For years, chromate-containing coatings have been used to provide corrosion protection on steel and aluminum alloys. Hexavalent chromium has since been identified as a health threat and is highly regulated due to its toxicity. In order to meet new Occupational Safety and Health Administration and federal/state environmental regulations and to protect worker safety, alternative coating systems that are hexavalent chromium free and reduce or eliminate volatile organic compounds (VOC) and hazardous air pollutants (HAP) are urgently needed. Electroactive polymer (EAP) coatings provide such an opportunity to reduce these hazardous materials, eliminate chromium (VI) from coating formulations, facilitate compliance with new environmental regulations, and reduce hazardous disposal costs while ensuring mission readiness and worker safety.
This project addressed the environmental limitations of current chromate-containing corrosion-protection coatings using new EAP coating materials as environmentally compliant formulations. The objectives of the research were fourfold: (1) synthesize, characterize, and scale up novel EAPs; (2) apply films of these EAPs onto aluminum and steel substrates via environmentally friendly applications; (3) test these coatings in accelerated weathering chambers and measure their performance against known standards; and (4) understand the mechanism by which these novel EAPs can retard or inhibit corrosion when exposed to corrosive environments.
Once multi-gram/kilogram quantities of EAP materials were prepared, coating formulations were developed and applied to metal coupons. Several different coating processes (solvent, powder, carbon dioxide, and water-borne) were demonstrated. The EAP formulations were coated onto 2024-T3 aluminum alloy and 1010 steel coupons. Coated samples were tested against current coatings and evaluated for their performance. Testing protocols included qualitative adhesion tests, accelerated weathering tests, analysis of corrosion mechanism/inhibition, and coating degradation.
The synthesis, scale-up, and characterization of new monomers based on a bis-amino derivative of poly-p-phenylene vinylene (PPV) called poly(2,5-bis(N-methyl-N-hexylamino)phenylene vinylene (BAM-PPV) and oligomers of polyaniline have been completed. These polymers have been thoroughly characterized using advanced spectroscopic and analytical methods. BAM-PPV has been processed using a variety of conditions including high VOC and zero VOC processing conditions. BAM-PPV has also been incorporated into full military coating systems that include non-chrome primers and topcoats. These coatings have been tested against known controls such as chromium primers and trivalent chromium process (TCP) pretreatment with non-chrome primer and topcoat. BAM-PPV performed as well as the TCP pretreatment with non-chrome primer and topcoat. Additionally, BAM-PPV has been incorporated into a zero VOC powder formulation and has matched a commercial polyester powder resin (control) performance in neutral salt fog. Studies showed evidence that BAM-PPV provides more than simply barrier protection to corrosive environments and may passivate the metal surface. To further determine the viability of BAM-PPV pretreatment as a field-tested alternative to chromate conversion coatings and TCP, demonstration/validation efforts are ongoing under ESTCP project WP-200527.
The benefits of using electroactive polymers as corrosion-protection coatings include: (1) increased environmental safety by reducing toxic metals; (2) increased endurance of military equipment subject to corrosive conditions including humidity, seawater, and salt spray; (3) increased readiness; and (4) significant cost savings by reducing painting/depainting waste treatment. (Project Completed – 2006)