Eliminating the Formation of Hexavalent Chromium in Chrome Stripping Operations Using Pulse Reverse Processes

Abstract

Chromium electrodeposits are commonly applied to aeronautical and automotive components as protective coatings from harsh and/or corrosive operational environments. Through normal use, these coatings may become damaged. During repair and refurbishment of these components, the parts are stripped of the existing chromium deposit before being recoated and reinstalled into various systems. Traditionally, the chrome stripping process has been facilitated by applying a potential bias to a workpiece in an alkaline electrolyte per MIL-STD-871B. While the alkaline pH may help to protect the underlying steel surface from corrosion, it preferentially favors the formation of the hexavalent chromium species, which is a carcinogen and an environmental toxin. In the conventional stripping process, hexavalent chromium ions build up in the stripping electrolyte over time, resulting in decreased stripping rates, high energy consumption and possible damage to the part. To minimize these effects, the bath must be purified or discarded and replaced. Prior to waste disposal all hexavalent chromium in the electrolytes must be reduced to trivalent form as the trivalent species is non-toxic, non-hazardous and more stable compared to the hexavalent species. Therefore, if the oxidation of chromium metal can be limited to the trivalent species during stripping operations, air quality regulations will be easier to meet and working conditions personnel would be greatly improved. Faraday is working to develop an electrolytic stripping process intended as a drop-in replacement for the conventional stripping process. The process seeks to eliminate the formation of hexavalent chromium while maintaining the material properties of the underlying substrate. Faraday will present work that demonstrates removal of chrome from high strength steel substrates using a weak acid electrolyte that results in the dissolution of the metal to its trivalent state. The effect of pulse and pulse/reverse electric fields on the efficacy of the stripping process will be presented in comparison to operation under constant voltage electric fields. The stripping process based on use of a weak acid electrolyte in conjunction with pulse/pulse reverse electric fields was demonstrated for flat and round specimens in a pilot scale cell using a simple oxalic acid electrolyte. In addition, studies were conducted to assess how age of the oxalic acid electrolyte impacts the stripping process using constant voltage electric fields. Faraday also developed a process for the recycling of spent alkaline stripping electrolytes loaded with hexavalent chromium ions. This process relies on the use of catalytic Au-Pd-Cu cathodes and pulse reverse process conditions. Results of the process in bench scale validation studies and lessons in the scaling of this technology are discussed. Acknowledgement: Funding for this work is gratefully acknowledged from Air Force SBIR Contract Number FA8222-16-C-0006.