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

Porous anodic oxide formation on an Al-1.4wt%Fe alloy containing finely dis persed Al6Fe particles has been examined by transmission electron microscop y of ultramicrotomed sections, with particular attention directed towards t he local film formation over the Al6Fe particles. It was found that anodic oxide formation over the Al6Fe phases involves initial oxidation of alumini um and interfacial enrichment of iron, with the enrichment confined to a th in layer, about 1-2 nm thick, in the Al6Fe phase immediately beneath the ox ide. With EDX analysis, using an electron probe of 0.8 nm diameter, the com position of the iron-enriched layer has been determined to be about Al80Fe2 0. Additionally, close examination of the iron-enriched layer, using an ult ra-high resolution transmission electron microscope, has revealed that the layer has a heavily distorted structure with the occasional presence of ord ered domains, only 1 MI diameter, with the Al6Fe structure. No evidence was obtained to suggest the presence of nano-sized iron clusters in the enrich ed layer. These observations are discussed in terms of the Gibbs free energ y criteria for alloy oxidation proposed recently by several of the authors. (C) 2000 Elsevier Science Ltd. All rights reserved.