Embedded space charge in porous alumina films formed in phosphoric acid

The embedded charge in the barrier layers of porous alumina, formed potentiostatically in phosphoric acid was studied as a function of anodizing voltage (30-57 V) and bath temperature (18 and 21 °C). For that, the polarization measurements of as-grown and annealed alumina/Al samples were conducted in the same anodizing bath by anodic potential sweep at a scan rate of 2.6 V/min. The plane capacitor model was used for the assessment of the charge density in the barrier layers of as-grown porous alumina. For the barrier layers of films formed at 18 °C this value equals to 0.747 μC cm⁻² and does not depend on the anodizing voltage. Increase in electrolyte temperature rises the embedded charge density. Polarization measurements carried out in this paper clearly present that the barriers of phosphoric acid films grown at the anodizing voltages lower than 39 V contain a layer of virtual cathode while at higher voltages this layer disappers. The obtained results allow speaking about promising opportunities of potentiodynamic polarization measurements of alumina films in the same anodizing solution before and after annealing for the studies of charges embedded within the alumina barriers and for the regularities of ion transport.