Silicon contamination effects in the oxidation of carbide-containing cobalt-chromium alloys

Abstract

Austenitic Co-25Cr(wt pct) and two phase γ + M 7 C 3 alloys of composition Co-25Cr-xC were reacted with pure, dry oxygen at 1000°C. All alloys reacted according to relatively fast parabolic kinetics if they were prepared without silicon contamination. However, if the alloys were contaminated with silicon during annealing in silica ampoules, or if 0.05 wt pct silicon was added to Co-25Cr-1.0C, parabolic oxidation kinetics two orders of magnitude lower resulted. In the case of the rapid reactions, the scales consisted of an inner layer of CoCr 2 O 4 + CoO overlaid by CoO. The slow reactions corresponded to growth of a thin scale of Cr 2 O 3 overlaid by CoCr 2 O 4 In the latter case the selective oxidation of chromium led to chromium carbide dissolution in a subsurface zone of the two-phase alloy, but the rates were the same as for the single-phase alloy. Consideration of gas phase conditions in the silica annealing ampoules showed that p SiO values were high enough to transfer substantial amounts of silicon to the alloy if p O2 was low enough. This situation arose in well evacuated ampoules where oxygen was consumed by reaction with alloy chromium, or in titanium gettered capsules. In contrast, annealing the alloys under moderate oxygen pressures led to the growth of a protective oxide film which prevented silicon contamination of the oxide surface. It is concluded that the presence or absence of carbon in Co-25Cr is irrelevant to the oxidation mechanism and that the silicon effect is critical. An approximate diffusion analysis shows that bulk alloy properties are not affected by the silicon, and it is concluded that silicon has its effect at the alloy surface, by promoting Cr 2 O 3 nucleation.