Journal of Electroanalytical Chemistry, Vol.510, No.1-2, 59-66, 2001
Interface physicochemical processes controlling sulphate anion incorporation in porous anodic alumina and their dependence on the thermodynamic and transport properties of cations
Aluminium was anodised in H2SO4, LiHSO4, NaHSO4, KHSO4, Mg(HSO4)(2) and Al(HSO4)(3) electrolytes. The kinetics of growth of porous anodic alumina films and of the pore wall oxide dissolution during anodisation was studied. Based on the derived kinetic parameters, suitable physicochemical processes in the barrier layer I electrolyte interface controlling the anion incorporation in the barrier laver were suggested and relevant models were formulated. According to these processes Al3+ and H+ ions are rejected from the pore base surface in the attached double layer, where Al3+ ions are solvated, and are transferred to the pore filling solution. The strongly different mobilities of Al3+ and H+ and the necessary space negative charge density distribution in the double layer result in similar concentration distributions of Al3+ and anions inside it, which differ strongly from that of H+. These Al3+ and anion concentrations increase with decreasing mobility of the main cations in the solution which depends on their hydration enthalpy and transport mechanism. The concentration of incorporated anions inside both a thin surface layer of the barrier layer and the double layer vary similarly. For identical surface density and base diameter of pores the decrease of the above mobility reinforces anion incorporation.
Keywords:porous anodic alumina;sulphate anion incorporation;interface processes;cation mobility effect