Carburization of 9 %Cr Steels in a Simulated Oxyfuel Corrosion Environment

Laboratory corrosion tests, simulating fireside corrosion of 9 mass%Cr steels in an oxyfuel boiler of combusting bituminous 0.6 %S coal, were conducted. Thermodynamic calculation of the tube deposit chemistry indicated that sulfates, instead of carbonates, should be the stable corrosive salts. Specimens of 9 %Cr steels were coated with a synthetic salt mixture of 37.5 m/o Na2SO4–37.5 m/o K2SO4–25 m/o Fe2O3 and were exposed to 57.6 vol%CO2–22.4 %H2O–1.6 %O2–0.16 %SO2–18.2 %N2 at 500–700 °C for 96 h. A two-layered scale of outer “pure” iron oxides and inner oxide/sulfide mixture of Cr and Fe formed on the specimens. The two-layered scale became thick at temperatures where the deposits were fused. The tested steals also underwent internal carburization, with the carbide-zone thickness increasing with increasing temperature for the given exposure time. Growth rate of the carburization zone was much less than the estimation derived from diffusion constants of carbon in α-Fe.