A novel approach for the study of the migration of Cl − ions in anodic alumina

Abstract

High purity aluminium surfaces, electropolished in a perchloric acid–ethanol bath, are covered with a thin, so-called `polishing film' which contains a significant Cl − ion content. Through novel use of the polishing films as a source of Cl − ions, their migration in anodic alumina has been examined successfully, with inward migration occurring as expected. The rate of migration of Cl − ions, relative to that of O 2− ions, was 0.62 during anodic oxide growth at a constant current density of 5 mA/cm 2 in 0.1 M ammonium pentaborate solution at 20°C. Additionally, the migration of Cl − ions is not independent of, but appears to be correlated with, the migrations of Al 3+ and O 2− ions. The rate of migration of Cl − ions determined is in excellent agreement with those obtained previously using Cl − ions introduced artificially into the metal, or into thin pre-formed alumina films, by ion-implantation. Thus, any radiation damage to the substrate, associated with relatively low dose ion-implantation, does not significantly affect the behaviour of implanted Cl − ions or the ionic transport processes during anodic oxide growth generally.