EFFECT OF ALKALIS AND OF A REDUCING ATMOSPHERE ON THE CORROSION OF REFRACTORIES BY MOLTEN ALUMINUM

The corrosion of aluminosilicate refractories by molten aluminum, such as in melting and holding furnaces, generally leads to the formation of an alumina deposit on the refractory. In the presence of alkalies and under a reducing atmosphere the extent of corrosion is increased. This could be related to the transformation of the alumina to sodium aluminate, whose formation kinetics are increased under a reducing atmosphere, promoting the formation of aluminum nitride. In melting and holding furnaces, the alkalies could originate from the presence of a cryolitic bath in the metal charge and/or from the refractories. In the latter case, it was shown that aluminum nonwetting refractory castables containing aluminum fluoride as the nonwetting agent could lead to severe corrosion in the above applications if there alkali content is such that it promotes the formation of beta-alumina.