Effects of current density and electrolyte temperature on the volume expansion factor of anodic alumina formed in oxalic acid

The formation of porous anodic alumina in 0.4 M oxalic acid is investigated over a range of current density and electrolyte temperature using sputtering-deposited substrates containing tungsten tracer layers. The findings reveal volume expansion factors and efficiencies of film growth that increase with the increase of the current density and decrease of the temperature. Pore generation by the flow of the anodic alumina in the barrier layer toward the pore walls is proposed to dominate at relatively high current densities (above ∼2 mA cm−2), with tungsten tracer species being retained within films. Conversely, losses of tungsten species occur at lower current densities, possibly due to increased field-assisted ejection of Al3+ ions and/or field-assisted dissolution of the anodic alumina.