Improved Test Methods and Models for Localized Corrosion Assessment of Stainless Steels in Aqueous Chloride Media with Low Dissolved Oxygen

Abstract

Experimental foundation has been established for improving corrosion models for stainless steels exposed to neutral aqueous chloride media with low levels of dissolved oxygen (DO). Accurate repassivation potentials were measured for UNS S31603 and UNS S32205 alloys in chloride electrolytes at room and elevated temperatures using Tsujikawa-Hisamatsu electrochemical (THE) and cyclic potentiodynamic polarization (CPP) methods. It was observed that the repassivation potential values measured using the CPP method were lower than the ones obtained from THE method for the same environmental conditions. Cyclic polarization scans in aqueous chloride media were conducted at different reverse scan rates resulting in varied repassivation potentials. Medium-term corrosion potentials were measured in chloride media with controlled dissolved oxygen levels ranging from 20 ppb to 400 ppb. These results were used as inputs for improving model prediction for repassivation and corrosion potentials. In an extension of previous modeling studies, an improved mechanistic repassivation model was developed based on a new generalized theory that explicitly accounts for the adsorption of water rather than treating it as an ever-present background. With a flexible definition of the repassivation current density, the new repassivation model reproduces the experimental repassivation potentials obtained using THE and CPP methods. The dependence of corrosion potential on DO has been modeled using a mixed-potential model. It has been demonstrated that the effect of agitation (caused by the bubbling of oxygen) needs to be accounted for to describe the corrosion potential, especially at low DO, where mass transport is important. The combined model correctly predicts the occurrence of localized corrosion when tested against exposure data from the literature.