Mechanisms and inhibition of dealloying in an alpha brass

MECHANISMS AND INHIBITION OF DEALLOYING
IN AN ALPHA BRASS







By


WILLIAM CLARENCE FORT, III


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



UNIVERSITY OF FLORIDA


1975




























Dedicated to my wife, Deborah.












ACKNOWLEDGEMENTS


I would like to express my appreciation to the

chairman of my supervisory committee, Dr. Ellis D. Verink,

Jr., for his encouragement and inspiration throughout the

course of my studies and research. Thanks are extended to

Dr. R. T. DeHoff, Dr. R. W. Gould, and Dr. G. M. Schmid

for serving as members of the supervisory committee, and to

Dr. C. M. Chen for his many helpful suggestions.

I wish to acknowledge the financial assistance made

available by the College of Engineering of the University of

Florida, the NDEA Title IV fellowship Program, the Office of

Naval Research, and the International Nickel Company. In

addition, certain of the equipment used in these investiga-

tions was purchased with funds from the Office of Saline

Water.

I also extend special appreciation to Mr. P. M.

Russell, Mr. M. H. Froning, Mr. J. T. Healey, and Dr. R. G.

Connell for their unselfish assistance in the final moments.

I am indebted to Mr. W. A. Acree, Mr. E. J. Jenkins, Mr.E.C.

Logsdon, Mr. C. J. Minier, and Mr. C. Simmons for their

expert technical assistance during the course of the inves-

tigation.,










TABLE OF CONTENTS


Page


ACKNOWLEDGEMENTS .

LIST OF TABLES . .

LIST OF FIGURES. .


iii


. vi

. vii

. xiii


ABSTRACT


CHAPTER 1 INTRODUCTION .

CHAPTER 2 LITERATURE REVIEW .


2.1. Selective Leaching Mechanism ... .. .. 5
2.2. Dissolution and Replating Mechanism .... 20
2.3. The Role of Arsenic as an Inhibitor of
Dezincification. . 29

CHAPTER 3 EXPERIMENTAL PROCEDURE ... .34

3.1. Potentiokinetic Polarization Experiments 34
3.2. Selective Leaching Experiments ... 37
3.3. Controlled Anodic Dissolution Experiments 38
3.4. Artificial Occluded Cell Experiments 39
3.5. Auger Electron Spectroscopic Analyses 42
3.6. X-Ray Identification of Corrosion Products 51


CHAPTER 4 RESULTS AND DISCUSSION . .

4.1. Electrochemical Characterization of
Copper, Zinc, and Cu30Zn . .
4.2. Dealloying of Cu30Zn by Selective Leaching
4.3. Dealloying of Cu30Zn by Dissolution and
Replating . . .
4.4. Effect of Arsenic Additions on Dealloying
of Cu3OZn . . .

CHAPTER 5 CONCLUSIONS . .

CHAPTER 6 RECOMMENDATIONS FOR FURTHER RESEARCH .


S52


S52
66

. 103

S150

. 165

S169











TABLE OF CONTENTS (Continued)


Page


APPENDICES

1

2

3

4

5

6

7


SAMPLE CHARACTERIZATION AND PREPARATION. .

SOLUTION CHARACTERIZATION AND PREPARATION.

STANDARD POLARIZATION CELL .

LIST OF EQUIPMENT . .

AUGER ELECTRON SPECTROSCOPY .

CALIBRATION OF THE ARGON SPUTTERING RATE

FREE ENERGY DATA USED IN CALCULATED EQUI-
LIBRIA . . .


BIBLIOGRAPHY . .

BIOGRAPHICAL SKETCH .


S171

S174

S175

178

179

S185


S187

S188

S193














LIST OF TABLES


Table

1 Conditions of Exposure for the Cu30Zn
Selective Leaching Samples .

2 Rate of Zinc Dissolution from Cu30Zn
Under Selective Leaching Conditions .

3 Thicknesses of the Zinc-Depleted Surface
Layers . . .

4 Summary of Diffusion Analysis Data .

5 Plateau Values of Dissolved Copper Con-
centrations Measured in Controlled Anodic
Dissolution Experiments .


Page


. 67


. 72


. 93

. 99



.132













LIST OF FIGURES


Figure Page

1 Superposition of calculated equilibria for
the Cu-O.1M C1--H20 and Zn-H20 systems and
experimental potential versus pH diagram
for Cu30Zn alloy in 0.1M chloride solutions,
after Verink and Heidersbach (18) 7

2 Kink-step-terrace model of a dissolving
alloy surface, after Pickering and Wagner. 10

3 Schematic of the equipment set-up for
generation of potentiokinetic polarization
curves . . 35

4 Artificial occluded cell electrode assembly 40

5 Schematic of the electrical circuit for
the artificial occluded cell experiments .. .43

6 Schematic of the AES system used to analyze
selective leaching samples . .45

7 Typical Auger spectrum from the surface of
a Cu3OZn sample .. . 46

8 Effect of the composition of Cu-Zn alloys
on the Cu/Zn ratio obtained by AES analysis. 49

9 Auger composition profile obtained for
Cu-15 w/o alloy . 50

10 Experimental potential versus pH diagram
for pure copper in nitrogen saturated,
0.1M chloride solutions .. 54

11 Superposition of polarization data for pure
copper in 0.1M chloride solutions and the
equilibrium potential versus pH diagram
calculated for the system Cu-O.lM C1'-H20
at 250C, after Van Muylder et al. (71) 57


vii










LIST OF FIGURES (Continued)


Figure Page

12 Experimental potential versus pH diagram
for pure zinc in nitrogen-saturated 0.1M
chloride solutions . ... 59

13 Superposition of polarization data for
pure zinc in 0.1M chloride solutions and
the equilibrium potential versus pH dia-
gram calculated for the system Zn-H20 at
250C, after Pourbaix (72) . 61

14 Experimental potential versus pH diagram
for Cu30Zn in nitrogen-saturated 0.1M chlo-
ride solutions . ... 62

15 Variation of the corrosion potential of
Cu30Zn with time of exposure in 0.1M
chloride solutions of pH 2 and pH 4. 64

16 Zinc dissolved from Cu30Zn samples poten-
tiostated at -0.700 VSCE in 890C, 0.1M
chloride solutions of pH 4 . 70

17 Zinc dissolved from Cu30Zn samplespoten-
tiostated at electrode potentials between
-0.500 and -0.900 VSCE at 890C . 71

18 Pitting of Cu30Zn sample exposed 4 days
at -0.450 VSCE. 3700X . 75

19 Typical surface of Cu30Zn samples poten-
tiostated below -0.450 VSCE. From a
Cu30Zn sample exposed 4 days at -0.600
VSCE. 3500X . ... 76

20 Auger composition profile obtained from
an unexposed Cu30Zn sample . 78

21a Auger composition profile obtained from
a Cu3OZn sample potentiostated at -0.450
VSCE for 1 day . 80

21b Auger composition profile obtained from
a Cu30Zn sample potentiostated at -0.450
VSCE for 4days . ... 81


viii










LIST OF FIGURES (Continued)


Figure

21c Auger composition profile obtained from
a Cu30Zn sample potentiostated at -0.450
VSCE for 7 days . .

22 Auger composition profile obtained from
a Cu3OZn sample potentiostated at -0.500
VSCE for 4 days . .

23 Auger composition profile obtained from
a Cu3OZn sample potentiostated at -0.600
VSCE for 4 days . .

24a Auger composition profile obtained from
a Cu3OZn sample potentiostated at -0.700
VSCE for 2 days . .

24b Auger composition profile obtained from
a Cu3OZn sample potentiostated at -0.700
VSCE for 4 days . .

24c Auger composition profile obtained from
a Cu3OZn sample potentiostated at -0.700
VSCE for 7 days . .
24d Auger composition profile obtained from
a Cu3OZn sample potentiostated at -0.700
VSCE for 10 days . .

25 Auger composition profile obtained from
a Cu30Zn sample potentiostated at -0.800
VSCE for 4 days . .

26 Auger composition profile obtained from
a Cu30Zn sample potentiostated at -0.900
VSCE for 4 days . .

27 Infinite diffusion model used in zinc
diffusion analysis . .

28 Log-log plot of the zinc diffusion data
taken from composition profiles obtained
by AES from Cu30Zn samples selectively
leached at -0.700 VSCE . .


Page


. 82



. 84



85


. 86



. 87



. 88


. 90



. 91


S. 95


100









LIST OF FIGURES (Continued)


Page


Figure


29 Simplified potential versus pH diagram
for Cu30Zn in 0.1M chloride solutions
outlining conditions of exposure for
controlled anodic dissolution experi-
ments . .

30 Etched surface of a Cu30Zn sample
potentiostated 13 days at -0.200 VSCE
in pH 4, 0.1M chloride solution.
1500X . .

31 Curves showing the concentrations of
copper and zinc present in solution
during dissolution of Cu30Zn at
-0.200 VSCE. . .

32a Pitting observed on etched Cu30Zn sam-
ple exposed 10 days at -0.100 VSCE.
Two copper crystals formed on the sur-
face are marked by arrows. 125X .

32b Detail of one of the crystals found on
the sample described in figure 32a. The
tetrahedral shape is outlined in the
accompanying sketch. 300X .


32c Cross section of one of the copper
crystals detailing the Cu30Zn grain
boundary artifacts visible in the de-
zincified structure. 75X .

33 Concentrations of copper and zinc mea-
sured in the solution of a cell contain-
ing a Cu30Zn sample polarized 10 days
at -0.100 VSCE . .

34 Dezincification noted on a Cu30Zn sam-
ple exposed 3 days at 0.000 VSCE. 100OX

35 Dezincification noted on a Cu30Zn sam-
ple exposed 13 days at 0.000 VSCE. 60X

36 Results of solution analysis for Cu30Zn
samples potentiostated at 0.000 VSCE
for 7 days and 13 days . .


. 104




. 107




. 108




. 111


112


. 113




. 115


117


. 118



119









LIST OF FIGURES (Continued)


Figure

37 Dezincification noted on a Cu30Zn sample
exposed 10 days at +0.100 VSCE. 100X .

38 Results of solution analysis for a Cu30Zn
sample potentiostated 10 days at
+0.100 VSCE. . .

39 Detail of sample surface of a Cu30Zn sam-
ple exposed 13 days at +0.200 VSCE, cov-
ered by a layer of cuprous chloride. 1500X

40 Results of solution analysis for a Cu30Zn
sample potentiostated 13 days at
+0.200 VSCE . .

41 Results of solution analysis for a Cu3OZn
sample potentiostated 10 days at
+0.500 VSCE . .

42 Extent of dezincification noted on a
Cu30Zn sample exposed 10 days at
+0.500 VSCE. 60X . .

43 Equilibria calculated for the Cu-C1--H20
system at pH = 4 showing agreement between
equilibrium calculations and constant cop-
per concentrations observed in the control-
led anodic dissolution experiments .

44 Cu30Zn sample potentiostated 4 days at
+0.500 VSCE, showing separation between
the brass and the copper sponge structure.
250X . . .

45 Results of artificial occluded cell experi-
ments showing occluded cell potential and
occluded cell current measured as a func-
tion of time . .

46 Results of solution analysis for arsenical
Cu30Zn potentiostated at -0.200 VE in
pH 4, 0.1M chloride solution .


Page


. 122



123



. 125


. 126



. 127





. 133




. 138




. 141



. 154








LIST OF FIGURES (Continued)


Figure

47 Results of solution analysis for arseni-
cal Cu30Zn potentiostated at -0.100 VSCE
in pH 4, 0.1M chloride solution .

48 Results of solution analysis for arseni-
cal Cu3OZn potentiostated at -0.000 VSCE
in pH 4, 0.1M chloride solution .

49 Results of solution analysis for arseni-
cal Cu30Zn potentiostated at +0.100 VSCE
in pH 4, 0.1M chloride solution .

50 Results of solution analysis for arseni-
cal Cu30Zn potentiostated at +0.200 VSCE
in pH 4, 0.1M chloride solution .

51 Results of solution analysis of arseni-
cal Cu30Zn potentiostated at +0.500 VSCE
in pH 4, 0.1M chloride solution .

52 Standard polarization cell .

53 Energy level diagram depicting a KLM Auger
transition . .

54 Schematic energy spectrum and experimental
AES curve for silver excited in a 1000 eV
electron beam . .

55 Composition profile of a 102625A copper
film deposited on a zinc substrate, used
to calibrate the argon sputtering rate


Page


. 157


158


. 159


161


. 176


. 180



S 182


. 186


xii









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


MECHANISMS AND INHIBITION OF DEALLOYING
IN AN ALPHA BRASS


By

William Clarence Fort, III

July, 1975


Chairman: Dr. Ellis D. Verink, Jr.
Major Department: Materials Science and Engineering


Dealloying of a single-phase binary alpha brass in

pH 4, 0.1M chloride solution was investigated in terms of

the operative mechanism. Selective leaching of zinc from

the brass was produced under conditions of cathodic polari-

zation. Auger electron spectroscopy was employed to mea-

sure composition versus depth profiles of the resulting

zinc-depleted surface layers. The selective leaching mecha-

nism was found to be based on enhanced volume diffusion of

zinc to the brass surface. Dealloying by dissolution of the

brass as a whole, followed by redeposition of the copper,

was found to occur exclusively under occluded cell conditions.

The mechanism was examined in terms of the copper-chloride

equilibria established at the dissolving brass surface. The

redeposition of copper was found to proceed by reduction of

cuprous chloride near the brass surface. The inhibitive ef-

fect of arsenic was determined to be the prevention of the

cuprous chloride reduction reaction.


xiii












CHAPTER 1


INTRODUCTION



Since the initial description of the dealloying phe-

nomenon in brasses more than 100 years ago (1), the metals

literature has witnessed an ongoing debate as to the mecha-

nism(s) responsible. Notwithstanding that rather early in

the century the practical problem of dezincification of

alpha brasses was solved by alloying with arsenic (2), a

considerable amount of research effort is even now being

directed towards understanding the phenomena involved.

This can be at least partly attributed to ongoing

discoveries of dealloying behavior in a host of diverse

alloy systems (3). In addition, several reports in more re-

cent years have linked dealloying behavior with stress cor-

rosion cracking in several systems of interest (4-8). It

remains the goal of researchers in this area of investigation

to apply a basic understanding of the mechanisms of dealloy-

ing in brasses to solve the dealloying problems encountered

in other systems, and to predict when dealloying might be

expected in new applications.









Two principal mechanisms for dealloying* of single

phase binary alloys in aqueous environments have gained wide

acceptance:

(1) Selective leaching of the less noble component of the

alloy from the metal matrix into solution without cor-

responding dissolution of the more noble component; and

(2) Total dissolution of the alloy (both components enter-

ing solution at rates proportional to their respective

concentrations in the alloy) followed by redeposition

of the more noble component metal on or adjacent to the

dissolving alloy surface.

Almost without exception, one or the other of these

mechanisms has been assigned to each reported dealloying

occurrence. Efforts to resolve the nature of the mechanism

operating for any one system have resulted, for the most

part, in confusing and often contradictory conclusions.

It is the goal of this research to clarify the mecha-

nisms responsible for dealloying in a defined system -

Cu-30 w/o(weight percent) Zn [hereafter designated as Cu30Zn]

exposed in aqueous solutions containing 0.1 molar of chloride

ion. The choice of this system is based on several factors.

Cu30Zn has been and remains the most important binary brass

alloy. For this reason, if none other, it has also received

the majority of research attention. Consequently, a large


*
Dealloying is defined as the corrosion process whereby one
constituent of an alloy is preferentially removed from an
alloy, leaving an altered residual structure (3).








volume of literature concerning its corrosion and physical

properties is available. The fact that it is a single phase

alloy eliminates complications which might otherwise result

from galvanic interactions between dissimilar phases. The

possibility of phase changes under corrosion conditions is

likewise eliminated.

Further, a large proportion of the dezincification

failures reported have occurred in saline environments. Thus,

the role of the chloride ion in causing or in enhancing de-

zincification cannot be overlooked.

This investigation is divided intofour main sections:

(1) Definition of the polarization behavior of the system

in order to establish a firm basis for predicting the

conditions which lead to dealloying, and the effects of

these conditions on the mechanism;

(2) Elucidation of the detailed mechanism involved in se-

lective leaching of zinc from the brass matrix through

use of surface sensitive techniques;

(3) Clarification of the chemical and electrochemical equi-

libria which govern dezincification of alpha brass in

chloride-containing environments; and

(4) Clarification of the role played by arsenic additions

to alpha brass in inhibiting dezincification.

Special attention is payed to the role of crevice

or occluded cell phenomena in the initiation and propaga-

tion of the attack.












CHAPTER 2


LITERATURE REVIEW



Investigations of dealloying mechanisms have been

limited almost entirely to the binary copper-zinc and copper-

gold systems. The research popularity of non-commercial

copper-gold alloys is due to the following factors: (1) the

system is isomorphous, hence phase changes with accompanying

compositional changes need not be considered; (2) gold ex-

hibits a very noble single electrode potential, and does not

dissolve under ordinary circumstances (the dissolving compo-

nent in this case is copper); (3) there is little overlap

in the X-ray spectra of copper and gold; and (4) the melt-

ing points of copper and gold are similar, and alloying is

relatively simple.

Dealuminification of the commercially important alu-

minum bronzes has received only limited attention regarding

mechanisms, primarily because the problem has been remedied

through slight compositional adjustments and improved heat

treatment procedures (9-11). The literature concerning de-

alloying in other systems is almost entirely limited to re-

ports of in-service failures. Several excellent reviews

covering the entire dealloying spectrum are available (3,

12-15).





5


2.1. Selective Leaching Mechanism


The theoretical aspects of dealloying by a selective

leaching mechanism have been described most eloquently by

Pickering and Wagner (16). They state that preferential re-

moval of the less noble component of a binary alloy under-

going anodic dissolution requires that: (1) there be a

sufficiently large difference (i.e., several RT/F) between

the single electrode potentials of the two constituent metals;

and (2) the electrode potential of the alloy be higher than

that of the less noble component and significantly lower than

that of the more noble component. Such is the case for Cu-

Au alloys and beta brass. The electrode potential of alpha

brass, however, is nearly the same as that of its more noble

component, copper (17).

The important condition here is that, at a given

alloy electrode potential, there be a much greater tendency

for the more active component atoms to dissolve than for the

noble component to dissolve, based on the respective single

electrode potentials of the component metals. Thus, alpha

brass polarized several RT/F below the single electrode po-

tential for copper (i.e., cathodically) meets this require-

ment.

Verink and Hei'dersbach (18) have proposed that the

electrochemical conditions outlined above as necessary for

dealloying by a selective leaching mechanism can be rather








simply predicted by comparing the calculated and experimen-

tal potential versus pH diagrams for the system of interest.

Thus, referring to figure 1, selective leaching of zinc from

a Cu30Zn alloy exposed in neutral and acidic 0.1M chloride

solutions free of copper ions would occur at potentials more

noble than the calculated Zn/Zn+(10-6M) equilibrium and

less noble than the experimentally determined corrosion po-

tentials for Cu30Zn (area of small dots).

The authors tested this hypothesis by exposing

Cu30Zn samples in buffered 0.1M chloride solutions of pH 4

for up to 13 days at several potentials in the predicted

range (19). They report that zinc is dissolved into the

electrolyte with no discernable accompanying copper disso-

lution. However, the formation of a dezincified layer was

not confirmed although tarnishing was observed. The iden-

tity of tarnish layer could not be determined by any of the

then currently available techniques.

The results of Verink and Heidersbach are consistent

with earlier investigations of selective dissolution of Cu-

Zn and Cu-Au alloys. Pickering and Byrne (20) measured par-

tial currents (by solution analysis) for the dissolution of

copper and zinc from alpha, gamma, and epsilon brass speci-

mens held at potentials between -1.000 and +0.500 VSHE in

acetate buffered (pH = 5) 1N Na2SO4 alloy where only zinc

was dissolved in detectable quantities. At potentials more

noble than certain critical potentials, simultaneous













































3 4 5 6 7 8 9 10 II 12 13


Figure 1.


Superposition of calculated equilibria for the
Cu-O.lM C1--HzO and Zn-H20 systems and experi-
mental potential versus pH diagram for Cu30Zn
alloy in 0.1M chloride solutions, after Verink
and Heidersbach (18).


-1.4








dissolution of the alloys occurred.

Similar results were reported by the same authors

for Cu-13a/o (atomic percent) Au and Cu-18a/o Au alloys

potentiostated between -0.350 and +0.875 VSHE in IN Na2SO4-

0.01N H2SO4 solutions (21). In this case, however, simul-

taneous dissolution does not occur above the critical po-

tentials. Instead, severe roughening of the alloy surface

was noted. These results are in substantial agreement with

those of earlier investigators (22) for the same system.

Ionization of the less noble component results in

rapid accumulation of the other components' atoms on the

alloy surface. If selective dissolution is to continue,

some means must be established to provide the atoms of the

more active component access to the liquid-solid interface.

The two likely possibilities are

(1) Aggregation of the more noble component atoms by sur-

face diffusion, exposing new alloy surface containing

active atoms; and

(2) Transport by volume diffusion of active metal atoms

to the liquid-solid interface.



Simultaneous dissolution is defined by both alloy components
entering solution at rates proportional to their respective
concentrations in the alloy.
tImplicit in this argument is that the ionization of the ac-
tive atoms at the liquid-solid interface and their conse-
quent diffusion into the bulk electrolyte is fast and not
rate-controlling. This assumption has been recently dis-
cussed in detail by Rubin (17).








Pickering and Wagner (16) favor the latter possibi-

lity and have proposed a divacancy diffusion mechanism which

counters the classical argument against such a concept (i.e.,

the sluggishness of transport by simple bulk diffusion at

ambient temperatures). The mechanism is based on a kink-

step-terrace model of the surface of the dissolving metal

alloy A-B, figure 2.

Ionization of the less noble component atom A at

kink position 1 proceeds by its moving along the step and

onto the terrace as an adsorbed atom, from where it passes

into solution as an ion. The more noble component atom B

initially at step position 2 may then move along the step,

and become adsorbed, but not ionized, or it may desorb. As

this process continues, the concentration of adsorbed B

atom increases, thereby increasing the probability that ad-

sorbed B atomsdesorb to kink sites, and consequently de-

creasing the probability that active A atoms occupy kink

sites. This leads to increased electrochemical polarization

and increased driving force for ionization of A atoms. The

increased driving force makes possible direct adsorption of

A atoms (position 6) from step sites and, eventually, direct

adsorption from terrace sites. The step and surface vacan-

cies resulting from such events may in some cases be filled

by subsurface atoms, thereby introducing excess monovacan-

cies and divacancies into the alloy matrix. The buildup of

an excess vacancy concentration in the vicinity of the sur-

face would enhance chemical diffusion (i.e., diffusion in















dissolved ion



6 A
34 A

step position 2 B
kink position I

L /
erroce .
position

77 absorbed atom










Figure 2. Kink-step-terrace model of a dissolving alloy
surface, after Pickering and Wagner.








the concentration gradient due to enrichment of the surface

in component B) of A atoms towards the surface and B atoms

into the subsurface layers.

The authors have presented calculations of the cur-

rent density and effective diffusion zone thickness expected

for dissolution of copper from a Cu-10 a/o Au alloy at 250C.

Several assumptions have been made which are open to ques-

tion (17), among them:

(1) the concentration of divacancies near the surface is
-2
taken to be as high as 10- a/o;

(2) the accuracy of divacancy diffusion coefficient values

extrapolated to 25C are realistic;

(3) the diffusion coefficient value used is assumed to be

composition independent; and

(4) contributions to the total diffusion rate by short-

circuit diffusion paths (subgrain boundaries and dis-

locations) are ignored.

Notwithstanding these criticisms, the authors calculate a

current density of about 2 x 10-4 amp/cm2 and an effective

diffusion zone thickness equal to about 106 cm (0.01 um)

after 1000 seconds'exposure.

Experimental verification of the model calculations

presented above were undertaken by the authors. Cu-10 a/o Au

sheets dissolved at 1 ma/cm2 for 320 min (19.2 coulombs/cm2

passed) in 1N H2SO4 or in buffered 1N NaC1 solution developed

gold-rich surface layers approximately 8 pm thick. Based on









the amount of charge passed in the experiments, 8 pm is very

close to the depth from which total dissolution of copper

would occur. Calculation of the effective diffusion zone

thickness from the divacancy diffusion model gives the maximum

thickness as only about 0.7 pm for 320 min exposure.

These conflicting results can be reconciled by con-

sidering the surface roughening noted to have occurred in the

gold-rich layers (23). As the attack proceeds, the surface

roughening effect introduces channel-like penetrations and

porosity, thereby significantly increasing the surface area

of the gold-rich surface layers. Consequently, a much larger

surface area is exposed to the solution, and can undergo se-

lective dissolution. This mechanism results in the formation

of a leached structure significantly thicker than could be

formed without roughening.

The operation of a selective leaching mechanism does

not preclude simultaneous operation of dissolution and replat-

ing mechanism. Pickering and Wagner have discounted the possi-

bility that significant numbers of gold atoms are ionized on

the basis of rotating ring disk electrode measurements on a

Cu-10 a/o Au alloy specimen dissolved at polentials up to

+0.900 VSHE. The limitations of this method in measuring very

small concentrations has been pointed out by Rubin (17). At

any rate, the effects of increased overpotentials on the mo-

bilities of adsorbed gold atoms and any resultant surface

diffusion effects are unknown.









Under appropirate polarization conditions, complete

dissolution and selective leaching might be expected to occur

simultaneously. Such would be the case if a Cu-Au alloy were

dissolved at potentials close to the single electrode poten-

tial for gold, or if alpha brass were dissolved at very low

anodic overpotentials. The consequent effect of the reced-

ing metal-solution interface on the interdiffusion zone thick-

ness has been treated by Holliday and Pickering (24) for the

case where the rate of selective leaching and the rate of

total dissolution are similar. After an initial period during

which the metal diffusion profiles are established, a station-

ary state is reached, wherein simultaneous dissolution from

the surface critically enriched in the more noble component

occurs. Thereafter, the rate of selective leaching of the

more active component is proportional to the total alloy dis-

solution rate, and the diffusion profiles adjacent to the

surface (i.e., the level of surface enrichment of the more

noble component) remain constant as dissolution continues.

The role played by surface diffusion in the selec-

tive dissolution mechanism has received relatively little

attention. Pickering and Wagner (16) discount its importance,

pointing out that noble component atoms migrating on the

metal surface would agglomerate and form small crystals. As

these crystals grow to impingement, blockage of the active

atoms' access to the liquid-solid interface occurs, and the

leaching process is stifled.








The ease with which blockage would occur presumably

is related to the alloy composition, i.e., alloys containing

larger percentages of the noble element would be more immune

to significant leaching than those containing more of the

active component. This possible explanation of the fact

that high brasses, containing less than about 10 w/o of zinc,

have not been observed to dezincify has been hinted at by

Feller (25). Feller discounts normal unaided isothermal sur-

face diffusion as being too slow to produce measurable ef-

fects. Instead he proposes that under appropriate high

anodic potentials, surface diffusion is enhanced. This view-

point is supported by Tischer and Gerischer (26), who report

that surface mobility of gold depends on the electrode po-

tential.

Rubin (17) has recently proposed that noble and ac-

tive atoms might combine via surface diffusion to form

patches of alloy having a composition more stable under the

prevailing surface condition than the initial alloy compo-

sition. As is pointed out, the mechanism could be used to

explain the detection of intermediate compositions in de-

alloyed Cu-Au alloys, to be discussed subsequently.

Direct experimental evidence for the operation of

the selective leaching mechanism in dealloying is rare.

Furthermore, quantitative data concerning leaching rates,

layer thickness, and the detailed diffusion processes in-

volved are almost completely lacking.








The optical metallographical evidence for selective

leaching is primarily speculative in nature, and has in most

instances been successfully challenged (3). Aside from the

obvious comment, that the extent of attack found in most

dealloying experiments cannot be accounted for by even the

most optimistic diffusion mechanism, consideration is rarely

given to the factors and processes which result in the for-

mation of the grossly porous residual metal sponges often

found.* Explanation of the formation of such structures by

a selective leaching mechanism is difficult.

X-ray and electron diffraction examinations of de-

alloyed metals have yielded the best evidence to date. The

applicability of the method to dealloying measurement rests

on the assumption that the lattice parameter of the alloy

undergoing selective leaching will evolve in some orderly

fashion from its initial value towards that of the more

noble component. Thus, the diffraction pattern obtained

from a partially leached alloy would contain "bands" of

intensity limited by the normal diffraction line positions

of the alloy and the noble metal. Furthermore, if dealloy-

ing has taken place by a dissolution and replating mechanism,

then only the lines corresponding to the initial alloy and

to the more noble metal should be detected.


*Some experiments by Lucey (27) are the notable exceptions.








Graf's early X-ray diffraction experiments (28, 29)

indicated the formation of a series of crystals containing

from 60 to 80 atomic percent of gold on Cu-10 a/o Au alloy

specimens exposed in alcohol-picric acid solutions or in

alkaline solutions of sodium tartrate at 25C. He found,

however, that Cu-25 a/o Au specimens, dealloyed in more

highly oxidizing nitric acid solutions at 70C, formed no

regions of intermediate composition;only diffraction spectra

for the original alloy and for pure gold were found.

Pickering and Wagner (16) refined Graf's method and

applied it to Cu-10 a/o Au foils dissolved in room tempera-

ture 1N H2SO4 and in 1N NaC1 solution at current densities
2
up to 20 ma/cm. They report the emergence of maxima in

diffracted intensity between the Cu-10 a/o Au peaks and the

pure copper peaks. These maxima move towards the gold peaks

as the amount of dissolution increases. Later electron dif-

fraction experiments on a series of copper-rich Cu-Au alloys

exposed under the same conditions were reported to confirm

the findings for the earlier stages of dissolution (30, 31).

The positions of the intensity maxima observed in patterns

from dealloyed Cu-Au alloys containing 3, 5, and 10 a/o of

gold were observed to be nearly the same, and to correspond

to those of an alloy of approximate composition Cu-69 a/o Au.

Similar formation of equivalent intermediate compositions

by a series of alloys has been described in the Cu-Ni









system (17). In this case the stability of an intermediate

composition was attributed to altered thermodynamic condi-

tions present at the surface of the dissolving metal.

Similar X-ray diffraction determinations were car-

ried out by Heidersbach and Verink (19) on alpha (Cu-30 w/o

Zn) and beta (Cu-48 w/o Zn) brass specimens exposed in 5N

HC1 for 20 and 30 days and 2 days, respectively. Their

X-ray patterns from the alpha brass show a band of increased

intensity between the peaks due to copper and to the Cu30Zn

alloy. A similar pattern reported for a beta brass sample

shows peaks for beta brass and copper and a peak whose posi-

tion corresponds to an alpha brass containing approximately

36 w/o of zinc (32).

The diffraction results described above embody the

only undisputed evidence for the operation of a selective

leaching mechanism in dealloying. The diffraction methods

are limited, however, in that they yield very little infor-

mation about the distribution in the dealloyed structure of

new alloy compositions.

In both investigations discussed above, the electron

microprobe was employed to obtain profiles of composition

across all or part of the dealloyed layers. The gold and

copper profiles reported for dealloyed Cu-lO a/o Au speci-

mens show a monotonic increase in gold concentration and a

corresponding decrease in copper concentration over the








width (,10 pm) of the dealloyed layer (30, 31). Zinc composi-

tion profiles reported for alpha and beta brass were taken

only close to the interface between the bulk metal and the

dezincified layer (19, 32). The widths of layers in which

the zinc contents drop from 30 w/o to zero are less than

10 pm for alpha brass exposed in 1N NaC1 for 7 days at room

temperature and in 5N HC1 for 10 days at 750C, but about

30 pm wide for beta brass exposed in 5N HC1 for 2 days at

75C.

It should be pointed out that concentration profiles

obtained by spectrographic techniques reflect only the aver-

age composition of the material in the volume of measure-

ment. Thus, a profile obtained from a binary alloy sample

having a uniform gradient of depletion in one element may

be indistinguishable from a sample composed of fine particles

of each of its constituents and having only a uniform gra-

dient in the fraction of particles of each element. For

this reason, such profiles by no means represent unequivocal

proof of the operation of a selective leaching mechanism.

The possibility that selective dissolution might

result in the formation of stable phases having compositions

intermediate to the starting alloy and the more noble compo-

nent has been tested in two investigations of dealloying in

high zinc content brasses. Stillwell and Turnipseed (33)

used X-ray diffraction to study epsilon brass (. Cu-80 w/o Zn)

samples exposed in a series of acid solutions. They found









that in acetic acid or dilute HC1, gamma brass (% Cu-

65 w/o Zn), beta brass, and alpha brass or copper were

successively formed on the epsilon brass substrate. How-

ever, in stronger acids (HNO3, H2SO4, and concentrated HC1),

only copper was formed, or complete dissolution took place.

Pickering (34) has extended the analysis to epsilon

(Cu-86 a/o Zn) and gamma (Cu-65 a/o Zn) brasses dissolved

anodically (1 to 5 ma/cm2) in IN H2SO4 and in a series of

buffered (pH = 5) solutions. He reports the formation of

alpha brass on the gamma samples and alpha and gamma brass

on the epsilon brass samples. In addition, the starting

brass compositions were observed to shift towards higher

copper contents, thus reflecting depletion of zinc.

In summation, several important conclusions can be

drawn about our knowledge of the selective leaching pheno-

menon as it pertains to metal corrosion.

(1) The theoretical aspects of dealloying by selective

leaching of the less noble component from the alloy

matrix are rather well-developed, detailed models having

been proposed.

(2) Undisputed experimental evidence for the occurrence of

selective leaching in the Cu-Au and Cu-Zn alloy systems

exists in the form of X-ray diffraction measurements of

lattice parameter changes.

(3) No direct measurements of the physical scale of deal-

loyed layers resulting from the operation of a selec-

tive leaching mechanism exist.









(4) Consequently, little is known of the operational de-

tails of the selective leaching mechanismss.

These conclusions clearly point out the need for a

new experimental approach in analyzing leached structures.

Such an approach is contemplated in Auger electron spectro-

scopy, a highly surface-sensitive experimental technique.


2.2. Dissolution and Replating Mechanism


In contrast to the Cu-Au alloys which undergo selec-

tive dissolution rather easily, the alloys of the Cu-Zn sys-

tem normally dissolve simultaneously in aqueous environments.

This observation led early investigators to conclude that

the porous layers of copper remaining on brass after dezinci-

fication must certainly be the result of redeposition (at

favorable sites on the brass surface) of copper which had

previously passed into solution.

Pickering and Wagner (16) have analyzed the condi-

tions for simultaneous dissolution and replating of a binary

alloy for the case where no completing reactions are possible

and no solid products are formed. Consider dissolution of a

binary Cu-Zn alloy by the reactions


Cu[Cu-Zn] Cu+n + ne-, n = 1, 2 (1)

Zn[Cu-Zn] Zn++ + 2e (2)


If the reactions are independent, then the single electrode








potentials for dissolution of each component are given by

a
+n
E Eo + RT In Cu (la)
Cu[Cu-Zn] Cu nF a u[Cu-
aCu[Cu-Zn]


and by

a+2
EZC- = Eo + T n Zn (2a)
Zn[Cu-Zn] Zn 2F aIn
2F aZn[Cu-Zn]


Subsequent redeposition of dissolved copper ions at copper-

rich surface sites, by the reaction

Cu+n + ne- Cu[Cu sites], (3)


occurs at an electrode potential given by

+n
E Eo RT n Cu (3a)
Cu[Cu sites] u nF au[Cu sites]


The activity of copper on the brass surface,aCu[Cu-Zn], is

less than unity. Consideration of equations (la) and (3a)

for these conditions leads to the conclusion that the single

electrode potential for dissolution of copper from the brass,

ECu[Cu-Zn], is more noble than the potential for deposition

of copper, ECu[Cu sites]. Consequently, redeposition does

not occur.

If, however, the copper and zinc dissolution reac-

tions are not independent, then copper atoms may tend to

remain adsorbed on the surface as the zinc dissolves. These








adsorbed copper atoms tend to aggregate on the surface. An

isolated adsorbed copper atom not surrounded by other copper

atoms possesses a higher activity than a copper atom which

has aggregated and been surrounded by other copper atoms.

Due to this activity reversal, the single electrode poten-

tial for deposition of copper at copper-rich sites, equa-

tion (3a), becomes more noble than the single electrode

potential for dissolution of copper from the Cu-Zn surface,

equation (la). Under these circumstances, copper dissolving

from the brass surface may be plated out on copper-rich

areas on the surface.

The necessity of copper-rich sites as areas for ini-

tiation of replating is intuitively attractive. Formation

of such cathodic sites has been attributed to selective

leaching of zinc from zinc-rich impurities at grain bound-

aries (35), traces of beta brass, or compositional inhomo-

genieties (36, 37).

This rather simple argument has a basic flaw in that

it fails to explain why dezincification of high brasses

(i.e., brasses containing less than 15 w/o of zinc) has not

been observed. Surely for such dilute alloys, the differ-

ence in single electrode potentials for dissolution and

replating of copper, equations (la) and (3a), respectively,

is much smaller than in the case of brasses with higher

zinc content, which do dezincify.







Additionally, Pickering and coworkers have shown,

both through analysis of metal ion pickup in solution (20)

and by a soft X-ray diffraction technique (24), that simul-

taneous dissolution of Cu-30 a/o Zn in acid sulfate solu-

tion is preceded by an enrichment of the brass surface in

copper. Similar results were obtained by Bumbulis and

Graydon (38) for dissolution of a Cu-15 w/o Zn alloy. In

each of these cases, no tendency towards further dezinci-

fication by redeposition of copper was observed. Thus the

existence of copper-rich sites on a dissolving brass sur-

face does not necessarily represent a sufficient condition

for the occurrence of copper redeposition.

The situation discussed above is critically altered

if the concentration of dissolved copper ions adjacent to a

portion of the surface is increased above the equilibrium

value corresponding to the average surface potential. Then,

ECu[Cu sites] may become larger than ECu[Cu-Zn], and depo-
sition of copper will occur. In connection with dealloying

failures in service, copper concentrations higher than the

equilibrium concentrations have been attributed to: (1)

stagnant conditions (15); (2) decreased flow rates (19, 36);

(3) porous surface films or deposits (39, 40); and (4) cre-

vices or pits (41-43). It should be noted that the porous

copper deposits resulting from dezincification produce stag-

nant conditions in the solutions adjacent to the corroding









brass surfaces beneath. Deposits of copper (27, 44, 45)

and of nickel (46) have been employed to initiate dezinci-

fication on brasses.

The traditional, extensive use of brasses in saline

environments has made the effects of chloride ions in pro-

moting dezincification well known. Several early investi-

gators recognized the importance of the complex salts of

copper and chlorine which are commonly observed on dezinci-

fication failures.

Bengough, Jones, and Pirret (2) observed that brass

immersed in aerated sodium chloride solution becomes covered

by a film of sparingly soluble cuprous chloride


Cu[Cu-Zn] + Cl- 1 CuCl + e- (4)


which eventually is oxidized to cuprite and cupric chloride

by the reaction


4CuC1 + 02 Cu20 + 2CuC12 (5)


The authors hypothesized that the cupric chloride formed by

this reaction attacks the brass according to the reactions


CuC12 + Cu 2CuC1 (6a)


and


CuC12 + Zn ZnC12 + 2Cu.


(6b)









Of these reaction products, copper is deposited at the brass

surface, soluble ZnCl2 passes into solution, and sparingly

soluble CuCl is available to continue the reaction cycle.

Abrams (39) demonstrated in a classic series of

experiments the ability of cuprous chloride reaction pro-

duct membranes as well as artificial membranes to pro-

duce dezincification. He was also able to show that condi-

tions which retard the formation of dense CuCl films, e.g.,

low pH and agitation, also retard preferential dissolution.

More recently, Lucey (27) noted the physical simi-

larities between pits in copper and dezincified areas in

brasses formed in chloride containing environments. He

reasoned that most of the copper dissolved from a pit is

precipitated in the pit as cuprous chloride, some of which

is subsequently oxidized to cuprite. A more detailed dis-

cussion of pit formation and morphology has been presented by

Van Muylder et al. (47). In the case of dezincification,

however, the cuprous chloride formed through dissolution

of the brass is reduced to copper. Further, beta brass is

capable of direct cathodic reduction of cuprous ions. Alpha

brass, having a much higher electrode potential, can reduce

only cupric ions, which are formed by disproportionation of

cuprous chloride


2CuC1 Cu + CuC12


(7)









Therefore, according to this reasoning, dezincification of

alpha brass is dependent upon the formation of cupric ions.

Unfortunately, Lucey's rather low experimental value for the

potential of a film-free alpha brass surface is unsubstan-

tiated, making the validity of this mechanism questionable.

Heidersbach and Verink (18, 19) have also recognized

the importance of the copper-cuprous chloride reaction in

producing dealloying in copper alloys. They observed the

deposition of copper on Cu30Zn and CulONi samples polarized

below a certain critical electrode potential during cyclic

polarization in acid O.1M chloride solutions. This critical

potential, +0.200 VSHE' also observed by Efird (48), corre-

sponds closely to the equilibrium potential of the copper-

cuprous chloride reaction


Cu + Cl- CuCl + e- (8)


On this basis, the authors have predicted the conditions of

electrode potential for which copper alloys having polariza-

tion behavior similar to that of pure copper may undergo

dealloying by a dissolution and replating mechanism in neu-

tral and acid chloride environments. Referring to figure 1,

Cu3OZn undergoes simultaneous dissolution at electrode po-

tentials more noble than its corrosion potential, the area

of large dots. However, at potentials less noble than this

critical potential, cuprous chloride formed on the brass

surface as a result of dissolution may be reduced to copper.









Thus, in the potential range bounded by the corrosion poten-

tial of the brass and the critical potential, the area of

crosshatching, dealloying by dissolution and replating may

take place.

Substantiation of this prediction was undertaken by

potentiostating alpha and beta brass samples in deaerated

0.1M chloride solutions of pH 4 at several potentials in

the predicted potential range. All beta brass samples were

dezincified after a few days' exposure. The alpha brass

samples dezincified only if their solutions were unstirred

or after several weeks in stirred solutions. This result

is consistent with earlier observations of agitation on the

formation of adherent cuprous chloride layers (39). Essen-

tially identical potential dependence has been reported by

Marshakov and Bogdanov (49) for dealloying of several alpha,

beta, and duplex brasses polarized anodically in 0.5M NaCl

solutions. Comparable results have been obtained by Robinson

and Shalit (50) and by Sugawara and Ebiko (51) for similar

series of brasses potentiostated in 31% NaCl solutions.

Additionally, Heidersbach and Verink exposed alpha

and beta brass samples in solutions known to produce dezinci-

fication. The steady state electrode potentials were mea-

sured, and in each case were found to fall within the pre-

dicted potential range for dealloying.









The uncertainties involved in specifying electrode

potentials required to produce, or to inhibit, dealloying

have been pointed out by several investigators (19, 21,

48, 51). Potential drops due to anodic films, dealloyed

layers, and crevices can alter the electrode potential at

the attack interface. For example, as dealloying attack

proceeds, the formation of a thick, porous residual metal

sponge over the sample surface changes significantly the

conditions adjacent to the dissolving alloy surface. This

process is in many ways analogous to the development of

pitting or crevice attack (52). By the same token, as the

initiation and the propagation stages of pitting attack

can be treated as separate processes (53), so ought the

initiation and propagation stages of dealloying attack.

A case in point is the initiation of dezincification in

brass by deposition of copper onto the brass surface. Early

investigators (54) referred to such a deposit as "apparent

dezincification." The resulting dezincification which occurs

beneath the deposit is true dezincification, and the condi-

tions which sustain the attack are very probably quite dif-

ferent from those initiating it. The determination of condi-

tions at the dealloying attack interface is the prerequisite

necessary for understanding how the attack proceeds and,

ultimately, how to stop it. Unfortunately, heretofore very

little work has been directed towards this area of investi-

gation.









2.3. The Role of Arsenic as an Inhibitor of Dezincification


Extensive research early in this century concerning

the interplay of compositional variables and dezincification

of brass condenser tubes resulted in the discovery of arsenic's

inhibitive properties (2). Since that time, the addition of

0.02 to 0.04 w/o of arsenic to alpha brasses has become the

standard means of preventing their dezincification (3). Re-

search into the causes) of the effect, however, has proceeded

haltingly, and no consensus has been reached concerning the

identity of the mechanisms) involved. The several theories

which have been proposed to explain the effect can be loosely

classified into two groups:

(1) Arsenic acts as an anodic inhibitor, interfering with

dissolution of the brass or its zinc component; and

(2) Arsenic acts as a cathodic inhibitor, interfering with

copper deposition.

Fink (36) first proposed the former mechanism, stat-

ing that arsenic dissolved from the brass combined with cop-

per to form copper arsenide films over the most "active" or

zinc-rich points on the brass surface. The protective action

of the arsenic films prevented any subsequent enrichment in

copper of the active spots by dealloying, thereby eliminating

potential sites for deposition of copper from solution. The

small amount of arsenic found in commercial brasses precludes

the formation of a continuous arsenical film over the entire









surface (12, 55), hence, arsenic affects only dezincifica-

tion attack, and has little influence on general dissolu-

tion rates.

More recently, West (56) has proposed that the

effect of arsenic under certain conditions of pH may be to

render the brass surface passive. According to this theory,

arsenic in solution is stable as arsenite, As(OH)4 under

the local slightly alkaline conditions present during oxy-

gen reduction. These arsenite ions readily adsorb onto

the positively charged metal surface, forming strong bonds

with the copper atoms, and particularly with the zinc atoms,

at the surface. The effect of this bonding is to stabilize

the surface by rendering the surface atoms in an essentially

passive state. West points out, however, that in the acid

environments expected within pits, some other mechanism must

be operative. Under these conditions the arsenyl ion, AsO+,

is the stable dissolved arsenic species. This ion would

possess little tendency to adsorb onto a positively charged

metal surface.

The theory that arsenic prevents redeposition of

copper at the brass surface has been proposed in many forms

over the years. Masing (57) found that the overpotential

for hydrogen reduction on brass surfaces was less in solu-

tions containing dissolved arsenic than in arsenic-free

solutions. On the basis of his observation he proposed

that for arsenical brasses undergoing corrosion, the cathodic








copper reduction reaction was replaced by the hydrogen reduc-

tion reaction, hence, no dezincification (i.e., no copper

redeposition) occurred. More recent polarization data re-

ported by Sugawara and Shimodaira (58) point out a possible

flaw in Masing's experiments. They found that anodic and

cathodic polarization of copper and copper-zinc alloys is

influenced by additions of sodium arsenite to the test solu-

tion (3% NaCl). However, arsenical brasses exposed in solu-

tions without arsenic additions exhibited polarization data

identical to those of the non-arsenical brasses. The dis-

agreement between the two works is by no means conclusive.

The detection of the influences of trace element additions

to alloys by electrochemical polarization curves can be

insensitive to very subtle effects (59). Nevertheless,

similar indications that the source of arsenic, whether from

the alloy or from the bulk solution, influences its inhib-

itive effects have been reported by other investigators

(27, 60, 61).

Hollomon and Wulff (60) have presented the only

experimental data concerning the identity of the black or

gray films normally found on the surfaces of corroded ar-

senical brasses. Their film-structure experiments consisted

of immersing samples of copper and Cu30Zn containing up to

about 0.5 w/o of arsenic in 2N and 5N nitric acid and in

2N and 5N hydrochloric acid, followed by examination of the

corroded surfaces by electron diffraction. Arsenious oxide,









As203, films were observed to form on arsenical copper and

Cu30Zn in all solutions tested. Significantly, elemental

copper was also found on the Cu30Zn samples. These results

were interpreted in terms of a dissolution and replating

mechanism. As the brass dissolves, redeposition of copper

and, later, of arsenic takes place. The arsenic deposits

are subsequently oxidized to arsenious oxide.

The detection of copper on the arsenical brass sur-

faces in this investigation has significant implications,

namely: either arsenic additions have limited effects in

inhibiting redeposition of copper on Cu30Zn (contrary to

experience); or dezincification of brass by a selective

leaching mechanism may proceed even when arsenic is present.

Hollomon and Wulff later extended their investiga-

tions, exposing arsenical and non-arsenical Cu30Zn samples

in a series of environments designed to produce dissolution

of the copper component of the brass to either cuprous or

to cupric ions. They concluded that non-arsenical alpha

brass undergoes simultaneous dissolution and that dezincifi-

cation occurs by the reduction of cuprous ions to copper on

the brass surface. In the case of arsenical brass, the

arsenic dissolved from the alloy is subsequently deposited

on the brass surface. The arsenical film thus formed

increases the effective electrode potential of the brass

surface such that reduction of cuprous ions to copper is

impossible.









Lucey (27) has also stated that arsenic interferes

with the copper deposition step of dezincification. Accord-

ing to this author's mechanism, copper deposition at the

advancing corrosion interface proceeds by the disproportion-

ation of cuprous ions to form cupric ions and copper,


2Cu Cu + Cu. (9)


When arsenic is present at the interface, the dispropor-

tionation reaction is suppressed by the following pair of

reactions:


3Cu+2 + As 3Cu+ + As+3 (10)
+3 +


3Cu + As+3 3Cu+ + As (10a)


A noteworthy feature of this mechanism is that the arsenic
+3
is recycled between the As and metallic states. Conse-

quently, only a very small amount of arsenic is required for

protection. This is consistent with the fact that only

traces of arsenic are required to protect alpha brasses

against dezincification.

In summary, it can be said that the mechanisms pro-

posed to explain arsenic's role in the inhibition of dezinci-

fication have been influenced by the investigators' choices

for the detailed dezincification mechanism. Thus, it is

clear that complete clarification of the inhibition phenom-

enon awaits final determination of the mechanisms) of

dezincification.













CHAPTER 3


EXPERIMENTAL PROCEDURE



Characterization of the metals used in this investi-

gation and the details of sample preparation are discussed

in Appendix 1. Details of solution compositions and prepa-

ration are given in Appendix 2. The standard polarization

cell as used in potentiokinetic testing, and its modifica-

tion for subsequent tests, are described in Appendix 3. A

list of the equipment used in these investigations is given

in Appendix 4.



3.1. Potentiokinetic Polarization Experiments



Potentiokinetic polarization tests on copper, zinc,

and Cu30Zn were conducted in the standard polarization cell,

described in Appendix 3. Details of the potentiokinetic

technique and theory have been presented elsewhere (62, 63).

A schematic of the experimental set-up is presented in fig-

ure 3.

Each test was conducted in a fresh solution of about

500 ml volume. Samples were introduced into the cell only

after the solution was thoroughly purged with nitrogen.





















































CIII ENT NS ITY


Figure 3. Schematic of the equipment set-up for generation
of potentiokinetic polarization curves.









Equilibration of the sample corrosion potential was monitored

with an electrometer. Zinc samples achieved stable poten-

tials within a few minutes. Copper and Cu30Zn samples, how-

ever, were sluggish, exhibiting changes in potential after

several hours. It was soon discovered that a prepolarization

treatment, consisting of a 5-minute cathodic polarization to

-1.000 VSCE, resulted in much faster attainment of steady

corrosion potentials. A sample was considered to have

achieved a steady corrosion potential when its potential

changed less than about 30 mV in 30 minutes.

Polarization curves were obtained by scanning the

sample's electrode potential from low cathodic overpotentials

to the appropriate anodic overpotential, both in the forward

(i.e, active-to-noble potential) and in the reverse (noble-

to-active) directions. The curves were plotted on potential

versus log current coordinates on an X-Y recorder. The

effect of scan rate on the polarization curves was investi-

gated for copper by comparing curves obtained at scan rates

varying from 10 to 70 mV/min with step scans made in steps

of 50 mV every 5 min (64). As a result of these comparisons,

1 V/hour (16.67 mV/min) was chosen as the optimum scan rate

consistent with reproducible characteristic potential fea-

tures on the polarization curves.









3.2. Selective Leaching Experiments



Potentiostatic selective leaching experiments were

carried out in the modified standard polarization cell, as

described in Appendix 3. All experiments were on polished

Cu30Zn samples immersed in 0.1M chloride solutions of

pH = 4.0+0.1. Cells containing 500 ml of solution were

brought up to temperature and purged with purified nitrogen

for 12 hours prior to introduction of the samples. The

potentiostat was switched on simultaneously with the intro-

duction of the sample.

Cell temperature was monitored throughout the tests.

A constant temperature bath was employed to maintain cell

temperature at 89+0C throughout the tests. Cell potential

and cell current were noted frequently.

Solution aliquots of 25 ml volume were pipetted from

each of the 890C cells before introduction of the sample and

at approximately 24-hour intervals during the test. The

solution drawn off was replaced with 25 ml aliquots of fresh,

nitrogen saturated solution. Solution samples were submitted

to atomic absorption spectrophometric analysis for dis-

solved copper and zinc. Details of the analytical procedure

have been presented elsewhere (32). The experimental limits

of detection were 0.10 ppm for copper and 0.05 ppm for zinc.

At the conclusion of each test, the potentiostat was

switched off as the sample was removed from the cell. Each









sample was rinsed in a stream of distilled and deionized

water and dried with a blast of compressed dusting gas.

The sample connecting wire was ripped away, and the stop-

off lacquer was peeled off. Each sample was inspected

scrupulously for evidence of leaks in the stop-off lacquer.

These were usually easily recognized as patches of deposited

copper on the sides or back of the sample. Any indication

of leaks was cause for rejection of the experiment.

The exposed (projected) surface area of each sample

was measured by direct comparison with a celluloid sheet

inscribed with a 15 mm x 15 mm grid. The accuracy of these
2
measurements was about +0.005 cm Most samples were care-

fully sectioned with a jeweler's saw across a diameter.

Half of the sample was submitted to Auger analysis; the

other half examined by scanning electron microscopy (SEM).

Solution pH was checked before and after each ex-

periment, and was found in every case not to have changed.

Final solution volume was measured in order to correct

atomic absorption data for evaporation losses.


3.3. Controlled Anodic Dissolution Experiments


Potentiostatic anodic dissolution experiments were

conducted in the standard polarization cell, modified as

described in Appendix 3. Experiments were carried out on

Cu30Zn and Arsenical Cu30Zn specimens in pH 4, 0.1M chloride

solutions of 600 ml volume. Cell temperature was maintained










by a constant temperature bath at 30+1"C. Details of

cell operation are as described for the selective leaching

experiments, section 3.2.

At the conclusion of tests, samples were sectioned

across diameters. One half was mounted and polished for

metallographic examination; the other half was examined by

SEM. Analysis of solution samples was accomplished through

atomic absorption analysis. The experimental lower limit

of detection for arsenic was 0.01 ppm.



3.4. Artificial Occluded Cell Experiments



The artificial occluded cell was designed to simu-

late the restricted flow and electrochemical conditions ad-

jacent to the internal dissolving brass surface of a partial-

ly dezincified sample.

The electrode assembly, detailed in figure 4, con-

sisted of a Cu30Zn sample positioned horizontally to, and

shielded from the bulk solution by, a porous copper screen.

The sample holder was constructed from three pieces of

0.0625 inch thick polycarbonate sheet, bonded together with

epoxy. A length of 0.7 mm inside diameter "spaghetti" tube,

inserted through a slot cut in the middle sheet, connected

the 1 cm2 hole between the Cu30Zn and the copper screen with

the top of the assembly. The entire sample assembly was in-

serted in a slot in the rubber stopper. The end of the














CONNECTING WIRES



















Cu 30 Zn
SAMPLE --


STANDARD CALOMEL
ELECTRODE CAP


-RUBBER STOPPER









COPPER
SCREEN


SPAGHETTI TUBE


Figure 4. Artificial occluded cell electrode assembly.









spaghetti tube was inserted into the bottom of a standard

calomel electrode storage cap, which was bonded to the top

of the assembly.

The copper screen was fabricated from a 18 mm x

18 mm, number 100 mesh bronze, copper-plated in a saturated

CuSO4 bath to a total thickness of about 0.6 mm. The plati-

num auxilliary electrode, Haber-Luggin probe, gas disperser,

and gas outlet, all similar to those used on the standard

polarization cell, were inserted through holes in the rub-

ber stopper to complete the assembly. The cell container

consisted of a glass jar. A saturated calomel reference

electrode,SCE 2,was connected to the Haber-Luggin probe

through a salt bridge of the test solution.

Before each experiment, a fresh Cu30Zn specimen and

copper screen were carefully attached to opposite sides of

the 1 cm2 hole with a quantity of Miccroflex stop-off lacquer.

The connecting wires were fed through glass tubes set in the

stopper. The volume of the occluded cell cavity between the

two electrodes was approximately 0.154 ml.

All experiments were conducted potentiostatically at

room temperature in approximately 300 ml of stirred, nitrogen-

saturated, 0.lM chloride solutions of pH = 4. The assembled

cell was purged with nitrogen prior to introduction of the

solution and during the test. A saturated calomel electrode,

SCE 1, was inserted into the electrode cap at the top of the

cell. The occluded cell cavity, spaghetti tube, and electrode









cap were then filled by drawing solution up from the cell

with a hypodermic syringe and needle inserted through the

side of the electrode cap.

The Cu30Zn sample and copper screen were connected

through a zero-resistance ammeter (53). The electrode

assembly was connected to the working electrode terminal

of the potentiostat. The bulk cell potential, EB, was

monitored by an electrometer connected between the working

electrode assembly and SCE 2. The occuded cell potential,

EC, was similarly monitored by a second electrometer con-

nected between the working electrode assembly and SCE 2.

The occluded cell current, i passing between the Cu30Zn

sample and the copper screen was measured by the zero-

resistance ammeter. A schematic diagram of the electrical

circuit is given in figure 5. When the potentiostat was

switched on, the occluded cell current and occluded cell

potential were recorded on identical strip-chart recorders.


3.5. Auger Electron Spectroscopic Analyses



Auger electron spectroscopy (AES), coupled with

argon sputtering,was employed to determine the thickness of

the dezincified layers formed on the Cu30Zn samples subjected

to selective leaching as described in section 3.2. The

theory of AES analysis and of the argon sputtering process

is discussed in detail in Appendix 5.



























AMMETER


Cu30Zn Cu SCREEN


Figure 5. Schematic of the electrical circuit for the
artificial occluded cell experiments.









The AES system in the Department of Materials

Science and Engineering of the University of Florida is

employed routinely in the determination of glass surface

compositions and in the measurement of near-surface com-

positional profiles of metals, glasses, and composites. The

system consists of a high vacuum chamber and accompanying

pumping systems, an electron gun, a cylindrical-mirror

energy analyzer, a sputter-ion gun, and accompanying control

and data treatment and acquisition systems. A schematic

diagram of the AES system is presented in figure 6.

As many as six samples were attached (by copper

clips) to the externally manipulated carrousel sample holder.

The vacuum chamber was evacuated to a base pressure no higher
-8
than 5x10 atm. Auger surface composition measurements

were made with the electron gun operating at 3 keV. The

electron beam diameter was about 0.5 mm, and the beam current

about 25 pa. The angle of incidence was about 45 degrees

from normals to the sample surface. Energies from zero to

1800 eV were normally scanned, however, only the portions of

interest were scanned as generation of the composition pro-

files proceeded. A typical spectrum obtained from the sur-

face of a Cu30Zn sample is shown in figure 7.

Composition profiles were obtained by successive

removal of layers of metal by argon sputtering and recording

the Auger spectra of freshly exposed surfaces. Argon was

leaked into the evacuated chamber until a pressure of






45











X-Y Recorder
or Oscilloscop,


Lock-In
Electron p er
Gun

S Sweep f ____
Supply




Carrousel Target / S
Holder Electron \ Electron
Gun n Multiplier
y-,-,




SMagnetic
Shield

Sputter Ion Gun











Figure 6. Schematic of the AES system used to analyze
selective leaching samples.






















oo
E


0
-o



o



4-
0
O



o a
0 ru









0
t-"





O


Z 0

0 '4-



LiL
01










-,
U
a)



o -



lo
CM.










I.-
o ^L
0 -







-5
5-6x10 atm was reached. The sputter-ion gun was then

switched on and the Argon sputtering begun. The area of

the surface affected by the sputtering was about 5 mm in

diameter. The electron gun and the sputter ion gun were

aligned so that the incident electron beam was concentric

with the sputtering crater. The sputter-ion gun was

switched off while the Auger spectra were measured. The

peak-to-peak heights measured for each element of interest

were plotted as a function of sputtering time to produce

the composition profiles.

Calibration of the Auger peak-to-peak heights with

absolute concentrations was not attempted in this investi-

gation. The primary reason for this was the uncertainty

in measurement introduced by the argon sputtering process.

In particular, the effects of differential sputtering rates

of copper and zinc from the brass surface on the measured

peak-to-peak heights are unknown. Also, smearing or averag-

ing of the surface due to sputtering is likely. Secondly,

the introduction of excess vacancies or porosity into the

near surface layers of brass samples by the selective leach-

ing process could lead to false results. Consequently, the

ratio of the peak-to-peak heights for copper and zinc

(Cu/Zn ratio) was introduced as a qualitative indicator of

the level of selective leaching determined at each depth

from the sample surface.









An indication of the peak-to-peak ratios to be ex-

pected from surface layers leached of part of their zinc

contents can be seen in figure 8. The data points on this

plot represent the Cu/Zn ratios measured from binary copper-

zinc alloys having 90, 85, 80, and 70 w/o of copper. Two

important points are to be noted from the curve.

(1) The Cu/Zn ratio measured for the Cu30Zn alloy has a

value of about 5.

(2) A small change in alloy composition away from 30 w/o

of zinc towards pure copper is accompanied by a signi-

ficant increase in the measured Cu/Zn ratio.

Referring to figure 9, the composition profile re-

corded for the Cu-15 w/o Zn alloy, one can see that a steady

Cu/Zn ratio is reached after about four minutes of sputter-

ing. The initial period of sputtering, before the ratio

becomes constant, corresponds to a "cleaning" of the surface

of adsorbed gases, fingerprints, etc. which are the inevita-

ble results of handling samples outside the vacuum chamber.

This "steady state" Cu/Zn ratio reflects any effects which

may be present due to differential sputtering rates.

Calibration of the argon sputtering rate was under-

taken in order to assign numerical values to the depth axes

of the composition profiles obtained from selectively leached

Cu30Zn samples. Details of the calibration procedure are

given in Appendix 6.















20




15





* 10
0
N



5




0


WEIGHT PERCENT COPPER






Figure 8. Effect of the composition of Cu-Zn alloys on
the Cu/Zn ratio obtained by AES analysis.


I I













OlIv O uz/no

-) 0 o


1HO13H >iV3d-o:-AV3d









3.6. X-Ray Identification of Corrosion Products



Solid corrosion products were examined by the X-ray

powder diffraction method (65). Standard Debye-Scherrer

powder cameras of 57.3 mm of 114.6 mm diameter were employed.

Nickel-filtered copper radiation or manganese-filtered iron

radiation was used in all cases. Product identification

was made by the comparison of d-spacings derived from the

X-ray films with those listed in the ASTM Powder Diffrac-

tion Files.

Identification of some complex hydroxy-chlorides of

copper was simplified considerably by reference to the

d-spacings published for these compounds by Feitknecht and

Maget (66).













CHAPTER 4


RESULTS AND DISCUSSION



4.1. Electrochemical Characterization of
Copper, Zinc, and Cu30Zn


Electrochemical characterization of copper, zinc,

and the Cu3OZn alloy in deaerated O.1M chloride solutions

was undertaken in order to establish a firm basis for pre-

dicting the experimental conditions which lead to dealloy-

ing behavior in the system. Further, the effects of these

experimental conditions on the choice of dealloying mecha-

nisms were explored.

Potentiokinetic polarization experiments described

in section 3.1., were employed throughout this phase of

the investigation. Wherever possible, experimental polari-

zation results were compared with calculated equilibria.

Identification of corrosion products was accomplished

through X-ray powder diffraction experiments.


4.1.1. Polarization of Copper in 0.1M Chloride Solutions


Cyclic polarization curves were generated for pure

copper samples immersed in a series of room temperature








(2530C), nitrogen-saturated, 0.1M chloride solutions having

pH values between 1 and 13. The characteristic features of

the polarization curves are summarized in figure 10 as an

experimental potential versus pH diagram, after the method

of Pourbaix (67). The general regions of immunity, passi-

vity, and corrosion are so marked. A similar diagram has

been published (59).

The diagram can be divided into two parts according

to the change in the corrosion behavior of the system from

general corrosion to active-passive at pH 8. In solutions

of pH between 3 and 8, copper exhibited pH-independent

polarization behavior. Its corrosion potential was about

-0.260 VSCE in this pH range. General corrosion behavior

was noted at more noble (higher) potentials. A maximum in

anodic current density (- 5 ma/cm ) occurred at about

0.060 VSCE. The formation of a film of white cuprous chlo-

ride, CuCl, on the sample surface preceded the current

maximum (68, 69). At electrode potentials more noble than

about 0.100 VSCE, increases in corrosion current with po-

tential were again noted. If the polarization scan was

reversed at this point, the current fell with decreasing

electrode potential until the protection potential

(-0.060 VSCE) was reached, whereupon the current changed

polarity to cathodic. As the potential was decreased


Strictly speaking, the protection potential has no signifi-
cance under active corrosion conditions.

























LEGEND

o corrosion potential
lAa4passivation potential
a rupture potential
+protection potential


*A
-a




A N







A


4. 0__ 0


, 4


_-- ----- ---,-- --+,-- -- ---L -,



* A *


-.2 -


-.3


N


IMMUNITY


-7'


,111 ~1-s


' 0 I


I I I I I


2 3 4 5 6 7 8 9 10 II 12 13 14
pH





Experimental potential versus pH diagram for
pure copper in nitrogen saturated, 0.1M
chloride solutions.


L 2
(.)



0
U

S .1
_1J


I-
0
_ --.i


Figure 10.


-


"'a


-.5


-.6-








further, the CuCl on the copper surface was reduced to copper.

In solutions of pH > 8, copper exhibited pH-dependent

corrosion potentials and active-passive polarization behav-

ior (70). When polarized anodically, the copper became

covered by a tarnish film, identified (by X-ray diffraction)

as Cu20, and the current density began to decrease at poten-

tials about 90 mV above the corrosion potential. One or

more current maximum features was observed as the electrode

potential was subsequently increased. Such maxima are label-

led in figure 10 as passivation potentials. Breakdown of

passivity occurred when the rupture potential was exceeded.

Initial breakdown of the passive film occurred at small,

apparently random, spots on the surface. The spots quickly

became covered with voluminous corrosion products identified

variously as different forms of copper-hydroxy-chloride com-

bined with CuCl. The action of stirring sometimes dislodged

the products, causing them to smear over the sample face.

This action resulted in the breakdown of passivity on the

copper surface beneath the product. The likely mechanism

involved is acidification of the solution held between the

hydroxy-chloride corrosion product and the passive metal sur-

face resulting from copper dissolution,


Cu + 2H20 Cu(OH)2 + 2H + 2e" (1a)









The resulting pH decrease, in turn promotes dissolution of

the Cu20 passive film,


Cu20 + 2H+ + 4C1- 2CuC12 + H20 + 2e- (llb)


In figure 11, the polarization data for copper in

0.1M chloride solutions are superimposed over the equilib-

rium potential versus pH diagram calculated for the

Cu-O.1M Cl- H20 system at 250C. Similar calculations

have been presented elsewhere (18, 71). A list of the free

energy data employed in these and later equilibrium calcu-

lations can be found in Appendix 7. Excellent agreement is

noted between the experimentally measured corrosion poten-

tials and the calculated lines corresponding to the equili-

brium between copper and the CuCl2 ion (ion activity = 106)

and between copper and cuprite, Cu20. The measured value of

the protection potential coincides well with the calculated

equilibrium between copper and cuprous chloride. The locus

of the rupture potentials for solutions of pH < 9 indicates

that the current increases observed at more noble potentials

are due to oxidation of cuprous ions at the sample surface

to cupric ions (69).

It is significant that the protection potential has

the same value in both acid and alkaline solutions up to

pH 11. This indicates that the Cu-CuCl equilibrium is es-

tablished under localized corrosion conditions (47). The




































Uj



T-

I-
0i
a.

z*
>-
*_
UJ


2 3 4 5 6 7 8 9 10 I1 12 13

pH


Figure 11.


Superposition of polarization data for pure
copper in 0.1M chloride solutions and the
equilibrium potential versus pH diagram
calculated for the system Cu-O.1M C1--H20
at 25C, after Van Muylder et al. (71).









equilibrium was at least partially confirmed by the detection

(by X-ray diffraction) of CuC1 in the corrosion products.


4.1.2. Polarization of Zinc in 0.1M Chloride Solutions


Cyclic polarization curves were generated for pure

zinc at room temperature in stirred, nitrogen-saturated

0.1M chloride solutions having pH values between 2 and 14.

The data are plotted on potential versus pH coordinates in

figure 12. Apparent from the shape of the diagram is the

fact that zinc exhibits amphoteric behavior, undergoing

general corrosion in both acid solutions and in highly basic

solutions. Equally obvious is that zinc corrodes freely in

deoxygenated solutions, its corrosion mixed potentials fall-

ing well below the domain of stability for water (with re-

spect to the H20/H2 equilibrium, line a).

The region of passivity between pH values of about

7.5 and 13 is virtually insignificant since the pitting

potentials also fall almost entirely below the lower limit

of stability for water. When zinc samples are polarized

above these potentials, severe pitting occurs. No regions

of complex ion formation were suggested by the polarization

data. The voluminous white corrosion products resulting

from pitting attack could not be identified by X-ray dif-

fraction but are assumed to be amorphous zinc hydroxides

as predicted by previous calculations (72).






























-.7


Lj

ui1





0
a-
i-

(n,


.0


I.I


-1.2


-1.3


1.4


-1.7'
I


LEGEND
So corrosion potentlol
. AD passivation potential
rupujre po;enterriol
\ t protection potentrol







\ PITTING

NN*
*

















IMMUNITY PTTNG

IM UNITY ot

0x


I I I


2 3 4 5 6 7 8 9 10 11 12 13 14


pH





Figure 12. Experimental potential versus pH diagram for
pure zinc in nitrogen-saturated 0.1M chloride
solutions.


3 -------- ---- -- --- ----- -. -- -------1...- .. ---- ----- __ -- --~,,. ,ii- .,~.,,i,,,.,. ,-,, -- --- --


-1.5









In figure 13, the experimental polarization data

for pure zinc in 0.1M chloride solutions are superimposed

over the stable equilibria calculated for the Zn-H20 sys-

tem at 25'C, after Pourbaix (72). Again, excellent agree-

ment is found between calculated and experimental behavior.

It can be concluded from these data that there is a

very large tendency for zinc atoms to pass into solution at

all electrode potentials more noble than about -1.100 VSCE.


4.1.3. Polarization of Cu30Zn in 0.1M Chloride Solutions


Cyclic polarization curves were generated for the

Cu30Zn alloy in nitrogen-saturated 0.1M chloride solutions

at room temperature and at 890C. The characteristic elec-

trode potential features of the polarization curves gener-

ated at room temperature are presented in figure 14.

With a few exceptions, these results are very simi-

lar to those obtained for pure copper. The locus of corro-

sion potentials between pH 4 and pH 8 is -0.260 VSCE, as it

was for pure copper. There is, however, a sharp drop in

measured corrosion potentials between pH 3 and pH 4 to a

steady value of about -0.450 VSCE at lower pH's. The drop-

off in corrosion potential was confirmed by exposing two

Cu30Zn samples in nitrogen-saturated pH 2 and pH 4 solutions

for 34 days and 50 days, respectively. The variation in

measured corrosion potential was recorded as a function of















































-1.8
3


Figure 13.


4 5 6 7 8 9 10 11 12 13 14 15
pH


Superposition of polarization data for pure
zinc in 0.1M chloride solutions and the
equilibrium potential versus pH diagram
calculated for the system Zn-H20 at 250C,
after Pourbaix (72).






















LEGEND DN
ocorrosion potential \
ADspoassivation potential
rupture potential N
+protection potential







CORROSION



0 00
A : S
.----a--- --


a-
4
*+
*- ,
---
+ +

o B.
6

N. 8


0 I
N\


S *O
*


N1
N *
N
N
N<


PASSIVITY


a 0


0
o


IMMUNITY


N
I I


I 2 3 4 5 6 7
pH


8 9 10 11 12 13


Experimental potential versus pH diagram for
Cu30Zn in nitrogen-saturated 0.1M chloride
solutions.


Li


.1
-J

z

i~


Figure 14.


j L


-.5


-.6


-.7L








time, figure 15. The value of the corrosion potential mea-

sured in solutions of pH < 3 can be explained by considering

the equilibrium between copper and the CuC13 ion:

-2 -
Cu + 3C1 CuC13 + e- (12)


Using the free energy data compiled in Appendix 7, the equi-

librium potential for reaction (12) at 25C is given by:


a ucl -2
aCuC1d2
3
E = -.011 + .0591 log 3 .(13)
(aCl-)


Assuming aul-2 = 106, the equilibrium potential of the

-2
Cu-CuCl1 reaction is -0.427 VSE. This value is in fair
3 SCE
agreement with the experimental corrosion potential value.

Indications of the establishment of this equilibrium on cop-

per in acid chloride solutions have been reported elsewhere

(73).

The locus of the protection potential determined by

cyclic polarization of Cu30Zn samples is -0.070 VSCE, vir-

tually the same as that found for copper. The protection

potential takes on a new significance in the case of the

brass (48), since when the electrode potential of the cuprous

chloride-covered brass sample was brought below the protec-

tion potential, deposition of copper occurred. This was

found to be the case not only in acid and neutral solutions,

but also in basic solutions when the rupture potential was






64









-.20-

Cu30Zn
S CI-3 = 10-


-.30 -

ui

UU
c pH =4
u0 o ii
> o o^












t1 20 30 40 50
_E -.40 day
I-

W








-.60 10 20 30 40 50

TIME, days



Figure 15. Variation of the corrosion potential of Cu30Zn
with time of exposure in 0.1M chloride solutions
of pH 2 and pH 4.









exceeded and copper-hydroxy-chloride corrosion products were

formed over part of the surface. Upon passing through the

protection potential, deposition of copper took place on the

brass surface beneath the corrosion products, but not on the

areas of the surface still remaining passive. If the rup-

ture potential was not exceeded during the polarization scan,

then no deposition of copper was detected when the sample

electrode potential was brought below the protection poten-

tial.

In the context of producing dezincification of the

Cu30Zn alloy, the breakdown of passive behavior can be

thought of as an activation step. The concept of activation

has received considerable attention in connection with the

initiation of pitting on Fe-Cr alloys (53, 62, 74). Van

Muylder et al. (47) have described the initiation of pitting

of copper in aqueous chloride solutions by the introduction

of oxygen at local sites on copper surface. This action of

the oxygen in destroying passivity was attributed to its

ability to increase the electrode potential of the copper

surface above its critical potential.

A limited number of cyclic polarization experiments

were conducted on Cu30Zn in 0.1M chloride solutions at 890C.

Through these experiments the corrosion potential of Cu30Zn

at 890C in solutions of pH 4 was established at -0.580 VSCE.









4.2. Dealloying of Cu30Zn by Selective Leaching


4.2.1. Conditions of Exposure


The electrochemical requirements for operation of

a selective leaching dealloying mechanism were outlined in

section 2.1. According to the proposal of Verink and Hei-

dersbach (18), selective leaching of an alloy's active com-

ponent may be expected when the alloy is polarized to elec-

trode potentials between its corrosion potential and the

corrosion potential of its active component.

In the previous section (section 4.1.), the elec-

trochemical characterization of copper, zinc, and Cu30Zn

was undertaken in order to establish the domain of electrode

potentials in which selective leaching of the Cu30Zn alloy

could be studied. Referring to figures 12 and 14, this re-

gion is bounded in room temperature neutral and weakly acidic

solutions by the corrosion potential of Cu30Zn, -0.380 VSCE,

and by the corrosion mixed potential of zinc, -1.100 VSCE.

At 890C, the upper boundary is displaced to a somewhat lower

value, -0.480 VSCE.

Accordingly, potentiostatic selective leaching experi-

ments, described in section 3.2., were conducted on samples

of Cu30Zn exposed in pH 4, 0.1M chloride solutions at 89i0iC.

Table 1 is a list of the applied electrode potentials and times

of exposure for the samples reported. Note that three samples













Table 1


Conditions of Exposure for the Cu30Zn
Selective Leaching Samples


Electrode Potential Time
volts (S.C.E.) days


-0.450 44
-0.450 1
-0.450 7
-0.500 4
-0.600 4
-0.700 2
-0.700 4
-0.700 (2 samples) 7
-0.700 10
-0.800 4
-0.900 4









were exposed at -0.450 VSCE, slightly above the measured

corrosion potential of the Cu30Zn alloy. It is expected

that these samples would show both the effects of anodic

(simultaneous) dissolution at low rates and of selective

leaching.


4.2.2. Results of Solution Analysis


Atomic absorption spectroscopy was employed to ana-

lyze the copper and zinc contents of solution samples taken

from the cells before the start of and during each experi-

ment. As stated previously, the experimental limits of

detection were approximately 0.10 ppm for copper and 0.05 ppm

for zinc. Smaller amounts, or traces of copper and zinc

could be detected, but the accuracy of measurement of con-

centrations below the limits of detection is questionable.

For the conditions of exposure outlined in table 1,

no detectable amounts of copper are expected to enter solu-

tion. Such was found to be the case for all experiments

reported. Whenever significant quantities of copper were

detected in solution, failure of the stop-off lacquer to

protect unexposed portions of the sample was determined to

be the cause, and the experiment was rejected.

Calculation of the amount of zinc leached from each

Cu30Zn sample was made from measurements, by atomic absorp-

tion analysis, of the zinc content of the solution during

the course of the experiment. Corrections were made for








changes in the solution volume due to evaporation and for the

quantities of zinc removed from the cells in aliquots taken

for analysis. The total amount of zinc dissolved at any time

during an experiment is reported in micrograms per square

centimeter of sample surface (projected) area (pg/cm2). Cal-

culations were completed for all samples listed in table 1,

except those exposed at -0.450 VSCE.

Figure 16 summarizes the zinc dissolution data cal-

culated for the five samples exposed at -0.700 VSCE. Con-

siderable variation in the rates of zinc dissolution from

sample to sample is evident. However, the data denote a

marked tendency towards linear zinc dissolution kinetics.

Figure 17 summarizes the zinc dissolution data from

samples exposed at potentials between -0.500 and -0.900 VSCE

The data points plotted for -0.700 VSCE at each time of ex-

posure represent the average of the data points shown in

figure 15 for the five -0.700 VSCE samples. Again, linearity

is evident in each of the plots, at least for four days of

exposure. The rates of zinc dissolution measured at each

electrode potential are summarized in Table 2. The third

column gives the partial current density for the dissolution

of zinc to its divalent ion from Cu30Zn, as calculated from

the zinc dissolution rates.

Pickering and Byrne (20) have presented data for

dissolution currents of zinc from Cu30Zn exposed potentio-

statically at 23C in an acetate-buffered 1N Na2SO4 solution















180-

160-


NE 140


=120-
w
100 -
o
m80


S60

40-


20


0



Figure 16.


Cu 30 Zn T=89C
E= -0.200 VSE/-0.455SHE


TIME, days


Zinc dissolved from Cu30Zn samples potentio-
stated at -0.700 VSCE in 890C, 0.1M chloride
solutions of pH 4.












100

90


Cu 30Zn, T=89C
; E= -0.500 VCE
SCE
0 E =-0.600
A E=-0.700
0 E = -0.800
0 E=-0.900


E
S 60 0

S50- A
O
>,
o 40


5 30-
z

20 0




0
0 I 2 3 4
TIME, days



Figure 17. Zinc dissolved from Cu30Zn samples potentio-
stated at electrode potentials between -0.500
and -0.900 VSCE at 890C.

















Table 2

Rate of Zinc Dissolution from Cu30Zn Under
Selective Leaching Conditions


Electrode Potential
volts (S.C.E.)


Rate of
Zinc Dissolution
g/cm2/day


Zinc Dissolution
Current Density
a/cm2


-0.500 3.5 1.2 x 10-7
-0.600 14 4.8 x 10-7
-7
-0.700 22 7.5 x 107
-7
-0.800 7.0 2.4 x 107
-0.900 3.3 1.1 x 10-7
-0.900 3.3 I.I x I0








of pH 5. In the region of electrode potential between -0.600

and -0.200 VSHE, they report zinc dissolution current values
8 7 2
between 7 x 10 and 2 x 10 a/cm. These current values

are in qualitative agreement with those measured in the pres-

ent investigation for selective leaching of zinc from Cu30Zn

at 890C.


4.2.3. Surface Morphology


Visual examination of the Cu30Zn samples after expo-

sure under selective leaching conditions indicated that de-

alloying had occurred in all cases. The unmasked portions

of the sample surfaces exhibited the pink or reddish color

characteristic of dezincified brass. For the two series of

samples exposed for different periods of time at -0.450 and

-0.700 VSCE, a definite deepening of the surface color with

increasing times of exposure was noted. In all cases, the

masked surfaces of the samples showed no detectable change

in color from the characteristic yellow color of the Cu30Zn

alloy.

Further examination of the surfaces of the selec-

tively leached Cu30Zn samples was accomplished by scanning

electron microscopy. Cu30Zn samples were polished through

1 micron diamond paste before being subjected to selective

leaching. In this way, any gross surface effects resulting

from the selective leaching process could be easily recog-

nized.








Figure 18 shows a typical area on the exposed sur-

face a Cu30Zn sample potentiostated 4 days at -0.450 VSCE.

It is apparent from this photograph that the sample has

undergone anodic dissolution, as was expected for samples

potentiostated above the corrosion potential measured for

the Cu30Zn alloy. The anodic dissolution appears to have

taken place primarily in pits, which tend to lie on polish-

ing scratches. Similar surface appearance was noted for

the Cu30Zn sample exposed 7 days at the same electrode poten-

tial.

Figure 19 shows an area from the surface of a Cu3OZn

sample exposed 4 days at -0.600 VSCE. The surface was fea-

tureless and could not be distinguished from the unexposed

surface, except by the fact that it developed a pinkish color.

Figure 19 exemplifies the appearance of Cu30Zn surfaces after

selective leaching at electrode potentials below -0.450 VSCE.

It should be noted that the resolution in figure 19 is better

than 0.2 vm.

Surface roughening has been cited as a factor con-

tributing to selective leaching in copper-gold alloys (16,

21, 31), and predicted as a possible factor in the case of

copper-zinc alloys (16). In agreement with the results of

Pickering and Byrne (20), no evidence for surface roughen-

ing of Cu30Zn exposed under conditions expected to produce

selective leaching has been found in this investigation.

Only in the case of the samples exposed under slightly anodic















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Figure 19.


Typical surface of Cu30Zn samples potentio-
stated below -0.450 VSCE. From a Cu30Zn
sample exposed 4 days at -0.600 VSCE. 3500X


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conditions (at -0.450 VSCE) were any changes in surface

appearance noted.


4.2.4. Auger Composition Profiles


Composition profiles of the near-surface layers of

the dealloyed samples listed in table 1 were obtained

through Auger electron spectroscopic analysis coupled with

argon sputtering, described in section 3.5. The present

study describes the first reported use of Auger electron

spectroscopy in the characterization of dealloyed structures.

Figure 20 is a composition profile of an area on the

unexposed surface of a Cu30Zn sample which has undergone

selective leaching. The dimensions on the abscissa of the

profile are based on the measured argon sputtering rate of
O
25 Angstroms/min, determined as in Appendix 6. The sharp
0
increase in the copper peak-to-peak heights below 100 Ang-

stroms is characteristic of all the profiles measured during

the investigation. This initial period of sputtering can

be assigned to cleaning of the metal surface of gases and oil

adsorbed on the surface during handling. After the first

few minutes of sputtering the "cleaned" metal surface is un-

covered and Auger electron spectra characteristic of the

metal structure are measured.

As stated previously, the ratio of the peak-to-peak

heights of copper and zinc serves as an indication of the

relative level of enrichment of the measured surface in these























UNEXPOSED


Cu 30 Zn


0 200 400 600


I-
-10
E:o 5

N


Cu/Zn -5


Zn


0 I 0
800 1000 1200


DEPTH FROM SURFACE, Angstroms


Figure 20.


Auger composition profile obtained from an
unexposed Cu30Zn sample.


100





80


60




40




20.









two components. The relationship between the composition of

a brass surface uncovered during sputtering and its measured

peak-to-peak ratio was presented in figure 8, in section 3.5.

A measured Cu/Zn ratio % 5 is indicative of a brass surface

containing 30 w/o zinc. A higher measured Cu/Zn ratio sig-

nifies decreased zinc content, and a lower ratio would indi-

cate enrichment of the surface in zinc. Referring again to

figure 20, a constant Cu/Zn ratio % 5 is reached after sput-
0
tering about 5 minutes (125 A). If the initial four minutes

of sputtering is assigned to cleaning of the surface, then

it is evident that no detectable enrichment of the Cu30Zn

surface in either component has taken place.

Figures 21a, b, and c are the composition profiles

measured for Cu30Zn samples potentiostated at -0.450 VSCE

for 1, 41, and 7 days, respectively. In each case, deple-

tion of the zinc content of the near-surface layers is evi-

denced by initially low peak-to-peak heights for zinc. With

increasing depth from the sample surface, the amount of zinc

measured increases until a constant value is reached. The

peak-to-peak heights measured for copper remain relatively

constant (after the initial "cleaning" period) with depth.

In some cases, e.g., figure 21c, the copper peak-to-peak

heights show a decrease after reaching maximum values. The

cause of this effect is unclear, but is thought to be a re-

sult of the argon-sputtering process.



















0
0




E = 0.450 VSCE
I day


200 400 600 800
DEPTH FROM SURFACE, Angstroms


100





80
I-


I
S60
w
I
0
0
v 40
Li.
a-



20





0


Figure 21a.


Auger composition profile obtained from a
Cu30Zn sample potentiostated at -0.450
VSCE for 1 day.


100
1000


15




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The Cu/Zn ratio curves plotted on each profile indi-

cate the level of dealloying at each point in the composi-

tion versus depth profiles. Comparison of these curves in

figures 21a, b, and c shows that the level of zinc depletion

at the surface increases with increasing time of exposure.

Similarly, the depths of the zinc-depleted surface layers,

indicated by the attainment of constant values % 5 by the

Cu/Zn ratio curves, increase with increasing time of expo-

sure.

Figure 22 and 23 are the composition profiles ob-

tained from Cu30Zn samples exposed 4 days at -0.500 VSCE

and -0.600 VSCE, respectively. Depletion of the zinc con-

tent of the surface is evident, but appears to be less

drastic than in the -0.450 VSCE sample exposed 44 days.

Figures 24a, b, c, and d are the composition profiles

obtained from Cu30Zn samples potentiostated at -0.700 VSCE

for 2, 4, 7 and 10 days, respectively. Comparison of the

Cu/Zn ratio curves for this series shows that the levels of

zinc depletion at the surface and the depths of zinc-depleted

layers fall roughly in order according to the times of expo-

sure.

Figure 25 and 26 are the composition profiles obtained

from Cu30Zn samples potentiostated 4 days at -0.800 VSCE and

-0.900 VSCE. The sample at -0.800 VSCE gives a profile

similar to those measured for the previous samples. The

-0.900 VSCE sample exhibits a radically different profile.








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