document: Anodizing processes are widely used for protecting aluminium alloys against corrosion [1]. The resultant films are composed of amorphous alumina and consist of a relatively thick, porous, outer region and a thinner, non-porous, inner region [2,3]. The porous region contains the major pores of the film, which extend from the film surface to the barrier layer. Near the film surface, shorter, incipient pores are also present, whose growth stopped in the early stages of anodizing. The diameter of the major pores and the thickness of the inner, barrier region are dependent on the potential applied during anodizing, with typical proportionalities of ∼1nmV−1 [3,4]. Studies of ionic migration in barrier-type and porous anodic alumina films have usually found a transport number of O2− ions of ∼0.6 [5,6]. During the formation of porous films, the outward migrating Al3+ ions, constituting the remainder of the ionic current, are ejected to the electrolyte at the pore bases [7]. The electronic current in the barrier region is generally considered to be negligible. The thickness of the barrier region, which is relatively constant during the growth of a film under either a constant potential or constant current density, is maintained by a balance between growth of the barrier layer by continued oxidation of the aluminium substrate and thinning of the barrier layer by either field-assisted dissolution of the alumina at the pore bases [8] or field-assisted flow of alumina from the barrier layer to the pore walls [9–13]. The pores may be widened toward the film surface by chemical dissolution to an extent dependent on the anodizing conditions.
extractive_keyphrases: alumina aluminium substrate anodizing barrier layer barrier region chemical dissolution corrosion ejected electrolyte extend from the film surface film incipient pores ionic current ionic migration major pores o2− ions oxidation pore bases pores pore walls porous films protecting aluminium alloys resultant films
abstractive_keyphrases: amorphous alumina anodic alumina films anodizing processes field-assisted flow of alumina film surface growth of the barrier layer by continued oxidation growth stopped in the early stages of anodizing outward migrating al3+ ions thinning of the barrier layer by either field-assisted dissolution