Elemental composition and corrosion resistance of plasma nitrided stainless steels

Abstract Cr18Ni9Ti and Cr17Ni2 stainless steels were plasma nitrided in N2/H2 mixtures with 25 vol% N2 or 80 vol% N2 at 585 and 440°C. Surface analysis by glow discharge optical emission spectrometry (GDOES) and by AES showed an accumulation of carbon in the transition region from nitrided layer to substrate, where nitrogen concentration steeply dropped. Corrosion behaviour through the depth of the nitrided layers was determined in 0.05 M Na2SO4 at pH 3.0. AES analysis of surface films for various depths was made on taper sections. Corrosion resistance of the outer layers (nitride zones) of nitrided steels was only slightly lower than that of the untreated steels, but the resistance of the deeper layers (diffusion zones) was much lower. Anodic films on the diffusion zones were composed mainly of iron oxides. In the transition region there occurred an intergranular corrosion which suggests that C was accumulated at grain boundaries in the form of Cr carbides.