Effects of Fe(III) on MgCl2 electrolysis and its cathodic processes on W electrodes

Abstract

In this article, the cathode passivation in the MgCl2 electrolysis caused by iron ions was characterized first time by scanning electron microscopy (SEM), in which the magnesium with the shape of caviar could be seen intuitively. The cathodic processes of Fe (III) on tungsten electrode in MgCl2-NaCl-KCl-CaCl2 melts were investigated with different methods including square wave voltammetry (SWV), cyclic voltammetry (CV), chronoamperometry (CA), and chronopotentiometry (CP) at 973 K. The results showed that the reduction of Fe (III) was a diffusion control reversible reaction process with one single step and that the nucleation mechanism of Fe (III) deposition on tungsten electrodes was instantaneous, consistent with the formation and growth mechanism of hemispherical nuclei. Besides, Arrhenius treatment was employed to obtain the activation energy for the diffusion process in the molten salt mixtures. The value of activation energy for the diffusion of Fe (III) was derived as 32.50 kJ mol−1 by the method of CA electrochemical technique. The equilibrium potential of Fe (III)/Fe(0) was determined by open circuit chronopotentiometry. In addition, the standard apparent potential of the Fe (III)/Fe(0) and Fe (III) activity coefficient in melts were estimated at 973 K.