GROWTH OF ANODIC FILMS ON VALVE METALS

By analysis of transport number and F-log j data for valve metals under galvanostatic anodization, obedience is found to the kink dependent Cabrera-Mott equation for irreversible transfer of metal ions from metal to oxide and to the defect dependent high-field ion migration equation for transport of both metal and oxygen ions in the growing anodic film. For Al, the kink and defect dependences are d log s/d log j = 0.5 and d log N/d log j approaching 0.67 at low current densities (oxygen ion transport) and 0.5 at high ones (metal ion transport), and the ion transfer valence is n = 3. The denser is the oxygen packing in the oxide, the higher is the electric field needed to drive a given ionic current through it.