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 degrees 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/AB(2)O(4)/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 microcrystalline 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. (C) 2007 Elsevier B.V. All rights reserved.