PLATING ON ANODIZED ALUMINUM .2. THE EFFECT OF THE METAL, THE ANION AND THE ALUMINUM-ALLOY

The mechanism of plating on anodized aluminum having a porous anodic film is studied. It is concluded that both copper and silver ions cross the barrier layer and are deposited at the Al/oxide interface. Transport through the barrier layer is dominated by migration. Diffusion occurs at a much lower rate (about two orders of magnitude) and it is the mechanism by which plating takes place at open circuit. The relative resistance in the barrier layer and in the much thicker porous layer is important in determining the uniformity of plating into the pores, although the effect is operative only in the very early stages of plating. The barrier layer at the bottom of the pores can be partially dissolved by local increase of pH, due to hydrogen evolution or the reduction of a nitrate ion, which may occur simultaneously with metal deposition. While this is not an essential step for metal deposition to occur, it seems to improve the bonding between the aluminum substrate and the metal being deposited (Cu or Ag in the present experiments). The basic parameters which must be controlled to achieve uniform plating are discussed. Studying the surface after small amounts of metal have been deposited may provide a tool for fast and efficient evaluation of optimal conditions required to obtain highly adherent platings on anodized aluminum. © 1990.