Development of a theory for the determination of the composition of the anodizing solution inside the pores during the growth of porous anodic Al2O3 films on aluminium by a transport phenomenon analysis

The determination of the composition of the anodizing solution inside the pores during the growth of porous anodic Al2O3 films is of great importance for the study of the kinetics and mechanism of growth and the structure of oxide. This composition can be determined only by considering the mass and charge transport phenomena inside the pores. In the present work the general and complex equations governing the mass and charge transport phenomena inside the pores during the growth of porous anodic Al2O3 films on aluminium in H2SO4 anodizing bath solution were suitably transformed to equations which involve partial (migration and diffusion) transport numbers of ions, total transport numbers, electrolytes concentrations and elementary parameters like limiting equivalent ion conductivities, diffusion coefficients for the electrolytes H2SO4 and Al-2(SO4)(3) etc. A numerical solution of these equations for the growth of oxide in a stirred bath at bath temperatures 20 to 40 degrees C and current densities 1 to 105 mA cm(-2) was found for film thicknesses up to 150 mu m. It describes the variation of the electrolyte concentrations at pore bases with film thickness and along pores in the simplified but representative case of cylindrical pores. The Al-2(SO4)(3) concentration always increases towards the pore bases. It also increases with increasing current density and decreasing bath temperature. Nevertheless, the H2SO4 concentration decreases, increases or passes through a minimum at a position along the pores; the kind of variation depends on the anodizing conditions by a complex but well described manner. Predictions for their variations along the pores during the real growth of the anodic Al2O3 films were also made. (C) 1998 Elsevier Science S.A. All rights reserved.