Electrodeposition of Tantalum in NaCI-KCI-K2TaF7 Melts

Electroreduction f K2TaF7 dissolved in KC1-NaC1 at 720 ~ was investigated by transient electrochemical techniques. The primary reaction involves a single reversible five electron step Ta(V) + 5e ~ Ta. In addition to the metal deposition, other insoluble compounds were generated during the reduction step; their influence increases as the potential becomes more negative. The formation of these compounds was progressively inhibited by addition of sodium fluoride; simulta-neously, a shift of the deposition potential towards more negative values was observed. This result was interpreted as a consequence of the formation of an increasing amount of fluoride rich complexes such as TaFt- or TaFt-. When oxide ions were present in the bath, perturbations appeared, which indicates that the reduction of electroactive species containing oxygen occurs at a potential more negative than for oxygen free complexes. The diffusion coefficient of the complex ion containing Ta(V), measured by chronopotentiometry andcyclic voltammetry, is about 1.5 x 10-5 cm 2 s-1. The characteri-zation of the tantalum deposits bbtained at different overpotentials was determined by x-ray diffraction and by scanning electron micrography. It is generally considered that the electrolytic route using fused electrolytes i the most promising technique for production of refractory metals such as tantalum (1)