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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.