Credit: © 2008 Wiley

Self healing is a property we all know; scratch or cut yourself and within days the damaged area has been replaced with fresh skin. Other materials are not so functional, but now Daria Andreeva and co-workers at the Max Planck Institute of Colloids and Interfaces, Germany, have developed1 a coating made of alternating nanolayers sandwiched together, which can guard against corrosion and heal itself after being scratched.

The researchers deposited the coating on an aluminium alloy surface through the stepwise, layer-by-layer assembly of oppositely charged polyelectrolyte layers that encapsulate a central layer of 8-hydroxyquinoline — a known corrosion inhibitor. To test the properties of the coating, both treated and untreated aluminium alloys were scratched and exposed to a salt solution. The treated surface showed no corrosion after 16 hours, in contrast to the untreated aluminium surface that was severely corroded after only 6 hours of exposure.

The coating works through three separate mechanisms that act together to inhibit corrosion and heal the surface. When the surface is mechanically deformed, the central layer of 8-hydroxyquinoline is released and able to passivate the exposed region by forming an insoluble chelate with the aluminium. The polyelectrolyte layers act as an acid–base buffer system that stabilizes the pH of the corrosion site and, because of their mobility, they can also slide into the deformed area, restoring the sandwiched nanolayer structure and preventing any further corrosion.