High Temperature Steam Environment Research Articles

Reducing the oxidation rate of Cr-coated Zr alloys under high temperature steam environment: An approach of an outer Zr coating

Cr-coated Zr alloys are widely regarded as promising accident-tolerant fuel (ATF) cladding materials. However, the rapid consumption of the Cr coating through both oxidation and diffusion demands a thick coating, which is detrimental to neutron economic. In this study, a thin Zr coating was fabricated on the top of the Cr coating to act as a protective layer, aiming to reduce the oxidation consumption rate of Cr coatings. In-situ weight gain measurements were performed to determine the oxidation kinetics. The microstructural evolution of the coating/substrate interface and the oxide/coating interface were analyzed. The results show that the weight gain rate of the Zr-Cr coated Zr alloy sample is lower than that of the bare Cr-coated Zr alloy samples. An equiaxed grain structure composed of Cr/Zr mixed oxides was observed in the coating, which helps to inhibit elements diffusion and reduce the oxidation rate of Cr coatings. After steam oxidation at 1000 °C for 4 h, the sample structure consisted of the Zr substrate, a Cr2Zr layer, a residual Cr coating, a Cr2O3 layer, a Cr/Zr mixed oxides layer, and a ZrO2 layer.

Effects of Mn addition on oxidation behaviour of heat-resistant steel in a high-temperature steam environment

A comparative study of high chromium (Super304H) and high manganese low chromium heat-resistant steel (HT630) was carried out at 650 °C in steam. Results indicate that the high manganese content in HT630 significantly improved its oxidation resistance. It is ascribed to the formation of a dense protective MnCr2O4 oxide layer during oxidation. MnCr2O4 layer effectively inhibits the diffusion of iron, suppresses the growth of iron-rich oxides, promotes the formation of an iron-rich protective interface with the matrix, and improves the overall oxidation resistance.

Tailored high-temperature corrosion behavior of Cr coatings using high power impulse magnetron sputtering on ZIRLO alloys for accident-tolerant fuel application

To further enhance the oxidation resistance of Zirconium alloys during Loss of Coolant Accident (LOCA), protective Cr coatings were deposited in synchronized pulse bias mode through high power impulse magnetron sputtering (HiPIMS). The results showed the weight gain of the Cr coatings with synchronized pulse bias was reduced by approximately 61.7 % compared to that of the Cr coatings without synchronized pulse bias after 90 min of steam oxidation at 1200 °C. This was mainly attributed to the formed intense (200) preferred orientation of the Cr coatings with synchronized pulse bias during deposition, accelerating the recrystallization growth of the residual Cr layer in a high-temperature steam environment, thereby reducing the rapid diffusion pathways for oxygen. Moreover, the synchronized pulse bias induced Cr grains refinement, favoring the formation of a more compact Cr2O3 scale, and therefore efficiently inhibiting the diffusion of external oxygen to the underlying alloy.

Microstructural evolution in 12% Cr heat-resistant steel during compression deformation at 650°C

Microstructural evolution in 12% Cr heat-resistant steel during compression deformation at 650°C

Effect of Si on the structure and oxidation behavior of Cr-Si coatings on Zry-4 under high temperature steam environment

Under loss of coolant accident (LOCA), zirconium alloy (Zry-4) is easily oxidized by high temperature steam, which makes its performance deteriorate. To improve the oxidation resistance of Zry-4 under LOCA, Cr-Si coatings with different Si content were deposited on Zry-4 by magnetron sputtering, and the effects of Si content on the steam oxidation resistance of the Cr-Si coatings were investigated. With the fluctuation of Si content inside the coating, there appears different lattice structure. Some amorphous phases and lattice distortions in the coatings with moderate Si content were discovered by TEM, which were beneficial to limit O diffusion in the lattice. However, in the coatings with the higher Si content, the lattice distortions transform into dislocations, which contributes to the O diffusion. The oxidation kinetics of the coated samples proved that the Cr-Si coating with lattice distortion was more consistent with the parabolic law and showed excellent oxidation resistance. In addition, the results of first-principles calculations supported the above view that the O diffusion energy in the lattice increases after the introduction of Si.

Effect of long reaction distance on gas composition from organic-rich shale pyrolysis under high-temperature steam environment

When high-temperature steam is used as a medium to pyrolyze organic-rich shale, water steam not only acts as heat transfer but also participates in the chemical reaction of organic matter pyrolysis, thus affecting the generation law and release characteristics of gas products. In this study, based on a long-distance reaction system of organic-rich shale pyrolysis via steam injection, the effects of steam temperature and reaction distance on gas product composition are analyzed in depth and compared with other pyrolysis processes. The advantages of organic-rich shale pyrolysis via steam injection are then evaluated. The volume concentration of hydrogen in the gas product obtained via the steam injection pyrolysis of organic-rich shale is the highest, which is more than 60%. The hydrogen content increases as the reaction distance is extended; however, the rate of increase changes gradually. Increasing the reaction distance from 800 to 4000 mm increases the hydrogen content from 34.91% to 69.68% and from 63.13% to 78.61% when the steam temperature is 500 °C and 555 °C, respectively. However, the higher the heat injection temperature, the smaller the reaction distance required to form a high concentration hydrogen pyrolysis environment (hydrogen concentration > 60%). When the steam pyrolysis temperature is increased from 500 °C to 555 °C, the reaction distance required to form a high concentration of hydrogen is reduced from 3800 to 800 mm. Compared with the direct retorting process, the volume concentration of hydrogen obtained from high-temperature steam pyrolysis of organic-rich shale is 8.82 and 10.72 times that of the commonly used Fushun and Kivite furnaces, respectively. The pyrolysis of organic-rich shale via steam injection is a pyrolysis process in a hydrogen-rich environment.

Mechanical degradation of duplex SiC-fiber reinforced SiC matrix composite tubes under a controlled high-temperature steam environment

This paper describes results from the mechanical evaluation of unirradiated SiC fiber–reinforced SiC matrix composite tubes under a controlled high-temperature steam environment. The experiments were aimed at identifying key material degradation behavior under environments relevant to loss-of-coolant accidents of light water reactors. Mechanical tests of the SiC composite tubes at 1000 °C under steam and inert environments were conducted using a unique test capability. The material tested was a duplex tube with a thick monolithic SiC layer on the outer surface. The tubes were subjected to preloading at ∼100 MPa in tension before exposure to high-temperature steam with up to 75 % of the preload at a constant displacement. In the presence of matrix cracks, the steam exposure caused embrittlement of the SiC composite tubes and failure at a stress level below the pretest stress. The material degradation was explained by a fiber oxidation model, which can be applied to various SiC cladding concepts. The embrittlement could be a limiting factor for using SiC cladding subjected to loss-of-coolant accident conditions.

Study on eutectic-oxidation coupling reaction of Cr-Zr system in high temperature steam environment

The isothermal steam oxidation behavior of Cr-coated Zr-1Nb alloy at 1350 ℃ and 1400 ℃ was investigated. The temperatures are above the Cr-Zr eutectic temperature. The reliability of the experimental setup and methodology was confirmed by conducting isothermal steam oxidation experiments on bare Zr-1Nb specimens at 1100∼1400 ℃, and the corresponding oxidation kinetics correlation was obtained. After the eutectic-oxidation reaction, the surface of the coated specimens exhibited a “crocodile skin” morphology composed of “ridges” and “hollows”, which was attributed to the non-uniform eutectic reaction. Due to the much higher O affinity of Zr compared to Cr, and the higher solubility of Cr in β-Zr compared to α-Zr(O) and ZrO2, the ZrO2 and α-Zr(O) layers initially grew on the Cr-Zr eutectic during oxidation. Cr continuously diffused inward and eventually precipitated as Cr-rich phases within the metallic matrix. Three types of Cr-rich phases were observed, with varying oxygen content and reflecting the oxidation process of the Cr-rich phases. The Cr-rich phases were ultimately oxidized into a mixture of Cr2O3 and ZrO2. The parabolic weight gain rates for the oxidation of Cr-Zr eutectic are compared with the oxidation kinetics correlation derived from the bare Zr-1Nb specimens, and the model applicability of the oxidation kinetics correlations of Zr alloys in the simulations of Cr-Zr eutectic-oxidation reaction is discussed.

Design, synthesis and characterization of multilayer environmental barrier coatings

The formation of cristobalite thermally grown oxides (TGO) is a major life limiting factor for environmental barrier coatings (EBCs) with Si bond coat exposed to high temperature steam environments due to its reversible phase transformation and the resultant cracking behaviour during thermal cycling. Therefore, controlling or avoiding cristobalite TGO formation by engineering coating chemistry and architecture will be desirable for a highly durable EBC system. In this work, a multilayer Si/Al-Si-N/Yb2Si2O7 EBC was produced using a hybrid vapor deposition process. The oxidation behavior of the EBC was evaluated in flowing 90% H2O (g) + 10% air at 1316 °C. After 100 h and 510 h steam test, crack-free mullite and Al-Si-O-N layers formed above Al-Si-N layer and no SiO2 scale formation of Si bond coat was observed. Detailed TEM analyses show that the Al-Si-N layer successfully prevented the oxidation of underlying Si bond coat and avoided the formation of detrimental cristobalite phase.

Degradation of the Cr2O3 layer protectiveness caused by grain refinement during high-temperature oxidation of Cr coatings

To enhance the understanding of the failure mechanism of Cr coatings under simulated loss of coolant accident (LOCA) conditions in pressurized water reactors, an investigation into the isothermal oxidation behavior of Cr coatings in 1200 °C steam was conducted. The structure evolution of Cr coatings was characterized by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) methods. The obtained results reveal the formation of a three-layer structure during the 1200 °C steam oxidation process. This structure comprises a Cr2O3 layer, a residual Cr layer, and a ZrCr2 layer. Over time, the grains in the Cr2O3 layer undergo refinement, leading to an increase in grain boundaries. This phenomenon promotes the inward diffusion of oxidizing medium, contributing to understanding of the degradation of Cr2O3 layer protectiveness after prolonged oxidation. These findings offer valuable insights into the failure mechanism of Cr coatings in high-temperature steam environments.

Systematic investigations on the oxidation mechanism of Cr coated Zr-4 alloy under different high-temperature steam conditions

Chromium (Cr) coating is a potential candidate material for improving the oxidation resistance and extending rail service life of zirconium (Zr) alloys used in nuclear fuel cladding materials. This study investigates the effect of temperature on the effectiveness of Cr coating on Zr-4 alloy deposited using arc ion plating, revealing the microstructure evolution mechanism of Cr coating in high temperature steam environments. The research results indicate that the oxidation of the Cr coating is slow at 900-1000 °C and that the surface oxide layer, to some extent, hinders the diffusion of oxygen from the outside to the inside. However, at higher temperatures, the oxidation rate of the Cr coating is accelerated, and large bubbles and voids are formed at the interface between the Cr2O3 layer and the residual Cr layer, resulting in the fracture of the oxide layer. At this time, the protective performance of the Cr coating is completely lost. Note that the simultaneous inward diffusion of oxygen and outward diffusion of Zr trigger the occurrence of redox reactions, generating a ZrO2 network. Changes in steam temperature significantly affect the oxidation behavior of Cr coating. As a driving force, temperature accelerates the diffusion of oxygen to the substrate. Conversely, the formed zirconia network also promotes the failure of the Cr coating.

Study on oxidation behaviors of Zr–Sn–Nb alloy in water steam at 1250 °C

To investigate the effect of different oxidation times on the oxidation behavior of Zr–Sn–Nb alloy in a high-temperature steam environment at 1250 °C, steam oxidation tests were conducted on the Zr–Sn–Nb alloy at 1250 °C for durations ranging from 100 to 5000 s. Microstructural and elemental composition analyses of the Zr–Sn–Nb alloy were carried out using scanning electron microscopy, energy-dispersive x-ray spectroscopy, and optical microscopy. The results showed that in the initial stages of oxidation (0–1000 s), the weight gain per unit area followed a parabolic trend. After 2500 s of oxidation, the weight gain rate significantly increased. In the later stages of oxidation (after 4000 s), the weight gain curve transitioned from a parabolic shape to a linear law. With increasing oxidation time, the thickness of the oxide layer gradually increased. In the early stages (0–2500 s), the growth rate of the oxide layer thickness was relatively slow, but the appearance of micro-pores and cracks was observed. However, after 2500 s, the steam oxidation rate of the Zr–Sn–Nb alloy significantly accelerated, leading to fracture and failure of the alloy specimens. The change in oxide layer thickness over time followed a parabolic law before 2500 s and a linear law after 2500 s. The growth curve of the α-Zr(O) layer within 5000 s also followed a parabolic law.

Comparing the oxidation of FeCrAl alloys in isothermal and cyclic high-temperature steam environments

This study investigates the oxidation behavior of FeCrAl (APMT and C26M) alloys in high-temperature steam at 1000 °C using isothermal and cyclic conditions. The FeCrAl alloys exhibited a fully ferritic body-centered cubic structure with no significant phase transformation, and the manufacturing procedures of the alloys resulted in varying grain sizes and porosity. In the 5 h isothermal test, the protective alumina film provided corrosion resistance without cracking in the high-temperature steam environment. However, thermal cycling seemed to compromise passivity; expansion and contraction during heating and cooling allowed oxygen penetration into the underlying metal in local regions, leading to transgranular crack-like features. Spallation of oxides was also observed. Post-exposure analysis of the oxide film formed on the FeCrAl alloys revealed two distinct oxide layers on the alloy: an outer Al-rich oxide film and an inner mixed oxide enriched in Cr and Fe.

Oxidation behavior of Cr1-xAlx coated Zr-4 alloy under 1200 °C high temperature steam environment

In present study, the 1200 °C high temperature steam (HTS) oxidation behavior of Cr1-xAlx coating with different Al content (0, 35 at.%, 50 at.% and 70 at.%) deposited on Zr-4 alloy by multi-arc ion plating (MAIP) has been investigated. The results demonstrate that Cr65Al35 coating has optimal protectiveness due to the formation of compact and continuous double oxide layer, consisting of outer Al2O3-rich layer and inner Cr2O3-rich oxide layer. The severe consumption of Al element for Cr50Al50 coating during the oxidation process, leading to vacancies formation and a decrease in enrichment degree of the Al2O3 in outer oxide layer. The protectiveness of pure Cr coatings is reduced by the formation of volatile CrO3, which induces the micro voids and provides a channel for the oxygen diffusion. The Cr30Al70 coatings shows the worst protective effect because of the serious diffusion behavior of Al into Zr alloy, providing the diffusion channel for O element and causing the substrate to oxidize essentially. The effect of Al content on the oxidation resistance of Cr1-xAlx coatings is discussed in detail.

Stability assessment of alumina and SiC based refractories in a high temperature steam environment as potential thermal energy storage materials

Solar energy can be utilized not only for electricity generation but also for synthetic fuel production, making it a versatile option in the transition to a low-carbon and sustainable energy system. However, to enable economically viable solar fuel production, the development of efficient thermal energy storage (TES) materials is crucial. Ceramic materials have been identified as a potential solution for high-temperature TES applications. In this study, two commercially available refractories, alumina coated recrystallized silicon carbide and alumina-based refractory were investigated for their stability under long-term steam corrosion tests to evaluate their suitability as TES materials for a potential TES unit. X-ray diffraction (XRD) revealed that recrystallized silicon carbide is composed of two different SiC polytypes (moissanite 6H and moissanite 4H) while alumina-based refractory contained corundum and mullite. Upon being exposed to steam, both materials exhibited distinct characteristics. SiC experienced significant mass gain, a subtle lightning of its surface color and erosion of a porous alumina coating on the SiC. Amorphous silica phases were detected as an indication of oxidation of SiC from X-ray diffractrograms. These remarkable changes clearly indicate that SiC is unsuitable for use in a potential thermal energy storage (TES) unit. On the other hand, there was no significant visual changes in the alumina-based refractory and the weight change was only marginal after corrosion tests. However, XRD indicated mullite completely disappeared after steam corrosion tests possibly due to its decomposition into Al2O3 and Si(OH)4.

Improved oxidation resistance of Cr/FeCrAl coating on Zr alloy in high-temperature steam environment

A new bi-layer Cr/FeCrAl coating was successfully fabricated on the Zr-4 plates through multi-arc ion plating in order to take advantage of the two materials of FeCrAl and Cr. The high-temperature steam oxidation behavior of the Cr/FeCrAl-coated Zr-4 at 1000–1200 °C was investigated and compared with that of bare and single-layer FeCrAl-coated Zr-4. Various analytical techniques including scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used for the microstructural and compositional analysis of the as-deposited and oxidized samples. Results indicated that the FeCrAl-coated Zr-4 could only withstand up to 1000 °C because of the severe FeZr interdiffusion. Whereas the introduction of the Cr interlayer effectively prevented fast FeZr interdiffusion and consequently improves the overall oxidation resistance of the Zr substrate below 1200 °C steam environment. After 1100 °C steam oxidation, the weight gain of the Cr/FeCrAl-coated Zr-4 was about 8 times and 2 times lower than that of the uncoated and FeCrAl-coated Zr-4 alloys, respectively. These facts make possible the use of FeCrAl coating on Zr cladding which has excellent high-temperature mechanical properties and oxidation resistance.

Oxidation behavior and microstructural evolution of Cr coatings prepared by multi-arc ion plating on Zry-4 in steam environments up to 1400 °C

This study aimed to investigate the effectiveness of the multi-arc ion plating method in depositing Cr coatings on Zry-4 substrate and to study the oxidation behavior and microstructural evolution of the Cr coatings in steam environments up to 1400 ℃. The results showed that the Cr-coated sample exhibited excellent high-temperature resistance at 1100-1200 ℃, but accelerated oxidation occurred at 1300 ℃, leading to the formation of bubbles and voids in the Cr2O3 layer and the development of a new Cr-Zr-O layer between the residual Cr layer and the ZrCr2 layer. At 1400 ℃, the coating failed completely. These findings provide valuable insights into the oxidation behavior and degradation mechanism of Cr-coated zirconium alloys under accident conditions. The novelty of this study lies in the investigation of the microstructural evolution and failure mechanism of Cr coatings on Zry-4 using multi-arc ion plating method in high-temperature steam environments, which has not been extensively explored in previous research.

Comparative study on protective Cr coatings on nuclear fuel cladding Zirlo substrates by AIP and HiPIMS techniques

After the Fukushima nuclear accident, the concept of accident-tolerant fuel (ATF) cladding coating was proposed with the aim of increasing the rescue time in the case of an accident. Cr coatings are considered to be promising ATFs cladding materials for loss-of-coolant-accident conditions. The oxidation behavior of a Cr coating is dominated by the microstructure and defects formed during the deposition process. To endow the Cr coatings with excellent oxidation resistance under a high-temperature steam environment, we deposited Cr coatings on Zirlo substrates by high-impulse-power magnetron sputtering (HiPIMS), a technique that can optimize the microstructure of the coating compared to the traditional arc ion plating (AIP) technique. The results revealed that the HiPIMS-Cr coatings exhibited a better steam oxidation resistance, and their oxidation weight gains were 14.6% and 42.4% lower than those of AIP-Cr coatings at 1100 °C and 1200 °C, respectively.

Oxidation behavior of CrAl coating with and without Nb addition in high temperature steam environment

CrAl coatings with and without Nb addition are fabricated and their oxidation behavior at 1000 °C and 1100 °C is investigated. Results reveal that Nb addition has a positive effect on forming the stable Al2O3 layer. The oxide layer changes from a porous Al2O3 layer with internal Al2O3 oxides in the CrAl coating into a single dense Al2O3 layer in the CrAl-1.5Nb coating. Adding Nb can significantly maintain the Al inventory of the CrAl coating as well as suppress the outward diffusion of Zr by grain boundary pinning effect, and increases the activity of Al to form a stable Al2O3 layer on the surface, which improve the oxidation performance of CrAl coatings.

High temperature steam oxidation behavior of textured Cr coatings with different grain structures

In order to improve the oxidation resistance of Zr-4 alloy in high temperature steam environment, Cr coatings with the 〈001〉 fiber texture were prepared on Zr-4 substrates by the high-power pulsed magnetron sputtering (HIPPMS) and direct current magnetron sputtering (DCMS) methods, and their structure and high temperature steam oxidation behavior at 1200 and 1300 °C were systematically investigated. The results showed that the fine grains of Cr coatings combined together to form large grains while the texture of the coatings was not changed during oxidation. Furthermore, cracks or defects formed during the combination of grain boundaries in the grain growth of DCMS Cr coating, providing paths for short-circuit diffusion of the oxidizing medium, which accelerated the oxidation of the Cr coating.