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Si3N4-SiC materials have considerable strength, yet in Al reduction cells the decay of SiC side lining takes place due chemical corrosion and oxidation, sometimes accompanied and enhanced by physical erosion. N-SiC lining materials in reduction cells should be corrosive resistant to electrolyte and oxidation resistant. In Aluminium reduction cell the upper part of SiC lining is subjected to interaction with Oxygen from air. However other parts, covered with side ledge and near the bus bar windows, also may be oxidized during the service. Probably oxidation resistance and corrosion resistance of Si3N4-SiC materials may be governed by different factors. The priority of oxidation resistance and corrosion resistance to cryolite of Silicon Carbide over Silicon Nitride leaves many questions. Anyway, Silicon Nitride and Silicon Carbide slowly oxidize during the service in N-SiC side lining of reduction cell. The question on priorities of oxidation and corrosion resistance of one over another has aspect not only in the service life time in reduction cell, but also in uncertain optimum of concentrations, slight porosity and concentration gradients.