A way for the damaged surfaces of metals to repair themselves has been devised

Sadly for engineers, inanimate objects cannot yet repair themselves. But scientists have invented a way of healing damaged metals.

The surfaces of many metal objects are coated with other metals for protection. Iron, for instance, is frequently galvanized with zinc. The basic idea of the new technology is to infiltrate this coating with tiny, fluid-filled capsules. When the metal coating is punctured or scratched, the capsules in the damaged area burst and ooze restorative liquids, in the form of compounds called trivalent chromates. These react with nearby metal atoms and form tough, protective films a few molecules thick to ameliorate the damage.

The idea of doing this has been around for years, but it has proved difficult in practice because the capsules used were too big. Surface coatings tend to be about 20 microns thick. The capsules were 10-15 microns across - large enough to disrupt the coatings, and thus do more harm than good. The trick worked out by scientists is how to create capsules a few hundredths of this size.

The capsules researchers have come up with are made by mixing butylcyanoacrylate, a chemical found in superglue, with an oil carrying the healing compounds. This mixture is then, itself, mixed with dilute hydrochloric acid. The result is an emulsion of droplets between 100 and 300 nanometers across. Each droplet has an oil core surrounded by a thin layer of butylcyanoacrylate molecules. To make the droplets stable, phosphate is added to the emulsion. This triggers the polymerization of the butylcyanoacrylate into a tough plastic, which forms the outside of the capsule.

The greatest challenge, however, was not making the capsules in the first place, but stabilizing them during the plating process. Though galvanization is often done by dipping steel in liquid zinc, it is sometimes done by electrolysis - and nickel and copper plating are normally done this way. The capsules, though, tend to stick together in the liquids used as electrolytes during electroplating, and are also destroyed by the extreme acidity or alkalinity that is often involved in the process. To overcome these problems special detergents that stick to polybutylcyanoacrylate were used, and thus both stop the capsules sticking together and protect them from the electrolytes. The techniques were proved in electroplated layers of copper, nickel, and zinc, and self-repairing metals are believed to commonly be available in the years ahead.

The Economist

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