Flow modulated ionic migration during porous oxide growth on aluminium

The ability to tailor the morphology of porous anodic alumina (PAA) drives applications in the important architectural, aerospace, packaging, lithography and nanotechnology sectors with the mechanisms regulating oxide growth being pursued vigorously. Here, we have probed the barrier layer of PAA during film growth, using high voltage, in situ, electrochemical impedance spectroscopy. Our new findings account for aluminium oxidation at the metal/oxide interface, ionic migration and interaction of charge carriers through the layer, and oxide viscous flow under the field. Separation of these processes by appropriate modelling discloses that ionic migration, being driven by the electrical field only, is independent of the electrolyte. The incorporated electrolyte-derived species modify the mechanical properties of the oxide, by regulating the oxide displacement from the barrier layer. The approach developed provides the vital foundations for an integrated theory of porous film growth on a range of metals in various film-forming electrolytes.