Role of hexavalent chromium in the inhibition of corrosion of aluminum alloys

Selection of alternatives to hexavalent chromium for inhibitors and inhibiting films for Al and its alloys requires a clear understanding of how hexavalent chromium inhibits corrosion. To this end, the following hypotheses have been proposed: residual chromate in conversion coatings provides self healing; hydrated Cr(III) oxide films are uniquely inert and hydrophobic; oxyanions of Cr(VI) rapidly passivate secondary phases in the highly unstable Cu-rich aluminum alloys; and adsorption of hexavalent chromium on passive aluminum oxide makes it less susceptible to chloride attack. Evidence supporting these hypotheses come from X-ray spectroscopic analysis (XANES) of conversion coatings on aluminum, characterization of Cr(VI) oxyanion adsorption on anodized aluminum via a piezo–electrokinetic (PEK) method, and the electrochemical current vs. voltage response model intermetallic compounds.