DEVELOPMENT OF A STRICT KINETIC-MODEL FOR THE GROWTH OF POROUS ANODIC AL2O3 FILMS ON ALUMINUM

A strict kinetic model, governing the growth of porous anodic Al2O3 films, was developed in a form easily and directly applicable to a galvanostatic anodization in a stirred bath at constant temperature. It was applied to the experimental results obtained from film growth at 25 degrees C, 15 mA cm(-2) and in a wide range of H2SO4 concentrations from 2 to 105% w/v. Its application provided, consistently with other experimental evidence, that a critical electrolyte concentration exists between 5 and 15% w/v. Above the critical concentration the normal mechanism of oxide production and film growth is valid and the model applies; below the critical concentration, where the model does not apply, a deficient growth of the oxide is observed. The experimental results and their treatment by the developed model showed that the electrolyte concentration generally affects parameters such as the weight 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, the parameters involved in the developed kinetic model, etc. Their variation with electrolyte concentration was well justified by the fact that the rate of the dissolution of the anodic Al2O3 in an open circuit has a maximum at a specific concentration and the electrolyte concentration is variable along pores being maximum at pore bases.