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

Abstract

An ultra-thick nitrided layer (>80 μm) 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 sub-layers: the upper thick layer composed of N-expanded austenite and CrN, and the lower thin layer mainly composed of γN 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 μm. 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.