This thesis focuses on the influence the fireside environment poses on selected austenitic and nickel-based alloys dedicated i.a. for construction of superheaters and reheaters in high efficiency pulverized coal boilers. Within the study six alloys are exposed in three power plants, one pilot scale combustion test rig and simultaneously tested in technical and laboratory conditions at metal temperatures in the range 580 – 750°C in order to determine their performance in different coal combustion conditions. The corrosion behaviour is estimated using various analytical techniques. Subsequently corrosion rate is calculated with respect to determined corrosion product and thereof deflected metal loss observed at varying exposure times and environments. Fireside corrosion is studied having in focus the synergy effect of combustion gas atmosphere, real fly ash deposits and the alloy itself. Influence of the deposit is studied carefully with respect to corrosion and finally the synergy effect of metal-gas and metal-gas-deposit is discussed supported by the outcomes of the high temperature corrosion tests performed under mono-deposits – selected mineral phases typical for coal combustion.
Generally for both material groups, austenitic and nickel-based alloys, the data on metal loss determined for the specimens exposed in power plants confirm the results generated from the laboratory tests. Whereas for the studied austenitic alloys the corrosion resistance increases with rising chromium content, the same tendency has not been confirmed in the case of the examined nickel-based alloys. The superalloys show better performance with increasing titanium and decreasing molybdenum content. Nickel-based alloys with one exception withstand very well the SOx-rich conditions in the studied temperature range. Intergranular oxidation is often noticed at all the three nickel-based alloys and it appears to originate from the manufacturing processes rather than from exposures to corrosive environment.
Moreover this work shows the crucial role the deposits play in a fireside corrosion process. Type of the present deposit plays a significant in corrosion. Due to their impact on corrosion process deposits can be divided into protective, neutral and corrosive. On the one hand the deposits may act as corrosion accelerator (i.e. K2SO4, Na2SO4, FeS2, NaCl) leading to catastrophic corrosion in the studied temperature ranges, on the other hand they may act as corrosion inhibitor (CaCO3) due to binding potential of SOx from the surrounding gaseous environment making the corrosive gases unavailable for the sulfidation of the metal tubes. The deposits may as well possess neutral role as observed in the case of mineral phases stable at the operation conditions like CaSO4. In the case of protective fly ash deposits the saturation of deposits with sulfur plays a significant role in observed corrosion process.
The thesis was published by epubli.