Evaluation of the Effect of CO₂ Cover Gas on the Rate of Oxidation of an AlMgSi Alloy

Small additions of beryllium (Be) to aluminum magnesium (AlMg) alloys have proven to decrease their oxidation rate during industrial liquid metal handling. As Be can cause respiratory health issues, it is desirable to evaluate alternative methods to inhibit the oxidation rate. Earlier work has revealed that small amounts of carbon dioxide (CO₂) to the surrounding atmosphere has a positive effect. In the present study the oxidation behavior of an aluminum magnesium silicon (AlMgSi) alloy has been investigated using a Differential Scanning Calorimetric (DSC) unit equipped with a Thermogravimetric Analyzer (TGA). Changes in both the heat flux and the mass have been monitored during exposer to 20% argon (Ar) and 80% synthetic air, 99.999% pure Ar, and a gaseous mixture of 20% Ar, 76% synthetic air and 4% CO₂ at 750 °C for 7 h. The results revealed a one-step mass gain when heated in synthetic air, giving a total mass gain of 12.33% and an oxide layer thickness of >15 µm. Pure Ar had a positive effect on the oxidation rate lowering the mass gain to 2.80% and a thickness of ~10 µm. A mass gain of only 0.46% and a continuous dense oxide layer of 200–400 nm, with an additional granular discontinuous oxide layer of ~2 µm underneath, was obtained during heating in 4% CO₂. This confirms that even in the case of the AlMgSi alloy, small amounts of CO₂ have a significant inhibiting effect on the oxidation rate.