Corrosion of 85 Ni-15 Cr Alloy at 1600 F in Controlled Atmospheres Containing O2, SO2, SO3, H2S, and N2

Reactions involving synthetic gas mixtures of SO2, O2, SO3, H2S, and N2 with an 85 Ni-15 Cr alloy were studied in a controlled atmosphere furnace at 870 C (1600 F). Reaction of SO2 (in N2) with the alloy resulted in sulfidation-oxidation, the severity of which was much greater at very low concentrations (0.2%, 0.8%) than in pure SO2. Addition of 0.01% O2 to a SO2-N2 mixture considerably reduced the corrosion. Addition of 0.01% SO3 to a SO2-N2 mixture did not change the degree or type of corrosion, indicating that the importance of SO3 should perhaps be considered only in connection with formation and stabilization of corrosive sulfate deposits. Exposure of the sample to a 2% H2S-N2 mixture for one hour caused severe sulfidation with formation of Cr3S4 and Ni3S2. Subsequent exposure of this sample to oxygen or argon at 870 C resulted in a conversion of the Ni3S2 to Cr3S4, with an attendant tenfold increase in the depth to which sulfides were found in the base metal.