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Future technologies require structural materials resistant to environmental degradation in high-temperature CO2-rich environments. Herein, we exposed several commercially available Ni-based alloys (230, 263, 282, 617, 625, and 740H) to atmospheric pressure gases intended to simulate the compositions expected in future direct-fired supercritical CO2 power cycles. The alloys were exposed to 95% CO2 + 4% H2O + 1% O2 and the same gas containing 0.1% SO2 at temperatures of 600, 650, 700, 750, and 800 °C for 2500 h. With minor exceptions, chromia scales formed on all alloys at all temperatures in the SO2-free gas, yielding parabolic growth rates that followed an Arrhenius temperature dependence. Behavior in the SO2-containing gas was more complex. Generally, the alloys performed well at temperatures of 650, 750, and 800 °C. While some alloys further performed relatively well across the whole temperature range, several of the alloys experienced chromia failure resulting in non-protective duplex oxide scales and high oxidation rates, at temperatures of 600 and 700 °C. Deviation from protective behavior was associated with internal sulfide formation and, additionally for the case of 600 °C, external sulfate formation. Extensive carburization accompanied growth of the non-protective duplex oxide scales, which made it more difficult for the alloy to recover after initiation of the sulfur-induced accelerated corrosion process. The thermodynamic and kinetic factors influencing the accelerated corrosion in the presence of sulfur are discussed. The results suggest that caution is required when assessing compatibility of Ni-based alloys for CO2-based systems when sulfur-based impurities are expected. [ABSTRACT FROM AUTHOR]

Next-generation supercritical CO2 (sCO2) power cycles will require different classes of alloy throughout the operational temperatures to optimize tradeoff of creep strength, oxidation performance and cost. This will necessitate joining methods such as welding, which might pose compatibility concerns at the joined interfaces. In this study, similar and dissimilar metal welds were generated from a variety of candidate alloys for sCO2 systems including ferritic/martensitic steels, austenitic steels, and Ni-based superalloys. Samples were extracted from different regions of the welds and exposed to sCO2 at 550 °C and 20 MPa for 2500 h and then characterized to understand their behavior in this environment. Unsurprisingly, the local oxidation behavior was largely dictated by the Cr content in the underlying metal. High-Cr austenitic steels and Ni alloys formed slow-growing Cr-rich oxide scales with minimal carburization of the underlying metal, while low-Cr ferritic/martensitic steels formed fast-growing Fe-rich oxide scales with significant carburization. Most welds did not show any unusual oxidation behavior at the interfaces, considering the local Cr content. The one exception was the 347H similar metal weld, where a larger grain size and complex grain structure in the fusion zone led to a significantly higher rate of Fe-rich oxide nodule formation compared to the base metal. This suggests that microstructural changes at joined interfaces can play an important role on the oxidation-limited lifetimes in future sCO2 systems. The composition changes across the interfaces enabled study of the effect of Fe on the growth rate of Cr-rich oxides and of the origins of the subsurface recrystallization zone that forms beneath them. [ABSTRACT FROM AUTHOR]

Next-generation supercritical CO2 (sCO2) power cycles will require different classes of alloy throughout the operational temperatures to optimize tradeoff of creep strength, oxidation performance and cost. This will necessitate joining methods such as welding, which might pose compatibility concerns at the joined interfaces. In this study, similar and dissimilar metal welds were generated from a variety of candidate alloys for sCO2 systems including ferritic/martensitic steels, austenitic steels, and Ni-based superalloys. Samples were extracted from different regions of the welds and exposed to sCO2 at 550 °C and 20 MPa for 2500 h and then characterized to understand their behavior in this environment. Unsurprisingly, the local oxidation behavior was largely dictated by the Cr content in the underlying metal. High-Cr austenitic steels and Ni alloys formed slow-growing Cr-rich oxide scales with minimal carburization of the underlying metal, while low-Cr ferritic/martensitic steels formed fast-growing Fe-rich oxide scales with significant carburization. Most welds did not show any unusual oxidation behavior at the interfaces, considering the local Cr content. The one exception was the 347H similar metal weld, where a larger grain size and complex grain structure in the fusion zone led to a significantly higher rate of Fe-rich oxide nodule formation compared to the base metal. This suggests that microstructural changes at joined interfaces can play an important role on the oxidation-limited lifetimes in future sCO2 systems. The composition changes across the interfaces enabled study of the effect of Fe on the growth rate of Cr-rich oxides and of the origins of the subsurface recrystallization zone that forms beneath them. [ABSTRACT FROM AUTHOR]

Current and future power systems require steels resistant to high-temperature oxidation in CO2-rich environments. The introduction of structural defects by various surface treatments can profoundly affect the oxidation/corrosion behavior of steels in many environments. This effect is largely unexplored for steels exposed to high-temperature CO2, which is the focus of this work. We prepared Grade 22, Grade 91, 347H, and 310S steels with three different surface finishes, ranging from little to substantial surface damage, and exposed the steels to 1 bar CO2, 200 bar supercritical CO2, and laboratory air at 550 °C for up to 1500 h. Surface finish had little impact on the oxidation behavior of low-Cr (2 wt%) Grade 22 and high-Cr (25 wt%) 310S steels. In contrast, intermediate-Cr steels Grade 91 (8 wt%) and 347H (17 wt%) generally showed improved oxidation and carburization resistance with increasing extent of surface damage, which was attributed to the delay or prevention of the onset of Fe-rich oxide nodule growth. Comparison between exposure environments suggests that this effect is more complex for CO2 compared to air and that it is additionally affected by CO2 pressure. The results suggest that surface treatments should be considered as one approach to achieve improved corrosion resistance in high-temperature CO2, particularly for steels containing Cr levels near the transition that is required to form and maintain a protective Cr-rich oxide scale. [ABSTRACT FROM AUTHOR]

The oxidation behavior of several Fe and Ni commercial alloys was evaluated in high-temperature environments expected in next-generation supercritical carbon dioxide power cycles. The alloys were exposed to pure CO2 at 0.1 and 20 MPa and to CO2 containing H2O, O2, with and without SO2 at 0.1 MPa, at temperatures of 550°C (Fe alloys) and 700-750°C (Ni alloys). For comparison, the alloys were also exposed to air and supercritical steam. Low-Cr ferritic Fe alloys formed Fe-rich oxides leading to high oxidation rates in pure CO2, while high-Cr austenitic Fe alloys formed Cr-rich oxides leading to low oxidation rates. H2O and O2 impurities had negligible effect on the oxidation behavior of all Fe alloys. The addition of SO2 had negligible effect on the oxidation of ferritic Fe alloys, but significantly enhanced the rate for austenitic Fe alloys, where S in the oxide and underlying alloy contributed to the transition from Cr-rich to Fe-rich oxide growth. Ni alloys formed Cr-rich oxides and showed low oxidation rates in all CO2-containing environments which were very similar to those obtained in air and supercritical steam. Little or no effect of pressure was observed for any of the alloys exposed to pure CO2. [ABSTRACT FROM AUTHOR]

Abstract: Atomic layer deposition (ALD) of controlled-thickness TiO2 films was carried out on particle substrates in a fluidized bed reactor for the first time. Films were deposited on 550 nm SiO2 spheres and 65 nm ZnO nanoparticles for enhanced optical properties. Nanoparticles were fluidized with the assistance of a magnetically-coupled stirring unit. The metalorganic precursor titanium tetraisopropoxide was used here followed by either H2O or H2O2 to deposit TiO2 at various substrate temperatures. Growth rates of 0.01 nm/cycle and 0.04 nm/cycle were achieved when using H2O and H2O2 as the oxidizer, respectively. These conformal TiO2 films were verified using HRTEM, ICP–AES, XPS and UV absorbance measurements. The specific surface area changed appropriately after the particle size increased by the deposition of films with a given density, which showed that primary particles were not agglomerated together due to the coating process. In situ mass spectrometry was used to monitor reaction progress throughout each ALD reaction cycle. Bulk quantities of powder were successfully functionalized by TiO2 nanofilms without wasting excess precursor. [Copyright &y& Elsevier]