Oxidation of Fe–Si, Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO2, with and without H2O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe3O4, an intermediate Fe3O4 + FeO layer, an inner FeO + Fe2SiO4 (or FeAl2O4) layer and internally precipitated SiO2 (or FeAl2O4). Ternary alloys and Fe–6Al developed a protective Al2O3 layer beneath Fe2O3 in Ar–20% CO2. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H2O-content gas, and its scale was non-protective.

► Additions of water vapour to CO2 accelerate the corrosion of dilute Fe–Si and Fe–Al alloys.
► Fe–6Al passivates in Ar–20CO2, but undergoes internal oxidation and breakaway when H2O is added.
► Ternary Fe–Si–Al alloys passivate in both wet and dry CO2.