Structural evolution in nanoporous anodic aluminium oxide

Nanoporous and self-organized layers of aluminium alloys are used in many applications as membranes, templates for nanometric objects or corrosion protection for aluminium alloys. The use of this nanometric structure widely remains empirical, especially in the case of very small pores (<20 nm). The objective of this study is to present the structural evolution of nanoporous anodic aluminium oxide during “sealing” processes and ageing on aluminium alloy. The probing on the chemical bonding environment of aluminium by XANES spectroscopy, combined with different analytical techniques demonstrate that the oxide structure is constituted by 2/3 of aluminium in tetrahedral coordination 1/3 in octahedral coordination and sulphate species. In contact with water electrolyte, the hydrolysis of AlO4 into AlO6 cluster and a partial release of sulphate ions are an important chemical transformation of the amorphous structure. This structural transformation defines the chemistry (pH and surface charge) inside the nanopores, the ageing behaviour and the possible incorporation or diffusion of chemical species in the nanostructure.