Analysis of porous oxide film growth on aluminum in phosphoric acid using re-anodizing technique

The effects of the anodizing voltage on the porous alumina film formation on Al foil in 4% phosphoric acid at 20 °C have been studied. The barrier layer thickness of porous films was determined by a re-anodizing technique. A digital voltmeter with a computer system was used to record the change in the anode potential with re-anodizing time. It was established that the change in the porous film growth mechanism occurred at 38 V in phosphoric acid. We explained this phenomenon by the surface charge of anodic oxide film and its dependence on the anodizing voltage in the electrolyte. It was shown that the surface charge of oxide film during anodizing in phosphoric acid to 38 V was negative. The charge was equal to zero at 38 V. Above 38 V, the surface of oxide film had a positive charge and this charge increased with the anodizing voltage.