On the Ratio of Cathodic and Anodic Processes during Electroreduction-Electrooxidation of Hafnium in Molten Salts

Abstract

Investigation of hafnium cathodic and anodic processes in chloride melts containing HfCl4 revealed that two-hafnium electroreduction processes are corresponded two electrooxidation processes. The only difference between these processes was the presence of salt passivation under certain dissolution conditions. In chloride-fluoride melts containing K2HfF6 the number of cathodic and anodic processes can be different due to the amount of hafnium ions passing into the melt during anodic dissolution exceeding the amount of fluorine anions necessary to bind them into fluoride complexes. Based on the experimental data it was found that the conclusion about the mirror reflection of cathodic and anodic processes is invalid in the case of a heteroligand composition of melts. It was determined that the dissolution in the NaCl-KCl-K2HfF6 (10 wt%) melt at the anodic current density of >0.02 А∙cm-2 was accompanied by the formation of Hf(IV) fluoride complexes and Hf(II) chloride complexes. It was shown that the appearance of hafnium complexes with different valence during anodic process in the NaCl-KCl-K2HfF6 (10 wt%) melt is a highly undesirable phenomenon, which leads to the formation of metallic film on the surface of the melt decreasing the current efficiency for coating deposition.