Aluminium anodising in oxalate and sulphate solutions. Comparison of chronopotentiometric and overall kinetic response of growth mechanism of porous anodic films

Aluminium anodising was performed in saturated and unsaturated H2C2O4 solutions and in H2SO4 and saturated H2SO4 + Al2(SO4)3 solutions at similar temperatures, concentrations and current density to reveal the effect of anions and condensed electrolyte nanoparticle micelles formed on pore base and wall surfaces on the mechanism of growth of porous anodic alumina films. Chronopotentiometry and overall kinetics showed that the kinetics and mechanism of growth and nanostructure of films are similar in oxalate and sulphate solutions. But differences also appear, like higher field strength across the barrier layer and much lower pore wall dissolution rate in oxalate solutions. These are attributed to dissimilar kind of anions, smaller amount of incorporated ones and depth of penetration in barrier layer and pore walls during the film growth in oxalate solutions and to their participation in a step of oxide dissolution mechanism that becomes the rate controlling in this case. In saturated solutions the condensed H2C2O4 and Al2(SO4)3 micelles affect the kinetics and mechanism of film growth, more H2C2O4, allowing a better design of structure. Results promote the knowledge on growth mechanism and kinetics of anodic films enabling methods to optimise the structure for their many scientific or technological applications.