Passive Film Characteristics Formed on Nitrogen-Containing Austenitic Stainless Steel at Different pH and NaCl Concentrations

Austenitic stainless steels exhibit excellent corrosion resistance owing to the formation of passive films composed of a dense Cr-rich inner layer and porous Fe-rich outer layer. The corrosion resistance and passive film characteristics of N-containing austenitic stainless steel (NASS) were investigated in various pH and Cl−-containing environments and compared with those of commercial 304 SS. Microstructural analysis revealed that NASS had larger grains but more favorable crystal structures for the adsorption of passivating species. NASS exhibited a lower corrosion current density, higher pitting potential, and superior repassivation behavior in acidic and neutral environments, whereas 304 SS exhibited better corrosion resistance under strongly alkaline conditions. NASS formed a passive film with lower defect density and a higher fraction of compact Cr-rich species, contributing to its enhanced passive film stability and repassivation ability. Immersion tests demonstrated that pit initiation was delayed in the NASS group compared with the 304 SS group. These results indicate that the corrosion resistance of NASS in acidic and neutral environments originates from the improved stability and protective characteristics of the passive film.