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Abstract
Understanding high temperature interfacial phenomena with Al-alloys is essential for improving corrosion performance of refractories in melting/holding furnaces. Both physical and chemical properties are known to influence wetting and corrosion behaviour. However, uncertainties exist regarding the influence of SiO2 in refractory compositions on interfacial reactions/mechanisms, particularly when present along with non-wetting chemical additives like BaSO4, CaF2 and AlF3.
An experimental study was conducted to clarify the interfacial phenomena of Al-alloy7075 with high-alumina refractories at extreme furnace temperatures of 1250ºC and 815ºC, using classical sessile drop approach and industrial cup tests respectively.
At 1250ºC, Al-alloy reacted more intensely with SiO2 compared to Al2O3. The interfacial behaviour of SiO2-Al2O3 system with Al-alloy was strongly dependent on SiO2 percentage, such that when upto 25% silica was present, wetting was reduced due to the presence of both original and newly formed corundum. Formation of mullite and originally present silica, along with decreasing corundum contents increased wetting in systems where silica varied from 25-45wt% and more than 45wt% respectively. Moreover, the nature of silica did not influence wetting when present in concentrations less than 20wt%.
Different additives produced varying interfacial reactions in the Al-alloy/high-alumina refractory system. AlF3 did not improve the wetting resistance, except when present in high concentrations (>10wt%) in the refractory; this improvement attributed to corundum-rich matrix formation resulting from silica loss as gaseous fluorides. Low CaF2 amounts (<3wt%) improved the wetting resistance due to corundum presence and anorthite formation in the refractory. As CaF2 content exceeded 5wt%, proportion of glassy phases increased, hence enhancing interfacial reactions.
However unlike CaF2, low BaSO4 levels (<5wt%) decreased the wetting resistance due to barium silicate formation, while high BaSO4 concentrations (≥10wt%) increased the wetting resistance due to celsian formation. Also, CaF2 dominated interfacial mechanisms when present along with BaSO4 in the refractory.
The effect of additives on modifying wetting resistance was found to strongly vary with SiO2 levels of the refractory. The study demonstrated that additive effect is also influenced by treatment temperatures such that generally higher additive amounts are required at lower temperatures for improving the wetting resistance of high-alumina refractories.