The effect of water vapor in increasing growth stresses in the oxide scale on martensitic steam plant alloys

Investigation of the oxidation behavior of 9% Cr steels at 650°C in dry air and air + 10% H2O have shown that oxide scale growth stresses may play a significant role in breakaway. For this reason, preliminary studies on the development and characteristics of growth stresses in oxides on these materials have begun. These studies include in situ acoustic emission (AE) for monitoring scale cracking, deflection testing in monofacial oxidation (DTMO) and in situ X-ray measurements. The measurements were complemented by detailed post-experimental metallographic investigations. From the DTMO measurements the stresses in the oxide on the specimens tested in humid environment are increased by about a factor of 5 compared to dry air at the beginning of oxidation for P91. In a humid environment the stresses decrease with oxidation time while in dry air they remain almost constant. Significant acoustic emission (AE) occurs in humid air for oxidation coupons while virtually no AE is observed for thin foils in a humid environment and for coupons in dry air. These first results seem to be in good agreement with weight gain data characterizing the breakaway behavior in these environments, indicating that, indeed, oxide growth stresses play a key role for oxidation resistance and, thus, component lifetimes of such steels. The first X-ray results indicate that for E911 the scale structure and composition changes completely between the two environments. Furthermore, in humid air, a breakaway effect is observed with a change from protective spinel type oxide to locally non-protective Fe-rich oxide.