Investigation on corrosion behavior of plasma nitrided AISI 420 martensitic stainless steel

The effects of plasma nitriding temperature on corrosion behavior of nitrided AISI 420 stainless steel were investigated. The microstructure, phase composition and microhardness of the nitrided layer were examined by means of optical microscope, X-ray diffraction technique and microhardness tester. The corrosion behavior of plasma nitrided AISI 420 stainless steel was evaluated by potential dynamic polarization and salt spray corrosion tests in a simulated industrial environment. The results show that the microstructure of nitrided layer obtained at 350°C consists of ε-Fe₃N and α_N, both of which are supersaturated nitrogen solid solution phase. The corrosion resistance of AISI 420 martensitic stainless steel is considerably improved by plasma nitriding at 350°C due to the synergistic effect of dissolved Cr and high chemical stabilities phase of ψ-Fe₃N and α_N. When plasma nitriding is carried out at 450°C or 550°C, the α_N phase is decomposed into α and CrN phases which results in the depletion of Cr in matrix. Therefore, the corrosion resistance of the nitrided steel is reduced. Plasma nitriding at low temperature not only increases the surface hardness, but also improve the corrosion resistance of the martensitic stainless steel.