Role of Nitrogen in Pitting Corrosion Resistance of a High-Nitrogen Face-Centered-Cubic Phase Formed on Austenitic Stainless Steel

The role of nitrogen in pitting corrosion resistance of Fe-Cr-Ni austenitic stainless steel was investigated using a high-nitrogen face-centered-cubic phase formed on the plasma source ion-nitrided 1Cr18Ni9Ti (18-8 type) austenitic stainless steel. No pitting corrosion for the phase layer was confirmed by electrochemical polarization measurement in 3% NaCl solution. In order to explore the mechanism of pitting corrosion resistance, the passive film on the phase was analyzed by Auger electron spectroscopy and X-ray photoelectron spectroscopy in conjunction with ion-beam sputtering. The passive film with a duplex character was by two to three times thicker than that of original stainless steel. It was essentially composed of two regions: the iron hydroxide∕oxides in the outer region and the chromium hydroxide∕oxide and iron oxides accompanying the chromium and iron nitrides in the inner region. The conventional bilayer substructure, e.g., outer hydroxide sublayer and inner oxide sublayer, was also detected in each region. During anodic polarization a part of the chromium with weak Cr-N ionic-type bonds together with the iron in minor amount in the phase was prone to chemical oxidation, releasing sufficient nitrogen to form stable oxides and ammonia on the passivating surface. The thick iron hydroxide∕oxides region from preferential solution of the iron in metallic state was formed on the stable chromium and iron oxides due to increase of alkalinity in the aqueous solution buffered by ammonia formation. The protective passive film could completely barrier the penetration of localized attack of aggressive ions.