Rust never sleeps, in the words of songwriter Neil Young. But, if materials scientists cannot put corrosion into a deep slumber perhaps there is a way to produce smart materials that can heal themselves and so stop the spread of a rust patch in its tracks.
There are already several passive methods for protecting metals from corrosive attack. Galvanization and polymer coatings, for instance, simply add a waterproof 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 be banned in Europe from 2007. With any of these techniques there is a serious drawback: once the protective coating is damaged, air and moisture can reach the exposed metal and do their corrosive worst. The answer would be a coating that could heal itself if it cracks or suffers damage.
The self-healing metal is not such a far-fetched idea. Writing in the latest issue of Advanced Materials, Dmitry Shchukin and Helmut 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, describe how they have coated metals with a very thin gel-like layer. If this coating is damaged, the metal would normally be exposed to the elements. However, by incorporating an additive in the gel that spreads to fill any microscopic cracks and holes that appear, the wounded coating heals itself and continues to provide the metal with protection from attack.
The protection process developed by Shchukin and colleagues is 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—each containing a corrosion inhibitor. The first step is to make the nanocontainers. Nanoscopic particles of silicon dioxide, or silica, are coated with a thin layer of two electrically charged polymers, polyethylene imine and polystyrene sulfonate. Next, a layer of the corrosion inhibitor compound, benzotriazole, 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 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 their own. The charged polymers normally keep the corrosion inhibitor in place, but damage to the coating releases them and the corrosion inhibitor 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 of the work is the development of the self-healing coatings is to adapt them for other metals, such as steel," Shchukin adds, "and to provide faster release of the inhibitor from the nanoreservoirs resulting in more healing of the defects."
Author: Dmitry G. Shchukin and Helmuth Möhwald, Max Planck Institute of Colloids and Interfaces (Germany), http://www.mpikg.mpg.de/en/gf/
Title: Layer-by-Layer Assembled Nanocontainers for Self-Healing Corrosion Protection
Advanced Materials 2006, 18, No. 13, 1672–1678, doi: 10.1002/adma.200502053
Posted 29th June 2006