High temperature corrosion of alloy 617 OCC at 700 °C in simulated USC power plant environment

High-temperature corrosion of the nickel based superalloy 617 OCC (Optimised Chemical Composition) was studied in the context of proposed application in ultra-supercritical (USC) power plants. Studies were carried out in USC power plant environment simulated in the laboratory. Simulation was performed by coating the alloy with a salt mixture consisting of K2SO4, Na2SO4, Fe2O3, SiO2 and Al2O3 and exposing it to a flowing gas mixture comprising of 15% CO2, 3.5% O-2, 0.25% SO2 and the rest N-2 (Vol%) at 700 degrees C. For the sake of comparison, corrosion studies were also carried out under the same conditions, but without the gas mixture. Progress of corrosion was monitored by thermogravimetry, XRD and cross sectional studies on corroded specimens using SEM/EDAX analysis. There was evidence for both external and internal sulfidation with the flowing gas mixture, suggesting that the SO2 present in the gas mixture plays a dominant role. Chromium present in the alloy reacted with the sulfur-containing species in the environment forming chromium sulfide. No internal oxidation of aluminum and titanium was observed in the presence of flowing gas mixture. Internal oxidation was observed, in contrast, without flowing gas mixture at longer exposure times. The paper discusses the findings and possible corrosion mechanisms, coming into play.