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Stanford Report, May 17, 2000

Discovering why airplanes don't rust: In the atmosphere, aluminum oxide provides protection


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