Electrochemical Formation of Calcium Hexaboride and Boronizing of Metal Electrodes in CaCl₂-Based Molten Salt

Abstract

The electrochemical behaviors of boron species on Mo and Ni electrodes in molten NaCl–CaCl₂–CaO–B₂O₃ at 1123 K were investigated to establish a process for CaB₆ production using molten salt electrolysis. Electrolysis was performed at four different potentials (− 2.97, − 2.77, − 2.37, and − 2.07 V vs Cl⁻/Cl₂) on Mo and Ni plates. Boronized Mo or Ni was observed under all experimental conditions. At a potential of − 2.37 V, the boronized layer was a few micrometers thick for Mo, whereas it was approximately 15 µm for Ni under the same electrolytic conditions. Moreover, CaB₆ was formed at − 2.37 V or a more negative potential in the case of Mo, although it was formed at − 2.77 V or a more negative potential in the case of Ni. Boronizing proceeded more rapidly on Ni than on Mo, indicating that boronizing tends to be predominant over the formation of CaB₆ in the case of Ni. The results show that Mo is a more suitable electrode material than Ni for CaB₆ production. Furthermore, even though the melt contains Ca²⁺ ions, the metal can be boronized under appropriate conditions by molten salt electrolysis without the formation of CaB₆.