Autoclave Treatment of Anodic Oxide Film of Aluminum in Zinc Acetate Solution and Improvement of Alkali Resistance

In order to improve the alkali resistance of anodic oxide film of aluminum, anodized aluminum was autoclaved in 0 mass% and 0.1-10 mass% zinc acetate dihydrate (ZnAce) added in 0-100 mass% 1, 4-butanediol (BD) aqueous solution at 130°Cto 250°Cfor 4 houres. In result, over 150°Cand with the exception of 100 mass% BD, anodic oxide film was sealed by ZnAl2O4 spinel (ZAS) phase above 1 mass% ZnAce, and by complex phase with ZAS and pseud-boehmite between 0.1 and 1 mass% ZnAce, and by pseud-boehmite phase only without ZnAce. In the case of 100 mass% BD, ZAS or pseud-boehmite was not produced, then, it was thought that these phases were prepared through dissolve/precipitation reaction of anodic oxide film by contained water. By means of SEM observation of cross-section of film, the film was composed of three layers, outer dense layer and a middle layer and inner columnar layer, or two layers, outer dense layer and inner columnar layers. Alkali resistance time of this film in 10 mass% of NaOH aqueous solution by means of the electromotive force method (JIS H-8681-1) was 100-1000 times as long as the boiling water sealing film. The alkali resistance time increased with increasing of the dense layer thickness. From this result, it was thought that the effect of the ZAS phase sealing on alkali resistance was about 10-times bigger than the pseud-boehmite phase sealing, and, the effect of the complex phase sealing was 4-times bigger than the pseud-boehmite phase sealing.