POROUS BARRIER COMPOSITE FILMS FORMED ON ALUMINUM BY RE-ANODIZING/

The structure and the formation behavior of porous / barrier composite oxide films formed on aluminum by re-anodizing in a boric acid / ammo nium berate solution have been studied by transmission electron micros copy combined with ultramicrotomy. The porous film was formed in an ox alic acid solution. The porous / barrier composite film is amorphous, whereas a crystalline film grows when the porous film is hydrated befo re the re-anodizing to form hydrous alumina platelets in the pores. Th e size and the volume of both crystals and voids increase with anodizi ng time. The thickness of the various types of barrier films formed at a given current density up to 350 V in the berate solution decreases in the order: the amorphous region of the barrier film > the barrier / porous amorphous composite film > the hydrated porous / barrier cryst alline composite film > the crystalline region of the barrier film.