High field kinetic processes in anodic oxide films on aluminium. I. Current transients for linearly changing electric field strength

Abstract

Anodic oxide films were formed on high purity aluminium (99.996 %) under steady-state conditions (current and field strength constant) in a glycol–borate electrolyte until the film reached a predetermined thickness at which point the anodic overpotential was changed rapidly and in a linear manner. As little film growth occurred during these linear sweeps, the conditions corresponded to linearly changing field strength. From these data, the transient differential field coefficient, β1, defined by[Formula: see text]where i and E are the ion current density and electric field strength and Es the steady-state formation field strength, was determined β1 was found to vary linearly with Es in the manner [Formula: see text] with [Formula: see text] A recent theory proposed by one of us (M. J. D.) predicts that the parameter [Formula: see text] should have the same value as that deduced from the field dependence of the steady-state differential field coefficient,[Formula: see text]Such agreement was indeed found, two independently determined 'steady-state' values of [Formula: see text] being 3.53 ± 11% and 3.11 ± 14% ÅV−1. A direct comparison of the present results with previous steady-state results gave βs/β1 = μs/μ1 = 3.09. More complex features of the transients were also found to be in accord with the above theory, but could be accounted for almost as well by an earlier theory, the so-called high field Frenkel defect theory.Dielectric constant values determined from the current discontinuity appearing upon application of the linearly increasing field gave K1 = 8.35 ± .1 for transients commencing from steady-state conditions and K1 = 8.85 ± .2 for films formed then 'aged' at E = 0 before measurement. Certain anomalies with regard to the charging current were apparent.