Effect of SO2 on corrosion behaviour of Ni-base alloys in a simulated combustion environment

Alloys of Ni-25Cr-(2Mn-1Si) under deposits of sulphate or ash+ sulphate were exposed to a wet CO2 gas with or without 0.5 vol.% SO2 at 650 °C for 300 h. Without SO2, multi-layered reaction products formed. Under sulphate, they were external NiO, inner NiCr spinel+ Cr2O3 band, internal oxidation zone and internal CrxS precipitates. Under ash+ sulphate, they were external NiO+ Ni above the internal oxidation zone. Introducing SO2 in the gas promoted alloy passivation, forming thin chromia scales under both deposits. Alloying with Mn and Si produced only minor improvements. These effects are examined in light of gas-deposit-alloy interactions.