Wild One-Minute Video Clearly Demonstrates Why Mercury Is Banned On Airplanes

There are plenty of items that you are not allowed on planes, and usually for fairly obvious reasons. You cannot, for instance, bring ammunition on board. Nor slingshots, firearms, or harpoon guns. Flammable liquids are also a no no, as well as any item containing its own internal combustion engine.

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But a ban that sometimes seems weird to people is that you are not allowed to bring a mercury-filled thermometer onto a commercial aircraft. So what is the danger here? Can thermometers be weaponized? Or does "The Man" just not like you knowing the cabin temperature?

Well, it's probably easier to just show you the problem.

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As you can see, mercury (Hg) and aluminum (Al) do not mix well together. Or they do, but not usefully when your plane is made (largely or partly) of this lightweight metal.

"A mercury-aluminum amalgam is a mixture of mercury and aluminum. Aluminium in air is ordinarily protected by a molecule-thin layer of its own oxide (which is not porous to oxygen)," Chem Europe explains. "Mercury coming into contact with this oxide does no harm. However, if any elemental aluminium is exposed (even by a recent scratch), the mercury may combine with it, starting the process described above, and potentially damaging a large part of the aluminium before it finally ends."

It is this runaway reaction, known as amalgam corrosion, that is the main problem, meaning that only a small amount of mercury can lead to a lot of damage.

"Amalgam corrosion is the combined action of Hg and moisture on susceptible materials, primarily aluminum and tin. The difference between this mode of attack and simple amalgamation is that the corrosion process propagates with minuscule amounts of mercury," a paper on the topic explains.

"Amalgam corrosion regenerates the reactant and hence is self-propagating. If sufficient moisture and mercury are present, aluminum structural components can be penetrated fairly rapidly."

In short, you can't take mercury thermometers on planes in case they break and begin corroding the aircraft. You wouldn't want it to make a hole; those belong on plane windows.