Why You Shouldn't Take a Mercury Thermometer in Your Carry-On
Aluminum (Al) is a popular structural material because of its strength-to-weight ratio, ease of fabrication, low cost and importantly it does not easily corrode. Right on all accounts except the corrosive one. Aluminum is actually highly reactive, but has a unique ability to protect itself. When iron is exposed to oxygen, it forms a scaly barrier that easily breaks away and exposes a new surface ready to oxidize and also fall away. Al, on the other hand, forms an extremely hard layer of aluminum oxide (Al2O3), which is related to corundum, alumina, sapphire, and ruby. The Al2O3 bonds solidly to the aluminum and protects it. Effectively, it coats itself with ruby armor. Anodized Al has been chemically etched and connected to a current supply to force the growth of a thick layer of Al2O3. The layer forms even on molten Al. Enter mercury (Hg), the archenemy of Aluminum. Hg easily penetrates the Al2O3 and attacks the subsiding Al. An Aluminum beam exposed to Hg will corrode as much in a few hours as an equivalent iron beam would in many years. This is the main reason Hg is banned from being onboard aircraft. Stories exist of infiltrators painting a mercury paste on the airframes of enemy planes during WWII, causing them to fail inexplicably in midair. It may be of some concern to you that the airplanes you ride in are literally kept in the air by a few microns of Al2O3.