Magnesium is the lightest structural metal in engineering use, with a density only about two-thirds that of aluminum, so every application that pursues weight reduction takes a special interest in it. In transportation, magnesium-alloy die castings are used for steering-wheel frames, instrument-panel brackets, seat frames and transmission housings, helping whole vehicles lose weight and save fuel; in aviation and aerospace, magnesium alloys are used in some fuselage panels, landing-gear components and helicopter gearbox housings, where their specific strength stands out among lightweight structural materials. Consumer electronics are another fast-growing market: magnesium-alloy die castings are used for laptop shells, mobile-phone frames and video-camera bodies, combining thinness, lightweight, heat dissipation and electromagnetic shielding. In chemicals, magnesium is widely used as a sacrificial anode for cathodic protection of buried pipelines, storage tanks and ship hulls, protecting steel by corroding preferentially. In metallurgy, magnesium is a desulfurizer and nodulizer; adding a small amount of magnesium to cast iron transforms graphite into nodules, yielding excellent ductile iron. Magnesium is flammable, so special fire precautions are required during processing and storage; this limits some of its uses, but it also makes it a traditional fuel in fireworks, flares and pyrotechnic compositions.
Magnesium oxide (MgO), commonly called magnesia, has a melting point as high as 2,800°C and is the principal basic refractory material for steelmaking furnaces, cement kilns and electric-furnace linings; it is also used to make magnesia cement and as a thermally conductive plastic filler. Magnesium hydroxide (Mg(OH)2) is a weakly alkaline flame retardant that decomposes endothermically on heating and releases water; it is widely used to retard flame and suppress smoke in plastics, rubber and cable compounds, and is also used as an acid wastewater neutralizer. Magnesium sulfate (MgSO4), commonly encountered as the heptahydrate Epsom salt, is taken orally as a laxative and applied externally to reduce swelling; in agriculture it is used as a magnesium-supplement fertilizer, and in industry it is used in dyeing and fermentation. Magnesium chloride (MgCl2) is an intermediate in extracting magnesium from seawater and brine lakes; electrolyzing it yields metallic magnesium, and its aqueous solution is also used as a road dust suppressant and deicer, and in the manufacture of magnesia oxychloride cement. In addition, magnesium oxide and magnesium hydroxide are used in large quantities in flue-gas desulfurization and acidic wastewater treatment, and magnesium oxide is also a magnesium supplement in animal feed and plant fertilizers.
