Effect of SO2 on corrosion behaviour of Fe-Cr-(Mn, Si) and Fe-Cr-Ni-(Mn, Si) alloys in CO2-H2O gases at high temperatures

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Abstract
Oxyfuel process makes CO2 capture feasible but raises corrosion problems for materials used. It is known that CO2 leads to a rapid breakaway oxidation of Fe-Cr alloys, which however, form a protective chromia scale in air/O2 gas. In coal combustion process, sulphur is unavoidable which could affect the corrosion behaviour of material in CO2-rich gas atmosphere. However, the kinetics and mechanism of sulphur on CO2 corrosion is still unknown. This work aimed at examining the influence of SO2 in CO2-rich gas on the corrosion behaviour of chromia forming alloys at 650 C and 811 C, in comparison with the results in SO2-free gas atmospheres. The effects of alloy doping of Mn and Si in Fe-Cr and Fe-Cr-Ni on corrosion behaviour were also investigated in the gases with and without SO2. Model alloys Fe-9Cr, Fe-20Cr and Fe-20Cr-20Ni (all in wt.%) and these alloys doped with Mn and Si were exposed to Ar-CO2-H2O-SO2 gas mixtures. The reaction kinetics were determined by weight change and corrosion products were examined using conventional metallography techniques, x-ray diffraction, scanning electron microscopy, focused ion-beam, x-ray photoelectron spectroscopy and transmission electron microscopy. The results showed that the presence of small amount of SO2 in wet CO2 gas promoted the formation of a protective chromia scale of all Fe-Cr and Fe-Cr-Ni alloys at 650 C. However, this beneficial effect became insignificant in dry CO2 gas. Although not all alloys were able to avoid breakaway oxidation in wet CO2 gas at 811 C, a clear reduction in oxide scaling and the incubation time to breakaway oxidation was observed. In all cases, carburisation underneath breakaway region was significantly reduced if SO2 was present. The slowing of carbon diffusion through chromia scale due to the competing role of sulphur upon adsorption was proposed to account for the increasing corrosion protection of alloys. The relative importance of reaction temperature and the presence of water vapour were discussed in determining the mass transport across the scale and the overall alloy performance. There exist mutual interactions between doping elements (Si/Mn) and gas species (S) occurring across the oxide scales, which depend on reaction temperature, humidity, Cr content and phase constitution of the alloys. Overall, Si-doping alloy plays more beneficial role than its detrimental role while Mn-bearing alloy degraded the ability of ferritic alloys to form a protective chromia scale in sulphur-bearing gas. The results provide a fundamental understanding of the sulphur effect on corrosion behaviour of chromia-forming alloys, which issues an effective way to mitigate CO2 corrosion by sulphur competitive adsorption in oxide grain boundaries.
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Author(s)
Yu, Chun
Supervisor(s)
Zhang, Jianqiang
Young, David
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Publication Year
2017
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Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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