Phase Investigations Under Steam Oxidation Process at 800 °C for 1000 h of Advanced Steels and Ni-Based Alloys

The aim of this study was to investigate phase formation under steam oxidation conditions and steam oxidation behaviours of two solid-solution-strengthened alloys—HAYNES® 230® and 617 alloy—two gamma-prime (γ′)-strengthened alloys—263 and HAYNES® 282®—and finally three austenitic steels rich in Cr—309S, 310S and HR3C. The samples were tested under 1 bar pressure in 100 % water–steam–water system at 800 °C for 1000 h. It was found that Ni-based solid-solution-strengthened alloys, HAYNES® 230® and 617 alloy, developed mainly Cr2O3 and MnCr2O4 phases on their surfaces, whereas the γ′-strengthened alloys, 263 and HAYNES® 282®, formed Cr2O3, TiO2 and MnTiO3 phases, indicating slightly higher corrosion degradation rate. The austenitic steels exposed for 1000 h formed Cr2O3, Fe3O4 and MnCr2O4 with traces of Fe,Mn(SiO)4. Some sites on the austenitic steel surfaces were enriched with Fe3O4 with little amount of Cr. During the test, no spallation of the external oxide scale was observed. The kinetic data showed that only the 263 alloy oxidized according to parabolic behaviour, whereas the other alloys deviated from the parabolic rate law, with time exponents of 0.4–0.6 or 0.3. Finally, cross-sectioned investigations of the exposed samples revealed that Ni-based alloys underwent extended internal oxidation, with the highest extent in HAYNES® 282® reaching 25 μm and the lowest in HAYNES® 230® alloy reaching 7 μm. The austenitic steels showed no internal oxidation phenomena.