Abstract
Porous anodic oxide films formed on Al in an oxalic acid solution were hydrated in water at 99.5°C with and without additions of chromate, phosphate or nickel acetate. The hydrated films were then dissolved in a chromic acid-phosphoric acid solution. The changes in film structure during the hydration and dissolution experiments were followed by gravimetry and by impedance measurements in a neutral borate solution.
(1) As in pure hot water, pores of the film were filled up (or sealed) with hydrous oxide in a short time in waters containing chromate and nickel acetate, although slight dissolution of the film was observed. The water with phosphate did not hydrate the film but dissolved it to some extent.
(2) In the chromic acid-phosphoric acid solution, films hydrated in chromate-containing hot water dissolved faster and films hydrated in hot water containing nickel acetate dissolved slower than those hydrated in pure hot water. For all films, the dissolution rate was controlled by the dissolution of hydrous oxide which fills up the pores. The dissolution rate in general decreased with increasing electric resistance of the hydrous oxide.
The significance of the three existing methods of verifying the degree of “sealing”, the coloring method, the impedance measuring method and the acid dissolution method, is discussed.
