Evaluation of the diffusion coefficient of fluorine during the electropolishing of niobium, Phys

Future accelerators, such as the envisioned international linear collider (ILC), require unprecedented cavity performance, which is strongly influenced by interior surface nano-smoothness. Electropolishing (EP) is the technique of choice being developed for high–field SRF cavities. Previous study has shown that the mechanism Nb electropolishing proceeds by formation and dissolution of a compact salt film under fluorine diffusion-limited mass transport control. We pursue an improved understanding of the microscopic conditions required for optimum surface finishing. The viscosity of the standard electrolyte has been measured using a commercial viscometer, and the diffusion coefficient of fluorine was measured at a variety of temperatures from 0ºC to 50ºC using an Nb rotating disk electrode. In addition, data indicate that electrode kinetics is becoming competitive with the mass transfer current limitation and increases dramatically with temperature. These findings are expected to guide the optimization of EP process parameters for achieving controlled, reproducible and uniform nano-smooth surface finishing for SRF cavities