LITHIUM-ALUMINUM-CARBONATE-HYDROXIDE HYDRATE COATINGS ON ALUMINUM-ALLOYS - COMPOSITION, STRUCTURE, AND PROCESSING BATH CHEMISTRY

A new corrosion resistant coating, being designed for possible replace ment of chromate conversion coatings on aluminum alloys, was investiga ted for composition, structure, and solubility using a variety of tech niques. The stoichiometry of the material, prepared by immersion of 11 00 Al alloy into a lithium carbonate-lithium hydroxide solution, was a pproximately Li2Al4CO3(OH)(12).3H2O. Processing time was shown to be d ependent upon the bath pH, and consistent coating formation required s upersaturation of the coating bath with aluminum. The exact crystal st ructure of this hydrotalcite material, hexagonal or monoclinic, was no t determined. It was shown that both the bulk material and coatings wi th the same nominal composition and crystal structure could be formed by precipitation from an aluminum supersatured solution of lithium car bonate.