Oxidation Behaviour of Coating Alloy CoCrMoSi & CoCrMoNiSi in Superheated Steam at 800˚C

Friday, May 28, 2021: 8:15 AM
Dr. Bingjie Xiao , Carleton University, Ottawa, ON, Canada
Dr. Xiao Huang, Professor , Carleton University, Ottawa, ON, Canada
Dr. Matthew Yao , Kennametal Stellite Inc., Belleville, ON, Canada
As steam power plants continue to move towards higher operating temperatures in order to improve efficiency, materials exposed to the working fluid are subjected to accelerated degradations in the forms of surface oxidation and reduced mechanical properties. In this study, the oxidation behaviour of two cobalt base alloys, CoCrMoSi (T14) and CoCrNiMoSi (T19), was evaluated in superheated steam (SHS, 0.1MPa) at 800ºC for up to 500 hours. After the exposure, both T14 and T19 alloys experienced weight gain caused by oxidation. Visual observation and SEM surface analysis revealed that T19 had greater extent of surface oxide spallation than that seen on T14. From the cross-sectional evaluation, however, a thin, adherent oxide layer was found to have formed on T19. T14 in fact had suffered from excessive internal oxidation and the surface oxide was uneven. Based on the results obtained so far, it is believed that the finer Laves phase combined with greater amount of Cr in alloy T19 have enabled the formation of a protective oxide layer and thus reduced the extent of internal oxidation. Due to the extensive oxidation ingress along the large Laves phase, it is concluded that T14 is not suitable for applications in SHS at 800ºC.