Electrolyte Maintenance Technology Platform: Applying Learning Across Electrochemical Machining and Stripping Processes
Authors: Inman Maria E. , Taylor E. J. , Skinn Brian , Hall Timothy D , Snyder Stephen , McCrabb Heather , Lucatero Savidra , Garich Holly 
Published: February 2020

In industrial electrochemical processes, there is commonly a need to maintain the electrolyte, so as to avoid excess costs associated with electrolyte replacement and waste disposal. Faraday is engaged in the development of a range of technologies aimed at electrolyte maintenance: 1) Recycling Electrochemical Machining ((R)ECM) for maintenance of electrochemical machining electrolyte to enable a zero-discharge process, 2) adjustment of a chrome stripping process to ensure that hexavalent chromium is not formed in the electrolyte during stripping, and 3) recycling of the components of a high velocity oxy-fuel coating during stripping. In each case, we are applying the principles of pulse-reverse electric fields, as necessary, as well as innovative racking and fixturing design to ensure good primary current distribution, to achieve the desired performance results. We will discuss how lessons learned in each project are applied to the other projects to enhance the chance of success.

Electrochemical machining (ECM) is suited for machining parts fabricated from “difficult to cut” materials and/or parts with complicated and intricate geometries. However, the conventional DC ECM process generates large amounts of sometimes hazardous waste. To develop a (R)ECM process, we select an electrolyte whereby the machined material is soluble and avoids sludge formation by precipitation. The ECM unit operation is coupled to an EW unit operation where the machined material is collected as metal. The ECM and EW unit operations are adjusted to keep the soluble metal concentration in a range where the ECM process is not adversely effected and the EW operation is efficient and economical. Consequently, metals are recovered, waste is avoided and water usage is minimized. We have demonstrated integrated operation of the (R)ECM process where the soluble metal concentration is maintained in a pre-determined range by 1) simultaneous (R)ECM operation by adjusting the ECM unit operation in the presence of constant EW unit operation, 2) simultaneous (R)ECM operation by adjusting the EW unit operation in the presence of constant ECM unit operation, and 3) sequential operation of the ECM and EW unit operations. Faraday recently installed a β-scale Recycling Electrochemical Machining ((R)ECM) system at the U.S. Army Benet Laboratory. The (R)ECM system is sized to recover up to 0.5 m3per year of metal from an electrochemical machining operation. The (R)ECM system couples an electrochemical machining unit operation with an electrowinning (EW) unit operation for recovery of metals, elimination of waste, and minimization of water usage. Consequently, the RECM system is consistent with the U.S. Army’s “Vision for Net Zero”.

Faraday is developing a Chrome Stripping technology for rapid removal of chrome from landing gear parts and other aircraft components, while maintaining the functional requirements of those parts, and eliminating or reducing hexavalent chromium formation to below OSHA PELs. Similarly to the (R)ECM technology, by integrating an Electrowinning unit, we will enable direct recovery of the “stripped” chromium metal from the electrolyte, return the electrolyte to the stripping process, and be a drop-in replacement for the current stripping process. Work has demonstrated removal of chrome from high strength steel substrates using a weak acid electrolyte while eliminating hexavalent chromium, without adversely affecting the substrate surface. We are optimizing the technology, and will design and build a pilot-scale facility to strip larger parts, perform compatibility testing for functional performance metrics, demonstrate replating after stripping, recover the chrome and enable a zero-discharge process

Finally, Faraday has demonstrated the feasibility of an ElectroStripping/ElectroWinning technology, able to decrease the electrolytic stripping time of a high velocity oxygen fuel (HVOF) coating from an Inconel 718 substrate from 72 hours to 4 hours, without adversely affecting the substrate surface. The capability to maintain the electrolyte and recover stripped metals was also demonstrated. In future work, we will optimize the stripping process for WC-Co coatings, by improving the cell configuration and racking and fixturing, electrolyte, and waveform parameters. 

Acknowledgements: This study is supported by the US Army (W15QKN-12-C-0116), and the US Air Force SBIR Program (FA8222-16-C-0006, FA8117-15-C-0019). The financial support of Faraday Technology, Inc. corporate R&D is also gratefully acknowledged.