Interpretation of the promoting effect of sulphate salt additives on the development of non-uniform pitted porous anodic Al2O3 films in H2SO4 electrolyte by a transport phenomenon analysis theory

The equations describing the mass and charge transport phenomena and composition of the solution at pore bases during the regular/uniform growth of porous anodic alumina films in pure H2SO4 solution were suitably solved to describe them in H2SO4+Al2(SO4)3 solution. The current densities at which the H2SO4 concentration becomes minimum at low film thicknesses were determined which coincided with the experimental ones above of which pitting appears. They decreased with the Al2(SO4)3 concentration. A theory was formulated explaining the promoting effect of sulphate additives on pitting appearance which is important to develop methods for producing regularly grown high quality films.