Sulfidation of three two-phase Cu-Cr alloys in H2-H2S mixtures at 400-600°C

The sulfidation of three Cu-Cr alloys with nominal Cr contents of 25, 50, and 75 wt.% and of the two pure metals has been studied at 400-600 degrees C in H-2-H2S mixtures under sulfur pressures of 10(-12) atm at 400 and 500 degrees C and 10(-10) arm at 500 and 600 degrees C, slightly above the Cu-Cu2S equilibrium All the alloys were two-phase, containing a mixture of the solid solution of chromium in copper with nearly pure chromium. The corrosion rates of the three materials under the same conditions were similar and intermediate between those of the two pule metals and increased with temperature and sulfur pressure. The scales had a complex composition, often containing an external Cu2S layer, which became discontinuous or even disappeared, in some cases, followed by an intermediate layer of the double Cu-Cr sulfide CuCrS2 and an innermost complex layer, which generally consisted of a mixture of the double Cu-Cr sulfide CuCr2S4 with the chromium sulfide CrS and also commonly contained unsulfidized chromium metal particles. No chromium depletion was developed in the alloys beneath the corrosion-affected region. Moreover; no internal sulfidation of chromium was observed in the alloy richest in copper and no exclusive external sulfidation of chromium in those richest in chromium, in spite of the large difference in the thermodynamic stabilities of the sulfides of the two pure metals. These peculiar scale features are interpreted by taking into account the special two-phase nature of these alloys.