Chemical composition and electronic structure of the oxide films formed on 316L stainless steel and nickel based alloys in high temperature aqueous environments

Auger analysis, capacitance measurements and photoelectrochemical measurements have been used in order to study the oxide films formed on 316L stainless steel, Alloy 690 and Alloy 600 in high temperature aqueous environments at pH 8 and 10. The analytical study reveals the duplex character of the films formed on 316L stainless steel and Alloy 690. For these materials it is assumed that the capacitance and the photoelectrochemical results can be related to the semiconducting properties of mixed iron-nickel oxide and chromium oxide which represent the outer and the inner parts of the film, respectively. In the case of the Alloy 600 the presence of nickel oxide controls the semiconducting properties of the film formed at pH 10, but for the film formed at pH 8 the presence of mixed iron-nickel oxide in the outer layer still controls those properties. Finally, it is demonstrated that the capacitance behaviour of the thin passive films formed at room temperatures could serve as a useful basis for interpreting the higher temperature results. (C) 2000 Elsevier Science Ltd. All rights reserved.