The corrosion behavior of Co-Mo alloys in H2-H2O-H2S environments |
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Authors: | C C Shing D L Douglass F Gesmundo |
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Affiliation: | (1) Department of Materials Science and Engineering, University of California, 90024-1595 Los Angeles, California;(2) Istituto di Chimica, Universita di Genova, 16219 Genova, Italy |
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Abstract: | The corrosion behavior of Co alloyed with up to 40 wt.% Mo alloys was studied in H2-H2O-H2S gas mixtures over the temperature range between 600C and 900C. The parabolic rate constants for corrosion decreased with increasing amounts of Mo. The compositions of all gas atmospheres fall in the sulfide(s stability region of the ternary M-O-S phase diagrams at all temperatures investigated. All the corrosion scales were composed of sulfides, while no oxide was detected. The sulfide scales formed were duplex at all temperatures except at 900C. The outer layer consisted primarily of cobalt sulfide, while the inner layer was complex and heterophasic, the phases formed being highly composition dependent. MoS2 predominated in the inner layer for all alloys. However, a metallic Mo layer was formed in the innermost layer of Co-40 Mo. Activation energies were different for all alloys, increasing with increasing Mo content. Identical kinetics were observed for Co-30Mo corroded at 700–800C. A Chevrel-phase Co1.62Mo6S8 was present in scales formed on the samples exhibiting the temperature-independent kinetics. A possible model in which Co1.62Mo6S8 forms preferentially in H2-containing mixed gas is suggested. Alloys corroded at 900C formed a lamellar-structure scale which contained Co and CoMo2S4 layers perpendicular to the alloy surface. A eutectoid decomposition of an unknown Co-Mo sulfide may be responsible for the presence of the lamellar structure. |
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Keywords: | mixed gas Co1 62Mo6S8 lamellae multilayered scale sulfidation |
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