The corrosion of technically relevant alloys, like stainless steel, causes damage that amounts to about 3 percent of the global gross national product. Although this every-day phenomenon has such broad consequences, its fundamental microscopic processes are still largely not understood -- most of all how corrosion begins and develops at an atomic level.
Now, Andreas Stierle and his colleagues at the Max Planck Institute for Metals Research and the University of Ulm, Germany, as well as the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, have succeeded in observing so-to-speak "live" the atomic processes behind the corrosion of an alloy. To the great surprise of the researchers, despite the destructive nature of the corrosion process, a perfect crystalline protective layer was formed. The scientists were able to determine the layer's structure and chemical composition using highly brilliant synchrotron radiation. Their observations show how technologically relevant alloy surfaces can be nanostructured using targeted corrosion processes (Nature, February 9, 2006).
The scientists from the Max Planck Institute for Metals Research and the European Synchrotron Radiation Facility have chosen an alloy, Cu3Au, whose two components exhibit very different corrosion behaviour. Copper, on the one hand, goes into a solution containing sulphuric acid when confronted even with small corrosion potentials, which yield a voltage between the sample and a reference electrode applied through the electrolyte. Gold, on the other hand, is much more corrosion resistant.
Using highly brilliant synchrotron radiation as a non-destructive, high resolution probe, the scientists have now observed the onset of corrosion in the alloy Cu3Au. They were thus able to do the first-ever analysis of the interface between the liquid electrolyte and the alloy crystal during the corrosion process, with a resolution in the picometre range (10-12 metres, 1 nanometre = 1,000 picometres).
If only a little bit of copper is dissolved from the interface, it builds up a crystalline, 3-atom-layer thick, Au-rich passivation layer. The layer protects the surface of the material from further corrosion (see image 1). Interestingly, the passivation layer does not mimic the crystal structure of the substrate one-to-one. The material's border to the electrolyte rather functions like a mirror, which causes the film to develop with a structure that is a twin of the substrate.
If the corrosion potential is increased by changing the voltage between the sample and the reference electrode, the rest of the copper from the protective passivation layer is dissolved, and the remaining gold atoms form gold islands about 2 nanometres high, which no longer completely cover the surface (see image 2). This process, also called dewetting, is already known in nature, when raindrops come together on a leaf. The corrosion progresses now on the Cu3Au surface, which is directly in contact with the electrolyte. This creates a foam-like, porous structure .
Materials scientists can use these results to understand that it is possible to optimise surface passivation in alloys by setting the corrosion potential above the surface in such a way that a passivation layer is created. Furthermore, controlled corrosion is, at higher potentials, an elegant method of chemically structuring material surfaces at nanometric scales. If the corrosion continues further, eventually a nanoporous gold film is formed, which can be e.g. potentially used as a catalyst because of its very large surface area.
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