Most metals, with the notable exception of gold, tend to oxidize when exposed to air and water. This reaction — which produces rust on iron, tarnish on silver, and verdigris on copper or brass — can weaken the metal over time and lead to cracks or structural failure. There are three known elements that produce an oxide that can actually serve as a protective barrier to prevent any further oxidation: aluminum oxide, chromium oxide, and silicon dioxide.
Researchers have found that a solid oxide protective coating for metals can, when applied in sufficiently thin layers, deform as if it were a liquid, filling any cracks and gaps as they form. The thin coating layer should be especially useful to prevent leakage of tiny molecules that can penetrate through most materials such as hydrogen gas that could be used to power fuel-cell cars, or the radioactive tritium (a heavy form of hydrogen) that forms inside the cores of nuclear power plants.
Metals under stress from pressure inside a reactor vessel and exposed to an environment of superheated steam can corrode quickly if not protected. Even with a solid protective layer, cracks can form that allow the oxygen to penetrate to the bare metal surface, where it can then penetrate into interfaces between the metal grains that make up a bulk metal material, causing further corrosion that can penetrate deeper and lead to structural failure. It turns out that aluminum oxide can have just that liquid-like flowing behavior, even at room temperature, if it is made into a thin enough layer — about 2 to 3 nanometers thick.
This approach demonstrated that an aluminum oxide layer that is normally so brittle it would shatter under stress, when made exceedingly thin is almost as deformable as a comparably thin layer of aluminum metal — a layer much thinner than aluminum foil. When the aluminum oxide is coated onto a surface of a bulk piece of aluminum, the liquid-like flow keeps the aluminum covered with its protective layer.
The aluminum with its oxide coating could be stretched to more than double its length without causing any cracks to open up. The oxide forms a uniform conformal layer that protects the surface, with no grain boundaries or cracks, even under the strain of that stretching. Technically, the material is a kind of glass, but it moves like a liquid and fully coats the surface as long as it is thin enough.