(Invited) Oxidation Mechanisms of Alloys for Advanced Ultra-Super Critical Steam Applications

This research supports efforts to design and build safe and cost-effective advanced ultra-supercritical (A-USC) steam boilers and turbines, with increases in energy production efficiency and reductions in emissions, including CO₂. A 1 liter flowing steam autoclave, rated to 346 bar/704°C and 228 bar/760°C, was constructed and steam oxidation tests conducted.   Comparison tests were run at 1 bar.  Parabolic rate constants were estimated from mass change data and compared with literature values from longer duration tests—but none at this high of combined temperature and pressure.  Test materials included Ni-base alloys H230, H263, H282, IN617, IN625 and IN740.  At high pressures the oxide scales were thicker and parabolic rate constants one–to-two orders of magnitude higher than at 1 bar.  The 1 bar data matched literature values for chromia, while the 267 bar data had a higher oxidation rate.  Possible causes for increased oxidation rates with increased pressure were examined.  Increased solid state diffusion within the oxide scale is quantifiable and accounts for a modest increases in expected corrosion rate and scale thickness.  Increased amounts of hydrogen in the alloy, injected into the metal during steam oxidation at high pressures, may increase oxygen permeability and thus lower the ability of the alloy to establish a chromia scale—an increase in the critical Cr content for chromia scales.