Mercury-aluminum amalgam

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). 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 (Ornitz 1998).

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For this reason, restrictions are placed on the use and handling of mercury in proximity with aluminium. In particular, mercury is not allowed aboard aircraft under most circumstances because of the risk of it forming amalgam with exposed aluminium parts in the aircraft. In the Second World War, mercury was used to sabotage aircraft.^{[citation needed]}

This amalgam is also used as a chemical reagent to reduce compounds, such as the reduction of imines to amines. Since this reaction produces waste mercury metal, it is best avoided in favor of more environmentally friendly reagents such as hydrides. The reaction tailings must be properly disposed of by a hazardous waste management company.

This reaction was popularized by Alexander Shulgin in his book PiHKAL, but has few virtues beyond being inexpensive and using chemicals that are readily available.

Normally pieces of aluminum aren't very reactive because they are covered with a thin layer of inert aluminum oxide (Al₂O₃). The mercury in the solution allows this protective layer to be removed, then prevents its (otherwise very rapid) re-formation by creating a thin layer of mercury over the bare aluminum. The net result is similar to the mercury electrodes often used in electrochemistry, except instead of providing electrons from an electrical supply they are provided by the aluminum (which becomes oxidized in the process.) The reaction that occurs at the surface of the amalgam may actually be a hydrogenation rather than a reduction.

The presence of water in the solution is reportedly helpful (even necessary); the electron rich amalgam will reduce water to hydroxide, creating aluminum hydroxide (Al(OH)₃) and hydrogen gas (H₂).