Film formation and detachment during anodizing of Al–Mg alloys

Anodic oxidation of binary Al–3 wt.% Mg and Al–5 wt.% Mg alloys has been undertaken in ammonium pentaborate electrolyte to develop barrier-type anodic films at relatively high current efficiency. During anodizing, aluminium and magnesium are oxidized at the alloy/film interface, entering the film as Al 3 and Mg 2 ions. Such behaviour is anticipated for an alloying element with a Gibbs free energy of oxide formation per equivalent less than that of alumina; further, the greater outward migration rate of Mg 2 ions relative to that of Al 3 ions in anodic alumina is expected from consideration of the respective single metal–oxygen bond energies. However, unexpectedly, with continued anodizing, the essentially alumina film detaches from the alloy surface, which is followed by growth of new film on the exposed substrate. The film detachment from the alloy is associated with void formation, considered to result from the significantly reduced Pilling–Bedworth ratio for formation of anodic MgO compared with that for anodic alumina.