| 其他摘要 | Even though the oxidation, sulfidation, chloridation and hot corrosion of metallic materials have been widely studied in the past years, providing a basis for understanding their basic chemical reactions and corrosion mechanisms, the corrosion behavior of these materials exposed to complex multi-oxidant atmospheres such as those containing oxygen、sulfur and chlorine have not been well understood up to now. These strongly corrosive multi-oxidant atmospheres are encountered in many practical high temperature industrial applications, therefore, it is of great importance to study these phenomena in order to gain insight into their nature. On the basis of this idea, this thesis mainly investigated the corrosion behavior of several model Fe-Cr and Fe-Cr-Al alloys in multi-oxidants corrosive atmospheres containing O、S and Cl as well as the corrosion beneath chloride deposits by using thermogravimetry, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Microanalysis (EDX), Electron Probe Micro Analysis (EPMA) and so on. The aim of this study is to understand the degradation processes of the protective oxides scales under the synergistic effect of sulfur and chlorine and to study the role of sulfur on the oxidation-chloridation reactions. Finally, this work investigated also the evolution of the impedance spectra during the corrosion of metallic materials beneath chloride deposits and established also a reasonable equivalent circuit to interpret the electrochemical corrosion mechanism.
Firstly, the corrosion of Fe-Cr alloys was examined at 600-700oC in reducing atmospheres containing HCl and H2S with two different H2S contents and compared with the behavior of the same materials in H2-CO2 and H2-HCl-CO2 mixtures free of H2S but providing approximately the same oxygen and chlorine partial pressures as those produced by mixtures containing H2S. Increasing the H2S level accelerated the corrosion of all the alloys, but particularly that of Fe-18Cr, which underwent a change from the growth of chromia scales to the formation of mixtures of Fe and Cr oxides. An increase of the Cr content reduced the corrosion rate in both H2S-containing gas mixtures, possibly due to the occurrence of Fe-Cr spinel or a larger volume fraction of Cr2O3 in the inner scale. The equilibrium partial pressures of chlorine, oxygen and sulfur of the mixed gases were calculated to estimate the possible reactions between the alloy components with the multi-oxidant atmosphere. The scale structure and the corrosion mechanism have been interpreted on the basis of the stability trend of metal chlorides, sulfides and oxides by using the thermodynamic stability diagrams of the M (Fe/Cr)-Cl-O-S systems.
Secondly, the corrosion of Fe-xCr-10Al alloys with a constant Al content of 10 at. % and Cr contents ranging from 0 to 20 at.% was examined at 600-700oC in reducing atmospheres (H2-H2S-HCl-CO2) and compared with that of the same materials in H2-CO2 mixtures producing approximately the same oxygen partial pressures. The simultaneous presence of sulfur and chlorine in a H2-HCl-H2S-CO2 atmosphere accelerated the corrosion of alloys with respect to their corrosion in a simple H2-CO2 mixture. The thick and porous scales grown on the surface of the alloys might allow an easy inward penetration of chlorine and sulfur which particularly suppresses the formation of the rather protective scales composed of mixtures of the oxides of the three alloy components on the three ternary alloys (Fe-10Cr-10Al, Fe-15Cr-10Al and Fe-20Cr-10Al) at 600oC. At 700oC, the presence of sulfur and chlorine in the reducing atmosphere did not only accelerate the corrosion of three Fe-xCr-10Al alloys (x = 0, 5, 10 at. %), but produced also a strong internal oxidation of aluminum plus chromium, absent in the oxidation of the same materials in a simple reducing atmosphere providing the same oxygen pressure. The increase of the chromium content of the alloys decreased slightly the corrosion rate by increasing the volume fraction of chromia in the inner scale region.
Thirdly, the corrosion of NF616, 12CrMoV and SS304 in a reducing atmosphere containing HCl plus H2S in the presence of ZnCl2-KCl deposits has been investigated at 400-500oC and compared with that of the same materials in a similar gas mixture free from H2S. The presence of H2S in the gas accelerated the corrosion of the three commercial steels beneath ZnCl2-KCl deposits. The corrosion attack in the presence of H2S was characterized by a rapid growth of scales more porous and poorly adherent. The considerably poor adherence of the scale to the steels may be related to the presence of chlorides and traces of sulfur at their interface.
Finally, a two-electrode probe system was adopted to investigate the time evolution of the electrochemical impedance spectroscopy (EIS) for the corrosion of Fe, Ni, 12CrMoV and SS304 beneath deposits of a 0.55ZnCl2-0.45KCl mixture at 400oC in comparison with their EIS in a deep molten salt. The corrosion processes as well as the nature of the changes of the impedance spectra were analyzed during the reaction of these substrate materials with the salt deposits and the gaseous oxidants.
The corrosion of pure iron beneath a ZnCl2-KCl deposit showed the features of a Warburg resistance at low frequencies. The features of the impedance spectra of the corrosion of pure iron in deep salt were similar to those for the corrosion beneath salt deposits, but with much larger impedance.
The corrosion of pure nickel beneath a ZnCl2-KCl deposit showed the features of a Warburg resistance at low frequencies. With extended times, the impedance spectra changed into two capacitive loops at low and high frequencies. This change of the impedance spectra is closely related to the growth of porous scales on the surface of nickel. The corrosion of nickel in deep molten salt always presented the features of Warburg resistance at low frequencies, but with much larger impedance than beneath the ZnCl2-KCl deposit. This may be due to the considerably small solubility of oxygen in molten chlorides and to a larger resistance to its diffusion in molten salts. The corrosion of nickel in deep molten salt is not more severe than beneath salt deposits, showing that it is not significantly affected by the different rate of oxygen supply under the two conditions.
The corrosion of 12CrMoV beneath a ZnCl2-KCl deposit consisted of two stages with different impedance spectra features. The initial Nyquist plots were composed of a depressed semi-circle at high frequency and a line at low frequency indicating a diffusion-controlled reaction. The subsequent Nyquist plots were composed of a depressed semicircle at high frequency and a large semicircle at low frequency, which were connected by a line at intermediate frequencies. The change of the impedance spectra with time may be associated with the growth of the porous scales on the metal surface. The impedance spectra of SS304 during the whole corrosion duration did not undergo any change, similar to the behavior of the impedance spectra of 12CrMoV during its later corrosion stage. |
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