Formation behavior of multi-layer oxide films on etched al foil by vacuum infiltration and anodization

A vacuum infiltration method was proposed to coat uniform zirconia (ZrO₂) films on etched aluminum (Al) foil. The formation behavior of the composite oxide films on aluminum (Al) foils with etch pits was different from that of flat Al foils. The multi-layer structures were formed on the ZrO₂-coated Al foil after being anodized. The formation mechanism of the multi-layer films on etched Al foils was discussed. The structure and composition of each layer were investigated by scanning electron microscopy and transmission electron microscopy. The multi-layer films consisted of an inner aluminum hydrate layer, middle Al₂O₃-ZrO₂ (Al-Zr) composite layer, and outer Al₂O₃ layer. Many Al³⁺ ions passed through the coating layer and transferred into the inner layer. A particle-like and layered structure was observed in the inner layer. The formation of multi-layer films was mainly affected by the volume expansion of the outer Al₂O₃ layer and the dense middle Al-Zr composite oxide film. The ZrO₂ coated foils exhibited 26% higher specific capacitance than foils with only Al₂O₃ layer when being anodized at 600 V.