Elektrochemické a spektroskopické studium oxidù na žáropevných ocelích exponovaných v superkritické vodì/ The electrochemical and spectroscopic study of oxides on creep resistant steels exposed to supercritical water

The use of supercritical water in the power industry is motivated by the increase of heat efficiency of the energetic cycle. Increased efficiency decreases CO2 emissions. Therefore, supercritical energetic cycles were included among the clean coal technologies (CCT). Considering the corrosion aggressiveness of supercritical water, the fundamental requirement is the selection of the right structure material. All types of steels tested in this study were used for the construction of components of a supercritical power plant. Four types of creep resistant steels were chosen for the study, two of which were austenitic steels and the other two were ferritic-martensitic steels. Specimens of the steels were exposed to an experimental loop under supercritical conditions (580°C, 25 MPa) for 12 to 50 hours. At the same time, the impact of the starting phase of the exposure, i.e. the impact of the heating from the ambient temperature to the operation parameters, was studied. Chemical composition and transport properties of oxidic layers being formed during the interaction with the supercritical water (SCW) were studied ex-situ by electrochemical methods ad surface analyses (XPS and Raman spectroscopy). Electrochemical tests included Mott- -Schottky analysis based on the measurement of impedance vs. the electrode potential. The Mott-Schottky curve helped quantitatively characterise the semi-conductive behaviour of oxides. Both types of the semi-conductive behaviour, N and P, were observed for the oxides on all types of steel. The properties of the oxides, namely the density of defects and chemical composition, significantly depend on the way of the initiation of the short-term supercritical exposure.