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

Highlights: • Oxide scale formed on Fe-Al protects against aqueous corrosion. • Double-layered Al₂O₃ scale consists of top Fe-rich equiaxed and bottom pillar-like pure α -alumina grains. • Columnar alumina layer is only formed at high temperatures and after prolonged oxidation. • Incoherent metal-oxide interface is facetted, preventing spallation. 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.