Fabrication of Single-Crystalline CoCrFeNi Thin Films by DC Magnetron Sputtering: A Route to Surface Studies of High-Entropy Alloys.

High-entropy alloys (HEAs) with their almost limitless number of possible compositions have raised widespread attention in material science. Next to wear and corrosion resistive coatings, their application as tunable electrocatalysts has recently moved into the focus. On the other hand, fundamental properties of HEA surfaces like atomic and electronic structure, surface segregation and diffusion as well as adsorption on HEA surfaces are barely explored. The lack of research is caused by the limited availability of single-crystalline samples. In the present work, the epitaxial growth of face centered cubic (fcc) CoCrFeNi films on MgO(100) is reported. Their characterization by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) demonstrates that the layers with a homogeneous and close to equimolar elemental composition are oriented in [100] direction and aligned with the substrate to which they form an abrupt interface. X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and angle-resolved photoelectron spectroscopy are employed to study chemical composition and atomic and electronic structure of CoCrFeNi(100). It is demonstrated that epitaxially grown HEA films have the potential to fill the sample gap, allowing for fundamental studies of properties of and processes on well-defined HEA surfaces over the full compositional space.

Corrosion of carbon steel and the passivating properties of corrosion films formed under high-PT geothermal conditions.

Corrosion is a major obstacle to a safe implementation of geotechnical applications. Using a novel approach that includes vertical scanning interferometry (VSI) and electrochemical impedance spectroscopy (EIS) we discuss time-dependent carbon steel corrosion and film formation at geothermally relevant temperatures (80-160 °C) in CO2-saturated mildly acidic NaCl brine. Iron dissolution kinetics follows a logarithmic rate at 80 and 160 °C and a linear rate at 120 °C. At 80 °C, high initial corrosion rates (first 24 h) generate H2 at a minimum rate of 12 µmol h-1 cm-2 and lead to the formation of a continuous ~100 µm thick porous corrosion film. It exhibits a duplex structure with a crystalline outer FeCO3 layer and an inner layer composed of a skeletal network of Fe3C impregnated with FeCO3. Being an electrical conductor we hypothesize the Fe3C to strongly enhance corrosion rates by providing additional cathodic sites. Pseudo-passivity due to an anodic film-forming reaction (presumably Fe-oxide) was observed at 120 and 160 °C, soon followed by the initiation of pitting at 120 °C. Steady-state corrosion rates at 160 °C are at least one order of magnitude lower than for 120 °C. Our experimental approach demonstrated potential for general applicability in studying corrosion-related phenomena.