Study of HF-Free Pulse Reverse Electropolishing of Nb Surface with an Alkaline Electrolyte

Abstract

Chemical etching or electropolishing of Nb superconducting radio-frequency (SRF) cavities to achieve a clean and smooth internal surface require the removal of the passive Nb2O5 film. Typically, an aggressive chemical, such as hydrofluoric acid is included in the solution to accomplish this. The problem with using HF is that it is dangerous both to people and to the environment. As an alternative, in order to depassivate the surface, cathodic pulses may be interspersed between anodic pulses which passivate the Nb surface forming Nb2O5 film, in place of or in conjunction with off-times. Use of such cathodic pulses eliminates the need for HF and/or fluoride salts or other chemicals to actively remove the surface oxide. A present understanding of a pulse reverse electropolishing of Nb in alkaline electrolyte-sodium hydroxide will be reported. In this study, 10~30 % concentration of NaOH has been used as the electrolyte. In order to better understand the mechanism of a pulse reverse electropolishing of Nb with sodium hydroxide, and optimize the process parameters for Nb cavity process, the effect of cathodic pulse duration and amplitude, off time, and anodic pulse duration have been systematically studied, the surface of Nb samples have been characterized chemically and morphologically. We expect the results will enable application of this safer and greener process to future accelerator projects using Nb SRF cavities.