Electrochemical Reaction Processes of Niobium Ions in FLINAK melt

To investigate the mechanism and influence of oxide and niobium metal on the electrodeposition of niobium, the electrochemical reaction mechanisms of niobium ions in LiF-NaF-KF eutectic (FLINAK) melt at 973 K was studied by chronopotentiometry, chronoamperometry and cyclic voltammetry. The electrochemical reduction process of Nb(V) (K₂NbF₇) in FLINAK melt relates to the complex anions. A two-step electrochemical reduction with disproportionate and catalytic reactions is proposed. The metallic niobium in K₂NbF₇-FLINAK melt can reduce the catalytic reaction. The influence of oxide (Na₂O) on the electrochemical processes of Nb(V) in K₂NbF₇-FLINAK-O^2- melts without and with niobium metal present was also studied. When oxide present in K₂NbF₇-FLINAK melt, a much stable oxyfluorocomplex ion is formed. Cyclic voltammetry showed that the reduction potential of oxyfluoro-complex ion is more negative than that of fluorocomplex ion (NbF_62-). The electrochemical stability of NbOF_63- is higher than those of NbF_72- and NbF_62-. An optimum composition for a coherent niobium metal coating on low-carbon steel is found to be 2.7%K₂NbF₇-0.5～2.0%Na₂O-FLINAK in molar fraction. The cathodic current density is about 90 mA/cm².