Localized Corrosion of Alloy 600 in Thiosulfate Solution at 25°C Available to Purchase

The integrity of the passive film formed on an untreated alloy in thiosulfate solution was investigated by adding small amounts of dilute chloride ions. Linearity of corrosion potential was obtained in the range of 1 to 0.001 M sodium thiosulfate (Na₂S₂O₃) as a function of the concentration of chloride ions. The chloride content needed to generate pitting decreased as the logarithm of the deaerated thiosulfate concentration decreased. Fractography showed ductile fractures for an untreated UNS N06600 alloy (alloy 600) and intergranular stress corrosion cracking (SCC) or intergranular attack for sensitized alloy 600. These were dependent upon potential. The failure time showed a minimum at 10⁻⁴ M Na₂S₂O₃. The fracture mechanism was correlated with the active loop regime, where vigorous dissolution reactions were produced during repassivation. Unsensitized alloy 600 in thiosulfate ions was not susceptible to SCC when potential was controlled at -300 mV_SCE or at +400 mV_SCE