It's better to burn out, than it is to rust...
— Neil Young, "My My, Hey Hey (Out of the Blue)", Rust Never Sleeps
Aluminum is rusty; that's what makes it useful. Really. There is a fine surface coat of aluminum oxide — rust — that protects the rest of the metal from oxidation. Without that layer, aluminum would be useless, because it would corrode (oxidize) while we watched. Copper is similar, which is why it was used for roofing. (That very same green coating on copper roofs is identical to the oxide coating the Statue of Liberty.)
Iron is quite different, because its oxide coating flakes off instead of tightly adhering to the surface. This means that new, unreacted iron is constantly being made available to oxygen's deadly embrace. Aluminum, to contrast, always has a hard layer of aluminum oxide (Al2O3) on the surface. To give you an idea how hard it is, this same molecule is the building block for abrasives and gemstones like corundum, alumina, sapphire, and ruby.
I found it interesting that the delicate oxide coat can be disrupted by mercury. Once this happens, the protective oxide layer fails to form and the aluminum literally crumbles before our eyes. This photograph shows what happens when an I-beam comes in contact with mercury; it corrodes as the seconds tick by. The photo above was taken after only an hour after mercury was applied. The problem is not just in the lab; it exists for any critical structure that might come into contact with mercury. Like, oh, say, airplanes. Yup, airplanes. Boeing's maintenance manual for the 747 specifically sets forth the risks:
The spillage of mercury or a mercury compound, within an airplane, requires immediate action for its isolationg and recovery to prevent possible corrosion damage to and possible embrittlement of aluminum alloy structural components. C. All metallic aircraft structure which is wetted by elemental mercury suffers severe degradation in strength. The rate of diffusion of mercury into a metal is dependent on the specific metal contacted and the protective finish applied; however, once diffusion has started it cannot be stopped.Boeing 747 Maintenance Manual Guidelines for Mercury Spills
The problem was recently written up by Popular Science, with some hype about terrorists. I've come across stories from the 1970s, during the peak of the terrorist-hijacking epidemic, about professors who were more worried about mercury being applied to aircraft than they were about bombs.
Unless you are a representative of a national meteorological bureau licensed to carry a barometer (and odds are you’re not), bringing mercury onboard an airplane is strictly forbidden. Why? If it got loose, it could rust the plane to pieces before it had a chance to land. You see, airplanes are made of aluminum, and aluminum is highly unstable.
Applied to aluminum’s surface, mercury will infiltrate the metal and disrupt its protective coating, allowing it to “rust” (in the more destructive sense) continuously by preventing a new layer of oxide from forming. The aluminum I-beam below rusted half away in a few hours, something that would have taken an iron beam years.
I’ve heard that during World War II, commandos were sent deep into German territory to smear mercury paste on aircraft to make them inexplicably fall apart. Whether the story is true or not, the sabotage would have worked. The few-micron-thick layer of aluminum oxide is the only thing holding an airplane together. Think about that the next time you’re flying. Or maybe it’s better if you don’t."The Amazing Rusting Aluminum", by Theodore Gray, Popular Science
This is not news; there's an old magic trick called "hypno heat" which involves taking a piece of aluminum foil, typically from a stick of gum a cigarette pack, and reacting it with HgCl2 (mercury bichloride) which used to be widely available as an antiseptic. (Before people realized that getting mercury into the body was very, very bad.) The aluminum oxidizes, giving off heat, which is attributed to the abilities of the magician. Viking Magic, to my amazement, still sells it by special request:
Question: I have a document created by you in 1989, and revised in 1995 titled: "Hypno heat/hot & cold-The tin foil trick". I was given hypno heat by an old friend in both solid pellet, and liquid form, but cannot find any suppliers in the UK. Could you tell me if you, or anyone you know supplies it please. Thank you in advance.
Answer: Hypno-Heat is a mercury by-product as as such can be dangerous if mishandled. I have been using HH for my own use for over 40 years with no adverse affects but then I am cautious and I know how to handle it. This item is not available to the general public any more but if you write me directly, I can put you in touch with it: NOSPAMhaenchen@msn.NOSPAM.com Do NOT use the liquid form. This is very dangerous as it is absorbed into the skin on contact. As with all chemmicals, keep this out of the hands of children or anyone not professional enough to handle it.Viking Magic FAQ
One has to be really, really, really stupid to handle any mercury compound, even if you aren't on an aircraft.
It's better to burn out, 'cause rust never sleeps...
— Neil Young, "Hey Hey, My My (Into the Black)", Rust Never Sleeps