Résumé du preprint Irfu-09-176

More information concerning electro-polishing mechanisms in hydrofluoricsulphuric acid mixtures
F. Eozénou, S. Berry, Y. Gasser, J.P. Charrier
The use of a rotating disc electrode for electrochemical measurements gives precious information concerning mass transport of species. Measurements were performed with a rotating niobium sample in HF-H2SO4 media. Intensity is found as a linear function of the square root of the rotation speed ω which is the signature of an electro-polishing (EP) controlled by the diffusion of the fluorine ion. The value of the related diffusion coefficient D has been estimated from 4 to 1410-8 cm2s-1 for different electrolyte compositions. D has also been found lower in aged mixtures. Electrochemical Impedance Spectroscopy measurements were also performed with different EP mixtures with a rotating disc electrode. Both voltamperemetric and EIS measurement prove the central role of fluorine during EP and show that EP mechanisms are modified with the aging of the bath. The observed characteristics can be used to evaluate the quality of acids and to improve the life time of EP mixtures. However, a too high fluorine content in niobium could be the origin of decreased performances noticed on single-cell cavities after EP using modified parameters.