Military corrosion protection coating systems typically consist of a metal pretreatment conversion coating followed by an epoxy primer and a polyurethane topcoat. Although very effective at preventing corrosion of the underlying metallic substrate, these coatings contain volatile organic compounds (VOCs), hazardous air pollutants (HAPs), and chemicals listed on the toxic release inventory (TRI). The chromate corrosion inhibitors used in the conversion coating in direct contact with the metallic substrate, as well as in the epoxy primer, present a health risk to workers that paint and de-paint military assets.
The objective of this project was to research, develop, characterize, evaluate, integrate, and demonstrate an environmentally friendly, chromate-free, zero-TRI/VOC/HAP two-layer coating system based on nontoxic, rare earth inhibitors and ultraviolet (UV)-curable coatings that meet or exceed current military corrosion requirements for metallic substrates.
UV-curable coatings are a class of coatings that can be formulated to be solvent-free, and therefore zero-VOC. Successful incorporation of existing and novel corrosion inhibitors into UV-curable coatings was critical. Potential chrome-free corrosion inhibitors evaluated in the UV self-priming topcoat included a University of Missouri-Rolla/Deft, Inc. rare-earth inhibitor, Wayncor 204 from Wayne Pigments, and a Boeing proprietary hydrotalcite-based inhibitor doped with an organodisulfide compound. Acrylated polyurethanes that have demonstrated good flexibility and weatherability in previous work were included. Oligomers with moderately low glass transition temperatures (Tg) were evaluated due to the beneficial effect of low Tg materials on flexibility properties important for aerospace applications. High molecular weight acrylate monomers were evaluated for their contribution to flexibility and toughness, and different functionality monomers were blended to optimize cure parameters and physical properties. Combining the non-chromate pretreatments and corrosion-inhibiting UV-curable coatings into an integrated coating system for aluminum alloys was the primary technology developed, with additional work demonstrating the system on nonaluminum substrates.
Cerium conversion coatings (CeCCs) were able to pass 336 hours of ASTM B117 salt spray testing on Al 2024-T3 and Al 7075-T6 with proper surface preparation, deposition conditions, and post-treatment. Substrate preparation significantly altered the surface chemical and electrochemical properties of high-strength aluminum alloys. The majority of CeCC depositions were done using a spray method, but brush application, immersion, and electrolytic processes were also demonstrated to be effective. In almost all cases, it was found that an approximately 400nm thick CeCC was the most effective corrosion coating. Phosphate solution post-treatment of CeCCs could significantly improve the corrosion protection of the films. All of the knowledge gained during the project was used to develop and publish a deposition mechanism for CeCCs that has gained recognition and acceptance within the scientific community.
Development and demonstration that UV-curable coatings with inorganic corrosion inhibitors could be fully cured in just a few seconds; have good flexibility, adhesion, and fluid resistance; and pass corrosion testing was a major breakthrough. The multifunctional UV (MUV) coatings were capable of passing most of the required aerospace requirements, and adjustments in formulation were able to alter properties as needed.
The results of this project indicate that a two-layer coating system consisting of a non-chromate CeCC and UV-curable polymer with inorganic corrosion inhibitors is capable of meeting aerospace coating requirements, as demonstrated in the first round of testing. Additional research is needed to understand and optimize the CeCC process and MUV formulation. The results of this project have also shown that the MUV can meet the important performance criteria for coatings used on military aircraft.
Benefits of this technology include zero-VOC, HAP-free corrosion coatings that fully cure in seconds, resulting in faster throughput and labor savings. UV technology is recognized as a best available control technology (BACT) and a lowest achievable emission rate (LAER) technology by the Environmental Protection Agency. A UV-curable coating qualifies as a "Superclean" technology, as defined by the Southern California South Coast Air Quality Management District (SCAQMD). This rating allows immediate installation of UV processes without the need for a permit.