Effect of water vapour partial pressure on the chromia (Cr2O3)-based scale stability

The kinetics of the Cr2O3-based scale oxidation and volatilisation were studied in the presence of water vapour (H2O). A commercial Cr2O3-based scale forming Type 310S stainless steel was examined at the ambient pressure (0.1 MPa) and 550°C in relatively low and high H2O-containing environments of air-10% H2O and air-70% H2O, respectively. The increase in the partial pressure of H2O (pH2O) from 10 to 70% resulted in the transition of the oxidation and volatilisation kinetics from the parabolic rate law in air-10% H2O to the paralinear rate law in air-70% H2O. The kinetics transition was attributed to the increase in the Cr loss rate from the base scale after coupons exposure in air-70% H2O. The significant role of Mn alloying element in the base scale protectiveness was also discussed in the context of the Cr2O3-based scale stability.