Browsing by Author "Renner, Frank Uwe"

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    In-situ X-ray study of the initial electrochemical corrosion of Cu 3 Au(111)
    (2004) Renner, Frank Uwe; Dosch, Helmut (Prof. Dr.)
    Corrosion is causing each year a tremendous loss of about 3-6% of a national GDP. Increasing the basic knowledge of corrosion processes and the formation of inhibiting passivation layers is therefore highly desirable. Corrosion is basically an electrochemical process. Electrochemical etching and micro-structuring are similar processes. The single crystal surface of the binary noble metal alloy Cu3Au(111) was chosen as a model system to study the potential dependence of the corrosion process in acidic aqueous electrolyte. With X-ray diffraction especially crystalline materials can be characterised. The crystalline structure can be obtained on the scale of atomic distances. In this way important questions can be answered: how thick are obtained passivation layers, which composition and which lateral dimensions do they have? Which crystallographic orientation do they exhibit? The X-ray diffraction experiments were performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble, at the Hamburger Synchrotronstrahlungslabor (HASYLAB) and at the beamline of the Max-Planck-Institut fur Metallforschung at the Angstrom-Quelle Karlsruhe (ANKA). In addition ex-situ AFM was used. Using a mobile X-ray-UHV-chamber at ANKA we could study the clean Cu3Au (111) surface. The ordering of the Cu3Au L12 superstructure was controlled by recording a superstructure Bragg reflection. In this way the ordering temperature could be optimised. The intensity along several crystal truncation rods (CTR) was measured and a model for the surface structure was obtained by using the ANAROD program. The main features of this model are a slightly reduced first atomic layer distance and a top layer with 70% occupation to account for atomic roughness. There was no segregation, i.e. all the layers showed a stochiometric occupation. For the corrosion experiments the sample was transferred from UHV in air to the used electrochemical X-ray cell. The sample was the immersed into the 0.1 M H2SO4 electrolyte at a cathodic potential of -100mV vs. Ag/AgCl. At more anodic potentials selective dissolution of Cu occurs and a Au enrichment on the surface is observed. In-plane and out-of-plane X-ray diffraction showed at about +100mV the growth of an ultra-thin epitaxial fcc-like Au-rich CuxAu1-x layer, with a lattice constant in-between of Cu3Au and epitaxial bulk Au. At potentials below 350mV this surface layer was stable. The peak width was pointing to lateral dimensions of layer islands of the order of 15-20nm. The out-of-plane data shows the epitaxial relation of this surface layer and its ultra-thin nature. The 111 layer normal is parallel to the substrate normal, but the stacking sequence is inversed. CTR measurements of the ultra-thin crystalline layer could be understood by a slightly distorted fcc-like model. Using bulk values for the elastic properties of Au the data can be interpreted as strained and nearly pure Au. On the other side, anomalous X-ray diffraction revealed a contribution of still 40% of Cu to the diffracted intensity. At higher (more anodic) potentials we then observed the formation of about 3nm thick pure Au islands. These thicker Au films showed still the inversed stacking sequence. In an electrolyte solution with an addition of 5mM HCl to the 0.1M H2SO4 solution we observed the same structures. But the transformation of the ultra-thin layer to the thicker Au islands occurred at about 150mV more cathodic potentials. The formation of islands with the selective dissolution of Cu could also be observed in ex-situ AFM images. These images show a hexagonal correlation of the visible islands.