Anodic oxidation of aluminium

Abstract

Key aspects of anodic film growth on aluminium at ambient temperatures in aqueous electrolytes, encompassing barrier- and porous-type films, are described. By direct observation of film sections, incorporating inert marker layers and tracers, in the transmission electron microscope and appropriate analysis, the locations of solid-film growth and electrolyte anion effects can be determined precisely. Thus, during barrier-film formation, at high current efficiency, Al³⁺ ion egress and O^2-/OH⁻ ingress proceed across the pre-existing air-formed film to develop solid material at the film/electrolyte and metal/film interfaces respectively. With decrease in current efficiency, the former contribution declines through a mechanism of direct ejection of Al³⁺ ions at the film/electrolyte interface. At a critical current density, all outwardly mobile Al³⁺ ions are lost to the electrolyte. Concerning anions of the forming electrolyte, such species (or more strictly their transformation products) may be mobile inwards or outwards or immobile under the field. Whatever their mobility, the resultant film displays a region of relatively pure alumina adjacent to the metal/film interface, separated from the electrolyte by a region of anion incorporated alumina. Typical levels of anion incorporation in barrier-type films lie in the range 0˙6–5˙5 at%. Porous anodic films develop at current densities below the critical value, through O^2-/OH⁻ ingress, where no healing mechanism (through Al^{3 +} egress) exists to repair electrolyte penetration paths into the outer film regions. The steady-state porous anodic film then develops at the metal/film interface, with incorporation of electrolyte anions which are normally mobile inwards under the field. The development of penetration paths and, ultimately, major pores, proceeds through field-assisted dissolution, where locally the film material is weakened, possibly through high electrostriction pressures.