Electropolishing of Titanium in an Amide-Type Ionic Liquid Containing Chloride Ions

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Titanium is widely used in the medical, food, aerospace, semiconductor, and chemical industries due to its low density, high corrosion resistance, and biocompatibility. A smooth Ti surface is often required for these applications. Electropolishing is one of the techniques used to smooth and brighten the metal surface without forming the Beilby layer. However, electropolishing of Ti has conventionally been performed in alcohol-based electrolytes and highly concentrated acidic solutions with the large applied voltage, resulting in the low current efficiency for the anodic dissolution of Ti with the oxygen evolution. Therefore, a safe alternative electrolyte for electropolishing of Ti with high current efficiency is needed. Amide-type ionic liquids (ILs) composed of bis(trifluoromethylsulfonyl)amide (TFSA – ) are attractive candidates as alternative electrolytes for electropolishing of metals and alloys due to their negligible vapor pressure and non-flammability. Surface smoothing and brightening of Sn has been reported to be possible in BMPTFSA (BMP + : 1-butyl-1-methylpyrrolidinium), due to the formation of the viscous layer near the electrode [1]. However, the native oxide film on the surface was reported to hinder the electropolishing of Ti in TMHATFSA (TMHA + : trimethyl- n -hexylammonium) [2]. On the other hand, we have previously reported that Ti can be dissolved in BMPTFSA containing Cl – to form tetravalent complexes, [TiCl 6 ] 2– , without removing the native oxide film. In the present study, the mechanism of anodic dissolution and electropolishing of Ti covered with the native oxide film was investigated in BMPTFSA containing Cl – . The electrolyte was BMPTFSA containing 0.5 M BMPCl. BMPCl and BMPBr were synthesized by the reaction of 1-methylpyrrolidine with butyl chloride and butyl bromide, respectively, in acetonitrile. BMPTFSA was prepared by the metathesis of BMPBr and LiTFSA in distilled water. The water content in BMPTFSA after drying under reduced pressure was determined to be less than 10 ppm by Karl Fischer titration. The electrochemical measurement was conducted using a three-electrode cell in an Ar-filled glovebox at 353 K. A Ti plate and disk were used as a working electrode. A Pt wire and a glassy carbon plate were used as a counter electrode. An Ag wire immersed in BMPTFSA containing 0.1 M AgCF 3 SO 3 was used as a reference electrode (Ag|Ag(I), +0.45 V vs. ferrocene|ferrocenium at 353 K). The Ti surface was characterized using a scanning electron microscope and a laser microscope. The Ti surface was uniformly dissolved after potentiostatic anodic oxidation at 0 V without agitation. In addition, the surface roughness estimated by the laser microscope decreased after electrolysis. The average depth of the electropolished Ti surface was consistent with the calculated value assuming the four-electron transfer reaction, suggesting that the apparent current efficiency was approximately 100%. The present work was partially supported by JSPS KAKENHI grant Number 25K08358. Yuza, N. Serizawa, and Y. Katayama, J. Electrochem. Soc. , 168 , 036509 (2021). Uda, K. Tsuchimoto, H. Nakagawa, K. Murase, Y. Nose, and Y. Awakura, Mater. Trans ., 52 , 2061 (2011). K. Yoneda, N. Serizawa, and Y. Katayama, ECS. Trans , 114 , 97 (2024).

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