Interaction of Molybdate Anions with the Passive Film on Aluminum

Abstract

Using x-ray photoelectron spectroscopy (XPS), the interaction of sodium molybdate with Al was studied under a number of different conditions, including variations on molybdate concentration, pH, and applied potential. By comparison with a database of Mo binding energies, the results show several different oxidation states present in the film. Mechanisms were developed to describe how changes in passive film chemistry lead to improved resistance to pitting in chloride solutions. In contrast to earlier investigations, it was found that the formation of Mo+4 did not improve passivity. Improvements in pitting resistance were associated with a molybdate-rich region on the film surface that inhibited the ingress of Cl~ anions to the metal/film interface.