| Abstract Scope |
Alloys used at moderate temperatures are designed to form passivating chromia scales. This succeeds in dry air or oxygen, but can fail when additional corrodents(C, S, Cl, N, H2O) are present. The examples of 9Cr1Mo steel exposed to O2-H2O and to pressurized CO2 show that competition between oxygen and H2O is important and that the steel is internally carburized under its oxide scale. To understand these effects, results on pure Cr are revisited. In CO/CO2 mixtures the metal grows Cr2O3 with an underlayer of Cr7C3. When N2 is added an inner layer of Cr2N forms. Clearly both C and N can penetrate Cr2O3. The presence of SO2 prevents N penetration and slows C entry. The original hypothesis of interactions between corrodent species resulting from shared occupancy of grain boundaries within the scale has now been confirmed by APT imaging. Alternative approaches to mitigation of secondary corrodent effects are also explored. |