Recently, researchers at MIT have discovered a protective coating of solid oxide that can be used in metal containers. When the coating is very thin, it deforms like a liquid, filling all possible cracks and gaps. The study was published in the journal Nano Letters.
Most metals (except gold) are usually oxidized when exposed to air and water. Oxidation will gradually weaken the metal and eventually lead to cracks or structural failure. For example, when oxidized, the iron surface produces rust, the silver becomes dull, and the copper/bronze surface produces copper rust. However, there are three known elements that are oxidized to produce the protective oxide, which prevents further oxidation. They are aluminum oxide, chromium oxide, and silicon dioxide.
“We have been trying to study why alumina and silica have excellent corrosion resistance.” Ju Li said, the nuclear engineering, materials science and engineering professor at the MIT.
The metal is subjected to pressure in the reaction vessel or exposed to superheated steam, which will be corroded rapidly without protection. Even if it is coated with a solid protective coating, it still cracks, which make oxygen penetrate to the metal surface and then penetrate into the metal particle interface, leading to deeper corrosion and ultimately resulting in structural failure. “We want an oxide that acts like a liquid, preventing cracks,” Yang said, a graduate student at MIT.
The results suggest that if it is thin enough to be about 2-3nm, alumina can behave like a liquid, even at room temperature. The alumina coating is usually so fragile that it can be damaged under pressure. And if it’s a very thin coating, it can be as deformed as a metal aluminum thin layer (thinner than aluminum foil). When the alumina coating is coated on the surface of a block of aluminum, the flow of similar liquid causes it to cover the aluminum surface and form a protective layer.
With E-tem, the researchers observed that aluminum coated with alumina could stretch to more than 2 times the length of the metal without any cracks. The oxide forms a very uniform layer (no grain boundary or crack), and metal surfaces can be protected even when stretched. Professionally speaking, the material is a glass, but as long as it is thin enough, it can move like a liquid and completely cover the metal surface.
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