Abstract
Model Ni-Cr alloys containing 5, 10, 15, 20, 25 and 30 wt% Cr were oxidized in Ar-20 vol% CO2 gas mixtures at temperatures of 650, 700 and 800°C. In general, multi-layered oxide scales were observed on the surface after reaction. With increased alloy Cr content, the oxide structure changed from external NiO, plus intermediate inner oxides and an internal oxidation zone, to forming a thin chromia band at the base of the oxide scale and, at higher Cr levels, an exclusive chromia scale. Increasing temperature accelerated the oxidation kinetics of low chromium-containing alloys, but significantly reduced the oxidation rate of high chromium alloys by promoting formation of the chromia band/scale. The critical Cr concentration required for chromia scale formation and maintenance decreased with increasing temperature, in accord with diffusion theory. Intergranular carbides were formed in high Cr content alloys, indicating elevated carbon activities beneath the oxide scales.
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