Ingenieurwissenschaften und zugeordnete Tätigkeiten
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P92 is one of the important steels for super heater tubes used in combustion plants. However, in fossil fuel fired environments the formation of oxides and sulfides are challenging the material. To understand early corrosion mechanisms, which are important for process prediction, high temperature in-situ ageing experiments of Fe-Cr-Mn alloys in SO2 and SO2+H2O atmosphere were performed, applying energy dispersive X-ray diffraction (EDXRD) analysis.
The ageing of Fe-Cr model alloys in 0.5 % SO2 and 99.5 % Ar atmosphere was investigated to aim in a fundamental and systematic analysis of the combined oxidation and sulfidation mechanism. The crystallization and reaction paths for oxide and sulfide formation were followed in-situ by energy dispersive X-ray diffraction (EDXRD) in an early stage of corrosion (30 s – 24 h). For this technique, high energetic white synchrotron X-ray radiation (10-100 keV) was used as radiation source. Diffraction pattern were collected continuously in an early stage of corrosion up to 24 h during the complete ageing experiment.
The crystalline phases, growing on top of the coupons, were identified directly via their specific dhkl values. The evolution of the extracted integral intensities of specific reflections of the corrosion products as a function of time access direct information about the kinetics of the nucleation and growth. The results presented here show for iron with 2 wt% Cr wuestite formation first. Wuestite vanishes after 10 min of reaction only and magnetite and hematite appear. Magnetite formation proceeds simultaneously with sulfide formation. Sulfides growth proceeds after an incubation time, which differs for low alloyed (2 wt% Cr) and high alloyed (9 wt% Cr) material.
The ageing of Fe-Cr model alloys in 0.5 % SO2 and 99.5 % Ar atmosphere was investigated to aim in a fundamental and systematic analysis of the combined oxidation and sulfidation mechanism. The crystallization and reaction paths for oxide and sulfide formation were followed in-situ by energy dispersive X-ray diffraction (EDXRD) in an early stage of corrosion (30 s – 24 h). For this technique, high energetic white synchrotron X-ray radiation (10-100 keV) was used as radiation source. Diffraction pattern were collected continuously in an early stage of corrosion up to 24 h during the complete ageing experiment.
The crystalline phases, growing on top of the coupons, were identified directly via their specific dhkl values. The evolution of the extracted integral intensities of specific reflections of the corrosion products as a function of time access direct information about the kinetics of the nucleation and growth. The results presented here show for iron with 2 wt% Cr wuestite formation first. Wuestite vanishes after 10 min of reaction only and magnetite and hematite appear. Magnetite formation proceeds simultaneously with sulfide formation. Sulfides growth proceeds after an incubation time, which differs for low alloyed (2 wt% Cr) and high alloyed (9 wt% Cr) material.
Ferritic-martensitic high temperature alloys are widely used as boiler tube and heat exchanger materials in combustion based power plants. All technologies have in common that the applied materials are exposed to different temperatures, process pressures and reactive atmospheres which lead to a change of the material properties and a further degradation of the material.
To date corrosion analytics mainly proceeds via the use of various microscopic techniques and the analysis of the corrosion products after the reaction is completed. Comprehensive efforts have been made to study high temperature corrosion by the use of environmental SEM’s or in-situ TEM technologies. The here presented work will show a different approach to study high temperature gas corrosion in a multiple gas atmosphere by energy dispersive X-ray diffraction (EDXRD). For this technique high energetic white X-ray radiation (10-100 keV) was used as radiation source instead of conventional monochromatic radiation. It enables us to study crystallization procedures on short and medium time scales (1 min < t < 24 h) and the collection of Bragg-Signals of the phases of interest as a function of process time. Their occurrence can directly be correlated with thermodynamic and kinetic parameters. A special designed corrosion reactor was used to combine high temperature gas corrosion experiments with the collection of diffraction patter. The crystallization and reaction paths for oxide and sulfide formation was followed in-situ on Fe-Cr and Fe-Cr-Mn model alloys in a hot SO2 containing (T=650 °C) atmosphere.
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.
Multielectrodes are arrays of single electrodes arranged in a particular geometry. In our work, all the single electrodes are identical stainless steel X5CrNi18-10 (1.4301) wire electrodes. Using a multielectrode analyser all single electrodes are connected via zero resistance ammeters, simulating a galvanically coupled single electrode surface. The advantage of the multielectrode analyser (MMA) is that the currents flowing between single electrodes can be measured. Thus, real-time maps can be generated indicating where anodic and cathodic areas lie on the surface of the multielectrode and how they behave.
The combination of the multielectrode analyser with the scanning electrochemical microscopy (SECM) enables the identification of corrosion sites and the detailed electrochemical analysis.
Geothermal brine is considered to be an aggressive
environment as it contains various dissolved salts.
Even though carbon steel is commonly used as a
construction material due to its machinability and
economical reason, it is susceptible to uniform and
localized corrosion in a high temperature and high
pressure system. Therefore, a coating system is
introduced to protect the carbon steel against
corrosion in such environment.
1. Carbon steel is susceptible to uniform corrosion in presence of oxygen, with corrosion rate of immersion at 150 °C of 0.39 mm/a.
2. Exposure test of carbon steel at 150°C and 10 bar (pressurized by Ar) results in pitting corrosion with depth of 19 µm and width 4 µm on average.
3. OCP measurement shows that the stability of measurement is reached after 40 hours of immersion in the solution.
4. EIS measurement is useful to observe the effect of temperature, immersion time, and coating composition on the corrosion processes of coated metals for use in geothermal application.
Methanogenic Archaea
(2017)
Different environmental samples reveal that methanogenic Archaea are part of a multi-species biofilm on corroding metallic structures. Studies on microbial influenced corrosion (MIC) focus mainly on sulphate reducing Bacteria (SRB), leading to the assumption that they are exclusively responsible for metal corrosion. In fact, methanogenic Archaea are known to be involved in metal corrosion as well (e.g. Methanococcus maripaludis DSM 2067). In some cases SRB and methanogenic Archaea have comparable high corrosion rates. However, the underlying mechanisms causing corrosion are still unknown.
The goal of this study is to develop suitable methods for analyzing two environmental isolates (M. maripaludis DSM 2067, M. maripaludis KA1) and two human-related isolates (Methanobrevibacter oralis and Methanobrevibacter smithii) for their ability to deteriorate/transform metals, which are relevant for technical and clinical applications. Moreover, the studies will provide essential information on the interaction mechanisms of human-related Archaea, which are frequently found in peri-implantitis, with dental material such as implants, crowns and bridges leading to their degradation and transformation.