Hochtemperaturoxidationsstudien zum Wasserdampfeinfluss auf thermisch wachsende Chromoxidschichten

Most high temperature alloys rely on the formation of a protective chromia scale during service. In this thesis the kinetic data of oxidation is strongly connected to a microstructural analysis and to stress measurements of the Cr2O3 layer formed on Cr, Ni-25Cr, Fe-10Cr and Fe-18Cr. The alloys were exposed to different dry and wet atmospheres with different oxygen and water vapour contents. Samples with different thicknesses were oxidized for 24 hours in the temperature range from 700°C - 1000°C. Thermogravimetric data were obtained using a microbalance system. The microstructures of oxidized samples were analysed using various experimental techniques such as scanning electron microscopy, X-ray diffraction and transmission electron backscatter diffraction in conjunction with transmission electron microscopy analysis.
Significant differences in the oxidation rates, observed during oxidation experiments, can be explained by different atmospheric conditions and compressive stresses within the scales. Obviously, the defect structure of chromia changes depending on the factors mentioned above. The differences of chromia growth in dry and water vapour containing atmospheres are a result of interactions between chromia and various gas species, such as hydrogen and hydroxide and also protons. It can be concluded that some oxidation phenomena cannot be explained by classical oxidation theory. However, it is reasonable to assume that water vapour influences the point defect structure in the chromia scale notably. As an important consequence of the precisely adjusted gas compositions, the effects of the oxygen partial pressure and the water vapour addition on the oxidation behaviour and the corresponding defect structure of chromia have been studied thoroughly and systematically.