Cr 2 AlC high temperature oxidation under dry and wet air: understanding of the oxidation mechanism

Cr 2 AlC is one of the most interesting MAX phase compositions to operate under aggressive environments up to 1400 °C, but the effect of humidity at high temperatures is still poorly investigated. Cr 2 AlC high-temperature oxidation tests under controlled humidity are carried out on fined-grained samples for 40 hours in the [1000-1400 °C] temperature range and compared to a previous study performed on single-crystals, fine and coarse-grained samples oxidized under dry air. Oxidation products are characterized and mass gain curves are analysed using both cubic and paralinear oxidation models to extract kinetics and characterize gas emission during the oxidation tests. These experimental data are supported by thermodynamic calculations in order to propose a quite complete oxidation mechanism for Cr 2 AlC. The nature and the arrangement of the oxidation products formed during wet and dry air oxidation tests are similar-a continuous Al 2 O 3 layer and a Cr 7 C 3 sublayer with pores-which demonstrates that the water vapor does not affect significantly the high-temperature oxidation of Cr 2 AlC. The acceleration of the oxidation process during wet air tests is correlated to the higher partial pressure of gaseous species produced in the presence of humidity which is associated with the chromium carbide's volatilization in the form of carbon oxide.