THE OXIDATION OF A CO-15 WT-PERCENT CE ALLOY UNDER LOW-OXYGEN PRESSURES AT 600-800-DEGREES-C

The scaling of pure cerium and of a cobalt-base alloy contain:ng appro ximately 15 wt% cerium was studied at 600-800 degrees C in H-2-CO2 mix tures providing at equilibrium an oxygen pressure of 10(-24) atm at 60 0 degrees C and of 10(-20) atm at 700 and 800 degrees C. Scaling of Co -15Ce formed internal oxidation region where the Ce-rich phase produce d a mixture of cobalt metal and cerium oxide. The kinetics of internal oxidation were very much faster than calculated from the known values of the diffusivity/solubility of oxygen in pure cobalt. The microstru cture of the internal oxidation region followed closely that of the or iginal alloy, while cerium was not depleted in the alloy. At 700 and 8 00 degrees C a thin outermost layer of pure cobalt metal was also prod uced. These results are examined by considering the low solubility of cerium in cobalt and the presence of intermetallic compounds in the al loy. (C) 1998 Elsevier Science Ltd. All rights reserved