| 其他摘要 | Supercritical water oxidation (SCWO) is a promising technology which can effectively and safely dispose various organic wastes. This method mainly uses the outstanding properties of supercritical water (SCW) that organics, such as hydrocarbons, and gases like oxygen, become completely miscible with SCW. In SCW, organics can be destructed in a few minutes. However, severe reactor corrosion occurs in SCWO processes since reactors capable of accommodating elevated temperatures and pressures must be used and, potentially, very aggressive environments are present when anions like chlorides or sulfates exist. Studying the corrosion and protection of the materials in SCWO has become a key problem to the widespread application of SCWO. The chemical compositions and structure of the corrosion scales are important for understanding of corrosion process and possible environmentally assisted cracking.
In this study, the morphologies, microstructure and chemical compositions of corrosion scales grown on 316L stainless steel and alloy 625 exposed to H2O2-containing supercritical water were investigated using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer, an X-ray photoelectron spectroscopy analyzer and an X-ray diffraction analyzer.
The morphologies, microstructure and chemical compositions of corrosion scales of 316L stainless steel exposed to SCW at different temperatures and for different time were studied. With an increase of temperature, the mass of 316L stainless steel increased. The results from SEM images indicate that the morphologies of corrosion scales changed with the temperature. The corrosion scales were found to become less continuous with the increase of temperature. There were even many big pores among the grains in the corrosion scale formed at 500 oC. From the cross sections, the corrosion scales formed in SCW consisted of double layers similar to those observed previously in high temperature water. The outer layer was loose and mainly composed of Fe oxide, while the inner layer was compact and composed of mixed Cr and Fe oxides. Ni enrichment was observed at the interface between the metal matrix and corrosion scale. XPS results reveal that with an increase of the SCW temperature, Cr oxides in corrosion scales tended to change from Cr(OH)3 to Cr2O3, Fe oxides changed from α-FeOOH to mixed α-FeOOH and γ-FeOOH. With an increase of time, the mass of 316L stainless steel increased and the velocity of mass gain enhanced first and then lessened. With the increase of time exposed to SCW, the oxide grains grew much larger. Fe, Cr and Ni of 316L stainless steel preferentially dissolved. The possible growth mechanism of the corrosion scales in SCW environment is also discussed. The outer layer grows via a metal dissolution/oxide precipitation mechanism and the inner layer grows via a solid-state growth mechanism.
The morphologies, microstructure and chemical compositions of corrosion scales of alloy 625 exposed to SCW at different temperatures and for different time were studied. With an increase of temperature, the mass of 625 alloy increased. The changing trends of 625 alloy was similar to 316L stainless steel, however, mass gain of the latter was much lager than that of the former. The results from SEM images indicate that the morphologies of corrosion scales looked relatively compact at different temperatures. The results of XPS showed that with the increase of temperatures, Cr oxides tended to change from Cr(OH)3 at 400 oC to CrO3 at 500 oC, indicating that Cr in corrosion scales tended to change from trivalence to hexavalence. Ni oxides changed from Ni(OH)2 to Ni2O3, indicating Ni changed from bivalence to trivalence. With an increase of temperature, the mass of 625 alloy increased first and then kept steady. When time increased to 150 h, the velocity of mass gain of 316L stainless steel was almost three times than 625 alloy. With the increase of time exposed to SCW, there was little change of the corrosion scales, indicating time plays a weak role in the corrosion of alloy 625 in SCW. The corrosion scales grow via a metal dissolution/oxide precipitation mechanism.
Compared the characterization of corrosion scales of alloy 625 with 316L stainless steel exposed to SCWO, it is indicated that alloy 625 has better corrosion resistance than 316L stainless steel. The former can be a candidate material for SCWO equipment, and the latter is not suitable for application in this environment. |
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