Effects of chlorides and sulphates on molten salt hot corrosion of dissimilar welded joints of 70 MW circulating fluidised bed combustion power plant super-heaters tubes

In the coal fired thermal power plants, super-heater tubes are subjected to corrosive environment containing aggressive chloride and sulphate salts, which results in early hot corrosion and shortened life. The study explores the hot corrosion characteristics of the dissimilar high-chromium ferritic (SA335 P91) and low-chromium ferritic heat-resistant steel (SA218 P22) joints produced with the PCGTAW (pulsed current gas tungsten arc welding) process and E70S-2 filler metal. The welded samples were subjected to a hot molten salt (K2SO4 + NaCl) environment at 700 ± 2°C for 50 thermal cycles. Scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, and energy-dispersive spectroscopy were used to investigate the corrosion behaviour, oxide scale evolution and degradation mechanisms. The weld metal showed superior hot-corrosion resistance due to the formation of compact, Cr2O3-rich protective oxide scales, while spallation of the oxides and localised hot corrosion attack was observed in the P22 region. The relationship between the corrosion kinetics, phase evolution, and microstructural changes in different weld areas is established, which is useful in making recommendations to enhance the durability and reliability of super-heater weldments in high-temperature molten salt environments.