Electrochemical Deposition Using Highly Concentrated Aqueous Electrolytes

This review summarizes the previous research conducted by the author's group on the electrodeposition of metals using highly concentrated aqueous solutions, or hydrate melts, of calcium chloride (CaCl₂) or lithium chloride (LiCl) as the "solvent" of the deposition baths. In an attempt to use the melts as novel trivalent chrome plating baths, crystalline, hard, and thick chromium layers that could not be obtained with conventional trivalent chrome plating were deposited rapidly with high current efficiency. This process is expected to serve as a new functional chromium plating method that does not use hexavalent chromium baths. The high current efficiency of the chrome plating stems from the suppression of hydrogen ion hopping diffusion (i.e. the Grotthuss mechanism) due to the extreme depletion of free water within the melts. Silver chloride (AgCl) and lead chloride (PbCl₂), which are generally considered to be poorly soluble chlorides, can also be dissolved in the hydrated melts. These were employed as electrolytic baths in attempts at novel cyanide-free silver displacement plating and novel lead electrolysis without fluoride-based electrolytes. The resulting film exhibits a certain degree of smoothness even without additives, likely because the hydrate melts are about one order of magnitude more viscous than those of typical aqueous solutions.