Transport Numbers and the Structure of Porous Anodic Films on Aluminum

Abstract

Experiments are described in which aluminum samples, covered with a porous oxide layer by anodization in oxalic acid, are subsequently re‐anodized in boric acid ("forming"). The experimental techniques employed combine standard voltage and current measurements with gravimetric and optical methods for film thickness determination and measurements of the rate of film dissolution. The results show that for a given anodizing electrolyte the porosity depends only on the anodizing voltage and that it decreases with increasing voltage. The growth of the porous films is determined solely by the formation of oxide at the oxide/metal interface. This can be used to define a transport number for the positive ions during anodization, which was found to be . During forming in boric acid of a sample already covered with a porous oxide layer some of the oxide is formed in the pores and some at the metal/oxide interface. For this process a transport number was obtained. The average pore diameter, cell diameter, and barrier layer thickness vary linearly with the anodizing voltage, the cell wall thickness being somewhat thinner than the barrier layer.