Oxidation of molten Al-Mg alloy in air, air-SO₂, and air-H₂S atmospheres

Abstract

Rates of oxidation of 8 g samples of molten Al-Mg alloys in air, air-SO₂, and air-H₂S atmospheres were determined at 750°C by gravimetry. Weight gains in air containing 10 or more volume percent SO₂ averaged 0.2 % in up to 100 hr, whereas samples heated in air alone gained over 5% weight in 20 hr and 10% in 70 hr. Heating for only 1 hr in SO₂ concentrations of 10% or greater prevented extensive oxidation during additional heating in air alone. H₂S was more effective than SO₂ heating for as little as 1/2hr in 0.25 vol. % H₂S prevented extensive oxidation during subsequent heating for at least 90 hr in air alone. The inhibiting effect of either SO₂ or H₂S probably involves oxidation to SO₃. This reacts with portions of the initial protective amorphous MgO film to form MgSO₄, which has a high volume quotient and maintains protection as nonprotective crystalline MgO forms at the end of an induction period.