Identification of degradation mechanisms in coatings for supercritical steam applications

摘要

The development and qualification of coatings for materials used in modern steam power plants stems from the increased demand for higher efficiency, and hence higher operating temperatures. Within the EU funded project 'SUPERCOAT', several coatings, both overlay and diffusion type, were investigated. Seven different coatings are presented in this work. They included two commercially available HVOF coatings (Ni-20Cr and Ni-50Cr), an aluminium-based slurry coating (IPCOTE), together with two further variations of this slurry coating containing sputter-coated interlayers. An overlay slurry coating consisting of silica particles embedded in a matrix of alumina and chromia was also examined. The final coating to be investigated was a pack-aluminised sample of P92. All the coating systems examined showed superior oxidation resistance compared to the 9%Cr steel substrate (P91 or P92) in extended exposures to a steam environment at 650℃. However, in service component lifetime will be limited by degradation of the coating, therefore it is essential that the mechanisms controlling this behaviour are understood. This paper reviews several degradation mechanisms that have been observed during long-term exposure of these coatings. The mechanisms that have been observed include depletion of active alloying elements, diffusion of aluminium into the substrate from the coating, formation of Kirkendall porosity and mechanical failure of the coatings. Examples of each of these mechanisms will be presented. Possible processing routes to avoid these degradation mechanisms will also be discussed.