The corrosion of a Co-15 wt% Ce alloy in mixed oxidizing-sulfidizing gasesat 600-800 degrees C

The corrosion behavior of a Co-Ce alloy containing approximately 15 wt% Ce has been studied at 600-800 degrees C in several H-2-H2S-CO2 mixtures, prov iding sulfur pressures of 10(-8) atm at 600, 700 and 800 degrees C and of 1 0(-7) atm at 800 degrees C, and oxygen pressures of 10(-24) atm at 600 degr ees C and 10(-20) atm at 700 and 800 degrees C. At 600 degrees C, the alloy corrodes more slowly than pure cobalt but more rapidly than pure cerium wh ile, at 700 degrees C, it corrodes at about the same rate as pure cerium, b ut much faster than pure cobalt. At 800 degrees C under 10(-8) atm S-2, i.e . a value below the stability of the cobalt sulfides, the alloy corrodes ra ther slowly but, under 10-7 atm St, the rate is very high, although slightl y lower than that of pure cobalt. The scaring kinetics are generally interm ediate between linear and parabolic but are sometimes irregular. The corros ion of this alloy produces multilayered scales, containing an outermost lay er of almost pure cobalt sulfide, an intermediate complex layer composed of a mixture of compounds of the two metals and, finally, an innermost region of internal attack of cerium by both oxygen and sulfur. Cerium is not able to diffuse outwards and remains in the alloy consumption region. In the in termediate region cobalt sulfide forms a continuous network which allows th e growth of the external CoSy layer, although at rates that are reduced wit h respect to those of pure cobalt. Thus, a cerium content of 15 wt% is not sufficient to prevent or even to significantly reduce the sulfidation of th e base metal. These results, as well as the details of the microstructure o f the scales, are interpreted by taking into account the limited solubility of cerium in the base metal and the presence of an intermetallic compound, rich in cerium, in the alloy. (C) 1999 Elsevier Science Ltd. All rights re served.