Why You Shouldn't Take a Mercury Thermometer in Your
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.