Coupling effect of temperature and Cr content on the steam oxidation of Ni-Fe-Cr alloys

Excellent resistance to steam oxidation is a key required property for heat-resistant alloys used in next-generation fossil power plants. In order to clarify the degradation mechanism of Ni-Fe-Cr alloys in high temperature steam, four kinds of Ni-Fe-Cr model alloys with various Cr content were prepared and their long-term steam oxidation were investigated at 650 oC and 700 oC. The microstructure and composition of oxide scales were characterized by SEM equipped with EDS, and the oxide phases were identified by XRD. The results showed significant dependence of temperature and Cr content in alloys on the oxidation kinetics, cross-section morphology and elemental section-distribution. For Ni-Fe-Cr alloys with low Cr contents (12~16 wt.%), the increase of temperature made the oxide scale change from breakaway scale morphology (nodule-crater microstructure with external exfoliation) to protective scale morphology (uniform layer and internal oxidation). For Ni-Fe-Cr alloy with 18wt.% Cr, the effect of temperature was greatly reduced. The oxidation mechanism was discussed from the perspectives of selective oxidation and the effect of alloying elements.