Influence of the Second Coordination Sphere on the Roughness of Niobium and Tantalum Coatings Obtained in Chloride-Fluoride Melts

The influence of the second coordination sphere complexes on the roughness of niobium and tantalum coatings was studied. It was found that the discharge potential of niobium oxofluoride complexes has a more positive potential than fluoride ones in the NaCl-KCl-NaF (10 wt%)-K2NbF7 melt. For the CsCl-CsF (10 wt%)-CsNbF6 molten system the inversion of potentials corresponding to discharge of oxofluoride and fluoride complexes was observed. Due to micropassivation by oxygen impurities as niobium suboxides and oxygen-niobium solid solution the coating roughness in the NaCl-KCl-NaF (10 wt%)-K2NbF7 (8 wt%) melt diminishes in comparison with CsCl-CsF (10 wt%)-CsNbF6 (8 wt%) melt. In molten salts NaCl-KCl-NaF (10 wt%)-K2TaF7 and CsCl-CsF (10 wt%)-CsTaF6 tantalum oxofluoride complexes are discharged at more negative potentials than fluoride ones and changes in the composition of the second coordination sphere from sodium to cesium led to a lower level of roughness. For melts containing in the second coordination sphere cesium cations, the roughness of the tantalum coatings was less than that of niobium due to the greater strength of the fluoride complexes of tantalum. It was found that pulsed current electrolysis for deposition of niobium and tantalum coatings has significant advantages in comparison with direct current electrolysis. (C) 2020 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.