Migration of sulphate ions in anodic alumina

The migration rate of sulphate ions, incorporated into anodic alumina from the electrolyte, has been determined by analysis of films, containing a boron species marker, by glow discharge optical emission spectroscopy. The relative distributions of boron and sulphur species in the depth profiled film indicate that the inward migration of sulphate anions proceeds at a rate of about 0.62 times that of O2- ions. However, significant broadening of the sulphur profile with depth of migration was observed, which is attributed to statistical fluctuation of jump distances. The factors determining the rates of inward migration of anion species in amorphous anodic oxides are considered with respect to influences of ionic size, ionic charge and local film structure, although there is at present no general explanation of behaviours. (C) 2000 Elsevier Science Ltd. All rights reserved.