Transport phenomena inside the pores involved in the kinetics and mechanism of growth of porous anodic Al2O3 films on aluminium

The mass and charge transport phenomena inside the pores during the growth of porous anodic Al2O3 films, which are involved in the kinetics and mechanism of the oxide growth, were investigated and a relevant system of mathematical equations was formulated describing these phenomena in each general case. The solution of the above system of equations in the simplified case of a stirred bath and films with cylindrical pores, where it could be solved, showed that, during the oxide growth, the concentrations of H2SO4 and Al-2(SO4)(3) are highest and remain constant at pore bases and vary linearly between the pore base and film surface. Equations, correlating the electrolytes concentrations at pore bases during the oxide growth and the bath bulk concentrations with the charge transport numbers of the different ions, were also derived. These, referring to cylindrical pores, can be considered to be approximately applicable to the case of the real shape of elongated pores. Also, it was shown that the charge is transported inside the pores mainly by the migrating H+ and Al3+ ions white the contribution of the SO42- ions to the charge transport is insignificant.