Effects of Different Calcium Compounds on the Corrosion Resistance of Andalusite-Based Low-Cement Castables in Contact w
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TRODUCTION
MELTING and holding furnaces in aluminum casthouses generally have used aluminosilicate refractories containing different chemical additives from the late 1970s.[1] Several additives have been used in the past to improve the resistance of the refractory to corrosion by molten aluminum, such as zircon, barium sulfate, calcium fluoride, glass frits,[1] wollastonite,[2–4] aluminum fluoride, aluminum borate,[5] chromium oxide, aluminum titanate, cryolite,[6,7] barium celsian,[8–10] and ceria.[11] The additives work by decreasing the wettability of the molten aluminum alloys with the refractory, thereby decreasing the extent of corrosion and associated molten metal penetration.[12] Although these additives have been used for several decades, ambiguity ESMAEIL ADABIFIROOZJAEI, M.Sc. Student, ALI SAIDI and AHMAD MONSHI, Professors, are with the Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran. Contact e-mail: e.adabifi[email protected] PRAMOD KOSHY, Doctor, is with the School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia. Manuscript submitted June 11, 2010. Article published online January 11, 2011. 400—VOLUME 42B, APRIL 2011
exists in regard to the mechanisms responsible for the improvement in the corrosion resistance. The nature and quantity of the materials used is believed to be influenced by the silica level of the refractories[13] and the operating temperatures. However, the addition of these materials to aluminosilicate refractories also influences the physical, mechanical, and corrosion properties of the refractories.[12,14] Previous research on calcium-containing additives has focused on CaF2, which has been effective in improving the wetting resistance of aluminosilicate refractories.[5,8,12–14] The efficiency of CaF2 addition has been shown to depend on the quantity used and the temperatures involved.[14] The addition of CaF2 to an aluminosilicate composition (close to mullite) was shown either to increase or to decrease the corrosion resistance. This response is dependent on the amounts of additive used as a result of the formation of noncrystalline phases that easily undergo reactions with the molten aluminum alloy.[5] Previous studies have shown that the presence of both high CaF2 contents and higher firing temperatures tend to favor the formation of larger quantities of anorthite, leading to an increase in the corrosion resistance. However, the use of 3.5 wt pct calcium fluoride resulted in extensive cracking, which METALLURGICAL AND MATERIALS TRANSACTIONS B
enhanced metal penetration and contributed to severe corrosion in the refractory.[12] However, Koshy et al.[13] showed that high-alumina refractories containing less than 5 wt pct CaF2 showed good resistance to reactions with the molten aluminum alloy because of the presence of corundum and anorthite at the interface, although the temperatures were higher than the former study (~1523 K [1250 C]). With continued increase in the additive content, a glas
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