Environmentally Benign Electrofinishing Process for Selective Material Removal and Reduced Surface Roughness of Materials (like Nb, Ti, Ta, SS and Mo) in Low Viscosity Water Based Electrolytes
Authors: Hall Timothy D , Garich Holly , Snyder Stephen , Lucatero Savidra , McCrabb Heather , Taylor E. J , Inman Maria E. 
Published: February 2020

Post finishing processes are used to within every material manufacturing process. Various techniques are used to remove burrs, radius edges, and polish to a desired surface finish. These techniques improve the overall performance of the material for its application and generally improve the overall appearance of the material. This paper will discuss a range of applications and materials in which a pulse reverse electrochemical approach, in water based HF-free electrolytes, has been used to obtain these desired finish properties.  

A specific application of interest that requires a high quality surface finish to obtain the desired performance is the Niobium superconducting radio frequency cavities (SRF) required for the International Linear Collider and other high energy physics projects. In order for these cavities to achieve the required particle acceleration gradients, electropolishing is conducted as a final surface finishing operation. Conventional electropolishing of SRF cavities is based on the well-established viscous salt film theory[1] and utilizes a viscous electrolyte consisting of a mixture of sulfuric acid (95-98%) and hydrofluoric acid (49%), in a 9:1 volume ratio.[2] The HF is included to depassivate the niobium oxide film during electropolishing.[3]

Based on prior surface finishing activities using pulse reverse waveforms in low viscosity aqueous electrolytes,[4] we speculated that a new paradigm was evolving and could be applied to electropolishing of SRF cavities. Within this activity Faraday used guidelines and observations from 1”x1” (6.4 cm2) and 3”x3” (58.1 cm2) coupons to scale the bipolar EP process to single cell SRF cavities of ~1,800 cm2 surface area. We recently reported these results at the 16th International Conference on RF Superconductivity in collaboration with Fermi National Accelerator Laboratory (FNAL) who declared that this cavity demonstrated the highest performance of any cavity previously processed at FNAL.

This presentation will focus pulse/pulse reverse electrofinishing processes being developed by Faraday Technology. It will explore various strongly passive materials and their applications specific surface finishing requirements. These materials include but are not limited to titanium alloys, tantalum alloys, nickel alloys, stainless steels, niobium, and molybdenum alloys.

References:

[1] P.A. Jacquet, Trans. Electrochem. Soc., 69 629 (1936).

[2] H. Tian, S. Corcoran, C. Reece, M. Kelly, J. Electrochem. Soc.,
155, D563-568 (2008).

[3] MacDougall, B. (1995), The Importance of Surface Oxide Films in Corrosion, Semiconductor and Environmental Research, Proceedings of the Symposium on High Rate Metal Dissolution Processes, Electrochemical Society Proceedings Vol. 95-19, Eds. M. Datta, B. MacDougall and J. Fenton, The Electrochemical Society, Pennington, NJ, pp 16-31.

[4] E.J. Taylor “Adventures in Pulse/Pulse Reverse Electrolytic Processes: Explorations and Applications in Surface Finishing” J. Applied Surface Finishing,
3(4) 178-189 (2008).