Corrosion and passivation of plasma nitrided stainless steels

Corrosion behaviour of nitrided steels was examined in chloride free and chloride containing sulphate solutions acidified to pH 3.0. Conventional nitriding (550-600 degrees C) strongly deteriorates the corrosion resistance, however, high reactivity of nitrided layers enables the formation of thick conversion coatings to protect against corrosion (oxidation and phosphating) or to improve lubrication (phosphating). Low-temperature nitriding (380-450 degrees C) produces thin layers of a supersaturated solid solution of nitrogen in austenite (S phase) of high resistance to pitting corrosion in chloride containing solutions. After nitriding at 450 degrees C for 30 h, the near-surface parts of layers with > 7 wt-% N have a deteriorated resistance to general corrosion in the acidified solution, but deeper parts with lower nitrogen contents have the resistance comparable with that of untreated steels. It is proposed that the improved pitting resistance is due mainly to the accumulation on corroding surface of large amounts of passivating products and of Cr-N species.