Anodic film growth on Al–Li–Cu alloy AA2099-T8

AA2099-T8 aluminium alloy was anodized at a current density of 5 mA/cm2 to selected voltages in 0.1 M ammonium pentaborate electrolyte at 293 K. It was found that the growth of barrier-type anodic film on the alloy was accompanied by oxidation of intermetallics, formation and rupture of oxygen gas-filled voids in the anodic film and healing of the anodic film at the sites of rupture. It was revealed that the formation of oxygen gas-filled voids was related to the oxidation of copper-rich nanoparticles in the copper-enriched layer at the film/alloy interface, and that the oxidation process of copper-rich nanoparticles depended on grain orientation. Further, the significantly reduced Pilling–Bedworth ratio for formation of anodic lithium oxide compared with that for formation of anodic alumina resulted in the formation of fine voids at the alloy/film interface and sequentially, detachment of the anodic film from the alloy surface.

► Void formation in the anodic film is related to oxidation of Cu-rich nanoparticles.
► The oxidation process of Cu-rich nanoparticles depends on grain orientation.
► Intermetallics are preferentially oxidized prior to the oxidation of alloy matrix.
► Fine voids formed at the alloy/film interface result in detachment of anodic film.