A study on the dissolution of non-porous alumina films on aluminium

Abstract

The dissolution behaviour in sulphate solutions of so-called non-porous anodic alumina films and films formed in dry oxygen and i n a moist oxygen atmosphere on chemically polished aluminium has "been investigated. Evidence is presented which indicates that the non-porous anodic films, formed at 7-30 volts, consisted of two regions, a thin, less soluble region adjacent to the metal and a thicker, more soluble outer region into which hydroxyl ions were incorporated. The structure might be related to the initial method of surface preparation adopted. It is suggested that films formed in dry oxygen at 500 C consist of crystalline Y-alumina, whilst those formed in the moist oxygen atmosphere a t 500 C possibly consist of amorphous alumina with someadsorbed water and are less faulted than the crystalline oxide. Again, there is some indication that the structure of these filmsis strongly influenced by the initial method of surface preparation used. The dissolution behaviour of the anodic films and films formed in dry oxygen at 500 C can be understood in terms of the extent of sulphate ion adsorption, apparently greater under the same conditions for the former type, and its effect on rate of transfer of protons, hydroxyl ions and water across the oxide/solution interface. A reaction scheme, proposed earlier to account for the behaviour during dissolution of the barrier layer of porous films formed anodically in sulphuric acid, appears adequate to explain the dissolution behaviour observed in the present studies. However, the type of behaviour, although rather similar for both' anodic non-porous and films formed in dry oxygen at 500 C, was quite different from the barrier layer of porous films formed anodically in sulphuric acid.