Structure and corrosion behavior of ultra-thick nitrided layer produced by plasma nitriding of austenitic stainless steel

An ultra-thick nitrided layer (>80 mu m) was formed by hot wire enhanced plasma-assisted nitriding of AISI 316L austenitic stainless steel under the higher pulsed bias to investigate the structure and the corrosion performance. The phase structure, the cross-sectional morphology, the element distribution, hardness and modulus, as well as the corrosion behavior of the nitrided layer were investigated by XRD, SEM, EDS, nano-indentation tester and electrochemical workstation, respectively. The results showed that the nitrided layer was divided into two sublayers: the upper thick layer composed of N-expanded austenite and CrN, and the lower thin layer mainly composed of gamma(N) with a trace of CrN precipitate. Nitrided at -400 V, CrN and high N-expanded austenite formed in the upper layer, which exhibited a high hardness and an excellent corrosion resistance. As the bias increased to -500 V, more nitrogen atoms precipitated as CrN phase and low N-expanded austenite phase began to form, the thickness of nitrided layer reached a maximum of 84 mu m. To -600 V, high N-expanded austenite almost completely transformed to low N-expanded austenite, accompanied by the increase of CrN, which caused the thickness and hardness of the nitrided layer to decrease, and the pitting corrosion occurred at -4.1 mV.

Automated summary

Nitriding of AISI 316L austenitic stainless steel under various pulsed bias conditions resulted in different phase structures, thicknesses, and hardness of the nitrided layer, which in turn affected the corrosion resistance of the material.