Abstract
High temperature corrosion of binary alloys Fe-(1, 2, 3)Si and Fe-(2, 4, 6)Al, and ternary alloys Fe-(2, 3)Si-(4, 6)Al (all in wt%) has been investigated by using isothermal experiments at 550, 650, and 800℃. The mixed gases dry Ar-20%CO2 and wet Ar-20%CO2 containing 5, 10, 20% H2O (all in vol.) at total pressure of 1 atm were used.
At 800℃, all alloys were oxidized in dry Ar-20%CO2 and wet Ar-20%CO2-(5, 10, 20)% H2O gases. The oxidation kinetics of binary alloys (except Fe-6Al) were roughly linear in dry CO2, and approximately parabolic in high H2O level gases. Binary alloys except Fe-6Al oxidized internally in all gases. However, for Fe-(2, 3%) Si, the internal oxidation zone disappeared after 50 hour reaction in wet CO2. For Fe-6Al and ternary Fe-Si-Al alloys, a protective Al2O3 layer grew underneath an outer Fe2O3 layer in dry Ar-20%CO2. Water vapour affected the oxidation of ternary alloys only slightly. However, the Fe-6Al alloy oxidized internally in high H2O-content gas, and the associated rapid nodule growth rendered the scale non-protective.
At 650℃, all alloys were oxidized in dry Ar-20%CO2 and wet Ar-20%CO2-(5, 20)% H2O gases. The oxidation kinetics of binary alloys were approximately linear in the first 240 hour reaction and then slowed down. All binary alloys oxidized internally. Ternary Fe-2, 3Si-4Al alloys oxidized internally and the oxidation rates were approximately parabolic. However, ternary Fe-2, 3Si-6Al alloys were protective with an Al2O3 layer formed at this temperature. Water vapour has only a slightly effect on the oxidation of all alloys.
At 550℃, all alloys were oxidized in dry Ar-20%CO2 and Ar-20%CO2-5%H2O gases. The oxidation kinetics of all alloys were approximately parabolic, but the rates of ternary alloys were slower than those of binary alloy. All alloys oxidized internally. -carbides were formed near the internal oxidation zone front and along the grain boundaries of Fe-6Al and ternary alloys in the form of fine needles or short rods.
The effects of temperature and water vapour on the oxidation mechanisms of these alloys are discussed, and the possibilities of carburization are analysed thermodynamically.