics.

The permanence of such a layer depends on several factors, being the more important:

* The concentration and diffusion of the chromium within the bulk.
* The diffusion of oxygen within the bulk.
* The chromia growth rate.

If the chromium content is low, a continuous layer does not develop, and a steady

state is reached in which iron oxides and chromia continue to form. Simultaneously, a

solid-state reaction between the FeO and Cr203 occur yielding FeCr204. Because iron

oxides grow at a slower rate than Cr203, the resulting oxide film structure consists of

several layers (Figure 2-7) which are

* An outer layer formed by sublayers of iron oxides in the order starting with the one
next to the substrate and moving outwards: FeO, Fe304, and Fe203.

* An internal layer consisting of FeO and FeCr203.

* An internal oxidation layer consisting of Cr2O3 particles within the bulk.

Thus, the main diffusion process involved in the overall oxidation of Fe-Cr alloys is

considered the outward diffusion of iron ions through the inner and outer layers of the









oxide film. Within the inner oxide layer, voids, cavities, and pores develop as iron ions

diffuse outward. This aids the inner oxide layer to grow into the bulk due to the inward

dissociative diffusion of oxygen across these defects. The iron within the internal

oxidation region reacts with the oxygen forming FeO, which in turns reacts with the

chromia particles to form FeCr204. The result of this overall reaction process is that the

transition boundary between the oxide's inner and outer layers is delimited by the

substrate's original surface. The amount of FeCr204 within the inner oxide layer

increases with increase in chromium content until part of the diffusion cross-section is

blocked, resulting in a decrease in oxidation rate.


Original Substrate Surface Voids, Pores, Cavities


Oxide Film4 Outer Layer of Iron Oxides

SInner Layer of FeO and FeCr203
: *;* :.*.*:.** :*.*..*: *: *.**. -Internal Oxidation Layer
Substrate
Cr203 Particles

Figure 2-7. Schematic illustration of the oxide film structure for Fe-Cr alloys of low
chromium content. The outer layer consists of iron oxides in the following
sequence starting closest to the metal substrate and moving outwards: FeO,
Fe304, and Fe203.

The mode in which the internal oxidation particles nucleate is temperature

dependent. [3] At high temperatures, volume diffusion predominates resulting in a more

homogenous nucleation of the oxide particles. Grain boundary diffusion predominates at

low temperatures, thus the oxide particles will preferentially nucleate along grain

boundaries. This can be detrimental to the mechanical properties of the alloy. On the

other hand, it can be beneficial if the internal oxides are connected to the oxide film.









They can "anchor" the oxide film to the substrate, thus improving adherence and

avoiding spallation.

Literature Review on the Oxidation of Stainless Steel in Air

The oxidation behavior of stainless steel is considered to obey parabolic kinetics,

but this may be an oversimplification. [16] Douglass and Rizzo-Assuncao studied the

isothermal oxidation of a Fe-19.6Cr-14.Mn alloy in air for 24 hours in the temperature

range from 7000C to 10000C and observed that the general oxidation curves were

parabolic, but true parabolic behavior was not observed due to the simultaneous

formation of several oxides as the protective oxide was forming. [17] Hoelzer et al.

observed that the oxidation of ODS Fe-Cr stainless steel resembled parabolic kinetics

when investigating the oxide films formed in lab air for 10,000 hours at 7000C, 8000C,

and 9000C. [18] During the investigation on the oxidation of stabilized ferritic Fe-Cr

stainless steels with different chromium content in water vapor, Henry et al. observed

that parabolic kinetics were followed when the samples were oxidized in flowing

Ar+15% 02 at 9000C up to 400 hours. [19] Brylewski et al. investigated the oxidation

kinetics of DIN 50049 in air from 750C to 9000C from 70 to 480 hours, and observed

parabolic oxidation over the studied temperature range. [20] Other researches have

observed non-parabolic kinetics or even change in behavior during an oxidation scan.

Saeki et al. studied the oxidation in of 430 stainless steel with different manganese

content in 0.165 atm 02-N2 at 10000C from 0 to 1800 seconds, and observed that the

oxidation rates did not follow parabolic kinetics. [21] This was also observed by Vosson

et al. when studying the limits of oxidation resistance in dry lab air of different types of

stainless steels at 6500C for 30 to 3000 hours. [22] When investigating the composition,

structure, and thickness of the oxide films formed on MANET I when exposed to various









oxidizing condition for different exposure times, Iordanova et al. observed that a simple

kinetics law was not followed during the entire oxidation process for samples oxidized in

air at 6000C up to 2 hours and at RT up to 1450 hours. [23] Doilnitsyna stated that the

oxidation kinetics might vary qualitative and quantitative due the various oxidation

conditions. [24] Possible causes for the deviation from parabolic kinetics according to

some of the researchers are

* Different type of oxides simultaneously forming with the protective chromia, thus
altering the properties and structure of the oxide film. [16, 17, 21, 23]

* Chromium content at the oxide/substrate interface and/or variation in the cation
diffusivity during the oxidation process. [22, 23]

* Local oxidation gradient is different from the general oxidation gradient across the
oxide film because of transfer in different directions due to grain curvature and
internal grain oxidation. [24]

Factors Affecting Oxidation Behavior

The oxidation behavior and resulting oxide films of stainless steels is influences by

several factors. Some of these factors are composition (i.e., chromium content and

alloying additions), surface treatment, alloy microstructure, and oxidizing environment

(i.e., presence of water vapor).

Effect of alloy composition

It has been demonstrated that oxidation rates and oxide thickness are chromium

content dependent. [17, 25-27] For example, high-Cr alloys exhibit lower oxidation rates and

thinner oxide films than low-Cr alloys [17, 25, 26] Although the general guideline is a

minimum chromium content of 10 wt% to 12.5 wt% for the development of a protective

oxide (i.e. chromia) in dry air, the minimum value varies with stainless steel family,

temperature, and alloying elements. [8, 17, 19, 22, 28]









Stainless steel type. For austenitic alloys to form a continuous chromia oxide the

chromium content should be within the 16 wt% to 20 wt% range. [28] In the case of

martensitic alloys, the chromium content is generally between 9 wt% and 12 wt%. If the

martensitic alloy contains more then 12 wt% Cr, it is difficult to austenitize the steel and

develop a fully martensitic structure during the heat treatment processes. [29]

Temperature. The critical chromium content at which the transition from

protective to non-protective oxidation occurs is temperature dependent. [8, 17, 27] Douglass

and Rizzo-Assuncao observed that the minimum chromium content for the development

of a protective chromia oxide increased with increasing temperature from 12 wt% Cr at

8000C to 21 wt% at 10000C. [17] According to Colson and Larpin, high temperature

oxidation resistance in the 9000C to 12000C temperature range depends on the chromium

content. [8] If the content falls below the minimum value of 10 wt%/, the protective oxide

may be destroyed by the formation of non-protective oxides, such as Fe203 or

(Fe, Cr)203, on top of Cr203 film.

Alloying elements. Martensitic alloys and ferritic alloys with less than 12 wt% of

chromium are considered marginal chromia former, requiring addition of selectively

oxidizing elements to aid in the development of protective oxides. [20, 29, 30] These

elements are generally silicon and manganese, which also are considered to enhance

oxide adhesion. [20, 29] Silicon is considered to enhance oxidation resistance
[10, 18, 19, 22, 31] and delay breakaway oxidation [32] by forming a silica (Si02) film, which

acts as a diffusion barrier for iron and chromium ions. Silica may form as a continuous

or discontinuous layer, or as oxide precipitates at the oxide/substrate or spinel/corundum

interface. [10, 18, 19, 32] Manganese is a common addition to ferritic and martensitic alloys









in substitution of nickel. [33] Manganese monoxide (MnO) has a more negative AGO than

Cr203, thus considered to influence the oxidation kinetics. [21, 23] It is still debatable

whether manganese addition is beneficial or detrimental to the oxidation process.

Manganese is beneficial for it reduces chromium vaporization slowing the oxidation

process, but it is detrimental for it forms a spinel with chromia, leading to higher

oxidation rates than pure Fe-Cr alloys. [20, 21] Other alloying additions are the reactive

elements (i.e. cerium, yttrium, hafnium, and thorium) which improve high-temperature

oxidation resistance by allowing inward diffusion of oxygen to predominate and

hindering the outward diffusion of cations. [8,34, 35] When alloying element are added,

complex spinel oxides may develop unless the chromium content increases such that only

a Cr203 film develops. [17] These complex spinels may serve as a protective oxide film,

but its growth may differ from that of pure chromia resulting in a complex non-ideal

oxidation behavior. [17,21-23]

Effect of surface treatment

Oxidation is influenced by the surface treatment. Different procedures induce

variation in the morphology and structure of the initial oxide, which defines the oxidation

resistance. [34-36] Mechanical polishing, grinding, and milling results in preferential

distribution of the alloying elements within the oxide film [36], and enhances chromium

diffusion due to the introduction of fast diffusion paths such as dislocations and subgrain

boundaries. [25, 36-38] Cold working also promotes faster chromia formation due to the

numerous nucleation sites introduced during cold working. [25, 36]

Ostwald and Grabke observed differences in the formed oxides and bulk

diffusivities during their study on the effects of surface treatment on the oxidation of 9-20









wt% Cr stainless steels in different oxidizing environment at 6000C from 1 to 100 hours.

[37] Samples were electropolished (EP), polished (P), grinded (G), and sandblasted with

fine (SBF) or coarse (SBC) particles. The study demonstrated that for all environments,

the oxide formation increased the with degree of surface deformation in the order

EP < P < G< SBF < SBC with the thickest film forming on the most severely deformed

surface (i.e., SBC finish). [37] Bulk diffusivity was also observed to follow the same trend

as the oxide formation. Depletion of selectively oxidized elements beneath the

oxide/substrate interface decreased with increased surface deformation, with sharpest

depletion in the EP samples and shallowest depletion in the SBF/SBC samples. [37]

Guillamet et al. studied the effects of surface preparation on the oxidation of AISI

304 and AISI 316 in air from 9000C to 11000C, and observed differences in the structure

and the composition of the oxide films formed on the mechanically polished surfaces and

the etched surfaces. [39, 40] The oxide film of the mechanically polished samples mainly

consisted of Cr203 and MnCr204 with small amount of a-Fe203 for both type of stainless

steels. For the etched samples, both alloys developed an oxide film consisting ofFe304

and a-Fe203 at 9000C, but at higher temperatures, the oxide film of the AISI 316

consisted of Cr203 and MnCr204 and the oxide film of the AISI 304 consisted of

a-Fe203. Kuiry et al. also observed this when investigating the oxidation of

mechanically polished and electropolished AISI 316 under isothermal and non-isothermal

conditions. [36] Mechanically polished sample exhibited a Cr-rich, uniform, fine-grained

oxide film. The oxide film of the electropolished sample was a Fe-rich, non-uniform,

coarse-grained oxide film with a nodular appearance and an internal oxidation zone of

Cr/Ni-rich particles. Graham et al. observed that Fe-26Cr alloys samples that were









electropolished had a different oxide structure than those that were electropolished and

vacuum annealed, although the film thickness for both conditions were similar. [34, 35]

The former developed an amorphous oxide and the oxide of the latter consisted of large

cubic grains.

Effect of alloy microstructure

Alloys with a fine grain microstructure exhibit better oxidation resistance than

those with a coarse grain microstructure. [25, 38] This is attributed to the higher number of

grain boundaries in the fine-grained structures that act as short-circuit diffusion paths,

thus allowing faster chromium diffusion from the bulk to the oxide film. Grain size

effect was also observed to influence the inner layer thickness of the oxide films formed

in alloys with less than 20 wt% Cr. [25] The inner oxide layer of fine-grained alloys

consisted of an uniform layer with uniform healing along the grain boundaries at the

oxide/substrate interface. Two types of structures were observed for the coarse-grained

alloys: a uniform layer with no healing, or a non-uniform layer due to the local healing

along the grain boundaries at the oxide/substrate interface, which inhibited the growth of

the inner oxide layer at these sites.

Diffusivity of chromium is higher for ferritic and martensitic stainless steels than

for corresponding austenitic alloys due to the higher grain boundary density in the first

two types of stainless steels. [8, 30, 37, 38] During the oxidation study of X20 (11CrlMoV)

at 6000C in various environment from 1 hour to 672 hours, Segerdahl et al. attributed the

better performance of the alloy to its complex microstructure when compared to

austenitic alloys. [30] X20 is a tempered martensitic steel with martensitic and/or ferritic

microstructure, in which lathe-like martensitic grains of micron size form within the

former austenitic grains, resulting in a high density of low-angle grain boundaries that act









as fast diffusion paths. Peraldi and Pint studied the influence of chromium and nickel on

the oxidation behavior of austenitic and ferritic alloys, and observed differences in the

oxidation behavior. [38] Ferritic alloys and austenitic alloys with high nickel content did

not show oxide spallation, although it did occur for low-Ni austenitic alloys. It was also

noticed that ferritic alloys performed better than austenitic alloys at 8000C, which was

attributed to a higher chromium diffusivity within the bulk. [38]

Effect of water vapor

Most stainless steel form and maintain a chromia layer in dry air, but the presence

of water vapor can have a detrimental effect, inducing rapid and catastrophic oxidation

such as breakaway oxidation. [19, 30, 32, 33, 38, 41-43] During this phenomenon, slow growing

chromia is formed during the initial protective stage, after which the oxidation suddenly

increases due to the formation of thick Fe-rich oxides. [19, 32] It is still not well understood

the sudden formation of the Fe-rich oxides and there are various theories

* Development of cracks within the passive oxide. [19, 32]
* Vaporization of Cr203 as volatile chromium species. [19, 30, 32]
* Chromium depletion in substrate region beneath the oxide/substrate interface. [19, 32]
* Change in the oxide's ionic conductivity due to hydrogen dissolution in the
oxide. [30]

The onset of breakaway oxidation varies with alloy composition and water vapor

partial pressure (pH20). [19, 30, 32] The onset is delayed by increasing the content of

chromium or silicon content, or by decreasing the pH2o. During the protective stage, the

parabolic rate constant in the presence of water vapor is higher than in dry air. [19, 30]

Henry et al. attributed this observation to an increasing in the concentration of hydroxide

species in the oxide film due the presence of the water vapor, which has a higher

diffusivity because of it smaller ionic radius. [19]









General Oxidation of Stainless Steels

Stainless steel alloys develop a native passive oxide that is typically 1-2 nm thick.

[43-45] The composition mainly consists of chromium oxides and oxyhydroxides [44] and it

has a double layer structure in which the outer Fe-rich layer is thicker than the inner

Cr-rich layer [45]

Upon oxidation, stainless steels follow the general oxidation behavior of binary

Fe-Cr alloys. The composition and structure of the resulting oxide film depends on the

oxidizing environment and the composition alloy. It is known that stainless steels form

protective films of mixed oxides rather than pure chromia, although pure chromia films

have formed under the proper conditions. [8, 10, 17, 19, 22, 23, 28, 30, 33, 38, 46] These alternate

protective oxides have been identified as

* Chromia with impurities of iron, manganese, or nickel. Example: (Fe, Cr)203.

* Mixed oxides of the main alloying elements, particularly the spinel type. Example:
FeCr204, MnCr204.

The oxide film is generally a double-layer structure with an inner Cr-rich layer at

the oxide/substrate interface, and a Fe-rich layer at the gas/oxide interface. Example of

some of the oxides identified within the inner and outer layers are [10, 17-19, 23, 25, 27, 29, 36-38,

44, 47- 49]

* Inner Layer: Cr203, FeO, Fe203, Mn203, SiO2, (Fe, Cr, Ni)i+xO, (Mn, Fe)203,
NiCr204, NiFe204, (Fe, Cr, Mn)304, (Fe, Cr, Ni)304,

* Outer Layer: FeO, Fe203, Fe304, y-Mn203, (Mn, Fe)203, (Mn,Cr)203, MnCr204,
(Fe, Cr)304, (Mn, Fe)Cr204

In some studies, a third middle layer was detected, which was identified as Fe203 [17]

MnFe204 [23], or (Fe, Cr, Ni)l-xO [38]. Other studies observed a continuous or

discontinuous thin silicon oxide layer at the oxide/substrate interface consisting of









amorphous SiO2 [10, 19, 22, 44] or a Si-rich oxide [18, 28, 42] film. Internal oxidation was not

observed in some of the oxidation studies. [36] When internal oxidation was observed, it

consisted of Cr203 and/or Si02 precipitates within the substrate [17 19 25 28], or a chromia

film along the martensitic lathe or grain boundaries [25, 29]

HT-9 Ferritic/Martensitic Stainless Steel

Fe-Cr stainless steels are currently used as structural material for gas turbine

components [26], and for fossil fuel and nuclear power plants components [13, 26, 41, 50-53]

This alloy is being considered for fusion power systems [26, 41, 54], advanced accelerator

systems [55], and solid oxide fuel cell (SOFC) application [20]. For nuclear applications,

ferritic and martensitic stainless steels are preferred over austenitic stainless steel due to

their better mechanical and thermal properties. [13, 26, 50-52, 54]

* High resistance to radiation induced void swelling, creep, and He embrittlement.

* High thermal conductivity and low coefficient of expansion reduce the
development of thermal stress and fatigue.

* Excellent dimensional stability at high displacement with less than 0.5 % swelling.

HT-9 (DIN X20CrMoWV12 1 or Fe-12Cr-MoVW) is high-chromium

heat-resistant alloy that may have a martensitic and/or ferritic microstructure depending

on the austenitizing and tempering treatments. Sandvik (Sweden) first developed the

stainless steel alloy for high temperature application that did not require the inherent

corrosion resistance of austenitic stainless steel. [41] HT-9 evolved for the alloying of

AISI 410 with molybdenum (Mo), vanadium (V), and tungsten (W) to obtain an alloy

with improved creep rupture strength as presented in Figure 2.8. [13, 26] These alloying

elements are known to increase the mechanical strength by solid solution and









precipitation strengthening. [13] The typical composition of HT-9 is presented in Table

2-3 along with the composition from different steel manufacturers. [11, 13, 26, 41, 50, 52]

105 hours Creep-Rupture Strength at 6000C


Add Mo Add Mo & V Add W

Fe-12?CF L Fe-12Cr-IMoV Fe-12Cr-IMoWV
AISI 410 Fe-12Cr-0.5Mo HT-91 HT-9
(DIN X20CrMoV12) (DIN X20CrMoWV12)

35 MPa 60 MPa

Figure 2-8.Development of a 12% Cr ferritic/martensitic stainless steel. Figure based on
Viswanathan and Bakker [13], and Klueh and Harries [26].



Table 2-3. Composition of HT-9 steel in weight percent according to different references.
Carpenter
Typical [26 41] Vallourec Carpenter
poitin [11] Sandvik [26, 41] [13] Technology
Composition Mannesman Cp t [50, 52]
Corporation
Cr 11.5 11.5-12 12 11.8
Ni 0.5 0.5-0.6 0.5 0.5-0.6
Mn 0.6 0.6 0.6 0.5
Si 0.4 0.4 0.4 0.2-0.3
C 0.2 0.2 0.2 0.2
Mo 1 1 1 1
P 0.030 max 0.001-0.012
S 0.020 max 0.002-0.003
Cu 0.04
W 0.5 0.5 0.5 0.4-0.5
V 0.3 0.3 0.25 0.33
Al 0-0.035
Fe Balance Balance Balance Balance

The typical microstructure of HT-9 consists of a tempered martensite with

predominantly M23C6 precipitates and possibly, small amount of 6-ferrite. [50-52] M23C6 is

a Cr-rich carbide that in addition, may contain iron, tungsten, molybdenum, vanadium,

and nickel. This carbide precipitates along high-energy regions such as prior-austenite

grain boundaries and martensitic lathe boundaries. [52, 56a, 57a] Based on the constitutional










diagram for Fe-Cr alloys [29] and the Schaeffler diagram[58al shown in Figure 2-9, the

microstructure will generally consists of a martensitic phase and a ferritic phase. The

amount of each phase will depend on the austenitizing and tempering treatments

Constitutional Diagram for Fe-Cr Alloys
1600

1400-

o 1200-
1 Austenite
S1000-

2 800-
a) Ferrite
H 600-

400 Ms, martensite start temperature
A
200 I A
0 5 10 12 15

Cr content or Cr requirement, wto


Schaeffler Diagram

Nickel Equivalent = %Ni + 30 x %C + 0,5 x %Mn

28







\ ,- ---,---"-.- ---z_-
4
uslele I B
20 ,











Predicted microstructure forerr
0 4 8 / 2 16 20 24 28 32 36 40
/1 Chromium Equivalent = %Cr + %Mo + 1.5 x %Si + 0.5 x %Nb
Predicted microstructure for
a Fe-12Cr stainless steel alloy.

Figure 2-9.Constitutional diagram for Fe-Cr alloys (A) and Shaeffler diagram [58a] (B).
Figure 2-9A based on Ennis and Quaddakkers [29].









General Applications of HT-9 Stainless Steel

HT-9 is a well-established alloy widely used in Europe and South Africa for the

fabrication of boiler and heat exchanger tubes, pipes, and header for steam temperatures

of up to 5400C in fossil fuel power generation industries. [13, 55] It has an extensive stress-

rupture database exceeding 100,000 hours of operating within the 5000C to 6000C

temperature range with operating experience of over 20 years in Germany, Belgium,

Holland, Scandinavia and South Africa. [13] Regardless, HT-9 has found little use in the

US, England, and Japan due to fabrication difficulties, especially during welding and

post-welding heat treatments. [13, 55] The difficulties arise due to the high carbon content

and low martensitic transformation temperature (TM) of the alloy that could promote

austenite phase retention after welding, high residual stresses, and cracking before and

during the stress relief treatment. [13,59] Even though the difficulties have been overcome

by careful control of the heat treatment processes, it still has not found much application

in the US. [13]

HT-9 is considered as a substitute for austenitic alloys in the fabrication of in-core

and out-of-core components for nuclear power applications, particularly for liquid-metal

cooled fast-breeder reactors (LMR). [26, 41, 50-54] HT-9 cladding and ducts are currently

used in the Fast Flux Test Facility (FFTF), a LMR system located at Richmond,

WA. [26, 41] It is a candidate alloy for first wall and tritium breeding blanket components

in fusion reactor applications. [26, 41, 54], and for pipes and molten metal vessels in the

accelerator-driven system (ADS) applications. [55]

ADS Applications of HT-9 Stainless Steel

The need to address the issues related to the management of nuclear waste has

prompted the search for alternate options of handling nuclear waste. One option is the









transformation of long-lived radiotoxic material into products that are short-lived or non-

radioactive by means of a fission reaction, a process known as transmutation. [60]

Transmutation is the concept behind the development of advanced accelerator systems

(ADS), which in the United States operates under the Department of Energy's (DOE)

Advanced Fuel Cycle Initiative (AFCI) program, an outgrowth of the DOE's Advanced

Accelerator Applications (AAA) program. [61] In ADS, a target material is bombarded by

high-energy protons, generating high-energy neutrons called spallation neutrons. When

the generated spallation neutrons are absorbed by the nuclear waste, fission reactions are

induced, thus transmuting the nuclear waste. Some of the benefits of ADS are the

minimization of nuclear waste, the reduction of plutonium by-product in nuclear power

plant, and the energy generated during the transmutation process can be used to generate

electrical energy. [60, 61] Unfortunately, the transmutation process imposes stringent

material requirements. The materials selected for ADS application should be able to

withstand high neutron fluxes, elevated temperature, and corrosion.

Lead and lead alloys, such as lead-bismuth eutectic (LBE), have been identified as

candidate materials for ADS application of target material (generating spallation

neutrons) and coolant (for removing heat during transmutation). However, molten lead

and molten LBE is corrosive to HT-9, one of the stainless steel candidates considered for

piping and for molten metal containers. [55] Liquid metal corrosion can manifest itself as

liquid penetration along grain boundaries, stainless steel dissolution, and formation of

undesirable compounds at the LBE/steel interfaces that can reduce the life expectancy of

the steel components and lead to catastrophic failure. [62-64]









The Russians have used LBE as a coolant in their nuclear submarines for over

several decades. Their experience has shown that the steel is protected from LBE

corrosion by an oxide layer and that controlling the oxygen concentration within the

molten metal is important for the development and maintenance of the protective oxide

film. [65] Barbier et al. has studied the behavior of austenitic and martensitic steels in

flowing LBE under controlled oxygen concentration (levels within the 10-6 wt%) with

exposure times of up to 3116 hours. [62, 66] Their results indicate that the development of

an oxide layer on the steel surface can protect the metal from dissolution, an effect of

liquid metal corrosion. The behavior of passivated (pre-oxidized) austenitic steel and

martensitic steel in static LBE under isothermal conditions have been investigated by

Soler Crespo et al. and Lillard et al. [67, *] In both studies, the oxide layer developed prior

to LBE exposure protected the steel from liquid metal corrosion. This has given way to

the hypothesis that the oxides films of passivated stainless steel may protect the alloy

from LBE corrosion, thus prompting interest in understanding the oxidation kinetics and

mechanisms of HT-9 stainless steel, and in elucidating the structure and composition of

its oxide films.


Lillard, R.S., C. Valot, M.A. Hill, P.D. Dickerson, and R.J. Hanrahan, Personal Communication. 2003.














CHAPTER 3
EXPERIMENTAL PROCEDURE

This chapter discusses the thermogravimetric analysis (TGA) methodology used for

the isothermal and non-isothermal oxidation scans, including the equipment used during

the scan and experimental parameters. Sample preparation and oxide characterization

techniques are also presented.

Sample Preparation

HT-9 (martensitic/ferritic, Fe-12Cr-1MoVW or DIN X20CrMoWV12 1) stainless

steel alloy used for this research was provided by the University of Michigan (UM). It is

part of an ingot previously provided to UM by Oak Ridge National Laboratory and later

heat treated at Argonne National Laboratory-West. The provided steel alloy piece was

cut into 20 mm x 4 mm x 1.5 mm flat bars by Microcut, Inc. (York, PA) using electrical

discharge machining (Wire-EDM).

Samples for the various oxidation scans were obtained by cutting the flat bars in

half on a low speed diamond wheel saw (South Bay Technology, Model 650) using a

cubic boron nitride (CBN) wafer blade (South Bay Technology, 4 inch x 0.012 inch

coarse/low blade). The cut samples were wet polished from 320 grit to 1200 grit on all

the exposed surfaces using silicon carbide (SiC) grinding paper (LECO 8 inch wet/dry C

weight grinding paper). The two main surfaces were further polished to a 1 |jm finish

using an alumina (A1203) powder suspension (LECO alpha alumina powder) on a

synthetic rayon polishing cloth (Buehler MicroCloth or equivalent). After polishing, the

samples were ultrasonically cleaned for 5 minutes in methanol followed by 5 minutes in









acetone after which they were thoroughly dried and stored in a dessicator cabinet until

needed. Prior to an oxidation scan, a sample would be retrieved from the dessicator

cabinet and its length, width, and thickness measured with a caliper (Scienceware). The

initial weight is obtained using a precision balance (Sartorius Research Semimicro

Balance, model R 160 P).

Wavelength Dispersive Spectroscopy (WDS) for compositional analysis was

performed on a polished sample using an EPMA JEOL Superprobe 733 located at the

Major Analytical Instrumentation Center (MAIC) at the University of Florida (UF).

Measurements were taken on seven different locations on the sample's surface. The

detected elements and the average weight percent (wt%) for each element are presented

in Table 3-1. The wt% of each element is based on the average of the seven

measurements. The results obtained by WDS are in good agreement with the

composition provided by UM and the typical composition found in the literature [11]

Table 3-1. Composition of HT-9 steel in weight percent (wt%).
Standard Deviation
Composition Typical Average of WDS of WD
Provided by UM Composition [11] Measurements
Measurements
Cr 11.63 11.5 11.43 1.22
Ni 0.5 0.5 0.73 0.24
Mn 0.52 0.6 0.65 0.03
Si 0.22 0.4 0.10 0.02
C 0.2 0.2 0.11 0.27
Mo 1.0 1 0.95 0.29
P 0.02
S 0.006
Cu 0.04
W 0.52 0.5 0.30 0.28
V 0.3 0.3 0.27 0.42
Co 0.08 0.16 0.03
Fe Balance Balance Balance 1.79









The HT-9 sample used for WDS analysis was further polished to a 0.25 rtm finish

using an alumina (Al203) powder suspension (LECO alpha alumina powder) on a

synthetic rayon polishing cloth (Buehler MicroCloth or equivalent). After polishing, the

sample was ultrasonically cleaned for 5 minutes first in methanol followed by acetone,

and then thoroughly dried. The surface was etched using Marbles Reagent to reveal the

microstructure of the as-received material. The sample was mounted on an aluminum

(Al) scanning electron microscope (SEM) stub using copper tape and the etched surface

characterized with a JEOL Field Emission Gun (FEG) SEM (Model JSM-6335F) located

at MAIC-UF. The microstructure consists of a mixture of ferritic and martensitic phases

with carbide particles along the boundaries between the two phases, as shown in Figure

3-1. A line scan across a particle reveals an enrichment of carbon and chromium, and

depletion of iron, thus the particles seem to be Cr-rich carbides. (Figure 3-2.)






















Figure 3-1. Secondary electron (SE) image of the microstructure of the as-received HT-9
stainless steel alloy after etching the surface with Marble's Reagent.























SE, 1.1B366E*.l 7 Cra 54


SA..53 K.. 64

_VKa, 31 M.,K 214



MniK. 49 Fia. 287
!^s '


Figure 3-2. SE image and line scan of a particle located along the boundary between the
martensite phase and the ferrite phase. The line scan profile indicates that the
particle is enriched in carbon (C) and chromium (Cr), and depleted in iron
(Fe).

Oxidation Scans

Equipment

The equipment used for the experimental scans can be grouped into three main

categories:

1. Thermogravimetric analyzer (TGA) system and supporting accessories.
2. Data acquisition system.
3. Gas delivery system and supporting equipment.

The schematic of the equipment set-up is shown in Figure 3-3 and images of the


actual equipment are shown in Figure 3-4.



































33
3Ct


S '*


H


a)

C



c >

















o

C)
o


a)

0 -
C)





SE
5F4


C

S


~C)



0F


CA





C)

-A













C)j

C








-e










-e
-e











C4t
























C)
en























en
C4











Double-Tube
Flow Meter


Gas Switching
Accessory


















Data Acquisition
System


TGA 2050


SArgon and Air
Gas Cylinders






Furnace Heat
1 Exchanger


SThermal Gas
Purifier


B

Figure 3-4.Photograph of the actual equipment used during the oxidation scans. The
flow meter in A is a double-tube configuration. The scaled tube on the left
side of the flow meter regulates the furnace purge gas flow rate and the one on
the right side regulates the balance purge gas flow rate.

TA Instruments TGA 2050. The TGA consists of two main components, a

desktop cabinet and a heat exchanger. The cabinet houses the balance assembly, the

furnace assembly, the Platinel II thermocouple, the purge gas inlet for the furnace and for

the balance, and the system's electronic and mechanical parts. The TGA is controlled by

a computer to which it is connected via a GPIB cable. The balance assembly consists of










a balance arm with two hang-down wires, one for the sample pan and one for the tare

(counterbalance weight) pan, and a balance arm sensor unit. (Figure 3-5.)


Balance ..
Electronics *: -rll 1 *i. ':::: ::i l
CBalance Arm
Cover
Balance Assembly
Hang-Down
Balance A ly Hang-Down --Balance Purge
W ire \II ll W Gas Inlet

I 1 Tare Pan
I Thermocouple
I .. .. Sample Pan





Balance Purge Rubber Gasket
Gas In
Quartz Tube
Water Cooled
TGA 2050 Jacket Electric Resistance
Heater
Furnace Assembly Off-Gases
Outlet
Furnace Core Furnace Purge

Quartz Liner Gas Inlet
Furnace Base

Rubber Cap

Figure 3-5. Cross-section view of the TGA 2050 balance assembly and furnace assembly.

The sensor unit consists of a transducer which is coupled to the balance arm, an infrared

LED light source, a pair of photodiodes, and a printed circuit board assembly. As the

balance arm moves due to weight changes, the amount of light that hits each diode is

unequal. This causes a changes in the amount of current the transducer requires to keep

the balance arm in the horizontal reference (null) position, thus the sample's weight

change is directly proportional to the variations in the transducer current flow. The

furnace assembly consists of a water-cooled jacket that houses the quartz sample tube, the









quartz liner, the furnace core (refractory ceramics), and the resistance heating elements.

(Figure 3-5.) The quartz liner protects the heating elements and the furnace core from

corrosive gases that could be present within the sample tube. The furnace assembly

moves vertically for sample loading and unloading. Although the furnace has a vertical

configuration, the purge gas flow is horizontal. The purge gas enters through the right

side gas inlet, flowing directly across the sample placed on an open sample pan, and out

the left side gas outlet. A spectrometer can be hooked up to the gas outlet to analyze the

gases evolved during the oxidation process. The heat exchanger dissipates the heat

generated by the furnace and consists of a fan, a radiator, a water reservoir, a pump, and

temperature and flow sensors.

Data acquisition system. The data acquisition system consists of a Windows 2000

based computer running two TA Instruments software: Thermal Advantage and

Universal Analysis. Thermal Advantage is the controller software through which the

user inputs the experimental parameters that controls the operation of the TGA, and

receives and saves in a file the raw data. Universal Analysis is the data analysis software

that reads that raw data file, allowing the user to plot, manipulate and save the data in

other file formats. Both programs are proprietary of TA Instruments.

Gas delivery system. The gas delivery system consists of a TA Instruments Gas

Switching Accessory (GSA), a Restek Corporation single-tube thermal gas purifier, a

double-tube flow meter, and two gas cylinders with their regulators. The GSA is a

solenoid-controlled gas manifold that allows for manual or automatic furnace purge gas

switching during an experimental run. It is connected to the TGA via an interconnecting

cable, which controls the switching when the GSA is in automatic mode. The thermal









gas purifier consists of a small furnace with a zirconium-granule filled stainless steel

converter tube. The purifier removes oxygen, water, carbon monoxide, carbon dioxide,

and hydrocarbons (except methane) to parts per billion (ppb) levels in the gas stream.

The left-side tube of the double-tube flow meter controls the furnace purge gas flow rate

and is connected to the GSA purge gas inlet. The right-side tube controls the balance

purge gas flow rate and is connected to the TGA balance purge inlet. The gases used are

from Praxair, which are Ultra High Purity Argon (Ar) identified as Gas 1 and Grade Zero

Air identified as Gas 2. The Ar gas constantly flows through the thermal gas purifier

prior to flowing into the furnace and balance to minimize the presence of oxygen in the

gas stream.

Non-Isothermal Scan Procedure

Prior to loading a sample into the TGA, the non-isothermal parameters are input

into the controller software which will ramp the furnace from room temperature (RT) to

950C at 2C/minute or at 5C/minute, and then cooled down to RT. The data sampling

rate is set to 6 data points/minute (i.e. sampling rate of 10 seconds/point). An example of

a non-isothermal program is shown in Table 3-2 and an example of the temperature

profile for this type of scan is shown in Figure 3-6. The furnace purge gas is switched to

dry air prior to loading the sample for Ar is the default purge gas that constantly flows

through the furnace and balance when the TGA is not in use. The furnace purge gas is

dry air during the non-isothermal scan (i.e. ramp-up to selected temperature) and Ar

during the cool down. During the experimental scan, the furnace purge gas flow rate is

kept at a constant 90 cm3/minute and the balance purge gas flow rate at 10 cm3/minute.







46


Table 3-2. Example of a non-isothermal program.
Program Segment Command Meaning/Description
1. Data Storage: On Starts data recording.
2. Ramp 2.00C/minute to 950.000C The furnace is ramped from RT to 9500C at
2C/minute.
3. Select Gas: 1 The gas is switched from dry air (Gas 2) to Ar
(Gas 1) prior to the cool down segment.
4. Equilibrate at 30.000C This is the cool down segment. The controller
automatically sets the cooling rate.

Sample NH1 Fie' C: i usersIorayalUonlsoHT IN _H1 001
Size: 25.2190 mg TGA Operator: S Beritez
Method: Nonisothermal Run D.Wtr 27.Ma,-0 17:42
Comment: Nonsio: 2CImin up to 950C Inrtrumern 2050 TGA V5.4A

/


Non-Isothermal Scan /-
(Ramp-up Segment)





/ '- Cool Down
/ I Segment
Q I



Time (mil) UnNtsaV3 1ETAslnr~bnts

Figure 3-6.Example of a non-isothermal temperature profile. This plot was generated by
Universal Analysis using the raw data file for a sample oxidized in dry air
from RT to 9000C at 20C/minute.

To initiate a non-isothermal scan, a sample is placed on the raised edge of an

alumina pan and loaded into the TGA. The sample is held for an hour within the closed

furnace in which dry air is flowing prior to initiating the scan, which allows for the

removal of lab air that could have been introduced into the furnace when loading the

sample. After an hour, the program is started, in which the furnace is ramped to the

selected final temperature at the selected ramp rate, and then cooled down to RT. The

sample is then unloaded, its final weight recorded, and the surfaces visually inspected.









Isothermal Scan Procedure

Prior to loading a sample into the TGA, the isothermal scan is programmed into the

controller software and the data sampling rate is set to 6 data points/minute (i.e. sampling

rate of 10 seconds/point). An example of an isothermal program is shown in Table 3-3

and its corresponding temperature profile is shown in Figures 3-7 and 3-8. The various

combinations of isothermal temperature and holding time used during this research are

shown in Table 3-4.

Table 3-3. Example of an isothermal program.
Program Segment Command Meaning/Description
1. Data Storage: On Starts data recording.
2. Equilibrate at 950.000C This is the ramp-up segment. The controller
automatically sets the ramping rate.
3. Mark end of cycle 0 An indicator is inserted in the data file to
identify the end of the equilibrate segment.
4. Isothermal for 5.00 minutes Holding time to allow temperature to stabilize.
5. Select Gas: 2 The gas is switched from Ar (Gas 1) to dry air
(Gas 2) prior to the isothermal scan.
6. Isothermal for 5.00 minutes Holding time to allow gas flow to stabilize.
7. Mark end of cycle 0 A marker is inserted in the date file to identify
the end of the holding time.
8. Isothermal for 360.00 minutes Isothermal holding time of 6 hours.
9. Select Gas: 1 The gas is switched from dry air (Gas 2) to Ar
(Gas 1) prior to the cool down segment.
10. Equilibrate at 30.000C This is the cool down segment. The controller
automatically sets the cooling rate.

Table 3-4. Isothermal scan temperatures and holding time combinations.
Temperature Holding Time
6000C, 7000C, and 8000C 48 hours
8250C, 8500C, and 8750C 24 hours
8630C 24 hours and 48 hours
9000C 30 minutes, 90 minutes, and 24 hours
9500C 6 hours








48



Curve tjerluv lerr eraule y I rrie


Urlvrsilw3LI L [A l AliurriuLrlL


25
InJrirr.IlV3 1E TA IisLrurrnLiIts


Figure 3-7.Example of an isothermal scan temperature profile. The plots were generated
by Universal Analysis using the raw data files for the 7000C/48hrs,
8500C/24hrs, and 9500C/6hrs samples. B is a closer view of the ramp-up
segment of the scans.


To start an isothermal scan, a sample is placed on the raised edge of an alumina pan


and loaded into the TGA. Once the sample is loaded, it is held for three hours within the


Time (min)


lflflfl


8s 24 o001
8- 526 001











Ramp-Up
Segment


Temperature Gas Flow
Stabilization Stabilization
Segment Segment


Isothermal
Hold Segment


Time (min)









To start an isothermal scan, a sample is placed on the raised edge of an alumina pan

and loaded into the TGA. Once the sample is loaded, it is held for three hours within the

closed furnace with a constant Ar flow. This waiting period allows the removal of lab air

that could have been introduced into the furnace when loading the sample. After the

waiting period, the program is initiated and the furnace is ramped to the selected

temperature by at a ramping rate selected by the controller, held for a period of time at

the selected temperature, and then allowed to cool down to RT before unloading the

sample. During the isothermal scan, the furnace purge gas is automatically switched

between dry air and Ar according to the programmed instructions. The purge gas is dry

air during the isothermal segment, and Ar during the ramp-up and cool down segments.

The flow rate is kept at 90 cm3/minute at all times, regardless of the type of gas. The

balance purge gas is Ar flowing at 10 cm3/minute. At the end of the scan, the sample is

unloaded, its final weight recorded, and all the surfaces visually inspected.

Weight Change Calculations

The weight change (AW) is calculated using Equation 3-1 and the raw data

associated to the non-isothermal or isothermal segment. The calculated data is then

plotted as AW versus temperature for non-isothermal scans and as AW versus time for the

isothermal scans.

W -W
AW = W Equation 3-1
A

* AWis the weight change, mg.

* W, is the weight at time i, mg.

* Wo is the weight at the beginning of the scan (i.e., for the isothermal scans it is the
time dry air is introduced into the furnace, and for the non-isothermal scan it is the
start of the ramp-up), mg.









* A is the total surface area of the samples, cm2

Oxide Characterization

The morphology and composition of the oxides developed during the isothermal

scans were characterized by scanning electron microscopy (SEM) equipped with an

energy dispersive spectroscopy (EDS) system. The phases and compounds present in the

oxides were elucidated by X-ray diffraction (XRD).

Surface Characterization

Secondary electron (SE) imaging and elemental composition of the surface of the

oxidized HT-9 samples were characterized with a JEOL Field Emission Gun (FEG) SEM

(Model JSM-6335F) equipped with an Oxford Link ISIS system for EDS analysis

(MAIC-UF). Samples were mounted on aluminum SEM sample holders with the aid of

conductive copper tape. Oxide from the backside of the sample was removed prior to

mounting to ensure good contact between the sample and the sample holder.

Cross-Section Characterization

After analyzing the oxidized surface, portion of the samples were cut off with

South Bay Technology low speed diamond wheel saw (Model 650) equipped with a

4 inches x 0.012 inches CBN wafer blade. The pieces were hot compression mounted

using a thermosetting mounting compound (Buehler Epomet F mounting powered).

Small flat bars of stainless steel were also mounted with the HT-9 samples to provide

edge retention protection during polishing. After the mount had cooled down to room

temperature, it was metallurgically polished from 320 grit to 1200 grit (LECO 8 inch

wet/dry C weight SiC grinding paper) followed by a 1.0 |tm to 0.25 |tm finish (LECO

alpha alumina powder suspension on a Buehler MicroCloth or equivalent synthetic rayon

polishing cloth). After polishing, the mount was ultrasonically cleaned to remove









polishing residue and then thoroughly dried. The ultrasonic cleaning was first done in a

soapy distilled water bath followed by a distilled water bath with a distilled water rinse

between baths. The mount was carbon coated at MAIC to provide sample conductivity

during SEM observation. The thickness, composition, and structural layers for the

developed oxide films were characterized with the JEOL JSM-6335F SEM coupled with

an Oxford Link ISIS system at MAIC-UF.

Oxide Compound Characterization

A non-oxidized sample (i.e. reference sample) and samples oxidized from 8250C to

950C were mounted on glass slides with the aid of double sided adhesive tape. A glass

slide was inserted into the chamber of a Phillips APD 3720 diffractometer (PW 1710

Based system) connected to a Windows based computer running Phillips PC-APD

controller software located at MAIC-UF. The diffractometer was set to perform a

continuous scan from 15020 to 70020 in 0.04020 increments. After the scan was

completed, a peak search was done using the PC-APD software and the results were run

through Phillips PC-Identify software for peak matching. PC-Identify compared the

provided data to its database of known compounds and generated a list of potential

candidates. Once the list was generated, each candidate was evaluated for best fit to the

obtained results.

After characterizing the samples in bulk form, the oxide of the samples oxidized

from 8500C to 9000C was scrapped off from the surface and mounted on glass slides.

(Figure 3-10.) The oxide fragment that spalled during handling of the 950C sample was

also mounted on a glass slide as shown in Figure 3-10. The mounted oxide powders and









oxide fragment were characterized following the same procedure as for the samples in

bulk form.

The 6000C to 8000C samples were characterized on a Phillips X'Pert MRD System

operated by MAIC staff. The diffractometer was set to perform a continuous scan from

20020 to 80020 in 0.02020 increments at a 30 glancing angle. After the scans were

completed, the data was evaluated following the same procedure used to evaluate the data

obtained with the APD 3720 diffractometer













Figure 3-10. Oxide scrapped from the surface mounted for XRD characterization. The
fragment for the 9500C/6hrs sample is an oxide fragment that spalled during
handling.

Platinum Marker Experiment

A thin platinum (Pt) line is painted on the surface of a non-oxidized HT-9 sample

prepared as previously mentioned using Pt ink from Heraeus Circuit Division. The

sample is placed in a desiccator cabinet for 24 hours to allow the ink to thoroughly dry.

Figure 3-11 shows the HT-9 sample with the dried Pt ink marker. After 24 hours, the

sample is loaded into the TGA and oxidized in dry air at 9000C for 24 hours following

the procedure for an isothermal oxidation scan. After the scan is completed, the sample

is unloaded and the surfaces visually inspected. To find the location of the Pt marker, a

cross-sectional sample is prepared by cutting a portion of the oxidized sample and









metallurgically polishing the cross-section from 320 grit to 1200 grit (LECO 8 inch

wet/dry C weight SiC grinding paper) followed by 1.0 |tm to 0.25 |tm finish (LECO

alpha alumina powder suspension on a Buehler MicroCloth or equivalent synthetic rayon

polishing cloth). After polishing, the cross-section sample was ultrasonically cleaned to

remove polishing residue and then thoroughly dried. The ultrasonic cleaning was first

done in soapy distilled water followed by a distilled water rinse, then in acetone. The

sample was vertically mounted on an aluminum SEM sample holder to which a hole had

been previously drilled for the purpose of holding the sample in a vertical position. To

avoid charging during the SEM characterization, conductive copper tape was used to

ground the sample to the sample holder. The tape was attached to the backside of the

sample, from which the oxide had been removed prior to mounting. The location of the Pt

marker was determined from the SE images, elemental mapping, and line scans obtained

during the characterization process using the JEOL JSM-6335F SEM coupled with an

Oxford Link ISIS unit (MAIC-UF).

HT-9
Sample

SPt inlmk







Figure 3-11. HT-9 sample with the Pt marker after drying for 24 hours in a dessicator
cabinet and prior to the isothermal scan.















CHAPTER 4
RESULTS

The results from the experimental oxidation scans and the various characterization

techniques are presented in this chapter. The discussion of the results is presented in

Chapter 5.

Non-Isothermal Scan Results

Obtaining kinetics information from non-isothermal scans is a controversial issue.

One school of thought argues that it is possible, while others do not agree. The general

consensus is that non-isothermal curves provide on overview of the oxidation kinetics

and mechanisms by indicating where changes in oxidation behavior occur. It is for this

reason that non-isothermal scans were performed.

HT-9 samples were prepared and subjected to non-isothermal scans according to

the experimental procedure detailed in Chapter 3. The ramping rate for samples 1 and la

was 20C/minute, and 5C/minute for samples 2 and 2a. The weight change (AW) of each

sample was calculated using Equation 3-1 and the raw data generated during their

respective scans as detailed in Chapter 3. The resulting AW versus temperature curves

are shown in Figure 4-1. The overallAW and surface area is presented in Table 4-1.

Table 4-1. Surface area and weight change for HT-9 samples oxidized under
non-isothermal conditions. Average surface area is 0.99 cm2
Sample Surface Area, cm2 Weight Change, mg/cm2
1 0.94 0.730
la 1.04 0.575
2 0.92 0.220
2a 1.05 0.163










Non-Isothermal Scans of HT-9 in Dry Air from RT to 9500C

0.8
Ramping Rates
Dashed Lines: 20C/minute
0.7 Solid Lines: 5C/minute i

0.6
h la
0.5

0.4- -
Change in predominant oxidation
0.3 kinetics/mechanisms.

0.2
., 2a
0.1 -

0.0 '
0 100 200 300 400 500 600 700 800 900 1000
Temperature, C

Figure 4-1. Calculated weight change versus temperature plots for HT-9 samples oxidized
under non-isothermal conditions. Samples were oxidized dry air and heated to
950C at a ramping rate of 20C/minute (1 and la) or 5C/minute (2 and 2a).

The non-isothermal plots show a change in profile between 8000C and 9000C,

which is associated to changes in the oxidation behavior. The significance of this

observation is that two different behaviors could be expected during the isothermal

oxidation scans from 6000C to 9600C. Thus, the predominant oxidation kinetics and the

oxide film of he samples oxidized above the 8000C to 9000C transitional range could be

different than the samples oxidized below this transitional range.

Isothermal Scan Results

Isothermal scans are useful in elucidating the oxidation kinetics and mechanism.

Reaction rate constants and activation energies can be derived from an analysis of the

resulting plots, aiding in defining the oxidation behavior.









HT-9 samples were prepared and subjected to isothermal scans according to the

experimental procedure detailed in Chapter 3. The AW of each sample was calculated

using Equation 3-1 and the raw data generated during their respective scans as discussed

in Chapter 3. The calculated AW versus time curve for each sample is shown in Figure

4-2. The overall weight gain and surface area is presented in Table 4-2.

After evaluating the calculated isothermal plots, two additional samples were

oxidized at 9000C for 30 minutes and 90 minutes following the procedure for isothermal

oxidation scans. The purpose of the two additional scans at shorter holding times is to

aid in determining the initial oxidation behavior of the samples oxidized at temperatures

above 863C. The AW versus time plots are shown in Figure 4-3, and the overall weight

gain and surface area are included in Table 4-2.

Table 4-2. Surface area and weight change for HT-9 samples oxidized under isothermal
conditions. Average surface area is 1.00 cm2.
Sample Surface Area, cm2 Weight Change, mg/cm2
6000C/48hrs 1.00 0.027
6000C/48hrs (a) 1.08 0.013
7000C/48hrs 1.02 0.045
8000C/24hrs 1.07 0.096
8000C/48hrs 1.00 0.147
8250C/24hrs 0.98 0.183
8500C/24hrs 0.95 1.230
8500C/24hrs (a) 0.97 1.975
8500C/24hrs (b) 0.98 1.612
8630C/24hrs 0.98 12.720
8630C/48hrs 0.99 23.481
8750C/24hrs 0.98 17.714
9000C/30mins 0.99 0.196
9000C/90mins 1.07 0.580
9000C/24hrs 0.97 17.706
9500C/6hrs 0.96 8.919







57


Isothermal Scans of HT-9 in Dry Air


24 T


9500C/6 hrs
/ /


8750C 8630C
24 hrs 24 hrs

9000C/24 hrs /J-

/ /


863C/48 hrs


-, y


0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000
Time, minutes


I I 8500C/24 hrs (a)
S850'C/24 hrs (b)
.11 /



I l //850C/24hrs
I I / /



SI /


Ill /

I ]/ 8250C/24hrs 8000C/24hrs


I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I


Zoom














800C/48 hrs
80600C/48 hrs
...... ,600C/48 hrs (a)


0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000
Time, minutes

Figure 4-2. Calculated weight change versus time plots for HT-9 samples oxidized under
isothermal conditions. Samples were oxidized in dry air at different
temperatures for different holding times.


'/ /


J


----------------










Isothermal Scans of HT-9 in Dry Air at 9000C

20.0 -

10.0


24 hours
0.6

S0.5 90 minutes

0.4

0.3

0.2 30 minutes

0.1

0.0

0 10 20 30 40 50 60 70 80 90 100 500 1000 1500
Time, minutes

Figure 4-3.Isothermal scan plots of HT-9 samples oxidized at 9000C in dry air for 30
minutes, 90 minutes, and 24 hours.

Two methods were used to determinate which kinetics rate equation best fits the

isothermal data, as previously discussed in Chapter 2. In one method, identifying the

possible oxidation behavior is based on the results of a linear regression performed on a

double logarithm plot of the isothermal data. This method is only valid for identifying

linear, parabolic, and cubic behavior. The second method consists in validating the

experimental data to each kinetics rate equation. Thus, this second method can identify

linear, parabolic, cubic, logarithmic, and inverse logarithmic behaviors.

For the first method, the logarithm of the AW data was calculated and plotted as a

function of the calculated logarithm of time as shown in Figure 4-4. Straight lines are

drawn on the calculated double logarithm plot, dividing the plot into segments. The

intersections of the drawn lines define the approximate moment during the isothermal










scan in which a change in the predominant oxidation behavior occurred. Linear

regressions are applied to each segment to obtain the line equation, and the resulting line

equation is then compared to Equation 2-8 to find the value of m. An example of the

application of this method is shown in Figure 4-5 for a sample oxidized at 8500C for 24

hours.

Isothermal Scans of HT-9 in Dry Air
Log(Weight Change) vs. Log(Time)

1.5
9000C/24 hrs ,
10, 863C/48 hrs
1.(0 950C/6 hrs /

/ --8750C/24 hrs
0.5 / /
S / 8630C 4s
/ / / 850C/24 hrs
S0.0 -/ / 24hrs
-0.5
-o.5 / / / /
/ / 8250C/24 hrs
/ 8000C/48 hrs
-1.0 / 8000C/24 hrs

700C/48 hrs
-1.5 -6000C/48 hrs

-2.0 . . . 16000C/48 hrs (a)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Log(Time, minutes)

Figure 4-4. Calculated double logarithm plots of the isothermal data.

Identification of the kinetics rate equation is based on the parameter m, which is the

inverse of the slope obtained during the linear regressions. This parameter has values of

1, 2, and 3, corresponding to linear, parabolic, and cubic kinetics, respectively. If the

calculated m values are vastly different than the values of 1, 2 and 3, the kinetics for the

oxidized sample in question cannot be determined by this method. Calculated m values

for the isothermally oxidized HT-9 samples are presented in Table 4-3.










Isothermal Scans of HT-9 in Dry Air at 8500C for 24 hours (a)
Log(Weight Change) vs. Log(Time)

0.50

0.25
Segment 2
0.00 y = 2.06x + -6.26
R2 = 0.9935
0 -0.25

" -0.50

-0.75
~ Segment 1
S-1.00- y0.41x+-1.79
R2 = 0.9957 I
-1.25- 462 minutes

-1.50

-1.75
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Log(Time, minutes)

Figure 4-5.Example of identifying the oxidation kinetics based on the results of the linear
regressions applied to each segment of a double logarithmic plot.

For the second method, the AW data is plotted according to the rate equation being

validated. For example, to validate the parabolic rate equation the data is plotted as the

AW squared (AW2) versus time. The plot is divided into the segments previously defined

in the double logarithmic method and linear regressions are applied to each segment. The

resulting coefficients of determination (R2 values) are evaluated to find which kinetics

rate equation best fits the experimental data. As an additional step, AW data is calculated

based on the possible kinetics rate equations and the resulting plots compared with the

experimental data. Thus, the possible oxidation kinetics is based on the R2 value and

how well the calculated kinetics rate equation's plot follows the experimental data.

Figure 4-6 illustrates an example of validating a kinetics rate equation (parabolic rate









equation) for a sample oxidized at 8500C for 24 hours. Figures 4-7 and 4-8 present the

possible oxidation behaviors according to the two methods.

Table 4-3. Calculated m values based on linear regressions of the segments of double
logarithmic plots.
Segment
Sample Segment
1 2 3
6000C/48hrs 5.23 -11.10
6000C/48hrs (a) 7.03 201.81 -2.61
7000C/48hrs 4.28 75.91
800C/24hrs 2.73=3 7.80
8000C/48hrs 2.62=3 3.90
8250C/24hrs 2.23=2 3.65=
8500C/24hrs 2.57=3 0.70=1
8500C/24hrs (a) 2.44=2 0.48
8500C/24hrs (b) 2.29=2 0.53=1
8630C/24hrs 1.92=2 0.50=1
8630C/48hrs 1.92=2 0.66=1
8750C/24hrs 1.64=2 0.71=1
9000C/30mins 1.25=1
9000C/90mins 1.37=1 0.36
9000C/24hrs 1.16=1 0.99=1
9500C/6hrs 0.78=1 1.17=1

The reaction rate constants kL, kp, kc, kLog, and kinv are obtained from the slope of

the linear regression performed during the kinetics rate equation validation. The obtained

rate constants for each model are presented in Table 4-4. Activation energies (Ea) for the

linear and parabolic models are calculated by plotting the natural logarithm of the rate

constants (Ln(k)) versus the inverse of the temperature (1/T). Linear regressions are

applied and the slopes are multiplied by the Universal Gas Constant (R=8.315 J/mol-K)

to obtain the activation energy. Figure 4-9 presents how the activation energies were

obtained from the linear regressions of the Ln(k) versus 1/T plot. The data for the

samples oxidized at 6000C for 48 hours were not included in the linear regressions.
















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SEM Surface Characterization Results

The morphologies and compositions of the surface oxides were elucidated with a

JOEL FEG-SEM (model JSM-6335F) equipped with an Oxford Link ISIS System

(MAIC-UF). The SEM was operated in secondary electron (SE) image mode with

smallest aperture setting for maximum field-of-depth due to the tortuous surface

topography. Presentation of the results is grouped into three temperature ranges based on

the observed morphologies: 6000C to 8250C, 8500C to 8630C, and 8750C to 9500C.

Energy dispersive spectroscopy (EDS) was performed at the center of the images or at the

center of the structure of interest.

6000C to 8250C Temperature Range

The samples oxidized at 6000C and 7000C have similar surface structure consisting

of small platelets of approximately 100 nm to 200 nm. The platelets of the 6000C sample

are rounded, having a fish-scale appearance while the platelets of the 7000C are

polygonal shaped. The composition of the oxide film for both samples is mainly oxygen,

iron, chromium, and manganese, with small amounts of silicon, sulfur, and vanadium.

The 7000C also presents small oxide particles about one |tm in size. The composition of

these particles consists of vanadium, manganese, iron, chromium, and oxygen with a

trace amount of silicon. The surface morphology of the oxide film for the 8000C and

8250C samples consists of Cr-rich oxide crystals less than one |tm in size with dispersed

clusters of large smooth oxide grains. These grains are rich in vanadium and manganese

with some iron and chromium. SE images and the corresponding EDS spectra for the

samples oxidized in this temperature range are shown in Figure 4-10 to 4-13.

















10000-
loooo-
F

C i L s F
0 2 4 6


Figure 4-10. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 6000C for 48 hours. A is a closer view of the surface morphology of the
oxide film.


AI
100nm
\^^^^^H


ol


Figure 4-11. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 7000C for 48 hours. A and B are higher magnification images of surface
areas that appear to have different morphologies when viewed at a low
magnification. C is a higher magnification view of one of the dispersed small
particles.













iLA:


Figure 4-12. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 8000C for 48 hours. A is a cluster of smooth oxide grains and B is a
higher magnification image of the surface of the oxide film consisting of small
oxide crystals.


Aiv l


Figure 4-13. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 8250C for 24 hours. A is a higher magnification image of the surface of
the oxide film, consisting of small oxide crystals as observed in the 800C
sample. B is a higher magnification view of a cluster of smooth oxide grains
and C is a closer view of one of the grains in the grain cluster shown in B.


I i:


U


-~--


II;;









8500C to 8630C Temperature Range

The samples oxidized at 8500C and 8630C have a more complex surface

morphology consisting of a combination of features observed in the samples oxidized

below 8500C and the samples oxidized above 863C. The surface of the 8500C sample is

covered with Cr-rich oxide crystals with dispersed clusters of large smooth oxide grains

and Fe-rich oxide nodules as presented in Figure 4-14. The composition and structure of

the grain clusters are almost identical to those observed in the 8000C and 8250C samples.

C


A



?r1pm


1011m


Figure 4-14. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 8500C for 24 hours. A and B are higher magnifications views of the
oxide film's surface and of an oxide grain cluster, respectively. C and E are
closer view of the Fe-rich oxide structures. D is a closer view of the interior
of the spalled Fe-rich oxide nodule shown in C.


]O


ii
....,,.,









The surface of the 8630C sample is covered with a Fe-rich oxide except for a

narrow perimeter band of Cr-rich oxide crystals separating the oxide developed at the

center of the sample from the oxide developed at the edges of the sample. (Figure 4-15.)

The morphology, size, and composition of the Cr-rich oxide crystals is similar to those

observed on the 8000C to 8500C samples with the exception that a small amount of

copper was detected during EDS analysis.

Oxide at the Center of the Sample
B






100nm

Oxide at the Edge of the Sample
A





10pm


Figure 4-15. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 8630C for 24 hours. B and C is the Fe-rich oxide at the edge and center
of the sample, respectively. D is a higher magnification view of the Cr-rich
oxide crystals within the narrow perimeter band.

The Fe-rich oxide observed at both 8500C and 8630C have several distinguishable

features that could be different growth stages of the oxide. EDS analysis of these features

indicate a composition of oxygen and iron with trace amounts of chromium and/or

manganese. SE images and corresponding EDS spectra of the Fe-rich oxide structures

developed on the 8500C and 8630C sample are presented in Figures 4-16 and 4-17

respectively.






















Gfl

x0^l


Figure 4-16. SE images and corresponding EDS spectra of the various features of the
Fe-rich oxide structures observed on the sample oxidized in dry air at 8500C
for 24 hours. The image in the middle corresponds to image E of Figure 4-14.


"i ,


ip
i;
~nil~uO~iYI


I ....


'"
''


~I
















B1gm
j^^^^H um \


A


l1m
*o


Figure 4-17. SE images and corresponding EDS spectra of the various features of the
Fe-rich oxide surface of the sample oxidized in dry air at 8630C for 24 hours.
The image in the middle corresponds to image B of Figure 4-15.


IljlYL~----------~NV\ II -II









8750C to 9500C Temperature Range

Samples oxidized at and above 8750C are entirely covered in iron oxide. The

950C sample was the only one to have visible cracks as shown in Figure 4-18. The

surface morphology of the 8750C to 9500C samples is almost identical to the surface of

the Fe-rich oxide observed on the 8630C sample. No Cr-rich oxide crystals were

observed on the surface of the samples oxidized within this temperature range.



HT-0 Sample
05(i'C 6hrs












Cracks


Figure 4-18. Image of the cracks on the surface of the Fe-rich oxide film developed on
the sample oxidized in dry air at 9500C for 6 hours.

During SEM sample preparation of the 8750C and 9000C samples, part of the oxide

spalled from one of the covers revealing three distinct oxide layers as seen in Figures

4-19 and 4-20 respectively. The outermost layer (1) is a Fe-rich surface oxide that

exhibits various features which could be different stages of oxide growth. (Figures 4-21

and 4-22 for the 8750C and 9000C samples respectively.) The composition of this layer

mainly consists of iron and oxygen with a trace amount of manganese, and only one

feature exhibited small amounts of copper. The structure of the innermost layer (3) is









similar for both samples, consisting of small equiaxed grains with what appear to be prior

grain boundary structures. (Image D in Figure 4-19 and image E in Figure 4-20.) The

composition of this layer is oxygen, chromium, and iron, with small amounts of silicon,

sulfur, manganese, and vanadium.


Legend
1: Outermost Layer
2: Middle Layer
3: Innermost Layer


V
Prior Grain
Boundary Structures


- I


Figure 4-19. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 8750C for 24 hours. A is the outermost layer 1 and D is the inner most
layer 3. B (in-plane view) and C (cross-section view) are closer views of the
three distinctive layers observed after the oxide spalled during sample
preparation. E and F are higher magnification images of the structures of the
innermost layer 3.


""
'""""*'"

















A





1Ogm






10om

B






Legend
















Figure 4-20. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 900C for 24 hours. A is the outermost layer 1, and B (cross-section
view) is a closer view of the three distinctive layers observed after the oxide
spalled during sample preparation. C, D, and E are higher magnification
images and corresponding EDS spectra of each layer.













E






1g


H

0- e


Figure 4-21. SE images and respective EDS spectra of the various features of the
outermost oxide layer of the sample oxidized in dry air at 8750C for 24 hours
The image in the middle corresponds to image A of Figure 4-19.



ii~-


1 I I]



























G


o~III ..",


A.


Figure 4-22. SE images and respective EDS spectra of the various features of the
outermost oxide layer of the sample oxidized in dry air at 9000C for 24 hours.
The image in the middle corresponds to image A of Figure 4-20.


Il


B!


I-

t oA


A



A-o--H^H /
IBIHB^-4

'^L,^ ,L


:I .









The oxide of the 950C sample spalled along one of the cracks formed during the

oxidation process (Figure 4-18) during sample preparation, revealing two different

structure layers as seen in Figure 4-23. The composition and morphology of the bottom

layer is similar to the innermost layer (3) of the 8750C and 9000C samples. (Figure 4-23.)

The surface of the top layer of the oxide film exhibits the least number of surface features

when compared to the surface of the Fe-rich oxide of the 8630C to 9000C samples.

(Figure 4-24.) The composition of the surface oxide is oxygen and iron with a trace

amount of manganese according to the EDS spectra.

C B D


Crack
"I~a41


I
Crack Boundary


Figure 4-23. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 9500C for 6 hours. A is the top layer and E is the bottom layer. B is a
cross-section view of the spalled oxide, revealing oxide layers with different
structures. C and D are higher magnification images of the top and bottom
layers respectively.


!.4






80

B


C




I


A D
10gm

10om 1gM




ei A

Figure 4-24. SE images and respective EDS spectra of the various features of the
outermost oxide layer of the sample oxidized in dry air at 9500C for 6 hours.
The image in the middle corresponds to image A of Figure 4-23.

The samples oxidized at 9000C for 30 and 90 minutes have similar surface

morphologies consisting of small oxide grains with dispersed Fe-rich oxide structures as

shown in Figures 4-25 and 4-26 respectively. The composition of the small oxide grains

mainly consists of chromium, iron, and oxygen, with some manganese and a trace

amount of vanadium. Silicon was only detected in the 30 minutes sample. The

composition of the Fe-rich oxide mainly consists of iron and oxygen with some

chromium and manganese, and a trace amount of vanadium. The 90 minutes sample also

exhibits large Fe-rich oxide structures, whose surface exhibits various features similar to

those observed on Fe-rich oxide of the 8500C to 9500C samples. (Figure 4-27). These

features are mainly composed of iron and oxygen, with some chromium and manganese.


. r
....,,.,

































Figure 4-25. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 9000C for 30 minutes. A is a higher magnification of the surface of the
oxide film consisting of small oxide crystals. B is closer view of one of the
Fe-rich oxide nodules.


Figure 4-26. SE images and corresponding EDS spectra for the sample oxidized in dry
air at 9000C for 90 minutes. A, D, and E are closer views of the various Fe-
rich oxide structures. C is a higher magnification view of the surface of the
oxide consisting of small oxide crystals.


rl
Mn
ii
Enrlgllbr\n


'""""


~cli~:I,









B C


1gm 1gm








A D












Figure 4-27. SE images and respective EDS spectra of the various features on the
surface of the Fe-rich oxide structures of the sample oxidized in dry air at
9000C for 90 minutes. The image in the middle corresponds to image B of
Figure 4-26.

SEM Cross-Section Characterization Results

The thickness, composition, and layer structure of the oxide films were elucidated

with a JOEL FEG-SEM (model JSM-6335F) equipped with an Oxford Link ISIS System

(MAIC-UF). The SEM was operated in SE image mode. The elements considered for

the elemental mapping and line scan are based on the recurring elements observed during

the EDS analysis. Carbon was not included in the list of elements. It was not possible to

clearly define the oxide thickness of the samples oxidized from 6000C to 8250C using the

FEG-SEM, thus a Strata Dual Beam-Focused Ion Beam (FIB) (model DB 235) operated

by MAIC staff was used to obtain cross-section images of these samples. To prepare the









samples for the FIB, portion of the samples were cut off, cleaned and dried. The pieces

were mounted onto a FIB stub using carbon tape and then carbon coated at MAIC. Once

the samples were placed inside the FIB, a thin strip of Pt is deposited on the surface of

the sample to protect the surface from damage during the ion milling process. Trenches

were ion milled at the edge of the Pt line, the samples tilted, and the thickness measured

using the FIB's software which has a correction feature for sample tilting. The cross-

section images are divided into temperature ranges based on the observed morphologies:

6000C to 8250C, 8500C to 8630C, and 8750C to 9500C.

6000C to 8250C Temperature Range

The oxide film of the 6000C sample could not be resolved, although a very thin

oxide layer did develop as indicated by the surface's bluish cast as shown in Figure 4-28,

which is associated to the low temperature oxidation process known as tarnishing. The

oxide thickness of the 7000C sample is less than half a micron, while the thickness for the

8000C and 8250C samples is approximately one micron. The cross-section images for

the sample within this temperature range are shown in Figure 4-29.















Figure 4-28. HT-9 sample on alumina pan after isothermal oxidation in dry air at 6000C
for 48 hours. The sample has a bluish cast which is associated with the
process of tarnishing.































on
C) ~
S
~ ~

Ct r~


C)
C)
-am


a a a
so u .u 4 0

OU -O$
I i I I









8500C to 8630C Temperature Range

The 8500C sample has a thin compact oxide film approximately 1.5 rtm thick as

presented in Figure 4-30. The composition of the film is oxygen, chromium, and

manganese based on the results of the elemental mapping and line scan. The oxide of the

8630C sample is more complex for it appears to be two types of oxides, a thin oxide film

and a thick oxide film which are identified as spot 1 and spot 2 respectively in the low

magnification view of the cross-section. (Figure 4-31.) The thin oxide (spot 1) is almost

identical in thickness, layer structure, and composition to the oxide observed in the 850C

sample. (Figure 4-32.) The thick oxide (spot 2) appears to be compact with cavities and

voids along the center of the oxide film and at the oxide/substrate interface as observed in

Figure 4-33. The outer region of the oxide also presents some cavities and voids with

cracks along the width of the sample. No cavities or cracks are seen in the inner region

of the oxide. The approximate oxide thickness is 130 |tm and the composition varies

across the thickness. According to the elemental mapping and line scan results, the oxide

film is divided into two compositional regions and the boundary appears to be along the

center of the oxide film, where the cavities and voids are located. The outer region is

mostly iron and oxygen, while the inner region is chromium, iron, and oxygen. A closer

view at the oxide/substrate interface, identified as spot 3 in Figure 4-33, reveals the

presence of internal oxidation. Oxide is observed along the grain boundaries and

particles or precipitates are observed within the grain structure of the substrate adjacent to

the oxide/substrate interface. (Figure 4-34.) Based on the elemental mapping, oxygen is

detected within the grain boundaries, which are enriched in chromium and depleted in

iron.








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