German researchers at the Max Planck Institute for Colloids and Interfaces in Potsdam, Germany, have designed a new coating capable of repairing minor scratches and preventing metal corrosion. Writing in the journal Advanced Materials, Helmut Mohwald and his colleagues have produced the mechanical equivalent of a slow-release medical preparation which is active only where needed. The team take tiny silica balls measuring 100 nanometres across and coat them with two polymers, polyethylene imine and polystyrene sulfonate, and a corrosion-killing agent called benzotriazole. The balls are then mixed into a zirconium oxide gel which causes the imine to become positively charged and the sulfonate to become negatively charged. This glues the two layers together, immobilising the benzotriazole between them. After it is applied to a surface, if the surface is scratched the silica balls fragement around the damaged area, discharging the benzotriazole which then blocks any subsequent corrosion. To test the new gel the researchers applied it to pieces of aluminium which were then scratched, both in the open air and in salty water. No corrosion occurred even with scratches up to 0.1 mm wide, which in a traditional setting are sufficient to trigger serious corrosion problems, particularly in the aerospace industry. The researchers are now working to modify the system to also protect iron and steel, perhaps even as an alternative to chrome-plating.