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

Porous anodic oxide formation on an Al-1.4wt%Fe alloy containing finely dispersed Al6Fe particles has been examined by transmission electron microscopy of ultramicrotomed sections, with particular attention directed towards the local film formation over the Al6Fe particles. It was found that anodic oxide formation over the Al6Fe 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 Al6Fe 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 Al80Fe20. 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 MI diameter, with the Al6Fe 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. (C) 2000 Elsevier Science Ltd. All rights reserved.