Investigation of the effects of time and temperature of oxidation on corrosion behavior of plasma nitrided AISI 4140 steel

In this paper, the effect of the time and temperature of the post oxidation process on corrosion behavior of AISI 4140 plasma nitrided steel has been studied. Plasma nitriding was carried out at 550 °C for 5 h with a gas mixture of N2/H2:3/1. The plasma nitrided samples were post oxidized for 30 and 60 min with a gas mixture of O2/H2: 1/5 at temperatures of 400 and 450 °C. The treated samples were characterized by optical microscopy, SEM, XRD, roughness testing, electrochemical polarization and salt spray testing. The X-ray analysis revealed the formation of the iron-nitride phases ε and γ′ during plasma nitriding and the iron oxide phases of magnetite and hematite through the post-oxidizing treatment. It was also demonstrated that the corrosion characteristics of the nitrided compound layer were further improved by post-oxidation treatment and that the least corrosion rate and optimum condition relative to pitting corrosion belongs to the oxidized sample at 450 °C for 30 min.

► Plasma nitriding with post oxidation is a good treatment for corrosion application.
► Oxidized samples at 450 °C for 30 min have the least corrosion current density.
► Oxidized samples at 450 °C and 30 min obtain the optimum pitting corrosion resistance.