Self-healing Metal Oxides Could Protect Against Corrosion
Hema Nookala posted on April 24, 2018 |
Solid oxide protective coatings on metals can deform like liquids and fill any cracks.

Metals are one of the most powerful and versatile materials in the industrial world. From cars to nuclear reactors to hair clips, metal is abundant. In some of the more extreme situations, the metal undergoes strenuous conditions of high temperature or pressure. Such conditions tend to create the potential for cracks, gaps and leakage of toxic liquids and gases.

Recently, researchers have found a solution. When used in thin layers, solid oxide protective coating on metals can deform in a similar way to liquids and thus fill any cracks and gaps as they form in the metal. Most metals oxidize when exposed to air, which weakens the metal to the point of structural failure. However, three known elements can provide a protective barrier when oxidized: aluminum oxide, chromium oxide and silicon dioxide. Naturally, aluminum reacts with oxygen to create a thin barrier layer, aluminum oxide, which stabilizes the surface and prevents further reaction to the environment.

A 2D model of fracture propagation, highlighting stress in the material.
A 2D model of fracture propagation, highlighting stress in the material.

In case you’re interested in watching an educational video from Cornell University about how and when metals fail, click here.

In many cases, researchers do not know the extent of a material’s usefulness until extensive research is performed under many different controlled environments. MIT Professor Ju Li and Yang Yang, the graduate student team lead, used specialized instruments from Brookhaven National Laboratory to observe the surface of metals coated with these specific oxides when exposed to oxygen and high stress. This particular experiment breaks boundaries as the team used an environmental transmission electron microscope (E-TEM) that allows samples to be studied in the presence of gases or liquids—whereas regular testing methods would require the sample to be in a vacuum. They used the E-TEM to test stress corrosion cracking; this is when metals corrode under pressure, allowing oxygen to penetrate through any protective layer and into the bare metal, where it corrodes to the point of structural failure.

Through testing it was determined that using the well-known coating material, aluminum oxide, in a layer about 2 to 3 nanometers thick can exhibit liquid-flow behavior.

Prior to this, aluminum oxide was known to be brittle at room temperature, mainly determined through various unrealistic testing environments, such as in a vacuum. Now that it has been tested at close-to-atomic resolution, in suitable environments, it was demonstrated that aluminum coated with aluminum oxide could be stretched to double its initial length with no cracks. The layers of aluminum oxide fully coat the surface and prevent grain boundaries and cracks from forming and propagating.

A thin layer of aluminum oxide (arrow) that stretches when oxygen (right) pushes against the aluminum grains (left).
A thin layer of aluminum oxide (arrow) that stretches when oxygen (right) pushes against the aluminum grains (left).

This behavior sounds unbelievable when you look at aluminum oxide’s other forms. Sapphires and rubies are made of the same material, but in a bulk crystalline form, making it strong but brittle. The crystal structure of the oxide is extremely important in determining its characteristics.

While on the surface it may look like we’ve found a way to increase the lifespan of our metal equipment and machinery, it’s about so much more than that. Using aluminum oxide as a protective layer can increase the safety of new technological advancements. Using this protective layer can also prevent the leakage of molecules that can penetrate through most metals—hydrogen gas in power-fuel cell cars or radioactive tritium that forms inside nuclear power plant cores. In addition, it can prevent slow liquid leakage, which can create hazardous situations for machine operators. Using corrosion protection is efficient and safe and can reduce the financial burden of constantly replacing machinery.  

Brookhaven National Laboratory has patented the cost-efficient method for applying the ultrathin coating of metal oxide nanoparticles. It was researched to provide an alternate for the toxic chromium compound that was traditionally used, as it provided the best corrosion resistance. If you want to learn more about the process from Brookhaven National Laboratory, click here.

For more research into corrosion protection, check out Getting a Look at Corrosion in Real Time.

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