| Description |
Molten magnesium exposed to an atmosphere of air will oxidize rapidly, resulting in burning on the metal surface, melt loss and handling difficulties. If magnesium is to be used as a casting metal, the melt must be protected from this severe oxidation. The objective of this work was to study the oxidation of molten magnesium in various protective atmospheres to obtain qualitative and quantitative data on the rate and mechanism of protection. Measurements of the kinetics of the protective layer formation in various atmospheres, additive gas concentrations in air and temperatures were made by monitoring the weight gain of the samples with time. To obtain knowledge of magnesium melt protection and to find the best practical protection condition, samples were examined in atmospheres of SF6/air, CO2/air, SO2/air, and SF6/CO2/air. Experiments were performed using a thermo-gravimetric analysis (TGA) unit in the temperature range of 670 - 770°C (943 - 1043K). Reaction times of 2, 3 and 7 minutes were selected to have the closest condition to that of industry. The morphology, chemical composition and thickness of the surface films were studied using SEM/EDS and a kinetic model for the process was developed. Results showed that all the additives contributed to protection of molten magnesium. Among them, SF6 and SO2 showed best protection by forming a denser and more uniform surface film. However, the use of SO2 is not recommended due to the violent behavior observed at longer times and higher temperatures. In addition, there are some drawbacks due to its high toxicity and corrosiveness, which demand additional handling and ventilation procedures. Despite the requirement of a higher concentration in air, CO2 showed the lowest protection capabilities among all. However, addition of small amount of CO2 to a mixture of SF6 and air revealed a high inhibiting effect to molten magnesium. The effects of additive concentration, temperature and reaction time were further investigated. Increasing the additive concentration and reaction time resulted in formation of a more protective surface layer that retarded the rate of magnesium oxidation. No burning was observed in the protected samples. At 720°C (993K), the surface layer showed a better protection behavior than at lower temperatures. The samples prepared at 770°C (1043K) suffered from impaired protection due to increased vapor pressure of magnesium and rupture of the surface film. Complete rate expressions were determined for the samples treated under SF6/air and CO2/air mixtures in the temperature range of 670°C to 770°C (943K to 1043K). |
