Ferritic-martensitic high temperature alloys are subject to devices for different power plant technologies. 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 further degradation of the material. The mechanisms can be understood as a combination of different competing processes. Two of these processes, the combined oxidation with sulfidation, was studied here in pure Fe and FeCr model alloys within the first 250 h of reaction time in Ar+SO2 atmosphere at different temperatures and time scales. The corrosion areas are subdivided into an inner and an outer corrosion zone and consist of higher iron-oxides (Fe2O3 and Fe3O4) and iron sulfides (see figure 1). An exemplarily chemical mapping of Fe, O and S within a cross section of the corrosion area is illustrated in Figure 1. Iron sulfides are found in a well crystallized state at the metal/scale interface as well as in the outer scale within a dense intergrown network of Fe3-xO4 and Fe1-xS. A lift off of the scale took place at the area where the highest amount of sulfides was detected. The former sample surface is marked by the red horizontal line, deviding the corrosion area into an outer and an inner zone. Especially the dimensions of sulfide phases vary from nm to µm scale. Since the corrosion products show different characteristics on different length scales, multi-scale analytical methods were used to understand fundamental aspects of the growth mechanism. Electron microscopy techniques such as TEM and SEM as well as synchrotron X-ray diffraction were used to study chemical and structural characteristics of the oxide and sulfide phases. The presentation will illuminate combined oxidation and sulfidation on different dimensions and discuss the potential of multi-scale approaches for studying complex material degradation mechanisms.
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|Präsentation||Figure 1||Figure 1. BSE micrograph and chemical mappings of Fe, S and O of pure iron after exposure for 250 h at T= 650°Cin 99.5 % Ar and 0.5 % SO2 reactive atmosphere. The red horizontal line indexes the original sample surface.||2 MB||Download|