Effect of sulphate and chloride ions on the crevice chemistry and stress corrosion cracking of alloy 600 in high temperature aqueous solutions

Abstract

Concentration-pH diagrams for Alloy 600 which may suffer from crevice corrosion at 316°C in sulphate or chloride aqueous solutions have been constructed. The steam generator made of Alloy 600 is one of the key components in a PWR nuclear power plant. If steam generator degradation occurs, the nuclear power plant will be forced to shutdown or derate. Crevice corrosion of samples was studied under [SO 4 2−] ≥ 15,000 ppm, pH 25 ≤ 4 and [Cl −] ≥ 10 ppm, pH 25 ≤ 4 conditions and easily occurred in acidic media. Heavily etched surfaces resulted from the coalescence of creviced regions on the specimen. With the decrease of pH value, the critical chloride concentration for generating crevice corrosion was lowered. However, the presence of sulphate ions (8000 ppm) suppressed the anodic dissolution reactions, perhaps through the formation of sulphate salts. Their presence also increased the endurance of chloride attack on Alloy 600 by two orders of magnitude. Only in extremely acidic conditions (pH 25 ≥ 1.5) was the stress corrosion cracking of Alloy 600 observed.