Protection of cobalt-based refractory alloys by chromium deposition on surface: Part II: Behaviour of the coated alloys in oxidation at high temperature

A high chromium content, typically 30 wt.%, is generally considered as necessary for cobalt-based alloys to allow them to resist oxidation by hot gases. Cobalt alloys with a bulk poorer in chromium may become resistant against high temperature oxidation if they are enriched in chromium on surface. The aim of this second part of the work is to expose to air at 1200 °C three low-chromium cobalt-based alloys (Co–10Cr, Co–10Cr–0.25C–4.4Ta and Co–10Cr–0.5C–8.7Ta in wt.%), previously enriched in chromium on surface by pack cementation, in order to examine if the applied cementation treatments are enough to ensure a good behaviour for a long time. The initial surface enrichment in chromium of the Ta-containing alloys, characterized by a maximal content on surface of about 30 wt.% Cr and a depth of enrichment of several hundreds of micrometers, gives them a good resistance to oxidation at 1200 °C, despite the very low-chromium content in the bulk. This is also due to the interdendritic TaC carbides which may facilitate outward diffusion of chromium during oxidation as they facilitated its inward diffusion during cementation. In contrast, the CoCr alloy, enriched in chromium in the same conditions did not display a so good oxidation behaviour.