Pitting corrosion resistance of high nitrogen f.c.c. phase in plasma source ion nitrided austenitic stainless steel

Plasma source ion nitriding has emerged as a low-temperature, low-pressure nitriding approach for low-energy implanting nitrogen ions and then diffusing them into metal and alloy. In this work, 1Cr18Ni9Ti (18-8 type) austenitic stainless steel was treated at a process temperature of 380°C during a nitriding period of 4 h. A single high nitrogen f.c.c. phase (γN) with a high nitrogen concentration of 32 at.% was characterized using Auger electron spectroscopy, electron probe microanalysis, glancing angle X-ray diffraction, and transmission electron microscopy. The pitting corrosion resistance of the γN phase layer was measured by cyclic polarization in a series of 3% NaCl solutions buffered to pH from 0.4 to 13. In the potential–pH diagram of the γN phase layer, the extended immunity and perfect passivity zones and the narrowed imperfect passivity and pitting zones were obtained, compared with that of the original austenitic stainless steel. No pitting corrosion resistance was observed for the γN phase layer in the solutions of pH 4–13. The high supersaturation of nitrogen in the γN phase led to the improvement in the pitting corrosion resistance.