Electrode Potential Explaining the Growth of Anodic Oxides

Abstract:

The formation mechanism of porous anodic oxides remains unclear till now. The classical field-assisted dissolution (FAD) theory cannot explain the relationship between the current curve and FAD reaction, and the influence of the electrode potential on anodization is rarely reported. The electrode potential theory, oxygen bubble model and the ionic current and electronic current theories were introduced to explain the growth of porous anodic oxides of three metals (Ti, Zr and Fe). Taking the anodization of Ti in aqueous solution containing 0.5wt% NH₄F as an example, the electrode potential was calculated, and the morphology of porous anodic oxides was investigated at low voltages. Results show that the growth of porous anodic oxides is determined by the ratio of the ionic current to the electronic current. During the anodization, metals are classified into two groups: one is easy to form the compact oxide layer, and the other is easy to induce oxygen releasing, thus forming oxygen bubbles. The electrolyte is also classified into two groups correspondingly: compact oxide layer-assisted electrolyte and releasing oxygen-assisted electrolyte.