Effect of presulfidation on the oxidation behavior of co-based alloys. Part II. Presulfidation at sulfur partial pressures below the dissociation pressure of cobalt sulfide

Abstract

A range of Co-Cr binary alloys, containing from 1 to 25 wt.% Cr, and Co-25 wt.% Cr-C alloys, containing 0.5, 1.0, and 2.0 wt.% C, have been oxidized at 1000°C after a preliminary sulfidation treatment under conditions where only chromium sulfide, CrS, was formed. Presulfidation has little significant effect on the subsequent overall oxidation rate, although it markedly increases the rate of deterioration of the alloy structure. The sulfur liberated by the oxidation does not escape to the atmosphere, but diffuses inward into the alloy, producing morphologies similar to those observed in hot corrosion and in laboratory tests involving molten sodium sulfate. The activity of sulfur beneath the oxide increases as the oxidation proceeds and, in some cases, becomes high enough to form liquid cobalt sulfides. This leads to an irregular oxidation front penetrating deeply into the alloy causing its fragmentation and incorporation into the oxide scale. This, too, is characteristic of hot corrosion behavior. From this it appears that the role of the sulfide may possibly have been underestimated in recent work and the importance of the molten salt chemistry overemphasized.