Electropolishing of Niobium in a Sulfuric Acid Solution by Pulse Potential Control

Abstract

Using a three-electrode system, effect of pulse period of controlled potential on the electropolishing behavior of Nb in 30 mass％ H₂SO₄ was examined. The potential pattern consisted of cathodic pulse （－3 V vs. Ag/AgCl for t_c）, anodic pulse （＋1 V vs. Ag/AgCl for t_a） and rest time （0 V vs. Ag/AgCl for 1 ms）. The results of various combinations of t_c and t_a showed that both t_c and t_a were needed to be longer than 0.5 ms for successful polishing. Good performance was obtained when each pulse was balanced and the etching rate became maximum for t_c＝t_a＝ 2 ms. The surface observation indicated that small pits were generated in the early stage of polishing and extended to form dimpled faces. However shorter t_a than t_c resulted in rugged surface, suggesting mechanical damage of Nb matrix by hydrogen. Thus, the combination of t_c and t_a in good balance is important in this polishing.