Degradation and protection phenomena of high-alloy Fe-Cr alloys in hot, multi-component gas systems

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The proposed project aims at the fundamental investigation of the damage mechanisms of high-alloyed Fe-Cr alloys in a hot gas atmosphere under SO2 and H2O and under chlorine-containing deposits (KCl). These materials are used as materials for heat exchanger tubes in various combustion power plants. The damage to the flue gas side of the materials used in biomass and biomass co-fired power plants is a serious problem and limits the operating temperature of such plants. Fundamental aspects of crystallization and growth of corrosion products will be investigated at an early stage of degradation by means of time-dependent ageing experiments. High purity model alloys (Fe-Cr) and model gas compositions are to be used to focus on the reaction of the elements Fe, Cr, S and Cl involved. The qualitative and quantitative phase analysis of the formed corrosion zones is performed using a broad spectrum of classical microscopy techniques and advanced characterization methods, such as X-ray near-edge structure absorption spectroscopy (XANES). The combination of classical microscopy with XANES leads to an extremely precise verification of the existing reaction products. This approach should help to show diffusion and transport paths of metal ions and gas components over the crystal structure of the reaction products. Crystallization and growth at a very early stage of corrosion will be followed directly during the reaction by in-situ energy dispersive X-ray diffraction in a specially designed light reactor.The project aims at three questions. The first objective is to investigate the formation of protective Cr2O3. For this purpose, an experimental setup is used that allows materials to be exposed to a hot flue gas. First, ageing tests are carried out on three different high-alloyed (Cr>15Ma%) model alloys with reaction times 12h