In Situ Study of the Surface Oxidation of FeCr Alloys Using Grazing Incidence X-ray Absorption Spectroscopy (GIXAS)

The surface oxidation of FeCr alloys with 18, 28, and 43 mass-% Cr was investigated in situ using grazing-incidence Xray absorption spectroscopy (GIXAS) at the chromium and iron K-edges. Oxidation in air was monitored in situ in the temperature range from 290 K to 680 K. The standard GIXAS data analysis is extended for the treatment of a single layer model in order to estimate the chromium concentrations of the oxide layer and of the near-interface substrate as well as the oxide layer thickness. XANES analysis shows transitions from b.c.c. Fe to corundum type Fe2O3 and from b.c.c. Cr to corundum type Cr2O3. The initial oxide layers are 1.1-1.4 nm thick and contain 60-90 mass-% chromium, while the near-interface substrate is depleted in Cr. During heating, iron oxide growth dominates up to 560-600 K. Then the chromium oxide layer loses its passivation effect and Cr oxidation sets ill.