PLANES, trains and automobiles could one day last a whole lot longer thanks to a smart coating that stops rust in its tracks. The material reacts to damage by releasing a substance that blocks corrosion then reseals the breach.
Today, metals are protected from rusting either by galvanising them with a thin layer of zinc, or by applying a waterproof polymer film. But these only stop corrosion if their surface remains pristine. The moment it is punctured, moist air or water can get in and do its worst to the naked metal underneath.
Now Helmut Möhwald and colleagues at the Max Planck Institute for Colloids and Interfaces in Potsdam, Germany, have designed a coating that not only acts as a moisture barrier but also copes with cracks and scratches (Advanced Materials, vol 18, p 1672).
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“Our system is an example of controlled-release technology,” Möhwald says. “Just like medical implants that leach a drug into someone’s system, we are releasing a corrosion inhibitor precisely where and when it is needed.”
First silica balls measuring 0.1 micrometres across are coated with two polymers – polyethylene imine and polystyrene sulfonate – along with benzotriazole, a corrosion inhibitor. The balls are then mixed into zirconium oxide gel, which causes the imine molecules to become positively charged and the sulfonate ones negatively charged. This traps the charged benzotriazole molecules in between and ensures they are locked onto the silica balls.
If anything damages the surface that the gel is applied to, the silica balls in that area will break into pieces, spreading the benzotriazole molecules over the damaged metal. The oxidation reaction that causes rusting then draws the benzotriazole out of the polymer coatings to block the reaction.
The team tested the gel by applying it to an aluminium alloy, which they then scratched in open air and under salty water. They found that cuts up to 0.1 millimetres across suffered no corrosion even in salt water. Scratches of this size can allow serious corrosion to take hold.
The aerospace industry, a big user of aluminium, could be one of the first to adopt the technology, according to Möhwald. His colleague Dimitry Shchukin demonstrated the material to aerospace giant EADS, which owns the aircraft maker Airbus, in Munich last week. The team also plans to approach several automobile makers.
Next, the researchers hope to modify the gel to work against corrosion in iron and steel. The process could provide an alternative to chrome plating for cars, which is associated with health risks and is currently being phased out of manufacturing in Europe.
