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Abstract
Ni-based alloys are required for advanced power generation because of their superior creep strength and oxidation resistance at high temperatures. Oxyfuel process makes CO2 capture feasible but raises corrosion problems which are unclear for Ni-based alloys. This work aims at investigating the corrosion behaviour of model Ni-Cr and Ni-Cr-Fe alloys in dry and wet CO2 in the temperature range 650‒800 °C. In dry CO2 gas, multilayered oxide scales were formed on Ni-Cr alloys with low Cr concentrations. Increasing Cr level, the protection of corrosion scales was gradually developed. Elevating temperature accelerated the corrosion kinetics of low Cr alloys, but significantly reduced the corrosion rates of high Cr alloys by promoting the formation of chromia band or scale. The critical Cr concentrations required for chromia scale formation and maintenance decreased with increasing temperature, in accordance with the diffusion theory. In wet CO2 gas, the presence of water vapour reduced the oxidation rate of pure Ni and changed the scale morphologies of Ni-Cr alloys. These effects were less significant at higher temperatures. The outer part of the corrosion scales was characterized by Ni metal nodules and/or loose NiO growths at 650 and 700 °C. The chromia scaling rate at 800 °C increased in the presence of water vapour, degrading the ability of high Cr alloys to maintain a protective chromia scale. At lower temperatures of 650 and 700 °C, Fe-containing Ni-20Cr alloys developed a non-protective corrosion scale after exposure to wet CO2 gas. As temperature was elevated to 800 °C, these alloys underwent scale buckling and spallation. On the other hand, Fe-containing Ni-30Cr alloys generally formed a chromia band or layer at the reaction front within the temperature range examined, which rendered the corrosion scales somewhat protective. Intergranular carbides were produced in Ni-Cr alloys with high Cr contents. The effect of water vapour on carburization varied with the Cr level. The extent of intergranular carbide precipitation tended to increase with increasing Fe contents of ternary Ni-Cr-Fe alloys. These phenomena were discussed based on the change of oxygen partial pressure and carbon activity as functions of oxide scale structure and Fe concentration.