Wet Oxidation Performance of Type 310S Stainless Steel

Abstract

High-temperature wet oxidation resistance of Type 310S stainless steel arises from the formation of a thin, adherent and compact external Cr2O3 scale that is more protective compared with the other oxides. Stability of the Cr2O3 scale is often at risk in the presence of water vapour at higher temperatures, which promotes the loss of oxidized Cr to volatilization. Continuous volatilization of the Cr2O3 scale accelerates the rate of oxidation and increases the risk of non-protective Fe-rich oxide formation that immensely contributes to the oxide thickening (breakaway oxidation). In this study, the possibility of surrogating high-pressure supercritical water with an ambient pressure air-10% H2O mixture is studied at temperatures associated with the predicted coolant outlet conditions in the current GEN IV design concepts. Factors influencing structure and composition of the Cr2O3 scale during wet oxidation are then examined in the wet environments. An increase in the total gas pressure, water vapour partial pressure and temperature is shown to accelerate the Fe-rich oxide formation by increasing the rate of oxidized Cr loss. A more complete physical description of the oxidation kinetics in terms of the evolution of the oxide scale structure and composition at the various exposure conditions is also reported. Presence of small amount of Mn in the alloy is shown herein to be beneficial as it assists the formation of a MnCr2O4 layer on top of the Cr2O3 scale, which serves to reduce the volatilization rate. It is shown however that the MnCr2O4 layer itself is only temporary protective and becomes prone to volatilization (loss of oxidized Cr) at relatively high temperatures, pressures and exposure times. The formation of a MnCr2O4 cap is therefore, only a temporary solution for delaying the onset of accelerated Fe-rich oxide formation. Addition of Si is proposed to be a more promising way of controlling the onset of the Fe-rich oxide formation. Increase in the Si content to ~6 wt.% results in the formation of a continuous SiO2 barrier layer under the Cr2O3 scale as well as Cr-rich silicide intermetallic phases in the starting microstructure that serve as effective Cr reservoirs in helping to maintain the structure and composition of the compact protective Cr2O3 scale despite the continued loss of oxidized Cr to volatilization.