TRANSIENT ELECTROCHEMICAL TECHNIQUES FOR STUDYING ELECTRODEPOSITION OF NIOBIUM IN FUSED NACL-KCL

The mechanism of the electroreduction of niobium chlorides dissolved i n fused NaCl-KCl was determined by transient electrochemical technique s at temperatures ranging from 670 to 870 degrees C. At temperatures < 750 degrees C it is shown that the stable oxidation states of niobium ions are Nb(V), Nb(IV), and Nb(III); reduction of Nb(III) gives rise t o metallic niobium. At higher temperatures, Nb(V) tends to decompose i nto Nb(IV) with chlorine evolution. Introducing fluoride ions stabiliz es the higher oxidation states. For a molar ratio F/Nb, r(F) > 2, Nb(V ) is stable over the whole temperature range and the direct reduction Nb(IV) --> Nb is obtained as the Nb(III) species is no longer stable. The standard potentials of the redox reactions are calculated for vari ous concentrations of fluoride ions. Presence of fluoride ions has a b eneficial effect on the quality of the niobium deposit, a ratio r(F) s imilar to 2 is enough for obtaining coherent deposits: