ELECTRODEPOSITION OF NIOBIUM FROM FLUORONIOBATE K2NBF7 SOLUTIONS IN FUSED NACL-KCL

The mechanism of the electrodeposition of niobium from fluoroniobate solution in molten NaCl-KCl at 720-degrees-C was studied by voltammetry and chronopotentiometry. It was shown that the reaction proceeds in two steps: first a reversible redox reaction, Nb(V) + e <-> Nb(IV), followed by a quasi-reversible reduction, Nb(IV) + 4e --> Nb(0). The standard potentials of the redox couples Nb(V)/Nb(IV) and Nb(IV)/Nb(0) are E5/4(0) = 0.394 V and E4/0(0) = -0.178 V vs. the Ni/Ni2+ reference electrode. The diffusion coefficients of the two electroactive species are D(Nb(V)) = 1.83 X 10(-5) cm2 s-1 and D(Nb(IV)) = 2.34 X 10(-5) cm2 s-1. On a nickel electrode the electrodeposition of well-crystallized metallic niobium occurred at a potential of -0.5 V vs. Ni/Ni2+. Better adherence and cohesion were achieved when pre-reduced solutions of niobium(IV) were used. At potentials lower than -0.7 V, a black powder was obtained. X-ray analysis indicated the presence of metallic niobium and of a new phage containing potassium and niobium.. In solutions containing niobium(IV) a thin layer of niobium carbide formed spontaneously on carbon. Voltammetric investigations indicated that the formation of niobium carbide occurred before the metal deposition; the redox reaction corresponding to the formation of the niobium carbide was found to be effectively irreversible.