Abstract
The isothermal oxidation behavior of Ni–15Cr sintered alloys with 1.0 mass% dispersion of various rare earth oxides, Y2O3, La2O3, CeO2, Sm2O3, Eu2O3 and Gd2O3, was studied in air at 1473 and 1523 K. The dispersion of La2O3, Sm2O3 and Gd2O3 greatly reduced the oxidation rate; whereas, the dispersion of CeO2 and Eu2O3 significantly increased the oxidation rate. The spalling of the scale was also dependent on the type of the dispersoid. The dispersion of La2O3, Sm2O3 and Gd2O3 greatly suppressed the spalling of the scale. The dispersion of Y2O3, CeO2 and Eu2O3 also suppressed the spalling of the scale, but the effect of these oxides was far less than that of La2O3, Sm2O3 and Gd2O3. The scale formed on the Ni–15Cr alloy without dispersoid consisted of an outer NiO scale and an inner Cr2O3 scale; whereas a single layered Cr2O3 scale without an outer NiO scale was maintained on the alloys with dispersion of rare earth oxides, even CeO2 and Eu2O3. However, the thickness of the Cr2O3 scale markedly increased at locations where the presence of Ce and Eu was detected by EPMA. La, Sm, Gd and Y were also detected in the Cr2O3 scale, but the thin Cr2O3 scale was maintained stably on the alloys with dispersion of these oxides.
The temperature and oxygen pressure dependences of the electrical conductivity of Cr2O3 doped with 1.0 mass% of rare earth oxides described above were measured at near atmospheric oxygen pressures in the temperature range from 773 to 1473 K. The introduction of these rare earth oxides changed the temperature and oxygen pressure dependences of the electrical conductivity of Cr2O3. These results suggested that the rare earth oxides induced into the scale might change the mechanism of the scale growth, but further detailed research is required before concluding whether or not the apparent dependence of the oxidation behavior of the Ni–15Cr sintered alloy on the kind of the dispersed rare earth oxides is due to the doping effect of the rare earth oxides on the defect structure of the Cr2O3 scale.
