Corrosion Behaviour of Stainless Steels Aner Plasma and Ammonia Nitriding

Abstract

Steels Cr17Ni2 and Cr18Ni9Ti were plasma nitrided and nitrided in ammonia at 585°C for 16h, and the layers obtained examined by X-ray diffraction and microhardness measurements. Corrosion properties were determined in 0.05M Na2SO4 at pH 3.0 and 5.0, with and without added chloride, using a potentiodynamic polarisation method. Nitriding resulted in a deterioration of corrosion resistance, especially in the acidic (pH 3.0) solutions, whereas in the near neutral (pH 5.0) chloride free solution the deterioration was slight and limitea to the transpassive region. Ammonia nitrided layers with ε +γ nitrides had lower corrosion resistances than plasma nitrided layers with γ nitride alone. This can be explained by the high porosity of the ε nitride layers and the resultant exposure of diffusion zones. The corrosion resistance of diffusion zones with CrN precipitates was markedly lower than that of outer nitride zones and of steel cores.