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Cathodic Protection Modelling of Buried Structures

Permanent Link: http://ufdc.ufl.edu/UFE0044291/00001

Material Information

Title: Cathodic Protection Modelling of Buried Structures
Physical Description: 1 online resource (85 p.)
Language: english
Creator: Shankar, Alok
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: bottom -- cathodic -- protection -- tank
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Petroleum products are of vital importance to the preservation of industrial civilization. These products have been transported over long distances by buried steel pipelines. The pipes are generally in soil environments which contain oxygen and therefore can undergo corrosion. Over the past years, several accidents have been reported which were caused due to corrosion, resulting in loss of property and lives. These pipes are protected against corrosion using coating and cathodic protection systems. As the demand for petroleum has increased over the years, multiple pipelines sharing the right of way have been used to transport the products. These products are stored in large above ground storage tanks. As each structure is provided with independent cathodic protection system, there arises the possibility of interaction between the different CP systems installed. This can lead to regions on the structure to being over protected or under protected. The effect is greater when there exits coating defects in the structures. The objective of this work was to use CP3D complex simulation tool to model different systems to study the effect of cathodic protection system interaction in pipelines. The potential and current distributions along the length of the structure were calculated to understand the interference effect and performance of cathodic protection system. The simulation tool is used to extend the recent modeling studies of tank bottoms to account for the presence of different coatings on the protection current distribution.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Alok Shankar.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Orazem, Mark E.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044291:00001

Permanent Link: http://ufdc.ufl.edu/UFE0044291/00001

Material Information

Title: Cathodic Protection Modelling of Buried Structures
Physical Description: 1 online resource (85 p.)
Language: english
Creator: Shankar, Alok
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: bottom -- cathodic -- protection -- tank
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Petroleum products are of vital importance to the preservation of industrial civilization. These products have been transported over long distances by buried steel pipelines. The pipes are generally in soil environments which contain oxygen and therefore can undergo corrosion. Over the past years, several accidents have been reported which were caused due to corrosion, resulting in loss of property and lives. These pipes are protected against corrosion using coating and cathodic protection systems. As the demand for petroleum has increased over the years, multiple pipelines sharing the right of way have been used to transport the products. These products are stored in large above ground storage tanks. As each structure is provided with independent cathodic protection system, there arises the possibility of interaction between the different CP systems installed. This can lead to regions on the structure to being over protected or under protected. The effect is greater when there exits coating defects in the structures. The objective of this work was to use CP3D complex simulation tool to model different systems to study the effect of cathodic protection system interaction in pipelines. The potential and current distributions along the length of the structure were calculated to understand the interference effect and performance of cathodic protection system. The simulation tool is used to extend the recent modeling studies of tank bottoms to account for the presence of different coatings on the protection current distribution.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Alok Shankar.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Orazem, Mark E.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044291:00001


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1 CATHODIC PROTECTION MODELLING OF BURIED STRUCTURES By ALOK SHANKAR A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UN IVERSITY OF FLORIDA 2012

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2 2012 Alok Shankar

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3 To my parents and brother

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4 ACKNOWLEDGMENTS I would like to sincerely thank my research advisor Prof. Mark Orazem for his constant encouragement, support and guidance throughout my research wor k. His valuable inputs brought in immense value to this work. Secondly, I thank my colleag ues in our research group Liu Ch ao, Christopher Cleveland, and Darshit for their help and meaningful suggestions during the course of this research work.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 9 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 14 2 LITERATURE REVIEW ................................ ................................ .......................... 17 Corrosion ................................ ................................ ................................ ................ 17 Electrode Kinetics ................................ ................................ ................................ ... 17 Application to Corrosion in Soil ................................ ................................ ............... 21 Corrosion Prevention Methods ................................ ................................ ............... 21 Cathodic Protection ................................ ................................ ................................ 22 Criterion for Cathodic Protection ................................ ................................ ............. 22 Anode Polarization ................................ ................................ ................................ .. 23 Galvanic Anodes ................................ ................................ .............................. 23 Impressed Current Anodes ................................ ................................ ............... 24 Tank Bottoms ................................ ................................ ................................ .......... 25 3 CATHODIC PROTECTION 3 D MODELLING SOFTWARE ................................ ... 28 So il Domain ................................ ................................ ................................ ............ 28 Pipe or Inner Domain ................................ ................................ .............................. 30 Domain Coupling ................................ ................................ ................................ .... 31 Numerica l Development ................................ ................................ .......................... 31 4 RESULTS AND DISCUSSION ................................ ................................ ............... 33 Dimensional Analysis of Anode Parameters ................................ ........................... 33 Influence of Anode Distance from Pipe ................................ ............................ 34 Influence of Anode Depth ................................ ................................ ................. 34 Influence of Anode Length ................................ ................................ ................ 35 Influence of Anode Diameter ................................ ................................ ............ 35 Cathodic Interference in Pipelines ................................ ................................ .......... 36 Ba se Case: Single Pipe ................................ ................................ .................... 36 Two Pipes in Cross Over Configuration ................................ ........................... 36 One pipe unprotected ................................ ................................ ................ 36 Both pipes protected with independent CP ................................ ................ 37

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6 Both pipes with independent CP systems and holiday on one pipe ........... 38 Effect of Coating on CP in Tank Bottoms ................................ ................................ 38 Tank Bottom with Coating Flaws ................................ ................................ ............ 40 5 CONCLUSIONS ................................ ................................ ................................ ..... 70 APPENDIX: POTENTIAL AND CURRENT DISTRIBUTIONS ON PIPELINES ......... 72 LIST OF REFERENCES ................................ ................................ ............................... 84 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 85

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7 LIST OF TABLES Table page 2 1 Parameters for the oxygen and chlorine evolution reactions [13] ....................... 27 4 1 List of Pipe and Anode Parameters ................................ ................................ .... 42 4 2 Ohmic resistance as a function of distance of anode from pipe with L a = 1m, D a = 0.1m, H a = 1m and V = 1V ................................ ................................ .......... 42 4 3 Ohmic resistance as a function of length of anode with D a = 0.1m, H a = 1m, D = 20m and V = 1V ................................ ................................ ........................... 42 4 4 Ohmic resistance as a function of diameter of the anode with L a = 1m, H a = 1m, D = 20m and V = 1V ................................ ................................ ............ 43 4 5 Ohmic resistance as a function of depth of the anode with L a = 1m, D a = 1m, D = 20m and V = 1V ................................ ................................ ........................... 43 4 6 Coating and Steel property parameters for the CP3D simulations ..................... 44 4 7 Simulation parameters ................................ ................................ ........................ 44 4 8 Anode parameters used in CP3D simulations ................................ .................... 44 4 9 Model parameters and result of single pipe configuration ................................ .. 45 4 10 Model parameters and result of two pipes single CP configuration .................... 45 4 11 Model parameters and result of two pipes with independent CP configuration .. 46 4 12 Model parameters and result of two pipes having independent CP with holiday on one pipe configuration ................................ ................................ ....... 46 4 13 Model Parameters and simulation results for tank bo ttoms with different coating properties ................................ ................................ ............................... 47 4 14 Non dimensional Current density as a function of non dimensional radius on tank bottoms with different coating properties ................................ .................... 48 4 15 Off Potential distribution as a function of non dimensional radius on tank bottoms with different coating properties ................................ ............................ 49 4 16 Model Pa rameters and simulation results for tank bottoms with different coating properties and holidays ................................ ................................ .......... 50 4 17 Current and On Potential distribution as a function of Distance along the Radiu s on tank bottoms with different coating properties and holidays .............. 51

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8 A 1 Potential and Current Distribution on Pipe 1 Single pipe ................................ .... 72 A 2 Potential and Current Distributions on Pipe1 and Pipe2 ................................ ..... 74 A 3 Potential and Current Distributions Pipe1 and Pipe2 ................................ .......... 76 A 4 Potential and Current Distributions on Pipe1 ................................ ...................... 79 A 5 Potential and Current Distribution on Pipe2 ................................ ........................ 81

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9 LIST OF FIGURES Figure page 4 1 Non Dimensional Ohmic resistance as a function of D with L a D a H a fixed ....... 52 4 2 R/R Dwight as a function of Distance of anode with L a D a H a fixed ....................... 52 4 3 Non Dimensional Ohmic resistance as a function of D a with L a D a D fixed ....... 52 4 4 R /R Dwight as a function of H a with L a D a D fixed ................................ ................. 53 4 5 Non Dimensional Ohmic resistance as a function of L a with D a H a D fixed ....... 53 4 6 R/R Dwight as a function of L a with D a H a D fixed ................................ ................. 53 4 7 Non Dimensional Ohmic resistance as a function of D a with L a H a D fixed ....... 54 4 8 R/R Dwight as a function of D a with L a H a D fixed ................................ .................. 54 4 9 Model of Single p ipe ................................ ................................ ........................... 54 4 10 On Potential and Off Potential on Pipe #1 as a function of distance along the pipe for single pipe configuration ................................ ................................ ........ 55 4 11 Current density on Pipe #1 as a function o f distance along the pipe for single pipe configuration ................................ ................................ ............................... 55 4 12 Model of two Pipes with Single CP ................................ ................................ ..... 56 4 13 On Potential and Off Potential on Pipe #1 as a function of distance along the pipe for two Pipes with Single CP configuration ................................ ................. 56 4 14 Current Density on Pipe #1 as a function of distance along the pipe for two Pipes with Single CP configuration ................................ ................................ ..... 57 4 15 On Potential and Off Potential on Pipe #2 as a function of distance along the pipe for two Pipes with Single CP config uration ................................ ................. 57 4 16 On Potential and Off Potential on Pipe #2 as a function of distance along the pipe around the point of ano de connection for two Pipes with one unprotected pipe configuration ................................ ................................ ........... 58 4 17 Current Density on Pipe #2 as a function of di stance along the pipe for two Pipes with one unprotected pipe configuration ................................ ................... 58 4 18 Current Density on Pipe #2 as a functi on of distance along the pipe around the point of anode connection for two Pipes with one unprotected pipe configuration ................................ ................................ ................................ ....... 59

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10 4 19 Model of two pipes with independent Cp ................................ ............................ 59 4 20 On Potential and Off Potential on Pipe #1 as a function of distance along the pipe for two Pipes with independent CP configuration ................................ ........ 60 4 21 Current density on Pipe #1 as a function of distance along the pipe for two Pipes with independent CP configuration ................................ ........................... 60 4 22 On Potential and Off Potential on Pipe #2 as a function of distance along the pipe for two Pipes with independent CP configuration ................................ ........ 61 4 23 Current density on Pipe #2 as a function of distance along the pipe for two Pipes with independent CP configuration ................................ ........................... 61 4 24 Model of 2 pipes with Independent CP and coating flaw on one pipe configuration ................................ ................................ ................................ ....... 62 4 25 On Potential and Off Potential on Pipe #1 as a function of distance along the pipe for two pipes with Independent CP and coating flaw on one pipe configuration ................................ ................................ ................................ ....... 62 4 26 Current density on Pipe #1 as a function of distance along the pipe for two pipes with Independent CP and coating flaw on one pipe configuration ............. 63 4 27 On Potential and Off Potential on Pipe #2 as a function of distance along the pipe for two pipes with Independent CP and coating flaw on one pipe configuration ................................ ................................ ................................ ....... 63 4 28 Current density on Pipe #2 as a function of distance along the pipe for two pipes with Independent CP and coating flaw on one pipe configuration ............. 64 4 29 Steel A ................................ ................................ ................................ ................ 64 4 30 Steel B ................................ ................................ ................................ ................ 65 4 31 Coating A ................................ ................................ ................................ ............ 65 4 32 Coating B ................................ ................................ ................................ ............ 66 4 33 Non dimensional Cu rrent Density as a function of non dimensional radius on Tank Bottoms with different coating properties ................................ ................... 66 4 34 Off Potent ial as a function of non dimensional radius on Tank Bottoms with different coating properties ................................ ................................ ................. 67 4 35 Coating A with holid ay exposing Steel B ................................ ............................ 67 4 36 Coating B with holiday exposing Steel B ................................ ............................ 68

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11 4 37 Current Density as a function of Distance along the Radius on Tank Bottoms with different coating properties and holidays ................................ ..................... 68 4 38 On Potential as a function of Distance along the Radius on Tank Bottoms with different coating properties and holidays ................................ ..................... 69

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12 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science CATHODIC PROTECTION MODELLING OF BURIED STRUCTURES By Alok Shankar May 2012 Chair: Mark Orazem Major: Chemical Engineering Petroleum products are of vital importan ce to the preservation of industrial civilization These products have been transported over long distances by buried steel pipelines. The pipes are generally in soil environments w hich contain oxygen and therefore can undergo corrosion. Over the past years, several accidents have been reported which were caused by corrosion, resulting in loss of property and life These pipes are protected against corrosion using coating and cathodi c protection systems. As the demand for petroleum has increased over the years, multiple pipelines sharing right s of way have been used to transport the products. These products are stored in large above ground storage tanks. As each structure is provided with independent cathodic protection system, there arises the possibility of interaction between the different CP systems installed. This interference can cause regions on the structure to be either over or under protected. The ef fect is greater when there are coating defects in the structures. The objecti ve of this work was to use CP3D, a numerical simulation tool to study the effect of interference in cathodic protection system s for pipelines. The calculated

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13 potential and current distributions along the length of the structure were used to understand the interference effect and performance of cathodic protection system. The simulation tool was also used to extend the recent modeling studies of tank bottoms to account for the presence of coatings and coat ing flaws on the ability of CP systems to provide adequate protection

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14 CHAPTER 1 INTRODUCTION Petroleum products are of vital importance to the preservation of industrial civilizati on These products have been transported over long distances by buried steel pipelines. The pipes are present in soil environments which contain oxygen and water. Therefore an unprotected pipe may undergo corrosion. Coatings are used as primary protection for buried pipelines and cathodic protection installations are used as secondary protection. Over the past years, several incidents have been r eported which were caused by corrosion or failure of protection system s resulting in loss of pro perty and lives [1] One such incident occurred in Plum Borough, Pennsylvania in 2008, where a natural gas explosion destroyed a residence, killing a man and seriously injuring a 4 year old girl. Two other houses were destroy ed, and 11 houses were damaged amounting to property damage and losses worth a million dol lars. The National Transportation Safety Board determined that the probable cause of the leak and explosion was excavation damage to the 2 inch natural gas distributi on pipeline that pti ble to corrosion and failure [2] Direct currents are deliberately introduced into the earth to apply cathodic protection to any buried st ructure. The current may damag e other structures which are present in the same earth. The corrosion engineer is responsible to prevent damage to the underground structure. Cathodic interference is a more manageable problem than stray current. A steady ex posure exists hence more accurate measurements and adjustments can be carried out. The source of current is under control and the rectifier

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15 can be switched on and off when desired. When the magnitude of the current involved is high, the exposure can be sev ere as all the current return is by the earth path than just at the portion where leaks occur [3] There are approximately 8.5 million regulated and non regulated aboveground storage tanks (ASTs) and underground st orage tanks (USTs) for hazardous materials (HAZMAT) in the United States. The total cost of corrosion for storage tanks is estimated to be $7.0 billion per year (ASTs and USTs). The cost of corrosion for all ASTs was estimated at $4.5 billion per year. A v ast majority of the ASTs are externally painted, which is a major cost factor for the total cost of corrosion. In addition, approximately one third of ASTs have cathodic protection (CP) on the tank bottom, while approximately one tenth of ASTs have interna l linings. These last two corrosion protection methods are applied to ensure the long term s tructural integrity of the ASTs [4] Petroleum products are generally stored in a collection of aboveground storage tanks ca lled tank farms. These tanks are cylindrical, built from steel, and placed on the soil. These tank bottoms are subjected to similar corrosion as the buried pipelines. The tank bottoms are generally made of thinner metal than pipelines as it is supported by ground and only subjected to hydrostatic pressures. The thinner metal is more easily prone to even slow rates of corrosion. Hence, it is critical to provide cathodic protection to tank bottoms which presents design challenges compared to buried pipelines [5] The type of coating on the tank bottoms plays an important role in the performance of cathodic protection system. It affects the distribution of the protection cur rent when different types of coatings exis t.

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16 Since the times cathodic protection was used, the engineers had to rely on experience and intensive monitoring to optimize the design to prevent corrosion. [3] The performance of the cathodic protection system d epends crucially on the anode parameters. The parameters such as distance of anode from the structure, the depth of the anode, diameter and length of the anode play a major role in determining the extent of protect ion of any buried structure. Wrong current s and positions can lead to unprotected or over protected areas on the buried structures [6]

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17 CHAPTER 2 LITERATURE REVIEW Corrosion C orrosion of metallic materials is classified into three groups. [7] The first classification is w et Corrosion where the liquid electrolytes namely water with dissolved species forms the corrosive environment. The second classification namely dry c orrosion also known as chemical corrosion comprises of dry gas as the corrosive environment. The third classification is c orrosion in other fluids such as fused salts and molten metals T he current study is restricted to Wet corrosion of steel. Electrode Kinetics Iron corrosion has been describ e d in two mechanisms depending o n the pH. The soil environment presents an alkaline environment for cathodic protection. [8] [9] Under high pH values, (2 1 ) ( 2 2 ) where R.D. S. stands for Rate Determini ng Step. The sum of reactions (2 1) and (2 2) gives the net reaction which is an anodic reaction when the reaction proceeds from left to rig ht. For iron deposition the reactions are written as ( 2 3 ) ( 2 4 ) where e quation ( 2 4 ) r epresent s the cathodic reactions from left to right.

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18 The anodic current density as given by [10] is written in equation ( 2 5 ) ( 2 5 ) w here is the kinetic rate constant, is the activity of OH ions, is defined as metal solution i.e the potential difference between the meta l and surrounding electrolyte, stant (96487 coulombs /equiv), is the Gas Constant and is temperature. The activity of the OH ions is linear with respect to pH. The cathodic current is given in equation ( 2 6 ) ( 2 6 ) where is the activity o f Fe 2+ ions The net current density is given by equation (2 7) ( 2 7 ) The equilibrium potential can be calculated by equating (2 6) and (2 7) ( 2 8 ) T h e exchange current density using the anodic term as is given in equation ( 2 9 ) ( 2 9 ) The same current density at would be for the cathodic term.

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19 T he expression for in equation (2 8 ) is used to obtain the following expression, ( 2 10 ) Rewriting ( 2 7 ) in terms of exchange current density gives equation ( 2 11 ) ( 2 11 ) ( 2 12 ) Upon changing the base from e to 10, it reduces the number of unknowns in the above equation (2 11 ). The Tafel slopes are defined as, ( 2 13 ) ( 2 14 ) Rewriting (2 11 ) using the above Tafel slopes the following expression is obtained for net current given in equation (2 15) ( 2 15 ) w here is fit to experimental data, estimated around 0.56 V referenced to Cu /CuSO4 by using large scal e experiments by ARCO personnel [11] For f reely corroding surface, T he anodic corrosion reaction is given as, ( 2 16) And the balancing cathodic react ion is given by

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20 ( 2 17) Equation (2 17) i s known as the oxygen reduction reaction. A Tafelian nature is assumed for the rate of oxygen r eaction. When a steel pipe is connected to another active metal anode (Zn or Mg) a second cathodic reaction occurs. Also, the potential shifts sufficiently negative. ( 2 18) The above reaction (2 18) is commonly known as the hydrogen evolution reaction. Since the hydrogen evolution reaction follows Taf el kinetics, an equation similar to the cathodic reaction is used [1] Corrosion potential can be defined as the potential difference between a reference electrode and a freely corroding surface. Corrosion pote ntial is different from equilibrium potential as more than one reaction takes place. It is a function of the oxygen content and transport characteristics for oxygen within the electrolyte of the system. The sum of the equations gives the total current whi ch can be written as, ( 2 1 9) The above equation can be further simplified by ignorin g certain terms depending on the potential range of interest. Upon setting the equation to zero and solving for V, we can calcu late the corrosion potential V corr This is the same potential as the one measured

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21 under zero current experimental condition. It is measured using a reference electrode just outside the surface of the metal. Application to Corrosion in Soil When two differ ent metals are interconnected in soil one metal will corrode at a higher rate than it would independently and the other would corrode at a lower rate. This can be used as boundary condition in a numerical method considering that the kinetics is known. A m ass transfer limited reaction would exist as the reactant comes from t he electrolyte. The equation (2 20 ) can be used to descri be the kinetics at the boundary. In a steady state soil system as discussed in [12] ba re steel takes the form, ( 2 20) w here is the mass transfer limited current density for oxygen reduction. It is termed as limiting as part of the current due to oxygen reduction cannot exceed the value of Since it is assumed that sufficient water is always available around the pipe in soil environment, no limitation is observed for hydrogen evolution reaction to proceed under kinetic limitation Corrosion P revention Methods Several measures can be implemented for removing o r reducing the effect of the con ditions tha t leads to corrosion [7] One way is to select a material d oes not corrode in the actual environme nt. The surrounding environment can be changed, e.g. we can

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22 remove oxygen from the environment or add anti corrosion chemical behaving as inhibitors Other method is incorporating design modifications which avoid corrosion The potential of the metal surfa ce can be changed to a more negative value Coatings can be applied to provide a barrier for the metal against the corrosive environment. Cathodic Protection The technique where an undesired reaction is replaced by a more desired reaction on a given metal surface is known as cathodic protection. For example the undesired reaction for buried metallic structure is the dissolution of metal. The principle of cathodic protection is to have a secondary surface within the conductive soil at a more negative potenti al than the pipe steel. The pipe steel is connect ed to this surface with a wire [1] The cathodic protection in general practice is coupled with coatings to protect the areas of defected coating or holidays fro m corrosion. The coating forms the primary form of protection. Cathodic protection installations can be used for buried metallic pipelines, buried ta nks, other offshore structures [7] The theoretical basis o f cathodic protection is provided by an Evans diagram. The activation process controls the metal dissolution reaction. The cathodic reaction diffusion is limited at higher density. When we increase the cathodic current density, the potential of the metal d ecreases and the anodic dissolution rate reduces. Since logarithmic sca les are used for current, for each unit in decrease of metal potential, the current requirements increase exponentially. [3] Criterion for Cath odic Protection The cathodic protection to be applied to a given structure depends on the material of the structure to be protected, the environment of the structure, the level of protection

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23 required, the potential at the electrolyte interface and the non uniform corrosion potentials of the corroding structures. If the level of protection is too little then excessive corrosion can take place, on the other hand excessive current can cause disbonding of the coatings For a buried steel structure, the potenti al criteria proposed are: Potential of the structure with respect to saturated Cu/CuSO 4 is 850mV under aerobic conditions. This is the most widely used criterion as it is easy to employ. 300 mV negative potential shift is observed when current is applied Since protection criteria is dependent on the potential of the structure at the soil interface, corrections have to be incorporated to the measurements performed when reference electrode is placed some distance away from the structure. The measurements a re thus carried out in ON and OFF condition of the CP system to satisfy the potential shift criteria. [3] Anode Polarization The typical anodes used in cathodic protection are Galvanic or Sacrificial anodes and im pressed current anodes. These undergo a similar electrochemical process as the buried structure like pipes or tank bottoms. The corrosion term comprises of an oxidation reduction reaction but the interest lies in corrosion (oxidation) of the anode material Galvanic Anodes These types of anodes are more active than the metal of the structure that needs to be protected. Typical galvanic anodes used in soil environment are carbon steel, zinc and magnesium. These have large driving forces in the highly resist ive soil environment to provide protection. [7]

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24 The galvanic anode model is described as ( 2 21 ) W here is current due to galvanic anode, is the mass transfer limited current density for oxygen reduction, is the voltage of the anode, is the voltage just outside the surface of the anode, is the free corrosion (equilibrium) potential of the anode and is the Tafel slope for the anode corrosion reaction. The three parameters of the model are generally known for all types of galvanic anodes. The c ontribution of the hydrogen reaction is small at the typical operatin g potentials and hence ignored [1] Since the driving force for reduction reaction is always higher than freely corroding steel, the operati ng conditions typically ensure that the reduction is always mass transfer limit. Thus, a constant term is assumed. Impressed Current Anodes Impressed current cathodic protection is applied through an external power current source. Usually, the outside curr ent source is a rectifier which provides the activity. The orders of current densities and power output range on impressed current anodes are usually higher than that of galvanic anodes thus greater driving force. The numbers of anodes required are fewer e ven in highly resistive environments. Larger areas can be protected using the impressed current cathodic protection system. They provide the user the ability to adjust the protection levels. The anode consumption levels are usually much lower than galvanic anodes and hence the system has a longer life. [3]

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25 A dimensionally stable anode connected to the positive terminal of a direct current source is the set up of an impressed current system. The pipe is connected to t he negative terminal of the DC source. The most probable reactions of ICCP system are water oxidation and chloride oxida tion as the anodes do not react [1] ( 2 2 2 ) ( 2 2 3 ) The impressed current anode model is given as, ( 2 2 4 ) w here is current density in ICCP anode, an additional term is added to the exponent to account for the poten tial setting of the rectifier, is the voltage of the anode, is the voltage just outside the surface of the anode, is the equilibrium potential for the oxygen evolution reaction, and is the Tafel slope for the oxygen evolution reaction. Tank Bottoms Cathodic protection of storage tanks against external corrosion require s good coating and low protection current density Effective cathodic protection with small protection current densities can be achieved with new installations. For older tanks, larger current densities are required. It also depends on the coating and the state of the

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26 tanks. The cost for protective installations and work is generally higher for older tank installations [5] The criterion for protection used for tank bottoms are similar to the ones used for pipel ines where o xygen reduction is assumed to be the standard cathodic reaction on the tank bottoms. The circular are a of a tank bottom has inherent non uniform current distribution. This leads to a compromise in the delivery of protection current to the cent er of the tank. A more uniform behavior can be observed if certain limitations to the kinetics and mass transfer are applied [5]

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27 Table 2 1 Parameters for the oxygen and chlorine evolution reactions [13] Reaction Equilibrium Potential (E), mV (CSE) Tafel Slop mV/decade O 2 evolution 172 100 Cl 2 evolution 50 100

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28 C HAPTER 3 CATHODIC PROTECTION 3 D MODELLING SOFTWARE CP3D is a complex, powerful, and computationally intensive piece of modeling software. The prediction of the performance of cathodic prote ction systems under conditions like modern use of coatings, localized failure of pipes at discrete coating defects requires a mathematical model that can account for current and potential distributions in both angular and axial directions. This model accou nts for current flow in the soil, pipes and the circuitry. Long pipes exhibit non negligible poten tial difference along the steel [1] The current software accounts for the flow of current through two separate domains. The first is the soil domain enclosed by the pipes and anodes surfaces, interface between soil and air and interface between soil and any buried surfaces if any. The second domain contains the metallic wall of the pipe, the volume of the anode an d connecting wires and resistors for the return of the protective current. These domains are electrochemically linked. Soil Domain This domain contains the material in which the pipes and anodes are buried. It does not comprise the interior volume of the p ipeline steel. The concentrations and potential within the soil needs a solution of a coupled set of equations, including conservation of each individual solute species [10] The material balance equation over a sma ll volume element is given in equation (3 1). ( 3 1 )

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29 and electroneutrality expression is given as, ( 3 2 ) where is the concentration of sp ecies i, is the rate of generation of species i due to homogeneous reactions, and is the net flux vector for species i. In a dilute electrolytic solution, the contributions to the flux are from convection, diffusion, and migration. Under the assumption that soil represents a dilute solution, the flux is given as ( 3 3 ) where is the fluid velocity, is the diffusion coefficient for species i, is the charge associated with species i, s constant, and is the potential. Under the assumption of a steady state and a uniform concentration of ionic species, the current density, expressed in terms of contributions from the motion of each ionic species, can be given coefficient by the Nerst Einstein equation. ( 3 4 ) The current current density is given by equation (3 5). ( 3 5 ) w here the conductivity in terms of individual species contributions is expressed as ( 3 6 )

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30 The conductivity h as a uniform value because the concentration is uniform. The assumption that concentrations are uniform yields Laplace equation given by equation (3 7). ( 3 7 ) This is commonly used in cathodic protection models. The uniform concentr ation assumption means that the gradients in concentration that are associated with pipeline and anode surface reactions are assumed to occur within a thin layer adjacent to the pipe and anode surfaces. This gradient in concentration within the narrow laye r is incorporated into the boundary condition which describes the electrochemical reactions. Pipe or Inner Domain When we consider long pipelines and large current levels, the potential drop i n the pipelines is significant [14] The flow of current through the pipe steel, anodes, and ( 3 8 ) where is the departure of the potential of the meta l from a uniform value and is the material conductivity. The conductivity of the pipe metal domain is not necessarily uniform. A simplified version of Laplace equation give n by equation (3 9) ( 3 9 ) can be used to account for the potential drop across connecting wires, where is the resistance of the wire, is the electrical resistivity, is the length of the wire, and is th e cross sectional area of wire.

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31 Domain Coupling The inner and outer domains described in sections 3.1 and 3.2, respectively, are linked by boundary conditions where the current density on metal surfaces is related to values of local potential. The type of surface governs the specific form of the relationship required like bare steel, coated steel, galvanic anodes, and impressed current ano des have been discussed earli er [1] Numerical Development The software can be used to solve this equation for arbitrary arrangements of pipes and anodes within the domain and the nonlinear boundary conditions that arise from the chemistry at the boundaries as discussed earlier [1] For corrosion problems where the activity is observed at the boundaries, the boundary element numerical technique is used for solving governing equation at the boundaries in soil domain. The finite element method is used to solve the pipe steel domain. The solutions for the current and potential distributions are available around the circum ference and along the length of the pipe. The domain is divided into elements using piecewise continuous polynomial isoparametric shape functions. A special type of thin shell elements is introduced here which are specifically designed for potential proble ms on shells where the is large. The pipe and anode are electrically connected using bonds and resistors. These go from connection node on one pipe to connection node on another pipe. Thus, no extra nodes are introduced. The resistor if specified within the wire, its resistance is added to the total resistance of the bond. The boundary element domain and finite

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32 Law is valid within either domain.

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33 CHAPTER 4 RESULTS AND DISCUSSION The following section covers the effect of anode parameters on the performance of cathodic protection system s the interaction between the cathodic protection systems in pipelines in cross ov er configuration and effect of coatings on the protection current in tank bottoms. Dimensional Analysis of Anode Parameters The current density is highest at the anode. It is important to understand the effect of anode parameters on the performance of cat hodic protection systems. The primary current distribution was used to calculate Ohmic resistance. The anode parameters are then validated using the Ohmic resistances. The Buckingham pi method is used to find the relation between non dimensional parameters as given i n Table 4 1 The relation obtained is (4 1) The oretically, the resistance at the anode (4 2) The resistances were then compared to u nderstand the effect of the anode parameters and determine the dominant ones. A pipe, 6 km long, 300 mm diameter at 1 m depth having equipotential surface connected to an equipotential anode of different length, diameter buried at different depths in soil havin g 10,000 ohm cm resistivity was modeled using CP3D software to

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34 understand the effect of primary current distribution and the associated Ohmic resistance on parameters associated with anode. Influence of Anode Distance from P ipe The variation of Ohmic resistance with distance of the anode from the pipe was studied. The simulations resulted in slight variations of Ohmic resistance with distance of anode from the pipe as seen in F igure 4 1 The resi stance increased due to the increase in soil section as t he distance of t he anode from the pipe increased. The current density was highe st near the anode and it reduced as distance from the anode increased Hence, for an equipotential an ode, the change in resistance was observed to be steeper near the anode than away from it. The resistance measured using the simulation software was compared with the Dwight resistance at the anode as shown in F igure 4 2 Better agreement between the resistances was observed as the distance of the anode from the pipe was increase d but complete agreement was not seen as the Dwight resistance assumes that the anode is placed infinitely away from the cathode where as in the simulation the cathode (pipe) is located at a finite distance from the anode. Influence of Anode Depth The var iation of Ohmic resistance with depth of the anode from the pipe was studied. The simulations resulted in considerable variations of Ohmic resistance with depth of the anode as shown in Figure 4 3 The amount of current at the anode would be larger upon in creasi ng the depth of the anode. As the depth of anode was increased greater section of soil is available for current to flow through the soil. Hence, for an equipotential anode, the resistance decreased as we increase d the depth of the anode.

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35 The resista nce measured using the simulation software was compared with the Dwight resistance at the anode as shown in Figure 4 4 It was observed that the agreement between the resistances decre ased as the depth of the anode was increased but complete agreement is n ot observed as the Dwight resistance does not take into account the depth of the anode. Influence of Anode Length The variation of Ohmic resistance with length of the anode from the pipe was studied. The simulations resulted in large variations of Ohmic r esistance with increase in the length of anode as shown in Figure 4 5 The amount of current at the anode would be larger upon increasing the length of the anode. Hence, for an equipotential anode, the Ohmic resistance measured at th e anode decreased as th e length of the anode was increased Upon comparing the simulation resistance with the Dwight resistance as given in Figure 4 6 it was observed that there was no complete agreement between the values as the Dwight resistance does not account for the finit e distance of the anode from the cathode (pipe) and the depth of the anode. Influence of Anode Diameter The variation of Ohmic resistance with diameter of the anode was studied. The simulations resulted in large variations of Ohmic resistance with increase in the diameter of anode as shown in Figure 4 7 The amount of current at the anode would be l arger upon increasing the diameter of the anode Hence, for an equipotential anode, the Ohmic resistance measured at the anode decreases as the diameter of the a node is increased

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36 Upon comparing the simulation resistance with the Dwight resistance given by Figure 4 8, it is observed that there is no complete agreement between the values as the Dwight resistance does not account for the finite distance of the anode from the cathode (pipe) and the depth of the anode. Cathodic Interference in Pipelines The following section describes the different cases where interaction between the cathodic protection installations for two pipelines in a cross over configuration was observed. Base Case: Single Pipe A single pipe, 2 km long, buried in the soil of resistivity 10000 ohm cm, connect ed to a zinc anode at a certain distance was modeled using CP3D software as shown in Figure 4 9 The model p arameters are given in Table 4 9 The potential distribution along the length of the pipe was observed to be uniform and in the protected range of potentials as shown in Figure 4 10 The current density distribution along the length of the pipe was cathodic and uniform shown in Figure 4 11 At the drainage point where the anode was connected to the pipe, a more cathodic behavior was observed. Two Pipes in Cross Over Configuration Two pipes of equal dimensions and same coating properties were modeled in a cross over configuration to unders tand the effect of interaction between different CP systems installed on the respective pipelines. One pipe unprotected Two pipes of equal dimensions and same coating properties were modeled in a cross over configuration as shown in Figure 4 12 The model parameters are given in Table 4 10 A cathodic protection system was connected only to one of the pipes. The

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37 potential and current distributions on the pipe with the CP system installed showed same behavior as the single pipe without the effect of the new pipe introduced The potential and current distributions were uniform along the length of the pipe and in the protection range as shown by Figure 4 13 and Figure 4 14 respectively The potential distribution on pipe which does not have a CP system installe d was outside the protection range but nearly uniform along the length of the pipe as shown in Figure 4 1 6 At the cross over region, a minor peak in the potential and current distributions was observed as sh own in Figure 4 1 7 and Figure 4 1 8 resp ectively This is attributed to minor anodic potential and associated anodic current on the pipe. As a result, localized corrosion can be observed at the cross over location. Both pipes protected with independent CP Two pipes of same dimensions and coating pr operties; each having independent CP system installed was modeled to understand the interaction between CP systems as shown in Figure 4 1 9 The model p arameters are given in Table 4 11 The potential and current distributions on both the pipes were in t he protected range as shown in Figure 4 20 and Figure 4 21 respectively On pipe #1 two dips were observed in the potential and current distributions. One dip is associated with the drainage point where the CP system is connected to the pipe #1 The other dip is the associated with the CP connected to the pipe #2 which are affec ting the potential distribution on pipe #1 This is indicative of interaction between the two CP systems. Similar behavior was observed in pipe #2 as well where the CP of pipe #1 is affecting the distributions on pipe #2 as shown in Figure 4 2 2 and Figure 4 2 3

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38 Both pipes with independent CP s ystems and holiday on one pipe The two pipe system in cross over configuration with a coating flaw on pipe #1 was modeled to understand the e ffect of interaction between CP systems as shown in Figure 4 2 4 The model pa rameters are given in Table 4 12 The specifications of the holiday are described in Table 4 12. The potential distribution on pipe #1 was observed to be non uniform. More positiv e potential was observed at the site where coating flaw is present as given in Figure 4 2 5 An anodic current as shown in Figure 4 2 6 is associated at the site of coating flaw indicating that this location is most prone to corrosion. Anodes with larger dri ving force should be used to ensure that the potentials are in the protected range along the entire length of the pipe. The potential distribution on pipe #2 is non uniform. A sharp positive peak in the potential distribution is observed at the site where the coating flaw is present on pipe #1 as shown in Figure 4 2 7 An anodic current is associated at the same location indicating that localized corrosion is possible at this location despite the absence of coating defect on pipe #2 as shown in Figure 4 2 8 This behavior can be attributed to the possible interaction between the 2 CP systems installed. Effect of Coating on CP in Tank Bottoms Tank bottoms with bare steel properties and different coatings connected to anode located far away were modeled to unde rstand the effect of coating properties on tank bottoms. Four configurations were studied namely Steel A Steel B C oating A and C oating B in a soil of uniform resistivity of 10000 ohm cm as shown in Figure 4 2 9 Figure 4 30 Figure 4 31 and Figure 4 3 2 re spectively. The model p arameters and anode specif ications are given i n Table 4 13 The protection current density as a function of non dimensional radius is as shown in Figure 4 3 3

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39 Riemer and Orazem [1] had reported that a tank bottom with no coatings ca nnot be protected by a deep well remote ground bed when the output o f the anode is 14.2 A mperes and anode is placed 250 feet from the tank bottom. In the case of Steel A and Steel B where no coatings are present the protection current distribution is obse rved to be non uniform with the outer most ring having higher current density than the middle of the tank bottom This behavior is seen when Tafel kinetics apply where the corrosion and hydrogen evolution play a role The center of the tank was poorly pola rized as the net current density would be below the mass transfer limited current density for oxygen reaction. Upon increasing the applied po tential to a larger value the output current from the anode also increased as shown in Table 4 12. The current d istribution was observed to be non uniform as the earlier result but the potential distribution in the tank bottoms were observed to be in the protected r ange. The center of the tank had a potential of 870 mV CSE and the outermost ring ha d a potential of 1065 mV CSE. The increase d net current is now above the mass transfer limited current density for oxygen reaction and hence the tank bottom with no coatings was protected. When we apply coatings in the tank bottoms, a near uniform current density was obse rved along the radius of the tank bottom. The protection current required for coated tank bottoms was lesse r than in the case of tank bottoms with no coatings as a reduction in transport of oxygen through a uniform barrier is seen. The potentials along the radius are observed to be in the protection range as shown in Figure 4 3 4 The uniform current density distribution helps in CP design where the maximum radius that

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40 can be protect ed with a coating is calculated by using equation (4 3) as described by [10] (4 3) where is the maximum over polarization of steel from the point of minimum protection which is 850 mV CSE. If the undesired hydrogen evoluti on is observed at 1200 mV CSE then is equal t o 350 mV CSE. The maximum radius protected for Coating A and Coating B are given in Table 4 12. Tank Bottom with Coating Flaws Simulations were performed for coated t ank bo ttoms wit h coating flaws that exposed bare steel. The coating defect was located at the center of the tank bottom, and, as was done for the previous section, the anode was placed at a large distance from the tank Two configurations were studied : Coating A with Ste el B exposed in the center of the tank and Coating B with Steel B exposed as shown in Figure 4 3 5 and Figure 4 3 6 respectively. The soil resistivity was assumed to be uniform with a value of 10,000 ohm cm. The model pa rameters are summarized in Table 4 16 The corresponding current distribution s are given in Figure 4 3 7 as a function of distance along the radius. The current distribution was found to be non uniform due to the presence of the coating defect. The current density was observed to be highest at the center of the tank bottom where the defect was present as the bare metal had a lower resistance to current flow as compared to the coated part of the tank bottom. To ensure that minimum protection of the entire tank bottom in the case of Coating A was achieved a large

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41 potential of 160V had to be applied. This resulted in large areas of tank bottom being over protected as shown by the potential values in Table 4 17 In the case of Coating B, a larger potential of 1600 V was applied to ensure minimum pro tection was achieved of the tank bottom. This also resulted in large areas of the tank bottom being over protected as shown by the potential d istribution in Figure 4 38

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42 Table 4 1. List of Pipe and Anode Parameters Parameter Symbol Basic units Length of pipe L p Length (meters) Diameter of pipe D p Length (meters) Depth of pipe H p Length (meters) Length of anode L a Length (meters) Diameter of anode D a Length (meters) Depth of anode H a Length (meters) Distance of anode D Length (meters) Ohmic Resistance R Ohm Resistivity of soil Ohm Length (meters) Total Current from anode i Amperes Potential difference V Volts Table 4 2 Ohmic resistance as a function of distance of anode from pipe with L a = 1m, D a = 0.1m, H a = 1m and V = 1V D (m) i (A) R (ohm) D/L p 1000 R Dwight (ohm) R/ R D wight 10 0.02292 2617.46 1.67 3229.6 0.810 20 0.02278 2633.66 3.33 3229.6 0.815 30 0.02274 2638.64 5 3229.6 0.817 40 0.02272 2640.96 6.67 3229.6 0.8177 50 0.02271 2642.36 8.33 3229.6 0.8181 60 0.0227 2643.17 10 3229.6 0.8184 70 0.02269 2643.75 11. 67 3229.6 0.8186 80 0.02269 2644.22 13.33 3229.6 0.8187 90 0.02268 2644.69 15 3229.6 0.8188 100 0.02268 2644.92 16.67 3229.6 0.8189 Table 4 3 Ohmic resistance as a function of length of anode with D a = 0.1m, H a = 1m, D = 20m and V = 1V L p (m) i (A) R (ohm) La/L p 10000 R Dwight (ohm) R/R dwight 0.5 0.0153 3935.46 0.83 17004.42 0.2314 1 0.0228 2633.66 1.67 7840.3 0.3359 1.5 0.0295 2034.24 2.5 4968.742 0.4094 2 0.0356 1677.71 3.33 3589.19 0.4674 2.5 0.0417 1437.47 4.17 2786.12 0.515 3 0.0475 1262. 95 5 2263.73 0.5579 3.5 0.0531 1129.58 5.83 1898.29 0.595 4 0.0586 1023.98 6.67 1629.12 0.6285 4.5 0.0637 937.98 7.5 1423.11 0.6591 5 0.0693 866.43 8.33 1260.68 0.6872

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43 Table 4 4 Ohmic resistance as a function of diameter of the anode with L a = 1m, H a = 1m, D = 20m and V = 1V D a (m) i (A) R (ohm) D a /L p 1000 R Dwight (ohm) R/R D wight 0.5 0.0183 3282.63 0.83 3891.51 0.8435 1 0.022 8 2633.66 1.67 3229.6 0.8154 1.5 0.0264 2269.63 2.5 2842.41 0.7984 2 0.0297 2021.56 3.33 2567.7 0.7873 2.5 0.03 2 7 1836.38 4.17 2354.6 0.7799 3 0.035 5 1690.43 5 2180.5 0.7752 3.5 0.038 2 1571.21 5.83 2033.3 0.7727 4 0.040 8 1471.2 6.67 1905.79 0.7719 4.5 0.0433 1385.65 7.5 1793.31 0.7726 5 0.045 6 1311.36 8.33 1692.7 0.7747 Table 4 5 Ohmic resistance as a funct ion of depth of the anode with L a = 1m, D a = 1m, D = 20m and V = 1V H a (m) i (A) R (ohm) H a /L p 10000 R Dwight (ohm) R/R Dwight 0.5 0.0221 2715.42 0.83 3229.6 0.8408 1 0.022 8 2633.66 1.67 3229.6 0.8155 1.5 0.023 2 2594.15 2.5 3229.6 0.8032 2 0.0233 2570 .58 3.33 3229.6 0.7959 2.5 0.0235 2554.93 4.17 3229.6 0.7911 3 0.023 6 2543.77 5 3229.6 0.7876 3.5 0.023 7 2535.49 5.83 3229.6 0.7851 4 0.0237 2528.99 6.67 3229.6 0.7831 4.5 0.0238 2523.87 7.5 3229.6 0.7815 5 0.0238 2519.63 8.33 3229.6 0.7802

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44 Table 4 6 Coating and Steel property p arameters for the CP3D simulations Type of coating Coating A Coating B Steel A Steel B Steel C Coating Resistivity ( ohm cm ) 10 8 50 2 Coating Thickness (mm) 0.508 0.508 Oxygen blocking (%) 99.9 99 A pore /A (%) 0.1 0.1 Corrosion Potential ( mV CSE ) 654.3 635.7 491.3 457.4 520.7 E Fe ( mV ) 522 522 522 522 522 Tafel slope Fe ( mV/decade ) 62.6 62.6 62.6 62.6 59 i lim O2 ( A/sq cm ) 1.05 1.05 3.1 10.764 1.05 E O2 ( mV CSE ) 172 172 172 172 172 Tafel slope O 2 ( mV/decade ) 66.5 66.5 66.5 66.5 61 E H2 ( mV CSE ) 942 942 942 942 942 Tafel Slope H 2 ( mV/decade ) 132.1 132.1 132.1 132.1 132.1 Table 4 7 S imulation parameters Steel Resistivity (ohm cm) 9.6 10 6 Soil Resis tivity (ohm cm) 10000 Steel Thickness 12.7 Number of circumferential nodes 8 Maximum ratio of adjacent element lengths 4.5 Maximum Aspect ratio 7500 Roundness of ends 25 Table 4 8 Anode parameters used in CP3D simulations Anode Type Standard poten tial Mg Zn ICCP Galvanic metal equilibrium potential (V) 1.5 1.1 Oxygen Equilibrium Potential (mV) 172 172 382 Oxygen Tafel Slope (mV/decade) 100 100 118 ICCP Operting Potential (V) 4

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45 Table 4 9 Model p arameters and r esult of single pip e configuration Pipe Number 1 Length of Pipe (m) 2000 Diameter of pipe (m) 0.3 Depth of pipe (m) 1 Coating on the pipe Coating B Length of Anode (m) 1 Diameter of Anode (m) 0.1 Depth of Anode (m) 1 Distance of anode from the pipe (m) 50 Type of An ode Zn Total Current (A) 0.0165 Total Area (m 2 ) 1875.1 Peak annual Metal Loss (mm/yr) 4.9848e 011 Table 4 10 Model p arameters and r esult of two pipes single CP configuration Pipe Number 1 2 Length of Pipe (m) 2000 2000 Diameter of pipe (m) 0.3 0.3 Depth of pipe (m) 1 1 Coating on the pipe Coating B Coating B Length of Anode (m) 1 Diameter of Anode (m) 0.1 Depth of Anode (m) 1 Distance of anode from the pipe (m) 50 Type of Anode Zn Total Current (A) 0.0165 1.3558e 017 Total Area (m 2 ) 1875.1 1875.1 Peak annual Metal Loss (mm/yr) 4.9848e 011 1.7554e 007

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46 Table 4 11 Model p arameters and r esult of two pipes with independent CP configuration Pipe Number 1 2 Length of Pipe (m) 2000 2000 Diameter of pipe (m) 0.3 0.3 Dept h of pipe (m) 1 1 Coating on the pipe Coating B Coating B Length of Anode (m) 1 1 Diameter of Anode (m) 0.1 0.1 Depth of Anode (m) 1 1 Distance of anode from the pipe (m) 50 50 Type of Anode Zn Zn Total Current (A) 0.00454 0.00454 Total Area (m 2 ) 1875.1 1875.1 Peak annual Metal Loss (mm/yr) 4.7753e 011 4.7758e 011 Table 4 12 Model parameters and r esult of two pipes having independent CP with holiday on one pipe configuration Pipe Number 1 2 Length of Pipe (m) 2000 2000 Diameter of pipe (m) 0.3 0.3 Depth of pipe (m) 1 1 Coating on the pipe Coating B Coating B Length of Anode (m) 1 1 Diameter of Anode (m) 0.1 0.1 Depth of Anode (m) 1 1 Distance of anode from the pipe (m) 50 50 Type of Anode Zn Zn Holiday Present Absent Location of ho liday Cross over junction Type of holiday Steel C Length of holiday (mm) 500 Width of holiday 300 Total Current (A) 0.0049 0.0045 Total Area (m 2 ) 1875.1 1875.1 Peak annual Metal Loss (mm/yr) 1.2573e 009 1.1509e 010 Peak annual Metal Loss a t holiday (mm/yr) 2.3542e 006

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47 Table 4 1 3 Model Parameters and simulation results for tank bottoms with different coating properties Tank Coating Steel A Steel B Coating A Coating B Tank Diameter (m) 45.72 45.72 45.72 45.72 Number of elemen t rings 12 12 12 12 Nodes in first ring 8 8 8 8 Length of Anode (m) 1 1 1 1 Diameter of Anode (m) 0.1 0.1 0.1 0.1 Depth of Anode (m) 1000 1000 1000 1000 Type of Anode ICCP 4750 ICCP 15500 Standard Potential Magnesium Standard Potential Magnesium Potential Applied (V CSE) 4750 15500 Output current of Anode (A) 111.62 364.34 0.009 2 0.0118 Cross section area of tank bottom (m 2 ) 1641.7 1641.7 1641.7 1641.7 i avg (A/sq m) 5.6*10 6 7.2*10 6 r max (m) 1721 1339

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48 Table 4 14 N on dimensional Current density as a function of non dimensional radius on tank bottoms with different coating properties Steel A Steel B Coating A Coating B r/R i (A/sq m) i/i avg i (A/sq m) i/i avg i ( A/sq m) i/i avg i ( A/sq m) i/i avg 0 0.03 4 0.50 0.110 7 0.50 5.53 0.99 6.94 0.96 0.06 0.0340 0.50 0.1108 0.50 5.53 0.99 6.94 0.97 0.08 0.034 1 0.50 0.111 0.50 5.53 0.99 6.94 0.97 0.16 0.0344 0.51 0.112 1 0.50 5.53 0.99 6.95 0.97 0.19 0.0345 0.51 0.112 5 0.51 5.53 0.99 6.95 0.97 0.23 0.0348 0.51 0.1134 0.51 5.53 0.99 6.96 0.97 0.36 0.036 2 0.53 0.1178 0.53 5.54 0.99 6.99 0.97 0.4 0.0368 0.54 0.1199 0.54 5.54 0.99 7.00 0.97 0.44 0.037 6 0.55 0.122 4 0.55 5.55 0.99 7.01 0.98 0.48 0.0384 0.57 0.125 3 0.56 5.55 0. 99 7.03 0.98 0.53 0.039 5 0.58 0.128 7 0.58 5.55 0.99 7.05 0.98 0.57 0.0407 0.60 0.132 8 0.60 5.56 1.00 7.07 0.98 0.7 0.0461 0.68 0.150 4 0.68 5.57 1.00 7.15 0.99 0.74 0.048 9 0.72 0.159 4 0.72 5.58 1.00 7.18 1.00 0.78 0.0522 0.77 0.1703 0.77 5.59 1.00 7.21 1.00 0.82 0.0570 0.84 0.186 0.84 5.60 1.00 7.25 1.01 0.87 0.062 8 0.92 0.204 6 0.92 5.61 1.00 7.30 1.02 0.91 0.074 4 1.09 0.2427 1.09 5.62 1.01 7.35 1.02 0.95 0.080 7 1.19 0.2622 1.18 5.63 1.01 7.40 1.03 0.98 0.124 2 1.83 0.4065 1.83 5.64 1.01 7.45 1.04

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49 Table 4 15 Off Potential distribution as a function of non dimensional radius on tank bottoms with different coating properties r/R Off Potential Steel A (mV CSE) Off Potential Steel B (mV CSE) Off Potent ial Coating A (mV CSE) Off Potential Coating B (mV CSE) 0.00 0.8735 0.8731 0.9079 0.9201 0.06 0.8745 0.8762 0.9079 0.9202 0.08 0.8757 0.8798 0.9079 0.9202 0.16 0.8813 0.8951 0.9079 0.9202 0.19 0.8833 0.9001 0.9079 0.9202 0.23 0.8 878 0.9105 0.9079 0.9203 0.36 0.9049 0.9431 0.908 0.9206 0.40 0.9114 0.9537 0.908 0.9207 0.44 0.9183 0.9642 0.9081 0.9208 0.48 0.9255 0.9745 0.9081 0.9209 0.53 0.933 0.9847 0.9082 0.9211 0.57 0.9408 0.9948 0.9082 0.9212 0. 70 0.966 1.0254 0.9084 0.9219 0.74 0.9755 1.0362 0.9084 0.9221 0.78 0.9855 1.0474 0.9085 0.9224 0.82 0.9969 1.06 0.9086 0.9227 0.87 1.009 1.073 0.9087 0.9231 0.91 1.0257 1.0908 0.9088 0.9235 0.95 1.0382 1.1037 0.9089 0.923 9 0.98 1.0663 1.1327 0.909 0.9243

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50 Table 4 16. Model Parameters and simulation results for tank bottoms with different coating properties and holidays Tank Coating Coating A with Steel B exposed Coating B with Steel B exposed T ank Diameter (m) 45.72 45.72 Number of element rings 12 12 Nodes in first ring 8 8 Length of Anode (m) 1 1 Diameter of Anode (m) 0.1 0.1 Depth of Anode (m) 1000 1000 Type of Anode ICCP 160 ICCP 16000 Potential Applied (V CSE) 160 16000 Output c urrent of Anode (A) 3.18 4 36.98 Cross section area of tank bottom (m 2 ) 1641.7 1641.7 Presence of Holiday on Tank Bottom Center Center

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51 Table 4 17. Current and On Potential distribution as a function of Distance along the Radius on tank bot toms with different coating properties and holidays Coating A, Steel B exposed Coating B with Steel B exposed Distance along the radius (m) On Potential (V CSE) Current Density (A/sq m) On Potential (V CSE) Current Density (A/sq m) 0 1.07 0.2034 1.07 0.2008 1.3188 0.92 0.114 0.92 0.1149 1.7585 1.19 9.90E 06 1.19 0.0002 3.6635 17.46 6.41E 04 15.48 0.0139 4.1397 18.41 6.78E 04 16.23 0.0146 5.0922 19.84 7.34E 04 17.34 0.0157 7.9497 21.88 8.14E 04 19.02 0.0174 8.9022 22.28 8.30E 04 19.43 0.0178 9.8547 22.59 8.42E 04 19.82 0.0182 10.8072 22.86 8.53E 04 20.21 0.0185 11.7597 23.09 8.62E 04 20.61 0.0189 12.7122 23.29 8.70E 04 21.04 0.0194 15.5697 23.77 8.89E 04 22.57 0.0209 16.5222 23.91 8.94 E 04 23.21 0.0215 17.4747 24.04 8.99E 04 23.93 0.0222 18.4272 24.16 9.04E 04 24.77 0.023 19.3797 24.29 9.09E 04 25.75 0.024 20.3322 24.41 9.14E 04 26.96 0.0252 21.1504 24.52 9.18E 04 28.22 0.0264 21.8342 24.62 9.22E 04 29.60 0.0276 22.3471 24.70 9.25E 04 30.87 0.0282

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52 Fig ure 4 1 Non Dimensional Ohmic resistance as a function of D with L a D a H a fixed Figure 4 2. R/R Dwight as a function of Distance of anode with L a D a H a fixed Figure 4 3. Non Dimensional Ohmic resistance as a function of D a with L a D a D fixed

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53 Figure 4 4. R/R Dwight as a function of H a with L a D a D fixed Figure 4 5. Non Dimensional Ohmic resistance as a function of L a with D a H a D fixed Figure 4 6. R/R Dwight as a function of L a with D a H a D fixed

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54 Figure 4 7. Non Dimensional Ohmic resistance as a function of D a with L a H a D fixed Figure 4 8 R/R Dwight as a function of D a with L a H a D fixed Figure 4 9 Model of Single pipe

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55 Figure 4 10. On Potential and Off Potential on Pip e #1 as a function of distance along the pipe for single pipe configuration Figure 4 11. Current density on Pipe #1 as a function of distance along the pipe for single pipe configuration

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56 Figure 4 12 Model of two Pipes with Single CP Figure 4 13. On Potential and Off Potential on Pipe #1 as a function of distance along the pipe for two Pipes with Single CP configuration

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57 Figure 4 14. Current Density on Pipe #1 as a function of distance along the pipe for two Pipes with Single CP configuration Fi gure 4 1 5 On Potential and Off Potential on Pipe #2 as a function of distance along the pipe for two Pipes with Single CP configuration

PAGE 58

58 Figure 4 1 6 On Potential and Off Potential on Pipe #2 as a function of distance along the pipe a round the point of anode connection for two Pipes w ith o ne unprotected pipe configuration Figure 4 1 7 Current Density on Pipe #2 as a function of distance along the pipe for two Pipes with o ne unprotected pipe configuration

PAGE 59

59 Figure 4 1 8 Current Density on Pipe #2 as a function of distance along the pipe a round the point of anode connection for two Pipe s with o ne unprotected pipe configuration Figure 4 1 9 Model of two pipes with independent Cp

PAGE 60

60 Figure 4 20 On Potential and Off Potential on Pipe #1 as a function of distance along the pipe for two Pipes with independent CP configuration Figure 4 21 Current density on Pipe #1 as a function of distance along the pipe for two Pipes with independent CP configuration

PAGE 61

61 Figure 4 2 2 On Potential and Off Potential on Pipe #2 as a function of distance along the pipe for two Pipes with independent CP configuration Figure 4 2 3 Current density on Pipe #2 as a function of distance along the pipe for two Pipes with independent CP configuration

PAGE 62

62 Figure 4 2 4 Model of 2 pipes with Independent CP and coating flaw on one pipe configuration Figure 4 2 5 On Potential an d Off Potential on Pipe #1 as a function of distance along the pipe for two pipes with Independent CP and coating flaw on one pipe configuration

PAGE 63

63 Figure 4 2 6 Current density on Pipe #1 as a function of distance along the pipe for two pipes with Independ ent CP and coating flaw on one pipe configuration Figure 4 2 7 On Potential and Off Potential on Pipe #2 as a function of distance along the pipe for two pipes with Independent CP and coating flaw on one pipe configuration

PAGE 64

64 Figure 4 2 8 Current densit y on Pipe #2 as a function of distance along the pipe for two pipes with Independent CP and coating flaw on one pipe configuration Figure 4 2 9 Steel A

PAGE 65

65 Figure 4 30 Steel B Figure 4 31 Coating A

PAGE 66

66 Figur e 4 3 2 Coating B Figure 4 3 3 Non dim ensional Current Density as a function of non dimensional radius on Tank Bottoms with different coating properties

PAGE 67

67 Figure 4 3 4 Off Potential as a function of non dimensional radius on Tank Bottoms with different coating properties Figure 4 3 5 Coati ng A with holiday exposing Steel B

PAGE 68

68 Figure 4 3 6 Coating B with holiday exposing Steel B Figure 4 3 7 Current Density as a function of Distance along the Radius on Tank Bottoms with different coating properties and holidays

PAGE 69

69 Figure 4 3 8 On Potential as a function of Distance along the Radius on Tank Bottoms with different coating properties and holidays

PAGE 70

70 CHA PTER 5 CONCLUSIONS The performance of the cathodic protection was analyzed using the dimensional analysis of the parameters associated with the anode. Several cases were studied to understand the interaction between two cathodic protection system installations in pipelines modeled in cross over configuration. The recent tank bottom studies were extended to understand the effect of protec tion current on coated tank bottoms and when coating flaws are present in the tank bottoms. The dimensional analysis of the a node parameters show ed that the length, diameter and depth of the anode are the dominant parameters which affect the performance of cathodic protection system. Cathodic int erference effect s in pipelines were observed in cases where the two pipes placed in a cross over configuration with one pipe connected to a cathodic protection anode both pipes having independent cathodic protecti on installations and both pipes having independent cathodic protection installations with a coating flaw present on one pipe. The potential and current distribution on one pipe was affected by the presence of CP on the other pipe. When the pipes in cross over configuration have independent CP and a holiday was present on pipe #1 the potential distribution indicated that the metal exposed by the coating defect was anodic and was subject to corrosion. On pipe #2 a similar anodic region was observed despit e the absence of a holiday. This is an indication of the interaction between the CP systems on both the pipes. The tank bottoms with no coatings present can be protected by using a deep well ground bed placed far from the tank bottom without over protecti ng any region o f the

PAGE 71

71 tank bottom The only concern is that the applied potential be sufficient to drive the necessary current. A non uniform potential distribution was observed along the radius of the tank bottom. The current density was highest at the out er ring of the tank bottom and least at the center. The off potential was more cathodic at the outer ring than the center of the tank bottom The coated tank bottoms had a uniform current distribution along the radius of the tank The magnitude of the pr otection current required was significantly smaller than the case when the tank bottom had no coating The protection current required for tank bottom with Coating A was lower than with Coating B as the resistivity and oxygen blocking property of Coating A was higher. The maximum radius protected was larger in the case of tank bottom with Coating A than Coating B. T he distribution of protection current was non uniform when coating flaws were present at the center of the tank in coated tank bottoms. Most of the protection current was directed towards the site of coating flaw. The protection current required in the case where tank bottom had Coating A and Steel B exposed in holiday was less than the case of tank bottom with Coating B with Steel B exposed as t he resistivity and oxygen blocking property of Coating A is higher When a large driving force was applied to ensure minimum protection of the center of tank bottom where coating flaw was present, the other regions of the tank bottom were overprotected.

PAGE 72

72 APPE NDIX POTENTIAL AND CURREN T DISTRIBUTIONS ON PIPELINES Table A 1. Potential and Current Distribution on Pipe 1 Single pipe Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) 0.00 0.8607 0.8582 2.43E 06 0.04 0.8607 0.8582 2.43E 06 0.19 0.8607 0.8582 2.43E 06 0.47 0.8606 0.8582 2.42E 06 1.14 0.8606 0.8582 2.42E 06 2.41 0.8606 0.8581 2.42E 06 5.45 0.8606 0.8581 2.42E 06 11.14 0.8606 0.8581 2.42E 06 24.81 0.8605 0.8581 2.42E 06 50. 44 0.8605 0.8580 2.42E 06 111.95 0.8605 0.8580 2.42E 06 200.82 0.8605 0.8580 2.42E 06 371.78 0.8605 0.8580 2.42E 06 628.22 0.8606 0.8581 2.42E 06 799.18 0.8607 0.8582 2.43E 06 888.05 0.8610 0.8585 2.44E 06 949.56 0.8613 0.8588 2.45E 06 975.19 0.8616 0.8591 2.46E 06 988.86 0.8618 0.8592 2.47E 06 994.55 0.8618 0.8593 2.47E 06 997.59 0.8618 0.8593 2.47E 06 998.86 0.8618 0.8593 2.47E 06 999.53 0.8618 0.8593 2.47E 06 999.81 0.8618 0.8593 2.47E 06 999.96 0.8618 0.8593 2.47E 06 1000.04 0.8618 0.8593 2.47E 06 1000.19 0.8618 0.8593 2.47E 06 1000.47 0.8618 0.8593 2.47E 06 1001.14 0.8618 0.8593 2.47E 06 1002.41 0.8618 0.8593 2.47E 06 1005.45 0.8618 0.8593 2.47E 06 1011.14 0.8618 0 .8592 2.47E 06 1024.81 0.8616 0.8591 2.46E 06 1050.44 0.8613 0.8588 2.45E 06 1111.95 0.8610 0.8585 2.44E 06 1200.82 0.8607 0.8582 2.43E 06 1371.78 0.8606 0.8581 2.42E 06

PAGE 73

73 Table. A 1 Continued D istance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) 1888.05 0.8605 0.8580 2.42E 06 1949.56 0.8605 0.8580 2.42E 06 1975.19 0.8605 0.8581 2.42E 06 1988.86 0.8606 0.8581 2.42E 06 1994.55 0.8606 0.8581 2.42E 06 1997.59 0.8606 0.8581 2.42E 06 1998.86 0.8606 0.8582 2.42E 06 1999.53 0.8606 0.8582 2.42E 06 1999.81 0.8607 0.8582 2.43E 06 1999.96 0.8607 0.8582 2.43E 06 2000.00 0.8607 0.8582 2.43E 06

PAGE 74

74 Table A 2. Potential and Current Distributions on Pipe1 and Pipe2 Pipe 1 Pipe 2 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) On Potential (V) Off Potential (V) Current Density (A/sq m) 0.00 0.8607 0.8582 2.43E 06 0.6355 0.6355 8.59E 10 0.04 0.8607 0.8582 2.43E 06 0.6355 0. 6355 8.59E 10 0.19 0.8607 0.8582 2.43E 06 0.6355 0.6355 8.59E 10 0.47 0.8606 0.8582 2.42E 06 0.6355 0.6355 8.58E 10 1.14 0.8606 0.8582 2.42E 06 0.6355 0.6355 8.58E 10 2.41 0.8606 0.8581 2.42E 06 0.6355 0.6355 8.58E 10 5.45 0.8606 0.8581 2.42E 06 0.6355 0.6355 8.58E 10 11.14 0.8606 0.8581 2.42E 06 0.6355 0.6355 8.57E 10 24.81 0.8605 0.8581 2.42E 06 0.6355 0.6355 8.56E 10 50.44 0.8605 0.8580 2.42E 06 0.6355 0.6355 8.53E 10 111.95 0.8605 0.8580 2.42E 06 0.63 55 0.6355 8.45E 10 200.82 0.8605 0.8580 2.42E 06 0.6355 0.6355 8.37E 10 371.78 0.8605 0.8580 2.42E 06 0.6355 0.6355 7.77E 10 628.22 0.8605 0.8581 2.42E 06 0.6356 0.6356 5.83E 10 799.18 0.8606 0.8582 2.42E 06 0.6356 0.6356 1.75E 10 888.05 0.8608 0.8583 2.43E 06 0.6357 0.6357 5.50E 10 949.56 0.8610 0.8585 2.44E 06 0.6359 0.6359 1.46E 09 975.19 0.8611 0.8587 2.44E 06 0.6360 0.6360 2.21E 09 988.86 0.8613 0.8588 2.45E 06 0.6361 0.6360 2.55E 09 994.55 0.8613 0.8588 2.45E 06 0.6361 0.6361 2.56E 09 997.59 0.8613 0.8588 2.45E 06 0.6360 0.6360 2.47E 09 998.86 0.8614 0.8589 2.45E 06 0.6360 0.6360 2.34E 09 999.53 0.8614 0.8589 2.45E 06 0.6360 0.6360 2.25E 09 999.81 0.8614 0.8589 2.45E 0 6 0.6360 0.6360 2.22E 09 999.96 0.8614 0.8589 2.45E 06 0.6360 0.6360 2.22E 09 1000.04 0.8614 0.8589 2.45E 06 0.6360 0.6360 2.22E 09 1000.19 0.8614 0.8589 2.45E 06 0.6360 0.6360 2.25E 09 1000.47 0.8614 0.8589 2.45E 06 0.6360 0. 6360 2.31E 09 1001.14 0.8614 0.8589 2.45E 06 0.6360 0.6360 2.49E 09 1002.41 0.8614 0.8589 2.45E 06 0.6361 0.6361 2.78E 09 1005.45 0.8614 0.8589 2.46E 06 0.6362 0.6362 3.27E 09 1009.88 0.8615 0.8590 2.46E 06 0.6363 0.6363 3.85E 09 1018.52 0.8616 0.8591 2.46E 06 0.6364 0.6364 4.70E 09 1031.48 0.8617 0.8592 2.47E 06 0.6366 0.6366 5.76E 09 1040.12 0.8618 0.8593 2.47E 06 0.6366 0.6366 6.30E 09 1044.55 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.44E 09 1047.59 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.52E 09 1048.86 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.54E 09 1049.53 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.54E 09 1049.81 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.55E 09

PAGE 75

75 Table A 2. Continued Pipe 1 Pipe 2 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) On Potential (V) Off Potential (V) Current Density (A/sq m) 1049.96 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.55E 09 1050.04 0.8618 0.8593 2.47 E 06 0.6367 0.6367 6.55E 09 1050.19 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.55E 09 1050.47 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.55E 09 1051.14 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.56E 09 1052.41 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.56E 09 1055.45 0.8618 0.8593 2.47E 06 0.6367 0.6367 6.54E 09 1061.14 0.8618 0.8592 2.47E 06 0.6367 0.6367 6.46E 09 1074.81 0.8616 0.8591 2.46E 06 0.6366 0.6366 5.78E 09 1100.44 0.8613 0.8588 2.45E 06 0.6363 0.6363 4. 00E 09 1161.95 0.8610 0.8585 2.44E 06 0.6359 0.6359 1.85E 09 1247.70 0.8607 0.8582 2.43E 06 0.6357 0.6357 3.65E 10 1406.16 0.8606 0.8581 2.42E 06 0.6356 0.6356 3.44E 10 1643.84 0.8605 0.8580 2.42E 06 0.6355 0.6356 6.66E 10 1802.3 0 0.8605 0.8580 2.42E 06 0.6355 0.6355 7.68E 10 1888.05 0.8605 0.8580 2.42E 06 0.6355 0.6355 7.86E 10 1949.56 0.8605 0.8581 2.42E 06 0.6355 0.6355 8.02E 10 1975.19 0.8605 0.8581 2.42E 06 0.6355 0.6355 8.07E 10 1988.86 0.8606 0.858 1 2.42E 06 0.6355 0.6355 8.10E 10 1994.55 0.8606 0.8581 2.42E 06 0.6355 0.6355 8.11E 10 1997.59 0.8606 0.8581 2.42E 06 0.6355 0.6355 8.12E 10 1998.86 0.8606 0.8582 2.42E 06 0.6355 0.6355 8.12E 10 1999.53 0.8606 0.8582 2.43E 06 0.6 355 0.6355 8.12E 10 1999.81 0.8607 0.8582 2.43E 06 0.6355 0.6355 8.12E 10 1999.96 0.8607 0.8582 2.43E 06 0.6355 0.6355 8.12E 10 2000.00 0.8607 0.8582 2.43E 06 0.6355 0.6355 8.12E 10

PAGE 76

76 Table A 3 Potential and Current Distributions Pipe1 and Pipe2 Pipe 1 Pipe 2 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) On Potential (V) Off Potential (V) Current Density (A/sq m) 0.00 0.8605 0.8581 2.42E 06 0.8605 0.8581 2.42E 06 0.04 0.8605 0.8581 2.42E 06 0.8605 0.8581 2.42E 06 0.19 0.8605 0.8580 2.42E 06 0.8605 0.8580 2.42E 06 0.47 0.8605 0.8580 2.42E 06 0.8605 0.8580 2.42E 06 1.14 0.8605 0.8580 2.42E 06 0.8605 0.8580 2.42E 06 2.41 0.8605 0.8580 2.42E 06 0.8605 0.8580 2.42E 06 5.45 0.8604 0.8580 2.42E 06 0.8604 0.8580 2.42E 06 11.14 0.8604 0.8579 2.42E 06 0.8604 0.8580 2.42E 06 24.81 0.8604 0.8579 2.41E 06 0.8604 0.8579 2.41E 06 50.44 0.8604 0.8579 2.41E 06 0.8604 0.8579 2.41E 06 111.95 0 .8603 0.8579 2.41E 06 0.8603 0.8579 2.41E 06 197.70 0.8603 0.8579 2.41E 06 0.8603 0.8579 2.41E 06 356.16 0.8603 0.8579 2.41E 06 0.8604 0.8579 2.41E 06 593.84 0.8604 0.8580 2.42E 06 0.8604 0.8580 2.42E 06 752.30 0.8606 0.8582 2 .42E 06 0.8606 0.8582 2.42E 06 838.05 0.8609 0.8585 2.44E 06 0.8609 0.8585 2.44E 06 899.56 0.8614 0.8589 2.45E 06 0.8614 0.8589 2.45E 06 925.19 0.8618 0.8593 2.47E 06 0.8618 0.8593 2.47E 06 938.86 0.8619 0.8594 2.48E 06 0.8619 0.8594 2.48E 06 944.55 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 947.59 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 948.86 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 949.53 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 0 6 949.81 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 949.96 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 950.04 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 950.19 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 950.47 0.862 0 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 951.14 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 952.41 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 955.45 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 959.88 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 968.52 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 981.48 0.8619 0.8594 2.47E 06 0.8619 0.8594 2.47E 06 990.12 0.8618 0.8593 2.47E 06 0.8618 0.8593 2.47E 06 994.55 0.8618 0.8593 2.47E 06 0.8618 0.8 593 2.47E 06 997.59 0.8617 0.8592 2.47E 06 0.8618 0.8592 2.47E 06 998.86 0.8617 0.8592 2.47E 06 0.8617 0.8592 2.47E 06 999.53 0.8617 0.8592 2.47E 06 0.8617 0.8592 2.47E 06

PAGE 77

77 Table A 3. Continued Pipe 1 Pipe 2 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) On Potential (V) Off Potential (V) Current Density (A/sq m) 999.81 0.8617 0.8592 2.47E 06 0.8617 0.8592 2.47E 06 999.96 0.8617 0.8592 2.47E 06 0.8617 0.8592 2.47E 06 1000.04 0 .8617 0.8592 2.47E 06 0.8617 0.8592 2.47E 06 1000.19 0.8617 0.8592 2.47E 06 0.8617 0.8592 2.47E 06 1000.47 0.8617 0.8592 2.47E 06 0.8617 0.8592 2.47E 06 1001.14 0.8617 0.8592 2.47E 06 0.8617 0.8592 2.47E 06 1002.41 0.8617 0.859 2 2.47E 06 0.8618 0.8592 2.47E 06 1005.45 0.8618 0.8593 2.47E 06 0.8618 0.8593 2.47E 06 1009.88 0.8618 0.8593 2.47E 06 0.8618 0.8593 2.47E 06 1018.52 0.8619 0.8594 2.47E 06 0.8619 0.8594 2.47E 06 1031.48 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1040.12 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1044.55 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1047.59 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1048.86 0.8620 0.8595 2.48E 06 0.8620 0.8 595 2.48E 06 1049.53 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1049.81 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1049.96 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1050.04 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 0 6 1050.19 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1050.47 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1051.14 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1052.41 0.8620 0.8595 2.48E 06 0.8620 0.8595 2.48E 06 1061.14 0.8619 0.8594 2.48E 06 0.8619 0.8594 2.48E 06 1074.81 0.8618 0.8593 2.47E 06 0.8618 0.8593 2.47E 06 1100.44 0.8614 0.8589 2.45E 06 0.8614 0.8589 2.45E 06 1161.95 0.8609 0.8585 2.44E 06 0.8609 0.8585 2.44E 06 1247.70 0.8606 0.85 82 2.42E 06 0.8606 0.8582 2.42E 06 1406.16 0.8604 0.8580 2.42E 06 0.8604 0.8580 2.42E 06 1643.84 0.8603 0.8579 2.41E 06 0.8604 0.8579 2.41E 06 1802.30 0.8603 0.8579 2.41E 06 0.8603 0.8579 2.41E 06 1888.05 0.8603 0.8579 2.41E 06 0.8603 0.8579 2.41E 06 1949.56 0.8604 0.8579 2.41E 06 0.8604 0.8579 2.41E 06 1975.19 0.8604 0.8579 2.41E 06 0.8604 0.8579 2.41E 06 1988.86 0.8604 0.8579 2.42E 06 0.8604 0.8580 2.42E 06 1994.55 0.8604 0.8580 2.42E 06 0.8604 0. 8580 2.42E 06 1997.59 0.8605 0.8580 2.42E 06 0.8605 0.8580 2.42E 06 1998.86 0.8605 0.8580 2.42E 06 0.8605 0.8580 2.42E 06

PAGE 78

78 Table A 3. Continued Pipe 1 Pipe 2 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) On Potential (V) Off Potential (V) Current Density (A/sq m) 1999.53 0.8605 0.8580 2.42E 06 0.8605 0.8580 2.42E 06 1999.81 0.8605 0.8580 2.42E 06 0.8605 0.8580 2.42E 06 1999.96 0.8605 0.8581 2.42E 06 0.8605 0.8581 2.42E 06 200 0.00 0.8605 0.8581 2.42E 06 0.8605 0.8581 2.42E 06

PAGE 79

79 Table A 4 Potential and Current Distributions on Pipe1 Pipe 1 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) 0.00 0.8409 0.8391 1.77E 06 0.04 0.84 09 0.8391 1.77E 06 0.19 0.8409 0.8391 1.77E 06 0.47 0.8409 0.8391 1.77E 06 1.14 0.8408 0.8391 1.77E 06 2.41 0.8408 0.8390 1.77E 06 5.45 0.8408 0.8390 1.77E 06 11.14 0.8408 0.8390 1.77E 06 24.81 0.8408 0.8390 1.77E 06 50.44 0 .8408 0.8390 1.76E 06 111.95 0.8407 0.8390 1.76E 06 363.97 0.8408 0.8390 1.76E 06 611.03 0.8408 0.8391 1.77E 06 775.74 0.8410 0.8392 1.77E 06 863.05 0.8414 0.8396 1.78E 06 924.56 0.8415 0.8397 1.79E 06 950.19 0.8413 0.8395 1.7 8E 06 963.86 0.8411 0.8393 1.77E 06 969.55 0.8409 0.8391 1.77E 06 972.59 0.8408 0.8390 1.77E 06 973.86 0.8408 0.8390 1.77E 06 974.53 0.8407 0.8390 1.76E 06 974.81 0.8407 0.8389 1.76E 06 974.96 0.8407 0.8389 1.76E 06 975.04 0.8 407 0.8389 1.76E 06 975.19 0.8407 0.8389 1.76E 06 975.47 0.8407 0.8389 1.76E 06 976.14 0.8407 0.8389 1.76E 06 977.41 0.8406 0.8388 1.76E 06 980.45 0.8404 0.8386 1.76E 06 985.11 0.8400 0.8383 1.74E 06 989.89 0.8392 0.8374 1.72E 06 994.55 0.8372 0.8356 1.67E 06 997.59 0.8333 0.8317 1.57E 06 998.86 0.8274 0.8260 1.43E 06 999.53 0.8181 0.8168 1.24E 06 1000.97 0.8180 0.8168 1.23E 06 1001.64 0.8275 0.8260 1.43E 06 1002.91 0.8333 0.8317 1.57E 06 1005.95 0 .8373 0.8356 1.67E 06 1010.71 0.8393 0.8375 1.72E 06

PAGE 80

80 Table A 4. C ontinued Pipe 1 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) 1020.63 0.8405 0.8387 1.76E 06 1030.56 0.8410 0.8392 1.77E 06 1040.48 0.8412 0.8394 1.78E 06 1055.37 0.8414 0.8396 1.78E 06 1065.29 0.8415 0.8397 1.79E 06 1070.05 0.8415 0.8397 1.79E 06 1073.09 0.8415 0.8397 1.79E 06 1074.36 0.8415 0.8397 1.79E 06 1075.03 0.8415 0 .8397 1.79E 06 1075.31 0.8415 0.8397 1.79E 06 1075.46 0.8415 0.8397 1.79E 06 1075.54 0.8415 0.8397 1.79E 06 1075.69 0.8415 0.8397 1.79E 06 1075.97 0.8415 0.8397 1.79E 06 1076.64 0.8415 0.8397 1.79E 06 1077.91 0.8415 0.8397 1.79 E 06 1080.95 0.8415 0.8397 1.79E 06 1086.64 0.8416 0.8397 1.79E 06 1100.31 0.8415 0.8397 1.79E 06 1125.94 0.8414 0.8396 1.78E 06 1187.45 0.8412 0.8394 1.78E 06 1271.63 0.8410 0.8392 1.77E 06 1423.84 0.8408 0.8390 1.77E 06 1652. 16 0.8408 0.8390 1.77E 06 1804.37 0.8407 0.8389 1.77E 06 1888.55 0.8407 0.8390 1.77E 06 1950.06 0.8408 0.8390 1.77E 06 1975.69 0.8408 0.8390 1.77E 06 1989.36 0.8408 0.8390 1.77E 06 1995.05 0.8408 0.8390 1.77E 06 1998.09 0.8408 0.8390 1.77E 06 1999.36 0.8408 0.8391 1.77E 06 2000.03 0.8409 0.8391 1.77E 06 2000.31 0.8409 0.8391 1.77E 06 2000.46 0.8409 0.8391 1.77E 06 2000.50 0.8409 0.8391 1.77E 06

PAGE 81

81 Table A 5 Potential and Current Distribution on Pipe2 Pipe 2 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) 0.00 0.8607 0.8582 2.43E 06 0.04 0.8607 0.8582 2.43E 06 0.19 0.8607 0.8582 2.43E 06 0.47 0.8607 0.8582 2.43E 06 1.14 0.8607 0.8582 2.43E 06 2. 41 0.8606 0.8582 2.42E 06 5.45 0.8606 0.8581 2.42E 06 11.14 0.8606 0.8581 2.42E 06 24.81 0.8606 0.8581 2.42E 06 50.44 0.8605 0.8581 2.42E 06 111.95 0.8605 0.8581 2.42E 06 196.13 0.8605 0.8580 2.42E 06 348.34 0.8605 0.8581 2.4 2E 06 576.66 0.8606 0.8581 2.42E 06 728.87 0.8607 0.8583 2.43E 06 813.05 0.8609 0.8585 2.44E 06 874.56 0.8612 0.8587 2.45E 06 913.86 0.8613 0.8588 2.45E 06 919.55 0.8613 0.8588 2.45E 06 922.59 0.8613 0.8588 2.45E 06 923.86 0.8 613 0.8588 2.45E 06 924.53 0.8613 0.8588 2.45E 06 924.81 0.8613 0.8588 2.45E 06 924.96 0.8613 0.8588 2.45E 06 925.04 0.8613 0.8588 2.45E 06 925.19 0.8613 0.8588 2.45E 06 925.47 0.8613 0.8588 2.45E 06 926.14 0.8613 0.8588 2.45E 06 927.41 0.8613 0.8588 2.45E 06 930.45 0.8613 0.8588 2.45E 06 935.21 0.8612 0.8588 2.45E 06 945.13 0.8612 0.8587 2.45E 06 955.06 0.8611 0.8586 2.44E 06 964.98 0.8608 0.8584 2.43E 06 979.87 0.8603 0.8578 2.41E 06 989.79 0.859 1 0.8567 2.37E 06 994.55 0.8570 0.8547 2.29E 06 997.59 0.8531 0.8510 2.15E 06 998.86 0.8475 0.8455 1.97E 06 999.53 0.8424 0.8406 1.81E 06 999.81 0.8396 0.8378 1.73E 06

PAGE 82

82 Table A 5. Continued Pipe 2 Distance along the pipe (m) On Pot ential (V) Off Potential (V) Current Density (A/sq m) 999.96 0.8388 0.8371 1.71E 06 1000.04 0.8388 0.8371 1.71E 06 1000.19 0.8396 0.8378 1.73E 06 1000.47 0.8424 0.8406 1.81E 06 1001.14 0.8475 0.8455 1.97E 06 1002.41 0.8531 0.8510 2. 15E 06 1005.45 0.8570 0.8547 2.29E 06 1010.21 0.8591 0.8567 2.37E 06 1020.13 0.8603 0.8578 2.41E 06 1030.06 0.8608 0.8583 2.43E 06 1039.98 0.8610 0.8585 2.44E 06 1054.87 0.8612 0.8587 2.45E 06 1064.79 0.8613 0.8588 2.45E 06 107 2.59 0.8613 0.8588 2.45E 06 1073.86 0.8613 0.8588 2.45E 06 1074.53 0.8613 0.8588 2.45E 06 1074.81 0.8613 0.8588 2.45E 06 1075.04 0.8613 0.8588 2.45E 06 1075.19 0.8613 0.8588 2.45E 06 1075.47 0.8613 0.8588 2.45E 06 1076.14 0.861 3 0.8588 2.45E 06 1077.41 0.8613 0.8588 2.45E 06 1080.45 0.8613 0.8588 2.45E 06 1086.14 0.8613 0.8589 2.45E 06 1099.81 0.8613 0.8588 2.45E 06 1125.44 0.8612 0.8587 2.45E 06 1186.95 0.8610 0.8585 2.44E 06 1271.13 0.8607 0.8583 2.43E 06 1423.34 0.8606 0.8581 2.42E 06 1651.66 0.8605 0.8581 2.42E 06 1803.87 0.8605 0.8580 2.42E 06 1888.05 0.8605 0.8581 2.42E 06 1949.56 0.8605 0.8581 2.42E 06 1975.19 0.8606 0.8581 2.42E 06 1988.86 0.8606 0.8581 2.42E 06 1 994.55 0.8606 0.8581 2.42E 06 1997.59 0.8606 0.8582 2.42E 06 1998.86 0.8607 0.8582 2.43E 06 1999.53 0.8607 0.8582 2.43E 06

PAGE 83

83 Table A 5. Continued Pipe 2 Distance along the pipe (m) On Potential (V) Off Potential (V) Current Density (A/sq m) 1999.81 0.8607 0.8582 2.43E 06 1999.96 0.8607 0.8582 2.43E 06 2000.00 0.8607 0.8582 2.43E 06

PAGE 84

84 L IST OF REFERENCES [1] D.P. Riemer, Modeling cathodic protection for pipeline networks, University of Florida, Gainesville, 2000. [2] Pipeline Accident Brief: Blumborough Pennsylvania, National Transportation Safety Board, Washington, D.C., 2008. [3] P.R. Roberge, Handbook of Corrosion Engineering, McGraw Hill, 2000. [4] G.H. Koch, M.P.H. Brongers, N.G. Thompson, Y.P. Virmani, J.H. Payer, Corrosion Cost and Preventive Strategies in the United States, NACE International, CC Technologies Laboratory, United States, 2001. [5] D.P. Riemer, M.E. Orazem, A mathematical model for the cathodic protection of tank bottoms, Corros. Sci. 47 (20 05) 849 868. [6] E. Santana Diaz, R. Adey, Optimising the location of anodes in cathodic protection systems to smooth potential distribution, Adv. Eng. Software. 36 (2005) 591 598. [7] E.1. Bardal, I. NetLibrary, Corrosion and protection, Sp ringer, London ; New York, 2004. [8] J.O. Bockris, M. Gamboa Aldeco, M.E. Gamboa Aldeco, I. NetLibrary, A.K.N. Reddy, Modern electrochemistry. Volume 2A, Fundamentals of electrodics, 2nd edition., Kluwer Academic, New York, 2002. [9] J. Morgan, Cathodic Protection, 2nd edition, NACE International, Houston, Texas, 1993. [10] J.S. Newman, Electrochemical Engineering, 2nd edition, Prentice Hall, Englewood Cliffs, New Jersey, 1991. [11] M.E. Orazem, J.M. Esteban, Phase II Final Report: A Three Dimensional M odel for the Cathodic Protection of an Underground Pipeline, Technical Report, University of Florida, 1994. [12] K. Kennelley, L. Bone, M. Orazem, Current and Potential Distribution on a Coated Pipline with Holidays Part I Model and Experimental Verific ation, Corrosion. 49 (1993) 199 210. [13] D.A. Jones, Principles and Preventions of Corrosion, Prentice Hall, Upper Saddle River, New Jersey, 1996. [14] M. Orazem, J. Esteban, K. Kenelley, R. Degerstedt, Mathematical Model for Cathodic Protection of an U nderground Pipeline with Coating Holidays: Part 1 Theoritical Development, Corrosion. 53 (1997) 264 272.

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85 BIOGRAPHICAL SKETCH Alok Shankar completed his undergraduate studies in chemical engineering from National Institute of Technology, Surathkal, Indi a in 2010. He then enrolled at the University of Florida, Gainesville for the Master of Science program in chemical engineering in August 2010. He received his Master of Science degree in May 2012.