Discovery by kinetic studies of the latent physicochemical processes and their mechanisms during the growth of porous anodic alumina films in sulfate electrolytes

Abstract

The kinetics of growth of porous anodic alumina films in pure H₂SO₄, in mixtures of H₂SO₄ and Al₂(SO₄)₃ and in Al(HSO₄)₃, NaHSO₄ and KHSO₄ electrolytes were studied. The latent physicochemical processes at the pore base surface/electrolyte interface, across the barrier layer, inside the metal/oxide interface and at the pore wall surface/electrolyte interface and their mechanisms were revealed. High field strength equations were formulated describing the ionic migrations from the pore base surface. These showed that, at constant current density and temperature, the inverse of the pore base square diameter depends linearly on the inverse of the H⁺ activity in the anodizing solution and that this diameter increases with H⁺ activity, in agreement with the experimental results. The mechanism of electrolyte anion incorporation inside the barrier layer and the real distribution of the anion concentration across both the barrier layer and pore walls were deduced. The effects of the different kinds and concentrations of the electrolyte anions and cations on both the above processes and their mechanisms were also examined.