Anodic polarization behaviour of cobalt-chromium alloys in sulphuric acid solutions

Abstract

Anodic polarization behaviour of annealed cobalt, chromium and nine cobalt-chromium alloys in 1, 5 and 10 N sulphuric acid solutions was investigated at 22°C using a potentiostatic technique. All specimens except pure cobalt and the 95Co-SCr alloy had an active-to-passive transition in all acid environments. Pure cobalt and the 95-5 alloy did exhibit marked “secondary” passivation in transpassive region just prior to visible oxygen evolution. “Secondary” passivation was attributed to adsorbed oxygen. The potential width of the passive region for specimens exhibiting passive behaviour was independent of acid concentration, but increased (especially for alloys containing 70 wt- % or more chromium) with increasing chromium content. Pure cobalt and the 95-5 alloy exhibited visible oxygen evolution in the transpassive region. The predominant transpassive electrochemical process for containing 15 wt- % or more chromium was chromate formation. The potential at which chromate formation occurred was composition-independent, but was shifted in the noble potential direction by increasing acid concentration. Tafel slopes for anodic dissolution, chromate formation and oxygen evolution were determined.Corrosion potentials for cobalt, chromium and the cobalt-chromium alloys were linear functions of pH over the pH range + 1·21 to −1·04. dEcorr/d pH varied from −0·050 to −0·060, independent of composition. Passivation potentials, Ep, also linear functions of pH, shifted in the active potential direction with increasing chromium content and increasing pH, dE/d pH being from −0·050 to −0·070, independent of composition. The critical current density for passivation decreased as chromium was added to pure cobalt; the minimum value of Ic for the cobalt-chromium system occured at 30 wt-% chromium. The passive current density also appeared to be minimal at the same chromium content.Anodic polarization behaviour is discussed in terms of the metallurgical structure of the cobalt-chromium system.