Electrochemical Aspects of Stress Corrosion Cracking of Ni-Cr-Fe Alloys in Acid Sulfate Solutions Relevant to Nuclear Steam Generators

Alloy 600 and Alloy 800 are Ni-Fe-Cr alloys used in nuclear steam generators. Sulfate is an impurity found in faulted secondary side water that can accumulate in crevices in the acidic-to-neutral pH range, possibly resulting in acid sulfate stress corrosion cracking. Electrochemical measurements were done on Alloys 600 and 800 exposed to acid sulfate solutions at 315 °C and 120 bar. Results indicate that the addition of sulfate significantly increased the corrosion rate of both alloys – possibly due to reduction of sulfate resulting in impairment of surface passivity by reduced sulfur species. The addition of Fe2+ in solution reduced the corrosion rate due to the formation of a magnetite film. The corrosion rate and anodic current density of the 800 alloy were greater than those of the 600 alloy at high sulfate concentrations but became comparable at lower concentrations, or with the addition of Fe2+. SCC experiments were done and were in broad agreement with electrochemical measurements.