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I recently saw a video on gallium infiltration on YouTube. Gallium is a metal with a melting point somewhere between standard room temperature and human body temperature, meaning if you pick up a block of it sitting on the table, it will melt in your hand. If this melted metal comes into contact with solid aluminum, it will seep into it and alloy with it, decreasing the strength of the aluminum to somewhere around that of tissue paper.

When I saw this, I immediately had a horrifying thought: aren't airplanes made of aluminum? And when I've flown in the past, I've seen a few places where there's exposed metal. This makes me wonder if a malicious person with no regards for their own safety wouldn't be able to bring down a plane with a novelty for children that anyone can buy for a few dollars.

If someone tried to do this, is there anything in the design of modern commercial airliners that would stop them?

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    $\begingroup$ I have read a number of times (but only reference I could quickly find here is aviation.stackexchange.com/questions/62028/… ) that there are big problems if mercury contaminates an aluminum-bodied airplane. Mercury is less common these days, but not that long ago you could buy a mercury thermometer anywhere. $\endgroup$ – manassehkatz-Moving 2 Codidact Jul 15 '20 at 3:17
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    $\begingroup$ There's a difference between "making a small hole in the fuselage" and "bring down a plane". $\endgroup$ – Sanchises Jul 15 '20 at 8:33
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    $\begingroup$ As we speak, liquid metals are not allowed with passingers. Only allowed in Checkin baggage. Atleast in some asian countries. Gallium has the capacity to damage ~10-15X the size of alluminium. To bring a plane down, you need much more than few drops of gallium to do some damage. Baggage compartments are rubberized and do not expose any aircraft metals, so it should be safe. $\endgroup$ – user46196 Jul 15 '20 at 10:18
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    $\begingroup$ @kris Do you have a source for this "rubberized baggage compartment" claim? Not a single one I've ever seen is rubberized... $\endgroup$ – Ron Beyer Jul 15 '20 at 11:56
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    $\begingroup$ @kris Yes, because a lightweight flame resistant insulation is used, not flammable rubber. $\endgroup$ – Ron Beyer Jul 17 '20 at 12:24
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Gallium does not interact strongly with steel so it is unlikely to damage hydraulic pipes (these are made from steel, not aluminium). Same source says it also does not interact much with polymers so unlikely to damage the insulated wiring or rubber hoses or epoxy floor panels. It may damage some sensitive point where important device is attached to the aluminium frame, but hopefully most of such places are not single points of failure.

All it can do is to make a hole in the fuselage under the very rare circumstances. Then the aircraft will be depressurized, descend and land. It is probably still not a good idea to allow gallium in the passenger compartment.

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    $\begingroup$ Wouldn't infiltration near the wing/body junction make it possible for the wing to snap off? $\endgroup$ – Jeffrey Jul 16 '20 at 0:11
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    $\begingroup$ @Jeffrey You'd have to have a lot of gallium (at least kilograms, likely 10+ kg) to have any chance of penetrating all the way through to the structural members and weakening them sufficiently (that's a lot of aluminum you have to alloy). You'd have to keep it molten all the time and in direct contact with aluminum. But the worst part is that diffusion in solid metals is an exceedingly slow process, so it'd take ages to affect anything except the surface of any bigger chunk of aluminum. $\endgroup$ – TooTea Jul 16 '20 at 12:05
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    $\begingroup$ Most importantly, all the aluminum in an aircraft is painted or similarly coated to protect it from corrosion. You'd first have to scrape off enough of the coating to expose a significant surface area to attack. Good luck trying to do that unnoticed during a routine passenger flight. $\endgroup$ – TooTea Jul 16 '20 at 12:10
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    $\begingroup$ Awesome, thanks! All reassuring comments. $\endgroup$ – Jeffrey Jul 16 '20 at 12:22
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    $\begingroup$ So this means it is still ok for me to fling gallium and mercury all about the passenger compartment for added entertainment when the in-flight movie is not sufficiently amusing? $\endgroup$ – quiet flyer Jul 16 '20 at 16:18

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