In vitro corrosion resistance of plasma source ion nitrided austenitic stainless steels

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 steel and alloy. In this work, a single high nitrogen face-centered-cubic (f.c.c.) phase (γN) formed on the 1Cr18Ni9Ti and AISI 316L austenitic stainless steels with a high nitrogen concentration of about 32 at % was characterized using Auger electron spectroscopy, electron probe microanalysis, glancing angle X-ray diffraction, and transmission electron microscopy. The corrosion resistance of the γN-phase layer was studied by the electrochemical cyclic polarization measurement in Ringerʹs solutions buffered to pH from 3.5 to 7.2 at a temperature of 37°C. No pitting corrosion in the Ringer’s solutions with pH=7.2 and 5.5 was detected for the γN-phase layers on the two stainless steels. The high pitting potential for the γN-phase layers is higher, about 500 and 600 mV, above that of the two original stainless steels, respectively, in the Ringer’s solution with pH=3.5. The corroded surface morphologies of the γN-phase layers observed by scanning electron microscopy are consistent with the results of the electrochemical polarization measurement.