The sulfidation of a Co-15 wt.% Ce alloy in H-2-H2S mixture at 600-800 degrees C

The sulfidation behavior of a Co-Ce alloy containing approximately 15 wt.% Ce has been studied at 600-800 degrees C in H-2-H2S mixtures providing a sulfur pressure of 10(-8) atm, but also of 10(-7) atm at 800 degrees C. At 600 and 700 degrees C the alloy corrodes more slowly than pure cobalt, but more rapidly than pure cerium. At 800 degrees C under 10(-8) atm S-2, which is below the stability of the cobalt sulfides, the alloy corrodes quite slowly, bur under 10(-7) atm S-2 it corrodes very rapidly and practically at the same rate as pure cobalt. The sulfidation kinetics are generally il regular, except for a few cases of nearly parabolic behavior. The sulfidation of the alloy produces duplex scales, containing an outermost layer of practically pure cobalt sulfide and an inner complex layer where the two components are simultaneously present. Cerium is not able to diffuse out of the alloy-consumption region, where it forms a cerium sulfide mixed with cobalt sulfide and an innermost region where cerium sulfide is mixed with cobalt metal. The cobalt sulfide forms a continuous network which allows the growth of the external CoSy layer, even though at rates reduced with respect to pure cobalt. Thus, a cerium content of 15 wt.% is not sufficient to prevent the sulfidation of the base metal. These results as well as the details of the microstructure of the scales grown on the alloy are interpreted by taking into account the limited solubility of cerium in the base metal and the presence in the alloy of an intermetallic compound rich in cerium.