Atomic force microscopy study of the initial stages of anodic oxidation ofaluminum in phosphoric acid solution

Aluminum foils with two different surface topographic textures were anodica lly oxidized at constant current in a phosphoric acid bath. In situ atomic force microscopy (AFM) was used to follow the initial development of surfac e topography on a 1 mu m scale, during the early stages of porous oxide fil m formation. Microscopic convex features such as ridges on both foils begin to increase in height and width when the anodic film thickness exceeds the initial feature height. Equations of a mathematical model are presented in corporating established interfacial reactions and oxide conduction behavior . The model indicates that the film-solution interface recedes into the met al during anodizing, since the current efficiency for oxide formation is sm aller than the oxygen ion transport number in the film. Ridge surfaces incr ease in height due to the higher local conduction resistance to the film-so lution interface, while film deposits rapidly at ridges because of the low local resistance to the metal-film interface. In agreement with the AFM res ults, enhanced oxide growth at ridges should start when the potential field in the film becomes two-dimensional, as a result of the film growing to a thickness larger than the ridge height. (C) 2000 The Electrochemical Societ y. S0013-4651 (99)04-054-9. All rights reserved.