An electrochemical study assisting the interpretation of acid sulfate stress corrosion cracking of NiCrFe alloys

Electrochemical measurements were made on Ni, Alloy 600, and Alloy 800, immersed in acid sulfate solutions at 315°C. For pure Ni, adding sulfate in relatively small concentrations to solutions containing only chloride ions resulted in significantly increased current density in the anodic range and a sharp drop in polarization resistance, suggesting sulfate as the aggressive anion in mixed sulfate and chloride systems. Electrochemical measurements were taken on Alloys 600 and 800 in solutions of varying sulfate concentrations and differences in dissolution kinetics and passivation tendency are reported. Electrochemical reduction of sulfate is likely occurring, accelerating the metal dissolution rate.