Anodic Dissolution and Electropolishing of Titanium in an Amide-Type Ionic Liquid Containing Halide Ions
Authors: Yoneda Kentaro , Serizawa Nobuyuki , Katayama Yasushi 
Published: December 2024

Titanium is widely used in the medical, food, aerospace, semiconductor, and chemical industries due to its low density, high corrosion resistance, and biocompatibility. The surface smoothing of the Ti components is often required in these fields. The smooth and bright surface without the Beilby layer can be obtained by electropolishing. However, electropolishing of Ti has been conducted in alcohol-based electrolytes and highly concentrated acidic solutions containing perchlorate acids with applying high voltage. Therefore, a safe and alternative electrolyte is required for electropolishing of Ti with high current efficiency.

Amide-type ionic liquids (ILs) composed of bis(trifluoromethylsulfonyl)amide (TFSA–) are attractive candidates as alternative electrolytes for electropolishing of metals and alloys because of the negligible vapor pressure and non-flammability. The 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]. It is reported that the anodic dissolution of Ti was suppressed by the native oxide film on the surface in TMHATFSA (TMHA+: trimethyl-n-hexylammonium)[2]. On the other hand, the electropolishing of type 316 stainless steel has been reported to be possible in BMPTFSA containing Cl–[3], although the Cr-rich oxide layer is known to act as the protective layer against corrosion. In the present study, the mechanism of anodic dissolution and electropolishing of Ti covered with the native oxide film were investigated in BMPTFSA containing Cl–. In addition, the anodic behavior of Ti in BMPTFSA was also investigated in the presence of Br–.

Electrolytes containing halide ions were prepared by dissolving BMPCl and BMPBr in BMPTFSA. Electrochemical measurement was conducted using a three-electrode cell in an Ar-filled glovebox at 353 K. A Ti plate and disc 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 AgCF3SO3 was used as a reference electrode (Ag|Ag(I), +0.43 V vs. ferrocene|ferrocenium at 298 K). The surface of a Ti electrode was observed using a scanning electron microscope. The dissolved Ti species in the electrolyte was identified using an ultraviolet-visible (UV-Vis) spectrometer.

The anodic current was observed in a linear sweep voltammogram of a Ti electrode in 0.5 M BMPCl/BMPTFSA, although the anodic current was negligible in BMPTFSA in the absence of Cl–. An absorption peak assignable to [TiCl6]2– was observed in the UV-Vis spectrum of the electrolyte after potentiostatic electrolysis at 0 V vs. Ag|Ag(I), indicating that Ti was anodically dissolved to form the tetravalent complex. The surface of a Ti electrode was brightened and smoothed after the potentiostatic anodic oxidation at 0 V vs. Ag|Ag(I), indicating that electropolishing of Ti covered with the native oxide film was possible in BMPTFSA in the presence of Cl–. The morphology of a Ti electrode changed after anodic polarization in BMPTFSA containing Br–, suggesting that the Ti surface was dissolved in BMPBr/BMPTFSA.

[1] N. Yuza, N. Serizawa, and Y. Katayama, J. Electrochem. Soc., 168, 036509 (2021).

[2] T. Uda, K. Tsuchimoto, H. Nakagawa, K. Murase, Y. Nose, and Y. Awakura, Mater. Trans., 52, 2061 (2011).

[3] N. Serizawa, S. Yamasaki, and Y. Katayama, 2023 Joint Symposium of Molten Salts, 1B06 (2023).