Nickel-base superalloys for ultra-supercritical coal-fired power plants: Fireside corrosion. Laboratory studies and power plant exposures

► Synergy effect of metal, gas and fly ash deposit in fireside corrosion. ► Sulphur-induced corrosion. ► Fly ashes as corrosion inhibitor or corrosive agent. ► Influence of chromium content on corrosion resistance of alloy. ► Influence of further alloying elements on corrosion resistance of alloy. The aim of the study was to determine the fireside corrosion performance of certain nickel-base superalloys dedicated for construction of superheater and reheater sections of a boiler operating at advanced ultra-supercritical conditions. For this purpose, three nickel-base alloys varying in chromium content from 20% to 25% (Alloys 263, 617 and 740) were selected for laboratory tests up to 1000h. Additionally, the chosen materials were exposed using a temperature-controlled corrosion probe in a 500kWth pulverized fuel test rig, one lignite-fired and two hard-coal-fired power plants. The specimens’ temperatures were in the range 640–760°C. The fireside corrosion was studied having in focus the synergy effect of combustion gas atmosphere, real fly ash deposits and alloy composition. Corrosion behavior of each alloy was determined using dimensional metrology and the obtained results were compared with data available from the literature. The values measured on the samples exposed in power plants fit well with the numbers generated from the laboratory tests performed at 24, 350 and 1000h. Moreover the values are in good agreement with results found in the literature and similar alloy ranking based on corrosion resistance is confirmed by the literature. Clear sulphur-induced corrosion was noticed after 1000h exposures in the laboratory furnaces at only one of the examined alloys, which is characterized by the highest molybdenum and lowest titanium content. Both of these elements are believed to play a significant role in the corrosion behavior of the examined alloys. In some metal rings exposed in power plants sulphur induced corrosion is witnessed. In contrast to iron-base austenitic steels no straight connection is observed between increasing chromium content and improved corrosion resistance in the nickel-base austenites. Intergranular oxidation with participation of alumina repeats and occasionally leads almost to a grain release. In laboratory conditions fly ash appears to partially inhibit the corrosive influence of the gas atmosphere, since it behaves to a certain extent as a protective barrier for the metal surface while acting as a “sulphur sink”.