Self-healing metal oxides can protect against corrosion
Researchers at the Massachusetts Institute of Technology (MIT) have discovered that a solid protective coating of metal oxide, when applied in a moderately thin layer, has the ability to deform like a liquid and fill any cracks and crevices that appear.
This special thin coating prevents the leakage of small molecules from penetrating through most materials, such as the hydrogen gas used to power fuel cell cars or the radioactive tritium (a heavy form of hydrogen) that forms inside the cores of nuclear power plants.
Most metals, except gold, tend to oxidize when exposed to air and water. This reaction produces rust on iron, tarnish on silver, and rust on copper, which can weaken the metal over time and lead to cracks or structural failure. However, there are three types of metal oxides, aluminum oxide, chromium oxide, and silicon oxide, that can actually be used as protective coatings to prevent oxidation.
This special thin coating prevents the leakage of small molecules.
The team, led by graduate student Yang Yang, used specially designed instruments to examine the surface of metals coated with these oxides in detail to see what happens when they are exposed to oxygen and under stress. While most transmission electron microscopes (TEMs) require placing samples in a vacuum, the team used a modified type of microscope called an environmental transmission electron microscope (E-TEM) that allows samples to be studied in the presence of gases or liquids. The instrument was used to study the process that leads to stress corrosion cracking.
Metals under the influence of internal stresses in the reactor vessel and exposure to the superheated steam environment can corrode rapidly if not protected. Even with a solid protective layer, cracks will form, allowing oxygen to penetrate the surface of the bare metal, specifically the interfaces between the metal particles that make up the large metal material, causing more severe corrosion and structural damage.
Yang said that previously, people thought that metal oxides were brittle and prone to cracking, and no one had been able to prove it because it was difficult to observe the behavior of the material under real conditions. Now, the research team has observed it for the first time at near-atomic resolution. This method has demonstrated that the aluminum oxide layer, which is usually very brittle and will break under stress, when produced in an ultra-thin form, almost deforms like a thin aluminum metal layer, much thinner than the aluminum core. When the aluminum oxide is coated on the surface of a large aluminum sheet, the liquid-like flow keeps the aluminum covered with a protective layer.
The researchers demonstrated inside the E-TEM that the oxide-coated aluminum could be stretched to more than twice its original length without any cracks. The oxide formed a uniform protective coating on the surface, with no grain boundaries or cracks, even when stretched. Technically, the material is a glass, but it moves like a liquid and covers the surface as long as it is thin enough.
Ju Li, professor of nuclear science and engineering and co-author of the study, said:"With the advantage of a soft surface and no cracks or grain boundaries to penetrate into the material, self-healing coatings have many potential applications."
