Effect of thiosulfate on passivity and corrosion properties of stainless steels

Abstract

The effect of thiosulfate on different aspects of pitting corrosion in chloride containing environments as well as the interaction between thiosulfate and stainless steels were studied in this research. Cyclic potentiodynamic polarization tests showed that the presence of thiosulfate hinders the repassivation of the pits. Preferential dissolution of ferrite phase was found on 2101. A statistical analysis of the metastable pitting events monitored by chronoamperometry revealed that thiosulfate promotes pit initiation and stabilizes the growth of metastable pits. Microstructure influenced the pitting corrosion behavior of 2101. The repassivation of metastable pits were closely related to the arrest of pit growth by phase boundaries. Mechanical scratch tests, designed to study the repassivation kinetics and interactions between a bare metal and thiosulfate, showed the effect of potential and microstructure on the growth of pitting in presence of thiosulfate. Interaction of alloying elements such as chromium, nickel, and molybdenum with thiosulfates were also evaluated. XPS showed formation of reduced sulfur species on the surface of pure alloying elements, and provided information regarding the overall corrosion process. Based on these results, a mechanism of pitting in the presence of thiosulfate was proposed.