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.