STUDY ON THE KINETICS OF GROWTH OF POROUS ANODIC AL2O3 FILMS ON AL METAL

A kinetic study on the growth of porous anodic Al2O3 films on Al metal, anodized galvanostatically at current densities 5-75 mA cm-2 in a stirred 15% w/v H2SO4 bath solution at bath temperatures 20-40-degrees-C, was performed. A strict kinetic model was formulated which was, nevertheless, rather complex. Thereafter, a simplified model was derived from the former on the basis of certain assumptions. The latter was found to apply satisfactorily to the experimental values of film mass at all bath temperatures and current densities employed. The model's application is valid approximately up to the anodization time at which maximum pore diameter approaches cell size. Treatment of results with this model yielded a satisfactory estimation of the parameters involved in the kinetics governing the thermally activated dissolution process effected by the electrolyte on either the pore wall oxide during anodization or on the oxide in an open circuit. It was verified that electrolyte concentration inside pores during anodization varies along the pore length, increasing in the direction from the film's surface down to the pore bases. At pore bases, the electrolyte concentration generally increases with decreasing bath temperature and increasing current density, being approximately a linear function of true current density at the corresponding surface of charge exchange. In addition, it was shown that a sufficiently accurate determination of structural features is possible to be obtained. A feature examined and being of specific interest is the real shape of pores which for stirred-bath films assumes that of an elongated spinning top.