TY - CONF A1 - Nützmann, Kathrin A1 - Frank, Thomas T1 - Influence of chromium on orientation dependent high temperature corrosion N2 - Ferritic steels with Cr-contents up to 13 wt. % are used as building components in combustion based power plants. These materials are subject to aggressive corrosion caused by the reaction of the steel with highly corrosive gases under high temperatures up to 620°C. Detailed knowledge about corrosion mechanisms regarding the attack of gaseous sulfur dioxide have a significant influence and are not entirely understood. The reaction with SO2 can be seen as two simultaneous processes; oxidation and sulfidation. Especially in the initial state, these processes are more kinetically controlled than thermodynamically. However, a detailed knowledge is a key aspect to understand the complex high temperature corrosion of high alloyed steels with a variety of alloying components ageing under combustion gases with a variety of gas components such as CO2, O2, N2, or SO2/SO3, and to better predict altering of building components and inspection intervals. To get more information about corrosion mechanisms in general and the simultaneous process of oxidation and sulfidation under SO2, initial stadia of the corrosion mechanism have to be studied. In order to minimize and control the free parameters of the corrosion procedure, simplified systems with model alloys and simplified gas compositions are a feasible tool. The current presentation will deal with the initial stages of high temperature corrosion on Fe-Cr model alloys under SO2+Ar atmospheres. Experiments took place using an infra-red light furnace with higher heating and cooling rates compared to regular tubular furnaces to preserve the high temperature phase distribution. Pure Fe, Fe with 2 wt. % and 13 wt. % Cr were used and aged for time scales between 30s < t < 6h under 0.5% SO2 and 99.5% Ar. Specific positions on each sample were marked with Vickers indentations and the crystal orientations were analyzed using ‘Electron Backscatter Diffraction’ (EBSD) to get the orientation of the ferrite grains in advance. After the corrosion experiment, the same positions were analyzed again using scanning electron microscopy (SEM) and electron microprobe analysis (EMPA) to correlate the crystal orientation with the corrosion product and the elemental distribution. Further, cross sections of the samples were prepared to analyze the phase distribution in depth. Even though Cr and Fe form a solid solution in the crystal lattice, a significant difference in oxide island growth was observed for orientation dependent corrosion on different Fe-Cr alloys. T2 - Junior Euromat CY - Lausanne, Switzerland DA - 10.07.2016 KW - crystal orientation KW - corrosion KW - sulfidation PY - 2016 AN - OPUS4-37784 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Nützmann, Kathrin A1 - Mosquera Feijoo, Maria T1 - Nucleation and growth of sulfur phases in grain boundaries N2 - Ferritic-martensitic high temperature alloys are used as building components for different power plant technologies. Depending on the type of fuel, the used power plant materials are exposed to different temperatures and reactive atmospheres containing e.g. CO2, O2, or SO2. Despite the sulfur chemistry is commonly present as an impurity in fossil or bio fuels; its role in high temperature corrosion is not entirely understood. During high temperature corrosion, high-alloyed steels often show sulfur precipitates with the ignoble alloy component(s) along grain boundaries within the base material. Sulfur precipitates are known to seriously influence the mechanical properties of the building component. In the case of VM12 and T92 steels, sulfur phases penetrate the base material along grain boundaries during the corrosion under oxyfuel atmosphere up to 20 µm within the first 960h (Fig. 1a). Figure 1a shows the oxide scale and (Cr, Mn, Fe)xSy grain boundary precipitates in the base material for a T92 steel aged for 960h under oxyfuel atmosphere. Figure 1b shows a thin oxide scale with nodules and also sulfur precipitates of (Fe, Cr)xSy along grain boundaries of the base material for a Fe13Cr model alloy aged for 24h under SO2 atmospheres. After 24h, sulfur precipitates already reached a depth of ca. 15 µm. The present work shows the corrosion behavior of Fe-Cr model alloys with Cr-contents similar to technical steels up to 13 wt%, aged under oxyfuel (27H2O/60CO2/1SO2/10N2/2O2) and SO2 atmospheres in the temperature range of 550 °C < T < 700 °C and for different time scales between 24 h < t < 960 h. During aging, the reactive gases were added when the experimental temperature was reached. To focus on the reaction of the intended elements Fe, Cr, S, and O, model alloys of high purity are used. Transport depths of sulfur and the nucleation of the precipitates are discussed for both, model alloys and technical steels. T2 - Materials Science & Engineering CY - Darmstadt, Germany DA - 27.09.2016 KW - Corrosion KW - Sulfide KW - Sulfidation PY - 2016 AN - OPUS4-37786 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Nützmann, Kathrin A1 - Schulz, Wencke A1 - Saliwan Neumann, Romeo A1 - Wollschläger, Nicole A1 - Stephan, Christiane A1 - Kranzmann, Axel T1 - The influence of sulfur on initial high temperature corrosion of Fe-Cr model alloys N2 - Ferritic steels with Cr-contents up to 13 wt. % are used as heat exchanger or boiler tube materials in combustion based power plants. These materials are subject to aggressive corrosion caused by the reaction of the steel with highly corrosive gases under high temperatures up to 650°C. The early stages of corrosion and sulfidation especially and the influence of the Cr-content in the alloy are thereby not understood but of fundamental interest. Our work shows corrosion mechanisms and presents corrosion models for Fe and different Fe-Cr-alloys under pure SO2 und SO2+O2 atmospheres for different time scales. Modell alloys of high purity are used to focus on the reaction of the intended elements: Fe, Cr, S, and O. Long-time experiments (≥12h) took place in tube furnaces and short-time experiments (≥5min) in a special designed light furnace. Heating and cooling took place under inert atmosphere. The reactive gases were added not until the experimental temperature was reached. Samples were analyzed using high resolution synchrotron X-ray diffraction using a micro focus setup and electron microscopy including FIB. Many earlier studies on corrosion and sulfidation on iron based alloys show a higher corrosion rate and material loss when sulfur was present in the atmosphere. In contrast, later studies propose an inhibiting effect of sulfur containing atmospheres. On the one side, most likely due to the formation of a protective layer of Mx(SO4)y -phases at the scale-gas interface. On the other side, a very recent study proposes the theory that MxSy-phases limit the further diffusion of elements. Up to now, no Mx(SO4)y –phases were detected in our samples, but sulfide phases nucleated within the oxide phases. For pure Fe our samples show FeS at the scale-metal interface working as a diffusion barrier for Fe-Cations from the base material in to the oxide scale. The above lying spalled off oxide-sulfide scale shows a wide area of small voids and accumulations of FeS-Crystals at the bottom. Under the main oxide scale a second generation of oxide-sulfide scale starts to form. For the Fe-Cr-alloys the inner oxide scale only shows FeS- and CrS-phases surrounded by hollow space. The here presented study will explain and discuss a new growth model for the shown phenomenon. T2 - 9th High Temperature Corrosion and Protection of Materials CY - Ile des Embiez, France DA - 15.05.2016 KW - Sulfidation KW - Diffusion barrier KW - Fe-Cr alloys PY - 2016 AN - OPUS4-38250 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Nützmann, Kathrin T1 - Corrosion mechanisms of Fe-Cr modell alloys in gas atmospheres containing SO2 N2 - In Kraftwerkstechnologien verwendete ferritische Fe-Cr-Stähle sind verschiedensten Temperaturen, Prozessdrücken und aggressiven, zum Teil schwefelhaltigen Verbrennungsgasen ausgesetzt, die zu Korrosion führen. Noch ungeklärt ist die Rolle der entstehenden Sulfide in der schützenden Oxidschicht und deren Auswirkung auf die Lebensdauer der Bauteile. Unsere Arbeit zeigt Korrosionsmechanismen ausgewählter Fe-Cr-Modelllegierungen unter Ar-SO2 Atmosphäre für verschiedene Zeitskalen. Der Focus dabei liegt auf Kurzzeitexperimenten (12h ≤ t ≤ 250h), um die initialen Stadien der Schwefelkorrosion zu untersuchen. T2 - Sektionstreffen der DMG Deutsche Mineralogische Gesellschaft CY - Bad Windsheim, Germany DA - 24.02.2016 KW - Fe-Cr Modell Alloys KW - Fe-Cr Modelllegierungen KW - Diffusion KW - Korrosion KW - Diffusion KW - Corrosion PY - 2016 AN - OPUS4-35388 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -