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