Crystal structure, thin films changes with oxidation

The thin films responsible for passivity are often amorphous, and since the extent of solid solubility is dependent on the crystal structure, the rigid compositions associated with the crystalline state are not necessarily operative within these thin films. It seems possible, therefore, that with films that are predominantly oxide a certain concentration of hydroxyl ions could be present, and likewise, films that are predominantly hydroxide could contain a certain proportion of oxygen ions. This view is supported by the corrosion behavior of such metals as aluminum and the stainless steels, where different degrees of passivity are obtained by alloying or by slight changes of concentration of the corroding solution. 

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