ROLE OF RARE-EARTH ELEMENTS ON HIGH-TEMPERATURE OXIDATION BEHAVIOR OF FE-CR, NI-CR AND NI-CR-AL ALLOYS

The influence of rare-earth elements (REEs) on oxidation behavior of Fe-Cr-Ni, Fe-Cr and Ni-Cr alloys has been studied and the role of REEs on oxidation behavior of chromia forming alloys discussed. The REEs were (a) applied superficially to Fe-18Cr-8Ni (AISI 304) and Fe-25Cr-20Ni (AISI 310) alloys, and (b) added as alloying additions or as oxide dispersions to Fe-20Cr and Ni-20Cr alloys. Superficial application of rare-earth oxides (REOs) and concentrates of Y and Ce improved the isothermal and cyclic oxidation resistance of the low Cr alloys, but not that of the high Cr alloy. The oxides of Y, Ce, Nd, La, Sm and Gd influenced oxidation resistance in decreasing order. The addition of 0.1% Ce and Y as well as 1% CeO2 or Y2O3 as dispersoids to Fe-Cr and Ni-Cr alloys improved their oxidation resistance and the alloy with Al and Y was found to be the most resistant. In the presence of Y or Ce (as alloying addition or as oxide on the surface) the oxide formed on the surface was convoluted and fine grained chromia. The increase in oxidation resistance of chromia forming alloys in the presence of REEs is due to changes in the cation diffusion process. This is brought about by segregation of REEs to the grain boundaries of the scale, and consequent variations in ionic defect concentrations.