Supercritical Water Environment Research Articles

Effect of temperature and dissolved oxygen on stress corrosion cracking behavior of P92 ferritic-martensitic steel in supercritical water environment

The effect of temperature and dissolved oxygen (DO) on stress corrosion cracking (SCC) of P92 martensitic steel in supercritical water (SCW) was investigated using slow strain rate test (SSRT) and fractography analysis. The SSRT was carried out at temperatures of 400, 500, 600 °C in deaerated supercritical water and at DO contents of 0, 200, 500 ppb at the temperature of 600 °C, respectively. The results of SSRT show that the decrease of ductility at the temperature of 400 °C may be related to the dynamic strain aging (DSA) of P92 steel. The degradation of the mechanical properties in SCW is the joint effect of temperature and SCC. Compared with the effect of temperature, DO in SCW has no significant effect on the SCC susceptibility of P92 steel. The observation of oxide layer shows that large numbers of pores are nucleated in the oxide layer, which is related to vacancy accumulation and hydrogen generated in the oxide layer. Under the combined action of the growth stress and tensile stress, micro cracks, nucleated from the pores in the oxide layer, are easily propagated intergranularly outward to the surface of specimen, and fewer cracks can extend inward along the intrinsic pores to the inner oxide/metal interface, which is the reason for the exfoliation of oxide films.

Effect of cold work on the stress corrosion cracking behavior of Alloy 690 in supercritical water environment

The stress corrosion cracking (SCC) behavior of Alloy 690 with 0, 20% and 30% cold work (CW) was studied in supercritical water (SCW) environment with an emphasis on CW and creep on the CGRs (CGR). SCC and creep CGRs increased with %CW, which correlated hardness very well. Microscopic characterization of the crack tip and fracture surface showed obvious cavity-like features, which is clear evidence of creep attack. The creep CGRs in inert gas were comparable to the SCC CGRs in SCW, indicating that creep is a major factor in crack growth. Increasing level of CW was found to increase the creep susceptibility, and high activation energies for creep crack growth were observed between 500 °C and 550 °C.

Conversion of organic matter in the carbonaceous medium in the supercritical water

The work is based on the modeling of geothermal transformations of hydrocarbons in oil-bearing formations of deep horizons of the earth's crust - under abnormally high pressure, in the presence of aqueous fluid and carbonaceous substances. The pressure and temperature in the experiments are typical for water in the supercritical state. The regularities of the conversion of heavy oil in supercritical water and in the presence of finely dispersed caustobioliths and metal oxides were shown. Aquathermolysis in the presence of proton provides blockage of free radicals of high-molecular weight hydrocarbons and saturation of unsaturated hydrocarbons, produced by cracking reactions, and inhibiting of condensation reactions of aromatic macromolecules. The hydrogen protons also promote hydrogenation reactions in the crude oil. The regularities of changes of the component, structural-group, fractional and elemental compositions of heavy oil during the conversion under the above conditions were established, rheological characteristics of the initial crude oil and converted oil were studied as well. As a result of carrying out aquathermolysis in the supercritical water environment and in the presence of initiating additives, the high-molecular weight components of the initial crude oil were degraded with the formation of light distillate fractions, which were scarcely present in the initial crude oil. Thus, the conversion rate for various samples amounted to 18–29%. It resulted in the significant reduction in the viscosity of the converted oil, up to 96% compared to the initial crude oil.

Catalytic Partial Oxidation of Methane in Supercritical Water: A Domain in CH4/H2O–O2/CH4 Parameter Space Showing Significant Methane Coupling

Partial oxidation of methane under supercritical water environment at 658 K and 26 MPa using a batch reactor was examined in the presence of metal oxide catalysts and H2O2 as a molecular oxygen source. Within a parameter space of feed concentrations (CH4/H2O and O2/CH4 ratios), there existed a domain that gave high C2+ hydrocarbon yield reaching ca. 4%. Obtained products were a mixture of hydrocarbons up to C4 with ethane as a major component, accompanied by CO, CO2, H2, and methanol. The domain that gave high hydrocarbon yield did not coincide with that which gave high methanol yield. Catalyst survey indicated that transition-metal oxides are effective for the production of C2+ hydrocarbons under the present conditions. In view of the oxidative coupling of methane in all reaction modes explored previously, presently reported level of hydrocarbon yield is unprecedented at this low reaction temperature.

Characterizing the effect of creep on stress corrosion cracking of cold worked Alloy 690 in supercritical water environment

The effect of creep on stress corrosion cracking (SCC) was studied by measuring crack growth rates (CGRs) of 30% cold worked (CW) Alloy 690 in supercritical water (SCW) and inert gas environments at temperatures ranging from 450 °C to 550 °C. The SCC crack growth rate under SCW environments can be regarded as the cracking induced by the combined effect of corrosion and creep, while the CGR in inert gas environment can be taken as the portion of creep induced cracking. Results showed that the CW Alloy 690 sustained high susceptibility to intergranular (IG) cracking, and creep played a dominant role in the SCC crack growth behavior, contributing more than 80% of the total crack growth rate at each testing temperature. The temperature dependence of creep induced CGRs follows an Arrhenius dependency, with an apparent activation energy (QE) of about 225 kJ/mol.

Demetalization of Heavy Oil Based on the Preferential Self-assembly of Heavy Aromatics in Supercritical Water

Demetalization of heavy oil through pyrolysis in the presence of subcritical water or supercritical water (SCW) was experimentally investigated. At a high water-to-oil ratio and high water density, the occurrence of pyrolysis can be transferred to the SCW phase. Driven by the π–π attractive interaction between aromatic sheets and the superb diffusivity in SCW, the coke-like self-assembly of metal-containing heavy aromatics occurs spontaneously and rapidly. The self-assembly behavior of aromatics in SCW depends not only on the thermodynamic state of SCW but also on the average scale of aromatics. With the aid of self-assembly in dense SCW, the condensation of metal-containing heavy aromatics, distributed mainly in a vacuum residue, to coke is significantly accelerated, by which the rate of demetalization is improved simultaneously. Owing to the preferential self-assembly of metal-rich heavy aromatics, an increasing yield of liquid products can also be obtained under an optimized SCW environment.

Catalytic cracking of 4-(1-naphthylmethyl)bibenzyl in sub- and supercritical water

The upgrading and reforming of coal-derived liquids are necessary for the application of high value-added liquid fuels, but the processing is difficult due to the amounts of impurities. In this paper, the catalytic cracking of 4-(1-naphthylmethyl)bibenzyl (NMBB) as a model reaction was studied deeply under sub- and supercritical water environment. Water might serve as a cheap and environmental H donor, which was more efficient than the supplied H2 in the processing of coal-derived liquids upgrading. Meanwhile, sub- and supercritical water could provide a homogeneous reaction environment for NMBB cracking as special solvents. The NMBB conversion under subcritical water was much higher than that under supercritical water. Furthermore, for H2 atmosphere, NMBB conversion was higher than that of N2 atmosphere, indicating that the process of in-situ H2 formation from subcritical water was more easily. Therefore, the cracking of NMBB under subcritical water and H2 atmosphere could be regarded as a reasonable reaction condition.

Analysis of localized oxides and oxide films formed on alloy 625 and 617 in supercritical water and superheated steam environments at 750°C

Analysis of localized oxides and oxide films formed on alloy 625 and 617 in supercritical water and superheated steam environments at 750°C

Atomistic modeling of nanoparticle generation in short pulse laser ablation of thin metal films in water

Laser ablation in liquids is actively used for generation of clean colloidal nanoparticles with unique shapes and functionalities. The fundamental mechanisms of the laser ablation in liquids and the key processes that control the nanoparticle structure, composition, and size distribution, however, are not yet fully understood. In this paper, we report the results of first atomistic simulations of laser ablation of metal targets in liquid environment. A model combining a coarse-grained representation of the liquid environment (parameterized for water), a fully atomistic description of laser interactions with metal targets, and acoustic impedance matching boundary conditions is developed and applied for simulation of laser ablation of a thin silver film deposited on a silica substrate. The simulations, performed at two laser fluences in the regime of phase explosion, predict a rapid deceleration of the ejected ablation plume and the formation of a dense superheated molten layer at the water-plume interface. The water in contact with the hot metal layer is brought to the supercritical state and transforms into an expanding low density metal-water mixing region that serves as a precursor for the formation of a cavitation bubble. Two distinct mechanisms of the nanoparticle formation are predicted in the simulations: (1) the nucleation and growth of small (mostly ⩽10nm) nanoparticles in the metal-water mixing region and (2) the formation of larger (tens of nm) nanoparticles through the breakup of the superheated molten metal layer triggered by the emergence of complex morphological features attributed to the Rayleigh-Taylor instability of the interface between at the superheated metal layer and the supercritical water. The first mechanism is facilitated by the rapid cooling of the growing nanoparticles in the supercritical water environment, resulting in solidification of the nanoparticles located in the upper part of the mixing region on the timescale of nanoseconds. The computational prediction of the two mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes is consistent with experimental observations of two distinct nanoparticle populations appearing at different stages of the ablation process.

A comparative study on the oxidation of austenitic alloys 304 and 304-oxide dispersion strengthened steel in supercritical water at 650 °C

This study concentrated on the investigation of oxide scale grown on alloys 304 and 304-oxide dispersion strengthened steel in supercritical water (650°C/25MPa) environment. The corrosion rate was evaluated by measuring the weight change of the samples and by cross-section examinations. Results showed that weight gains vs. supercritical exposure time follows a parabolic law for alloy 304-ODS contrary to alloy 304 which possibly follows a cubic rate law. The general weight gain after 550h exposure to the SCW was 131.8 and 621.6mg/dm2 for alloys 304-ODS and 304, respectively. Electron microscopy observations and elemental analyses as well as X-ray diffraction and time-of-flight secondary ion mass spectrometry results revealed that the oxide scales formed on 304-ODS alloy composed of three distinct layers including Fe3O4 (outer layer), FeCr2O4/(Fe,Cr)2O3 spinel structures (inner layer) and Cr2O3 (transition layer). However, alloy 304 had two layers including Fe3O4 (outer layer) and Fe-Cr spinel structures (inner layer). It is believed that the formation of Cr2O3 layer at the diffusion layer/metal interface becomes the rate-limiting step for oxide advancement, since this change in oxide structure also corresponds to a decrease in corrosion rate in the alloy 304-ODS compared to alloy 304. Based on the results and observations, the oxidation mechanisms are discussed.

Early oxidation of Super304H stainless steel and its scales stability in supercritical water environments

The early oxidation of Super304H stainless steel exposed to supercritical water at 450 °C, 540 °C, and 580 °C was characterized. After a 40 h exposure, distribution density of oxide grains produced on samples increased with experimental pressures, while initial aqueous pH had a negligible influence on short-term corrosion morphologies, implying that the outer oxide layer of scales may form via solid growth mechanism rather than by metals dissociation/oxides precipitation process. After a repeated 24 h cycles of exposure at condition of 540 °C/25 MPa followed by that of 500 °C/23 MPa up to 5 cycles, partial spallation of the scales appeared along inner and outer layer interface, not substrate/scales interface due to the enhanced adhesion of scales on the substrate by pinning effects and production and convergence of cavities at the inner/outer layer interface.

Corrosion and microstructural analysis data for AISI 316L and AISI 347H stainless steels after exposure to a supercritical water environment.

This article presents corrosion data and microstructural analysis data of austenitic stainless steels AISI 316L and AISI 347H exposed to supercritical water (25 MPa, 550 °C) with 2000 ppb of dissolved oxygen. The corrosion tests lasted a total of 1200 h but were interrupted at 600 h to allow measurements to be made. The microstructural data have been collected in the grain interior and at grain boundaries of the bulk of the materials and at the superficial oxide layer developed during the corrosion exposure.

Degradation of SPS-Fabricated YSZ and Nd2O3-YSZ Ceramics in Supercritical Water

Zirconia (ZrO2) ceramics are being considered as a candidate material for thermal insulating barriers in pressure tubes used in the supercritical water (SCW) nuclear reactors. However, the literature suggests that zirconia may undergo a detrimental phase transformation which is accelerated in aqueous environments. In this research, 8 mol% Yttria-Stabilized Zirconia (YSZ) ceramics with the addition of 5 and 10 mol% Nd2O3 were manufactured via spark plasma sintering (SPS) process and subsequently subjected to a SCW environment. The weight losses and microstructural evolutions of these materials during SCW exposure were studied. The results suggest that doping YSZ with Nd2O3 significantly decreased the degradation rate of the YSZ ceramic and improved its structural stability. X-ray diffraction studies revealed that after degradation testing, the Nd2O3 helped to retain the desirable cubic phase of YSZ matrix. In the case of pure YSZ ceramic, a phase change of the matrix toward the monoclinic lattice was observed and likely contributed to the ceramic’s disintegration in SCW environment.

Solvation of asphaltenes in supercritical water: A molecular dynamics study

To improve the understanding of the condensation of asphaltenes in supercritical water (SCW), the solvation of asphaltenes under SCW environments was investigated with molecular dynamics simulation. The simulation results indicate that the repulsive van der Waals interaction has a primary influence on the nature and the magnitude of the interaction between asphaltenes and SCW. A cavity short-range solvent structure surrounding asphaltenes thus is formed, and the solvation free energy (Gsol) of asphaltenes in SCW has a positive sign in most cases. The scale of cavities and the value of Gsol both are determined mainly by the bulk density of water. By the repulsive asphaltenes/SCW interaction and the attractive π–π aromatic interaction, the aggregation of asphaltenes in SCW occurs spontaneously. The asphaltene clusters formed, containing asphaltene monomers and nanoaggregates, present a coke-like supramolecular structure. Benefiting from excellent diffusivity in SCW, the aggregation of asphaltenes in SCW can be accomplished rapidly.

Comparison of Performance and Oxidation of Nitronic-50 and Stainless Steel 316 in Subcritical and Supercritical Water Environments

This work investigates two austenitic stainless steels, Nitronic-50 and stainless steel 316, for use in both subcritical and supercritical water (SCW) conditions. The mechanical characteristics of the materials were investigated using slow strain rate testing in a SCW test loop under the following conditions: nitrogen at ambient temperature and pressure, liquid water at 473 K (200 °C) and 8 MPa, liquid water at 573 K (300 °C) and 15 MPa, and SCW at 698 K (425 °C) and 27 MPa. The surfaces of the failed samples were characterized using Raman spectroscopy, and X-ray photoelectron spectroscopy. Nitronic-50 was found to have superior mechanical strength characteristics at all conditions compared to stainless steel 316. At all elevated temperature conditions, stainless steel 316 was found to have a surface film consisting of iron oxides, while the surface film of Nitronic-50 predominantly consisted of nickel-iron spinel.

High resolution microstructural studies of the evolution of nano-scale, yttrium-rich oxides in ODS steels subjected to ball milling, selective laser melting or friction stir welding

Coarse-grained ferritic Oxide Dispersion Strengthened (ODS) FeCrAlY alloys were initially developed for use in high temperature environments due to their excellent creep strength and oxidation resistance. More recently, ODS materials capable of forming protective α-alumina scales have attracted increasing interest for nuclear applications, due to their potential corrosion resistance in super-critical water and liquid metal environments. However, there is still controversy about how and when the fine dispersoids form; and whether they are transformed during fabrication processes.In this paper high spatial resolution electron microscopy is used to monitor the evolution of the dispersoids under different conditions including: heat treatment of as-milled powder, selective laser melting and friction stir welding.

Fuel Assembly Design for Supercritical Water-Cooled Reactor

The supercritical water-cooled reactor (SWCR) has been selected as one of the most promising reactors for Generation IV nuclear reactors due to its higher thermal efficiency and more simplified structure compared to the state-of-the-art light water reactors (LWRs). However, there are a large number of potential problems that must be addressed, particularly the fuel assembly design of the SCWR. SCWRs are a kind of high-temperature, high-pressure, water-cooled reactor that operates above the thermodynamic critical point of water (374°C, 22.1 MPa). Corrosion and degradation of materials used in supercritical water environments are determined by several environment- and material-dependent factors. In particular, irradiation-induced changes in microstructure and microchemistry are major concerns in a nuclear reactor. Many structural materials including alloys and ceramics have been proposed for use as SCWR components or materials for applying protective coatings in SCWRs. In this paper, the present status of supercritical fuel assembly design at home and abroad is reported. According to the special requirements of supercritical core design, a kind of configuration design of fuel assembly with two-flow core and using SiC as cladding material are proposed. The analysis results have shown that the design basically meets the requirements of fuel assembly design, which has good feasibility and performance.

Structural Characteristics of Asphaltenes Derived from Condensation of Maltenes in Supercritical Water

Asphaltenes derived from the condensation of maltenes under high-pressure N2 and supercritical water (SCW) environments were characterized with various approaches. The reaction kinetics of the condensation of asphaltenes under hydrothermal environments were also measured. By improving the diffusivity of hydrocarbon species and the isolation of aromatic carbon radicals from hydrogen donors in the SCW phase, the dealkylation of alkyl substitutes and the condensation of aromatic rings involved in the condensation of maltenes are promoted. Consequently, asphaltenes formed in the SCW phase (As-SCW) have a higher aromaticity and lower alkyl and naphthenic fractions than asphaltenes formed in the oil phase (As-oil). Nevertheless, As-SCW and As-oil both present a cokelike supermolecular structure with the stacking of polycyclic aromatic rings. Benefitting from the highly fused aromatic molecular structure and the cokelike supermolecular structure, the condensation of As-SCW to coke occurs readily under hydrotherm...

Catalytic bitumen cracking in sub- and supercritical water

The bitumen cracking reaction under sub- and supercritical water environments in H2 or N2 atmosphere using activated carbon (AC) supported nickel catalyst was studied. The sub- or supercritical water provided a unique homogeneous, acidic reaction system for the bitumen cracking reaction, which obviously promoted the bitumen conversion and selectivity of the middle distillates (kerosene and gasoline), and restrained the formation of carbon deposition, if compared with the conventional pyrolysis reaction without water addition. Moreover, the active in-situ H2 source was provided by the water–gas shift reaction in the sub- and supercritical water environment, thereby enhancing the catalytic activity of bitumen cracking.

Corrosion behavior of a 304-oxide dispersion strengthened austenitic stainless steel in supercritical water

The corrosion behavior of a 304 oxide dispersion strengthened (ODS) austenitic stainless steel after being exposed to supercritical water (SCW) with dissolved oxygen of 300 ppb at 600 °C/25 MPa was studied. By the means of a least-square fit, the weight gain (Δw) of the samples as a function of exposure time (t) could be described by the equation Δw = 8.011 × t0.459. An approximately parabolic time law in this specified SCW environment can be assumed. A triple protective oxide layer formed on the surface of the metal with the exposure time up to 1000 h. Furthermore, the result of slow-strain rate tensile (SSRT) test indicated that the 304-ODS austenitic stainless steel was not susceptible to stress corrosion cracking (SCC) in SCW at 600 °C/25 MPa.