High temperature corrosion of heat resistant nickel-based alloys in carburizing environments

Abstract

The main objective of this research is to understand the carburization mechanism and to assess the performance of two important alloys which are Incoloy 890 and FIR-160. The study involved cyclic exposure of alloy 890 and HR-160 as well as conventional alloys Incoloy 800HT and Incoloy 803 to a commercial 2%CH4-H2 gas mixture which contains a trace amount of oxygen impurity at 800°C and 1100°C for 500 hours. Carbon activities and partial pressures of oxygen in the gas mixture as well as dissociation pressures of various metal carbides and oxides were calculated to predict the phases formed due to exposure. Performance assessment was investigated by thermogravimetric measurements of test specimens during exposure. Thermogravimetric results indicate that alloy HR-160 exhibited better carburization resistance than other alloys at both temperatures. Alloys 890 and 803 showed excellent resistance while alloy 800HT was the worst. Microstructural characterization using XRD, SEM, and EDS indicates that the protection against carburization in alloy 890 and HR-160 at 800°C was achieved by the formation of a continuous scale of Cr-Mn oxide spinel. At 1100°C the protection in alloy HR-160 was primarily offered by the formation of a continuous scale of SiO2 while a semi-continuous scale of Al2O3 along a discontinuous Cr-Mn oxide spinel were responsible for the protection in alloy 890.