Trivalent Chromium Conversion Coatings for Aluminum Alloys

A new Navy process substitutes low toxicity trivalent chromium for toxic hexavalent chromium in aluminum conversion coatings. Further work is needed to investigate variations on the original formulations, with special consideration to formulas useful with common alloys of aluminum. Both a yellow and a blue-bright conversion film are desirable. Fractional Factorial Experimental Designs will be used to develop two alternative formulations, useful with 1100 series and with common aerospace aluminum alloys. The chemical structure of the film will be studied using wet chemical, instrumental, and electrochemical methods, including how it changes with improvements in formulations. Coated panels of 1100, 2024, 3003, and 7075 alloys will be evaluated by unpainted and painted salt spray and by painted water immersion testing, using a commercial "Alodine" hexavalent chromium conversion coating for comparison. The technical and patent literature for chromate conversion coatings on aluminum and zinc will be reviewed. The health aspects of hexavalent chromium and current regulatory constraints on its use will be reviewed to help clarify cost/benefit analyses, to help promote commercialization. This could reduce the use of hexavalent chromium conversion coatings by at least 25% within 5 years after commercial introduction. It may also prove valuable with phosphate and anodized coatings.