Study by Nuclear Microanalysis and O18 Tracer Techniques of the Oxygen Transport Processes and the Growth Laws for Porous Anodic Oxide Layers on Aluminum

Abstract

The method of nuclear microanalysis of stable oxygen isotopes by the reactions was used to investigate the mechanisms of growth, dissolution, and transport associated with oxygen during the formation of relatively thin (≤7000Aå) porous anodic layers on aluminum. The average charge associated with an oxygen ion fixed in the film was calculated on. the basis of the direct measurement of sulfur incorporation in the films. The current efficiency of the oxidation was determined (and found to be low, ∼55%) by comparing the amount of oxygen fixed on the metal surface deduced from nuclear microanalysis to that deduced from the amount of charge fed into the circuit. The rate of chemical dissolution of the oxide was measured in open circuit and under polarization and was found to be two orders of magnitude lower than that which would explain the low current efficiency. Hence the existence of an electrochemical process of cation transfer must be assumed. It was found, by using oxygen 18 tracer techniques, that the growing oxide layer is formed at the metal/oxide interface by oxygen transport through the base layer. The results show that about 60% of the ionic current is transported by oxygen ions and about 40% by cations during the growth of porous oxide layers on aluminum.