Corrosion Behaviour of Fe-Based and Ni-Based Alloys in Wet CO₂ Gas with and without Chloride Deposits at 750 C

Corrosion behaviours of Fe-25Cr, Fe-25Cr-2Mn-1Si, Fe-25Cr-20Ni, 310SS, Ni25Cr, and Ni-25Cr-2Mn-1Si (all in wt%) with and without NaCl-KCl deposits in Ar-60%CO2-20%H2O gas at 750 degrees C were studied. Without salt deposits, Fe-25Cr performed protectively, while Fe-25Cr-20Ni and Ni-25Cr underwent breakaway oxidation with multilayered scales formed. Adding alloy elements Si + Mn increased the corrosion resistance of all alloys by forming additional Mn-rich oxides and silica. Surface deposits of NaCl-KCl accelerated corrosion, forming porous Fe-rich oxide nodules for Fe-25Cr and thick, porous scales and internal oxidation zones for all other alloys. The protective effect of Si + Mn alloying disappeared in the presence of chlorides. Limited intergranular carbides were observed for all alloys in the gas-only condition. The extent of carburisation increased with the presence of chloride deposits for all Fe-based alloys, but remained unchanged for Ni-based alloys. Corrosion of these alloys at 750 degrees C is compared with that at 650 degrees C. The effect of chlorides in volatilising metals at 750 degrees C is discussed.