Conduction mechanism in oxide films on ferrous alloys studied by impedance spectroscopy in symmetrical and asymmetrical configurations

The electrical and electrochemical properties of the oxide films formed on Fe–25%Cr–x%Mo (x=0, 5 and 10) alloys by polarisation at −0.8–0.2 V vs. SHE in a 0.1 M Na₂B₄O₇ electrolyte at 200 and 300 °C are characterised by impedance measurements in both asymmetrical (alloy|oxide|electrolyte) and symmetrical (alloy|oxide|metal) configurations. The impedance magnitude at low frequencies in the symmetrical configuration has been found to be ca. more than one order of magnitude smaller than that in the asymmetrical configuration, indicating that the oxide films are mixed conductors with predominant electronic conductivity. The impedance spectra in both configurations comprised two time constants. The high-frequency time constant is ascribed to the electronic properties of the space-charge layer in the oxide, whereas the low-frequency time constant is associated with the solid-state transport of point defects. Kinetic parameters characterising the reactions of defect generation and consumption are estimated by fitting the equations of the Mixed-Conduction Model proposed earlier to the experimental impedance spectra in both configurations.