An O18 Study of Field‐Assisted Pore Formation in Compact Anodic Oxide Films on Aluminum

The mechanism of field‐assisted pore formation in 15% wt aqueous solutions was studied by O 18 tracing techniques. Compact base layers were first formed in an O 18 labeled solution and further oxidized in a nonlabeled solution, leading to field‐assisted pore formation. The O 18 content and depth distribution was measured in these duplex films. It was found that the initial number of O 18 atoms per square centimeter is conserved to within 2% and that the major part of the oxygen of the initial base layer is located on the top of the duplex film with the same O 18 isotopic concentration. This observation shows clearly that the pore formation does not take place through a simple oxide dissolution process whether chemical or field assisted. Pore formation consists of some kind of oxide decomposition involving only cation transfer into the solution. The values of the cationic and anionic currents are calculated. Possible mechanisms for ion movement are discussed.