The selective oxidation of Ni-15%Cr and Ni-10%Cr alloys promoted by surface-applied thin oxide films

Abstract

The effects of thin surface-applied Y 2O 3, Al 2O 3 and Cr 2O 3 coatings or films on the selective oxidation of chromium in Ni-15Cr and Ni-10Cr in air at 1000°C have been studied. The oxide films were deposited by an electrochemical method. It has been shown that establishment of a chromia scale can be promoted effectively by the presence of an Y 2O 3 film on Ni-15Cr and, at least locally, on Ni-10Cr. Such a scale is only established locally and maintained for short periods on the two alloys in the presence of surface-applied Al 2O 3 and Cr 2O 3 films. It subsequently breaks down and nickel-rich oxide can then develop and grow. The thin oxide films possibly influence the initial stages of oxidation in two ways, i.e. the Al 2O 3 and Cr 2O 3 films act as diffusion barriers for oxygen while all three films may provide preferential sites for oxide nucleation at the oxide film/alloy interface. It is also possible that a sintering process could be involved in the development of the oxide scale; this would be enhanced in the presence of a thin oxide film. Overall, these effects result in a decrease in the critical amount of chromium required to form a selective chromia scale. During subsequent oxidation, the thin oxide films are incorporated into the oxide scales on the alloys, where they may influence the transport mechanisms and stress conditions. In comparison with the cases of surface-deposited Al 2O 3 and Cr 2O 3 oxide films, the additional beneficial effect of the Y 2O 3 film in maintaining the selective oxidation of chromium is attributed to its ability to improve the mechanical integrity and adhesion of the chromia scale.