With funding from the Strategic Environmental Research and Development Program, researchers at the University of Missouri-Rolla (UMR), Rolla, MO, Boeing Phantom Works, St. Louis, MO, and Light Curable Coatings, Cleveland, OH, are developing environmentally friendly alternatives. The project, which began in May 2006, focuses on a two-layer coating system, including a non-chromate conversion coating layer and an ultraviolet (UV) light curable, self-priming coating containing nontoxic corrosion inhibiting materials (see Figure 1).
Previous efforts to replace the chromated corrosion coatings - in particular chromate primers - with more benign materials have had some success. For example, a chromate-free primer developed as part of a previous UMR-Boeing effort that has been commercialized by Deft Industrial Finishes is now being used on the U.S. Air Force fleet of F-15s. However, non-chromate primers and topcoats require a significant amount of time to cure and still contain undesirable volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). In contrast, UV- curable systems typically do not contain VOCs or HAPs and can be completely cured within seconds.
Incorporating environmentally benign, corrosion inhibiting compounds into UV-curable coatings is an attractive alternative to the existing systems, as it would reduce the complexity of the coating system by combining the primer and topcoat into a single layer while at the same time eliminating VOCs. When used with a non-chromate conversion coating, an environmentally friendly corrosion coating system would be obtained.
Cerium Oxide TechnologyConversion coatings are aqueous chemical surface treatments that provide metallic surfaces with increased corrosion protection and enhanced paint adhesion. Work done at UMR has identified cerium oxide-based conversion coatings (CeCC) as potentially viable corrosion inhibitors and replacements for chromate conversion coatings on aluminum alloys. The CeCCs show promise for corrosion protection and adhesion of paints to high-strength aluminum alloys, such as the 2024-T3 and 7075-T6 used in military and commercial aircraft.
The coatings are deposited from aqueous solutions containing cerium chloride using processing technology that is very similar to that currently used for the chromate system. To date, both a current-driven (non-spontaneous electrolytic) and spontaneous (immersion, spray, etc.) process have been developed (see Figure 2). The cerium oxide coatings are deposited on the aluminum alloy surface by an electrochemically driven precipitation process. Typically, the coatings are 100 to 500 nanometers thick with a characteristic grain size of a few nanometers in diameter. Depositions are achieved in less than 15 minutes and use commercially available materials and equipment, making the overall process compatible with industrial operations such as those employed by current aircraft manufacturers.
Coatings that result from these processes are composed mostly of cerium oxide/hydroxide phases. The operating parameters and conditions used to deposit the coating significantly influence the quality and performance of the deposited films. Three critical steps in producing the cerium oxide conversion coatings are surface preparation and cleaning, precipitation and coating formation, and post-deposition treatment and sealing. Studies to improve and optimize these critical steps will be conducted during the project, along with integration of the CeCC with the UV-curable coatings.