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 2 gas with or without 0.5 vol% SO2 2 at 650 degrees C for 300 h. Without SO2, 2 , multi-layered reaction products formed. Under sulphate, they were external NiO, inner NiCr spinel + Cr2O3 2 O 3 band, internal oxidation zone and internal CrxS x S precipitates. Under ash + sulphate, they were external NiO + Ni above the internal oxidation zone. Introducing SO2 2 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.