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Magnesium


Photograph of magnesium rod

U.S. Geological Survey. Via Wikimedia Commons

Magnesium, atomic number 12, is the lightest metal that is abundant enough, strong enough, and durable enough to be used as a structural metal. Though so reactive that it is never found in nature as the free metal, it resembles aluminum in forming a thin oxide coating that protects the underlying metal from further corrosion in air. The pure metal is fairly strong but quite soft (Mohs hardness 2.5), and it slowly corrodes in water with the release of explosive hydrogen gas. However, magnesium forms hard, lightweight, durable alloys with aluminum and other metals that are invaluable for aircraft components. The chief drawback of magnesium as a structural metal is its tendency to catch fire at a relatively low temperature, whereupon it burns rapidly and intensely. In fact, the brilliant white light of burning magnesium made it an ingredient for flares. Because of this fire danger, magnesium alloys were used primarily in aircraft engines, ailerons, turrets, and landing gear, rather than airframes.

Magnesium is essential to life and its sulfate was included in some fertilizers. The oxides were used to manufacture refractory bricks for high-temperature furnaces, such as those used to refine steel.

Magnesium is the eighth most abundant element in the earth’s crust, making up about 2% of its weight. It is even more abundant in seawater, and the United States turned to this source when the war increased demand for magnesium. Calcium hydroxide (slaked lime) was added to seawater, precipitating out the magnesium as magnesium hydroxide. This was filtered out and redissolved in hydrochloric acid to produce purified magnesium chloride. The magnesium chloride was then electrolyzed to produce the free metal, in a process not unlike that for aluminum. The process was expensive, and the price of magnesium was $507 per ton in 1941, significantly greater than aluminum at $374 per ton. Japan conducted only small-scale experiments with magnesium extraction from seawater, leaving her dependent on magnesite and other ores mined in her Asian empire.

U.S. production was just 4000 tons in 1940, in part because of German dominance of the market.  As a result, 97% of magnesium was imported before war broke out. Thereafter development of domestic magnesium production was centered in the western United States, where eight states accounted for 60% of American production. Production peaked at 167,000 tons in 1943, short of the original goal of 200,000 tons per year. Henry Kaiser built the Permanente magnesium plant, but this ran into considerable difficulties, not perfecting its carbothermic process until the summer of 1943, Production soared thereafter.

Peak production in Japan was just 6000 tons per year in May 1944. Because Japan never developed much magnesium smelting capacity, Japanese aircraft used relatively little magnesium compared with those of the other major powers.

References

Cohen (1949)

Klein (2013)

Miller (2007)

U.S. Geological Survey (accessed 2009-3-31)


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