Transformation of the overall strict kinetic model governing the growth of porous anodic Al2O3 films on aluminium to a form applicable to the non-stirred bath film growth

The strict and complex, overall kinetic model, governing the growth of porous anodic Al2O3 films, was transformed to a form easily and directly applicable to the galvanostatic anodization in a non-stirred bath at constant bath temperature. It was shown that the transformed model always acquires a form different from that of a stirred bath anodization. The transformed model was applied to the experimental results of film growth obtained at 25 degrees C, 15 mA cm(-2) and in a wide range of H2SO4 concentration, 5-85% w/v. The application of the transformed model provided, consistently with other experimental observations, the existence of a critical electrolyte concentration near 5% w/v above which the normal mechanism of oxide production and film growth is valid and below which a deficient growth of oxide is observed. The transformed model permitted some predictions for the real pore shapes. The experimental results and their treatment showed that the electrolyte concentration affects parameters such as the mass and porosity of the film, the pore base hemispherical surface area, the time at which the pore external diameter approaches cell width, the time interval in which the model applies and the parameters involved in the transformed kinetic model; the manner of the effect of electrolyte concentration on these parameters is significantly different from that in the stirred bath. Their dependence on the electrolyte concentration was well explained by the existence of a maximum in the rate of oxide dissolution in an open circuit at a specific concentration, of a maximum in the electrical conductivity at another specific concentration, and of some resulting slight changes of the temperature inside the pores and oxide bulk during anodization. Copyright (C) 1996 Elsevier Science Ltd