Formation and capacitance properties of Ti-Al composite oxide film on aluminum

Al specimens were covered with TiO₂ film by sol-gel dip-coating and then anodized in ammonium adipate solution. The structure, composition and capacitance properties of the anodic oxide film were investigated by transmission electron microscopy (TEM), Auger electron spectroscopy (AES), X-ray diffractometry (XRD) and electrochemical impedance spectroscopy (EIS). It was found that an anodic oxide film with a dual-layer structure formed between TiO₂ coating and Al substrate. The film consisted of an inner Al₂O₃ layer and an outer Ti-Al composite oxide layer. The thickness of layers varied with the number of times of sol-gel dip-coating. The capacitance of anodic oxide films formed on coated specimens was at most 80% higher than that without TiO₂. In film formation mechanism, it was claimed that the formation of composite oxide film was mainly affected by the structure of micro-pores network in TiO₂ coating which had an influence on Al³⁺ and O²⁻ ions transport during the anodizing.