Thermal alumina scales on FeCrAl: characterization and growth mechanism

High temperature oxidation resistance of alumina-forming materials is connected to the growth of dense, stable and protective alumina scales. Depending on temperature, impurities present in the base alloys, presence of water vapour in the oxidizing atmosphere, the alumina scales are composed of alpha-alumina (which is the stable phase obtained for temperatures over 1000 degrees C) or of transient alumina (gamma,theta,delta obtained for lower temperatures). It is generally considered that gamma-Al2O3 grows when T<850 degrees C. that theta-Al2O3 is present for 850 degrees C<T<1000 degrees C and that alpha-Al2O3 is stable when T exceeds 1000 degrees C The exact role played by transient alumina formation and/or transformation on the high temperature performances of alumina-forming materials is not exactly defined. Many works proposed that transient alumina phases grew during the first steps of the oxidation process and transformed into the stable phase after further oxidation. The transformation of transient phases in the stable alpha-phase is generally accompanied by a volume contraction of around 14%, In order to get better ox-dation resistance, the formation of transient alumina is not wished, because: 1) their growth rate is generally higher than that of alpha-alumina with, as a consequence, a huge Al consumption, detrimental for the material resistance after long exposures, 2) the change in volume during the transformation of transient phases into alpha-alumina can generate stresses in the oxide scale and can weaken its adherence to the underlying substrate, leading to massive spallation. The present study deals with the coupling of different characterization tools in order to precisely identify the transient phases grown on FeCrAl materials. The use of scanning electron microscope (SEM-FEG), transmission electron microscope (TEM), Photoluminscescence Spectroscopy(PLS), X-ray photoelectron spectrometry (XPS) and X-ray diffraction at different glancing angles (XRD) on model materials oxidized at two temperatures (850 and 1100 degrees C) could help the identification of transient phases. These techniques gave a better understanding of the alumina scale growth mechanism.