Contribution of electron microscopy to the study of sealing processes in oxalic anodised aluminium

Abstract

Transmission electron microscopy (TEM) of anodic coatings reveals a substructure in which three regions are clearly differentiated: a very thin band at the hexagonal cell boundaries; the cell walls contaminated with anions from the anodising bath; and the hydrated alumina that fills the pores with a certain amount of retained water, Scanning electron microscopy (SEM) appears to be a highly suitable tool to observe, at the surface and at more internal levels, the structural changes associated with each sealing degree. Both types of electron microscopy allow the possibility to visualise the different stages of the very complex sealing and ageing mechanisms indirectly determined by gravimetric and electrochemical techniques.