Abstract
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 °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 (fcc)γ-(Fe,Cr,Ni)4N and fcc nitrogen expanded austenite (γN) mixed phases, the γN without precipitation of the nitrides phase, and the α-ferrite, fcc CrN, and γN mixed phase. The hardness-depth profiles exhibit a distinct compound layer with a thickness between 1.5 and 10 μm and the Knoop microhardness varied from 1500 to 2400 HK (0.1 N load), depending on process temperatures. The γN 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 γ-(Fe,Cr,Ni)4N phase from the γN 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.
Published Version
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