Currently, the Department of Defense (DoD) uses cadmium (Cd) plating on high-strength steel components, fasteners, and connectors. Cadmium, however, is under increasing environmental, safety and health pressures worldwide given its high toxicity. Despite these environmental hazards, cadmium is still widely used in the plating industry because of its unique combination of properties. Cadmium provides sacrificial corrosion resistance in all weapons system environments, fungus resistance, lubricity, good adhesion, and consistent torque tension. There is no drop-in replacement for Cd-plated high-strength steels. Electroactive polymers (EAP), such as polyaniline and polythiophenes (PT), have been demonstrated to protect steel alloys in harsh environments. PTs and poly(pyrroles) (PPy) were examined in this project. PTs have demonstrated corrosion protection for 430 stainless and mild steel in 1N sulfuric acid and 3.5% aqueous sodium chloride solutions, respectively. These materials may enable the properties of EAPs to be tailored to provide strong adhesion to steel substrates, controlled lubricity and torque tension, and compatibility with flexible, environmentally friendly, non-line-of-sight (NLS) application techniques.
The objective of this project was to demonstrate, as a one-year proof of concept, an environmentally benign alternative to Cd plating on high-strength steels using EAPs.
The project focused on synthesizing monomers containing adhesion-promoting functionalities and/or cross-linkable groups. To promote adhesion to steel, carboxylate and phosphonium salts were used as pendent groups tethered to the polymer main chain. These groups also enhanced the water compatibility of the monomers and polymers, leading to more benign plating conditions. The monomers were polymerized using an electroless deposition process. This enabled a NLS approach to produce uniform films at room temperature without volatile organic compounds (VOC) or hazardous air pollutants (HAP) being released into the environment. The thickness, porosity, and uniformity of these films can be modified by controlling monomer and oxidant concentrations. The PTs and PPys can form uniform thin films on complex geometries by electroless deposition. Once the steel coupons were coated with the EAPs, several initial studies were conducted to determine the performance of these coatings. Coated steel coupons and fasteners were subjected to accelerated weathering using the neutral salt fog test in accordance with the requirements of ASTM B117 for 96 hours.
Novel PT and PPy compounds for improved corrosion inhibition and lubricity were synthesized and characterized. The new polymeric materials contain no heavy metals (Cd, Cr, Ni, Zn, Cu, etc.). New and known monomers, polymers, and co-polymers were prepared and processed to produce thin films on steel substrates. Thiophene-based co-polymers with adhesion promoting groups have been formed into good quality films by solvent casting and exhibit good adhesion properties. Aqueous electroless deposition (NLS technique) of pyrrole-based polymers containing adhesion promoting groups has been demonstrated. The EAP electroless deposition coating process does not introduce hydrogen into the specimens nor lead to hydrogen embrittlement during environmental exposure. EAP polymers with hard particle additives have measured galling resistance values of 6000-8000 psi, not as high as for cadmium but superior to coatings such as ion vapor deposited (IVD) aluminum. Several polymer-coated samples containing the pyrrole monomer with a long-chain aliphatic group have passed the military requirement of 96 hours neutral salt fog testing. The essential characteristic of the EAPs that have passed the neutral salt fog testing is the improved film quality.
Use of EAPs as a coating replacement for Cd plating will eliminate a major source of hazardous waste for automotive, aerospace, and weapons manufacturing. In addition, this research will help to launch a new generation of environmentally benign corrosion protection coatings for use throughout DoD and in numerous civilian applications. (Project Completed – 2008)