A mechanistic study on thiosulfate-enhanced passivity degradation of Alloy 800 in chloride solutions

This paper reports thiosulfate-enhanced effect of chloride ions on the passivity degradation of Alloy 800 and discusses how thiosulfate influences the semiconductor properties as well as breakdown of the passive films. Experimental results reveal that in concentrations of 10−6 to 0.075 mol/L, thiosulfate lowers the breakdown potential of Alloy 800 by 100–400 mV in solutions containing 0.6 mol/L chloride ions. This degradation is further verified by polarization resistance and noise resistance obtained from electrochemical impedance spectroscopy (EIS) and micro-nanoscale electrochemical noise (EN) data, respectively. Mott–Schottky analysis is employed to explore the corrosion mechanism and the results show that the passive film exhibits as p-type semiconductivity in a thiosulfate-only solution and n-type semiconductivity in a chloride-only solution, and the addition of thiosulfate into chloride solutions increases the concentration of oxygen vacancies in the passive film at the potential close to the pitting potential. Thiosulfate cannot be easily reduced on well-passivated surfaces in chloride-free solutions but can be readily reduced to sulfide in chloride solutions at active sites when the passive film breaks down – the reduced sulfur retards the healing process of repassivation, leading to an increase in oxygen vacancies.

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