Effects of Silicon on High Temperature Corrosion of Fe–Cr and Fe–Cr–Ni Alloys in Carbon Dioxide

Model alloys Fe–9Cr, Fe–20Cr and Fe–20Cr–20Ni (wt%) with Si (0.1, 0.2 and 0.5 %) were reacted in Ar–20CO2 gas at 818 °C. Undoped Fe–9Cr formed FeO and FeCr2O4, Fe–20Cr formed only Cr2O3, and Fe–20Cr–20Ni formed Fe3O4, FeCr2O4 and FeNi3. Silicon additions suppressed formation of iron oxides on Fe–9Cr and Fe–20Cr–20Ni, and dramatically slowed oxidation of all alloys. Scales on 0.2Si-alloys consisted of an outer layer of Cr2O3 and an inner layer of amorphous SiO2. Carburization occurred in all undoped alloys, but was suppressed in Si-bearing alloys. Wagner’s theory of alloy depletion is shown to provide a successful description of these phenomena.