Localized corrosion and repassivation behavior of Ni600 and SS304 under conditions relevant to SCC

It is well-known that Ni alloy 600 exhibits lower susceptibility to SCC than stainless steel 304 in chloride-containing environments. To understand potentially relevant electrochemical differences, the kinetics of localized dissolution and the repassivation behavior of the two alloys were compared under pit/crack-like environments. Downward potential scans were conducted at isothermal conditions on 1D pit samples to assess the kinetics of metal dissolution. Downward temperature scans under potentiostatic conditions were performed to determine the conditions of potential, temperature and pit depth at which each alloy repassivates. Alloy 600 exhibits slower activation-controlled dissolution kinetics than SS304 in pit/crack-like environments. However, Ni600 stays active at conditions of lower aggressiveness, like lower temperature, whereas SS304 exhibits a higher tendency to repassivate. DFT calculations help explain these conflicting results and develop an understanding of the behavior of these different materials under SCC conditions.