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.