Corrosion behavior of reaction bonded Si3N4-SiC and SiAlON-SiC composites in simulated aluminum smelting conditions

Abstract

Si3N4-SiC composites, which are widely used as sidewall refractories in aluminum smelting cells, have been produced by reaction bonding and their corrosion performance assessed in simulated aluminum electrochemical cell conditions. Additional samples were produced with the silicon nitride bonding phase replaced by β SiAlON with compositions ranging from Z = 1-4 in Si6-zAlzOzN8-z. The formation of the Si3N4 and SiAlON bonding phases were studied by reaction bonding of silicon powders in a nitrogen atmosphere at low temperatures to promote the formation of silicon nitride, followed by a higher heating step to produce β SiAlON composites of different composition. The corrosion performance was studied in a laboratory scale aluminum electrolysis cell where samples were exposed to both liquid attack from molten salt bath and corrosive gas attack. For the Si3N4 bonded samples, the corrosion resistance was shown to be strongly dependent on the environment during corrosion testing, with samples in the gas phase showing higher corrosion than those immersed in the bath. Samples that had been pre-soaked in the bath and then tested in the gas phase showed the highest corrosion due to the combined effects of bath penetration and gas attack. For the SiAlON bonded samples, the corrosion results showed similar trends but were complicated by the presence of a strongly adherent layer on the samples which influenced volume measurements. This layer is believed to be alumina and appears to form through an interaction with the SiAlON and the bath components, since no such layer was observed for the silicon nitride samples.