An impedance spectroscopy study of oxide films formed during high temperature oxidation of an austenitic stainless steel

Impedance spectroscopy has been used to study an oxide film formed on an AISI304 austentic stainless steel by oxidation at 800degreesC for 200 h. Impedance spectra of the oxide film clearly showed two semicircles, which correspond to two independent oxide layers present in the oxide film. The oxide film was also examined by using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Although it was difficult to identify the two phases in the oxide film using SEM, XRD showed the presence of Cr2O3 and MnCr2O4. In addition, surface analysis of the oxide film using XPS showed the presence of MnCr2O4. By comparing the relaxation frequencies of Cr2O3 and MnCr2O4 with those of the two semicircles, it is identified that the semicircles in the impedance spectra correspond to Cr2O3 and MnCr2O4. Furthermore, the electrical properties of both Cr2O3 and MnCr2O4 in the oxide film have been determined by impedance measurements, indicating that the chromia layer is apparently thicker than the spinel layer. (C) 2003 Kluwer Academic Publishers.