Patrick Keil

Initiation and inhibition of dealloying of single crystalline Cu3Au (111) surfaces

Dealloying is widely utilized but is a dangerous corrosion process as well. Here we report an atomistic picture of the initial stages of electrochemical dealloying of the model system Cu(3)Au (111). We illuminate the structural and chemical changes during the early stages of dissolution up to the critical potential, using a unique combination of advanced surface-analytical tools. Scanning tunneling microscopy images indicate an interlayer exchange of topmost surface atoms during initial dealloying, while scanning Auger-electron microscopy data clearly reveal that the surface is fully covered by a continuous Au-rich layer at an early stage. Initiating below this first layer a transformation from stacking-reversed toward substrate-oriented Au surface structures is observed close to the critical potential. We further use the observed structural transitions as a reference process to evaluate the mechanistic changes induced by a thiol-based model-inhibition layer applied to suppress surface diffusion. The initial ultrathin Au layer is stabilized with the intermediate island morphology completely suppressed, along an anodic shift of the breakdown potential. Thiol-modification induces a peculiar surface microstructure in the form of microcracks exhibiting a nanoporous core. On the basis of the presented atomic-scale observations, an interlayer exchange mechanism next to pure surface diffusion becomes obvious which may be controlling the layer thickness and its later change in orientation.

Investigation of Room Temperature Oxidation of Cu in Air by Yoneda‐XAFS

The structure of thin copper oxide layers which are formed on metallic Cu due to the exposure to air are investigated with Yoneda‐XAFS and ReflEXAFS. The measured Yoneda‐XAFS data were compared to quantitative model calculations in the framework of the distorted wave Born approximation (DWBA) assuming different model structures for the oxide layer. Yoneda‐XAFS fine structure spectra measured for various different grazing angles show that the experimental data are best described by a model structure consisting of a duplex type oxide layer with an outer layer of CuO (tenorite) in direct contact with the gas atmosphere and an inner Cu2O (cuprite) layer at the interface to the underlying Cu metal.

Eu oxidation state in fluorozirconate-based glass ceramics

The influence of InF(3) doping and remelting on Eu-doped fluorozirconate-based glass ceramics was investigated using near-edge x-ray absorption and optical spectroscopy. It was found that the addition of InF(3) to the melt decreases the Eu(2+)Eu(3+) mole ratio, while remelting leads to a significant change in the Eu(2+)Eu(3+) ratio in favor of Eu(2+). Photoluminescence spectroscopy shows that additional annealing steps lead to the formation of BaCl(2) nanoparticles in the glass. In as-made glass ceramics containing InF(3), a phase transition of the nanoparticles from hexagonal to orthorhombic structure is observed. This phase transition is not observed in the remelted glasses studied here.

UV-Curable Polyester Acrylate Coatings: Barrier Properties and Ion Transport Kinetics Along Polymer/Metal Interfaces

Zinc and iron substrates were coated with polyester acrylate films. The samples were cured by an exposure to different doses of ultraviolet (UV) radiation energy at different radiant fluxes of the UV light source. Infrared spectroscopy was used to estimate the resulting degree of macromolecular cross-linking. Its effect on the barrier properties of the coating was investigated by Electro-chemical Impedance Spectroscopy (EIS). A reduced water uptake and a reduced swelling of the coating as well as a decelerated diffusion of water through the polymer bulk were detected on polyester acrylate coated zinc samples that were exposed to increasing UV energy doses at low UV light intensity. With the Scanning Kelvin Probe (SKP), a similar tendency was detected for the resistance of polyester acrylate/iron oxide/iron interfaces against cathodic delamination. In contrast, curing with large energy doses at high UV intensity led to decelerated ion transport kinetics along the interface, but did not further improve the barrier properties of the coating. This indicated that SKP data gained on Fe substrates and EIS data recorded on Zn substrates can be correlated if UV curing does not result in strong tension within the polyester acrylate films.

Fundamentals of Electrochemistry, Corrosion and Corrosion Protection

This chapter introduces the basics of electrochemistry, with a focus on electron transfer reactions. We will show that the electrode potential formed when a metal is immersed in a solution is most of the time not an equilibrium potential, but a mixed potential in a stationary state. This mixed potential formation is the basis of corrosion of metals in aqueous solutions. Organic coatings are introduced as protecting agents, and several types of coatings are discussed: classical passive coatings, and active coatings as modern developments. Three electrochemical techniques, which are commonly used to asses the protecting properties of coatings, are shortly introduced as well: linear polarisation measurements, electrochemical impedance spectroscopy, and scanning Kelvin probe measurements.

Reflectivity studies on a synchrotron radiation mirror in the hard X-ray regime

Abstract The optical performance and roughness parameters of an X-ray mirror that was used for several years in a synchrotron radiation beamline are determined by studying its X-ray reflectivity and diffuse scattering behavior. These values are compared to the data derived from topographic measurements with an atomic force microscope (AFM).

Backside SERS studies of inhibitor transport through polyelectrolyte films on Ag-substrates

In situ backside surface enhanced Raman spectroscopy (in situ-SERS) was newly employed for the study of the transport of inhibiting molecules through a polymer film. The barrier properties of layer-by-layer polyelectrolyte films (PE) composed of polyacrylic acid and polyallylamine hydro-chloride layers on Ag-surfaces were compared between untreated, thermally crosslinked, and Ag-nanoparticles containing samples. IB-SERS enabled the study of the transport of 2-mercaptobenzimidazole (MBI) as an inhibitor through the film. Water barrier properties of the treated PE films determined by Electrochemical Impedance Spectroscopy were correlated to the MBI diffusion kinetics. The PE stability against MBI diffusion and thermal treatment was analyzed by Infra-Red Reflection Absorption Spectroscopy (IRRAS). IRRAS showed that the thermally treated PE films formed chemical crosslinking via amide bonds and lowered the diffusion of water and the water uptake in the films. Moreover, the MBI diffusion kinetics can be followed by means of SERS. However, MBI adsorption at the PE film/metal interface was not detected after the heat treatment. In this case the adsorbed PE on the Ag surface was not substituted by the competing adsorption of MBI. Moreover, the presence of Ag-nanoparticles in the film decelerated MBI diffusion to the SERS substrate due to the trapping effect of MBI molecules.

Selective Study of Atoms in Rough Gold Surfaces by Means of Yoneda-XAFS

A new method of grazing incidence X‐ray absorption spectroscopy using an off‐specular reflection geometry referred to as Yoneda‐XAFS is presented. Using an asymmetric setup, with the detector centered at the Yoneda peak, this technique is sensitive to lateral heterogeneities which typically occur at surfaces or interfaces. The Yoneda‐XAFS technique was used for the ex situ investigation of a sputter deposited thin gold films on a float glass substrate. The data analysis is done in the framework of the distorted wave Born approximation (DWBA).

XANES studies on Eu-doped fluorozirconate based glass ceramics.

The influence of adding InF3 as a reducing agent on the oxidation state of Eu in fluorochloro- (FCZ) and fluorobromozirconate (FBZ) glass ceramics was investigated using x-ray absorption near edge (XANES) and photoluminescence (PL) spectroscopy. For both materials, it was found that InF3 decreases the Eu2+-to-Eu3+ ratio significantly. PL spectroscopy proved that an annealing step leads to the formation of Eu-doped BaCl2 and BaBr2 nanocrystals in the FCZ and FBZ glasses, respectively. In the case of FCZ glass ceramics the hexagonal phase of BaCl2 could be detected in indium-free and InF3-doped ceramics, but only for InF3 containing FCZ glass ceramics a phase transition of the nanoparticles from hexagonal to orthorhombic structure is observed. For the FBZ glass ceramics, the hexagonal phase of BaBr2 can be formed with and without indium doping, but only in the indium-free case a phase transition to orthorhombic BaBr2 could be found.