Rust never sleeps, Neil Young once reminded us. Now Korean materials scientists have found a cheap, effective and readily available soporific for this most insidious of time's destructive agents. Sunlight, they report, can arrest the corrosion of steel
Wonyong Choi of Pohang University of Science and Technology and co-workers have immersed a steel plate in corrosive salt solution for days without it developing the usual red-brown rust coating. They wired the plate to a cheap kind of solar cell, and exposed the cell to sunlight.
The anti-corrosion process at work here is basically the same as that which protects galvanized steel. Galvanization gives steel (which is basically iron) a thin, 'sacrificial' coating of zinc. As zinc is a more reactive metal than iron, it is corroded first by reaction with the air.
The reaction of a metal with air involves the loss of electrons from the metal atoms. Zinc protects iron by supplying it with electrons: there is an electrical current flowing from zinc to iron as the zinc is corroded. So the iron atoms don't react to form rust (iron oxide).
Choi's group has simply replaced the zinc with a solar cell. This supplies electrons to steel by absorbing sunlight. The light energy mobilizes electrons in the cell, allowing them to travel through the wire connecting it to the steel plate. As long as light-stimulated electrons keep moving around the circuit between the solar cell and the plate, the iron is protected from rusting.
Solar cells are usually quite expensive, but for this application -- where high power output is not the prime concern -- cheap versions can be used. Choi's team made their cell by dipping a glass slide coated with a transparent, electrically conducting, material into a slurry of titanium dioxide powder. Titanium dioxide is the cheap pigment in white paint; it produces electrons when it absorbs the ultraviolet component of sunlight.
The solar cell doesn't have to be touching the steel it protects, as long as the two are wired together. A titanium dioxide solar panel could, for instance, be used to prevent an underground steel pipe from corroding.
The only problem, the researchers point out, is that the steel is left defenceless when night falls -- or even when the sky is overcast. One possible answer might be to use solar power to charge up a battery that would provide protective electrons when daylight fades -- but this increases the cost.