THE SELECTIVE OXIDATION OF NI-15-PERCENT-CR AND NI-10-PERCENT-CR ALLOYS PROMOTED BY SURFACE-APPLIED THIN OXIDE-FILMS

The effects of thin surface-applied Y2O3, Al2O3 and Cr2O3 coatings or films on the selective oxidation of chromium in Ni-15Cr and Ni-10Cr in air at 1000-degrees-C have been studied. The oxide films were deposit ed by an electrochemical method. It has been shown that establishment of a chromia scale can be promoted effectively by the presence of an Y 2O3 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 Al2O3 and Cr2O3 films. It su bsequently breaks down and nickel-rich oxide can then develop and grow . The thin oxide films possibly influence the initial stages of oxidat ion in two ways, i.e. the Al2O3 and Cr2O3 films act as diffusion barri ers for oxygen while all three films may provide preferential sites fo r oxide nucleation at the oxide film/alloy interface. It is also possi ble that a sintering process could be involved in the development of t he oxide scale; this would be enhanced in the presence of a thin oxide film. Overall, these effects result in a decrease in the critical amo unt of chromium required to form a selective chromia scale. During sub sequent oxidation, the thin oxide films are incorporated into the oxid e scales on the alloys, where they may influence the transport mechani sms and stress conditions. In comparison with the cases of surface-dep osited Al2O3 and Cr2O3 oxide films, the additional beneficial effect o f the Y2O3 film in maintaining the selective oxidation of chromium is attributed to its ability to improve the mechanical integrity and adhe sion of the chromia scale.