Study on the Corrosion Resistance Mechanism of Nitric Acid Passivation on 316L and SUS329J3L Duplex Stainless Steels in a Denitrification Agent

316L and DSS 2205 suffer from pitting corrosion in a denitrification agent, NaClO₂ solution, despite their excellent corrosion resistance and widespread utilization in plants. Herein, nitric acid passivation was successfully employed to enhance the corrosion resistance of 316L and DSS 2205 in 3% NaClO₂ solution. The 3% NaClO₂ solution, in which 316L and DSS showed the lowest corrosion rate, was chosen by electrochemically studying the effect of NaClO₂ concentration on the steel's corrosion from a series of solutions containing 1%, 3%, 6%, and 10% NaClO₂. The passive films with and without nitric passivation were investigated by chemical composition, electrochemical property, semiconductor behavior, and morphology observation analyses. The enhanced corrosion resistance of both 316L and DSS 2205 after nitric passivation was attributed to the Cr enrichment and decreased active area on the steel surface. The semiconductor property test and point defect model showed that both the reduced doping density and the decreased diffusivity contributed better resistance against ClO₂^- and Cl^-. The passivated 316L showed better corrosion resistance according to electrochemical tests and potentially is an option to replace high-cost DSS 2205.