Anodizing of Aluminum Coated with Zirconium Oxide by a Sol-Gel Process I. Effect of Heat Treatment on the Formation of the Anodic Oxide Film

Aluminum specimens were covered with zirconium oxide films by sol-gel dip coating at T_h, = 573 and 873 K, and then anodized galvanostatically in a neutral borate solution. The time-variation in the anode potential during anodizing was monitored, and the structure of anodic oxide films was examined by TEM, EDX, and RBS. During anodizing of specimens coated with ZrO₂ at T_h = 573 K, the anode potential, E_a, increased linearly with time, t_a, and the slope of the E_a vs. t_a curve increased with increasing the number of sol-gel coatings. An anodic oxide film, which was composed of an inner Al₂O₃ layer and an outer Al-Zr composite oxide layer, grew at the interface between the ZrO₂ layer and metal substrate during anodizing. The thickness of the inner layer increased with t_a, whereas the thickness of the outer layer showed a maximum before decreasing with t_a. Specimens coated with ZrO₂ at T_h=873 K showed E_a -jumps between 40 and 110 V just after the commencement of anodizing, and then a slow E_a increase. An anodic oxide film composed of a single layer of A1₂O₃, formed beneath the ZrO₂ layer during anodizing. The mechanism of the formation of anodic oxide films on aluminum coated with ZrO₂ films is discussed in terms of ion transport across the anodic oxide film.