Pulse Reverse Voltage Electropolishing of Superconducting Radio Frequency Niobium Cavities in Aqueous, Low Concentration Sulfuric Acid Electrolytes without Hydrofluoric Acid
Authors: Taylor E. J. , Inman Maria , Hall Timothy D , Snyder Stephen , Mammosser John , Furuta Fumio 
Published: February 2020

Niobium superconducting radio frequency cavities (SRF) are required for the International Linear Collider as well as 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 paradigm [[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 concentrated is included to establish a thick, viscous boundary layer to result in surface brightening or smoothing [[3]]. The hydrofluoric acid is included to remove the niobium oxide film formed during electropolishing [[4]]. Previously we described efforts directed towards demonstrating an innovative electropolishing process for niobium coupons in low concentration (5%) aqueous sulfuric acid without hydrofluoric acid enabled by pulse reverse voltage waveforms electrolyte [[5],[6]]. The advantages of pulse reverse voltage electropolishing are summarized in in Figure 1. Specifically, the forward (anodic) pulse on-time and peak voltage are tuned to eliminate the need for concentrated sulfuric acid; the off-time is adjusted to dissipate heat, and the reverse (cathodic) pulse on-time and peak voltage are tuned to eliminate the need for hydrofluoric acid. In this contribution, we review and describe continuing efforts leading to the scale-up of the electropolishing process to single-cell and three-cell niobium SRF cavities [[7],[8]]. Finally, we present a first order economic comparison of the pulse reverse voltage low concentration aqueous sulfuric acid process compared to baseline concentrated sulfuric acid-hydrofluoric acid process [[9]]. Acknowledgements: The financial support of Faraday corporate, DOE P.O. No. 594128 and DOE Contract No. DE-SC0004588 is acknowledged.
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[3] D. Landolt,(1987) “Fundamental Aspects of Electropolishing” Electrochimica Acta 32(1) 1-11 (1987).

[4] 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, Vol. 95-19, (Eds. M. Datta, B. MacDougall and J. Fenton) The Electrochemical Society, Pennington, NJ, pp 16-31.
[5] M. Inman, E.J. Taylor, T.D. Hall “Electropolishing of Passive Materials in HF-Free Low Viscosity Aqueous Electrolytes” J. Electrochem. Soc.,
160(9) E94-E98 (2013).
[6] E.J. Taylor, M.E. Inman, T.D. Hall (2015) “Electrochemical system and method for electropolishing superconductive radio frequency cavities” U.S. Patent No. 9,006,147 issued April 14, 2015.
[7] E.J. Taylor, T.D. Hall, M. Inman, S. Snyder (2013) “Electropolishing of Niobium SRF Cavities in Low Viscosity Aqueous Electrolytes without Hydroflouric Acid” Paper No. TUP054, Presented SRF2013, Paris, FRANCE.
[8] A.M. Rowe, A. Grassellino, T.D. Hall, M.E. Inman, S.T. Snyder, E.J. Taylor (2013) “Bipolar EP: Electropolishing without Flourine in a Water Based Electrolyte” Paper No. TUIOC02, Presented SRF2013, Paris, FRANCE.
[9] E.J. Taylor, M. Inman, T. Hall, S. Snyder, A. Rowe, D. Holmes (2015) “Economics of Electropolishing Niobium SRF Cavities in Eco-Friendly Aqueous Electrolytes without Hydrofluoric Acid” Paper No. MOPB092, Presented SRF2015, Whistler, CANADA.

Figure 1