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

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The sulfidation of three Cu-Cr alloys withnominal Cr contents of 25, 50, and 75 wt.% and of thetwo pure metals has been studied at 400-600°C inH2-H2S mixtures under sulfurpressures of 10-12 atm at 400 and 500°C and 10-10 atm at 500and 600°C, slightly above the Cu-Cu2Sequilibrium. All the alloys were two-phase, containinga mixture of the solid solution of chromium in copperwith nearly pure chromium. The corrosion rates of the three materialsunder the same conditions were similar and intermediatebetween those of the two pure metals and increased withtemperature and sulfur pressure. The scales had a complex composition, often containing anexternal Cu2S layer, which becamediscontinuous or even disappeared, in some cases,followed by an intermediate layer of the double Cu-Crsulfide CuCrS2 and an innermost complex layer, which generallyconsisted of a mixture of the double Cu-Cr sulfideCuCr2S4 with the chromium sulfideCrS and also commonly contained unsul fidized chromiummetal particles. No chromium depletion was developed in the alloys beneaththe corrosion-affected region. Moreover, no internalsulfidation of chromium was observed in the alloyrichest in copper and no exclusive external sulfidation of chromium in those richest in chromium, inspite of the large difference in the thermodynamicstabilities of the sulfides of the two pure metals.These peculiar scale features are interpreted by taking into account the special two-phase nature ofthese alloys.