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A role of CO2 in suppresing oxidation of molten Al-Mg alloys in air-CO2 atmosphere was studied. The surface film on the molten Al-Mg alloys in air-CO2 atmosphere is composed of MgO with a slight amount of carbon.Carbon does not form carbon compounds such as MgCO3. CO2 functions more effectively for the molten metal on which a thick MgO film covers, but does ineffectively for the molten metal on which thin MgO and/or MgAl2O4 films cover. The protecting ability of surface film depends on the concentration of CO2 at the film forming period, but does not on the gas applying time in the period. This phenomenon is attributed to prevention of O2 diffusion into the molten metal by the MgO film which adsorbs CO2 gas.

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