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The dissolution processes involved in the formation of porous films in sulfuric acid have been studied by suddenly lowering the formation voltag.e and observing changes in the current flowing and the barrier layer capacltance as steady-state conditions are re-established. It was found that current changes always follow capacitance changes, and that the dissolution processes involved are (a) initially a short time region of chemical dissolution is found up to some point where the minimum electric field strength required for ionic migration is approached, and (b) a second stage dissolution assisted by the presence of this electric field. The change from one dissolution rate to the other is continuous as this minimum electric field is approached. On the basis of the Keller model, the required pore base orientation is proposed to occur during the second stage field-assisted process.The structure of porous anodic oxide films on aluminum has been investigated for many years, the results of which have produced essentially two main proposals: (a) the classical structure of the cylindrical * Electrochemical Society Active Member.

The formation and dissolution of porous alumina films in sulfuric acid was studied by the incorporation of the radiotracer 35SO4= into the film during preparation. The results have shown that (a) the formation site of the porous oxide is the barrier layer‐porous layer interface, (b) the dissolution time for porous films is independent of thickness. Comparison with former work, on “duplex” barrier films, suggests a method of identifying barrier and porous type films. By the use of “duplex” films, dissolution studies have shown that a simple pore widening mechanism of dissolution is incompatible with the results. A pore widening‐concurrent pore shortening dissolution mechanism based on a truncated pore model is proposed, and has been found to be experimentally valid.

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