THE CATHODIC POLARIZATION OF ALUMINUM COVERED WITH ANODIC OXIDE-FILMS IN A NEUTRAL BORATE SOLUTION .2. FILM BREAKDOWN AND PIT FORMATION

Porous and barrier type oxide films were formed anodically on pure aluminum and dissolved chemically. The specimens were then cathodically polarized by the potential sweep method in a neutral borate solution. It was found that the cathodic current starts to increase at about -2 to -3 V(Ag/AgCl) and that at more negative potentials increases almost linearly, accompanied by hydrogen evolution. The cathodic polarization curve shifts in the anodic direction with decreasing film formation potential and increasing chemical dissolution time. During cathodic polarization, cubic-shape pits are formed by local corrosion of the metal substrate. The number of pits formed by cathodic polarization is larger for porous than for barrier type oxide films, while the pit number is independent of the film formation potential, it increases considerably with increasing chemical dissolution time. The ratio of the local corrosion rate to the electrochemical reaction rate during cathodic polarization is about 0.03 on specimens without chemical dissolution, and about 0.10 with chemical dissolution. The effect of film structure on the pit formation during cathodic polarization is discussed in terms of imperfections in the oxide.