Optimization of Electrolysis Conditions for Ti Film Electrodeposition from LiF – LiCl Eutectic Molten Salt
Authors: Unoki Makoto , Norikawa Yutaro , Yasuda Kouji , Nohira Toshiyuki 
Published: December 2020

1. Introduction
Titanium is known as a metal which has excellent properties such as high specific strength and high corrosion resistance. However, there are two problems that prevent widespread use of titanium. One is the high smelting cost and the other is the poor workability. As a method to solve these problems, titanium plating is an attractive technique because superior surface properties of titanium can be utilized. For some applications, physical vapor deposition (PVD) is practically used as a titanium plating method. However, the PVD method has high plating cost and can be used for only simple-shaped substrates. On the other hand, electrodeposition of titanium in high-temperature molten salt is a promising plating method which is expected to have lower cost and can be used for complex shaped substrates. Accordingly, there have been many reports on the titanium electrodeposition in high-temperature molten salts [1–3].
We have already reported the electrochemical behaviors of Ti(III) ions and the electrodeposition of titanium in KF–KCl and LiF–LiCl molten salts [4–7]. Furthermore, we found that LiF–LiCl has an advantage for electrodepositing smooth titanium films due to its lower melting point (774 K at the eutectic composition [8]); smoother titanium films were obtained at lower temperature by suppressing grain growth of titanium [9]. Thus, in the present study, we investigated the effect of electrolysis conditions such as current density and concentration of Ti(III) ions on smoothness of titanium films in LiF–LiCl eutectic molten salt.
2. Experimental
The experiments were conducted in LiF–LiCl eutectic molten salt in an Ar glove box at 823 K. Li2TiF6(0.50–5.0 mol%) and Ti sponge (0.33–3.3 mol%) were added to the bath and Ti(IV) ions were converted to Ti(III) ions by comproportionation reaction. Ni plate, Mo flag, and Au flag electrodes were used as the working electrode. The counter and reference electrodes were Ti rods. The potential of the reference electrode was calibrated by Cl2/Cl− potential measured at a glass-like carbon rod electrode. Samples were prepared by galvanostatic electrolysis of Ni plate substrates. The samples were analyzed by SEM/EDX after washing with distilled water and 1M Al(NO3)3 aqueous solution to remove adhered salts.
3. Result and Discussion
The electrodeposition was carried out by changing the added amounts of Li2TiF6from 0.50 to 5.0 mol% and Ti sponge from 0.33 to 3.3 mol%. The cathodic current density was also varied from 25 to 1000 mA cm−2. The surface roughness of the obtained samples was measured by SEM. As a result, smooth Ti films were obtained at high Ti(III) concentrations and low current densities. As a typical example of the smooth film, Fig. 1 shows an appearance of the sample prepared at 50 mA cm−2 and 60 C cm−2 after addition of 3.0 mol% Li2TiF6and 2.0 mol% Ti sponge. The electrodeposited titanium film had metallic luster. Fig. 2 shows a cross-sectional SEM image of the film, indicating the smooth and compact deposit.
In the poster session, the solubility of Ti(III) ions will be presented. Also, the effects of Ti(III) ion concentration and cathodic current density on the morphology will be discussed.
Acknowledgement
The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto University.
References
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Figure 1