Voltammetric study of the localized corrosion of Al-Zn-Mg alloys containing Cr and Nb in chloride solutions

Elecmchimico

Pergamon

ACM. Vol.41. No. 13, pp. 1933-1946. 19% Copyright 0 19% Elswicr Science Ltd. Printed in Great Britain. AU rights mervcd

00%4686/96 S15.00 + 0.00

VOLTAMMETRIC STUDY OF THE LOCALIZED CORROSION OF Al-Zn-Mg ALLOYS CONTAINING AND Nb IN CHLORIDE SOLUTIONS J. A. GARRIDO, P. L. CABOT,~ A. H.

Cr

R. M. RODRIGUEZ, P. T. A. Suhtomot and E. PEREZ

MOREIRA,?

Dept. de Quimica Fisica, Fat. de Quimica, UB, Marti i Franqds, l.-08028 Barcelona, Spain t Inst. de Quimica, Cidade Universitaria, USP, C.P. 20780, CEP 01498, SHo Paulo, Brazil (Received 1 August 1995; in revisedform 8 November 1995) Abstract-The resistance to localized corrosion of Al-5%Zn-1.7%Mg-0.23%Cu (alloy H) containing 0.14% Cr (alloy L), 0.053% Nb (alloy J) or both, 0.14% Cr and 0.053% Nb (alloy 0), annealed (A), cold-rolled (ST), quenched (F), quenched and aged (B) and quenched in two steps and aged (C), has been studied by means of co, optical microscopy, SEM, EDX and XPS. T’he cyclic voltammograms obtained at different sweep rates in NaCl solutions at concentrations in the range 0.1-l mol dm-” showed the same characteristics as those obtained previously for alloy H. One breakdown potential was found for the specimens F, which consisted of a supersaturated phase. One breakdown potential was also found for the specimens A and ST although they consisted of two phases: the matrix solid solution and big MgZn, precipitates (0.2-0.4pm in length). It was shown that the MgZn, precipitates disappeared from the alloy surface from potentials well before the breakdown potential. In contrast, an anodic maximum and a further breakdown potential were obtained for the age-hardened specimens (B and C) which consisted of the matrix solid solution and small MgZn, precipitates (about 0.02 pm in length). T’he anodic maximum was explained by a surface dealloying affecting all the alloy surface. Pit propagation was not detected. The breakdown potential after the anodic maximum was related with the further pit propagation in pits nucleated at random. The voltammograms of the specimens B and C corresponded to Al-alloys resistant to SCC, in agreement with previous experiments using the cut-edge method and breaking stresses measurements. The repassivation potentials were shown to characterize well the pitting corrosion resistance of the alloys, which was increased by the addition of Cr and Nb, independently or both at the same time. The effect of Cr and Nb was related with their good distribution in the alloy and in the oxide film. The repassivation potentials increased in the sequence H < J < L < 0 for a given heat treatment and in the sequence F c A z ST < B = C for a given composition, in agreement with previous corrosion potential measurements in chloride solutions containing hydrogen peroxide. Copyright 0 1996 Elsevier Science Ltd. Key words: Al-Zn-Mg alloys, localized corrosion, chloride solutions, cyclic voltammetry.

INTRODUCTION Al-(4.9 - 52%)Zn-(1.6 - 1.8%)Mg-(0.15 - 0.2O%)Cu(0.15 - 0.25%)Cr alloys are weldable and present good stress corrosion cracking (SCC) resistance as well as the minimum strengths required for a highstrength constructional material[l-7-J. While Nb increases the fatigue resistance and reduces the mean grain size of these alloys[8], Cr increases their SCC resistance[2]. Alloys H (Al-5%Zn-1.7%Mg-0.23%Cu) and J (alloy H containing 0.053% Nb), submitted to different heat treatments (A: annealing; ST: cold-rolled; F: quenched; B: quenched and aged and C: quenched in two steps and aged), were previously studied using cu and E,,, measurements in NaCl solutions[4-71. The microstructure, phase composition and E,,, in different chloride-containing solutions of alloys H, J, L (alloy H containing 0.14% Cr) and 0 (alloy H containing 0.14% Cr and 0.053%

$ Author to whom correspondence should be addressed.

Nb), submitted to the same heat treatments, were also determined using optical microscopy, TEM, EDX and the standard procedures to measure corrosion potentials[fl. Whilst the Nb addition slightly shifted the corrosion, pitting and repassivation potentials in the positive direction of alloy H[6, 71, addition of Cr significantly shifted its corrosion potential in the positive direction[7]. Previous works have shown the utility of the electrochemical methods, in particular cu, to study the susceptibility to localized corrosion (pitting, intergranular corrosion and SCC) of ALCu[9], 7075 AlZn-Mg-Cu alloy[lO, 111 and Al-S%Zn-1.7%Mg alloys containing small additions of Cu and Nb[463. Although SCC is not yet well understood, the essential result was that the specimens susceptible to SCC presented an anodic maximum in the voltammograms performed in chloride solutions at low sweep rates. Such anodic maxima were the consequence of two breakdown potentials, the most anodic of them corresponding to the grain boundaries (GBs) (SCC was related with preferential pitting in the GBs), and when the difference between these 1933

J. A. GAIUUWet al.

1934

breakdown potentials was lowered by means of suitable heat treatments, the specimens decreased their susceptibility to SCC. The anodic sweep of the cyclic voltammograms of the age hardened H and J alloys (specimens B and C)[4-61 presented an anodic maximum which overlapped the current increase corresponding to the pit propagation region at sweep rates equal to or higher than 1 mVs-‘. However, this anodic maximum was completely separated from such a current increase at sweep rates lower than 1 mVs-I. The SCC resistance of these alloys was then correlated with the small charge of the anodic maximum[S]. However, a further EDX microanalysis of these specimens in the TEM conditions did not show any significant compositional differences between the GBs and the matrix solid solution[7]. In this work, the surface transformation related with such anodic maxima and the effect of Cr and of Cr and Nb additions to Al-S%Zn-1.67%Mg0.23%Cu, that is alloys L and 0 respectively, on such anodic maxima and on the resistance to the localized corrosion of these alloys has been studied using cu, rde, SEM, EDX, XPS and previous microstructural analysis. These experiments have been performed in a variety of environmental conditions which have been shown to be useful to study the localized corrosion phenomena of aluminium alloys.

0.02pm in length) than for the specimens A (O-20.4pm in length). The precipitates in the specimens F are as small as the Guinier-Preston (GP) zones. On the other hand, the precipitates in the specimens ST are similar to those of A while the precipitates in the specimens C are even smaller than those of B. The thin film EDX microanalysis, published previously[7], indicated that the specimen F presented higher contents in Zn and Mg than the matrix of the specimens A and ST, while the matrix of B and C presented intermediate values. Any compositional differences between the matrix solid solution and the precipitate free zone (PFZ, near the GBs) of the specimens B and C (see Fig. l(b)) were not detected. Cr was detected in the matrix solid solution and also in the form of a fine dispersion of precipitates, while Nb was only found to be present dissolved in the matrix. Both, Cr and Nb were well distributed along the grains. In addition, the Cr-containing precipitates produced a Zn and Mg depletion of the matrix solid solution in the specimens A and ST when compared with the specimen without Cr. After the heat treatment, the alloy cylinders of 3mm in diameter were embedded in epoxy resin. Before the experiments, the specimens were polished using diamond paste up to 1 pm finish and cleaned with ethanol in ultrasonic bath. Only one base of the alloy cylinder was exposed to the electrolyte. The working electrolytes were 0.1, 0.5 and 1 moldmW3 NaCl (Merck p.a.) solutions prepared with water of Millipore Milli-Q quality. The CD experiments were performed using a PAR 273 potentiostat and the 342C PARC corrosion software, in the absence (after Ar bubbling) and in the presence of 0, dissolved (O,-saturated at latm), in stirred (rde at 2000rpm) and in quiescent solutions, just after the immersion of the electrode in the working solution and also after 3 h of immersion in the electrolyte itself at open circuit. The start potential in the CD experiments was -1.2V (cu. the open circuit potential in the absence of oxygen dissolved[4, 71) and the range of sweep rates was from 0.02SmVs-‘. The surface of the specimens were examined by optical microscopy (Olympus PMG3), SEM (Jeol JSM 840 SEM), EDX (Link Systems) and XPS (Physical Electronics PHI5500), at different points of the voltammograms in order to study the evolution of the surface morphology and composition. Those samples which had to be submitted to SEM and XPS examination were rapidly removed from the working solution, immersed and stirred in benzene (to eliminate the solution drops adhered on the metal surface), dried and stored under high vacuum. The surface area object of the XPS microanalysis

EXPERIMENTAL The electrochemical experiments were performed in a three-electrode cell at 25.0 f O.l”C and the reference and auxiliary electrodes were, respectively, a see (all the potentials given are referred to the see) and a Pt mesh. The alloys H, J, L and 0 (see the compositions in Table 1) were prepared, hot-rolled, cold-rolled and cut as cylinders having 3mm in diameter. Afterwards, different samples were submitted to the heat treatments ST (cold-rolled), A (annealed), F (quenched from 48o”C), B (quenched, naturally aged for 3 days at 25°C and then, artificially aged for 8 h at 90°C and for 24 h at 135°C) and C (quenched, interrupted for 2min at 4OO”C, and aged as B)[S]. The breaking stresses of HB, HC, LB and LC were 454 f 9, 435 k 2, 480 f 50 and 478 f 10 MN m-‘, respectively, while after boiling 3.5 h in aqueous solution containing 3 gdme3 NaCl + 36gdme3 CrO, + 30gdme3 K,Cr,O, , they were 307 f 30, 417 f 20,355 + 27 and 353 f 24 MN m-‘[3]. The microstructure of these specimens is exemplified in the TEM micrographs shown in Fig. 1. The MgZn, precipitates are clearly shown in this figure and are much smaller for the specimens B (about

Table 1. Compositions of the Al-Zn-Mg alloys under study, given in % in weight (spectrophotometric analysis). Fe, Ni, V and Mn contents were less than 0.001% Alloy

Zn

Mg

Cu

Cr

Nb

Si

Ti

H J L 0

5.0 5.0 5.0 5.0

1.7 1.7 1.7 1.6

0.23 0.23 0.24 0.24


co.01 0.053
0.006 0.006 0.007 0.001

co.001
Localized corrosion of Al-Zn-Mg alloys

Fig. 1. TEM

lg the M_ precipiiate free zone (PFZ) near the grain boundaries (GBs) of the s&en

was of 150 x 150pm2, the atomic profiles being obtained after controlled Ar sputterings. RESULTS Cv experiments of Al-5%Zn-1.67%Mg-0.23%Cu containing Cr and Nb in deaerated 0.1 mol dm - 3 NaCl Previous works using alloys H and J indicated that only minor changes in the voltammograms were found when the sweep rates were changed from 1 to 0.2mVs-‘[4-63. For this reason, a sweep rate of 1 mV s-r was found to be suitable to compare the corrosion behaviour of the present Al-Zn-Mg-Cu alloys containing Cr and Nb. The voltammograms obtained for alloys L and 0 in deaerated and quiescent solutions, just after introducing the alloy electrode in the cell, are exemplified in Fig. 2 and fit to

1935

Note the

i

one of the two types shown in Fig. 2(a), a or b (a’, on the one hand and b’ and b”, on the other hand, correspond to the types a and b, respectively). In the direct sweep, the current is very small up to’ a certain potential in which the oxide covering the metal is no longer protective and suffers breakdown by the chloride ion. This results in a sudden increase of the anodic current. The breakdown potential, Ebr, can be taken as the electrode potential extrapolation to zero current of such a sudden current increase. In the inverse sweep, the current decreases linearly from certain potential, thus permitting us to determine the repassivation potential, Err,, as the electrode potential extrapolation to zero current of such a current decrease. The differences between the voltammograms a and b in Fig. 2(a) are the presence of a maximum in the anodic sweep (M in curve b) and a hysteresis effect (curves a and a’). Heat treatments A, ST and F led to

1936

J. A. GARRIW et al.

voltammograms having the same form as for highpurity Al[ 123 (curves a and a’), while heat treatments

-0.60

-0.90

I I uA

(4

I I uA

(b) Fig. 2. Cyclic voltammograms at 1 mV s-r in deaerated and quiescent 0.1 mol dms3 NaCl solutions, (a) curves a, a’ and b corresponding to the specimens LF, LA and LB and (b) curves b’ and b” to the specimens OB and JB.

B and C, ie, those corresponding to the age hardened specimens, led to curves presenting a maximum (M in curves b, b’ and b”). Maxima of this type have been interpreted as a double breakdown behaviour[4-6, 9-111 and thus, two breakdown potentials can be measured, Ebrl and EbrZ. The breakdown and repassivation potentials of the specimens L and 0 obtained from the cyclic voltammograms at lmVs-’ are listed in Tables 2 and 3, where they are also compared with those corresponding to alloys H and J previously reported[461. It is noted that only the E,, of the predominant phase, ie that corresponding to Ebr2, is given in Table 3 because of the small charge associated with the process leading to Ebrl. The surface morphology of the specimen LA after an anodic sweep between - 1.2 and -0.85 V (point P in Fig. 2(a)), is shown in Fig. 3(a). This figure shows the selective dissolution of the MgZn, precipitates (compare with Fig. l(a)). As shown in Fig. 2(a) (curve a’), such a selective dissolution takes place with a very small current (-0.85V is more negative than the corresponding Ebr) and any anodic maximum is not found. The surface morphology after a sweep between - 1.2 and -0.6 V is shown in Fig. 3(b). At this latter potential, condition in which the current density is very high, the specimen exhibits a great number of pits, developed from zones where the MgZn, was initially present. As shown previously, such precipitates are distributed in parallel transgranular bands and present accumulation in the grain boundaries (GBs)[7]. These are just the zones which have suffered a greater attack. At 1 mVs- ‘, the anodic maxima M which appear for heat treatments B and C present a certain over-

Table 2. Breakdown, Es,, and repassivation potentials, E,,, given in mV vs see, measured at lmVs_‘, in deaerated and quiescent 0.1 moldm-” NaCl, of Al-Zn-Mg alloys submitted to heat treatments ST, A and F. The standard deviation of E,, was about 10 mV and that of E,, , about 5 mV

Ev

J&W Specimen

ST

A

F

y L 0

-835 -815 - 795

--840 820 -815 -815

-840 -830 -815 -795

ST

A

F

-885 -875 -850 -855

-885 -890 -865 -860

-915 -905 -885 -890

* Results taken from [4-61. Table 3. Breakdown, E,,, and I?,,, , and repassivation potentials, E,, , given in mV vs see, measured at 1 mV s-r, in deaerated and quiescent 0.1 moldm-’ NaCl, of Al-Zn-Mg alloys submitted to heat treatments B and C. Only the E, potentials corresponding to E,,, are given B Specimen

C

Es,,

&,,,

E,,

E,,,

Et,,,

E,,

-835 -825 -805 -790

-740 -730 -725 - 730

-815 -800 -740 -735

-830 -810 -800 -785

-730 -735 -725 -730

-805 -805 -740 -740

* Results taken from [4-61.

Localized

Fig. 3. SEM micrographs -0.85 V (point P in

corrosion

of the specimen

of Al-Zn-Mg

alloys

LA (a) after a potentiodynamic

1937

sweep between

- 1.2 and

Fig. 2(a))and (b) after a potendiodynamic polarization from - 1.2to -0.6 V.

lapping with the further current increase (Fig. 2, curves b, b’ and b”) and therefore, it is difficult to ascertain in these conditions the surface transformation related with such maxima. However, the maximum M disappeared in the second cycle when the anodic limit of the first cycle was set at the minimum following M in the anodic sweep (m in Fig. 2(a)). Effect of the Oz dissolved It was previously shown that 0, dissolved resulting from air saturation of the working solution shifted the breakdown potential of alloy H in the positive direction when the specimens were held for

several hours in the working solution at open circuit. In this work, this effect has been studied for alloys J, L and 0 using O,-saturated solutions, the Oz pressure being of 1 atm. The voltammograms obtained after 3 h of immersion in the electrolyte are of the types shown in Fig. 4. The essential result is that while voltammograms having the same form as in the absence of oxygen dissolved are found for specimens A, ST and F (Fig. 4(a), curves a and b), the anodic maxima are no longer found for specimens B and C (see curve d in Fig. 4(b) and compare with curve c). The corresponding E,, and E,, potentials of these alloys are collected in Table 4. It is shown that the E,, potentials in the absence and in the presence of Oz dissolved are coincident within the experimen-

J. A. GARRIW et al.

b

-0.80

i

!

,.’

:’

i

D*

I

I I

-200

1000

600

0

’ D,

._

‘.a

200

.-,

r

-0.65

200

400

600

800

I I PA

uA

(a)

(a) -0.65

-0.80

I I. ‘!

-0.95 -100

ii

-0.90

ii

-200

I

I

200

600

100

1000

300

I I

I 1 PA

500

PA

@)

@I Fig. 4. Effect of the 0, dissolved in the cyclic voltammograms at 1 mV s-i in quiescent 0.1 mol dm-3 NaCI. (a) specimen LA in deaerated (curve a) and after 3 h of immersion in the O,-saturated solution (curve b) and (b) specimen LC in deaerated (curve c) and after 3 h of immersion in the O,(l atm)-saturated solution (curve d).

tal error (better than 10mV). On the contrary, the breakdown potentials are much more positive (4050 mV) in the presence of O2 dissolved. Note that Es, in Table 4 must be compared with Ebr2 in Table 3 because the anodic maximum disappears in the presence of 0, dissolved. EJect of sweep rate and stirring on the cv experiments

The sweep rate in a very narrow and low range produces minor changes in E,, of alloys L and 0 and the heat treatments studied (see Table 5 and the examples shown in Figs 5(a) and (b)). The anodic

Fig. 5. Effect of sweep rate in the cyclic voltammograms in deaerated and quiescent 0.1 moldmm3 NaCl. (a) Curves a and b correspond to the specimen LA at 1 mVs_’ and 0.05 mV s-i, respectively and (b) curves c and d correspond to the specimen LB at 1 mV s-r and 0.05 mV s-i, respectively.

sweep of the voltammograms found for heat treatments B and C differ in the peak currents (curves c and d in Fig. 5(b)). The peak current increases with sweep rate. On the contrary, the anodic current in the most negative part of the cathodic sweep, that is from about -0.8 to - 1.1 V in Fig. 5(a) (region D,) and from about -0.7 to - 1 V in Fig. 5(b) (region DJ, increases when the sweep rate decreases. Note, however, that the anodic currents in the latter potential ranges are much higher for heat treatments A and ST (compare Figs 5(a) and 5(b)). This was also previously observed for alloy H[5]. The E,,, measurements in O,-saturated 0.1 mol drne3 NaCl and the ability of the 0, dissolved to polarize this alloy

Table 4. Breakdown, E,, and repassivation potentials, E,, , given in mV vs see, measured at 1 mV s-i, after the immersion of the Al-Zn-Mg alloys in O,-saturated (1 atm) and quiescent 0.1 moldme NaCl for 3 h E br

Er,

Specimen

ST

A

F

B*

C*

ST

A

F

BY

C*

J &

-800 - 780 770

- 790 -780 -765

-775 -775

-725 -660 -680

-710 -700 -640

-875 -850 -855

-885 -870 -860

-915 -895 -900

-815 -740

-815 --745 750

* These breakdown potentials are related with the pit propagation region (Ebrl in Table 3). ’ These repassivation potentials correspond to E,,, .

Localized corrosion of Al-Zn-Mg

alloys

1939

Table 5. Breakdown, E,, and repassivation potentials, Err, of the specimens JB and JC, given in mV vs see, together with the anodic charges corresponding to the anodic maxima M, given in mC, at different sweep rates in deaerated and quiescent 0.1 mol drnm3 NaCl JC

JB v/mVs-’ 0.05 0.2 0.5 1

E br1

E b&!

E,,

Q

- 845 -840 -830 -820

-765 -755 - 750 - 750

-810 -805 -800 -805

9.5 12 9.5 13

up to the pitting

potential Ds to the oxidation

permitted

to assign

'L,

4,

Q

-832 -835 -840 -825

-755 - 745 - 750 -735

-790 -780 -792 -805

6.7 11 11 12

0

100

the

of the MgZn, precipitates in the occluded cavities formed in the conditions of pit propagation and on the other hand, to determine E,, from the extrapolation of the region D, in Figs 5(a) and (b) down to zero current. As shown in Figs 5(a) and (b) and in Table 5, these extrapolated values were independent of sweep rate. The anodic charges (Q) of the anodic maxima M found for heat treatments B and C (Fig. 2) are given in Tables 5 and 6. It is noted that the anodic maxima M appear as processes which are separated from the further current increase (after the point d in Fig. 5(b)). Therefore, the anodic charge of such maxima were determined after a deconvolution of the total curve when they appeared overlapped with the further current increase corresponding to E,, . As shown in Tables 5 and 6 and Fig. 2(b), the anodic charges slightly depended on the alloy composition and heat treatment. The effect of stirring is exemplified in the rde experiments shown in Figs 6(a-c). As shown in these figures, & is more positive under forced convection conditions. This makes the anodic current of the anodic sweep higher in quiescent solutions. The anodic maxima of the voltammograms of the specimens submitted to heat treatments B and C are not practically shifted for very low sweep rates (Fig. 6(b)), while they are significantly shifted in the positive direction at higher sweep rates (Fig. 6(c)). It is also interesting to observe that for low sweep rates the anodic currents in the cathodic sweep are also much smaller in stirred (region D,’ in Figs 6(a) and (b)) than in quiescent solutions (region DJ. The difference is particularly very high for heat treatments A and ST (Fig. 6(a)). region

E,r,

EJ2ct of NaCl concentration

-0.95

-1.20 -100

I I

200

300

400

uA

(a) -0.60

1

I

I

I

0

loo

200

300

-0.85

-1.10

I -100

400

I I ).rA

(b) .0.65

r

0.80

To study the effect of chloride concentration,

voltammograms at different sweep rates using deaerated 0.5 and 1 mol dmW3 NaCl were obtained. As shown in Fig. 7, the anodic maxima at low sweep rates (M 0.95

Table 6. Anodic charges (QJ corresponding to the maxima M (see Fig. 1 and 3(b)),measured at 1 mV s-l in deaerated and quiescent 0.1 mol dme3 NaCl, for the Al-Zn-Mg alloys submitted to heat treatments B and C Specimen

Q/mC

Specimen

Qlmc

HB JB LB OB

17 13 12 10

HC JC LC oc

14 12 11 10

i -100

u

I

300

100

500

I I PA @I

Fig. 6. Cyclic voltammograms in deaerated 0.1 moldmm3 NaCl using the rde at 2000rpm to show the effect of stirring. (a) Curves a and b correspond to the specimen LA in quiescent and stirred solutions, respectively, at 0.05 mV s- r. (h) Curves c and d correspond to the specimen LB in quiescent and stirred solutions, respectively, at 0.05 mV s- ’ and (c) curves e and f correspond to the specimen LB in quiescent and stirred solutions, respectively, at 1 mV s-r.

J. A. GAIWDO et al. -0.70

,

/

I

I

I

I k

-0.90

-

-1.10 -30

0

I

I

I

30

60

90

120

I I uA (a) -0.60

,

I

I

k

i -0.85

-1.10

I

0

-100

100

I

200

300

I I /.tA (b)

Fig. 7. Cyclic voltammograms in deaerated and quiescent 0.5mol drnv3 NaCl (a) specimen HB at 0.05mV s-l and (b) specimen JB at O.O2mVs-‘. in Figs 7(a) and (b)) are also clearly separated from the current increase related with continued pit growth (region ik). The anodic charges of the anodic maxima M found for the specimens B and C were only slightly higher than those corresponding to 0.1moldm-3 NaCl (see Table 7 and compare with the data given in Tables 5 and 6). The experimental

results concerning the effect of chloride concentration and sweep rate on the E,, and E,, potentials are shown in Tables 7 and 8. SEM examinations of the specimens B and C submitted to different anodic sweeps between - 1.2 V and a potential just before the anodic maximum (point equivalent to m’ in Fig. 7(b)) and just after the anodic maximum (m) were performed in deaerated and quiescent 0.5 and 1 mol dm- 3 NaCl. In the point m’, the alloy surface did not present any transformations with respect to the polished sample, while a clear surface transformation was observed in the point m. Such a surface transformation consisted in a certain attack extended all over the alloy surface which produced general roughness (see Fig. 8(a)). Although the grains could be seen under the optical microscope, the grain boundaries did not show selective corrosion. SEM micrographs of the specimens B and C after a severe pitting attack showed that pits were spread at random on the alloy surface. Pits and precipitates were found all over the alloy surface, the EDX microanalysis of such precipitates indicating the presence of chloride, as in the case of the specimens F, A and ST. The SEM micrograph of the specimen JB in Fig. 8(b) shows the effect of a 5 h polarization at -0.78 V in 0.1 moldme NaCl. As shown in Fig. 2(b) (curve b”) and Table 5, this potential corresponds to the anodic maximum but it is more positive than E,,. For this reason and after a prolonged polarization at this potential, the two effects, ie, surface roughness (related with the anodic maximum) and pit propagation (- 0.78 V > E,, , as shown in Table S), can be clearly observed. Note that the GBs appear as slightly prominent lines, the surface presents certain roughness and pits are spread in the grain bodies. XPS results

The essential results of the XPS microanalysis after different sputtering cycles and different samples

Table 7. Breakdown, E,, , and repassivation potentials, E,, , of the specimens JB and JC, given in mV vs see, together with the charge corresponding to the anodic maxima M, given in mC, at different sweep rates in deaerated and quiescent 0.5 mol dn-’ NaCl JB v/mVs-’ 0.05

0.1 0.2 0.5 1.0 2.0

JC

E,,,

E bc!

Ev

Q

- 895 -900 -895 -895 -885 -885

-815 -815 -830 -810 -820 -810

-850 -855 -865 -855 -855 -860

11 11 16 15 15 16

-885 -885 -865 -875 -875 -875

-830 - 805 -810 -790 -790 -790

-850 - 860 -855 -850 -850 -850

6.9 8.5 8.7 11 13 14

Table 8. Breakdown, E,,, , and repassivation potentials, E,, of the different specimens submitted to the heat treatments B and C, given in mV vs see, at O.O5mVs-’ in deaerated and quiescent 0.5 mol dm- 3 NaCl

HB JB LB OB

-890 -895 -875 -860

-815 -815 -820 -805

-855 - 850 -825 -820

HC JC LC oc

-890 -885 -845 -840

-830 -830 - 780 -785

-840 - 850 -795 - 790

Localized corrosion of Al-Zn-Mg

alloys

Fig. 8. SEM micrographs of the specimen JB (a) after an anodic sweep at 0.05 mV s- 1from - 1.2 V up to the potential just after the anodic maximum (see Fig. 8(b)), in deaerated and unstirred OSmol dm-’ NaCl and (b) after being potentiostatically polarized for 5 h at -0.78V in deaerated and unstirred 0.1 mol dm - 3 NaCl (see curve b” in Fig. 2(b)).

are shown in Fig. 9. Figure 9(a) shows the XPS profiles for the specimen LB after a mechanical polishing and after a potentiostatic experiment in deaerated and quiescent 0.1 mol drnm3 NaCl at - 0.78 V for 2 min. This was a potential corresponding to the region of the anodic maximum (see Fig. 5(b)) and was more negative than the corresponding E,, (see Table 3). As shown in Fig. 9(a), a certain oxide growth takes place at this potential because oxygen appears for longer time of sputtering. A similar trend is observed for the specimen LA. The

Mg, Zn and Cl profiles corresponding to these experiments are shown in Figs 9(b) (LA) and (c) (LB). It is shown in these figures that chloride is present in the metal-oxide interphase (region of very low atomic concentration of oxygen, which corresponds to a very high atomic concentration of aluminium), in particular for the specimen LA. Note that for this specimen, -0.78V is more positive than the corresponding E,, (see Table 2). In addition, the XPS profiles were obtained for the specimens LB and JC after being submitted to

J. A. GARR~DOet al.

1942

0

LB (polished)

m

LB (2 min -0.78 V)

q

LA (2 min -0.78 V)

25

0 0

10

20

30

d

40

lb

20

3b

t/ min

t / min

(b)

(a)

75

0

LB (before

.

LB (afwr M)

M)

0

JC (&I

M)

25

0 0

10

20

30

40

t I min

0

2

6

4

6

10

12

t I min (d)

(c) Fig. 9. XPS microanalysis (atomic concentration vs time of sputtering) of the anodic flms formed in deaerated and unstirred NaCl. (a) oxygen profile, films on LB only polished and on LB and LA after being held for 2 min at - 0.78 V (see Fig. 5(a)), 0.1 mol dm - ’ NaCl. (bl atomic Drofiles of the same film as thatif Fig. 11(a), specimen LA. (c) atomic &files of the same fdm k that of Fig. 11(a), specimen LB. (d) oxygen profile, films on the specimens LB and JC obtained by potentiodynamic sweep at 0.2mV s-l in deaerated and quiescent 0.5 mol dm -’ NaCl, just before and just after the anodic maximum M (see Fig. 8(b)).

potentiodynamic sweeps in deaerated and quiescent 0.5moldm-3 NaCl at 0.2mVs-’ from -l.lV to the potentials corresponding to the points just before and just after the anodic maximum (Fig. 9(d)). In these conditions, the anodic maximum was clearly separated from the pit propagation region. Any difference in the oxide layer thickness cannot be detected between these two points. Note also that chloride was detected in the oxide film and that in O.Smol drnm3 NaCl, the oxide film is thinner than in 0.1 moldm-3 NaCl (compare with Fig. 9(a)). In the XPS microanalysis, the Nb and Cr profiles were also obtained. However, quantities in the detection limits were only found in the oxide film (note that only 0.075 atom percent Cr and 0.015 atom percent Nb were present in the bulk alloy). DISCUSSION Effect of the addition of Cr and Nb in the pitting corrosion behaviour of A1-5%Zn-1.6l%Mg-0.24%Cu

The voltammograms obtained for alloys L and 0 are of the same type of those corresponding to alloys H and J. Voltammograms of the type a shown in

Fig. 2(a) were previously obtained for alloys H and J submitted to the heat treatments A, ST and F, while those of the type b for the same alloys but heat treatments B and C. The heat treatment appears again responsible for the type of voltammograms obtained for alloys L and 0: type a for heat treatments ST, A and F and type b for B and C. However, their characteristic potentials are quite different due to the presence of Cr and Nb (see Tables 2 and 3). It is observed that chromium addition produces a significant shift of E,, and E,, in the positive direction in all the heat treatments performed. It was previously shown that the addition of Nb produced small shifts in the same direction[6]. However, Tables 2 and 3 show that its effect is much less marked than in the case of Cr. In addition, when Nb is added to the Cr-containing alloy, a further small shift in the positive direction of these potentials is also observed. The sequences in the E, potentials given in Tables 2, 3 and 8 for 0.1 and O.Smol drn-j NaCl are consistent with the sequences in E,,,, measured both, according to the ASTM standard procedure (which uses an aqueous solution containin& per liter, 53 g of NaCl and 9ml of 30% H,O,) and in Or-saturated (1 atm) 0.1 mol dm-’ NaCl, previously reported[n.

Locaked corrosion of Al-Zn-Mg alloys

It is noted that the E,, values given in Tables 2 and 3 are coincident within 20-30mV with the E,, data given in [7] in the case of using Or-saturated (1 atm) 0.1 mol drn-j NaCl, and then, the sequence in E,,, coincides with that of E,,. Note, however, that the E,,, data given in Ref. [7J when using O,-saturated NaCl were mean values (1 atm) 0.1 moldm-3 because sudden and very frequent changes in E,,, in a potential range of about f 25 mV were found. As is well known, the corrosion potentials by themselves do not always indicate the susceptibility to pitting of the alloys. Pitting is essentially related with the film resistance to breakdown and develops from certain anodic potential, while corrosion, different from pitting, may develop at more negative potentials. However, the alloys under study are rapidly polarized to the pitting potential in the presence of an oxidizing agent such as oxygenC4, 71. In addition, it has been shown that these alloys suffer pitting attacks at potentials between E,, and E,, (what makes E, the relevant quantity in pitting corrosion studies, this being in agreement with previous results of Aylor and Moran using different Al alloys[13]). Therefore, the E,,, and the E,, ranking of the present alloys also indicates their relative susceptibility to the pitting attack. The ASTM E,,, values given in [7] increased in the sequence H < J < L < 0 for a given heat treatment and in the sequence F < A x ST < B < C for a given alloy composition. These are generally the sequences of the E,, values obtained in the present work. The apparent exceptions in Tables 2, 3 and 8 are due to the experimental error in the measurement of the E,, values (less than 10mV). The ASTM standard procedure for the E,,, determination uses an excess of oxidant and leads to more precise data in the alloys classification. Therefore, the E, values obtained in this work permit us to cone Pude, in agreement with previous E,,, measurements, that Cr and Nb addition increase the pitting corrosion resistance of Al-5%Zn-1.7%Mg-0.24%Cu, the effect of Cr being dominant. The behaviour of the present alloys in front of the pitting attack can then be explained by the factors responsible for the E,,, ranking[7], that is the different compositions of the phases formed under different heat treatments and different alloying elements. As previously reported[7], the effect of Cr and Nb must be related with their partitioning in the alloys (see the experimental part): Cr is present in solid solution and as a very fine dispersion of Crcontaining precipitates, while Nb was present in the matrix solid solution. Although the presence of Nb and Cr in the oxide film was in the detection limits of XPS (due to the very small content of the alloys in these alloying elements), the Cr and Nb effect on shifting the E,, and ErP potentials in the positive direction can be explamed by their presence in the oxide film. In fact, the presence of Cr in the oxide film has been reported by supersaturated Al-0 alloys with higher content in Cr, from 6 to 15 atom percent[14]. In this case, a much greater shift of the breakdown potential in the positive direction was demonstrated and it was shown that Cr forms a CrOOH barrier layer that inhibits the oxidation of the Al substrate and restricts the chloride from

1943

reaching the metal/film interface. Ta and Zr, valve metals like Nb, also greatly increase the breakdown potential of Al because the oxidized solute protects the substrate by restricting the ingress of chloride[15]. Therefore, a similar effect can be foreseen for Nb. In any case, the small amounts of Cr and Nb only lead to a limited increase of the resistance to the pitting attack. The surface morphology of the specimens A and ST after being submitted to anodic sweeps between - 1.2 and -0.85 V (Fig. 3(a)) and between - 1.2 and -0.6V (Fig. 3(b)) shows that pitting corrosion in these specimens takes place principally where the MgZn, precipitates were present, that is in the CBS and in the inner part of the grains. Such precipitates disappear from the alloy surface before E,, and behave as pit nucleation centers. Such an intermetallic compound is more anodic than the matrix solid solution of the alloy and thus, it is the first species in suffering the electrochemical dissolution. The presence of holes in those places where such precipitates were present demonstrates such an electrochemical dissolution, which must lead to the Mgc2 and Zn +2 ions. These holes permit the formation of local surroundings more aggressive than the bulk solution because of the ion hydrolysis (which lowers to local pH) and the chloride accumulation. This also explains the hysteresis of the voltammogram a’ shown in Fig. 2. While E,, corresponds to the matrix solid solution, E,, is much more negative because of the presence of deep pits (containing a much more aggressive electrolyte), produced by the electrodissolution of the MgZn, precipitates. The specimens submitted to heat treatment F also exhibit important hysteresis (Fig. 2(a), curve a). In addition, the E,, potentials of the specimens F are more positive than those corresponding to the specimens A and ST despite the solid solutions of the former are the most anodic and the corresponding E,, the most negative. The specimens F have uniform composition and consequently, the induction time for pit nucleation is greater. Due to such an uniform surface, the local conditions which increase the pitting attack and lead to pit propagation are not easily achieved and thus, E,, is much more positive than E,, . It is interesting to note that specimens A and ST present E,, and E,, more negative than those corresponding to the specimens B and C despite the compositions of the matrix solid solution of the latter are intermediate between those of the specimens A and ST, on one hand, and those of the specimen F, on the other[7]. This can be explained by the size of the MgZn, precipitates in the specimens A and ST, which favour the formation of occluded cavities where the local environment enhances pitting corrosion. The specimens A and ST, which are not convenient for structural applications, are then susceptible to intergranular corrosion because of the presence of greater MgZn, precipitates in the CBS. As shown in Table 2, E,, and E,, of the specimen submitted to the heat treatments ST presents only minor differences with respect to those corresponding to heat treatment A and therefore, the effect of cold-rolling can not be detected from the voltammograms. The interpretation considering the size of the

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MgZn, precipitates is in agreement with the results concerning the effect of sweep rate and stirring. As shown in Figs 5(a) and (b) and in Table 5, the sweep rate only introduces minor changes in E, while EI, remains essentially the same. However, the anodtc current in the cathodic sweep is much higher at low sweep rates (region D2), in particular for the specimens submitted to heat treatments A and ST (Figs 5(a) and 5(b)). This was previously observed for alloy H and attributed to the oxidation of the MgZn, precipitates[5]. For low sweep rates, a deeper attack takes place in the regions where the MgZn, precipitates were present, thus permitting us to produce local environments having a very different composition from the bulk electrolyte. In these regions, MgZn, can be oxidised favouring the aggressivity of the local solution. In this work, it is shown that this behaviour is not eliminated by the presence of Cr and Nb in the alloy. However, the anodic currents in this potential region are lower when these latter alloying elements are present (see Fig. 2(b)). This can be explained by their incorporation in the oxide film when the repassivation takes place, thus increasing the pitting corrosion resistance of the alloy, as demonstrated by the shift of E,, and E,, in the positive direction. As shown in Fig. 6, stirring produces a shift of E,, in the positive direction. This appears to be due to a delay in the pit nucleation because the solution near the alloy surface becomes more homogeneous, thus making the local accumulation of chloride more improbable. Stirring also produces a significant decrease of the anodic current in the cathodic sweep of the voltammograms. This is clearly shown in Fig. 6, where the region D, is shifted to the region D,’ when stirring is introduced. The effect is particularly magnified in the case of the specimens A and ST because they present deeper cavities due to the size of the MgZn, precipitates. This is in agreement with the experiments changing the sweep rate. The vigorous stirring of the electrolyte makes the working solution near the alloy surface more homogeneous, thus precluding to be the local and bulk compositions of the electrolyte very different. As shown in Fig. 4 and Table 4, the voltammograms performed after 3 h of immersion of the alloys in O,-saturated (1 atm) 0.1 mol dm-’ NaCl presented important differences with respect to those obtained in the deaerated solution. The effects of O2 dissolved were a shift of E,, to potentials several tens of mV in positive direction and the suppression of the anodic maxima M appearing in the case of heat treatments B and C. Instead of this anodic maxima, a smoother current increase was found. However, any changes were not found in the E,, data. These results are coincident with previous data obtained for alloy H in air-saturated 0.1 mol dm-’ NaC1[4]. In the presence of Oz dissolved, the alloys suffered pitting attack[7]. This probably takes place with oxide growth because of the aqueous media and the effect of O2 polarization. Afterwards, when the anodic sweep starts from - 1.2 V, the alloy is repassivated and this makes pit nucleation and propagation somewhat more difficult. As a result, E,, is shifted in the positive direction. When pits propagate after the new E,, is achieved, the conditions of a pitting attack

are recovered and the alloy repassivates again at E, in the reverse scan. This reinforces the importance of E,, in pitting corrosion studies. SigniJcance of the anodic maximum in the localized corrosion of the age-hardened Al-5%Zn-1.67%Mg-

0.24%Cu As shown previously, the age-hardened Al alloys susceptible to SCC present an anodic maximum in the anodic sweep of the voltammograms[9-111. The anodic maximum was due to two breakdown potentials and therefore to the pit propagation in two different phases, the most anodic breakdown potential corresponding to the GBs in those specimens susceptible to SCC. In addition, a greater susceptibility to SCC was correlated with a greater difference between both breakdown potentials. The existence of anodic maxima in the voltammograms of alloys HB and HC was first interpreted according to these hypothesisC4, 51 and the high SCC resistance of these specimens (see the breaking stresses reported in the experimental part) was correlated with the small charge of such anodic maxima. However, the present results suggest that the anodic maxima found for alloys H, J, L and 0, submitted to the heat treatments B and C, are related with a surface dealloying affecting all the alloy surface instead of a pit propagation, this being also in agreement with their high SCC resistance. The charges corresponding to the anodic maxima of the specimens B and C slightly increase with sweep rate (Tables 5 and 7). This is contrary to the classical pitting corrosion behaviour, in which case the pit propagation current increases when the sweep rate decreases (see Fig. 5(b) and [4]). In addition, deep pits are produced when pitting attack takes place. However, any pits were not found when the specimens, submitted to an anodic sweep just after the anodic maximum, were observed under the SEM (see Fig. 8(a)). Instead, a general roughness appeared. On the other hand, the granular structure was clearly observed under the optical microscope. However, according to Fig. 8(a), this was not due to the GB deepening. These results indicate that the anodic maximum appearing in the present alloys submitted to heat treatments B and C corresponds to a transformation affecting all the surface, not to a pitting localized in the GBs. Such anodic maxima cannot be attributed to the specific dissolution of the MgZn, precipitates. As shown for the specimens A and ST, their selective dissolution takes place from potentials more negative than -0.85V (see Fig. 3(a)). In addition, any anodic maxima were not found for the specimens A and ST, which are those presenting the biggest MgZn, precipitates. It is also interesting to observe a close relation between the existence of two plateaus in the E,, vs time measurements[7], the first for more negative potentials, for the specimens submitted to heat treatments B and C, despite the composition of their PFZs were coincident with those of the matrix solid solution. The specimens submitted to heat treatments A and ST presented only one plateau despite presenting two phases: the matrix solid solution and

Localized corrosion of Al-Zn-Mg alloys

the MgZn, precipitates. The first plateau for the specimens B and C, which was attributed to a surface dealloying, would correspond to the anodic maximum, while the second, to EbrZ. Different experiments also bring information on the processes leading to these anodic maxima. These maxima did not appear in the second anodic sweep when the voltammograms were performed from - 1.2 V and the potential corresponding to the point m in Fig. 2(a). In addition, they disappear when the specimens remain immersed in the electrolyte for several hours in the presence of O2 dissolved. This shows that the same process takes place both, in the anodic maximum and in open circuit potential conditions in the presence of O2 dissolved. The anodic charges corresponding to such maxima are about 10mC. If all this charge corresponded to an homogenenous oxidation of the alloy surface, the oxidized layer would be about 0.05 pm in depth. According to the XPS microanalysis, the oxygen profiles before and after the anodic maximum when the potentiodynamic sweep was performed in 0.5 moldm3 NaCl are practically coincident (see Fig. 9(d)). This means that such maxima do not correspond to net oxide growth, at least for this NaCl concentration. Some oxide growth was detected for 0.1 moldme NaCl under potentiostatic polarization for a short time at potentials in the range of the maximum (Fig. 9(a)). On the other hand, the major process in 0.1 moldmm3 NaCl must coincide with that taking place in 0.5moldm-3 NaCl and the oxide film in 0.5moldm-3 NaCl is thinner than in 0.1 moldmm3 (see Fig. 9(a) and (d)). This suggests that oxide growth in the metal-oxide interface together with oxide dissolution in the oxide-electrolyte interface take place in the potential region corresponding to the anodic maximum. Chloride was also detected in the oxide film (see Fig. 9(c)). This is in agreement with previous results of Painot and Augustynski[l6], who showed for 99.99% Al and Al-S%Zn by XPS that the chloride content in the oxide film increased when the critical potential was approached. Apart from the information on the oxide film, the XPS results also show a Mg dealloying. The Zn and Mg percentages of the bulk alloy correspond to atomic concentrations of 2% for both elements. In the oxide film, such atomic concentrations, in the case that the elements were not dissolved into the electrolyte and assuming the formation of ZnO, MgO and Al,O,, would be approximately of 1%. However, Fig. 9(c) shows a Mg depletion in the oxide film near the oxide-alloy interface (the same was found for the specimen LA of Fig. 9(b)). It is then interpreted that the principal process in the anodic maximum is the oxide growth and dissolution together with the transport of Mg through the oxide film defects to the electrolyte, these defects being produced by the chloride penetration. All these results indicate that Ebrl, that is the anodic maximum M, is related with a surface dealloying instead of a pitting attack. Table 6 shows that the charge of the anodic maximum decreases slightly when Cr and Nb are added to the Al-5%Zn-1.7%Mg-0.24%Cu alloy. According to Table 3, this coexists with E,, and E,, shifts in the positive direction and therefore, can also

1945

be due to the Cr and Nb presence in the oxide film, which precludes the extent of the Mg dealloying and of the oxide dissolution. On the other hand, the voltammograms between - 1.2 V and potentials more positive than E,, show that pits in specimens B and C are not localized in the GBs, but they appear at random (Fig. 8(b)). This is also observed for the specimens F, in which only one breakdown potential is found. Therefore, the PFZs do not appear to be more sensitive to the localized attack than the matrix solid solution. The voltammograms obtained for the specimens B and C do not show any anodic maximum apart from M and, as M is due to a process affecting all the surface, the voltammograms of these age-hardened specimens show the characteristics of a SCC-resistant Al-ZnMg alloy, in agreement with previous breaking stress experiments of HB, HC, LB and LC[3].

CONCLUSIONS Co has been found to be a useful tool to study and characterize the localized corrosion of Al-S%Zn1.7%Mg-0.23%Cu, this technique being very sensitive to the compositional changes produced by small additions of Cr and Nb or different heat treatments. The breakdown potentials of the specimens presented changes under electrolyte convection or in the presence of 0, dissolved in the electrolyte. In contrast, the repassivation potentials did not depend on these conditions and they were shown to characterize well the pitting corrosion of the present Al-ZnMg alloys. The repassivation potentials increased in the sequence H (Al-5%Zn-1.7%Mg-0.23%Cu) < J (alloy H with 0.05% Nb) < L (alloy H with 0.14% Cr) < 0 (alloy H with 0.14% Cr and 0.05% Nb), for a given heat treatment, and in the sequence F (quenched) < A (annealed) x ST (cold rolled) < B (quenched and aged) z C (quenched in two steps and aged), for a given composition. This sequence is coincident with that, previously reported, based on the corrosion potentials measurements in chloride solutions containing excess of oxidant. This can be explained by the fact that the oxidizing agents are able to polarize these Al-Zn-Mg alloys to the corresponding pitting potentials. These sequences mean that the repassivation potentials in chloride solutions were shifted in the positive direction when Cr and Nb were added to the alloy, the effect of the first being much more marked. This has been interpreted to be due to the presence of a homogeneous distribution of Cr and Nb oxidized species in the oxide film, which improve the pitting corrosion resistance of these alloys. The big MgZn, precipitates in the specimens submitted to the heat treatments A and ST suffer selective corrosion before the corresponding breakdown potential. In these specimens, pits are initiated and propagate in the cavities were such precipitates were present, in particular in the grain boundaries. The deep cavities formed during pit evolution permit the development of local aggresive environments which produced an important hysteresis in the cyclic voltammograms of the specimens A and ST. In contrast,

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J. A. Glwu~o et al.

in the specimens F, pits were developed at random and the hysteresis found for these specimens are due to a delay in the pit propagation because of the homogeneity of the alloy surface. Any hysteresis was not practically found for the specimens B and C which presented MgZn, precipitates one order of magnitude smaller than those of the specimens A and ST. In this case, pits were also developed at random, not restricted to the grain boundaries. Therefore, the pit nucleation and propagation was related with the size of the MgZn, precipitates, such precipitates acting as pit nucleation centers only in the case of the specimens A and ST. Cr and Nb additions were not relevant in changing the distribution of pits because they were well distributed in the alloy. The anodic sweep of the cyclic voltammograms of the age hardened Al-5%Zn-1.7%Mg-0.24%Cu alloys, ie submitted to the heat treatments B and C, presented an anodic maximum prior to the pit propagation region. Such anodic maxima did not correspond to localized pitting in the grain boundaries and they were found also in the presence of small additions of Cr and Nb. However, the anodic charges of the maxima decreased when the latter alloying elements were added. This was related with the presence of Cr and Nb in the oxide film. The voltammograms of the age hardened specimens corresponded to alloys resistant to SCC, thus being in agreement with previous experiments using the cutedge method and breaking stresses measurements. Acknowledgements-The authors gratefully acknowledge the financial support of this work by the DGICYT (Spain), project No. PB91-0260 and the grant conceded to A. H. Moreira by the CNPq (Brazil). The authors also thank the “Servei Cientitico-T&mic de la Universitat de Barcelona”

for the facilities in the SEM and TEM observations and the EDX and XPS microanalysis.

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(1993).

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11. S. C. Byrne, in Aluminium Alloys. Physical and Mechanical Properties, Vol II (Edited by E. A. Stark and T. H. Sanders), pp 1095-l 107. Warley, U.K. (1986). 12. H. Kaesche, Werkst. Korros. 39,153 (1988). 13. D. M. Aylor and P. J. Moran, J. electrochem. Sot. 133, 868 (1986).

14. W. C. Moshier, G. D. Davis and G. 0. Cote, J. electro&em. Sot. 136,356(1989). IS. G. D. Davis, W. C. Moshier, T. L. Fritz and G. 0. Cote, J. electrochem. Sot. 137,422 (1990). 16. J. Painot and J. Augustynski, Electrochim. Acta 20,747 (1975).