Kinetics of growth of porous anodic Al 2O 3 films on A1 metal

Abstract

The kinetics of growth of porous anodic aluminium oxide films on A1 metal, anodized galvanostatically in a non-stirred bath solution 15%w/v H 2SO 4 at various current densities (5–75 mA/cm 2) and bath temperature (20–40°C), were studied. The kinetic model m = kit - λ 1 t exp (λ 2 t) was found to be valid for all the anodization conditions chosen and for anodization time intervals shorter than those beyond which limiting constant values of film mass and thickness are obtained. The model shows that the true shape of pores is that of a truncated cone, whereas parameters λ 1 and λ 2 are directly related to the surface area of the pore base section (λ 1 = 4 −1 kinπ D 2 0 and the dissolution rate of pore wall oxide (λ 2 = 2 k d D −1 0. With the aid of the model it was proven that, during anodization, oxide dissolution at the depth of pores is essentially a field-assisted process; while pore wall oxide dissolution is a first-order reaction, thermally activated, with an activation energy of 78.6 kJ/mol; electrolyte concentration inside the pores increases linearly with true current density ( C = C 0 + ai t). The open circuit oxide dissolution rate was found to be 0.52–4.1 Å/min for anodization temperatures of 20–40°C. Also, by applying the model, information was obtained on the oxide's main structural characteristics such as concentration of cells and pores, cell mean width, pore base mean diameter, thickness of barrier layer, total and specific real surface, porosity, etc.