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Magnesium is lighter than aluminum for the same strength. If the answer is corrosion, how much work has been done to improve the corrosion resistance of magnesium? How much lighter would a magnesium structure be than an equivalent aluminum structure?

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    $\begingroup$ How would a post-crash fire interact with a magnesium skin? $\endgroup$ – casey Feb 14 '15 at 8:36
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    $\begingroup$ Poor corrosion resistance is the major concern but it is used with aluminum to increase it's corrosion resistance.Recent studies found out yttrium also used for improving the resistance. $\endgroup$ – Katz Feb 14 '15 at 15:40
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    $\begingroup$ you obviously have never done the high-school chemistry experiment of setting a tiny piece of magnesium alight $\endgroup$ – Federico Feb 14 '15 at 16:42
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    $\begingroup$ You might want to look into aluminium magnesium alloys which have been used in aircraft. You perhaps (?) seem to have the idea that it's a very much one-or-the-other deal: Note that even aircraft aluminium contain a mix of around 10% other metals for alloying. There's a huge variety of aluminium alloys and correspondingly their properties. $\endgroup$ – Thunderstrike Feb 14 '15 at 17:55
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    $\begingroup$ Because it is expensive, a pain in the ass to machine and it burns like a son of a bitch. $\endgroup$ – Tyler Durden Feb 18 '15 at 5:00
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Magnesium has initially been held back by technical difficulties in the removal of chloride inclusions. These had been solved only after a lot of progress had been made in aluminum alloys. Also, magnesium parts should only be used in a dry environment - when the metal is exposed to water, rapid corrosion must be expected.

The first application of magnesium in aerostructures was in the 1930s (mostly under the trade name Elektron), with a small boom in the 1950s. Lockheed even built one F-80C fully from magnesium. Magnesium was widely used in the former Soviet Union, but eventually phased out in newer designs due to corrosion problems. Flammability, by the way, was never a real concern with magnesium.

With improved protection techniques, magnesium might make a limited come-back. It is already used in castings and extruded parts. Per kilogram, it is twice as expensive than aluminum, but very easy to machine. However, at higher temperature it will lose strength even faster than aluminum.

Parallel to that, aluminum sheet can be made lighter and stiffer by adding up to 2.5% of lithium. These Al-Li alloys have found their way into the newest aircraft designs and will raise the bar for magnesium applications.

Magnesium and water really don't mix well: Trying to extinguish a magnesium fire with water is a memorable experience, as adding water will intensify the reaction. Use iron chips or sand instead to cool down the burning metal and to keep oxygen away. Historically, though, magnesium fires on aircraft have been extremely rare.

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    $\begingroup$ @JayCarr: When Wright used cast magnesium crankcases for their Duplex Cyclones, early B-29s had the misfortune of the magnesium burning through the wing spar when the engine caught fire. The flammability issue is real, but rare. $\endgroup$ – Peter Kämpf Feb 14 '15 at 18:23
  • $\begingroup$ How much worse is a magnesium based alloy in a fire compared to aluminum or composites? $\endgroup$ – Brinn Belyea Feb 14 '15 at 19:40
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    $\begingroup$ @brinnb: Magnesium is certainly worse, if only for the fact that firefighting it is not easy and needs very targeted means. Composites can be made almost self-extinguishing by adding the right flame retardants, but this reduces their strength. But aluminum will also burn if heated enough - it is best to avoid a fire altogether. $\endgroup$ – Peter Kämpf Feb 14 '15 at 20:27
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    $\begingroup$ @PeterKämpf "it is best to avoid a fire altogether" I'm sure many many people agree with you, but that's a tall order $\endgroup$ – raptortech97 Feb 14 '15 at 20:55
  • $\begingroup$ @raptortech97: A lot can be done - look at how carefully the Zeppelins were designed to avoid sparks or the collection of hydrogen-oxygen mixtures. Using magnesium instead of aluminum wouldn't had changed the fire risk. $\endgroup$ – Peter Kämpf Feb 15 '15 at 6:55

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