Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Abstract (EDOC: 8013)
The aim of the present study was to investigate the early stages of protective oxide layer growth on Fe-15 at.%Al and Fe-40 at.%Al oxidized at 700 to 900 °C for 5 to 300 min in dry, synthetic air. Emphasis was put on the analysis of the oxide layer with respect to chemical and phase composition using X-ray photoelectron spectroscopy (XPS), grazing incidence XRD with synchrotron radiation and TEM investigations. The results indicate that at all conditions an external oxide layer grows and that this oxide layer consists of an outer, approximately 40 nm thin, Fe2O3-containing layer followed by a nearly pure Al2O3 layer which grows continuously with increasing temperature and time. The Fe2O3 content of the outer oxide layer decreases with time, temperature and Al content of the binary iron aluminide. The phase compositions of oxide layers less than 100 nm in thickness are determined, indicating the formation of hematite (?-Fe2O3) and corundum (?-Al2O3) on Fe-15 at.%Al at 700, 800 and 900 °C after 300 min of oxidation. Moreover, TEM analyses indicate that for both alloys at 700 °C a metastable Al2O3 layer, most likely consisting of ?-Al2O3 or ?-Al2O3 was formed epitaxially at the iron aluminide/oxide interface. Exposure times of 300 min at 700 °C lead to the transformation of metastable Al2O3 to stable ?-Al2O3. However, a very thin Al2O3 layer of about 5 nm thickness seems to remain at the iron aluminide/oxide interface, most probably a metastable Al2O3 modification.
(2005): Early stages of protective oxide layer growth on binary iron aluminides. Zeitschrift für Physikalische Chemie, 219, 11, 1489-1503.