Oxidation mechanism of cobalt based alloy at high temperatures (800-1100°C)

A cobalt based Phynox alloy has been oxidised in the 800-1100°C temperature range. Kinetic results show that the parabolic behaviour is followed under isothermal conditions. The scale growth mechanism of cobalt based Phynox alloy in air is consistent with a growth mechanism limited by the diffusion process in a growing Cr₂O₃ oxide scale. Thermal cycling tests show that the best scale adherence is found at 1000°C. This temperature permits a rapid chromium supply from the substrate to form a continuous chromia scale. A keying effect at the internal interface is promoted by the presence of silicon and molybdenum. At 900°C, CoCr₂O₄ cobalt containing oxide formation is favoured and leads to a bad scale adherence. At 1100°C, thermal cycling conditions lead to scale spallation and chromium depletion. Then, important weight losses are registered corresponding to the oxidation of cobalt and molybdenum to induce CoCr₂O₄ and CoMoO₄ formation.