THE SULFIDATION OF PURE CO, Y, AND OF A CO-15 WT.PERCENT Y ALLOY IN H-2-H2S MIXTURES AT 600-800-DEGREES-C

The corrosion of pure Co and Y and of a Co-15 wt.% Y alloy in H-2-H2S mixtures providing a sulfur pressure of 10(-8) atm. at 600-800 degrees C and also of 10(-7) atm. at 800 degrees C was was studied to examine the effect of yttrium on the sulfidation resistance of pure cobalt. T he alloy was nearly single-phase, containing mostly the intermetallic compound Co17Y2 plus a small amount of cobalt solid-solution. For all conditions except for 800 degrees C under 10(-8) atm. S-2, the alloy f ormed multilayered scales consisting of an outer region of pure cobalt sulfide, an intermediate region of a mixture of cobalt sulfide with y ttrium oxysulfide and finally an innermost layer of a mixture of yttri um oxysulfide with cobalt metal. At 800 degrees C under 10(-8) atm. S- 2, below the dissociation pressure of cobalt sulfide, the alloy formed only a single layer composed of a mixture of metallic cobalt with ytt rium oxysulfide. Pure yttrium produced only the oxysulfide Y2O2S, as a result of the large stability of this compound and of the presence of some impurities in the gas mixtures used. The corrosion kinetics were generally rather complex, but except at 800 degrees C under 10(-8) at m. S-2, the addition of yttrium reduced the sulfidation rate of cobalt , even though the formation of a continuous protective external layer of a pure yttrium compound was never achieved. Finally, when the gas-p hase sulfur pressure was above the dissociation of cobalt sulfide the corrosion rate of yttrium was significantly lower than that of Co-15 Y . The internal sulfidation of Y in Co-15Y was not associated with depl etion of Y in the alloy. This diffusionless kind of internal attack is typical of binary A-B alloys presenting a very small solubility of th e most-reactive component B in the base metal A, which restricts sever ely the flux of B from the alloy toward the alloy-scale interface.