| 其他摘要 | High temperature corrosion plays an important role in the selection of material for construction of industrial equipment used in high temperature environments, such as gas turbines, coal or oil-fired boilers. It is of great significance to develop exact and in-situ monitoring techniques for high temperature corrosion processes, both in theory and in practical applications. Electrochemical impedance spectroscopy (EIS) is a technique which has been proved effective in understanding reaction mechanisms as well as kinetics for corrosion processes, and by which more information such as interface, oxide scale, diffusion, etc. may be obtained. In this dissertation, EIS was employed to study the oxide scales formed on high temperature alloys, and the high temperature corrosion caused by solid or molten deposits. Moreover, high energy micro-arc alloying process was employed to prepare NiAl and FeAl coatings. The main achievements are as follows:
The effect of 0.2 and 1mass%Dy additives on the oxidation resistance of Fe-20Cr alloy at 900oC in air was studied, and EIS was used to investigate the electrical conductivity of oxide scales formed on the alloys at 900oC. Experimental results indicated that the addition of Dy to Fe-20Cr promoted the formation of a Cr-rich oxide scale, increased the adhesion of oxide scales, and thus improved its oxidation resistance, with a better effectiveness observed for 0.2%Dy. Electrochemical impedance spectra for the oxidized Fe-20Cr-Dy in neutral Na2SO4 solution bubbled with N2 were composed of double capacitive loops, suggesting the two-layered structure of the scales. The lowest electrical conductivity was observed for the scales formed on Fe-20Cr-0.2Dy, indicating a better oxidation resistance, as confirmed by kinetic measurements.
Global warming concerns have fostered interest in new energy sources. Biomass such as straw is considered as CO2-neutral energy sources since it consumes almost as much CO2 during growth as it liberates during combustion. Therefore, the use of biomass as a partial coal substitute is being under consideration. However, straw contains large quantities of potassium, chlorine and sulfur, which deposit on coal-biomass boiler heat exchange surfaces as KCl, K2SO4, etc., which cause severe hot corrosion. For this purpose, EIS was employed to study the corrosion behavior of Fe-Cr alloy beneath thick deposits of 0.5KCl+0.5K2SO4(mol.%) in air at 600oC-750oC. Experimental results showed that Fe-10Cr, Fe-20Cr, Fe-20Cr-0.2Dy and Fe-20Cr-1Dy underwent accelerated corrosion at temperatures lower and higher than the melting point of deposits, respectively, after an incubation period, which was shortened by increasing the temperature, finally producing a porous corrosion layer. Correspondingly, electrochemical impedance spectra for the corrosion at both 600oC and 650oC changed from a single capacitive loop in the initial stage to a Warburg impedance, followed finally by double capacitive loops, while those at 750oC were composed of double capacitive loops, and then of a capacitive loop at high frequencies and a Warburg impedance at low frequencies. The increase in the content of Cr and the addition of small amounts of Dy helped the formation of a Cr-rich scale, but did not have a regular effect on the extension of incubation period. The chlorination, sulfidation and oxidation were responsible for the accelerated corrosion of the alloys.
A simple, novel and cost-effective high energy micro-arc alloying process was successfully used to prepare compact microcrystalline Ni3Al(Cr) and NiAl coatings with a metallurgical bonding between the coating and the substrate alloy. Moreover, the preparation process is accompanied with a minimal thermal distortion or microstructural changes of the substrate. Air oxidation experiments at 900oC and 1000oC indicated that the Ni3Al(Cr) coating exhibited better oxidation resistance than the cast Ni3Al(Cr) and the substrate superalloy K3, forming continuous and compact Al-rich scales with good adhesion. This may be due to the fact that the microcrystallization of Ni3Al(Cr) enhances short-circuit diffusion paths and reduces the critical Al content for the formation of a continuous Al2O3 scale.
Finally, HEMAA was also employed to prepare FeAl coating to protect the wet seal area of 316 stainless steel bipolar plates for molten carbonate fuel cell. Corrosion measurements in molten (0.62Li,0.38K)2CO3(mol.%) at 650oC indicated that FeAl coatings exhibited better corrosion resistance than the substrate alloy, cast FeAl alloy and Ni3Al(Cr) coating prepared by HEMAA due to the formation of a continuous Al2O3-rich scale. |
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