Electrolytic and spontaneous spray deposition of organic assisted cerium conversion coatings for the corrosion protection of aluminum alloys

The primary goal of this study is to make, characterize and evaluate cerium conversion coatings on aluminum alloys prepared with the assistance of gelatin and certain amino acids. These coatings are potential replacements for chromate conversion coatings which, are very effective. Unfortunately, chromium is targeted for elimination from the workplace because it is very toxic. The important factors in the development of the cerium conversion coating process are cleaning temperature, current density, potential, pH, cerium concentration, additive concentration, coating time, and post deposition treatments. Both electrolytic and spray applied processes have been developed using gelatin and/or amino acids to assist the deposition of the cerium conversion coating on aluminum alloy 7075-T6 that gave greater than 80% pass rate for the ASTM B-117 salt fog corrosion test. The process included an alkaline cleaning step, application of an acidified containing cerium containing gelatins and/or amino acids solution by either an electrolytic or spray deposition, followed by sealing in a boiling aqueous sodium phosphate solution. These organic additive assisted cerium conversion coatings offer better corrosion protection, thicker coatings, more polarization, and better uniformity than those deposited from an aqueous electrolyte without organic additive. The conversion coatings were characterized by SEM, AUGER, XPS and TEM and were shown to be a mixed oxidation state (Ce III or Ce IV) for the as deposited cerium conversion coatings which were converted to primarily cerium (III) phosphate monohydrate after phosphate post treatment. A proposed mechanism for the deposition process is included.