Towards a better understanding of the high-temperature oxidation of MAX phase Cr2AlC

•Cr2AlC single crystal, fine grained and coarse grained polycrystals behave similarly during high temperature oxidation under dry air.•Preferential formation of Cr7C3 as compared to other chromium carbides results from the higher value of the Cr7C3 phase formation driving force.•Despite different microstructures, a continuous alumina layer and a chromium carbide Cr7C3 sublayer are formed on the Cr2AlC for T = [800;1400] °C.•For higher temperature or oxidation duration, a chromia-rich corundum phase is formed which is in agreement with thermodynamic calculations. High-temperature oxidation of bulk single crystal, fine and coarse grained polycrystals of Cr2AlC has been performed from 800 °C to 1500 °C. In the temperature range T = [800;1400] °C, an α-Al2O3 scale with a Cr7C3 subscale are in-situ formed independently of the initial microstructure on top of the bulk Cr2AlC. CALPHAD calculations were performed to explain the formation of Cr7C3 instead of the expected mixture of carbides, indicating a favourable driving force for Cr7C3 formation. The kinetic study of the oxidation is consistent with previous works with an activation energy of about 400 kJ.mol−1. Oxidizing for a longer time (1000 h) or at higher temperature (1500 °C) leads to the formation of chromia in the former alumina scale, which correspond to the calculated equilibrium phases.