Corrosion of high density graphite anodes during direct electrochemical de-oxidation of solid oxides in molten CaCl2 medium

Graphite anode used in the direct electrochemical de-oxidation of solid oxides in molten calcium chloride medium (FFC Cambridge process) degrades by formation of carbon oxides and erosion. The problem was studied by using graphite as anode in the electrochemical de-oxidation of powder compacted and sintered Nb2O5 pellet electrodes in molten CaCl2 at 1173 K and at a constant voltage of 3.1 V. The degradation of the graphite anodes, recovered from the electro-de-oxidation cells at predetermined intervals of time (2, 9, 22, 26, 35, 44 and 56 h) during the long-duration electrolysis, was determined by mass loss measurements and the morphological changes by SEM imaging. The graphite rods were found corroded differently along the lengths, with the maximum corrosion being at the melt-immersed surface of the electrode close to the melt-gas interface. The prominent carbon consuming reaction C + 2O2− → CO2 + 4e− apart, the impregnation of salt into the defects of the graphite anode and convection currents existing in the melt appeared to enhance the corrosion and hence the mass loss of graphite towards the later stages of electrolysis.