ELECTRODEPOSITION OF TUNGSTEN IN ZNBR2-NABR AND ZNCL2-NACL MELTS

The electrodeposition of tungsten has been investigated in the molten salt systems ZnBr2-NaBr-WBr5 and ZnCl2-NaCl-WCl6 at 450-degrees-C. Voltammetric results showed that the reduction of tungsten species is an irreversible process. Electrodeposits were mixtures of alpha- and beta-tungstens, the former being the thermodynamically stable phase. alpha-Tungsten exhibited the shape of cubic crystallites which tended to grow larger, whereas beta-tungsten always had a crystal size smaller than 1 mu-m. The beta-tungsten was formed during the initial stage of electrodeposition, irrespective of the substrate material, and remained predominant at high current densities or in pulsed electrolysis. It is likely that the phase transition of beta-tungsten to alpha-tungsten occurs when the size of the beta-tungsten crystallites becomes larger than a critical value.