Influence of surface composition on initial hydration of aluminium in boiling water

Eight aluminium samples were hydrated in boiling water for 5–60 s, which produces an adherent and porous pseudoboehmite (AlOOH) film on the surface. The progress of the hydration reaction was followed by IR reflection-absorption spectroscopy and a delay was observed owing to an induction period. The surface composition of the alloying elements was determined by XPS. AES depth profiling was used to analyse the sublayers of the hydrated oxide. Aluminium samples with Mg-enriched surface oxides were found to have the shortest induction periods, i.e. a shorter hydration time was required to obtain a certain thickness of the pseudoboehmite layer. The highest initial hydration rates were obtained in the interval 10%–20% Mg (determined by XPS) on the surface. XPS and AES analysis revealed that the hydrated oxide layer did not contain any Mg. By AES depth profiling Mg was found to be present at the metal/hydrated oxide interface. This indicates that Mg (as MgO or MgAl2O4) in the surface oxide is not dissolved by boiling water. The higher reaction rate for the Mg-enriched surfaces is suggested to depend on the disrupted Al-Mg oxide morphology, which probably increases the rate of diffusion and transport of soluble species. The Al-Mg oxide is also suggested to increase the number of nucleation sites and the nucleation rate.