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Novel Deposition Method for Chromium-Free Sealer for Metal Finishing Processes
Alexander Westbrook | NAWCAD
The objective of this project is to develop a chromium-free sealer for post treatment of finished metal surfaces, with a specific focus on zinc-nickel plated steel, since this particular metal finish is of importance in naval aviation applications where the replacement of cadmium remains an important goal. It is suggested that fluorozirconate-based conversion coatings doped with rare earth metal compounds and other corrosion inhibitors and deposited using a proprietary method developed at Naval Air Warfare Center Aircraft Division (NAWCAD) will yield a high-performance sealer offering protection against self-corrosion of the sacrificial plating. This material will also function as an effective chromium-free conversion coating for aluminum substrates. The method of deposition is considered the key to the success of this program. Recent work at NAWCAD has demonstrated that the corrosion performance of existing trivalent and hexavalent chromium pre-treatments can be considerably enhanced when deposited using this new method. Applying these methods to non-chromium treatments is the logical next step.
This process seeks to electrolytically deposit inhibitor-doped fluorozirconate coatings for use as a high performance sealer for zinc-nickel plated steel, with a particular focus on high strength applications. This involves optimizing both the formulation of the non-chromium pretreatment and the deposition process. Criteria such as corrosion resistance, electrochemical properties, paint adhesion, hydrogen embrittlement, and plating adhesion, will be used as measures of success. The electrolytic deposition process is considered key to the success of the approach. Since it requires only basic equipment, it is expected to be a drop-in replacement for existing hexavalent chromium based pretreatments.
Hexavalent chromium based chemistries have, for many years, been the standard posttreatment for many metal finishing processes within the Department of Defense (DoD). The health and environmental issues associated with hexavalent chromium materials are well known, their status as a human carcinogen long established. More than eight DoD depots have reported using carcinogenic post-treatment sealers for a variety of surface finishing processes, with over 30,000 gallons of dedicated infrastructure for chrome sealer operations. A chromium-free sealer implemented across multiple processes would dramatically reduce depot operations sustainment costs and eliminate worker exposure to known carcinogens. In addition, chromium-free sealers will enable the DoD to comply with Occupational Safety and Health Administration personal exposure limits. A recent recommendation from the American Conference for Governmental Industrial Hygienists (ACGIH) further revised the suggested chromium threshold limit values (TLV) setting the hexavalent chromium exposure limit to 0.0002 mg/m3 and a TLV of 0.003 mg/m3 for trivalent chromium. If adopted widely within the DoD, these new exposure limits will make it increasingly difficult to use any chromium based chemistries. To date, the best replacements for hexavalent chromium based sealer materials have been trivalent chromium based products, a replacement likely to be severely restricted when the updated guidance practices are adopted. While the scope of this project is focused primarily as a sealer of zinc-nickel, it is believed that an effective non-chromium chemistry could find use in numerous sealing applications including Zinc plating and anodizing.