Sergiy Borodin

Element-Resolved Corrosion Analysis of Stainless-Type Glass-Forming Steels

Rust Resistance The rusting of iron and steel can be prevented through the addition of 11% or more chromium. The addition of molybdenum can enhance the corrosion resistance, with a complex interplay between the Cr and Mo atoms. However, if chemical variations exist, corrosion can still occur in localized regions or if the surface layer is mechanically abraded. Duarte et al. (p. 372) studied the corrosive failure of an iron-based glassy alloy. A combination of atom probe tomography, electron microscopy, and x-ray diffraction was used to build up a near atomistic picture of local variations in the metal as it was heated and allowed to crystallize, and the impact these processes have on the corrosion resistance. Measurements of the local composition of a steel alloy are correlated with the corrosion resistance. Ultrathin passive films effectively prevent the chemical attack of stainless steel grades in corrosive environments; their stability depends on the interplay between structure and chemistry of the constituents iron, chromium, and molybdenum (Fe-Cr-Mo). Carbon (C), and eventually boron (B), are also important constituents of steels, although in small quantities. In particular, nanoscale inhomogeneities along the surface can have an impact on material failure but are still poorly understood. Addressing a stainless-type glass-forming Fe50Cr15Mo14C15B6 alloy and using a combination of complementary high-resolution analytical techniques, we relate near-atomistic insights into increasingly inhomogeneous nanostructures with time- and element-resolved dissolution behavior. The progressive elemental partitioning on the nanoscale determines the degree of passivation. A detrimental transition from Cr-controlled passivity to Mo-controlled breakdown is dissected atom by atom, demonstrating the importance of nanoscale knowledge for understanding corrosion.

Stabilization and Acidic Dissolution Mechanism of Single-Crystalline ZnO(0001) Surfaces in Electrolytes Studied by In-Situ AFM Imaging and Ex-Situ LEED

A combined approach of pH-dependent in-situ AFM topography and ex-situ LEED studies of the stability and dissolution of single-crystalline ZnO(0001)-Zn surfaces in aqueous media is presented. Hydroxide-stabilized and single-crystalline ZnO(0001)-Zn surfaces turned out to be stable within a wide pH range between 11 and 4 around the point of zero charge of pH PZC = 8.7 +/- 0.2. Hydroxide stabilization turned out to be a very effective stabilization mechanism for polar oxide surfaces in electrolyte solutions. The dissolution of the oxide surface started at an acidic pH level of 5.5 and occurred selectively at the pre-existing step edges, which consist of nonpolar surfaces. In comparison, the oxide dissolution along the ZnO(0001) direction proved to be effectively inhibited above a pH value of 3.8. On the basis of these microscopic observations, the mechanistic understanding of the acidic dissolution process of ZnO could be supported. Moreover, both the in-situ AFM and the ex-situ LEED studies showed that the stabilization mechanism of the ZnO(0001) surfaces changes in acidic electrolytes. At pH values below 3.8, the hydroxide-stabilized surface is destabilized by dissolution of the well-ordered radical3. radical3. R30 hydroxide ad-layer as proven by LEED. Restabilization occurs and leads to the formation of triangular nanoterraces with a specific edge termination. However, below pH 4 the surface structure of the crystal itself is ill-defined on the macroscopic scale because preferable etching along crystal defects as dislocations into the bulk oxide results in very deep hexagonal etching pits.

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.

Ultra high vacuum high precision low background setup with temperature control for thermal desorption mass spectroscopy (TDA-MS) of hydrogen in metals

In this work, a newly developed UHV-based high precision low background setup for hydrogen thermal desorption analysis (TDA) of metallic samples is presented. Using an infrared heating with a low thermal capacity enables a precise control of the temperature and rapid cool down of the measurement chamber. This novel TDA-set up is superior in sensitivity to almost every standard hydrogen analyzer available commercially due to the special design of the measurement chamber, resulting in a very low hydrogen background. No effects of background drift characteristic as for carrier gas based TDA instruments were observed, ensuring linearity and reproducibility of the analysis. This setup will prove to be valuable for detailed investigations of hydrogen trapping sites in steels and other alloys. With a determined limit of detection of 5.9×10(-3)µg g(-1) hydrogen the developed instrument is able to determine extremely low hydrogen amounts even at very low hydrogen desorption rates. This work clearly demonstrates the great potential of ultra-high vacuum thermal desorption mass spectroscopy instrumentation.

Initial oxidation of Fe–Cr alloys: in situ STM and ex situ SEM observation

AbstractIn order to understand the initial oxidation of Fe–Cr alloys a single crystal of Fe–15Cr (100) was oxidized at 440°C under controlled oxygen partial pressure in a UHV system and the surface morphology was observed using in situ STM (basic pressure 1×10−10 mbar); in addition, polycrystalline Fe&15Cr was oxidized at 400°C in an IR-furnace in atmospheric air and the morphology was observed using ex situ SEM. The chemistry of the surface oxide layers was studied by XPS.Preparation of the single crystal in the UHV system did not lead to segregation of Cr to the surface during heating. In situ STM investigation showed that oxidation of Fe–Cr commenced by nucleation of Cr oxide on the surface, due to selective oxidation of Cr. When the Cr at the surface and at the interface was completely consumed by nucleation of Cr oxide, Fe oxidized and covered the initial Cr oxide nuclei, resulting in an Fe oxide layer on the surface. Ex situ experiments showed that initial oxidation of the mechanically prepared poly...

Initial oxidation of Fe–Cr alloys:in situSTM andex situSEM observation

Abstract In order to understand the initial oxidation of Fe–Cr alloys a single crystal of Fe–15Cr (100) was oxidized at 440°C under controlled oxygen partial pressure in a UHV system and the surface morphology was observed using in situ STM (basic pressure 1×10−10 mbar); in addition, polycrystalline Fe&15Cr was oxidized at 400°C in an IR-furnace in atmospheric air and the morphology was observed using ex situ SEM. The chemistry of the surface oxide layers was studied by XPS. Preparation of the single crystal in the UHV system did not lead to segregation of Cr to the surface during heating. In situ STM investigation showed that oxidation of Fe–Cr commenced by nucleation of Cr oxide on the surface, due to selective oxidation of Cr. When the Cr at the surface and at the interface was completely consumed by nucleation of Cr oxide, Fe oxidized and covered the initial Cr oxide nuclei, resulting in an Fe oxide layer on the surface. Ex situ experiments showed that initial oxidation of the mechanically prepared polycrystalline alloy depended on the defect distribution in the surface. It started with formation of whisker-type Fe oxides along defects and proceeded with spherical-type nucleation and growth of Fe oxide. In both experiments, the final product on the surface was Fe2O3.

Thermodynamic Stability and Reaction Sequence for High Temperature Oxidation Processes in Steels

The grain boundary oxidation mechanism in hot rolled chromium – manganese steels during heat exposure at 700 °C was mathematically modeled. Given a fixed exposure time, the migration of the atomic species (iron, oxygen, chromium and manganese) has been calculated with the parabolic rate equation for diffusion. After each small time step, the data was transferred into the database ChemApp (GTT-Technologies, Germany) to calculate the oxide composition for each point in thermodynamic equilibrium. The concentration for each phase was illustrated in a phase map, similar to a cross section polish of the respective specimen. Total element concentration is shown as density plot to give a better comparison with experimental pictures from EDX or AES measurements. The obtained results are in good agreement with experimental data for alloyed steel samples with an element concentration below the critical concentration of protective oxide scale formation.