The relationship between copper component dissolution kinetics and the corrosion behaviour of monel-400 alloy in de-aerated NaCl solutions

Corrosion Science. Vol. 33, No. 7, pp. 1147-1159, 1992

0010-938X/92 $5.00 + 0.00 © 1992 Pergamon Press Ltd

Printed in Great Britain.

THE RELATIONSHIP BETWEEN COPPER COMPONENT DISSOLUTION KINETICS AND THE CORROSION BEHAVIOUR OF MONEL-400 ALLOY IN DE-AERATED NaC1 S O L U T I O N S J. A. ALl and J. R. AMBROSE* Department of Metallurgical and Materials Engineering, Obafemi Awolowo University, lie-Ire, Nigeria *Department of Materials Science and Engineering. University of Florida, Gainesville, FL 32611, U.S.A.

Abstract--Copper component dissolution behaviour of Monel-400 in de-aerated 0.5 M NaCI of pH 8.4 has been studied using a rotating ring-ring electrode. Both Cu(I) and Cu(II) species are generated during the anodic dissolution of copper within the potential region studied, Normalization of copper current with the corresponding total anodic current defines three potential regions within which the distribution of the copper dissolution products with respect to applied potential varies. Up to -150 mV (Region I) dissolution is predominantly via the Cu(I) species. Between - 150 mV and + 15(1mV (Region II) the major copper reaction is film formation, while above + 150 mV, production of the Cu(II) species dominates the copper reaction. Temperature and solution flow rate do not affect this distribution. It follows that the Cu(II) species is the major copper dissolution product in tile potential region where pitting of the alloy is likely to occur under conditions of low oxygen concentration.

INTRODUCTION

MONEL-400 alloy is known to suffer severe pitting attack when exposed to slowflowing or quiescent seawater environments. 1-3 Two forms of pit morphology have been found on the alloy. These are shallow trenched pit morphology and a morphology in which pits penetrate deeply and sometimes perforate the material. The latter morphology limits the application of this alloy in these environments. No theory exists which can satisfactorily explain this pitting behaviour. The conditions under which either form of the pit morphology develop are not known. In a programme to study the general corrosion behaviour of nickel-copper alloys in seawater environments with emphasis on the mechanisms of pit initiation and propagation on commercial Monel-400 alloy, an initial study was conducted in a noncomplexing (sodium sulphate) solution) It was found in that study that the corrosion behaviour of this alloy in a de-aerated sodium sulphate (Na2SO4) solution was heavily dependent on mass transport. It was observed that copper in the alloy dissolved solely via the Cu(II) species. The next phase of the study was conducted in de-aerated sodium chloride (NaCI) solutions. This paper summarizes the results obtained in de-aerated 0.5 M NaC1. EXPERIMENTAL METHOD Monel-400 alloy of the spectrochemical composition shown in Table 1 (supplied by INCO) was used in the as-received condition for this experiment. The collector ring electrode was fabricated out of a glassy Manuscript received 2 January 1991, in amended form 22 March 1991. 1147

1148

J . A . ALI and J. R. AMBROSE TABLE 1. MONEL-400 ALLOY COMPOSITION (wt%) Ni Cu Mn Fe C

66.20 31.50 1.14 0.70 0.17

Others

0.29

carbon block supplied by Astra Scientific International. The Monel-400 alloy ring had an active surface area of 0.059 cm 2 and the glassy carbon collector ring had an active surface area of 0.060 cm 2. Tacussel ring and disc electrode Model EAD 400, the detailed description of which was reported elsewhere, 5 was used. The polarization experiments were carried out using Tacussel Electronique bipotentiostat Model Bi-PAD 3 and function generator Model GSTP 3. Details of electrochemical cell and electrode preparation are described in Ref. 4. Solution analyses were accomplished using atomic absorption spectrophotometric analysis. A Perkin Elmer Model 460 atomic absorption spectrophotometer was used for the analysis.

Procedure The alloy electrode potential was linearly traversed through a potential region at a sweep rate of 5 mV s -l. In order to monitor the kinetics of the reduction or oxidation of any soluble reaction products generated at the central electrode, the collector electrode was simultaneously polarized at a potential within the diffusion-limited current region corresponding to the redox potential of the species of interest. In all cases, cathodic pretreatment at -500 mV for 10 min preceded anodic polarization of the alloy. Potential scanning was not begun until the collector electrode had attained a constant background current. Each polarization experiment was run in a fresh solution because of the small volume of the corrosion cell. For solution analysis, samples of solutions were taken in 50 ml aliquots after potentiostatic polarization for 30 min at given potentials. The concentrations of Ni, Cu, Fe and Mn in the sample solutions were measured. The influence of solution flow rate, chloride ion concentration and temperature on the anodic behaviour of Monel-400 alloy in this environment was investigated. All experiments were performed in de-aerated 0.5 M NaCI solutions of initial pH 8.2 prepared using demineralized water and reagent grade chemicals. EXPERIMENTAL

RESULTS AND DISCUSSION

Linear sweep voltammetry A t y p i c a l a n o d i c p o l a r i z a t i o n c u r v e f o r M o n e l - 4 0 0 a l l o y in d e - a e r a t e d NaC1 s o l u t i o n is s h o w n in Fig. 1. A T a f e l ( l i n e a r ) r e g i o n is o b s e r v e d b e t w e e n t h e p o t e n t i a l r e g i o n o f - 3 0 0 m V a n d 50 m V . T h e T a f e l s l o p e in this r e g i o n is

b = (6E/6 l o g i) = 119 m V d e c - l o f c u r r e n t . A T a f e l s l o p e o f 75 m V d e c - 1 has b e e n r e p o r t e d f o r N i in 0.5 M N a C I ( p H 0),6 w h i l e 60 m V d e c - 1 has g e n e r a l l y b e e n r e p o r t e d f o r C u in c h l o r i d e e n v i r o n m e n t s . A b o v e an a p p l i e d p o t e n t i a l o f 50 m V , t h e p o l a r i z a t i o n c u r v e d e v i a t e s f r o m l i n e a r i t y a n d t e n d s to a l i m i t i n g c u r r e n t v a l u e , a b e h a v i o u r s i m i l a r to d i f f u s i o n - l i m i t e d c u r r e n t b e h a v i o u r . A s will b e s h o w n p r e s e n t l y , this b e h a v i o u r a p p e a r s n o t to b e c o n t r o l l e d by diffusion.

Characterization of soluble reaction products W i t h t h e e l e c t r o d e r o t a t e d at 1600 r p m , t h e a l l o y p o t e n t i a l was s w e p t at a c o n t r o l l e d r a t e o f 5 m V s -1 f r o m - 5 0 0 m V to 400 m V w h i l e t h e c o l l e c t o r r i n g

Monel-400 in d e - a e r a t e d NaCI 106

I

i

I

[

I

1

1149 i

105 E

< '~ 10~'

z10 i.u

102

u

101

I I I I f ~_ I 100 -600 -300 -200-I00 0 100 200 300 400 HONEL ELECTRODEPOTENTIAL,mV(Ag/AgE[)

FIG. I.

Anodic polarization of Monel-400 alloy in de-aerated NaCI solution. 5 mV s-l, 1600 rpm.

currents were recorded as shown in Fig. 2. The potential scan was stopped at 400 mV because pitting of the alloy electrode was observed beyond this potential at the potential sweep rate used. The operation of the ring at the potential of +600 mV is shown in Fig. 2(a). The potential of +600 mV falls within the diffusion limited current region for the 106

I

I

I

i

I

f

lO5

i

(el

10/' ,=

<

103 106 lo 1

z C3

L_,

102t -.

s -lo5l

L

,

-300 -200 -100

,

0

I

,

L

Ic]

100 200 300 ~,00

HONEL ELECTRODEPOTENTIAL mVlAg/AgCl)

FIG. 2.

Collection ring current densities as a function of Monel-400 alloy potential, 5 m V s i. 1600rpm.

1150

J . A . ALI and J. R. AMBROSE

oxidation of Cu(I) to Cu(II). A preliminary experiment showed that no other species in solution was oxidizable or reducible at this potential. Thus the existence of the Cu(I) species in solution was established. It is well known that Cu(I) ions form complexes with chloride ions. Braun and Nobe 7 have reported that for [CI-] < 0.7 M, CuCI~- is the main complex, while at higher concentrations the complex CuCI 2- is dominant. Hurlen s similarly observed that anodic copper dissolution shows a reaction order of 2 with respect to chloride ions at chloride ion concentrations <1 M, and a reaction order of 3 at higher concentrations, suggesting the formation of CuC12 and CuCI 2- complex ions, respectively. From the foregoing, the more probable Cu(I) species in the 0.5 M NaC1 solution used in this study is CuC12. Figure 2(b) was obtained when the collector ring electrode was maintained at - 150 mV. This current could only be due to the reduction of Cu(II) species since a preliminary experiment shows that no other species in solution was reducible at this potential. This is evidence that Cu(II) ions are also produced during the dissolution of copper in this environment. Under the experimental conditions reported herein, the possible sources of Cu(II) species are: Cu(Monel) --->Cu2+(aq.) + 2e-

(1)

2Cu+(aq.) --. Cu + Cu2+(aq.)

(2)

CuC1--> Cu2+(aq.) + CI- + e Cu+(aq.) ~ Cu2+(aq.) + e-.

(3) (4)

Reaction (2) can occur only if the concentration of free Cu(I) ions in solution exceeds 10 -7 M (the stability constant for this reaction is 6.3 x 10-7). 9 Given the concentration of chloride ions in solution, it is doubtful whether a much higher concentration of free Cu(I) ions exist in solution. The standard potential for reaction (3) has been reported to be +250 mV (+538 mV(SHE)). 9 This reaction is unlikely to account for the Cu(II) current detected at -100 mV. The Cu(II) ion concentration must be as high as 1.3 x 10 -2 M for this reaction to occur at - 100 mV. Based on the foregoing analysis, only reaction (1) has the greatest probability of giving rise to the Cu(II) current in the lower potential region. As illustrated in Fig. 3, there is a sudden increase in Cu(II) current above about + 100 mV. It has been reported that the CuC1 film starts forming at the instant the anodic current is turned on. 1° Above +100 mV, the dissolution of the CuC1 according to reaction (3) becomes possible, and occurs in parallel with reaction (1). The ratios of ring current to the total anodic current are plotted as a function of applied potentials as shown in Fig. 3 for two reasons. The first is, that since one experiment is repeated two or three times, one for each soluble species in the solution, normalizing ring current with respect to the total anodic current insured that no artificial fluctuation in the alloy electrode current and, hence, in collector ring current, would lead to misinterpretation of data when two or more sets of data are compared. This is especially critical when the effects of certain parameters such as solution flow velocity, chloride ion concentration, temperature, etc., on soluble dissolution products are investigated. Secondly, the changes in the collector ring with respect to the total anodic current can be used to indicate where the effects of other competing anodic processes (such as film forming reaction) are predominant.

Monel-4()0 in de-aerated NaCI

1151

-20

123 -~-0

-60

u -80

,,=, N IK

-100

-120 I -200-100

t 0

l i 100 200

I 300

I 1.00

MONEL ELECTRODE POTENTIAL mV(Ag/AgCl) FIG. 3.

Normalized Cu(II) current density as a function of Monel-40() alloy potential. 5 mV s - I . 1600rpm,

Figure 2(c) was obtained with the collector electrode potential maintained at - 6 0 0 mV to reduce all soluble species, including nickel species. The figure is almost a mirror reflection of Fig. 2(a) and about equal in magnitude. This suggests that the contribution of nickel current to this curve is very small. The presence of nickel in the solution was, however, established by solution analysis using AAS technique (Table 2). Iron and manganese could not be detected in these studies. If present in solution, their concentrations were probably below the detection limits of these techniques. Figure 4 shows how the normalized total copper dissolution current density varies with the applied Monel potential. A minimum in the normalized current density is observed between - 5 0 mV and about +100 mV. The behaviour in this potential region could be ascribed to either a decrease in total copper current (denominator) or an increase in the total alloy dissolution current (numerator) relative to total copper current. As demonstrated in Fig. 5, the total copper current within the potential region where the current minimum is observed does not drop as it appears to do in Fig. 4. The minimum, therefore, is due to a larger increase in the total alloy TABLE 2.

ATOMIC ABSORPTION DATA FROM POTENTIOS'IATIC

TESTS IN DE-AERATED 0 , 5 M N a C I SOLUTION

Potential of polarization (Ag/AgCI) -50 mV 0 mV +50 m V + 100 m V

A m o u n t in solution (ppm) Ni

Cu

Fe

Mn

1.00 3.8(I 4.60 5.80

0.40 1.40 2.40 2.80

0. l0 0.10 0.10 0.10

nil nil nil 0.10

1152

J . A . ALl and J. R. AMBROSE I

I

[

f

I

I

T

~32.0

~

2B.0

~:2~.0 r,-

~20.0 (3-

o16.0 o 12.0

-~ 8.0 ~-o z

I

-30

-200

I

I

I

I

I

-100

0

100

200

300

I

&O0

MONEL ELECTRODEPOTENTIAL,mV(Ag/AgCI) FIG. 4.

Normalized total copper current density as a function of Monel-400 alloy potential, 5 m V s - ] , 1600 rpm.

dissolution current in that potential region. The possibility of increased nickel dissolution was considered. As shown in Table 2, the amounts of nickel and copper in a solution from an experiment carried out within the potential range being considered are close to their ratio in the alloy. Nickel, therefore, could not account for the behaviour in this region. There is little doubt then that the increased anodic current, largely due to formation of insoluble reaction products on the anode surface, accounts for the behaviour in this region. The increase in the total anodic 5

~

I

I

I

[

I

I

w

z

~ 3

~

2

eY

I

I

-30 -250-100

i

0

I

I

100 200 300 ~.00

MONEL ELECTRODEPOTENTIALmVlAg/AgCl) FIG. 5.

Total copper current density as a function of Monel-400 alloy potential, 5 m V s- z, 1600 rpm.

Monel-400 in de-aerated NaCI

1153

106

t05

E

I0~

>-

Io 3

~

j~ e~

102

• x

~.

500rpm BOOrpm

o 1~O0~pm

.~

e 2000rpm ~ 2500rpm

101 !

10

--

I

1

I

I

I

I

I

I

-400-300-200-100 0 100 200 300 /.00 MONEL ELECTRODE POTENTIAL ImV) FIG. 6.

The effect of electrode rotation speed on the anodic polarization of Monel-4OO alloy, 5 mV s ~.

current in the presence of the anodic film on the alloy is notionally ascribed to a decrease in the electrical resistance of the film. The role copper plays in the presumably increased electrical conductivity of the film is not very clear.

The influence of mass transport Anodic currents measured at different angular velocities (Fig. 6) show little dependence on rotation speeds. Tafel slopes obtained in the linear region under different rotation speeds vary from 117 to 122 mV dec -1 in no particular order. This suggests that the slopes are rotation speed-independent. What appears to be the diffusion-limited current region in the high overpotential region in Fig. 1 is, as evident from Fig. 6, not a diffusion-limited current region since the behaviour does not show any dependence on electrode rotation velocity. The levelling off of current density in this region may be due to an anodic process such as film formation limiting the anodic reaction. An inteffacial IR drop resulting from the presence of a salt film and/or solution resistance effects may also have a similar effect. The distribution of copper dissolution products as a function of solution flow rate is shown in Fig. 7. There is no mass transport dependence in the potential region where Cu(I) species are predominant. Above about + 150 mV where Cu(II) species predominate, a mass transport dependence is observed. The trend is that the higher the solution flow rate, the higher the proportion of Cu(II) species generated except for data at 2000 rpm which appear to be anomalous.

The effect of chloride ion concentration The effect of chloride ion concentration was investigated from 10 -3 to 0.6 M. The

1154

J . A . ALl and J. R. AMBROSE"

polarization curves exhibit active/passive/transpassive transitions for concentrations up to 1.0 x 10 -2 M (data not shown) with the only copper ionic species detected being Cu(II) ions, in agreement with the observation of Braun and Nobe. 7 Similar behaviour was observed for a pure copper electrode in sodium bicarbonate solution containing chloride ions.11 Not only does the alloy lose its ability to passivate in solutions containing chloride ions above about 5 x 10 -2 M (Fig. 8), but one also begins to detect Cu(I) species on the collector electrode in addition to Cu(II) ions (Fig. 9). The proportions of Cu(I) products detected above the concentration of 5 x 10 -2 M increases with increasing chloride ion concentration. It is not known whether there is a correlation between the inability of the alloy to sustain passivity and the presence of Cu(I) species in solution. It is possible that the occurrence of both phenomena (detection of Cu(I) species and loss of passivity) are purely coincidental. It is also possible that this chloride ion concentration is needed to raise the concentration of Cu(I) ions above the detection limit of this technique. The logarithmic plot of current density (obtained potentiostatically) is shown in Fig. 10. The curve is sigmoidal in shape with a linear portion in the middle. The slope of the linear portion is (6 log i/6 log [C1-])¢= 0 = -2.49

(5)

suggesting a reaction order with respect to a chloride ion concentration of 2.5. As discussed earlier, anodic dissolution of pure copper in chloride solutions of the concentrations used in this study shows a reaction order of 2 with respect to chloride ion concentration. Nickel has been reported to show a reaction order of 0.5 with respect to the chloride ion in chloride concentrations <1 M and a reaction order of 1.1 at higher concentrations. 6 The fact that the reaction order with respect to the chloride ion measured in this study is much closer to that for pure copper suggests

14

• 12 4 | | | ! / •

/ / / / / X / 0 / / J /

r

i

-20o

FiG. 7.

: -- 500rpm w w BOO rpm "-="°--1600rpm "°-o-2000rpm

tI

-1oo

-o

~oo

200

300

~oo

MONEL ELETF~DDE POTENTIAL(n'9/)

The effect of electrode rotation speed on the ratio of Cu(II)/Cu(I) current densities, 5 m V s - 1

Monel-400 in de-aerated NaCI

1155

106 _0.6m ..~c~.,.,j~'~r. , , ~ , ~ I 0 •3 m

105

~ ' ~ f ~ 0 . 1 m

~"~

O.~,m

~. 104

103

~

02

101

10 -/,00 FIG. 8.

I

l I ] ] I I - 200 0 200 MONEL ELECTRODE POTENTIAL

I /,00

The effect of chloride ion concentration on the anodic polarization of Monel-400 alloy. 5 m V s -I, 1600 rpm.

that corrosion behaviour of Monel-400 alloy may be controlled by the dissolution kinetics of the copper in the alloy.

The effect of temperature A general increase in the current density with increasing temperature has been

10 5 e 10&

z

2~

-~-

.

(Q)

.

.

.

---

~oTYl

~

I

102

101

m -101 z E: _102

~_ 103

t Ib)

SOmV

0.1M

8 -104

_I05

I I I I I ~ I Il -300 -200 -100 0 100 200 300 NONEL ELECTRODE POTENTIAL,mV {Ag/AgCI)

Fro. 9.

Effect of chloride ion concentration on the collector ring current densities, 5 mV s -I, 1600 rpm.

1156

J . A . ALI and J. R. AMBROSE

"~

35

_a z {=3

30

82s

8 2.

I

0.2

0.4

0.6

O.B

LOft [CI-x101 ] M FIG. 10.

Logarithmic plot of Monel-400 alloy current density as a function of chloride ion concentration, 5 mV s - l , 1600 rpm.

observed in this study (Fig. 11). Figure 12 shows the plot of the log of current density as a function of the reciprocal of temperature. The slope of the curve is ( 6 1 o g i / ~ l / T ) E .... = 1.86.

(6)

If m e t a l dissolution kinetics can be represented by equation (7),

K = Ko exp ( - Q/R T),

10 5

10~'

~

10 3

102

MONEL ELECTRODEPOTENTIAL(mVI FIG. 11.

Effect of temperature on the anodic polarization of Monel-400 alloy, 5 mV S- l , 1600 rpm.

(7)

Monel-400 in de-aerated NaCI

1157

E <

.a

~.0

= -

>-

rY el: LJ

-J

3.0

2

FIG. 12.

I 9

I

3 1

I

L

t

33 1000 OK-1 r

i

35

Arrhenius plot of current densities: (a) Monel-400alloycurrent; (b) collectorring current, 5 mV s l, 1600rpm.

then the activation energy for dissolution of Monel-400 alloy could be determined from the slope of log i vs 1/T. Thus using the slope of Fig. 12, the activation energy for Monel-400 alloy dissolution in 0.5 M NaC1 solution was determined to be 8.5 kcal mol -j. This is about one half the value reported for nickel in chloride solution (15 kcal mol 1).]2 The activation energy for copper dissolution ranging from 5.1 to 7.0 kcal mol -] , depending upon fluid flow conditions, has been determined in chloride solutions. 13 That of Monel-400 alloy determined in this study is closer to the values reported for copper. This is further evidence that the copper content dissolution kinetics control the anodic behaviour of this alloy. It has generally been reported that dissolution of copper and copper-base alloys in the presence of chloride ion concentration of the magnitude used in this study is entirely via the Cu(I) species. It is clear from the results in Fig. 2 that Cu(II) ions are also generated at potentials down to as low as - 1 0 0 mV in de-aerated 0.5 M NaCI solution. There is little doubt that a more sensitive technique than the one used may be able to detect Cu(II) species at a more negative potential. This is thermodynamically reasonable. The exact role the amount of nickel in the alloy plays in this mode of dissolution of copper has not yet been established. The information available in the literature indicates that copper-base copper-nickel alloys dissolve electrolytically solely via Cu(I) species at potentials below about +100 mV in chloride environments. 1 4 , 1 5 Based on the analysis of data reported here, the most probable description of Monel-400 alloy anodic reaction in this environment can be represented as follows: Region I (between the rest potential and - 1 0 0 mV): Generation of Cu(I) species as shown in equation (8) is the predominant copper dissolution reaction. Cu(Monel) --~ Cu+)aq.) + e -

(8)

Cu+(aq.) + 2C1- ~ CuCI;-. Formation of CuCI in this potential region as reported by Stephenson and

1158

J . A . ALl and J. R. AMBROSE

Bartlet 1° and generation of Cu(II) species in quantities that are below the detection limit of this technique may also take place. Region II (-100 to +150 mV): The major copper reaction is the formation of insoluble reaction products such as Cu(Monel) + CI- ~ CuCI + e-.

(9)

Generation of Cu(I) and Cu(II) ionic species continues but in small quantities and at constant level of current (Fig. 3). This region corresponds to the region of minimum normalized total current density in Fig. 4. Region III (above + 150 mV): Dissolution via Cu(II) ionic species according to either or both of the following reactions predominates: Cu(Monel) ~ Cu2+(aq.) + 2eCuCI---~ Cu2+(aq.) + C1- + e-.

(1) (2)

The likely nickel dissolution reactions at all potentials investigated are the following: Ni(Monel) + 20H- = NiOHad s + e-

(10)

Ni(OH)ads ~ NiOH + + e-.

(10a)

A good agreement has been found between the activation energy and the reaction order with respect to chloride ion concentration for the dissolution of Monel-400 alloy in de-aerated 0.5 M NaCI solution measured in this study and those reported for pure copper dissolution in the literature. It appears from the above that the overall anodic behaviour of Monel-400 in de-aerated NaC1 environment is strongly influenced by the dissolution kinetics of copper in the alloy. Values of the Tafel slope may suggest that the number of charges transferred in a given overall reaction. These have been widely used in the literature to infer mechanisms of dissolution. For example, we have measured a Tafel slope of about 120 mV dec -1 in Fig. 1 suggesting 1/4 electron transfer. Separation of the overall reaction in this region has revealed that in addition to the dissolution of nickel in the alloy, film formation (Fig. 4) and dissolution of copper in the alloy via Cu(I) and Cu(II) species (Fig. 2) do occur. The conclusion from this is that mere knowledge of Tafel slopes may not be sufficient for the determination of the reaction mechanism of anodic metal dissolution. As seen in Fig. 7, Cu(II) species are the predominant dissolution products of copper at potentials where pitting corrosion under conditions of low oxygen concentration is likely to occur. This is compatible with autocatalysis theory of pit propagation since hydrolysis of cupric ions would provide the driving force for continued pit propagation. Acknowledgements--The authors express their gratitude to Dr E. D. Verink, Jr, for his advice on this project and to the International Nickel Company for their support of this project through a fellowship (INCRA No. 880133vt-2) awarded to J. A. Ali.

Monel-40{I in de-aerated NaCI

1159

REFERENCES 1. L. L. SHREIR, Corrosion 1: Metal~Environment Reactions, 2nd Edition, Newnes-Butterworths, London (1977). 2. F. k. LAQUE, J. A m . Soc. Naval Engrs 53, 29 (1949). 3. F. W. W~NK and W. K. BOYD, The Corrosion qfMetals in Marine Environments. Bayer and Co., lnc,, C o l u m b u s Ohio (1970). 4. J. A. ALl and J. R. AMBROSE, Corros. Sci. 32,799 (1991). 5. J. A. ALl, Ph.D. Dissertation, University of Florida, Gainesville, FL (1983) 6. A. BENGAIA and KEN NOBE, J. electrochem. Soc. 126, 1 118 (1979). 7. M. BRAUN and KEN NOBE, J. electrochem. Soc. 126, 1666 (1979). 8. T. Ht:RI.EN, Acta Chem. Scand. 15, 1231 (1961). 9. WENDEll. M. LAI'IMER, The Oxidation States of the Elements and Their Potentials in Aqueous Solutions, Second Edition. Prentice-Hall, Englewood Cliffs, NJ (1952). l(I, L. STEPHENSON and J. H. BARTLETt, J. electrochem. Soc. 101,571 11954). 11, M. AI