Characterization by CEMS, XRD and XPS of oxidized layers formed on the surface of carbonitrided low-carbon steel

The surface structure and oxidation characteristics of carbonitrided low-carbon steel, prepared by oxidizing the surfaces in salt-baths after the carbonitriding process, have been studied using conversion electron Mössbauer spectrometry, X-ray diffractometry, and X-ray photoelectron spectrometry. The formation of an iron oxide layer in the carbonitrided surface improved the corrosion resistance significantly. The steel surface after carbonitriding for 3600 s consisted of ε- and γ′-nitride, ε-carbonitride, and cementite. After the oxidation, two sextets and one doublet were observed, attributable to the presence of magnetite, Fe3O4, and of fine crystals of ferromagnetic iron oxides such as α-Fe2O3, γ-Fe2O3, and Fe3O4. Crosssectional analysis or depth profiling of the carbonitrided layer revealed the presence of iron oxides not only in the outermost oxide layer but also in the carbonitrided layer. Oxygen diffused deep into the carbonitrided layer and formed fine crystals of iron oxides there, distorting the crystal lattices of ε-carbonitride and γ′-nitride.