The Raytheon Technologies (RTX) Research Center (RTRC), in collaboration with Naval Air Systems Command (NAVAIR), the Air Force Research Laboratory (AFRL), and Clean Corrosion Control Concepts (CCCC) LLC will conduct a three-year research program developing a chromium (Cr)-free composite conversion coating for zinc (Zn) and zinc nickel (Zn-Ni) plating. The project team seeks to determine the feasibility and cost-effectiveness of Cr-free composite conversion coating based on molybdate-silicate-phosphate (Mo-Si-P) system on Zn and Zn-Ni plating. The effort will explore enhanced formulations through the addition of chemical species as co-additives to identify the Mo-Si-P-X (X= enhanced performance additive) composite conversion coatings. The project team will down-select the additives for a lab scale technology/manufacturing readiness level (TRL/MRL) 5/4 demonstration to confirm corrosion performance that is at least comparable to commercial trivalent chromium (Cr(III)) baseline treatments. The demonstration will include both unpainted and non-chromate primed surfaces. Key performance metrics will include salt-fog corrosion resistance, primer adhesion, and hydrogen embrittlement and re-embrittlement. The Mo-Si-P-X composite coating formulation will comply with low volatile organic compounds industry guidelines and be free of hazardous air pollutants. Upon successful demonstration of the developmental Mo-Si-P-X composite coating technology, the team will also conduct a sustainability analysis to determine potential life cycle impacts and costs. The demonstration of the Crfree Mo-Si-P-X composite coating, as an alternative to the currently used trivalent Cr(III) technology in the Zn-Ni plating process, will further advance the Cr-free coating TRL/MRL.
The overarching hypothesis is that multi-element composite conversion coatings based on Mo-Si-P-X systems will provide significantly improved adhesion properties and enhanced corrosion resistance to Zn and Zn-Ni plating, with additions of inorganic, and organic co-additives/conversion agents to the Mo-Si-P system. The Cr-free alternative inhibitor chemistries and resulting conversion coating process are based on a multi-functional inhibitor mixture of molybdate, silicate and phosphate species. The team will leverage and build upon existing RTX intellectual property in the areas of non-chromate inhibitor pigments, nonchromate primers for Zn-Ni plated steel, and Cr-free surface treatments. The approach will be focused on enhanced formulations in the forms of Mo-Si-P-X composite coatings with the addition of inorganic and organic species as co-additives/conversion agents. Based on extensive prior research at RTRC, the combination of commercial Mo-Si-P inhibitive pigments have a uniquely effective capability to protect ZnNi plated steel components. The RTRC-led team’s broad and deep expertise will ensure on-budget and on-schedule execution of work scope, in particular: (i) RTX’s expertise in non-chromate pigments and primers for Zn-Ni plating and Cr-free surface coating process, and electrochemical measurement and testing capabilities; (ii) CCCC’s corrosion science and control experience; (iii) NAVAIR and AFRL expertise in critical mechanical property testing, and field testing capabilities.
Development, test, validation and implementation of Cr-free Mo-Si-P-X composite conversion coatings and non-chromate primer systems would allow government and industry facilities to eliminate worker exposure to chromium compounds, allow use across multiple plating processes and reduce the cost to sustain depot operations, as well as enable the Department of Defense to comply with Occupational Safety and Health Administration and American Conference of Governmental Industrial Hygienists exposure limits for chromium compounds.