Electrodeposition of magnesium from CaCl2-NaCl-KCl-MgCl2 melts

Electrodeposition of magnesium from calcium chloride-based melts was studied on metallic (tungsten and molybdenum) and glassy carbon electrodes. Pure CaCl2 melt and different alkali chloride solvents including binary CaCl2-NaCl, equimolar composition, and ternary CaCl2-NaCl-KCl melts with compositions 35:55:10 mol %, containing different MgCl2 concentrations, were used. In the case of the binary mixture, the operating temperature was both above and below the melting point of metallic magnesium (923 K) so that solid and liquid deposits were obtained. Underpotential deposition of calcium is an important process in melts containing CaCl2 due to formation of Ca-Mg alloys, which affects the magnesium electrodeposition process. Three-dimensional magnesium deposits that grow macroscopically on the studied substrates were found. This influences determination of the diffusion coefficient of Mg(II) ions. Formation of a magnesium monolayer on both electrodes, and especially on tungsten, was a stable process in the ternary mixture containing MgCl2, which was related to the presence of 10 mot % KCl. Moreover, in this melt a remarkable pseudopassivation process was found at high overpotentials. (C) 2004 The Electrochemical Society.