Corrosion Behaviour of Ni-Based Alloys 230, 617 and 601 in CO2 Gas at 750 and 850°C

Abstract

As a newly developed technology, oxyfuel combustion makes CO2 capture and sequestration feasible but raises a corrosion problem because of high concentrations of CO2 in flue gases at high temperatures. Conventional ferritic/martensitic heat resisting steels are sufficient to resist corrosion in oxygen or air but cannot survive in CO2-rich gases. To increase the process efficiency, higher temperatures will be used for energy production. As a result, austenitic steels and/or nickel-base alloys are required because of their superior corrosion resistance and mechanical properties at high temperature. This paper investigated the corrosion behavior of three nickel-based chromia-forming commercial alloys (230, 617 and 601) at 750°C and 850°C in a carbon dioxide environment for up to 500 h.Samples were cut to form 10mm x 8mm x (0.8-3) mm rectangular shaped coupons. After cutting, all surfaces of samples were ground up to 1200-grit finish, and further polished down to 3μm. Electro-polishing in 15% concentrated hydrochloric acid was used to remove the subsurface deformation zone. The grain sizes of 617 are larger (100-200μm) than those of 230 (20-80μm) alloy, and the 601 (10-30μm) alloy has the smallest grain size. All samples were ultrasonically cleaned with ethanol before being reacted in Ar-20%CO2 at 750℃ and 850°C, for up to 500 h. After reaction, samples were analysed by surface X-ray diffraction, metallographic cross-section analysis by optical microscopy, and scanning electron microscopy together with energy dispersive X-ray analysis.All three alloys showed good oxidation resistance by forming mainly a protective chromia layer with low weight changes. Internal Al2O3 was precipitated beneath a thin protective chromia layer for all cases. For 230 and 617 alloys, NiO and Cr-rich spinel outer layers were formed, but for 601 less iron and nickel outward diffusion was observed at both temperatures. Furthermore, some minor alloy elements (Mn, Ti, and Co) were also observed to diffuse into the chromia layers.Wagner’s Theory was applied to examine the critical chromium concentration for forming a protective chromia. This prediction indicated alloy concentrations were marginal for chromia formation at both temperatures for the test alloys. The observation of protective chromia formation can be attributed to the effect of other alloying elements, e.g. Al, Mn, Ti, Si etc. Because of the temperature effect on diffusion, the predicted result from Wagner’s Theory showed that the critical chromium concentration needed to form and maintain a protective chromia layer decreased with increasing the oxidation temperature, which is consistent with the experimental observation.The Al and Si oxides formed an additional protective barrier between the matrix and the chromia layer, preventing the diffusion of iron and nickel outwards and therefore increasing the oxidation resistance. Manganese effectively prevented carburization by combining with chromium oxide to form the outermost spinel oxide layer, increasing oxidation/carburisation resistance. Titanium diffused through the chromia layer, accelerating chromium diffusion and leading to higher kinetic rates. Tungsten reduced oxidation resistance by increasing the metal vacancies and therefore outward diffusion of Ni, Fe and Mn through the chromia scale.In general, no significant carbide formation was found in any of the Ni-base alloys in these reaction conditions. At 750℃, carburization due to CO2 reaction was found for 230 alloy but not for 617 and 601 alloys after 500h reaction. However, at 850°C, internal carbides from the reaction were observed for all three alloys. The variation in carburization extent among the different alloys can be attributed to the formation of oxides of aluminum, manganese and silicon, which can impede the inward diffusion of carbon, resulting in the reduction of carburizing kinetics.