MSE PhD Defense  - Yushu Wang

Date: Friday July 13th, 2015, 3 PM

Location: RBI Building 521 (the previous IPST Building)

Committee members:

Dr. Preet M. Singh (MSE, advisor)

Dr. Arun Gokhale (MSE)

Dr. Faisal Alamgir (MSE)

Dr. Lawrence Bottomley (Chem)

Dr. Yulin Deng (ChBE)

Title: Effect of Thiosulfate on Passivity and Corrosion Properties of Stainless Steels

Abstract:

Thiosulfate is present in many environments in a variety of industrial applications. It is known to cause localized corrosion such as pitting corrosion and stress corrosion cracking (SCC) of stainless steels, especially in the presence of chlorides. Stainless steels are widely used for their superior corrosion properties attributed to the formation of a passive oxide film on their surface. On the other hand, corrosion issues in stainless steels almost exclusively result from the breakdown of this passive film. Understanding how thiosulfate influences the passive behavior of the austenitic and duplex stainless steels is critical for the mitigation of these corrosion problems.
    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. A statistical analysis of the metastable pitting events monitored by chronoamperometry revealed that thiosulfate promotes pit initiation and stabilizes the growth of metastable pits. Mechanical scratch tests, designed to study the repassivation kinetics and interactions between a bare metal and thiosulfate, showed the effect of a number of environmental parameters 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 and EDS showed strongest interaction between iron and reduced sulfur species, and provided information regarding the overall corrosion process. Reduced sulfur species was found to form in presence of iron. Based on these results, a mechanism of pitting in the presence of thiosulfate was proposed.