材料期刊网

The corrosion of pure yttrium and of two Fe-base alloys containing approximately 15 and 30 wt.% Y was studied at 600-800 degrees C in H-2-CO2 mixtures providing an equilibrium oxygen pressure of 10(-24) atm 600 degrees C and 10(-20) atm at 700 and 800 degrees C. The corrosion of yttrium under these low oxygen pressures resulted in the growth of Y2O3 scales and presented two approximately parabolic stages at 800 degrees C, while at 600-700 degrees C it was faster and nonprotective. The corrosion of the two alloys followed approximately the parabolic rare law, except for Fe-15Y at 600 degrees C which oxidized nearly linearly. At 600 and 700 degrees C, when the gas-phase oxygen pressure was in the field of stability of iron oxide, the alloys formed also a thin external Fe3O4 layer, while at 800 degrees C, when the oxygen pressure was below the stability of FeO, a thin outermost layer of pure iron was observed to form. Under all conditions a region of internal oxidation formed in the alloy, in which the yttrium-rich phases were transformed into a mixture of iron metal and oxides, which included double Fe-Y oxides as well as Y2O3. The microstructure of the internal-oxidation region followed closely that of the original alloys, which moreover did not undergo any yttrium depletion. These results are examined by taking into account the low solubility of yttrium ib iron and the presence of intermetallic compounds in the alloys.