Elastic recoil detection analysis (ERDA), RBS and TEM study of barrier film formation on Al-4.5 at.% Mg-0.05 at.% Cu alloy

The influences of magnesium upon the formation of barrier-type anodic films on an Al–4.5 at.% Mg–0.05 at.% Cu alloy have been examined by elastic recoil detection analysis, Rutherford backscattering spectroscopy and analytical transmission electron microscopy. The results reveal that magnesium species are distributed uniformly throughout the anodic alumina film, with magnesium species migrating outwards in the film approximately twice as fast as Al3+ ions. Upon reaching the film surface, the magnesium species are ejected into the electrolyte under the particular conditions of anodizing, whereas Al 3+ ions are retained within the film. The oxidation of magnesium results in voids at the alloy/film interface, which are probably associated with the low Pilling–Bedworth ratio of Mg/MgO relative to that of Al/Al2O3. The voids are formed in the presence of a high enrichment of copper (to 1.0×1019 Cu atoms m-2 ) in the alloy region immediately beneath the anodic film, with copper species (which migrate outward faster than Al 3+ ions) present throughout the anodic film.