Study On The Corrosion Resistance Of Al₂O₃-CaO Castables Against Molten Aluminium Alloy

Al₂O₃-SiO₂ refractories are widely used in the aluminum industry due to their wide source,good mechanical properties and thermal shock resistance.However the performance of Al₂O₃-SiO₂ refractories will be degraded by the corrosion of the molten aluminum in service,which will damage the lining.The development of aluminum alloy smelting technology also makes Al2O3-SiO2 refractories face severely corrosive environments.It is known from thermodynamics that CaO can’t be reduced by Al,so the Al2O3-CaO castable is a potential refractory for the aluminum industry.However,few researches focus on the corrosion resistance of Al₂O₃-CaO castables against molten aluminum alloy in China.Calcium hexaluminate（CA₆）and Calcium dialuminate（CA₂）have attract more and more attentions as a new type of refractory raw materials with low thermal conductivity,good thermal shock resistance and low wettability with molten aluminum.In this paper,CA₆,CA₂ and fused corundum were used as main raw materials and the content of calcium aluminate cement,reactive alumina and micro-silica in the matrix were constant.The formulation of corundum-CA₆ castables with different CA₆ addition,corundum-CA₂ and CA₆-CA₂ castables with different CA₂ addition were designed by replacing fine powder and aggregates with different particle sizes.The effects of CA6 or CA2 addition on the physical properties and thermal properties of castables were studied.Static crucible method was used to evaluate the corrosion resistance with 7075 aluminum alloy at 850℃and1000℃.Based on the experiments,castables with different compositions and additives BaSO4,CaF2 and AlPO4 were selected to conduct anti-aluminum liquid wetting test respectively.Combined with the phase composition and micro-structure analysis,the effects of different additives on the corrosion resistance of castables against molten aluminum alloy were analyzed and compared.The main results showed that:（1）The amount of water required for molding the castables increased with the increase of CA₆ or CA₂ addition.The samples shrunk slightly,which was negligible after heat treatment at 900℃and 1200℃.The bulk density of corundum-CA₆ castables decreased,the apparent porosity increased,the cold modulus of rupture（CMOR）and cold crushing strength（CCS）decreased with the increase of CA₆ addition.And the strength of the samples increased after teat treatment at 900°C and 1200°C.With the addition of CA₂increase,the bulk density of corundum-CA₂ castables and CA₆-CA₂ castables decreased,the apparent porosity increased,and the CMOR after drying changed slightly while the CMOR decreased after firing.When CA₂ replace all the aggregates and fine powders the CMOR was obviously improved.The CCS of castables after drying could be greatly improved by adding a proper amount of CA₂,however the CCS of drying samples and firing samples decreased with the increase of CA₂.（2）With the increase of CA₆ addition,the thermal expansion coefficient of corundum-CA₆ castables decreased slightly while the thermal conductivity decreased significantly,and the flexural strength retention rate after thermal shock increased from13%to 28%.With the increase of CA₂ addition,the thermal expansion coefficient and thermal conductivity of corundum-CA₂ and CA₆-CA₂ castables decreased significantly:the thermal expansion coefficient decreased to 6.3×10^-6°C^-11 and the thermal conductivity decreased to nearly 0.6 W/（m·K）.The thermal shock resistance of castables with proper amount of CA₂ improved significantly.The flexural strength retention rates of corundum-CA₂ and CA₆-CA₂ castables increased to exceed 80%and 57%respectively.When the aggregates and fine powders were all replaced by CA₂ the strength of specimens D88 after thermal shock reduced greatly which indicated its’poor thermal shock resistance.（3）The addition of CA₆ couldn’t improve the corrosion resistance of corundum-CA₆castables,samples with different CA₆ addition were infiltrated by molten aluminum alloy seriously.Corundum growth occured in corrosion test conducted at 1000°C.The corrosion resistance of corundum-CA₂ and CA₆-CA₂ castables could be improved significantly with the addition of CA2.For corundum-CA2 castables,there was no obvious corrosion by molten aluminum alloy when the amount of CA₂ addition was more than 59%.For CA₆-CA₂ castables,when the addition of CA₂ was more than 60%,the castables were free from corrosion.The corrosion resistance of CA₆-CA₂ castables was better than corundum-CA₂ castables.The corrosion resistance of corundum castable was improved when the experimental temperature was 1000℃,while it decreased with the addition of CA₆ or CA₂.（4）Additives（BS,CF,AP）were effective for improving the corrosion resistance of castables.The effect of adding CaF₂ was the best,and it was equivalent for BaSO₄ and AlPO₄.（5）CaF₂ could promote the formation of anorthite.There were no metal penetration and corrosion occurred on the samples containing CaF₂ which was not wetted by molten aluminum alloy at 850℃and 1000℃.But the surface of the sample was attached with a discontinuous layer composed of MaF₂ and Mg-spinel after erosion at 1000°C.After firing at 900°C and 1000°C,AlPO₄ mainly undergoed a crystallographic phase change from the hexagonal to the orthorhombic structure.The sample containing AlPO₄ reacted with aluminum alloy at the interfacial layer to form dense corundum and Mg-spinel deposition,which could prevent further penetration of the metal.The presence of reactive alumina and microsilica in the matrix promoted the decomposition of BaSO4,so part of BaSO4 reacted with the matrix to form hexa-celsian after firing at 900°C,and the formation of hexa-celsian increased when temperature increased to 1000°C.The formation of hexa-celsian could reduce the wettability of the samples with molten aluminum alloy.