Effects of nitrogen on the repassivation kinetics, the metastable pitting behavior, and the semiconducting properties of austenitic stainless steels

Abstract

It is well known that a small amount of nitrogen improves significantly the resistance to pitting corrosion and mechanical properties of austenitic and duplex stainless steels in chloride solutions. Recently, the studies of metastable pitting have been performed to estimate the pitting corrosion. Metastable pitting occurs via a repetitive process of film breakdown-dissolution-repassivation, which suggests the repassivation behavior of an alloy can influence the metastable pitting behavior. According to point defect model (PDM), film breakdown is explained by the defect structure of the passive film that determines the semiconducting property of it. There have been few studies on the influence of nitrogen alloying on the metastable pitting resistance, the repassivation kinetics, and the semiconducting property of stainless steels. Moreover, in spite of a number of studies, the mechanism for nitrogen to improve the resistance to pitting corrosion has not yet been elucidated. In this study, nitrogen effects on the metastable pitting behavior, the repassivation kinetics, and the semiconducting properties of austenitic stainless steels were determined using electrochemical noise technique, rapid scratch electrode technique, Mott-Schottky plots, respectively. Then, based on the results, nitrogen effects on the pitting resistance were rationalized through the in-depth composition profile performed by XPS. 1. PSD (Power Spectral Density) Analysis PSD method is recently suggested as a quantitative measure of the pitting tendency. However, there have been opposite results on the standards to estimate the pitting tendency. To clarify the standards, the PSD analyses were performed in various conditions. With increasing Cl- ion concentration, applied potential, and solution temperature, the PSD intensity and the roll-off slope for 304L stainless steels were increased, which suggests that the increase in both PSD intensity and the roll-off slope are corresponded to high co...