STAINLESS steel is not meant to rust, but it does sometimes. Now we know that the corrosion is kicked off by a chain reaction in the steel鈥檚 surface 鈥� a finding that could lead to new ways of eliminating rust.
The surface of stainless steel is covered by a protective oxide layer, but in places where the layer is thin or defective, a 鈥渃orrosion pit鈥� may form. To study these pits, Jack Hudson of the University of Virginia in Charlottesville and his colleagues dropped pieces of steel into salty water, which speeds up corrosion. They then monitored the steel鈥檚 surface with an optical microscope.
Initially, just a few corrosion pits appeared as blackened holes about a thousandth of a millimetre across. But within minutes the number of pits began to multiply like mad, until there were thousands of pits per square millimetre (Science, vol 305, p 1133).
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The first few pits trigger a runaway reaction, says Hudson. Compounds leaking out increase the acidity around the pit, weakening the oxide layer and making the surface more susceptible to damage. Each pit can therefore spawn many more. Removing or inactivating the corrosive compounds somehow could stop the reaction in its tracks, says Hudson.