THE MECHANISM OF GROWTH OF POROUS ANODIC AL2O3 FILMS ON ALUMINUM AT HIGH FILM THICKNESSES

The mechanism of growth of porous anodic Al2O3 films at various bath temperatures, current densities and H2SO4 concentrations was studied for films produced in a vigorously stirred bath at anodization times higher than those at which the maximum pore diameter behind or at film surface approaches first the cell width. Three cases of film growth were observed: a fluctuating variation, a continuous increase and a case in which a maximum, a minimum and then a continuous increase of mass and thickness successively appear. A method was developed by which alternative, transformed and simplified forms of the developed strict kinetic model governing the growth of porous anodic Al2O3 films, applicable up to the above anodization times, were formulated capable of predicting an approximate pore shape. This pore shape, the concentration of the electrolyte and its variation along pores, the rate of the transport of Al-2(SO4)(3), produced by the field assisted pore base oxide electrochemical dissolution and the purely chemical pore wall oxide dissolution, and the electrolyte conductivity inside pores were found to be of decisive importance for the growth of oxide beyond these times. Also, suitable models governing the growth of the oxide beyond these times were formulated. In cases where they could apply to the experimental results and observations these were consistent with the predictions of the models. The existence of a maximum limiting constant film mass and thickness was also predicted to occur at specific anodization conditions.