Compositional evidence for flow in anodic films on aluminum under high electric fields

Using an Al-3.5 atom % W alloy, the formation of porosity in anodic alumina films is examined in phosphoric acid and borax electrolytes. During growth of major pores in phosphoric acid electrolyte, the film develops two main layers with respect to the cation species: an outer layer of tungsten-free alumina, located next to the electrolyte at the pore base and adjacent to the pore walls, and an inner layer of tungsten-containing anodic alumina, located next to the aluminum and the boundaries between the cells that contain individual pores. The W6+ ions in the inner layer do not reach the pore base regions, unlike expectations for pore generation by a dissolution mechanism. The porosity is attributed to electric-field-assisted flow of film material within the barrier layer region of the film toward the cell walls. In contrast, pore generation in anodic films grown in borax electrolyte occurs mainly due to field-assisted dissolution of film material. Accordingly, tungsten is present throughout the barrier layer and the cell wall regions of the major pores. The thickness of the anodic films formed in borax electrolyte is similar to that of the oxidized metal, whereas the thickness of anodic films formed in the phosphoric acid electrolyte exceeds that of the oxidized metal by a factor of about 1.42, consistent with flow of the film material in the latter electrolyte. (c) 2007 The Electrochemical Society.