CCS and Power Systems
Advanced Energy Systems - Hydrogen Turbines
Degradation of TBC Systems in Environments Relevant to Advanced Gas Turbines for IGCC Systems
Performer: University of Pittsburgh
Project No: FE0007271
- Commercial fly ash was found to cause extensive degradation of the YSZ top coat material at temperatures above 1300 °C. At relatively lower temperatures, a solidstate reaction occurred between the YSZ and ash which was less detrimental, but still relevant to TBC durability.
- Synthetic fly ash of varying compositions was used to assess TBC degradation at the lower temperatures. Solid-state reaction to form calcium zirconate (CaZrO3) was found to occur only if a minimum level of CaO was present in the ash.
- Realistic synthetic fly-ash compositions with additions of K2SO4 and FeS caused extensive damage, resulting in thicker reaction layers and much deeper infiltration into cracks within the YSZ. Using synthetic mixtures will provide an understanding of the specific role(s) of each ash constituent in accelerating the degradation process.
- Testing done in CO2+20%H2O environments showed very little difference in the formation of the CaZrO3 layer as compared to experiments in dry air. These initial results suggest that this particular attack on YSZ may not depend greatly on the gas atmosphere.
- The presence of CO2 or H2O in the reacting atmosphere was not found to alter the protective oxidation of state-of-the-art NiCoCrAlY bondcoat alloys at 1100 °C.
- A study of the reactive element level in NiCoCrAlY alloys revealed that while harmless in air, slightly excessive Y doping is detrimental in the presence of commercial fly ash, as Y-rich oxide inclusions in the scale act as initiation sites for rapid alloy degradation.
- Upon exposure of various NiCoCrAlY alloys to commercial fly ash, as well as synthetic ash mixtures (oxides with/without FeS or K2SO4) in air and in O2-SO2, the extent of metal loss could be directly related to the relative amounts of β-NiAl and γ-Ni phases in the alloy structure.
- Exposure of selected alloys to individual oxide deposits (CaO, MgO, Al2O3, SiO2, Fe2O3, Cr2O3) in air showed that, while all oxides altered the extent of Al2O3 growth, only CaO caused significant alloy degradation.
- CaO was shown to react preferentially with the Al-lean γ phase through the formation of a liquid calcium chromate. The β-γ phase distribution was found to be critical to the establishment of a continuous Al2O3 layer, which effectively mitigates alloy degradation. These observations provide an understanding for the better resistance of β-rich alloys in the presence of more complex ash deposits, and offer guidance for coating design.