Dumb materials succumb to rust, but smart materials might be able to heal themselves, thanks to researchers in Europe.

Dmitry Shchukin and Helmuth Möhwald of the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, and colleagues in the Department of Ceramics and Glass Engineering at the University of Aveiro in Portugal, have devised a nano coating for metals and alloys that forms a very thin gel-like layer on the material. If this coating is damaged, the metal would normally be exposed to the elements. However, nanoreservoirs of preservative instead spread to fill any microscopic cracks and holes that appear and quickly block them; preventing further attack.

Engineers and manufacturers already have several passive methods for protecting metals from corrosive attack. Galvanization and polymer coatings for instance add a water-proof layer to iron, steel, aluminum alloys, and other metals that are susceptible to corrosion. Chrome plating is also an effective method of protection, but comes with health risks during the manufacturing process and will soon be banned in 2007. The drawback of all these techniques is that once the protective coating is damaged, air and moisture can reach the exposed metal and do their corrosive worst. A smart coating that can heal itself has to be the answer.

The protection process developed by Shchukin and colleagues is fairly complex. However, the initial investment could reduce the enormous economic losses caused by rust and corrosion in dozens of industries the world over from automotive and aeronautics to chemical and construction.

Their approach involves loading up the self-healing coating layer by layer with tiny molecular containers, assembled nanoreservoirs, containing a corrosion inhibitor, benzotriazole. The first step is to make the nanocontainers. Nanoscopic particles of silicon dioxide, or silica, the stuff of sand, are coated with a thin layer of two electrically charged polymers, polyethylene imine and polystyrene sulfonate. Next, a layer of the inhibitor benzotriazole compound is wrapped around the particles. The nanoparticles are then deposited with a soluble silica gel containing zirconium oxide to cover the metal component. The researchers have successfully tested their metal-coating approach on aluminum.

Each chemical in the multilayered coating plays a key role in protecting the aluminum alloy from corrosion. The silica particles provide a support for the benzotriazole corrosion inhibitor and the charged polymers, while the zirconium dioxide makes them stick to the aluminum alloy.

Intact, the coating provides simple protection from corrosive agents in the same way as a conventional polymer coating. However, it is when the coating is damaged that its smart properties come into play. The charged polymers normally keep the benzotriazole in place, but damage to the coating releases them and the benzotriazole becomes free to diffuse through the gel layer, quickly plugging any tiny gaps that form before the corrosive agents can get to work on the metal.

In tests on aluminum alloy, Shchukin and his colleagues demonstrated that their smart coating could protect the metal from salty water even when they pricked the surface repeatedly with a needle. "The self-healing coating can protect aluminum alloy in salt solution for a long time," Shchukin explains, "When the coating is damaged, the defects of less than several tens of micrometers in size heal in less than 24 hours." He adds that, "This technique can already be adopted for the protection of aluminum alloys used in the aerospace industry."

The current materials can heal cracks of up to 100 micrometers in size in water and in salt solutions. "The next stage is to develop the self-healing coatings to adapt them for other metals, such as steel," Shchukin adds, "and to provide faster release of the inhibitor from the nanoreservoirs resulting in faster healing of the defects."

Adv Mater, 2006, 18, 1672-1678; http://dx.doi.org/10.1002/adma.200502053

http://www.mpikg.mpg.de/en/gf/