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.