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From: "Barry L. Ornitz" <ornitz@dpnet.net>
Subject: Re: Aluminium Alloys and Mercury & "FEATHERS"
Date: 15 Dec 1998
Newsgroups: sci.engr.chem

David Konneker wrote in message <36703C7A.5F7262FC@lmco.com>...
>I am interested in the reaction that would make aircraft aluminum
>unuseable after exposure to spilled mercury in the cargo compartment of
>an airliner.


I thought surely someone would answer this quickly.

Aluminum is a very reactive metal but its surface is normally protected by
a thin oxide layer that prevents further oxidation.

Mercury can amalgamate with aluminum if it can penetrate below the oxide
surface.

When an amalgamated aluminum surface is exposed to the oxygen in air, the
aluminum oxidizes leaving the mercury behind to further amalgamate with the
fresh aluminum surface below.  The oxide produced is very light and porous
(often called feathers which will be obvious if you perform the
experiment).  This oxide layer will build up and eventually flake off.  In
the mean while, the aluminum amalgam keeps being oxidized until it is all
used up (or until all of the mercury is lost to the "feathers").

To demonstrate this, take a small test tube and add a drop of mercury.
Then add a few CC of dilute hydrochloric acid.  Take a long aluminum wire
and dip it in the test tube.  Leave the end in the acid several seconds
until the oxide layer is dissolved before pushing it into the mercury.
Withdraw the wire and wash it in water.  Leave the wire hanging in a place
with very still air.  Within minutes, long "feathers" will begin growing on
the end coated with mercury.  These can grow to several feet in length if
the air is very still.  The "feathers" are aluminum oxide.  After a period
of time, note that the wire diameter that was coated in mercury will be
smaller from the amount of aluminum lost to oxidation.

But let me add a note of warning here.  Some mercury will be carried over
into the "feathers" so handling them is _quite_ dangerous.  Since these
"feathers" are so friable, it is easy to get mercury vapor in the air.
Remember the extreme toxicity of mercury vapor if you perform this
experiment.

I would expect a few drops of mercury to not be too harmful to an aircraft.
Remember the mercury will not amalgamate with the oxide-coated aluminum.
But a scratch or such might provide a place to start.  Once started, the
process continues for quite a while.

          Dr.Barry L. Ornitz          ornitz@dpnet.net




 
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