Silicon Carbide-Silicon Nitride Materials: Part 2—Corrosion Resistance and Oxidation in Al Reduction Cells and at Lab Testing

The key question for understanding the corrosion phenomena of Si₃N₄-SiC material in Al reduction cells is as follows: does the interaction with gases promote future step corrosion by molten cryolite (bath) or does cryolite interact with the Si₃N₄-SiC refractory and deteriorate the properties of the refractory material? More probably the reactions of silicon carbide and silicon nitride with gases, which result in the formation of silica, occur before the reactions of silica with molten cryolite. The corrosion of Si₃N₄-SiC material in the reduction cell may take place by “gas-solid” reaction and by “liquid-solid” reaction. There are several variants of lab corrosion tests for the evaluation of the corrosion resistance of Si₃N₄-SiC material to cryolite. The results of the investigation of Si₃N₄-SiC lab corrosion tests give no direct evidence of selective dissolution at a specific phase (Si₃N₄ or SiC, α-Si₃N₄ or β-Si₃N₄) in cryolite. The existing variants of lab corrosion testing require clarification.