Simulated Fireside Corrosion Behavior of a Wrought Ni-Fe-Based Superalloy for 700 °C-Class Ultra-supercritical Power Plant Applications

Laboratory corrosion tests were performed to assess the corrosion behavior of a newly wrought Ni-Fe-based superalloy for a 700 °C-class ultra-supercritical power plant application. The new alloy was investigated in a simulated coal ash/gas atmosphere at 700 and 750 °C (with and without NaCl addition) for exposure times up to 500 h, using scanning electron microscopy with energy-dispersive x-ray spectroscopy as well as x-ray diffraction. The results indicated that the new alloy exhibited an anti-corrosion behavior comparable to the currently used Ni-Fe-based alloys for A-USC plant applications. Layered corrosion products consisting of a Cr₂O₃ outer layer and thread-like internal oxides were observed at both 700 and 750 °C. The elevation of the temperature accelerated the growth of corrosion scale, which was characterized by the formation of thicker corrosion scales and more internal oxides and sulfides. The addition of NaCl led to significant scale spallation, where volatile metal chlorides may be responsible for these spallations and for more severe internal oxidation and sulfuration.