Discovering why airplanes
don't rust: In the atmosphere, aluminum oxide provides
protection
BY MARK SHWARTZ
Did you ever wonder why
airplanes never seem to rust, despite their constant
exposure to rain, sleet and snow?
The quick answer is that
most aircraft are made of aluminum -- a chemical element
that seems to resist corrosion even when exposed to air
and water.
But the fact is that pure
aluminum reacts so readily with water that, according to
the laws of chemistry, the aluminum shell of an airplane
should actually dissolve in the rain.
Fortunately for the
airline industry, when aluminum metal is placed in the
atmosphere, a thin layer, known as aluminum oxide, forms
on the metal's surface and acts like a protective,
rust-resistant shield.
Scientists have long known
that aluminum oxide does not corrode rapidly in water,
but they have been unable to fully explain why.
Now, for the first time,
researchers have shown that liquid H2O has a surprisingly potent effect
when it comes in contact with the surface of a metal
oxide, a finding that has industrial and environmental
implications.
"Water actually
changes the structure of the solid surface," says
Gordon Brown, Jr., the Dorrell William Kirby Professor of
Earth Sciences.
Writing in the May 12
issue of the journal Science, Brown, graduate
student Thomas P. Trainor and collaborators from the
University of Chicago and Lawrence Berkeley National Lab
present the first atomic-level model of what happens when
water and aluminum oxide meet.
Shifting atoms
Aluminum oxide consists of
atoms of aluminum and oxygen bonded together.
But Brown and Trainor
discovered that, when water molecules come in contact
with aluminum oxide, the aluminum and oxygen atoms on the
surface move apart -- in some cases separating by more
than 50 percent compared to their normal molecular
positions.
As a result, when the
outer layer of aluminum oxide gets hydrated or wet, its
structure changes just
enough to become chemically inert and thus unable to
react rapidly with additional water molecules or
atmospheric oxygen. This change in molecular structure is
why aluminum oxide metal resists corrosion.
Brown notes that these
findings have widespread applications for the
multi-billion-dollar catalysis and semiconductor
industries, which are concerned with the effects of
atmospheric water on metal oxides used in chemical
catalysts and silicon chips.
However, he adds, the real
driving force for this research is the important role
that hydrated metal oxide surfaces in soils and sediments
play in removing toxic metals like lead, mercury,
chromium, arsenic, and selenium from contaminated
groundwater.
"Understanding the
molecular structure of the particle surfaces with which
these metals react is essential for determining how
effectively they are removed from the environment, and
hence how available they are to organisms, including
humans.
"Now for the first
time we have a picture of the molecular structure of one
of these surfaces and a better idea of what controls its
reaction with environmental contaminants," Brown
concludes.
To conduct their analysis
of the surface of hydrated aluminum oxide, researchers
used the most powerful synchrotron x-ray source in the
U.S. - the Advanced Photon Source located at the Argonne
National Laboratory in Illinois.
"Our research
required the brightest synchrotron x-ray source
available," says Brown. "The biggest surprise
is that we could do it at all."
The other co-authors of
the May 12 Science article are Peter J. Eng,
Mathew Newville, Steven R. Sutton and Mark L. Rivers with
the University of Chicago's Consortium for Advanced
Radiation Sources; and Glenn A. Waychunas of the Lawrence
Berkeley National Laboratory. SR
|
