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Application of Schiff Base Chemistry to Surface Passivation and Sealing of Plated Steels, Anodized Aluminum and IVD Coatings
Waynie Schuette | The Boeing Company
Boeing Research and Technology (BR&T) will partner with the Missouri University of Science and Technology (Missouri S&T) and the Naval Air Systems Command to develop unique, chromium- free post-treatment finishing solutions for zinc (Zn), zinc-nickel (ZnNi), cadmium (Cd) plating, and anodized aluminum (Al) and ion vapor deposition (IVD) Al coatings. Currently, hexavalent chromium (Cr6+) containing sealers are primarily used to enhance corrosion protection of metal plating, Al anodizing and other sacrificial layers when exposed to corrosive atmospheric environments. The Department of Defense (DoD) estimates the use of over 1500 lb. of Cr6+ containing sealers for various processes across its depots. For select applications, trivalent chromium (Cr3+) based sealers are being transitioned, for example, for post-treatment of ZnNi plated steels. The need to reduce or eliminate the use of Cr6+ containing anodizing and passivating sealing solutions remains a top priority for the DoD to reduce risks associated with environmental, human health, and compliance issues. The objective of this project is to develop and mature organic Schiff base (SB) containing passivating and sealing post-treatment solutions. The SB passivating solution will be developed for corrosion protection of Zn, ZnNi, and Cd plating and the SB sealing solution for anodized Al and IVD Al coatings. To help transition and mature the technology, a life-cycle framework of the SB passivating solution will be outlined, and compared to sustainment analysis of the Cr3+ post-treatment of ZnNi plated steels at the Navy Fleet Readiness Center- Southeast depot.
BR&T has discovered a unique functionality of SB inhibitors that enables high polarization resistance of 2024-T3 Al alloys when exposed to chloride-containing corrosive media. Using this information, this work will further modify the chemical structure of the SB inhibitors to achieve specific properties on the plated metals and the anodized Al and IVD coatings. BR&T and Missouri S&T will work together to synthesize novel SB inhibitors and to characterize the chemical formula and corrosion resistance. Specialized electrochemical techniques will be used to study the interaction of the SB inhibitors at the metal interface. The best performing inhibitor candidates will then be formulated into passivation solutions for Zn, ZnNi, and Cd plating, and sealing solutions for anodized Al and IVD aluminum coatings. Analytical techniques and electrochemical testing will be employed to characterize the SB post-treatments. Performance testing will be done at Boeing on SB-treated test coupons per military specification requirements. Additionally, a sustainment analysis of these solutions will be developed to help transition and mature the technology.
The DoD and Boeing will benefit from elimination of chromium from its sealing and post-treatment processes which will decrease costs associated with workers health and safety, hazardous wastes management, and regulation compliance. This program explores new chromium-free chemistries providing fundamental knowledge of a new class of corrosion inhibitors. Coupling the technology development with a lifecycle analysis will facilitate the transition plans to commercialize the technology for easy adoption across the DoD depots and The Boeing Enterprise.