THE SULFIDATION OF 2 FE-CE ALLOYS IN H-2-H2S MIXTURES AT 600-800-DEGREES-C

The sulfidation behavior of two Fe-Ce alloys containing approximately 15 and 30 wt% cerium has been studied at 600-800 degrees C in H-2-H2S mixtures providing a sulfur pressure of 10(-8) atm. The alloys corrode more slowly than pure iron, but more rapidly than pure cerium. The su lfidation rates generally decrease with time and tend to become parabo lic after an initial stage intermediate between linear and parabolic, except for Fe-15Ce which corrodes linearly after an initial quasi-para bolic behavior. Both alloys produce complex scales, containing an oute rmost layer of practically pure iron sulfide and an inner complex laye r where the sulfides of both alloy components are simultaneously prese nt. Finally, a thin region of internal sulfidation of cerium is also g enerally present in contact with the alloy which is not depleted in ce rium. Cerium is not able to diffuse out of the alloy consumption regio n, where it forms a cerium sulfide mixed with iron sulfide. The iron s ulfide forms a continuous network which allows the growth of the exter nal FeS layer, even though at rates reduced with respect to pure iron. Thus, a cerium/content up to 30 wt% is not sufficient to prevent the sulfidation of pure iron. These results as well as the details of the microstructure of the scales grown on the two alloys are interpreted b y taking into account the limited solubility of cerium in the base met al and the presence of intermetallic compounds in the alloys. (C) 1997 Elsevier Science Ltd.