Alloys In Supercritical Water Research Articles

Comparison of the corrosion behavior of four Fe-Cr-Ni austenitic alloys in supercritical water

The corrosion behavior of four Fe-Cr-Ni austenitic alloys were investigated after exposure to deaerated supercritical water (SCW) at 650°C. Results show that the corrosion resistance follows the trend:Fe-29Cr-61Ni > Fe-16Cr-75Ni > Fe-25Cr-20Ni > Fe-21Cr-31Ni. The increase in Cr and Ni contents promote a transition from low-protective multilayer oxide scales to protective Cr2O3 scales. After surface grinding, the weight gains of all specimens dropped by one order of magnitude. Surface grinding induces the formation of ultrafine grains and a high density of dislocations, which enhances the corrosion resistance among all studied austenitic alloys. However, the beneficial effects of grinding diminish with increased Cr and Ni contents due to the pre-existing portion of Cr2O3 scales. The lower critical Cr content required to form Cr2O3 scales in high Ni austenitic alloys is primarily attributed to their lower solubility of O and the slower diffusion rates of Ni, which inhibit corrosion behavior.

Effect of Al addition on the corrosion behavior of Al CoCrFeNi high entropy alloys in supercritical water

The effect of Al addition on the corrosion behavior of high entropy alloys AlxCoCrFeNi (x = 0, 0.1, and 1.0) in supercritical water was investigated. The weight gain and oxide film thickness reduced with the increase of Al content. A duplex-structure oxide film, consisting of FeCr2O4 outer and Cr2O3 inner layer, was developed on CoCrFeNi alloy. A triple-layered oxide film consisting of FeCr2O4 outer, Cr2O3 middle, and Al2O3 inner layer was developed on Al0.1CoCrFeNi and AlCoCrFeNi alloys. The improvement of corrosion resistance by adding Al was attributed to the high Al concentration that accelerated the formation of Al2O3.

Corrosion characteristics and mechanisms of typical iron/nickel-based alloys in reductive supercritical water environments containing sulfides

The corrosion characteristics of stainless steel 316 and typical nickel-based alloys in supercritical water containing sulfides were investigated by a series of detection methods. Breakaway corrosion occurred on Incoloy 825 and Inconel 600 due to sulfide-formed tunnels or enriched microlayers in the inner layer of corrosion scales. In supercritical water containing sulfides, we find that the alloys with higher chromium and iron contents exhibited superior corrosion resistance than the nickel-rich alloys, which is further confirmed by an analysis of the thermodynamics and the effects of temperature and alloying elements. The corrosion mechanism of iron/nickel-based alloys in reductive supercritical water systems containing sulfides was proposed, especially the related breakaway corrosion mechanism and the effects of temperature and alloying elements. Additionally, prediction models of corrosion products are established to guide material selection.

Heterogeneous yielding mechanisms of body centered cubic iron for high resistance to chemical reaction-induced deterioration in supercritical water environments: A reactive molecular dynamics study

The atomic-scale yielding mechanisms of body centered cubic (BCC) iron in supercritical water have been an elusive problem that requires understanding the roles of chemical reactions with supercritical water in the mechanical behavior of iron. This work shows the combined effect of the supercritical water and tensile direction on the yielding mechanism of BCC iron using reactive molecular dynamics simulations. Our simulation results show that tensile strain along the [11¯0] direction of BCC iron may potentially exhibit much higher tensile strength and lower sensitivity to the environment compared with other directions. This is because yielding of iron along the [11¯0] direction originates from the homogenous generation of HCP precursors inside the iron bulk rather than at the surface, which limits the effects of surficial chemical reactions with supercritical water on the yielding behavior. This work is expected to contribute to the theoretical design of high-strength alloys in supercritical water.

Heterogeneous Yielding Mechanisms of Body Center Cubic Iron for High Resistance to Chemical Reaction-Induced Deterioration in Supercritical Water Environments: A Reactive Molecular Dynamics Study

The atomic-scale yielding mechanisms of body center cubic (BCC) iron in supercritical water have been an elusive problem that requires understanding the roles of chemical reactions with supercritical water in the mechanical behavior of iron. This work shows the combined effect of the supercritical water and tensile direction on the yielding mechanism of BCC iron using reactive molecular dynamics simulations. Our simulation results show that tensile strain along the [110] direction of BCC iron may potentially exhibit much higher tensile strength and lower sensitivity to the environment compared with other directions. This is because yielding of iron along the [110] direction originates from the homogenous generation of HCP precursors inside the iron bulk rather than at the surface, which limits the effects of surficial chemical reactions with supercritical water on the yielding behavior. This work is expected to contribute to the theoretical design of high-strength alloys in supercritical water.

High-resolution characterization of the internal and external oxidation of austenitic alloys in supercritical water

The surface oxide structures formed on two austenitic alloys after exposure in supercritical water were investigated via high-resolution characterization techniques. Results show that the corrosion processes of 19-Cr and 25-Cr alloys are internal oxidation and external oxidation, respectively. 19-Cr alloy shows a typical duplex thick oxide structure, while 25-Cr alloy has a much thinner protective oxide layer mainly composed of Cr2O3. The higher Cr content of 25-Cr alloy exceeds a critical value that enables the conversion from internal to external oxidation, thus facilitates the formation of a thin continuous Cr2O3 layer and protects the sub-surface from further oxidation.

Corrosion Characteristics of Typical Ni–Cr Alloys and Ni–Cr–Mo Alloys in Supercritical Water: A Review

The corrosion of supercritical water system materials under high-temperature and high-pressure conditions exists widely, which may result in the serious damage of the supercritical water reactor. T...

General Corrosion of Chromium-Coated Zirconium- and Titanium-Based Alloys in Supercritical Water at 500 °C

Abstract The 300 MWel small Canadian supercritical water-cooled reactor (SCWR), which is a scaled-down version of the original 1200 MWel concept, has a smaller core, uses low enriched uranium fuel instead of a plutonium–thorium fuel, and features a lower (maximum) cladding temperature of 500 °C. The lower cladding temperature may permit the use of different alloys, including zirconium alloys, which had been ruled out as candidates for the Canadian SCWR, whose cladding temperature may reach 850 °C. The potential to use zirconium alloys is exciting because they have a low neutron cross section, which in turn means that fewer neutrons are lost, and the fuel can be used more efficiently. One advantage, for example,, is that the fuel cycle can be lengthened. In this paper, we report on the results of corrosion experiments used to screen zirconium- and titanium-based alloys as well as corrosion-resistant coating materials such as Cr and Al as potential candidates for fuel cladding in the small Canadian SCWR. These experiments were conducted in a refreshed autoclave in deaerated supercritical water at 500 °C and 23.5 MPa. After exposure, the weight gain was measured, and the oxide thickness and the oxide phases were examined. Of all materials, the coated and uncoated Ti-grade 2 and Ti-grade 5 alloys met our screening qualification criteria, however, Al/Cr-coated zirconium coupons showed notable improvement and will be explored further in future testing.

Oxidation Processes and Involved Chemical Reactions of Corrosion-Resistant Alloys in Supercritical Water

Corrosion continues to be a major challenge in developing supercritical water-cooled reactor and supercritical water oxidation system. Corrosion behaviors of candidate alloy materials used in these...

Effects of dissolved oxygen on the oxide scales formed on ferritic-martensitic steel T12 exposed to supercritical water

Abstract The oxidation behavior of T12 after exposure to 560 °C/25 MPa supercritical water with different dissolved oxygen contents was investigated. The weight gain increases while the resistance of the oxide scale decreases with the increasing dissolved oxygen content. The oxide scale is a typical multi-layered structure including a Fe-rich outer layer and a Cr-rich inner layer. The Cr-rich hematite layer oxide was observed on specimen tested at high dissolved oxygen. The possible oxidation mechanism was proposed and the effects of oxygen on the oxidation of alloys in supercritical water were discussed.

Effect of Al/Si Content on Corrosion of Ni-based Alloys in Supercritical Water

Effect of Al/Si Content on Corrosion of Ni-based Alloys in Supercritical Water

Characterizing the effects of in-situ sensitization on stress corrosion cracking of austenitic steels in supercritical water

This work compared the stress corrosion cracking (SCC) susceptibility of two austenitic alloys in supercritical water (SCW), and revealed the quantitative difference of SCC growth rates through scanning transmission electron microscopy (STEM). Results showed that in-situ sensitization and creep occurred on the specimens during the SCC tests in SCW environment. Higher degree of Cr-depletion at the grain boundary ahead of the SCC crack tip was identified in the Fe-25Cr steel specimen, which has suffered severer in-situ sensitization because of the lower Cr content than the Ni-29.3Cr alloy specimen, and this in turn increased the creep susceptibility.

Corrosion Properties and Mechanisms of Austenitic Stainless Steels and Ni-Base Alloys in Supercritical Water Containing Phosphate, Sulfate, Chloride and Oxygen

The corrosion behaviors of 316 stainless steel (SS), 316L SS, Inconel 625, Incoloy 825 and Hastelloy C276 were studied in supercritical water (SCW) containing phosphate, sulfate, chloride and/or oxygen. The results show that the alloy corrosion rate in SCW containing oxygen and phosphate was increased by adding sulfate and/or chloride. A two-layer structure of oxide film on 316 SS consisted of an inner layer with Cr-rich oxides (i.e., Cr2O3) and an outer layer rich in phosphates (e.g., FePO4 and Ni3(PO4)2). A three-layer oxide film formed on Incoloy 825 including corrosion products Fe2O3, NiO, FePO4, Ni3(PO4)2 and NiCr2O4, with phosphates in each layer. A two-layer oxide film on Hastelloy C276 was composed of metal oxides in the inner layer and metal oxides as well as phosphates in the outer layer, including Fe2O3, Cr2O3, NiO, MoO2, FePO4, CrPO4, Ni3(PO4)2 and NiCr2O4. The existence of sulfate, chloride, and especially both of them destroyed the formation of water-insoluble phosphates on the oxide film of tested alloy in SCW containing oxygen and phosphate.

Oxide Film Growth Behavior of Ni-base Alloys in Supercritical Water at 750ºC

Oxide Film Growth Behavior of Ni-base Alloys in Supercritical Water at 750ºC

An hydrogen evolution method for the estimation of the oxide scale growth on reactor alloys in supercritical water

This study demonstrates the use of the hydrogen evolution method to estimate the weight gain and thickness of the oxide layer formed on stainless steel 316 and Alloy 800H tubing, exposed to supercritical water. The estimated weight gain of both materials follows a roughly parabolic rate law. Weight gain due to the oxidation after 300 h of exposure was found to be about 1.5 times larger in the SS316 compared to Alloy 800H. This method can be used for monitoring the corrosion behavior of nuclear reactor materials, without the need to interrupt the experiment and remove corrosion coupons.

An Industrial Perspective on Environmentally Assisted Cracking of Some Commercially Used Carbon Steels and Corrosion-Resistant Alloys

Commercial metals and alloys like carbon steels, stainless steels, and nickel-based super alloys frequently encounter the problem of environmentally assisted cracking (EAC) and resulting failure in engineering components. This article aims to provide a perspective on three critical industrial applications having EAC issues: (1) corrosion and cracking of carbon steels in automotive applications, (2) EAC of iron- and nickel-based alloys in salt production and processing, and (3) EAC of iron- and nickel-based alloys in supercritical water. The review focuses on current industrial-level understanding with respect to corrosion fatigue, hydrogen-assisted cracking, or stress corrosion cracking, as well as the dominant factors affecting crack initiation and propagation. Furthermore, some ongoing industrial studies and directions of future research are also discussed.

Corrosion and deuterium uptake of Zr-based alloys in supercritical water

To increase the thermodynamic efficiency above 40% in nuclear power plants, the use of supercritical water as the heat transport fluid has been suggested. Zircaloy-2, -4, Zr–Cr–Fe, Zr–1Nb and Zr–2.5Nb were tested as prospective fuel cladding materials in 30MPa D2O at 500°C. Zircaloy-2 showed the highest rates of corrosion and hydriding. Although Zr–Cr–Fe initially showed a very low corrosion rate, it displayed breakaway corrosion kinetics after 50h exposure. The best-behaved material both from a corrosion and hydrogen uptake point of view was Zr–2.5Nb. However, the Zr–2.5Nb oxide growth rate was still excessive and beyond the current CANDU®11CANDU – CANada Deuterium Uranium is a registered trademark of Atomic Energy of Canada Ltd. design allowance. Similar coupons, coated with Cr, were also tested. The coated layer effectively prevented oxidation of the coupons except on the edges, where the coating was thinner and had some flaws. In addition, the Cr-coated Zr–2.5Nb coupons had the lowest deuterium pickup of all the alloys tested and showed no signs of accelerated or non-uniform corrosion.

Oxidation of austenitic and ferritic/martensitic alloys in supercritical water

Supercritical Water Reactors (SCWRs), one of the concepts considered by the Generation IV International Forum, are an attractive option due to their high thermal efficiency, 45% vs. 33% for current Light Water Reactors (LWR) and their more simple design. The reference design for the European SCWR is a direct cycle system operating at 25MPa with core inlet temperatures of 280 and average core outlet temperature of 500°C. In this range of temperatures, shifting from subcritical to supercritical conditions, there is a sharp change in water density as well as in chemical properties, such as dielectric constant and ionic product. These changes in properties could influence the behavior of austenitic alloys and other materials to oxidation and stress corrosion cracking, providing unexpected responses in the light of the available knowledge for LWRs. Oxide layer characteristics are relevant to both stress corrosion cracking and corrosion product transport.Oxidation experiments on austenitic alloys, 316 L SS, Alloy 600 and Alloy 625, and F/M T91 were performed at two temperatures, 400°C and 500°C, and 25 and 30MPa in order to explore the influence of the chemical properties of supercritical water on the oxide layers characteristics. Results from the characterization of the oxide layers by optical, SEM/EDX and Auger spectroscopy as well as the oxidation kinetics are discussed.

Irradiation-assisted stress corrosion cracking of austenitic alloys in supercritical water

Four commercial austenitic alloys, 316L, D9, 690 and 800H were irradiated with 2.0 MeV protons to doses of up to 7 dpa at temperatures of 400 and 500 °C to study the role of irradiation and temperature on stress corrosion cracking in supercritical water. Constant extension rate tensile (CERT) tests were performed on the irradiated specimens in deaerated supercritical water at the same temperature as the irradiation. The results showed that irradiation significantly increased the severity of intergranular stress corrosion cracking in all four alloys. Cracking severity increased with dose and temperature. The severity of cracking correlated with both the hardness and the grain boundary chromium concentration, making attribution to a single irradiation feature difficult. However, a very significant increase in cracking of alloy 690 at 500 °C is likely due primarily to RIS rather than to hardening. Cracking of the irradiated alloys in supercritical water follows the same behavior as in subcritical water, implying that a common mechanism may be controlling.

Oxidation of ferritic–martensitic alloys T91, HCM12A and HT-9 in supercritical water

The oxidation behavior of ferritic–martensitic (F–M) alloys in supercritical water (SCW) was studied in order to evaluate the suitability of these alloys for use in the Gen IV supercritical water reactor (SCWR) concept. A series of exposure tests in SCW were performed with three F–M alloys: T91, HCM12A, and HT-9. The effect of temperature was evaluated over the range of 400–600 °C and the dissolved oxygen concentration was controlled at <10 ppb (deaerated condition), 100 and 300 ppb. The oxidation behavior was determined from weight gain measurements along with oxide structure analysis. The results indicated that the oxidation rate was strongly dependent on temperature and followed an Arrhenius behavior. Activation energies for oxidation were 172, 177, and 189 kJ/mol for HT-9, HCM12A, and T91, respectively. The time dependence of the oxidation rate followed an exponential law with time exponents ∼0.3–0.42. Reduction in oxidation rate was observed at intermediate values (100–300 ppb) of dissolved oxygen concentration. The oxide formed on the alloy surface consisted of an outer layer of porous magnetite (Fe 3O 4) and an inner layer of iron chromium oxide, (Fe, Cr) 3O 4 with spinel structure. A transition region lies beneath the inner oxide in which the metal content increases to bulk values and the oxygen content decreases to nearly zero. Iron chromium oxide, (Fe, Cr)O, with the wustite structure was observed in the transition layer at 600 °C. The relatively good agreement between the activation energies for oxidation and that for grain boundary diffusion of oxygen support an oxidation mechanism based on short circuit oxygen diffusion to the oxide–metal interface.