Initial stage oxidation of a FeCrAl alloy at 1000 degrees C. Influence of water vapor

Fe-21.5Cr-5.6Al alloy oxidation, at 1000 degrees C, in dry or wet oxygen shows that steam has an influence on the initial stage of the process. In dry oxygen, an initial linear kinetic regime is observed during one hour, before it follows the parabolic behaviour, Under wet conditions, weight gain curves follow the same parabolic regime all along the oxidation test. The scale structure is also strongly dependent on the steam presence in the gaseous environment. In dry oxygen, gamma-Al2O3 identified by ESCA and RHEED is initially formed. After some hours the oxide scale is mainly composed of alpha-Al2O3. Successive abrasion and RHEED analysis show that the transient stage corresponds to the necessary time for the internal part of the initial gamma-Al2O3 scale to transform into a continuous alpha-Al2O3 diffusion barrier. The transformation, starting at the internal interface, is induced by the external cation diffusion. Under wet oxygen conditions, transient oxides are identified as (Mp,Fe)(Cr,Al)(2)O-4, MgAl2O4(orthorhombic), Al2O3(hexagonal) and such a scale acts as an effective diffusion barrier. These oxides transform into MgAl2O4(cubic), Cr2O3, Fe2O3, alpha-Al2O3, after long term testings, leading to probable bad protection characteristics in more aggressive environments.