Development of hot corrosion resistant coatings for gas turbines burning biomass and waste derived fuel gases

Carbon dioxide emission reductions are being sought worldwide to mitigate climate change. These need to proceed in parallel with optimisation of thermal efficiency in energy conversion systems on economic grounds to achieve overall sustainability. The use of renewable energy is one strategy being adopted to achieve these needs; with one route being the burning of biomass and waste derived fuels in the gas turbines of highly efficient, integrated gasification combined cycle (IGCC) electricity generating units. A major factor to be taken into account with gas turbines using such fuels, compared with natural gas, is the potentially higher rates of hot corrosion caused by molten trace species which can be deposited on hot gas path components. This paper describes the development of hot corrosion protective coatings for such applications. Diffusion coatings were the basis for coating development, which consisted of chemical vapour deposition (CVD) trials, using aluminising and single step silicon-aluminising processes to develop new coating structures on two nickel-based superalloys, one conventionally cast and one single crystal (IN738LC and CMSX-4). These coatings were characterised using SEM/EDX analysis and their performance evaluated in oxidation and hot corrosion screening tests. A variant of the single step silicon-aluminide coating was identified as having sufficient oxidation/hot corrosion resistance and microstructural stability to form the basis for future coating optimisation.