The influence of manganese and silicon on the oxidation behavior of Co-20Cr

The initial oxide was CoO which formed very rapidly by outward cation diffusion. A chromium-enriched substrate layer subsequently reacted via internal oxidation to form a continuous film of Cr₂O₃. The formation of CoCr₂O₄ occurred rapidly by the reaction of CoO and Cr₂O₃. The presence of CoCr₂O₄ spinel in the oxide scale was found to offer less oxidation resistance than a layer of Cr₂O₃ in Co-20Cr. The large mass of CoO that existed, as well as the much more rapid rate of spinel formation compared to the rate of Cr₂O₃ growth, resulted in the Cr₂O₃ being used up faster than new Cr₂O₃ could form. This behavior was opposite that observed in Ni-Cr alloys. Manganese additions slightly reduced the oxidation rate as well as the solid-state growth rate of spinel. However, the spinel still formed faster than new Cr₂O₃, and hence the protective layer was used up. Silicon additions reduced the oxidation rate, but the layers were highly susceptible to spalling upon cooling as well as during isothermal oxidation. The scales contained both spinel and Co₂SiO₄. The ortho-silicate was present as isolated particles and did not form a continuous protective film per se. The reduced oxidation rate was associated with a thin inner film of Cr₂O₃.This work was performed at Stanford Research Institute, Menlo Park, California and was supported by the National Aeronautics and Space Administration, Contract NAS 3-11165.