Corrosion in metals can lead to the dangerous weakening of civil structures. Researchers at Northwestern University, Illinois, have developed a self-healing coating that can fix corrosion in a matter of seconds.
“Local corrosion in metal is quite dangerous because it is hard to predict, and when it happens, oftentimes it is hard to detect, thus left unattended,” explained Professor Jiaxing Huang, who led the research efforts. “There are [detection] methods based on visual inspection, or techniques such as those based on electrochemistry to monitor the rate of corrosion. But because corrosion is local, finding the spots in a large metal structure is not easy.”
Self-healing coatings powered by the sun and inspired by a snake's skin have been developed, but these can only repair microscopic defects. In order to heal bigger imperfections on the millimetre scale, Huang and his colleagues took a different inspiration.
“When a boat cuts through water, the water goes right back together,” says Huang. “The 'cut' quickly heals because water flows readily. We were inspired to realise that fluids, such as oils, are the ultimate self-healing system.”
The oil-based coating had to be fluid enough to quickly self-repair in a matter of seconds, but not so liquid that it would simply drip off the metal's surface. The ‘Goldilocks zone’ was achieved by adding microscopic and lightweight graphene capsules which thicken the oil and help it stick to the metal extremely well even in harsh environments, whether underwater or in acid baths.
The scientists showed that the material can heal quickly and repeatedly, even after scratching the same spot for 200 times in a row, on a number of different metals including aluminium, brass, and steel. Huang tells us that the coating should in principle work with any metal, since it is not based on a specific surface chemistry.