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Different Effects of Copper on the Pitting Corrosion of Stainless Steels 150 Imax = 100 Μa 120 Imax = 1 Ma

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Different Effects of Copper on the Pitting Corrosion of Stainless Steels 150 Imax = 100 Μa 120 Imax = 1 Ma Different effects of Copper on the pitting corrosion of stainless steels 150 Imax = 100 µA 120 Imax = 1 mA 1,2 2 1 1,2 ) 90 T. Sourisseau , E. Chauveau , B.Malki , B.Baroux E C S / V 60 m ( p (1) Institut National Polytechnique de Grenoble, e r GEDAI/LTPCM/INPG V 30 (2) Usinor Recherches, CMDI 0 Ugine Research Center, 73400 Ugine, France e-mail [email protected] -30 bNi+MnS bNi+MnS+Cu Figure 1 :. Repassivation potentials of 304, 304+Cu, Even if a review on the subject shows that the bNi and bNi+Cu in 0.02M NaCl pH = 6.6 at 23°C influence of Copper on pitting corrosion of austenitic (test s performed at a polarization rate of 100 stainless steels depends upon both experimental mV/min). conditions [1, 4] (pH, chloride concentration and temperature) and steel composition (especially Schema of figure 2 sums up the proposed Molybdenum and Sulfur contents), no comprehensive mechanism deduced from our observations performed mechanistical model has been proposed to account for on stainless steels, but since austenite structure is not the different results found in the literature. involved in any of these effects, this mechanism should also apply to ferritic steels. This work aims first of all to study the influence of the addition of 3% Copper on pitting of 304-type Finally, since also the founded phenomenon and low-Nickel austenitic stainless in 0.02M NaCl depends upon experimental conditions (chloride pH=6.6 at 23°C with and without small addition of concentration, temperature and steel composition), CuCl2. We try then to propose a model that specifies our model is likely to clarify some controversial the role of Copper particularly in the pit initiation results [4] published on the problem during the last steps. Different experimental techniques: current decade. transient analysis, polarization curves in acidic media, pitting and repassivation potential measurements, XPS and SEM observations have been used for this purpose. Repassivation by delayed Cu enrichment, Assuming that metallic Cu hinders Cr and Fe Generally, Copper is known to decrease the Fe, Cr metallic dissolution rate in acidic media and consequently to slower the propagation pit growth Adsorption Pit rate. However, Copper was found in our study to have both benefic and detrimental effects in the metastable step of pitting. The two major opposite effects are: Unsoluble Cu2S hindering Pit initiation 1) benefic effect: Usually, in stainless steel pit initiates on or close to sulfide inclusions, with a well known detrimental effect of dissolved sulfur species. However, by Matrix: Dissolved No C u nor adding CuCl2 to our electrolyte we have found that car bide the dissolution of copper allows to precipitate an prec ipitate Cu dissolution unsoluble Copper sulfide Cu2S preventing sulfur species from adsorbing on steel surface. This has the Low Cu effect on Cr effect of inhibiting pit initiation revealed clearly by a content significant increase in the pitting potential. Passive film 2) detrimental effect: Figure 2: A proposed mechanism that specify the role On the contrary, we have found that for an of Copper on pitting corrosion of stainless steel. appropriate metal-electrolyte potential difference at the bottom of the pit ie when it is more cathodic than References the thermodynamic dissolution potential VCu, Copper enrichment takes place on pit walls. Under such [1] H. Lin, W. Tsai, J. Lee, C. Huang, Corrosion conditions, metallic Copper is expected to prevent Science, 33 (5), 691 (1992). Chromium oxidation and hence pit repassivation. [2] M. Seo, G. Hultquist, C. Leygraf, N. Sato, Idem, this has the effect of lowering the pitting Corrosion Science, 26 (11), 949 (1986). potential (see figure 1). [3] DM Buck, Trans. Am. Electrochem. Soc., 39, 109 (1921). [4] G. Wranglen, Corrosion Science, 9, 585 (1969). [4] R.J. Brigham, E.W. Tozer, Corrosion, 30 (5), 161 (1974). .
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