Application of surface behaviour diagrams to the study of hydration of phosphoric acid-anodized aluminium

Abstract

The hydration of aluminium surfaces prepared for adhesive bonding by anodization in phosphoric acid has been studied using surface behaviour diagrams. These surface behaviour diagrams, which are similar to phase diagrams for equilibrium bulk phases, trace the evolution of the aluminium adherend surface composition, obtained by X-ray photoelectron spectroscopy, during the hydration process. When supplemented with high-resolution scanning electron micrographs and Auger depth profiles, the surface behaviour diagrams show that hydration proceeds in three steps. The first step is reversible and consists of the adsorption of water by the monolayer of AlPO₄ initially present on the surface. It involves no change in the oxide morphology. The second, which appears to be rate-controlling, involves the slow dissolution of the phosphate followed by rapid hydration of the exposed alumina to the oxyhydroxide, boehmite. During this stage, extensive morphological changes occur as the boehmite fills the pore cells and bridges the whiskers of the original surface. The third step consists of the nucleation and growth of the trihydroxide, bayerite, on top of the boehmite. Using these results as examples, we propose the surface behaviour diagram approach as a new tool for the study of surface reactions in general.