Deposition and characterization of cerium oxide conversion coatings on aluminum alloy 7075-T6

An improved process has been developed for the spontaneous deposition of cerium oxide conversion coatings for corrosion protection of aluminum alloy 7075-T6. Approximately 80% of panels prepared using the improved process inhibited corrosion up to two weeks (336 h) in ASTM B117 salt fog testing, compared to approximately 20% for previous processes. Coatings were deposited from water-based solutions of CeCl 3 and other compounds. Coating thickness, surface morphology, and performance in salt fog testing were sensitive to process parameters including surface preparation prior to coating, immersion time in the coating solution, and post-treatment. Surface preparation of the alloy was a critical first step in the deposition process. Desmutting followed by degreasing in a water solution of a commercial alkaline cleaner at a specific temperature was found to be an acceptable pre-treatment. Coating thickness, as determined by Auger electron spectroscopy depth profiling, varied depending on the surface pre-treatment and time that the panel was immersed in the coating solution. Immersion of desmutted, degreased panels for 15 min produced coatings 200 nm thick. Post-treatment, which converted Ce 4+ in the as-deposited coating to Ce 3+ as shown by X-Ray photoelectron spectroscopy, consisted of immersion in a phosphate sealing solution. Transmission electron microscopy analysis indicated that the as-deposited coatings were composed of a heretofore unidentified nano-crystalline cerium compound, presumably a hydrated Ce 4+ oxide or Ce 4+ hydroxide.