Effect of gas mixture of plasma post-oxidation on corrosion properties of plasma nitrocarburised AISI 4130 steel

Plasma post-oxidizing is a process to improve the corrosion resistance of nitrocarburised parts. In this study, the effect of gas mixture of post-oxidation process on corrosion resistance of AISI 4130 plasma nitrocarburised steel has been studied. Plasma nitocarburising was carried out at 520 °C for 5 h in an atmosphere containing 49 vol% nitrogen, 49 vol% hydrogen and 2 vol% carbon dioxide. The nitrocarburised samples were post-oxidized at 450 °C for 1 h under different O2:H2 ratios (5:1, 1:1, 1:3, 1:5 and 1:7). The treated samples were studied using XRD, SEM, surface roughness measurement, microhardness and potentiodynamic methods. X-ray diffraction patterns revealed that with decreasing the O2:H2 ratio from 5:1 to 1:5, the amount of magnetite phase and the corrosion resistance of the samples were increased. It was seen that with increasing the O2:H2 ratio, the thickness of oxide layer was increased and the surface roughness was decreased. Furthermore, a model for oxide formation during plasma post-oxidation is proposed.