The behavior of impurity and minor elements during electropolishing, chemical polishing, and anodizing treatments is examined for 99.99% Al and Al-0.2 atom % Mn alloy containing iron, copper, and silicon impurities in the parts per million range. Copper is enriched strongly, to at least 2­4×1014 copper atoms cm−2, during the two polishing treatments. Assuming that the copper is located in a metal layer of thickness 1 nm, just beneath the surface film, the enrichment corresponds to an average concentration of about 5 atom % copper. In contrast, iron and silicon impurities were not enriched significantly by the various treatments. Manganese was enriched in the Al-0.2 atom % Mn alloy to about 7×1014 atoms cm−2. Manganese appeared to interact with copper impurity in the alloy to reduce the level of copper enrichment. The factors determining the degree of enrichment include the Gibbs free energies per equivalent for formation of the various impurity and alloying element oxides, interactions between coenriching elements, and the location of the particular atoms in the metal, especially the proportions of the atoms in solid solution, in fine particulates, and segregated at cellular and grain boundaries. © 2002 The Electrochemical Society. All rights reserved.

Chemical polishing and electropolishing of aluminium result in fine cellular textures of nanometre dimensions, which play a role in the electrochemical behaviour of aluminium and its response to further treatment processes such as conversion coating and anodizing. Concerning the latter, the surface texture is highlighted by porous anodic film growth for protection of the substrate. The influence of residual impurities and low additions of alloying elements on the development of the textures is investigated here for superpure aluminium (99.99%) and an Al-0.4wt.%Mn alloy, respectively. Complementary analytical approaches, including atomic force microscopy and transmission electron microscopy, disclose scalloped cellular textures comprising adjacent peaks and troughs, with a dependence on the grain orientation of the aluminium substrate. In addition to texture development, enrichment of copper impurity in the alloy occurs adjacent to the alloy/oxide film interface, as revealed by in-depth elemental profiles generated by r.f. glow discharge optical emission spectroscopy. The absence of similar enrichments of iron and silicon in the alloy suggests that these elements are distributed mainly as fine segregates in the aluminium, unlike copper, which is in solid solution. At low alloying levels, manganese reveals limited enrichment and is also probably present mainly as segregates. The origin of the textures and their dependence on grain orientation are thus considered to be related to the presence and location of relatively high-melting-point elemental impurities in the alloy and their local electrochemical responses relative to the adjacent alloy surface.

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The development of regular, finely spaced, arrays of metal ridges and troughs, i.e. nanotex tures, by surface treatment of aluminium has been probed by atomic force microscopy (AFM) and transmission electron microscopy (TEM), assisted by radio frequency glow discharge optical emission spectrometry (rf GDOES). AFM reveals the grain orientation dependence of the tex ture and its increased dimensions with increased metal purity. TEM reveals atomic number contrast effects at the centres of metal ridges comprising the boundaries of the cells, thus implicating the composition and structure of the substrate. Enrichment of copper in the alloy immediately beneath the residual alumina film is evident in rf GDOES depth profiles; conversely, iron and silicon, present at similar levels as copper, are not detected. This suggests strongly that crucial impurities, i.e. silicon, are present as fine segregates in the alloy with a grain orientation dependent distribution originating from casting. The distribution of segregates controls the dissolution of the adjacent aluminium matrix in surface pretreatment, thus generating nanotex tures of regular arrangements of metal ridges and troughs with an amorphous alumina film developed over the macroscopic aluminium surface.