Effects of Temperature and Thiosulfate Concentration on the Crevice Corrosion of Alloy 800 Under Layup Conditions

Ni-Fe-Cr Alloy 800 used in steam generator tubes of nuclear power plants exhibits crevice corrosion in chloride solutions and thiosulfate-assisted pitting corrosion, a pitting submode that occurs at a lower potential than conventional chloride-assisted pitting corrosion. In this work, the effect of thiosulfate on crevice corrosion was systematically studied, with a discussion of the mechanisms of corrosion. For this purpose, a custom crevice former was designed to expose only the external, curved surface of the tube in crevice corrosion tests. The crevice corrosion resistance was characterized by the potentiodynamic-galvanostatic-potentiodynamic technique in 1 M NaCl solutions with thiosulfate additions ranging from 10−4 M up to 1 M and in temperatures from 30°C up to 90°C. It was found that thiosulfate-assisted pitting corrosion initiated at much lower potentials than crevice corrosion in 1 M NaCl solution, up to 400 mV lower for an addition of 5 × 10−4 M of thiosulfate at 90°C. The repassivation potential, measured during the backward potential scan, could decrease in solutions with thiosulfate by up to 200 mV, in comparison to 1 M NaCl solutions. Besides the effect on critical potentials, thiosulfate additions drastically changed the modes of corrosion, favoring the occurrence of pitting corrosion in the crevice corrosion test, with different morphology depending on thiosulfate concentration and temperature.