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

Plasma post-oxidizing was conducted immediately after plasma nitriding in the same equipment for AISI 4140 steel, and plain air was used as the oxygen bearing gas. The cross-sectional microstructures of the treated samples were observed by optical metallography and scanning electron microcopy (SEM), and the thickness of compound layer was measured accordingly. The phases were determined by X-ray diffraction (XRD), corrosion resistance was evaluated by electrochemical polarization, and the surface morphology before and after polarization test was also observed by SEM. Meanwhile, standard Gibbs free energy of the oxidation reactions existed in Fe-O system was calculated. The results show that a thin iron oxide layer composed of magnetite (Fe3O4) and hematite (Fe2O3) is formed on top of the compound layer during plasma post-oxidizing process, and the ratio of magnetite (Fe3O4) to hematite (Fe2O3) is depended on plasma post-oxidizing temperature and time. Highest ratio of Fe3O4 to Fe2O3 is obtained while post-oxidizing at 673 K for 60 min due to lower standard Gibbs free energy and appropriate forming rate for the formation of Fe3O4 at this temperature. The thin oxide layer brings out significant enhancement of corrosion resistance, especially at higher ratios of Fe3O4 to Fe2O3, due to the dense and adherent characteristic of Fe3O4 oxide. Surface images of the post-oxidizing specimen show that a dense oxide layer was formed when post-oxidized at 673 K for 60 min, and those after polarization test show that there exists no corrosion pit for the specimen post-oxidized at 673 K for 60 min, however some corrosion pits occur for those treated under other conditions. Meanwhile, the thickness of the compound layer formed during nitriding process tends to decrease with the increase of post-oxidizing temperature and time due to spallation by air plasma bombardment during post-oxidizing process.