The effect of molybdenum on corrosion of low-temperature nitrided stainless steels in sulphate–chloride solution

Resistance of stainless steels to pitting corrosion is strongly enhanced by nitriding at 380–450°C. In this work, anodic behaviour of steels with 0.13, 2.1, 4.5 and 6.1 wt% Mo was studied before and after nitriding at 450°C for 30 h which gave 13.0–15.8 wt% N at the surface. Electrochemical measurements were carried out in 0.1 M Na2SO4 + 0.4 M NaCl at pH 3.0. Nitriding of steels with lower Mo contents (0.13 and 2.1 wt% Mo) enhanced the resistance to pitting corrosion; steels with higher Mo contents retained their high resistance to pitting but underwent an activation. Nitrogen increased anodic reactivity in initial stages of polarisation, and it also increased the passivation rate. X-ray photoelectron spectroscopy analysis showed that anodic films on nitrided steels contained oxides at the outer surface and Cr–N species deeper from the surface. The enhanced resistance to pitting corrosion of low-temperature nitrided steels is explained by the increased anodic reactivity which leads to the formation of a salt-type film with Cr/Fe oxides and Cr–N species.