Enhancement of corrosion resistance for plasma nitrided AISI 4140 steel by plain air plasma post-oxidizing

•Plain air was primarily used for plasma post-oxidation for AISI 4140 steel.•A thin iron oxide layer composed of Fe3O4 to Fe2O3 was formed on top of the compound layer.•The ratio of Fe3O4 to Fe2O3 was closely related to the post-oxidizing conditions.•Post-oxidizing at 673K for 60min brought out highest ratio of Fe3O4 to Fe2O3 and optimum corrosion resistance.