1. Max-Planck-Institut für Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany
2. European Synchrotron Radiation Facility, Boîte Postale 220, F-38043 Grenoble, France
3. Abteilung Elektrochemie, Universität Ulm, Albert-Einstein-Allee 47, D-89081 Ulm, Germany

Correspondence to: F. U. Renner^1,2 Correspondence and requests for materials should be addressed to F.U.R. (Email: renner@esrf.fr).

Abstract

Corrosion destroys more than three per cent of the world's GDP¹. Recently, the electrochemical decomposition of metal alloys has been more productively harnessed to produce porous materials with diverse technological potential^{2, 3}. High-resolution insight into structure formation during electrocorrosion is a prerequisite for an atomistic understanding and control of such electrochemical surface processes. Here we report atomic-scale observations of the initial stages of corrosion of a Cu₃Au(111) single crystal alloy within a sulphuric acid solution. We monitor, by in situ X-ray diffraction with picometre-scale resolution, the structure and chemical composition of the electrolyte/alloy interface as the material decomposes. We reveal the microscopic structural changes associated with a general passivation phenomenon of which the origin has been hitherto unclear. We observe the formation of a gold-enriched single-crystal layer that is two to three monolayers thick, and has an unexpected inverted (CBA-) stacking sequence. At higher potentials, we find that this protective passivation layer dewets and pure gold islands are formed; such structures form the templates for the growth of nanoporous metals². Our experiments are carried out on a model single-crystal system. However, the insights should equally apply within a crystalline grain of an associated polycrystalline electrode fabricated from many other alloys exhibiting a large difference in the standard potential of their constituents⁴, such as stainless steel (see ref. 5 for example) or alloys used for marine applications, such as CuZn or CuAl.

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