Semiconducting properties of thermally grown oxide films on AISI 304 stainless steel

The oxide films formed on AISI type 304 stainless steel in the temperature range between 50 and 450 degrees C, have been studied by Auger electron spectroscopy, capacitance measurements and photoelectrochemistry. The analytical results show that the oxide films are in all cases composed of an inner region of a mixed chromium-iron oxide and an outer region of iron oxide. An increase in temperature of film formation changes most particularly the thickness of the outer iron oxide region. Capacitance results show that when the temperature of film formation is decreased the doping density increases and reaches, for films formed at 50 degrees C, the characteristic values of very thin passive films. It also appears that, although very different in thickness and doping density, thermally grown and passive films show identical capacitance and photoelectrochemical behaviour. The electronic structure model previously proposed to explain the semiconducting properties of very thin passive films can be applied to explain the semiconducting properties of the relatively thick well crystallised and nondegenerate oxides. (C) 2000 Elsevier Science Ltd. All rights reserved.