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Eingeladener Vortrag
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In service, most materials are operated under simultaneous corrosive and mechanical load and there are very few methods capable for testing material degradation under these conditions, especially when it comes to high resolution analysis. For this purpose, in this work, a tensile module capable of uniaxial stretching and compression with up to 5 kN force was integrated into an AFM stage. The elimination of the need for sample unmounting and remounting and the resulting possibility of keeping the sample under constant mechanical load during AFM measurements not only enables a precise positioning of the area of interest but also allows for the analysis of processes in the elastic deformation regime.
This methodology was demonstrated for two case studies. Scanning Kelvin Probe Force Microscopy (SKPFM) was used as the main tool to characterize the deformation behavior. Moreover, a flexible electrochemical measurement cell was used to enable electrochemical analysis by means of electrochemical impedance spectroscopy (EIS) and Linear Sweep Voltammetry (LSV) during AFM measurements at different levels of strain. The in situ AFM results are complemented by microstructure analysis by means of electron backscatter diffraction (EBSD).
In the first case study, the deformation induced delamination of a thin organic coating on AA2024 T3 aluminium alloy was investigated as a function of alloy surface treatment. The formation of cracks in the insulating passive film enabled an early detection of deformation processes by means of SKPFM. The second case study focused on the comparison of corrosion and deformation behavior of conventional and additively manufactured 316 stainless steels. In comparison to the conventional 316 stainless steel, the effect of processing was clearly detectable on the additively manufactured material as zones of inhomogeneous potential, which also affected the initiation of local corrosion processes. The contribution will provide detailed information on the new AFM setup and summarize our results from both case studies.
The presentation summarizes our recent results on the coupled electrochemical methods for high resolution corrosion studies. The combination of Scanning Electrochemical Microscopy (SECM) and multielectrode (MMA) based real-time corrosion monitoring was presented as a new method for achieving high time resolution in local electrochemical analysis. Correlative imaging by means of Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) was demonstrated as a tool for the investigation of local corrosion processes initiated by the intermetallic particles (IMPs) on AA2024-T3 aluminium alloy.
The performance of functional coatings and adhesively joined hybrid components relies strongly on the stability of the polymer-metal interface. With the increasing utilization of multi-material structures in the automotive and aerospace industry, it is of great scientific and technical interest to understand the processes leading to interface degradation and to develop novel strategies to increase corrosion and delamination resistance.
The aim of this project is to develop thin epoxy-based films and their carbon nanofiller loaded composites on aluminium alloy AA2024-T3 as a model system and to investigate their interfacial stability under corrosive and coupled corrosive-mechanical load. Spin coating was used for the layer-by-layer deposition of poly[(o-cresyl glycidyl ether)-co-formaldehyde] and poly-(ethylenimine) bi-layers. Atomic force microscopy (AFM) results indicate a very homogeneous and dense film with low surface roughness. Carbon nanofillers were introduced either by mixing into the coating components or in between individual layers to control the separation between the carbon nanofillers and alloy surface. The film chemistry and barrier properties were characterized by means of spectroscopic and electrochemical methods, respectively. The degradation and delamination behavior of the epoxy-based films was characterized by means of in situ AFM corrosion experiments. The quantitative imaging (QI) mode allowed the observation of hydrogen-generation induced blister formation during exposure to corrosive electrolyte and how the local corrosion processes evolved with exposure time. Complementary energy dispersive X-ray spectroscopy (EDX) analysis was performed to correlate the corrosion behavior with the different intermetallic particle chemistries and distributions. The presentation will summarize our results on the effect of interface chemistry and carbon nanofiller – alloy separation on the initiation of local corrosion processes on thin film covered AA2024-T3 aluminium alloys.
The efficient exploration of novel alloy compositions is crucial for advancing the development of new materials. One widely utilized approach for creating phase diagrams is the use of diffusion-controlled synthesis for gradient alloys. This method is also an effective means for rapidly identifying potential material combinations.
The present study focusses on the exploration of quaternary multi-principle-element alloys (MPEAs) using diffusion multiples. We established a diffusion system by combining an equimolar ternary alloy (FeNiCr) with single diffusing elements Mn and Mo. Using ThermoCalc simulations, we determined suitable temperature ranges where we can expect the formation of single-phase alloys. Depending on the diffusion constants of the selected metals, diffusion times were adjusted to obtain diffusion zones with a depth of greater than 50 μm. Microstructural and compositional information was gathered via scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) and correlated to local mechanical properties evaluated by means of nanoindentation. Local corrosion properties were examined using Atomic Force Microscopy (AFM) and Scanning Electrochemical Microscopy (SECM).
Our results indicate that the ThermoCalc simulations have a good predictive power for crystallographic phases. However, especially with the Mn-FeNiCr system, formation of Cr-rich secondary phases were observed, which led to Cr-depletion and thus to localized corrosion processes in the matrix at the vicinity of the secondary phases. Based on the results of the local electrochemical tests, we have selected single-phase compositions along the diffusion zone and synthesized these alloys in bulk for corrosion studies by means of potentiodynamic polarization and SECM.
The presentation will summarize our methodology using diffusion couples as an
efficient tool for exploring compositional spaces of MPEAs in the search for novel single-phase alloys and the results of our correlative study on the mechanical and corrosion properties of these materials.
The efficient exploration of novel alloy chemistries is crucial for advancing the development of new materials. Diffusion-controlled synthesis of gradient alloys is an intelligent approach for creating phase diagrams and to effectively identify potential material combinations with tailored properties.
This project focusses on the design of quaternary multi-principle-element alloys (MPEAs) using diffusion couples. Our diffusion system contains an equimolar ternary alloy (FeNiCr) and additional single diffusing elements e.g. Mn and Mo. We determined the optimal temperature ranges for the diffusion thermal treatment by means of ThermoCalc simulations with the aim to form single-phase MPEAs. Microstructure and chemical characterization of the diffusion couples were performed by means of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). For most alloy couples, the diffusion zone contained a single-phase alloy matrix with diffusion-induced compositional gradient as well as precipitation phases. This heterogeneity makes the diffusion couples interesting materials to investigate local mechanical and corrosion properties. Thus, local corrosion properties were examined using Atomic Force Microscopy (AFM) and Scanning Electrochemical Microscopy (SECM). Nanoindentation was used for the analysis of local mechanical properties.
Based on the results of the local corrosion analysis, we have selected single-phase alloy chemistries along the diffusion zone and reproducibly synthesized these alloys in bulk for detailed corrosion studies by means of potentiodynamic polarization and SECM.
The presentation will briefly summarize our methodology and motivation for using diffusion couples as an efficient tool for exploring phase diagrams of MPEAs in the search for new alloy chemistries and the results of our correlative study on the mechanical and corrosion properties of these materials.
Metal reducing bacteria (MRB) are capable of utilizing different metals, such as iron, chromium, manganese or uranium as well as many organic compounds, as electron acceptors for their metabolism. Via direct and indirect electron transfer processes MRB are able to convert insoluble passive film species like Fe(III)-oxides to soluble Fe(II)-oxides and hydroxides. This weakening of the passive film not only leads to an acceleration of the general corrosion processes, but also increases the susceptibility of stainless steels to pitting corrosion. Electron transfer mechanisms are not yet fully understood and the role of bacteria in corrosion processes is controversially discussed in the literature. Moreover, recent research indicates that the secretion of electron shuttles like riboflavins by MRB also contributes to the extracellular electron transfer.
This project aims at clarifying the chemical and electrochemical interaction mechanisms of MRB with stainless steel surfaces. To investigate the changes in the oxide chemistry on the stainless steel surface in the presence of biomolecules and MRB a new flow cell has been designed and constructed which enables the collection of XANES (X-ray Absorption Near Edge Structure) spectra in fluorescence mode at the Fe K-edge and electrochemical analysis. Availability of oxygen and the pre-exposure of the MRB to Fe(III) during cultivation have been investigated as parameters with significant effect on the corrosion rates. XANES analysis is supplemented by ex-situ X-Ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) to complete the surface characterisation in terms of the oxide chemistry and the composition of organic residues. Complementary electrochemical quartz crystal microbalance (e-QCM) measurements have been performed to quantify the kinetics of bacterial attachment and biofilm formation. Together with the frequency shift, the evolution of the dissipation signal has been analyzed to investigate the changes in viscosity and structure of the biofilm from initial stages up to maturation. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) has been used to study the structure and viscoelastic properties of the biofilms after e-QCM experiments.
The presentation will summarize our results on the effects of individual surface and environment related parameters on the chemical/electrochemical interaction mechanisms of MRB leading to passive film degradation on stainless steel surfaces and provide useful insights from a fundamental aspect for the development of novel mitigation strategies for microbiologically influenced corrosion.
The interface of a metal substrate and a biofilm can differ significantly from the surrounding environment. Metal reducing bacteria (MRB), for instance, are capable of utilizing various metallic compounds as electron acceptors. Besides chromium, uranium and manganese as well as many organic compounds, Fe(III) is converted to the soluble form Fe(II) during the bacterial metabolism. This could lead to a weakening of the protective passivation layer on stainless steel and thereby facilitate microbiologically influenced corrosion (MIC). Even tough the processes of electron transfer are not yet entirely explained, the contribution of flavins and other humic substances as electron shuttles is widely discussed in the literature. Moreover, the adsorption of organic thin films on steel surfaces can lead to surface preconditioning and thus to changes in adhesion behavior of bacteria.
The aim of this work is to understand the chemical and electrochemical interaction mechanisms of MRB with steel surfaces by combining electrochemistry and surface-analytical techniques. The investigations primarily focus on the effect of pre-adsorbed thin organic films and self-assembled monolayers (SAMs) on the electron transfer processes between bacteria and steel surfaces. Electrochemical Quartz Crystal Microbalance (eQCM) studies have been performed to investigate the adsorption/desorption kinetics of organic films as well as the formation of biofilms on FeCr electrodes. Furthermore, the evolution of the biofilms on steel surfaces has been analyzed by means of electrochemical impedance spectroscopy (EIS) to support the QCM studies with information on structural changes during different stages of biofilm growth. Electron transfer and corrosion processes have been analyzed by means of square wave voltammetry (SWV) and linear sweep voltammetry (LSV), respectively. The results of electrochemical studies are complemented with microscopic and spectroscopic characterisation of organic adsorbates and analysis of changes in the passive film chemistry and surface morphology.
This presentation will summarize our results on the chemical and electrochemical interaction mechanisms of MRB on steel surfaces leading to passive film degradation. The role of flavins in accelerating corrosion processes will be elucidated in detail to provide useful insights from a fundamental aspect for the understanding of the initial stages of microbiologically influenced corrosion in the presence of MRB.