Corundum (Al2O3) forms when molten aluminum is in contact with refractory, accelerated by oxygen.
External corundum grows above the metal line, outwardly from the hot face of the refractory lining, often appearing as mushroom-shaped corrosion. Here, corundum growth after four months is shown.
Heat and oxygen fuel growth
Corundum growth in the same furnace after 13 months. Heat and atmospheric oxygen continue to feed the corundum reaction in the furnace chamber.
Corundum under heat expansion cracks the refractory lining.