Electroactive Materials as Smart Corrosion Inhibiting Coatings for the Replacement of Hexavalent Chromium (Cr(VI)) Pretreatments and Primers

Most military coatings on aluminum alloys utilize hexavalent chromium (Cr(VI)) conversion coating (CCC), a Cr(VI) primer followed by a topcoat (non-Cr(VI). Cr(VI) is a well known inhibitor, exhibiting both barrier and self-healing properties. However, Cr(VI) is a carcinogen and is highly regulated material. The Naval Air Warfare Center Weapons Division (NAWCWD) in cooperation with its military partners (Army, Navy and Air Force) have developed and tested a non-Cr(VI) pretreatment for use with a total non-Cr(VI) military coating system. This pretreatment utilizes an electroactive polymer, poly(2,5-bis-(N-methyl-N-hexylamino) phenylene vinylene),(BAM-PPV) on aluminum alloys.  BAM-PPV used as the pretreatment coating with both Cr(VI) and non-Cr(VI) primer and topcoats survived 2,000 hours neutral salt fog exposure. Field testing of the best performing BAM-PPV pretreatment/primer/topcoat systems by each service (Air Force, Army and Navy) on non-critical military hardware has shown in several cases comparable performance to fully Cr(VI) military coatings after one year. Additionally, the NAWCWD in cooperation with its industrial partner, Crosslink has tested a non-Cr(VI) epoxy primer containing a corrosion-inhibiting compound (2,5-dimercapto-1,3,4-thiadiazole, DMcT) with BAM-PPV as the pretreatment coating. The laboratory results showed that this completely non-Cr(VI) pretreatment/primer military coating has survived up to 4,500 hours in neutral salt fog exposure testing. Continued testing with BAM-PPV, Crosslink non-Cr(VI) primer and the Air Force Advanced Performance Coating (APC) has resulted in poor performance with significant corrosion in the scribe and undercutting of the coating at 2,000 hours neutral salt fog exposure. Optimization of the BAM-PPV and Crosslink primer with APC are being investigated to improve the overall corrosion performance of this completely non-Cr(VI) military coating.