TEM investigation of pre-oxidised Fe–Al with improved aqueous corrosion resistance

Abstract

The protectiveness of an oxide scale on iron aluminides against aqueous corrosion has been controversially discussed in the literature. For clarification, Fe–25.8 at.% Al was oxidised in air at 1000 °C for 188 h and then immersed in H2SO4 (0.0126 M, pH 1.6). This experiment showed that pre-oxidation can substantially improve the aqueous corrosion behaviour of Fe-Al [31]. The present detailed investigation by transmission electron microscopy on the structure and constitution of the oxide scale after aqueous corrosion reveals why the oxide scale improved aqueous corrosion behaviour. During oxidation, a two-layered oxide scale formed. Corrosive attack by the acid is limited to the outer layer, consisting of equiaxed α-Al2O3 and a small amount of spinel. The inner layer of columnar α-Al2O3 grains with few grain boundaries apparently inhibits further ingress of the acid to the scale/metal interface. Faceting of the columnar α-Al2O3 grains at the scale/metal interface explains the good adherence of the oxide scale to the underlying alloy. Differences in oxide scale formation after short-time and prolonged oxidation are likely the reason for the discrepancy in previous observations.