Electropolishing of Type 304 Stainless Steel in an Amide Type Ionic Liquid Containing Chloride Ion

Abstract

Electropolishing of stainless steel (SUS) has been commercially performed in such aqueous acid solutions as phosphoric and sulfuric acid. The current process shows low current efficiency due to the oxygen evolution reaction. Amide type ionic liquids with wide electrochemical windows are expected as an alternative electrolyte for electropolishing of metals with high current efficiency. Although electropolishing of Ti, Cu, Pt, and Sn has been reported in amide type ionic liquids, there is no report for SUS. In addition, previous studies on electropolishing of SUS in nonaqueous electrolytes have suggested that chloride ions are necessary to promote the breakdown of the oxide film on SUS. In the present study, the anodic dissolution mechanism of type 304 SUS was investigated in an amide type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA), containing chloride ions. BMPTFSA containing Cl- was prepared by dissolving BMPCl. The anodic behavior of SUS was investigated by linear sweep voltammetry. The surface morphology and roughness of SUS before and after the anodic dissolution were evaluated by scanning electron microscopy and laser scanning microscopy. A smooth and shiny surface was obtained by the anodic dissolution with optimum conditions in BMPTFSA containing Cl-. The surface roughness decreased after the anodic dissolution. On the other hand, no anodic current corresponding to the anodic dissolution of SUS was observed in BMPTFSA without Cl-, indicating the presence of Cl- is necessary for the anodic dissolution of SUS in an amide type ionic liquid. The dissolved metal ion species in the ionic liquid were identified to be the chlorocomplexes of Fe, Cr, and Ni by ultraviolet-visible spectroscopy. The current efficiency of anodic dissolution was estimated to be 93 to 100% from the mass change of the electrode and electric charge, suggesting that electropolishing with high current efficiency is possible. The present work was partially supported by the Kato Foundation for Promotion of Science.