Anodizing of Aluminum Coated with Silicon Oxide by a Sol-Gel Method

Aluminum specimens were covered with SiO ₂ film by a sol-gel coating and then anodized galvanostatically in a neutral borate solution. Time variations in the anode potential during anodizing were monitored, and the structure and dielectric properties of the anodic oxide films were examined by transmission electron microscopy. Rutherford backscattering spectroscopy, and electrochemical impedance measurements. It was found that anodizing of aluminum coated with SiO ₂ films leads to the formation of anodic oxide films, which consist of an outer Al-Si composite oxide layer and an inner Al ₂O ₃ layer, at the interface between the SiO ₂ film and the metal substrate. The capacitance of anodic oxide films formed on specimens with a SiO ₂ coating was about 20% larger than without a SiO ₂ coating. In the film formation mechanism, the conversion of Al ₂O ₃ to Al-Si composite oxide at the interface between the inner and outer layers is discussed in terms of inward transport of Si-bearing anions across the outer layer.