Effects of carbon-bonded coatings on sodium expansion of the cathode in aluminium electrolysis

Effects of coatings consisting of Al4C3, TiC, TiB2, TiO2 or their mixtures and a carbonaceous binder on sodium expansion are investigated. In a basic alumina-saturated cryolitic melt with CR = 4.0 and for a current density of 0.70 A/cm2, a 180 % to 600 % increase in expansion was observed for the Al4C3-coated carbon samples compared to the expansion of the corresponding uncoated samples. This strong expansion enhancement is attributed to the low electronic conductivity and porous structure of the coating, which allows the cathodic reactions to take place at the sample surface at an increased sodium concentration. All coatings present a certain resistance to sodium penetration and slow down the sodium expansion initially for a short period of time. Additives increasing the electrical resistivity of the coating increase the sodium expansion. The TiB2 coating is best wetted by molten aluminium and reduces the sodium expansion by 20 % to 70 %.