Electrodeposition of Titanium in Water−Soluble KF−KCl Molten Salt

Abstract

１． Introduction Metallic titanium has superior properties such as corrosion resistance, heat resistance, specific strength, etc. However, its high production cost and poor workability hinder the wide spread use of titanium. Coating technology is a fascinating method to utilize the characteristics of high corrosion resistance and high strength of titanium metal or compounds. Currently, PVD methods are commonly used for a coating of some titanium compounds, however, the deposition onto a substrate with complicated shape is difficult. Electroplating in molten salts has been expected as a coating method of titanium even for any shapes. Until now, there are some reports on the titanium electrodeposition from molten salts [1-7]. In previous studies, fluoride [3,6], chloride [1,2,4], and fluoride-chloride [4,5,7] molten salts have been used as electrolytes. Generally, compact and smooth Ti films were obtained from fluoride melts such as LiF–NaF–KF [3,6] or fluoride-chloride melts such as NaCl–KCl–NaF [5,7]. For the fluoride-chloride mixtures, when F/Cl atomic ratio became higher, better deposits (grain size and purity) were obtained and higher current efficiency were achieved [5,7]. However, one of the problems in the previous studies employing fluoride-based molten salts was the difficulty in removing the salt adhered to the deposited Ti, because the solubilities of LiF and NaF in water are very low. Recently, we have proposed a new electrodepositon process of Si using a KF–KCl binary system [8]. Since both KF and KCl are highly soluble in water, the adhered salt can be removed easily by washing with water. In the present study, we investigated the electrodeposition of Ti in molten KF–KCl after the addition of K2TiF6and sponge Ti. ２． Experimental The experiments were conducted in eutectic KF–KCl in a dry Ar atmosphere at 923 K. K2TiF6 (0.1-2.0 mol%) and Ti sponge (10 g) were added to the bath as Ti sources, and Ti (IV) ions were converted to Ti (III) ions by proportionation reaction. A Ni plate electrode was used as a working electrode. The counter and quasi-reference electrodes were a Ti rod and a Pt wire, respectively. The potential of the quasi-reference electrode was calibrated by the deposition potential of K metal on a Mo wire. Samples produced by the galvanostatic electrolysis were analyzed using SEM/EDX and XRD after washed with water to remove the adhered salts. ３． Result and discussion The samples preparation was carried out by galvanostatic electrolysis under the conditions of the added amount of K2TiF6 of 0.1-2.0 mol% and the cathodic current density of 25-250 mA cm-2. As a typical example, Fig. 1 shows an XRD pattern of the sample prepared at 100 mA cm-2 for 100 min in the melt after addition of K2TiF60.1 mol% and sponge Ti. Electrodeposition of crystalline Ti metal was confirmed. As shown in the surface SEM image in Fig. 2, the obtained granules have a diameter about 20 μm. No element was detected except for Ti by EDX analysis. In the presentation, the effects of concentration of Ti (IV) ions and current density on the morphology and crystalline orientation will be discussed. Acknowledgement A part of this study was conducted as a collaborative research with Sumitomo Electric Industries, Ltd. R eferences [1] J. O’M. Bockris, G.J. Hills, I.A. Menzies, L. Young, Nature, 178, 654 (1956). [2] D. Inman, S.H. White, J. Appl. Electrochem., 8, 375 (1978). [3] J. De Lepinay, J. Bouteillon, S. Traore, D. Renaud, M.J. Barbier, J. Appl. Electrochem., 17, 294 (1987). [4] D. Wei, M. Okido, T. Oki, J. Appl. Electrochem., 24, 923 (1994). [5] N. Ene, S. Zuca, J. Appl. Electrochem., 25, 671 (1995). [6] A. Robin, R.B. Ribeiro, J. Appl. Electrochem., 30, 239 (2000). [7] J. Song, Q. Wang, X. Zhu, J. Hou, S. Jiao, H. Zhu, Mater. Trans., 55, 1299 (2014). [8] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara, T. Homma, J. Electrochem. Soc., 162, D444 (2015). Figure 1