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.

  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 sosurface.

  Aluminum oxide consists of atoms of aluminum and oxygen bonded together.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.