Anodizing of aluminum coated with silicon oxide by a sol-gel method

Aluminum specimens were covered with SiO2 film by a sol-gel coating and the n anodized galvanostatically in a neutral borate solution. Time variations in the anode potential during anodizing were monitored, and the structure a nd dielectric properties of the anodic oxide films were examined by transmi ssion electron microscopy, Rutherford backscattering spectroscopy, and elec trochemical impedance measurements. It was found that anodizing of aluminum coated with SiO2 films leads to the formation of anodic oxide films, which consist of an outer Al-Si composite oxide layer and an inner Al2O3 layer, at the interface between the SiO2 film and the metal substrate. The capacit ance of anodic oxide films formed on specimens with a SiO2 coating was abou t 20% larger than without a SiO2 coating. In the film formation mechanism, the conversion of Al2O3 to Al-Si composite oxide at the interface between t he inner and outer layers is discussed in terms of inward transport of Si-b earing anions across the outer layer. (C) 2001 The Electrochemical Society.