Effects of Different Calcium Compounds on the Corrosion Resistance of Andalusite-Based Low-Cement Castables in Contact with Molten Al-Alloy

Abstract

Andalusite containing low-cement castables (LCCs) have been used in aluminum casthouses for several decades. CaF 2 is commonly added to the refractory to improve its corrosion resistance mainly because of its role in the formation of anorthite (CaAl 2 Si 2 O 8 ); the latter has been reported to decrease the penetration of molten aluminum alloys into refractories. This article investigates the effect of the addition of different calcium containing compounds (CaO, CaCO 3 , CaSO 4 , CaF 2 , Clinker white cement, calcia feldspar, wollastonite, and Ca 3 (PO 4 ) 2 ) on reactions with the refractory constituents to form anorthite as well as the effect of the additives on both the subsequent physical properties and the corrosion resistance of andalusite LCC refractories. Corrosion tests using the Alcoa cup test at temperatures (1123 K [850°C] for 150 hours and 1433 K [1160°C] for 72 hours) were conducted to determine the extent of penetration, whereas immersion tests in boiling water were conducted to determine the extent of open porosity in the material. Scanning electron microscopy coupled with energy dispersive spectrometer, optical microscopy, and X-ray diffraction techniques were employed to characterize the phase formations in the materials after the tests. The study demonstrated that both calcia feldspar and clinker white cement had the potential to be used as new additives for decreasing the penetration of molten Al-alloy into the refractory materials. Anorthite formation (in the refractory matrix), along with the absence of glassy phases, were responsible for the improvement in the corrosion resistance of the castables containing calcia feldspar. However, in the sample containing cement, the presence of calcium silicate phases were observed to resist reactions with molten aluminum. The observed results were validated using thermodynamic calculations, which indicated that tricalcium silicates (3CaO.SiO 2 ) and dicalcium silicate (2CaO.SiO 2 ) phases were more resistant than wollastonite (CaSiO 3 ) for applications involving contact with molten aluminum.

Effects of Different Barium Compounds on the Corrosion Resistance of Andalusite-Based Low-Cement Castables in Contact with Molten Al-Alloy

“…Calcium aluminate phases are more resistant to molten aluminum alloy reactions than anorthite phase, which was concluded by our previous study [57] and is also shown by Eqs. [11] and [12] (Table I).…”

Synthesis of Al6Si2O13-BaAl2Si2O8-ZrO2-based composites and their wettability by molten Al and an Al-Si alloy

“…Different calcium compounds (CaO, CaCO 3 , CaSO 4 , CaF 2 , clinker white cement, calcia feldspar, wollastonite and Ca 3 (PO 4 ) 2 ) were used as additives in andalusite-based low-cement castables in contact with a molten Al-alloy, demonstrating that the addition of white cement clinker and calcia feldspar to the matrix is effective in decreasing the penetration of molten Al. The presence of CaO, CaCO 3 , CaF 2 , wollastonite and Ca 3 (PO 4 ) 2 did not assist in improving the corrosion resistance of the samples [21].…”

Effects of Different Boron Compounds on the Corrosion Resistance of Andalusite-Based Low-Cement Castables in Contact with Molten Al Alloy

“…The method of preparing the test samples and the procedure for the Alcoa cup test are listed in our previous publications. [7,8] All raw materials for the tests were provided by Amol Refractory Industries Company, Iran, and were of industrial purity. Figure 1 shows the images of the sectioned samples after testing at different temperatures.…”

“…The base refractory (BS1100) contains andalusite as the major phase, with kyanite, corundum, anorthite, and cristobalite as minors. Andalusite and kyanite are polymorphs that tend to coexist naturally [9] ; anorthite forms from reactions between cement and microsilica, [8,[10][11][12] whereas cristobalite forms from microsilica transformations. [1] The absence of nonwetting agents that are referred as additive material results in reactions of Al alloy with free silica (microsilica), and fine andalusite particles.…”

“…The calculated value for aluminoborate phase shows that the feasibility for this reaction is comparable with that of the reduction of silica (-128.22 KJ/mol Al), whereas it is more feasible compared with the reduction of both anorthie (-39.66 KJ/mol Al) and andalusite (-43.43 KJ/mol Al) with the Al alloy at this temperature. [8] A high heat of formation implies strong interatomic bonds and correspondingly weak interaction with metals, and in the case of oxides, wettability decreases with an increase in the free energy of oxide formation. An increase in ionic character generally leads to an increase in the thermodynamic stability of these compounds.…”