Metals may seem tough, but given enough time even plain old air can be their undoing. Oxygen reacts with it to form metal oxides like rust, but now it turns out a metal oxide may come to the rescue. An MIT team has found that aluminum can be protected from further oxidation by a thin coating of aluminum oxide, which acts like a liquid, flowing to fill any gaps as they form.
If metal oxides like rust or tarnish are allowed to build up, they can eat away at the metal and eventually cause it to crack. But not all metal oxides are so destructive. Three "special" oxides – aluminum oxide, chromium oxide and silicon dioxide – can actually work to protect metal from further corrosion. So the MIT team set out to explore why this was the case.
Under particularly stressful conditions, like those inside a reactor, corrosion can happen very quickly. The researchers tested how well coatings of these special oxides can protect the metals underneath, when exposed to oxygen and placed under mechanical stress. To observe in detail what was happening, the team used an environmental transmission electron microscope (E-TEM).
Aluminum oxide performed particularly well, and with the almost-atomic resolution of the E-TEM, the researchers were able to see why. Even though it's a solid, aluminum oxide acts like a liquid when it's applied in extremely thin layers of about two to three nanometers. As the aluminum is stretched out, the oxide layer elongates and keeps the metal covered up, preventing oxygen from getting in.
The MIT researchers were able to demonstrate this phenomenon at room temperature, stretching the aluminum more than twice its original length without it cracking. Aluminum oxide is normally very brittle, but a layer that thin was found to be almost as deformable as a layer of regular aluminum would be.
This kind of self-healing coating could be put to work in high-stress situations like fuel cells or nuclear power plant reactors. Since it's active on such a tiny scale, the researchers say it could be particularly well suited at containing tiny molecules like hydrogen gas, which can slip through most other materials.
The research was published in the journal Nano Letters.
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