Microstructure and corrosion resistance of plasma source ion nitrided austenitic stainless steel

Plasma source ion nitriding has emerged as a low-temperature, low-pressure nitriding approach for implanting nitrogen ions and then diffusing them into metallic materials. In this work, 1Cr18Ni9Ti austenitic stainless steel was nitrided at process temperatures from 280 to 480 degrees C over a range of nitriding conditions, and the resultant corrosion properties in 1% NaCl solution and in 1 N H2SO4 solution were investigated by cyclic polarization and anodic polarization measurements, respectively. Glancing angle x-ray diffraction and transmission electron microscopy studies were used to determine the crystal structures obtained at different process temperatures. It was found that three different near-surface microstructures were present, they were the face-centered-cubic (fee) gamma'-(Fe,Cr,Ni)(4)N and fcc nitrogen expanded austenite (gamma(N)) mixed phases, the gamma(N) without precipitation of the nitrides phase, and the alpha-ferrite, fcc CrN, and gamma(N) mixed phase. The hardness-depth profiles exhibit a distinct compound layer with a thickness between 1.5 and 10 mu m and the Knoop microhardness varied from 1500 to 2400 HK (0.1 N load), depending on process temperatures. The gamma(N) phase layer has a good pitting corrosion resistance in 1% NaCl solution and an equivalent homogeneous corrosion resistance in 1 N H2SO4 solution, compared with the original stainless steel. The formation of the gamma'-(Fe,Cr,Ni)(4)N phase from the gamma(N) phase layer can be the cause of the poor homogeneous corrosion resistance, while the precipitation of chromium nitride leads to deterioration of both the pitting corrosion resistance and the homogeneous corrosion resistance. (C) 1997 American Vacuum Society.