The Principle of Tantalum Electrodeposition on 316 L Stainless Steel in the LiF-NaF-KF-K 2 TaF 7 Molten Salt System

This research primarily investigates the structural characteristics of tantalum ions in FLiNaK-10 wt% K 2 TaF 7 molten salt and their corresponding reduction mechanisms. High-temperature Raman spectroscopy was employed to explore the structural configurations of tantalum ions at varying temperatures. It was found that two tantalum ion species, TaF 7 2− and TaF 8 3− , coexist in the molten salt at lower temperature, while only the TaF 7 2− structure remains at elevated temperature. Cyclic voltammetry and square wave voltammetry were utilized to probe the reduction steps of different ions, revealing that the reduction pathway of TaF 7 2− follows Ta F 7 2 − + 5 e − → Ta + 7 F − and TaF 8 3− proceeds via Ta F 8 3 − + 2 e − → Ta F 3 + 5 F − and Ta F 3 + 3 e − → Ta + 3 F − . Electrode kinetics studies using CV at varying scan rates demonstrated that the reduction of tantalum ions is a diffusion-controlled reversible reaction. The diffusion coefficient (D = 1.32 × 10 −5 cm 2 s −1 at 800 °C) and diffusion activation energy (42.32 kJ mol −1 ) were calculated. Chronoamperometry was further applied to investigate the phenomenon and process of tantalum nucleation and growth. Combined with SEM and EBSD characterization of the deposits, these results further confirm that the nucleation mechanism of tantalum ions follows instantaneous nucleation and the island growth model.