Thickness Uniformity in Dual-Layer Anodic Aluminum Oxide Films Achieved by Structural Control of Barrier Layer Using Current Recovery Method

To improve the adhesion and corrosion resistance of aluminum, the dual-layer anodic oxide film was formed by sequential anodizing in phosphoric acid (PA) and oxalic acid (OA) electrolytes. The upper-layer anodic oxide film, with large pore diameter and thick barrier layer, was obtained by applying a high voltage in PA electrolyte. While the large pore diameter could improve adhesion, the thick barrier layer hindered the uniform growth of the lower-layer anodic oxide film formed by anodizing in OA electrolyte. To achieve a uniform growth of the lower-layer film, the barrier layer of the upper-layer film was thinned using the current recovery method. Consequently, pores grew continuously at the interface between the upper-layer and lower-layer films, resulting in a uniform film thickness of the dual-layer film. When the dual-layer film was subjected to hot water sealing, few flaky hydrates formed in the upper-layer film and the pores remained open at the surface, whereas many granular hydrates formed in the lower-layer film. Thus, the dual-layer film could improve both adhesion and corrosion resistance of aluminum.