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Eingeladener Vortrag
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The corrosion resistance of martensitic stainless steels (MSS) depends strongly on the chemical composition and the applied heat treatment. Both determine the distribution of the alloying elements in the microstructure and the resulting material properties. The addition of nitrogen is known to be beneficial for the pitting corrosion resistance of stainless steels. In case of MSS this effect is not only connected to nitro-gen itself because nitrogen can be used to substitute carbon which also influences the result of the heat treatment process. This paper shows the effect of nitrogen on the corrosion resistance in relation to the hardening process of MSS. Therefore the effects of austenitization temperature and cooling rate on microstructure, hardness and corrosion resistance were studied on the MSS X30CrMoN15 1 and X50CrMoV15. The effect of different cooling rates was studied using the jominy end quench test. The changes in corrosion resistance were detected with electrochemical potentiodynamic reactivation (EPR) and by the determination of critical pitting potentials. Besides this experimental approach thermodynamic calculations with the software thermocalc will be presented and used for the interpretation of the effect of nitrogen on the corrosion resistance of MSS.
Microbiologically influenced corrosion (MIC) on steel occurs where the presence and activity of microbes change the localized conditions on the surface of a metal substrate. For instance, metal reducing bacteria (MRB) are capable of utilizing metal compounds in the passive layer on stainless steel as electron acceptors during their metabolism. 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. Even though the electron transfer mechanisms are not yet fully understood, recent research shows that the secretion of electron shuttles like flavins contribute significantly to the extracellular electron transfer (EET). Electron shuttle molecules like riboflavin (RB) or flavin mononucleotide (FMN) are secreted by MRB after the transition from planktonic to sessile mode and exist in the biofilm at low concentrations. Therefore, they are precise early phase indicators of bacterial settlement.
This project aims at clarifying the electrochemical interaction mechanisms of MRB with stainless steel surfaces, with a special focus on the role of the extracellular redox molecules. The analysis of corrosion processes as a function of chloride and flavin concentration have been performed by means of electrochemical methods. Due to the differences in their chemical structure, FMN and RB have shown significant differences in terms of their adsorption behavior and the stability of the formed films, which directly influences the electron transfer processes. Therefore, Electrochemical Quartz Crystal Microbalance (eQCM) studies have been performed on sputtered FeCr electrodes to investigate the adsorption/desorption kinetics of flavins.
The results of electrochemical studies are complemented by the analysis of the changes in the passive film chemistry and the chemical composition of the adsorbed films by means of Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS). Changes in surface morphology have been investigated by means of Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM).The presentation will summarize our results on the degradation mechanisms of passive films on stainless steel surfaces in the presence of flavins and provide useful insights from a fundamental aspect for the understanding of the initial stages of microbiologically influenced corrosion.
Electrochemically active bacteria such as iron oxidizing bacteria (IOB) or iron reducing bacteria (IRB) accelerate the corrosion of stainless steel via the oxidation and reduction of iron oxides in the passive layer. The exposure to medium containing IOB and IRB leads to pitting corrosion with deep pits on stainless steel surfaces. Improving corrosion control via a better understanding of localized corrosion processes is highly relevant especially for reasons of safety or environmental protection since advancing pitting corrosion can easily lead to unpredictable material failure. Classic electrochemical methods however, provide insufficient information about the spatial heterogeneity of a corroding sample and fall short in detecting localized corrosion.
The aim of this work is to develop methods for the analysis of localized corrosion, on stainless steel surfaces induced by IRB and IOB biofilms. It is quite challenging to address the complexity of the variations in the passive layer composition and microstructure as well as the biofilm’s complexity with its local differential aeration cells and electrochemical parameters both at the same time. Thus in this study, agarose artificial biofilms mimicking the physical properties of a natural biofilm have been used as model systems to be able to differentiate between individual effects. This artificial biofilm is applied on a multi-electrode stainless steel sample to identify local anodic sites during exposure experiments. For more detailed analysis of active sites, a scanning electrochemical microscopy (SECM) has been applied. Using the SECM as an amperometric sensor, we have investigated concentration gradients of iron ions or oxygen within the biofilm and its immediate vicinity on actively corroding electrodes.
The presented analytical approach delivers promising results in clarifying how localized corrosion of stainless steels develops chronologically and spatially in the presence of IRBs and IOBs. Our results on model systems do also provide the basis for the investigation of natural biofilms in the future.
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 novel heat treatment concept of “Quenching and Partitioning” (Q&P) enables producing low alloyed martensitic steels with high strength and formability. Therefore austenite, retaining from a quenching temperature between Mf and Ms, is stabilised by carbon diffusion. This stabilised austenite transforms in martensite (TRIP effect) under mechanical loading. Current investigations on the Q&P heat treatment of martensitic stainless steels reveal a further enhancement of mechanical properties due to higher amounts of austenite. Thus a tensile strength of 1.800 MPa and a maximum elongation of 20 % are possible, while mechanical properties under compression are much higher due to a distinct SD effect.
The presentation aims to gain a further understanding of the Q&P heat treatment and the resulting mechanical properties for the steel X46Cr13, especially the mechanical behaviour under dynamic mechanical loading condition. Results of drop work and Split Hopkinson Pressure Bar test confirms the findings of quasi static compression test. Furthermore, Charpy impact tests show higher impact toughness compared to the common heat treatment of quenching and tempering (Q&T). Complementary investigations indicate a higher corrosion resistance of Q&P compared to Q&T.
The quenching and partitioning (Q+P) heat treatment enables a higher deformability of high strength martensitic steels. Therefore, it is necessary to have some metastable austenite in the microstructure, which transforms in martensite during plastic deformation (TRIP-effect). This condition is guaranteed by the quenching and the additional partitioning treatment. Due to local carbon diffusion retained austenite is stabilized and a partial reversion of austenite from martensite occurs. The Q+P heat treatment was investigated for the martensitic stainless steel 1.4034 (X46Cr13) concerning the influence of partitioning time. In line with these efforts metallographic, XRD- and EBSD-measurements were performed to characterize the microstructural evolution. The mechanical experiments included mechanical testing with different strain rates. The reversion of austenite by the partitioning treatment could be detected with EBSD- and XRD-measurements. Further-more the results of the mechanical testing showed improved values of strength and de-formability because of the Q+P heat treatment.
The corrosion resistance of stainless-steels grades is of high importance for the technical safety in different environmental conditions. Processing and surface finish influences the corrosion resistance of ferritic stainless steels, which may be considered for some application in marine environment. Three ferritic stainless steel alloys (X2CrNi12, X2CrTiNb18 and X2CrMoTi18-2) were exposed in four different surface finishes (rolled, dry grinding, glass beaded and electro polished) for five years in marine environment to obtain reliable information about the corrosion resistance. All samples were assessed after 22 and 60 months according to DIN EN ISO 10289 in validation degrees from 1 to 10. Complementary investigation using accelerated chamber test (salt spray test followed by four weeks storage at 30 °C / 80 % relative humidity) and short-term electrochemical methods (electrochemical potentiodynamic reactivation, pitting potentials and the KorroPad method) were conducted to compare their results. The presentation gives an overview on all methods and the generated results, which show benefits and restrictions of long-term, accelerated and short-term corrosion tests.
The characteristics of different molecules chosen as representatives for specific functionalities in conditioning layers play an important role on attachment behavior and later biofilm formation of bacteria. The chemical composition is a major component influencing the attachment but there is a conglomerate of influences.
Nickel-free, nitrogen alloyed austenitic stainless-steels, with about 19 wt.-% Mn and 0,8 wt. % N, are an interesting alternative to classic CrNi austenitic stainless steels due to their superior mechanical properties (Rm > 900 MPa, A5 > 50 %, Av > 350 J) in the solution annealed condition. The formation of chromium-rich nitrides during suboptimal heat treatment, processing or application leads to an inhomogeneous distribution of alloying elements in the microstructure, which reduces the corrosion resistance. Consequently, an accurate knowledge of the sensitization behavior is indispensable for the use of nickel-free, high-nitrogen austenitic stainless steels. The relationship between artificial aging, phase formation and corrosion resistance was investigated on the alloys X8CrMnN18-19 (1.3815) and X8CrMnMoN18-19-2 (1.4456), both alloyed with 0,8 wt.-% Nitrogen, in the present work. The microstructural evolution was studied by LM and SEM while the corrosion resistance was characterized with the electrochemical potentiodynamic reactivation (EPR) and the KorroPad indicator-test. Both alloys showed increased corrosion susceptibility within critical aging parameters. Finally, a sensitization diagram was described successfully for both alloys showing the positive effect of molybdenum.
The copper alloyed stainless steel 1.4542 (X5CrNiCuNb16-4) is used in different areas due to its good mechanical properties and corrosion resistance. Strength and corrosion resistance can be adjusted by the heat treatment, which is of importance for the application of this alloy. The mechanical properties (strength and hardness) are attributed to the dispersive precipitation of the copper rich ε–Phase. The additional precipitation of chromium carbides can reduce the corrosion resistance. Different ageing states were produced to investigate the precipitation behaviour with various methods. Furthermore, the influence of cold-rolling on the precipitation behaviour was studied in comparison to a solution annealed state without deformation. The microstructure was studied by SEM and the variations of hardness and magnetic proportion were characterised. The electrochemical potentiodynamic reactivation (EPR) was used to determine the corrosion resistance and detect chromium depletion in all heat-treated states. The results show that a work hardening accelerates the precipitation rate, while ageing at 600 °C reduces the corrosion resistance due to chromium depletion.