Electroreduction of Hafnium and Electrodeposition of its Protective Coatings on a Molybdenum Substrate in Chloride-Fluoride Melt

The electroreduction of HfF 6 2− complexes to Hf on a molybdenum electrode in the NaCl-KCl-K 2 HfF 6 -NaF (5 wt%) melt in the temperature range 973–1123 K was studied by cyclic voltammetry and impedance spectroscopy. It was found that discharge of Hf(IV) complexes to Hf occurs in one step and is irreversible. The diffusion coefficients of Hf(IV) fluoride complexes were determined by cyclic voltammetry. The impedance spectroscopy was used for the determination of the standard rate constants of charge transfer. Electrodeposition of hafnium coatings on a molybdenum substrate was performed in the NaCl-KCl-K 2 HfF 6 (10 wt%)-NaF (5 wt%) melt at the cathodic current density 5–100 mA cm −2 and temperatures 973, 1023 and 1073 K. The influence of the current density and temperature on the roughness of hafnium coatings was studied. It was determined that hafnium coatings with significantly higher purity than that of the hafnium anode were obtained confirming that metal impurities were removed during electrodeposition. The oxidation kinetics data were obtained on a sample with hafnium and a sample of pure molybdenum, which were tested in a water/oxygen mixture at 773 K. The hafnium coating improves the corrosion resistance of coated plates in comparison with that of pure molybdenum.