Crystalline chromium electroplating with high current efficiency using chloride hydrate melt-based trivalent chromium baths

Chromium (Cr) plating using trivalent chromium has been investigated as a replacement for the highly-toxic hexavalent chromium bath. Herein, we report novel chromium plating baths using hydrate-melts. Hydrate-melts have been investigated in recent years as an electrolyte for aqueous Li-ion batteries with a wide electrochemical window, and can be used as an electroplating bath in which the reduction of protons, i.e., bath decomposition, is suppressed. By using a hydrate-melt for electroplating with trivalent chromium, the increase of local pH during electrolysis is suppressed and generation of the electrochemically-inert oligomer of chromium hydroxide can be avoided. Crystalline chromium electrodeposits were successfully obtained from the hydrate-melt-based aqueous baths, which had not been possible with the conventional trivalent chromium process requiring organic coordination agents. Moreover, a current efficiency greater than 80% was achieved. The hydrate-melt-based aqueous bath, with low material cost, is a promising candidate for industrial trivalent chromium plating.