The degradation of oxide layer on Cr-containing steels in simulated atmospheres on carbothermic reduction

Abstract

ABSTRACT High-temperature corrosion has been a critical issue for coal-fired boiler steels, particularly Cr-containing steels SA213-T22, for many decades. Hot combustion gases containing CO2, CO, N2, and unburnt carbon particles play a significant role in the degradation of oxide scale due to reduction reactions. However, carbide precipitation can occur in the oxide layer by carburisation reactions. The effects of reduction and carburisation on the oxide scale of SA213-T22 steel in a simulated combustion atmosphere by carbothermic reduction were investigated and compared with ferritic stainless steels SS430. The two steels were oxidised under an air atmosphere at 750 °C for 48 hours and heated afterwards in graphite powder at 950–1350 °C for 30–60 minutes. After the oxidation test, a thick porous oxide layer of Fe2O3 was found on the SA213-T22 and a thin Fe-Cr-Mn oxide film was found on SS430 as shown by X-ray diffraction analysis. Most oxide scales on SS430 were reduced and destroyed by carbothermic reduction at 1350 °C but the oxide scale on SA213-T22 transformed to compact mixed phases of Fe-C and Fe-C-Cr-O by carburisation and carbon diffusion. The compact oxide layer with heat resistant carbides can increase wear resistance and might withstand corrosive atmospheres in coal-fire boiler plants.