The oxidation of two Fe-Ce alloys under low oxygen pressures at 600-800 degrees C

The corrosion of pure cerium and of two iron-based alloys containing approx imately 15 and 30 wt% cerium was studied at 600-800 degrees C in H-2-CO2 mi xtures providing at equilibrium oxygen pressure of 10(-24) atm at 600 degre es C and 10(-20) atm at 700 and 800 degrees C. The corrosion of the two all oys produced an internal attack, which transformed the cerium-rich phases i nto mixtures of iron metal and cerium-rich oxides, including a double Fe-Ce oxide as well as CeO2. The rate of internal oxidation was much faster than expected on the basis of the solubility/diffusivity of oxygen in iron, esp ecially for Fe-30Ce, for which oxygen penetration probably occurs mainly th rough microcracks produced by the large mechanical stresses associated with the local volume increase. The microstructures of the regions of internal oxidation followed closely those of the original alloys, while no cerium de pletion was observed in the metal substrate. At 600 and 700 degrees C some iron oxide formed on the alloy surface, while at 800 degrees C a thin layer of pure iron metal formed on the surface of Fe-30Ce, These results are exa mined by taking into account the low solubility of cerium in iron and the p resence of intermetallic compounds in the two alloys.