Oxidation of aluminum-magnesium melts in air, oxygen, flue gas, and carbon dioxide

Abstract

Oxidation rates of aluminum—1 to 14 pct magnesium melts in air, oxygen, flue gas, and carbon dioxide at temperatures from 600 to 1100°C were measured with an automatic recording balance. For most conditions, a protective amorphous film on the melt surface kept the oxidation rate low initially. After an interval that was shortened by increasing temperature or magnesium content, the film crystallized to magnesium oxide and magnesium aluminate, accompanied by a sudden increase in oxidation rate. Known as breakaway oxidation, this phenomenon could be produced by adding crystalline magnesium oxide or magnesium aluminate seed to melts protected with amorphous oxide. Flue gas or carbon dioxide in the atmosphere, sodium or beryllium in the alloy, or boron dusted on the surface delayed crystallization unless seed was added. Beryllium was the most effective. Flue gas from burning natural gas delayed breakaway oxidation of unseeded melts containing up to about 4 pct magnesium at normal melting furnace temperatures near 750°C. Slow melting of solid aluminum-magnesium alloy concentrated magnesium in the first liquid to melt, sharply decreasing the interval before breakaway oxidation after melting. Rapidly melting the solid increased the protective interval. Homogenizing the solid just below the melting range before rapid melting further extended the interval before the onset of breakaway oxidation.