On the mechanism of carbide network oxidation and subsequent passivation interference in a high carbon-containing cobalt-based superalloy

Oxidation is a life-limiting challenge for components made from cobalt-based superalloys used in high temperature service. However, further elucidation of the oxidation mechanisms is required if new strategies to prolong service life are to be effective. In this study, the mechanism responsible for the preferential oxidation of the carbide network was revealed in the cobalt-based superalloy, Co-101. It was found that the preferential oxidation of the carbide network provided regions of low oxygen partial pressure, where gaseous chromium species could be evolved. This volatilisation resulted in enhanced wastage of chromium in the local microstructure surrounding the oxidised carbide network. Critically, this depletion of chromium from the matrix near the surface of the alloy could inhibit passivation of the alloy surface, a crucial requirement for long duration high-temperature service.