Corrosion rate measurements of electrodeposited zinc-nickel alloy coatings

Corrosion Science, Vol. 36, No. 7, pp. 1115-1131, 1994 Copyright ~) 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0010-938X/94 $7.00 + 0.00

Pergamon

CORROSION ELECTRODEPOSITED K. R.

BALDWIN,

RATE MEASUREMENTS ZINC-NICKEL ALLOY

OF COATINGS

M. J. ROBINSON* and C. J. E. SMITH

Structural Materials Centre, Defence Research Agency, Farnborough, Hampshire, U.K. * School of Industrial and Manufacturing Science, Cranfield University, Cranfield, Bedfordshire, U.K.

A b s t r a c t - - T h e corrosion rates of detached electrodeposited zinc-nickel alloys containing up to 30% nickel by weight were determined under total immersion conditions in NaCI solutions and on exposure to neutral salt fog. Weight loss m e a s u r e m e n t s and linear polarisation resistance (LPR) techniques were employed to determine corrosion rates under both experimental conditions. The corrosion rates of zincnickel alloys were found to decrease with increasing alloy nickel content u n d e r total immersion conditions in NaCI solutions, indicating that the barrier corrosion resistance of zinc alloys increases with increasing nickel content. This effect was attributed to both a reduction in the rate of zinc dissolution due to nickel enrichment at the metal surface and an increase in the stability of the corrosion product layer. Pure zinc was found to exhibit a higher corrosion rate in the salt fog environment than in quiescent NaCI solutions and this was thought to result from the higher level of available oxygen in the salt fog environment. In contrast, the corrosion rate of a zinc-14 wt% nickel alloy was lower in the salt fog environment than under quiescent conditions due it is suggested to a more rapid e n n o b l e m e n t of the surface and the formation of a protective corrosion product layer.

INTRODUCTION

ELECTRODEPOSITEDzinc-nickel alloys are finding increasing use as protective coatings for steel. The corrosion resistance of zinc-nickel alloys has been widely determined using the neutral salt fog test. Recent studies conducted at D RA Farnborough 1 have shown that the corrosion resistance of zinc alloys increases sharply as the nickel content is increased until an optimum resistance is reached for those coatings containing approximately 14 wt% nickel. Further increases in the nickel content, >14 wt%, resulted in a gradual decline in the level of corrosion protection afforded to steel. The initial rapid rise in performance as the alloy nickel content approaches 14 wt% has been attributed 1 to an increase in barrier corrosion resistance, with the subsequent decline above approximately 14 wt% nickel being due to the loss of sacrificial corrosion protection. In this paper the influence of nickel on the barrier corrosion resistance of zinc-nickel alloys in chloride containing media has been examined in detail. Information relating to the barrier corrosion resistance of a coating can be obtained by measuring its corrosion rate in isolation from any metallic substrate. In the present work, the corrosion rates of detached zinc-nickel alloy coatings have been determined in aqueous chloride environments using the electrochemical linear polarisation resistance (LPR) technique and conventional weight loss measurements. Manuscript received 18 N o v e m b e r 1993; in a m e n d e d form 18 January 1994. 1115

1116

K . R . BALDWIN, M. J. ROBINSON and C. J. E. SMrria EXPERIMENTAL

METHOD

Preparation o f electrodeposited zinc-nickel alloys Electrolytes were prepared by dissolving 0.92 mol 1-1 zinc sulphate and 0.58 tool 1- l nickel sulphate in distilled water. This gave a solution pH of 4.5 and the electrolyte was unbuffered. The electrolyte also contained a surfactant, polyoxyethylene sorbitan monolaurate (10 ml 1-1), which was added to minimise the formation of holidays (small pores) in the coatings. The electrolyte was aerated during plating to assist the formation of uniform coatings. Corrosion rate measurements were conducted using detached pure zinc and zinc-nickel alloy coatings. These were prepared by plating directly onto aluminium foil electrodes from which the coatings (0.5 mm thickness) were readily detached. Tb~" construction of the aluminium foil electrodes has been detailed elsewhere.2'3 Prior to plating, the aluminium foil electrodes were cleaned by immersion in dilute sulphuric acid (0.1 m o l l - 1 ) for 30 s, rinsed in distilled water and then totally immersed in plating electrolyte. The zinc-nickel alloy coatings were deposited at a current density of 10 A dm -2. Pure zinc and pure nickel coatings were also electrodeposited on to aluminium foil electrodes using commercially available electrolytes. Immediately after plating, the cathode assembly was removed from the electrolyte and rinsed in distilled water, followed by acetone solvent. The cathode assembly was then dismantled and the coating carefully detached from the aluminium foil. The detached coatings were further rinsed in distilled water and acetone to remove any traces of electrolyte. The detached coatings were stored in a desiccator until required for compositional analysis or specimen preparation. Corrosion rate determinations Weight loss measurements. Weight loss measurements were conducted using 0.3-0.5 mm thick detached pure zinc or zinc-nickel alloy coatings (15 mm × 20 mm dimensions). The detached specimens were each weighed and then suspended vertically from nylon thread either under total immersion conditions in quiescent 600 mmol 1-1 (3.5% w/v) NaCI solution (100 ml) at 25°C, or in a neutral salt fog cabinet operating in accordance with ASTM B 117.4 The neutral salt fog test employed a humidified 5% w/v NaCI spray at 35°C. Weight loss measurements were conducted in duplicate for each immersion or exposure period. After the above tests had been completed the specimens were removed from the test environment and rinsed in distilled water and acetone. For the pure zinc specimens, the corrosion products were removed by chemical cleaning in a hot chromic acid solution. This treatment was unsatisfactory for the zinc-nickel alloys and instead they were cleaned in a hot glycine solution, using a method described by Beccaria et al.5 After cleaning, the specimens were rinsed in distilled water, followed by acetone and then allowed to dry in air. The weight loss of each specimen was then calculated by subtracting the weight after chemical cleaning from the values obtained before immersion/exposure. Linear polarisation resistance measurements. LPR sweeps were carried out using electrodes constructed from detached pure zinc and zinc-nickel alloy coatings under total immersion conditions and during exposure to neutral salt fog. For the total immersion tests, a conventional three-electrode glass cell was employed which contained quiescent or aerated 600 mmol l-I NaCI solutions at 25°C. Prior to each LPR sweep, the test electrodes were permitted to stabilise for approximately 20 min in the solution. Each electrode was then cathodically polarised from its open circuit potential by 10 mV at a sweep rate of 0.1 mV s - l . This was immediately followed by a cathodic sweep over a 20 mV range at the same sweep rate and the electrode was then returned to open circuit. Several potentiodynamic sweeps were also performed over wide potential spans to obtain anodic and cathodic Tafel constants, using fresh detached coatings each time. For several zinc-nickel alloy specimens, the surface composition was estimated after 200 h immersion in the chloride solution. In each case, a final open circuit potential measurement was made and the alloy specimen removed from the test solution. The corrosion products were removed using the hot glycine cleaning treatment described above. The surface composition of each specimen was then determined using electron microprobe analysis (EMPA). A previous study 3 showed that the glycine treatment itself does not significantly affect the surface composition of zinc-nickel alloys. LPR sweeps were also conducted during exposure to neutral salt fog. For these experiments a novel flat three-electrode cells which is illustrated by Fig. 1 was used. The construction of the cell has been described in detail elsewhere. 2'3 Briefly, a pure zinc or zinc-nickel alloy coating was attached to a glass slide using an epoxy adhesive and a Luggin capillary, tipped with a ceramic frit, was positioned adjacent to

Corrosion rates of electrodeposited zinc-nickel alloys

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Diagram of cell used to conduct L P R m e a s u r e m e n t s u n d e r neutral salt fog conditions.

and level with the surface of the detached coating. The Luggin capillary was in solution contact with an external SCE. A n epoxy lip was constructed around part of the ceramic frit to trap a drop of solution over the surface of the frit and hence ensure solution contact with the coating. The cell was completed by attaching a platinum counter electrode to the glass slide approximately 0.5 m m from the opposite and lower edges of the detached coating. The fiat three-electrode cell was m o u n t e d in the test cabinet at an angle of 20 ° to the horizontal. The ohmic drop across the cell was found to fall to negligible values after several hours in the c h a m b e r as a salt film developed across the electrode surfaces. The L P R experiments were then conducted using the sweep parameters detailed above. EXPERIMENTAL

RESULTS

Potentiodynamic polarisation sweeps Anodic potentiodynamic polarisation sweeps obtained for detached electrodeposited pure zinc and zinc-nickel alloy coatings after 1 h immersion in 600 mmol 1-1 NaC1 solution are shown by Fig. 2. For each coating, a large increase in current density was obtained for a small increase in polarisation overpotential, indicating that they were essentially active in nature. Figure 3 illustrates the anodic polarisation behaviour obtained for an electrodeposited pure nickel electrode after 1 h immersion in quiescent 600 mmol 1-1 NaC1 solution. This plot clearly shows the presence of an extensive passive region, which is typical for nickel in near-neutral solutions. A comparison with Fig. 2 indicates that even those zinc alloys with high nickel levels did not exhibit passive corrosion behaviour. The anodic Tafel constant, b a , was obtained for each coating by measuring the slope of each anodic polarisation curve using a graphical method described by Mansfeld. 6 The values of b a obtained for the zinc alloys are plotted against nickel content by Fig. 4. This shows that the value of ba increased as the alloy nickel level was increased. This behaviour can be attributed to the alloys becoming gradually less active in nature as the level of the more noble nickel component in the alloys was increased. Polarisation sweeps were also conducted for pure zinc and Zn-14 wt% Ni alloys after extended immersion times in 600 mmol 1-1 NaCI solutions. The anodic polarisation characteristics of pure zinc did not change significantly over immersion times of up to 500 h. This indicated that the same anodic Tafel constants can be used

1118

K.R. BALDWIN,M. J. ROBINSONand C. J. E. SMrrH

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Lo9 .Current IrA/creel FI6.3. Anodic polarisation sweep of electrodeposited pure nickel after 1 h immersion in quiescent 600 mmol 1-1 NaC1 solution. Sweep rate: 2.5 mV/20 s. for pure zinc in corrosion rate calculations, i n d e p e n d e n t of immersion time. In contrast, the anodic polarisation behaviour of the Z n - 1 4 w t % Ni alloy was f o u n d to change considerably during immersion in chloride solutions. Figure 5 shows the anodic polarisation behaviour of a series of Z n - 1 4 w t % Ni alloys in 600 m m o l 1-1 NaC1 solution after immersion times of up to 450 h. This indicates that both the corrosion potential and the slope of the anodic polarisation curve increased with increasing immersion time. Zinc-nickel alloys are k n o w n to be susceptible to dealloying, or m o r e specifically, a form of de-zincification. 2'3'7'8 It is t h o u g h t that the selective dissolution of zinc from the alloy matrix p r o m o t e s nickel enrichment in the

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Flc. 5. Anodic polarisation sweeps of Zn-14Ni alloys in quiescent 600 mM 1-1 NaC1 solutions, after various immersion times. Sweep rate: 2.5 mV/20 s. surface layers. This p h e n o m e n o n would lead to gradual alloy ennoblement, thereby accounting for the shifts in potentials observed in Fig. 5. Anodic Tafel constants obtained from each curve indicated that b a increased from 39 m V after one hour immersion to a value of 45 m V after 450 h immersion, showing that due to changes in surface composition the apparent value of b a for the zinc-nickel alloys is time dependent. H o w e v e r , once again there was a complete absence of the passive

1120

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Relationship between open circuitpotential and alloy compositionfor zinc-nickel alloys after 1 and 200 h immersionin 600 mmol 1-1 NaCI solution.

corrosion behaviour observed for pure nickel. For zinc-nickel alloys, a single value of b a cannot therefore be employed in long-term corrosion rate measurements because of the time-dependent nature of b a. In the present work, it has been established that the surface composition of zincnickel alloys can be estimated from open circuit potential measurements. Figure 6 shows a plot of open circuit potential versus alloy composition for specimens which have been immersed in chloride solution for 1 and 200 h. After 1 h immersion it can be assumed that no significant de-alloying had occurred. For zinc-nickel alloys immersed for 200 h, the potential was measured immediately before removal of the specimens from the test solution. It was found from the analysis of the surface composition of the zinc-nickel alloys, that the potentials of the alloys after immersion in chloride solution were in close proximity to the potentials adopted by the compositions to which they had ennobled. Figure 6 shows that the potential data obtained after 200 h immersion fall on the potential-composition line determined after 1 h, confirming that surface composition can be accurately determined from potential data. In the L P R experiments, the open circuit potential of each zincnickel alloy was measured immediately prior to the sweep, which enabled the surface composition to be found from the calibration plot in Fig. 6. The appropriate value of b a was then obtained from Fig. 5 and used in the corrosion rate calculations. The cathodic polarisation curves obtained for Zn-14 wt% Ni alloys after various immersion times are shown in Fig. 7. The curves were found to shift to lower current densities during immersion in the chloride solution. This suggests that the rate of the cathodic corrosion reactions occurring on the alloys decreased with increasing immersion time, possibly as a result of ennoblement. The cathodic polarisation curves obtained for the Zn-14 wt% Ni alloys for each immersion time showed highly polarised behaviour, i.e. only a small increase in current density was obtained for a large increase in polarisation overpotential. This is indicative of cathodic reduction

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reactions that are under diffusion control, e.g. oxygen reduction. The cathodic Tafel constant, be, was found to vary from 450 to 520 mV decade -1 for the metal coatings studied. This indicates that bc is over ten times larger than ba (21--45 mV decade-I).

Linear polarisation resistance measurements LPR measurements under total immersion conditions. The L P R sweeps obtained for the detached coatings displayed a near-linear overvoltage-current density response in most cases, z'3 The slope of each AV vs Ai curve was calculated using a least-squares method, giving values for the polarisation resistance, Rp. The corrosion current density was calculated from the polarisation resistance using a form of the S t e r n - G e a r y 9'1° relationship: icorr =

b a × bc 1 2.3(b, + be) Rp

(1)

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ba 2.3Rp

(2)

The corrosion currents of zinc alloys containing up to 28 wt% Ni were determined after 1 h immersion in quiescent 600 mmol 1- t NaC1 solution. These values were considered to represent the actual corrosion currents of zinc-nickel alloys, before de-alloying had occurred to any significant extent. The relationship between the composition of the zinc-nickel coatings and the corrosion current density in quiescent 600 mmol 1-1 NaCI solution is illustrated by Fig. 8. This indicates that the corrosion current density of the zinc alloys decreased with increasing nickel content over the compositional range studied and suggests that the barrier corrosion resistance of zinc alloys increases with increasing nickel content.

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Fro. 8. Variation ofcorrosioncurrentdensity withinitial(pre-immersion) nickelcontent ~relectrodepositedzinc-nickelalloycoatings, determined by LPR.

The corrosion rate was calculated from the corrosion current using the following equation: CR(mdd) - icorr X t x W a 10 x F '

(3)

where C R = corrosion rate ( m g d m - E d a y - ] ) ; icorr = corrosion current density (ktA cm-2); t = time (86400 s); W = electrochemical equivalent of metal/alloy; F -- 96 483 C m o l - i. The corrosion rate of pure zinc in quiescent solution was 122 mg dm -2 day -1 which is approximately in the mid-range of values reported by Sunder and Boyd 12 (50-220 mg dm -2 day - i ) for zinc under total immersion conditions in sea water. The corrosion rate values obtained for the zinc-nickel alloys also compare favourably with those measured by Short et al. 7 in 5% NaCl solution. In the present work, the corrosion rate of the zinc alloys was found to decrease by approximately 4.4 mg dm -2 day -1 for each 1% by weight increase in the nickel content. The open circuit potentials of the zinc-nickel alloys were measured immediately prior to each LPR sweep and are plotted against the corrosion current density determined after one hour immersion in Fig. 9. The corrosion current density values were obtained by employing equation (2), i.e. using Rp values measured from L P R sweeps and b a values determined from Fig. 4. The plot shown by Fig. 9 indicates that, for the zinc-nickel alloys, the open circuit potential became more negative as the corrosion current density was increased. Similar observations have been made by Short et al. 7 for a range of zinc alloys. They concluded that corrosion was predominantly under cathodic control since the corrosion current density becomes larger as the potential becomes more negative. The gradient of the plot constructed in Fig. 9 can therefore be taken as an estimate of the cathodic Tafel constant (bc). This was found to have a value of 220 mV decade-1 which confirms the assumption made earlier that be is significantly larger than the anodic Tafel constant, b a.

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FI6.9. Relationship between corrosion current density and open circuit potential for detached pure zinc and zinc-nickel alloy coatings in 600 mmol 1-1 NaC1solution, showing the initial (pre-immersion)compositionsof the coatings. It has been found that zinc-nickel alloys are susceptible to a form of de-alloying (i.e. de-zincification) and it is therefore important to identify changes which may arise in their corrosion behaviour over extended immersion periods as a result of this phenomenon. The variation of corrosion current density with immersion time in quiescent 600 mmol 1-t NaCI solution for a range of zinc-nickel alloys is illustrated by Fig. 10. The corrosion current density of pure zinc is also shown on this plot and was found to be relatively stable during immersion in the chloride solution. In contrast, the corrosion current density of the zinc-nickel alloys were found to gradually decrease with immersion time, which suggests that the barrier corrosion resistance of the alloys would increase with increasing immersion time. This effect became more pronounced as the initial (pre-immersion) nickel content was increased. The decrease in corrosion current density observed for the zinc-nickel alloys during immersion in NaCI solution appeared to be associated with an increase in open circuit potential, i.e. an ennoblement effect. The relationship between corrosion current density and open circuit potential with immersion time is shown for pure zinc and a Zn-14 wt% Ni alloy by Fig. 11. This indicates that as the zinc-nickel alloy became more noble during immersion, the corrosion current steadily decreased and appeared to mirror even small changes in potential. In contrast, the potential of pure zinc was almost constant and the corrosion current remained very stable. The effect of oxygen concentration on the corrosion behaviour of the coatings was also investigated. The corrosion rates of detached pure zinc and Zn-14 wt% Ni alloy coatings were determined over an extended immersion period (500 h) in continuously aerated 600 mmol !-1 NaCI solutions. The average (mean) corrosion

1124

K . R . BALDWIN,M. J. ROalNSONand C. J. E. SMITH tO0.

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Variation in corrosion current density with time for detached electrodeposited coatings during immersion in quiescent NaCI solution.

rate of each coating was determined under aerated conditions and the values obtained are compared in Table 1. The average corrosion rate of pure zinc was about ten times higher in the aerated sodium chloride solutions than under quiescent conditions. In contrast, the average corrosion rate of the Zn-14 wt% Ni alloy was slightly lower in the aerated solution than under quiescent conditions. Also, the zinc-nickel alloy did not appear to produce the white corrosion products observed on the alloy under quiescent conditions but instead developed a thin grey surface film. The corrosion products formed by pure zinc and a Zn-14 wt% Ni alloy were analysed after 500 h immersion in quiescent 600 mmol 1-1 NaCI solution. The electrodes were removed from the test solution, rinsed in distilled water, followed by acetone. For the zinc electrodes, a sample of the voluminous white corrosion products was taken from the surface for analysis. This procedure was not possible for the zinc-nickel alloy as the corrosion products were not present in sufficient quantity. For this specimen, the corroded surface of the intact coating was analysed. The specimens were analysed using X-ray diffraction (XRD), which showed that the corrosion products from the pure zinc electrode were composed mainly of zinc oxide (ZnO). For the zinc-nickel alloy, the corrosion products were mainly a form of zinc

Corrosion rates of electrodeposited zinc-nickel alloys 100.

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Variation in corrosion current density and open circuit potential for detached pure zinc and Zn-14 wt% Ni alloy coatings (pre-immersion composition) in quiescent NaCI solution. F I G . 11.

TABLE 1.

AVERAGE CORROSION RATES OF DETACHED PURE ZINC AND

Zn-14 wt% Ni ALLOYCOATINGSDETERMINEDBYLPR Average corrosion rate of coating (mg dm -2 day i) Test environment

Quiescent 600 mmol 1- i NaC1 Aerated 600 mmol 1-1 NaC1 Neutral salt fog

Pure zinc

Zn-14 wt% Ni alloy

35.4 309.2 387.8

15.8 12.3 9.7

h y d r o x y c h l o r i d e ( Z n s ( O H ) 8 C I 2. H 2 0 ) a n d o n l y a trace o f Z n O was d e t e c t e d . T h e r e was also e v i d e n c e to suggest t h a t nickel o x i d e ( N i O ) was p r e s e n t on t h e alloy surface.

Linear polarisation resistance measurements under salt fog conditions T h e v a r i a t i o n in c o r r o s i o n c u r r e n t density with i m m e r s i o n t i m e f o r p u r e zinc an d

1126

K.R. BALDWIN,M. J. ROBINSONand C. J. E. SMITH I000

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(hours)

Fio. 12. Variation in corrosion current density with time for detached pure zinc and Zn-14 wt% Ni alloy coating (pre-immersion composition) on exposure to neutral salt fog. a Z n - 1 4 wt% Ni alloy measured in the salt fog cabinet using the L P R technique is shown by Fig. 12. This indicates that the corrosion current densities recorded for pure zinc were significantly higher in the salt fog environment than observed for the Z n - 1 4 wt% Ni alloy. The current densities measured for pure zinc remained relatively stable, as observed under total immersion conditions, whereas the corrosion current of the zinc-nickel alloy decreased sharply in the first few hours of exposure followed by a more gradual decline in/corr. For pure zinc under total immersion conditions it was found that if the solution was aerated the average corrosion rate obtained approached the value measured in the highly aspirated salt fog environment (Table 1). (The higher t e m p e r a t u r e in the salt fog cabinet may also have contributed to the increase in the corrosion rate of zinc.) Table 1 shows that the behaviour of the Z n - 1 4 wt% Ni alloy was completely opposite to that observed for pure zinc; the corrosion rate of the zinc-nickel alloy was lower in the salt fog environment than under quiescent total immersion conditions and the effect of solution aeration was to lower the corrosion rate still further. (Note: the small difference in the chloride levels used under each experimental condition has been found to have a negligible effect on corrosion rate. 3) The decrease in corrosion rate observed for the zinc-nickel alloys during

Corrosion rates of electrodeposited zinc-nickel alloys t00

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FIG. 13. Variation in corrosion current density and open circuit potential with time for detached Zn-14 wt% Ni alloy coatings (pre-immersion composition), under total immersion conditions in quiescent 600 mmol 1- 1NaCIsolution and on exposure to neutral salt fog.

exposure to neutral salt fog again appeared to be associated with an increase in potential. The variation in corrosion current density and open circuit potential with time for the Z n - 1 4 wt% Ni alloy under both total immersion and salt fog conditions is shown by Fig. 13. One obvious effect is that in the salt fog environment, the zincnickel alloy adopted open circuit potentials that were at least 100 m V more noble than found under total immersion conditions. Although the potentials measured under both experimental conditions were initially similar, under salt fog conditions, the rate of ennoblement over the first 50 h of exposure was considerably m o r e pronounced. This suggests that the initial rate of de-alloying was higher in the salt fog environment than under quiescent total immersion conditions.

Weight loss determinations The weight losses of detached pure zinc and Z n - 1 4 wt% Ni alloy specimens were determined under total immersion conditions in 600 m m o l 1-1 NaCI solution and on exposure to neutral salt fog. The variation of weight loss with time is shown for both

1128

K.R. BALDWIN,M. J. ROBINSONand C. J. E. SMITH

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FIG. 14. Weightloss versus time behaviour for detached pure zinc and Zn-14 wt% Ni alloy coatings (pre-immersion composition), under total immersion conditions in quiescent 600 mmol 1-] NaCI solution and on exposure to neutral salt fog showing mean, maximum and minimum values obtained for duplicate sets.

experimental conditions by Fig. 14. The weight loss of pure zinc under both total immersion and salt fog conditions was found to increase linearly with time. This indicates that the corrosion rate of pure zinc will be almost constant with time, which is consistent with the results obtained from the L P R experiments (Figs 10 and 11). However, for the Z n - 1 4 wt% Ni alloy, the increase in weight loss was linear for the initial 200 h immersion but then increased very slowly for the remainder of the experiment, indicating that the corrosion rate was initially stable but then gradually decreased. This type of behaviour was observed for the zinc-nickel alloy in both test environments. The average corrosion rates of the above test specimens were calculated over the 500 h test period to enable comparisons to be made with the values obtained using the linear polarisation resistance technique. The data obtained are shown in Table 2 below. The L P R and weight loss techniques were found to show reasonable correlation in both test environments. The correlation was particularly good for pure zinc under both experimental conditions and for the Z n - 1 4 wt% Ni alloy under total immersion conditions. The average corrosion rates determined by L P R were found to be less than 20% higher than those determined by weight loss.

1129

Corrosion rates of electrodepositedzinc-nickel alloys TABLE 2. AND

AVERAGE CORROSION RATES IN m g

Zn-14Ni wt%

dm- 2day- ~, oF DETACHED PURE ZINC

ALLOY COATINGS DETERMINED BY WEIGHT-LOSS AND

Total immersion Coating Pure zinc Zn-14 wt% Ni alloy

LPR

Neutral salt fog

Weight loss

LPR

Weightloss

LPR

31.2 379.0

35 388

13.1 6.2

16 10

DISCUSSION The barrier corrosion resistance of zinc-nickel alloys In the present work, a series of cathodic polarisation sweeps were performed on Zn-14 wt% Ni alloys during immersion in 3.5% NaC1 solution (Fig. 7). An examination of the cathodic polarisation curves shows that they differ from the purely diffusion limited case in that the corrosion current does not reach a constant value as the polarisation overpotential is increased. A possible explanation is that the diffusion limiting current is not attained at moderately small overpotentials due to a continued supply of oxygen to the metal surface. At higher overpotentials, the reduction of water would also occur to supply an additional source of current which would also contribute to the net cathodic current density. A consequence of the nonideal form of the cathodic curves is that a change in the slope of the anodic polarisation curve will influence the magnitude of the corrosion current density. Thus if the corrosion potential of the zinc-nickel alloy is increased, i.e. through an ennoblement process, a decrease in the magnitude of the corrosion current density would be obtained, as observed in Fig. 9. The presence of nickel at the surface of zinc alloys is also considered to contribute to an improved level of protection by increasing the stability of the corrosion product layer. In aqueous environments, the initial corrosion product layer observed on the surface of zinc rich coatings is known to be zinc hydroxide. 13--16The hydroxide is generally a continuous adherent white film on the surface of the metal. Zinc hydroxide is considered to be an effective insulator and its presence would tend to stifle the corrosion reactions occurring on the metal surface, thereby decreasing the rate of corrosion. For pure zinc coatings, the analysis of corrosion products by Miyoshi et al. 15 has shown that the hydroxide layer rapidly de-hydrates to form mainly zinc oxide, which is observed as characteristic voluminous white corrosion products. In the present work, such voluminous corrosion products were observed on pure zinc and zinc alloys containing low levels of nickel and were confirmed as being mainly zinc oxide by X-ray diffraction analysis. Zinc oxide can be considered to provide a less efficient barrier against corrosion than the hydroxide due to its poorer insulating properties and loosely adherent nature. In contrast to pure zinc, zinc alloys, particularly those containing high nickel contents, have been found by Miyoshi et al. ,15 to form mainly the more protective hydroxide, rather than the oxide. This has been confirmed in the present work by the analysis of corrosion products on the surface of a Zn-14 wt% Ni alloy. It was found that only small levels of zinc oxide were present on the surface of the zinc-nickel alloys and the corrosion product layer was mainly a form of zinc hydroxychloride.

1130

K.R. BALDWIN,M. J. ROBINSONand C. J. E. SMITH

The presence of different forms of binary phases of zinc-nickel alloys has also been considered to influence the corrosion rate. 17 Electrodeposited zinc alloys containing approximately 10-16% nickel by weight are known to exist mainly as a pure single gamma phase, whereas those containing higher or lower nickel levels exist as binary or multiphase alloys.18 Shibuya et al.17 proposed that the various phases present in electroplated binary zinc-nickel alloys adopt different equilibrium potentials in aqueous solutions. They suggested that alloys containing more than one phase would therefore be susceptible to local galvanic corrosion whereas the single phase alloys would not. Shibuya et al.17 concluded that the corrosion rates of the single phase alloys should therefore be lower than those of the binary or multiphase alloys. If local galvanic corrosion effects occurred for the zinc-nickel alloys studied in the present work, then the corrosion rates of those containing 10-16 wt% nickel would be expected to be lower than those containing alloying additions outside of this range. Figure 8 shows that the corrosion rate of zinc alloys decreased with increasing nickel content and no significant deviation from this trend was obtained for those containing 10-16 wt% nickel. Thus no evidence was found to support the above theory and although local galvanic action may have occurred on the surface of zinc-nickel alloys, it would be a minor effect compared with the bulk dissolution of zinc from the alloy matrix.

The effect o f oxygen concentration on corrosion rate The effect of oxygen concentration on the corrosion rate of zinc-nickel alloys was investigated. An increase in the oxygen concentration in solution appeared to increase the corrosion rate of pure zinc but decrease the corrosion rate of a Zn-14 wt% Ni alloy (Table 1). This behaviour can be interpreted by examining the effect of oxygen concentration on the cathodic polarisation characteristics of a metal coating. An increase in the concentration of dissolved oxygen will increase the limiting diffusion current. By increasing the limiting diffusion current, the cathodic reaction is depolarised resulting in an increase in the corrosion current density, accounting for the increase in corrosion rate observed for pure zinc when the test solutions were aerated. For the Zn-14 wt% Ni alloy, the corrosion rate decreased as the oxygen level in solution was increased. This effect is difficult to interpret as an increase in oxygen concentration will usually cause the corrosion rate of a metal to increase. It is, however, possible to comment on two unusual aspects of zinc-nickel alloy corrosion which may account for the observed behaviour: (1) The zinc-nickel alloy initially corroded at an increased rate in the oxygen-rich salt-fog environment, causing an accelerated rate of ennoblement and hence a reduction in the rate of corrosion; and/or (2) The increased level of oxygen in solution may have stimulated the formation of zinc hydroxide thereby promoting the development of a more protective surface film on the metal surface. For the Zn-14 wt% Ni alloy, an accelerated rate of ennoblement was observed in the neutral salt fog environment (Fig. 13) and thus the formation of an enriched nickel surface layer would have been stimulated, supporting mechanism (1) above. No direct evidence was found to support mechanisms (2) but it seems likely that the increased concentration of oxygen in solution would have stimulated the formation of the zinc hydroxide layer earlier identified by X-ray diffraction analysis.

Corrosion rates of electrodeposited zinc-nickel alloys

1131

CONCLUSIONS (1) T h e c o r r o s i o n r a t e s o f p u r e zinc a n d Z n - 1 4 w t % N i alloys d e t e r m i n e d by L P R u n d e r t o t a l i m m e r s i o n c o n d i t i o n s in NaC1 s o l u t i o n s a n d on e x p o s u r e to n e u t r a l salt fog w e r e f o u n d to offer r e a s o n a b l e c o r r e l a t i o n with c o r r o s i o n r a t e s d e t e r m i n e d b y w e i g h t loss. (2) T h e c o r r o s i o n r a t e s o f zinc alloys c o n t a i n i n g up to 28 w t % nickel b y w e i g h t w e r e f o u n d to d e c r e a s e with i n c r e a s i n g alloy nickel c o n t e n t , i n d i c a t i n g t h a t t h e b a r r i e r c o r r o s i o n r e s i s t a n c e o f zinc alloys will i n c r e a s e with i n c r e a s i n g nickel c o n t e n t . This effect was a t t r i b u t e d to a r e d u c t i o n in t h e r a t e of zinc d i s s o l u t i o n d u e to nickel e n r i c h m e n t at t h e m e t a l surface a n d an i n c r e a s e in t h e stability of t h e c o r r o s i o n p r o d u c t layer. (3) P u r e zinc was f o u n d to e x h i b i t h i g h e r c o r r o s i o n r a t e s in t h e salt fog e n v i r o n m e n t t h a n in q u i e s c e n t N a C l solutions. This was t h o u g h t to result m a i n l y f r o m t h e h i g h e r level o f a v a i l a b l e o x y g e n in t h e salt fog e n v i r o n m e n t . I n c o n t r a s t , t h e c o r r o s i o n r a t e o f a Z n - 1 4 w t % Ni n i c k e l alloy was f o u n d to be l o w e r in t h e salt fog e n v i r o n m e n t t h a n u n d e r q u i e s c e n t c o n d i t i o n s , d u e it is s u g g e s t e d to a m o r e r a p i d e n n o b l e m e n t o f t h e surface a n d the f o r m a t i o n o f a p r o t e c t i v e c o r r o s i o n p r o d u c t layer. Acknowledgements--The authors thank the DRA (Farnborough) Analytical & General Chemistry

Section for the analysis of zinc-nickel alloys.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

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