The Sulfidation of a Co-15 wt.% Ce Alloy in H2-H2S Mixtures at 600-800°C

The sulfidation behavior of a Co-Ce alloycontaining approximately 15 wt.% Ce has been studied at600-800°C in H2-H2S mixturesproviding a sulfur pressure of 10-8 atm, butalso of 10-7 atm at 800°C. At 600 and 700°C the alloy corrodes moreslowly than pure cobalt, but more rapidly than purecerium. At 800°C under 10-8 atmS2, which is below the stability of thecobalt sulfides, the alloy corrodes quite slowly, but under 10-7 atmS2 it corrodes very rapidly and practicallyat the same rate as pure cobalt. The sulfidationkinetics are generally irregular, except for a few casesof nearly parabolic behavior. The sulfidation of the alloy produces duplexscales, containing an outermost layer of practicallypure cobalt sulfide and an inner complex layer where thetwo components are simultaneously present. Cerium is not able to diffuse out of thealloy-consumption region, where it forms a ceriumsulfide mixed with cobalt sulfide and an innermostregion where cerium sulfide is mixed with cobalt metal.The cobalt sulfide forms a continuous network which allowsthe growth of the external CoSy layer, eventhough at rates reduced with respect to pure cobalt.Thus, a cerium content of 15 wt.% is not sufficient toprevent the sulfidation of the base metal. Theseresults as well as the details of the microstructure ofthe scales grown on the alloy are interpreted by takinginto account the limited solubility of cerium in the base metal and the presence in the alloy ofan intermetallic compound rich in cerium.