Structure and properties of anodic oxide films formed on niobium

The film thickness as well as the content and the penetration depth of contaminated phosphate anion of anodic films formed on niobium in phosphoric acid solution at constant current density up to an identical voltage has a linear relation with log of current density and electrolyte temperature. This suggests that the film structure and properties are controlled by the electric field strength E, since log of current density is proportional to E. When the film is formed at 10 Am-2 up to 60 V at 20degreesC, anodizing ratio is 2.3 nm/V and transport number of Nb5+ is 0.26, while the penetration depth of phosphorus is approximately 0.5. With the increase in current density and decrease in temperature, the film thickness decreases and the transport number of niobium, the phosphorus content and its penetration depth increase. Furthermore, it was confirmed that the film thickness was determined only by the final current density without relying on the initial formation process of anodizing. Anodic film is composed of three layers divided by different chemical dissolution rate. The dissolution rate is highest in the middle layer but is independent of current density, i.e., anion content.