Improvement of oxidation resistance of 9 mass% chromium steel for advanced-ultra supercritical power plant boilers by pre-oxidation treatment

•Pre-oxidation treatment in argon gas containing 0.1ppm to 1volume % oxygen improves oxidation resistance of 9 mass% Cr steel in steam at 650°C.•A thin scale of Cr2O3-rich oxides is formed on the surface of 9Cr steel by the pre-oxidation treatment.The addition of Si enhances the formation of Cr2O3-rich oxide scale. Effect of pre-oxidation treatment on oxidation resistance in steam at 650°C has been investigated for 9 mass% chromium (Cr) steel for advanced ultra-supercritical power plants. The formation of thin scale of Cr2O3-rich oxides is achieved by pre-oxidation in argon gas containing 0.3ppm oxygen, which significantly improves the oxidation resistance of the steel in steam at 650°C for a long time exceeding 20,000h. Effects of pre-oxidation conditions, such as oxygen concentration in argon gas and pre-oxidation temperature and time, and of silicon concentration in the steel on the oxidation resistance in stream are systematically investigated.