Effects of Protective Gases on the Oxidation Behavior of Mg-Ca Base Molten Alloys

The effects of inert gases, such as argon, nitrogen and carbon dioxide, on the liquid oxidation behavior of Mg-Ca and Mg-Ca-Al alloys were investigated. With increasing the flow rate of inert gases in oxidation chamber, ignition and burning of molten Mg-Ca base alloys were suppressed. Argon has no reaction with magnesium. Thus, its high flow rate was required to prevent the melt from ignition. Nitrogen was analyzed as the most effective as a protective gas among them. The surface of Mg-Ca and Mg-Ca-Al alloys after oxidation treatment at 700 degreesC was clean and smooth without even partial ignition. Oxide film having 1similar to2 mum in thickness was dense and compact. It was confirmed that the oxide film was mixed with nitrides such as Mg3N2 and Ca3N2 together with CaO and MgO. Thus, once the protective film was formed at surface further oxidation was suppressed. However, the use of carbon dioxide was quite different in reacting with the molten Mg alloys. The surface was covered with black thick film of 10similar to20 mum in thickness. Cross-sectional observation showed that there were two distinct porous layers of a carbon-rich layer identified as CaC2 and Mg2C3, and oxygen-rich layer of CaO and MgO.