Changes in the specific surface area of porous aluminium oxide films during sealing

This paper analyses the information on anodic film sealing and autosealing–ageing processes which can be provided by thermogravimetry and gas adsorption porosimetry techniques that are much less widely used in this type of study than electrochemical impedance spectroscopy (EIS) or scanning and transmission electron microscopy (SEM and TEM). During the porous wall dissolution stage, the pore volume is approximately doubled due to the dissolution of anhydrous alumina in the hexagonal cell walls, forming a bayerite gel that precipitates upon reaching the saturation point. The pore diameter increases from 140–150 to 200–220 Å and its specific surface area grows from approximately 20 m2/g to around 30 m2/g. The informations obtained by thermogravimetry, porosimetry, EIS and TEM are complementary and concordant, allowing an estimation of the effects of the dissolution and precipitation stages, which can be directly and quantitatively visualised by SEM.