Mechanism of aluminium and oxygen ions transport in the barrier layer of porous anodic alumina films

Aluminium was anodised in oxalic acid electrolyte at concentrations 0.125–0.5?M, current densities 25–100?A?m^?2 and low temperatures 0 and 5?°C. The efficiencies of Al consumption and oxide production in the metal|oxide interface and the transport numbers of Al³⁺ and O^2? in the barrier layer of porous anodic alumina films were determined. The Al consumption efficiency essentially coincides with that by Faraday’s law while that of oxygen evolution, visually detected at these temperatures, is negligible. The oxide production efficiency and O^2? transport number decrease with temperature, increase with current density and are almost independent of electrolyte concentration. The transport numbers combined with literature ones for oxalate and sulphuric acid electrolytes were treated by high field kinetic equations describing independent Al³⁺ and O^2? transport to penetrate its mechanism. The half jump activation distances were found comparable to ions radii. This mechanism embraces two steps, equilibrium established between ordinary oxide lattice hardly allowing transport and locally emerging transformed structure dispersed in barrier layer consisting of pairs of Al³⁺ and O^2? clusters enabling transport and the rate-controlling step of actual ion transport within clusters. The transformed structure then returns to ordinary while it emerges at other sites. The real activation energy of Al³⁺ transport is higher than that of O^2?, e.g. by ≈?19?kJ?mol^?1 at low current densities, but the fraction of really mobile Al³⁺ is ≈?10³–10⁴ times larger than that of O^2? justifying the not excessively different values of O^2? and Al³⁺ transport numbers.