| For the purpose of reduction of the greenhouse gas emission, the thermal powerplants, as a major greenhouse gas resource, are required to operate at highertemperatures.13Cr-5Ni (mass concentration) stainless steels are applied in the turbinerotors, thus it is essential to study the high temperature oxidation behavior of13Cr-5Nisteels in humid atmospheres. In this study, Fe-13%mass Cr-5%mass Ni alloys wereoxidized at800°C for43.2ks,86.4ks and172.8ks, in dry atmosphere (Ar-21%O2)and humid atmosphere(Ar-18%O2-15%H2O), respectively.Mass gain of the samples during the oxidation was measured. Optical Microscopy,XRD, SEM, EPMA and EDS methods were employed to clarify the phase of the scale,to observe the surface morphologies and the cross section morphologies and todetermine the element distribution along the cross section.It was found that the presence of water vapor in the atmosphere accelerated theoxidation rate by approximately2orders of magnitude. The scales formed in dryatmospheres exhibited fewer voids whereas the scales formed in humid atmospheresexhibited duplex structures with porous outer scales and gaps in between the innerscales and the outer scales. The scales formed in dry atmospheres consisted of Fe2O3and Cr2O3with Cr2O3near the scale/alloy interface, while the duplex scales formed inhumid atmospheres consisted of hematite outer scales and Cr enriched spinel innerscales.For the oxidation in humid atmospheres, at the initial stage, Fe reacted directlywith water vapor, forming FeO. Cr was oxidized to Cr2O3which dissoluted in the FeOto form FeCr2O4. In dry atmospheres, external oxidation occurred, leading to theestablishment of a continuous protective Cr2O3zone, while in humid atmospheres, aninternal oxidation mechanism resulted in a Cr enriched, but non-protective inner scale,thus accelerated the oxidation rate. Cr evaporation induced by water vapor was not adominant effect when a sample was oxidized in humid atmosphere. |
PDF Full Text Request