Nanostructural changes in porous anodic films on aluminum during aging

In the microstructure of sealed porous anodic films on aluminum, prepared in an oxalic acid solution, a substructure with three regions can be clearly seen: the cell boundary bands; the cell walls containing acid anions; and the central pore fill material with abundant retained water. Under the electron beam the first region remains practically unaltered, while rapid changes take place in the substructure of the other two regions, leading to their progressive homogenization. The results provided by transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) show a clear similarity between the transformations that take place in a few minutes under the effect of the electron beam, in a few days at moderate temperatures of 70–100 °C, and over months and years of atmospheric exposure at ambient temperature. Retained water plays a key role and unsealed anodic films, which are practically free of water, hardly experience any transformation under the electron beam; giving rise to identical impedance plots when recently obtained, after 24 h of aging in an oven at 100 °C, and after months and years of exposure in an atmosphere of low relative humidity.