Duplex structure of oxides produced on low-carbon steel surfaces after DC plasma jet treatment

Polished low-carbon steel treated with a DC plasma jet has been investigated by means of X-ray diffraction and conversion electron Mössbauer spectroscopy in order to determine the formed oxides. Nuclear backscattering has been used to follow the oxygen concentration as a function of the depth. It has been demonstrated that the dual oxide layer could be approximated by a duplex structure of hematite and magnetite. At the metal–oxide interface only magnetite is formed at a thickness of 200 nm. Above this layer, magnetite and hematite are mixed into a 300 nm thick layer and, at the oxide–air interface, only hematite is present. The apparent presence of oxygen down to 1200 nm below the surface is explained by a roughness effect due to the polishing process.