Chemical corrosion impacts on global commercial turnover significantly as equipment, buildings, and transportation systems have to be continually maintained to combat its effects. Chemists ever looking for the silver lining, however, have recognized that chemical attack of metal surfaces is not all bad and might be exploited to produce useful nanoscale surface features with potential technological applications in catalysis, sensors, and other areas.

Now, scientists from Germany and the European Synchrotron Radiation Facility (ESRF) have highlighted a self-organization process on the surface of a metal alloy, that could improve our understanding of corrosion processes leading to better conservation methods as well as novel methods for modifying surfaces. It also explains the ancient Inca technique of eking out gold supplies by mixing in copper and coating their ingots with salts. Their results provide a structural description on the atomic scale of the surface, revealing exactly how a protective surface layer forms and so hinders further corrosion.

Researchers from Max Planck Institute, the University of Ulm (Germany), and the ESRF used the European synchrotron light source to reproduce the onset of corrosion of a gold-copper alloy exposed to sulfuric acid. Gold does not corrode under everyday conditions, but copper is far less resistant to chemical attack, leading to an ambivalent alloy. At the first moments of corrosion revealed by synchrotron X-ray studies, explains team member Jorg Zegenhagen, an extremely thin protective layer of gold-rich material forms over the surface of the copper-gold alloy.

The protective layer has an unexpected crystalline and well-ordered structure of tiny islands of gold about 1.5 to 20 nanometers across. These islands eventually form a porous gold metal layer, which Zegenhagen reckons is ripe for technological exploitation. "Understanding and controlling the formation of the first layer and the nano-islands may help to produce nano-materials with specific properties", he explains. The new insight could help explain the properties of various other alloys.

Ancient Inca smiths eked out precious gold supplies by mixing it with copper, surrounding the alloy with salts to create an acidic environment that dissolved the copper from the top layer, leaving a gold-rich surface ready for polishing.

Nature, 2006, 439, 707-710; http://dx.doi.org/10.1038/nature04465

http://www.fit.edu/fip/jorg_zeganhagen_member.htm