Residual flaws due to formation of oxygen bubbles in anodic alumina

Flaws in anodic alumina films are localized sites where the film material is modified morphologically, structurally, or chemically compared with adjacent regions of uniform amorphous alumina. The flaws are preferred sites of pitting and dielectric breakdown, which are of importance to the corrosion resistance and electronic applications of aluminium respectively. Although flaws have been categorized in general terms as either mechanical or residual, there is no detailed understanding of the precise origin and nature of flaws and their mechanisms of influence in pitting and dielectric breakdown. Consideration is given here to the formation of oxygen bubbles within anodic alumina, by oxidation of O2- ions in the vicinity of the metal/film interface, which are suggested to be one type of residual flaw. The compositions of impurities or second phase particles, which may lead to generation of oxygen bubbles, are examined in the light of systematic investigations of anodizing of binary aluminium alloys. (C) 1999 Elsevier Science Ltd. All rights reserved.