The oxidation of micro-crystalline Co–Cr alloys under balanced oxygen pressure of Co2O3/Co powders

Surface micro-crystallizing was applied to Co–5Cr, Co–10Cr and Co–15Cr alloys by electro-spark deposition (ESD) and the oxidation behaviors of both cast and micro-crystalline alloys were investigated at 1000 °C under balanced oxygen pressure of Co2O3/Co-mixed powders. Surface and cross-section SEM morphologies, EDS and XRD analysis show that micro-crystallizing promotes both the external oxidation and internal oxidation of Co–5Cr alloy. Micro-crystallizing provides more grain boundaries so that the diffusion is enhanced. The inwards diffusion of O makes internal oxidation zone thicker while the outwards diffusion of Cr makes external oxidation more prominent for the micro-crystalline Co–5Cr alloy. For both the cast and micro-crystalline Co–10Cr and Co–15Cr alloys, 10 wt.% Cr is enough to form complex selective oxides (AO/AB2O4/BO) scales and 15 wt.% Cr is still not enough to form a single Cr2O3 oxide scale. Micro-crystallizing promotes the outwards diffusion of Cr and selective Cr2O3 scales are easier to form so that the external oxides scales of micro-crystalline Co–10Cr and Co–15Cr alloy are much thinner and their Cr/Co ratios are higher. The oxidation model of A–B alloy under oxygen pressure that both A and B can be oxidized is also proposed.