The protectivity of aluminum and its alloys with transition metals

The mechanism of the protectivity of aluminum and supersaturated aluminum alloys containing W, Mo, Ta and Cu has been investigated in chloride environments. The potential of zero charge (PZC) of the passive film was evaluated by a method based on impedance spectroscopy. The chloride ion adsorption on the passive film was measured by means of an in situ radiotracer technique. Constituents of the passive film as a function of depth were investigated by means of ex situ spectroscopic techniques including XPS, ISS and SIMS. The PZCs of the passive films of Al and Al alloys were calculated from the flatband potentials. A linear correlation between pitting potential and the PZC was found. Adsorption of the chloride ion on the Al-Ta surface starts at more anodic potentials than those of pure Al, and this shift is in agreement with the anodic shift of the PZC. A constant surface concentration of chloride ion was observed during the induction time for breakdown. A significant decrease of OH- concentration in the passive film of Al and its alloys has been found after the passive film has undergone breakdown. The mole fraction of the alloying elements in the surface region of the passive film is ca. 1-8%. The adsorption of the chloride ion on the surface of the passive film is influenced by the anodic PZC shift, which varies with the alloying element. However, retardation of the chloride penetration into the passive film by blocking of the entry site by oxide ions of the alloying element controls the rate of breakdown.