Corrosion of ferritic steels in supercritical CO2 at 450 oC
Başlık çevirisi mevcut değil.
- Tez No: 403207
- Danışmanlar: Dr. KUMAR SRIDHARAN
- Tez Türü: Yüksek Lisans
- Konular: Nükleer Mühendislik, Fizik ve Fizik Mühendisliği, Nuclear Engineering, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2017
- Dil: İngilizce
- Üniversite: Unıversıty Of Wısconsın-Madıson
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
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Özet (Çeviri)
The quest for higher efficiencies in energy producing systems is driving efforts for increasing operating temperatures of these systems. For example, the high temperature gas-cooled reactor (HTGR), fluoride salt-cooled high temperature reactor (FHR), and sodium fast reactor (SFR) are all expected to operate at higher temperatures than the present light water reactor (LWR). For all these high temperature reactors (and also fossil and solar power systems), the S-CO2 (super critical CO2) Brayton cycle is being seriously considered for power conversion as it provides for higher efficiency at higher temperatures, while also providing for smaller machinery and simpler cycle layouts compared to conventional Rankine steam cycle. Materials corrosion is an important concern in S-CO2 environment. While this is being addressed by the use of stainless steels and Ni-based alloys for higher temperature (> 550oC) experiencing components, relatively less emphasis has been placed on corrosion at lower temperatures. For these lower temperatures, ferritic steels are being considered because the use of Ni-containing stainless steels and Ni-based alloys can make the costs prohibitively expensive. With the above background, the present research focuses on the assessment of corrosion behavior of ferritic-martensitic steels, T22, T92, and T112 in research grade CO2 (99.999%) at 450 oC and 20 MPa. Additionally, pure Fe and Fe-12%Cr binary alloys were also tested for achieving a more fundamental baseline understanding. Tests were performed for exposure durations up to 1000h with samples being removed at 200 hours intervals for analysis. Post-test characterization was performed using a range of techniques including weight change measurements, optical microscopy, microhardness tests, scanning electron microscopy-energy dispersive spectroscopy (SEM), x-ray diffraction (XRD), glow discharge mass spectroscopy (GDMS), and glow discharge optical emission spectroscopy (GDOES). The experimentally obtained data was then used to establish theory and models that aided in achieving more fundamental understanding of corrosion degradation and for establishing long-term predictive capability. The research broadly encompasses four areas, namely, experimental evaluation of oxidation, application of the“available space model”to the experimental oxidation results, experimental evaluation of carburization, and theoretical treatment of carburization based on experimental data. Pure Fe developed a Fe3O4 magnetite layer which exhibited spallation, located at the edges of the samples. Fe-12Cr alloy showed a very a thin, protective surface chromia layer. Thick nodular structures with a duplex nature were also observed distinctly apart from the protective layer. Ferritic-martensitic steels T22, T92, and T122 all showed a duplex oxide layer structure consisting of an outer magnetite layer and an inner spinel oxide layer, CrxFe(3-x)O4. Modeling of oxide growth was performed were by applying the experimental data to the available space model which is predicated on the formation of vacancies and eventually voids and nano-channels due to the outward diffusion of Fe cations and inward permeation of CO2 through these voids. This fundamental physical process interlinks the growth of the magnetite and spinel oxide layers such that the layers grow while maintaining a constant ratio. This was indeed demonstrated experimentally for the three ferritic martensitic steels, and furthermore the ratio of spinel to magnetite oxide layer thicknesses increased with increasing Cr content in the steel. The value of x in the spinel CrxFe(3-x)O4 was determined by EPMA-WDS analysis and in turn used to calculate the diffusion coefficient of the Fe cation through the spinel oxide. The models were further refined by taking account both lattice and grain boundary diffusion in the oxide (i.e., an effective diffusion coefficient). The simulations with the effective diffusion coefficients were shown to predict the observed kinetics perfectly for the alloys with global mass balance in between the oxide layers. The depth of the carburized layer was developed underneath the oxide layer in the three ferritic martensitic steels were significant. Optical microscopy, SEM-EDS, microhardness testing as a function of depth, GDMS, GDOES and EPMA-WDS were used correlatively to determine carbon depth profiles as a function of exposure time and thus allowing for the determination of the kinetics of carburization. A direct correlation was observed between the thickness of the oxide layer and the depth of carburization, the two being interlinked again by the constraints of the available space model. For pure Fe where the available space model is not applicable, no carburization was observed. The depth of carburization was inversely related to the carbide forming alloy content (i.e. Cr) in the steel. The nature of the carbides formed during carburization was predicted by thermodynamic considerations.
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