Behavior of Impurity and Minor Alloying Elements during Surface Treatments of Aluminum

Abstract

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.

Microscopic Observations of Voids in Anodic Oxide Films on Aluminum

“…The same conclusion was drawn from the PAS study of anodic oxidation, from the invariance of S d with oxide film thickness. 5 These observations support the interpretation that, as interfacial metal voids are incorporated into the oxide, new metal voids are generated at the same sites.…”

“…Direct observations of interfacial voids in the metal using cross-sectional TEM may be possible. 5 However, given the typical number densities of pitting sites, the relatively small interface area sampled by these cross sections may not permit pit precursor defects such as voids to be effectively viewed. However, one can expect the interfacial voids in the metal to be incorporated into the oxide during anodic film growth as the metal layer containing voids is oxidized.…”

Influence of alloying elements and etching treatment on the passivating films formed on aluminium alloys

“…25,35 The condition that sodium atoms must be in solid solution for the phenomenon of enrichment to occur provides an explanation for the difference in behaviour between this alloy and the other tested materials. In this respect, it seems likely that sodium in the Al-Mg-Si, Al-Cu and pure Al materials is retained forming part of the composition of precipitated second-phase particles, such as NaAlSi x , 36,37 while in the Al-Mg alloy an important part of the Na remains dissolved in the matrix.…”

“…Owing to the higher Gibbs free energy for copper oxide formation than for alumina formation, copper atoms in solid solution in the aluminium matrix cannot initially be oxidized leading to an increased copper concentration in the alloy, in a film thickness of up to ¾2 nm. 32,35 All the studied alloys contain small but significant sodium contents in their bulk (Table 1), so it is curious that the superficial presence of Na is revealed in only one of them (Al-Mg) (Fig. 6).…”

Positron Annihilation Spectroscopy Study of Interfacial Defects Formed by Anodic Oxidation of Aluminum

“…The composition changes are due to the accumulation of noble impurities at the metal/oxide interface as the aluminum atoms are dissolved. 12,13 In the present study, the formation of interfacial voids during anodic oxidation of aluminum was investigated. During anodizing, the extent of metal consumption during film growth was controlled, and could be kept very small compared to that experienced in typical dissolution processes.…”