Selective oxidation of aluminium and interfacial enrichment of iron during anodic oxide growth on an Al6Fe phase

Abstract Porous anodic oxide formation on an Al-1.4wt%Fe alloy containing finely dispersed Al 6 Fe particles has been examined by transmission electron microscopy of ultramicrotomed sections, with particular attention directed towards the local film formation over the Al 6 Fe particles. It was found that anodic oxide formation over the Al 6 Fe phases involves initial oxidation of aluminium and interfacial enrichment of iron, with the enrichment confined to a thin layer, about 1–2 nm thick, in the Al 6 Fe phase immediately beneath the oxide. With EDX analysis, using an electron probe of 0.8 nm diameter, the composition of the iron-enriched layer has been determined to be about Al 80 Fe 20 . Additionally, close examination of the iron-enriched layer, using an ultra-high resolution transmission electron microscope, has revealed that the layer has a heavily distorted structure with the occasional presence of ordered domains, only 1 nm diameter, with the Al 6 Fe structure. No evidence was obtained to suggest the presence of nano-sized iron clusters in the enriched layer. These observations are discussed in terms of the Gibbs free energy criteria for alloy oxidation proposed recently by several of the authors.