Rotating ring-ring electrode study of copper component dissolution behaviour in air-saturated 0.5 M NaCl solution

Corrosion Science, Vol. 36, No. 5, pp. 773-783, 1994

Pergamon

Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0010-938x/94 $6.00 + 0.00

0010-938X(93)E0017-L

ROTATING RING-RING ELECTRODE STUDY OF COPPER COMPONENT DISSOLUTION BEHAVIOUR IN AIR-SATURATED 0.5 M NaC1 S O L U T I O N J. A . A L l Department of Metallurgical and Materials Engineering, Obafemi Awolowo University, lle-lfe, Nigeria Abstract--The anodic dissolution behaviour of the copper component of Monel-400 alloy in 0.5 M NaCI solution containing dissolved oxygen has been investigated using a rotating ring-ring electrode in combination with linear sweep voltammetry. The results reveal that both Cu(I) and Cu(II) species are generated simultaneously at all potentials between 0 and +400 mV, inclusive, when Monel-400 alloy corrodes in the presence of oxygen. Film-forming reactions occur between the rest potential and 0 inV. The distribution of Cu(1) and Cu(II) species with respect to applied potential is reversed in the presence of oxygen. Dissolution via Cu(II) predominates in the low overpotential region while dissolution via Cu(I) species predominates in the middle to high overpotential region. The reverse is the case in the absence of oxygen. Solution flow rate does not affect this distribution. The explanation for mechanisms of pitting and hence the pit morphology may be found in this dissolution behaviour.

INTRODUCTION

IT HAS been observed that Monel-400 alloy exhibits two forms of pit morphology in seawater.1 While some pits show a shallow trenched structure on the alloy surface, others penetrate deeply and, on occasion, perforate the materials. The conditions under which these morphologies form are not yet defined. In a continuing effort to determine the mechanism by which the two morphologies develop and to define the conditions under which they form, a study was conducted in air-saturated 0.5 M NaCI solution. This followed an earlier study conducted in de-aerated 0.5 M NaCI solutions. It was found in that study that Cu(I) and Cu(II) species were generated simultaneously during the anodic dissolution of Monel-400 alloy in de-aerated NaC1 solution within the potential region of -350 to +400 mV on the Ag/AgCl reference scale. 2 Three potential regions within which the distribution of copper dissolution products varied with respect to applied potential were identified. Up to -150 mV, Cu(I) species were the predominant copper corrosion products. Between -150 and +150 mV, film-forming reactions were predominant, while generation of Cu(II) species dominated the reactions in the potential range between + 150 and +400 inV. Both relative solution flow rate and temperature did not appear to influence this distribution. Akkaya and Ambrose, 3 in their study of anodic behaviour of copper in 1 N sodium bicarbonate solutions containing ammonium chloride, also observed that the distribution of anodic current amongst copper dissolution products varied with Manuscript received 27 August 1993. 773

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applied potential. Their own current distribution was, however, affected by relative solution flow rate. In this investigation, the influence of oxygen on the mechanism of dissolution of c o p p e r in t h e a l l o y a n d o n t h e d i s t r i b u t i o n o f c o p p e r c u r r e n t a m o n g s t c o p p e r c o r r o s i o n p r o d u c t s w a s s t u d i e d u s i n g a r o t a t i n g r i n g - r i n g e l e c t r o d e in a i r - s a t u r a t e d 0.5 M N a C I s o l u t i o n . EXPERIMENTAL METHOD The alloy ring electrode was made from a commercial Monel-400 alloy with a nominal composition (wt%) of 66.20 Ni, 31.5 Cu, 1.14 Mn, 0.70 Fe, 0.17 C and 0.29 others. This electrode had an initial active surface area of 0.059 cm 2. The collector ring electrode was fabricated out of glassy carbon block, supplied by Astra Scientific International. It had an active surface area of 0.060 cm2. The experimental collection efficiency of this electrode configuration was 0.27. The schematic diagram of the ring-ring electrode has been given previously.4.s A Tacussel electrode system Model EAD 400 along with Tacussel Electronic bipotentiostat Model BiPAD 3 and function generator Model GSTP 3 were used for the polarization experiments.4 Details of the electrochemical cell used and sample preparation are as reported elsewhere.4 Polarization procedures are the same as those described in previous papers.2'5 The air-saturated solution was prepared by bubbling air through 0.5 M NaCI solution which had been prepared with de-

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mineralized water and reagent grade chcmicals. The air, which was dried by passing it through indicator Drierite laboratory gas and an air drying unit, was bubbled through the solution for 30 min before the beginning of each experiment. An air atmosphere was maintained above the electrolyte during polarization by passing air through the gas inlet port which is situated at the shoulder of the cell above the solution. The initial pH of the solution was adjusted to 8.2 using NaOH solution. The influence of solution flow rate was also investigated in this environment. All experiments were run at 298 K. Potential data in this paper are reported relative to Ag/AgC1reference scale. E X P E R I M E N T A L RESULTS AND D I S C U S S I O N T h e a n o d i c p o l a r i z a t i o n curve for Monel-400 alloy in a i r - s a t u r a t e d 0.5 M NaCI solution is s h o w n in Fig. 1. T h e electrode r e m a i n s active t h r o u g h o u t the p o t e n t i a l range investigated. A Tafel region is o b s e r v e d b e t w e e n the applied p o t e n t i a l s of +25 a n d 125 m V ( A g / A g C l ) with a slope of 32 m V / d e c a d e , against 119 m V / d e c a d e o b t a i n e d in d e - a e r a t e d solution. 2 T h e collector ring c u r r e n t s for c o p p e r soluble dissolution p r o d u c t s are p r e s e n t e d in Fig. 2. C u r v e (a) was o b t a i n e d by m a i n t a i n i n g the collector electrode p o t e n t i a l at + 6 0 0 m V (Ag/AgC1) to oxidize C u ( I ) to C u ( I I ) species. T h e p r e s e n c e of C u ( I )

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species in solution is confirmed by this curve and data from previous studies. 4'5 Similarly, the presence of Cu(II) species in solution is established by curve (b) obtained by reducing Cu(II) species to Cu(I) species at - 1 5 0 mV. The figure reveals that both Cu(I) and Cu(II) species are generated simultaneously at all potentials between 0 and +400 mV (Ag/AgCi), inclusive, when Monel-400 alloy corrodes in the presence of oxygen under the experimental conditions reported herein. No collector ring current is recorded for either species below the applied potential of 0 mV, most probably because the amount generated is below the detection limit of this technique.

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A more probable reason is that film formation occurs between the rest potential and 0 inV. If this were the case it would support the observation that formation of insoluble copper corrosion products (Cu20 and/or CuCl) occur during the early stages of the anodic process. 6 Chernov et al. 7 also observed from their study of corrosion behaviour of copper in 3% NaCI solution and natural seawater that the main anodic process was the conversion of copper to Cu20 which formed Cu +, CuCI~ and CuCI32-. Figure 3 compares the anodic curve generated in air-saturated solution with that obtained in de-aerated 0.5 M NaC1 solution. A striking difference is the shift of the curve obtained in air-saturated solution to more noble potentials. Up to the applied potential of +200 mV (Ag/AgC1) the polarization current obtained in air-saturated solution is much lower than that for de-aerated solution, the difference being nearly up to two orders of magnitude at certain potentials. This p h e n o m e n o n can be ascribed to the presence of possibly a more resistant film on the specimen polarized in

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air-saturated solution. The lower polarization current in air-saturated solution recorded in this study does not support the results of Bjoradahl and Nobe s who found that the corrosion rate of copper in chloride media in the absence of oxygen was smaller than in the presence of oxygen. A t the applied potential of +200 m V (Ag/AgCI) and above, the two curves are essentially the same. It appears that the anodic processes in this potential region are no longer influenced by the presence of oxygen. Neither are they likely to be indicative of processes affecting the corrosion behaviour of this alloy in oxygenated chloride solutions because the potentials are far r e m o v e d from the corrosion potential. Figure 4 compares the collector ring currents obtained in de-aerated and airsaturated solutions. Figure 4a which is almost a minor reflection of Fig. 3 compares the Cu(I) currents in both media. As with the alloy current in Fig. 3, the Cu(I) current in air-saturated solution is shifted to more noble potentials and is lower than the Cu(I) current obtained in de-aerated solution for all potentials below +250 m V (Ag/AgCl). A b o v e +250 m V Cu(I) currents in the two environments are practically the same. T h e r e is little difference in the behaviour of Cu(II) currents in both environments (Fig. 4b).

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The distribution of the two soluble copper species generated at each applied potential was monitored (Fig. 5). It is interesting to note that the distribution of both species with respect to alloy dissolution potential is reversed in the presence of oxygen. As illustrated vividly in Fig. 5, Cu(II) species dominate in the potential region between 0 and +75 mV (Ag/AgCI) while the amount of Cu(I) species produced in the potential regime +75 and +400 mV is more than that of Cu(II) with the proportion of cupric species generated increasing steadily above the potential of + 250 mV. The reverse is true when dissolution occurs in de-aerated solution or when oxygen concentration is low. 2 More Cu(I) species are generated in the low overpotential region while more Cu(II) species are generated in the higher overpotential region. The detection of Cu(II) species and its dominance over Cu(I) species in this environment in the low overpotential region is at variance with the results of previous studies which show that both pure copper and copper in copper-based Cu-Ni alloy dissolve in the active region to form cuprous chloride complexes in oxygenated chloride solutions, s-~ t The discrepancy can probably be accounted for by the amount of nickel in Monel-400 alloy. There is no doubt that oxygen affects the concentration of both Cu(I) and Cu(II) species in solution through the homogeneous reaction in solution. But as the analysis in Ref. 4 shows, and given that this solution cannot dissolve more than 8 m3/1 of oxygen, t2 homogeneous oxidation of Cu(I) to Cu(II) species by oxygen in solution cannot account for the magnitude of Cu(II) current recorded in this study. Moreover, the homogeneous reaction does not exhibit dependence on applied potential, unlike the data recorded herein. Oxygen, therefore, accounts through the homo-

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geneous reaction with Cu(I) species for a small fraction of the cupric ion current recorded. It is noteworthy that in the air-saturated solution, the Tafel line falls in the potential region where Cu(II) species is the dominant soluble copper dissolution product; the Cu(I) species is the major soluble dissolution product of copper in the alloy in the Tafel region in de-aerated solution or in the presence of low oxygen concentration. Of equal significance is the observation that in de-aerated solution, the gener-

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ation of Cu(II) species dominates that of Cu(I) species in the high overpotential regime where pitting is likely to occur. A significant amount of Cu(I) is generated in the high overpotential domain where pitting is likely to occur in aerated solution. The implication of this for the mechanisms of formation of the two pit morphologies found on Monel-400 alloy exposed to seawater and the conditions under which they form is not yet clear and is the subject of current investigations. It is surmised, however, based on the observation herein, that the shallow-trenched pit morphology is likely to develop under conditions of high oxygen concentration. Under this condition and in the intermediate-to-high overpotentiai region, more Cu(I) species than Cu(II) species are generated. The autocatalysis model of Galvele for pit propagation is based on hydrolysis of cations in solution leading to acidification of the electrolyte inside the pit.13 It has been reported that monovalent ions are not easily hydrolysable. 14 If this is the case with Cu(I) species then it is conceivable therefore, that there are not enough hydrolysable cations in solutions to support deep propagation of pits on this alloy under high oxygen concentration. Conversely, under low oxygen concentration conditions, Cu(II) ions are the predominant copper cations produced in the high overpotential region. They are more easily hydrolysable and could conceivably support deep pit propagation by the autocatalysis mechanism. Figure 6 is the plot of the normalized total copper current as a function of Monel400 alloy dissolution potential. The proportion of anodic current used to generate soluble copper species rises initially but falls to a constant value between +75 and +300 mV (Ag/AgCI) after which it rises again. A similar phenomenon was observed in a previous study. 2 It was explained there that the anodic film forming processes dominated in the applied potential region where the current minimum occurred. The influence of solution flow rate on the anodic polarization of Monel-400 alloy in the presence of oxygen was investigated (Fig. 7). The behaviour observed was

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similar to the behaviour in de-aerated NaC1 solution.2 There appears to be very little dependence on rotation speed, suggesting that the rate determining step for the anodic reaction is not mass transport within the anolyte. The Tafel slope varied from 32 to 39 mV/decade, in no particular order. This is in sharp contrast to the behaviour of NazSO4 solution which exhibited strong dependence on solution flow rate. 5 Figure 8 shows the influence of electrode rotation speed on the distribution of copper dissolution products with respect to applied potential. The figure reveals that fluid flow velocity does not alter the distribution with respect to applied potential of copper corrosion products in this environment. Although solution flow rate does not affect the distribution with respect to corrosion potential of the soluble copper dissolution products, its effect on the individual products is clearly seen in Fig. 8. As was the case in de-aerated NaCI solution, the amount of soluble Cu(I) species generated in this environment does not appear to show any dependence on the electrode rotation speed. The situation with soluble Cu(II) species is, however different. In the low overpotential region where Cu(II) species is dominant, there is a marked dependence, though non-linear, of the amount of Cu(II) species generated on solution velocity. This behaviour was seen in de-aerated NaCI and Na2SO 4 solutions. A linear dependence was recorded in Na2SO 4 solution. The behaviour of Cu(I) species with respect to solution flow rate is also at variance with general observation. Akkaya and Ambrose, 3 Chernov et al.,7 Bjorndahl and Nobe s and many others have found the dissolution of copper via Cu(I) species to be mass transport dependent. The observation herein however, supports the results of Kato and Pickering who found that the anodic dissolution of Cu-9.4Ni-1.7Fe alloy in airsaturated 3.4% without NaC1 solution was not dependent on solution velocity.9 They proposed that diffusion through the porous surface layer controlled the rate of anodic dissolution. CONCLUSION

The observed anodic dissolution behaviour of the copper component in oxygenated 0.5 M NaC1 solution is somewhat different from what has been widely reported in the literature. Both Cu(I) and Cu(II) ionic species have been detected at all potentials investigated with the proportion of Cu(II) species considerably larger than that of Cu(I) species in the low overpotential region and more than can be accounted for by homogeneous reaction of Cu(I) species with oxygen in solution. Although the results from this study are not decisive, it can be surmised from the data reported herein and those from de-aerated 0.5 M NaCI solution that the mechanism of pitting and hence the explanation for the morphology of pits may be found in the concentration of oxygen in solution. This follows from the observation that, depending upon oxygen concentration, the generation of one species predominates over the other in the potential range where pitting is likely to occur. Acknowledgements--The author acknowledges with gratitude the support of the International Nickel Company (INCO) of this project through fellowship (INCRA No. 880133 Vt-2) awarded to him and expresses his thanks to Drs J. R. AMBROSEand E. D. VErUNK,JR for their advice on this project. REFERENCES 1. F. W. FINK and W. K. BOVD, The Corrosion of Metals in Marine Environment, Bayer and Co., Columbus, OH (1970). 2. J. A. ALl and J. R. AMBROSE,Corros. Sci. 33, 1159 (1992).

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M. AKKAYAand J. R. AMBROSE,Corrosion (Houston) 41,707 (1985). J. A. ALl, PhD. Dissertation, University of Florida, Gainesville (1983). J. A. ALl and J. R. AMBROSE,Corros. Sci. 32,799 (1991). R. MaY, J. Just Metals 32, 65 (1953-1954). B. B. CHERNOV,K. T. KUZOVLERAand A. A. OVSYANNIKOVA,Prot. Metals 21, 42 (1985). W. D. BJORNDAHLand K. NOBE, Corrosion 40, 82 (1984). C. KATOand H. W. PmKERIN~,J. electrochem. Soc. 131, 1219 (1984). H. P. L~:Eand K. NOBE, J. electrochem. Soc. 131, 1236 (1984). C. DESLOUIS,B. TRmOLLET, G. MENCOL1and M. M. MUSIAN1,J. appl. Electrochem. 18,374 (1988). C. W. DRANE, Corrosion 1: Metal~Environment Reaction 2nd Edn. (cd. L. L. SHRHR), p. 41. NewnesButterworths, London (1979). 13. ,1. R. GALVELE,J. electrochem. Soc. 123,464 (1976). 14. H. L. HEYS, Physical Chemistry 3rd Edn, Georgc G. Harrap and Co. Ltd, London (1966).