Factors Affecting Corrosion Resistance of Silicon Nitride Bonded Silicon Carbide Refractory Blocks

The lifetime of aluminum reduction cells is driven primarily by the lifetimes of two components of the cell lining: the carbon cathode and the sidewall refractory material. The current state-of-the-art sidewall material is a silicon nitride bonded silicon carbide (SNBSC) refractory and its corrosion mechanisms in the aluminum reduction cell environment have been examined in this study. Microstructural analysis of commercial SNBSC materials identified variations in porosity and a/beta Si3N4 ratio in the binder phase, with higher porosity levels and beta Si3N4 content found in the interior part of the block. Unreacted metallic silicon was observed only as a crystalline phase encapsulated inside SiC grains and not in the binder phase. The effects on the corrosion rate of porosity levels, amount of binder, a/beta Si3N4 ratio, and different factors in the environment, were examined in laboratory scale trials. High corrosion rates were associated with high porosity levels and a high beta Si3N4 fraction in the binder. The crystal morphology of beta Si3N4 is suggested as the main reason for the higher reactivity of this material. This morphology presents a higher surface area compared with a Si3N4 crystals. A corrosion mechanism for SNBSC materials in the aluminum reduction cell atmosphere is suggested.