A method for improving high-temperature water vapor oxidation resistance of FeCrAl alloy based on pre-etching

Abstract

The application discloses a method for improving high-temperature water vapor oxidation resistance of FeCrAl alloy based on pre-oxidation, and the method comprises the following steps: performing surface pretreatment on FeCrAl alloy material; placing the pretreated FeCrAl alloy material in a high-temperature and high-pressure water environment to perform pre-uniform corrosion treatment, so as to form a double-layer oxide film; and placing the FeCrAl alloy material with the double-layer oxide film in a high-temperature water vapor environment to test the oxidation resistance thereof. The pre-uniform corrosion oxide film formed under the normal working condition of a nuclear reactor improves the oxidation resistance of the FeCrAl alloy under the accident high-temperature condition, especially enables the low-Cr-content FeCrAl alloy (Cr 10-12%) to have high-temperature oxidation resistance equivalent to or even better than that of the high-Cr-content alloy, and meanwhile, the irradiation embrittlement problem caused by the high-Cr-content is avoided, thereby providing technical support for development of a new generation of nuclear materials and safe operation of a nuclear power station.

Description

Technical Field

[0001] This invention relates to the field of material surface modification and protection technology, and in particular to a method for improving the resistance of FeCrAl alloy to high-temperature water vapor oxidation based on pre-corrosion, applicable to the surface modification and protection of nuclear power plant reactor fuel cladding materials. Background Technology

[0002] Traditional nuclear reactor fuel cladding materials primarily use zirconium alloys, which exhibit excellent overall performance under normal operating conditions. However, in severe accidents such as core meltdowns, zirconium alloys undergo violent oxidation reactions in high-temperature (over 1000°C) steam environments, leading to rapid material failure and the generation of large amounts of hydrogen gas, posing serious safety hazards. FeCrAl alloys, due to their superior high-temperature oxidation and mechanical properties, are widely used in nuclear energy, aerospace, and other fields. Particularly in nuclear power plant accident scenarios, FeCrAl alloys, as nuclear fuel cladding materials, must maintain structural integrity in high-temperature steam environments to prevent the leakage of radioactive materials.

[0003] However, the FeCrAl alloy materials currently under research still have the following problems: On the one hand, in order to obtain good high-temperature oxidation resistance, it is usually necessary to add a high content of Cr (>13%), which not only makes the material brittle, but also exacerbates radiation hardening and embrittlement under reactor irradiation environment; on the other hand, reducing the Cr content will affect the oxidation resistance of the material under high-temperature conditions in an accident.

[0004] Studies have shown that high-temperature steam environments above 1200℃ accelerate the oxidation process of FeCrAl alloys, leading to rapid degradation. Under these extreme conditions, the formation rate, density, and stability of the α-Al₂O₃ protective film on the alloy surface play a decisive role in the material's service performance. Once formed, the α-Al₂O₃ protective film effectively prevents further diffusion of oxygen into the matrix, thus significantly improving the material's oxidation resistance. However, for FeCrAl alloys with low Cr content, the rapid formation of a continuous and dense Al₂O₃ protective film in a high-temperature steam environment remains a technical challenge.

[0005] Chinese patent CN117187705B discloses a heat treatment method for a low-Cr, high-strength and high-toughness alloy. Through alloy element design and heat treatment process optimization, good mechanical properties are obtained under conditions of low Cr content. However, this invention mainly focuses on the strength and toughness of the alloy and does not propose an effective solution for improving the oxidation resistance of FeCrAl alloys under accidental high-temperature conditions with low Cr content. The journal article *Journal of Nuclear Materials*, 2022, Vol. 10, pp. 555-563, studied the influence of different oxidation environments on the high-temperature oxidation kinetics of FeCrAl alloys, but did not address the relationship between prior nuclear environment corrosion and subsequent high-temperature oxidation resistance.

[0006] Pre-corrosion treatment involves forming a stable oxide film on the material surface in a specific corrosive environment. This oxide film serves as a "seed" or "template" for the formation of a protective oxide film during subsequent high-temperature oxidation, effectively improving the oxidation resistance and corrosion resistance of the alloy material. The unique advantage of this method lies in its ability to significantly improve material properties through the microstructure control of the surface oxide film without altering the matrix composition. It features simple processing, compatibility with actual working conditions, and strong adaptability.

[0007] Under normal operating conditions (325℃, 15.5MPa water environment), the FeCrAl cladding material undergoes natural, uniform corrosion, forming a unique double-layer oxide film structure on its surface. This naturally formed precursor corrosion oxide film in the nuclear environment differs fundamentally in its microstructure, compositional distribution, and interfacial properties from artificially prepared oxide films, potentially significantly impacting the material's oxidation resistance under subsequent high-temperature accident conditions. More importantly, this precursor corrosion process is significantly influenced by water chemistry parameters such as dissolved oxygen (DO) and dissolved hydrogen (DH) in the nuclear environment; different water chemistry conditions lead to the formation of oxide films with varying structures and properties.

[0008] Currently, no research systematically explores the influence of the pre-corrosion uniform oxide film formed under normal nuclear reactor operating conditions on the oxidation resistance of FeCrAl alloys in high-temperature steam (e.g., 1200℃) environments under accident conditions. Furthermore, few studies have proposed how to utilize pre-corrosion to improve the oxidation resistance of materials under accident conditions. While pre-corrosion treatment of alloy materials can improve their oxidation resistance to some extent, it is necessary to comprehensively consider the synergistic effects of multiple factors, including temperature control, dissolved oxygen (DO) conditions, dissolved hydrogen (DH) conditions, time control, and oxide film composition and structure, when dealing with alloy materials with different compositions and ratios. How to systematically study the key parameters of pre-corrosion treatment in nuclear environments, and establish the intrinsic relationship between pre-corrosion and high-temperature oxidation resistance, thereby providing technical support for the safe application of low-Cr content FeCrAl alloys under nuclear accident conditions, is a critical scientific and technological problem that urgently needs to be solved. Summary of the Invention

[0009] The purpose of this invention is to provide a method for improving the high-temperature steam oxidation resistance of FeCrAl alloys based on pre-corrosion, thereby addressing the issues of low oxidation resistance in FeCrAl alloys and how to utilize pre-corrosion to enhance the material's oxidation resistance under accident conditions. This invention's use of pre-corrosion technology to solve this problem has significant engineering and practical value: on the one hand, it provides a technical approach to reduce the Cr content in FeCrAl alloys, reducing the risk of radiation embrittlement and improving the material's service performance in reactor environments; on the other hand, by utilizing the naturally formed corrosion oxide film during reactor operation to enhance the material's safety performance under accident conditions, it eliminates the need for additional surface treatment processes, demonstrating significant economic and practical advantages. This is of great importance for improving the safety level of nuclear power plants, preventing serious accidents, and promoting the engineering application of next-generation accident-tolerant nuclear materials.

[0010] To achieve the above objectives, the present invention provides a method for improving the high-temperature water vapor oxidation resistance of FeCrAl alloys based on pre-corrosion etching, comprising the following steps:

[0011] (1) Pretreatment of FeCrAl alloy material to remove surface impurities;

[0012] (2) The pretreated FeCrAl alloy material is placed in a high temperature and high pressure water environment for pre-treatment uniform corrosion treatment to form a double oxide film.

[0013] Existing technologies often improve the oxidation resistance of alloy materials by limiting their composition. However, these methods only focus on improving one aspect of the material's performance, and have many shortcomings in terms of overall performance, especially in maintaining performance under specific harsh environments. To address these issues, this invention employs a pre-treatment uniform corrosion process to modify the surface of FeCrAl alloy materials, forming a double-layer oxide film. This is crucial for improving its oxidation resistance, especially enabling the prepared alloy materials to be used in nuclear environments.

[0014] Preferably, in step (1), the pretreatment process includes mechanical polishing, chemical cleaning and ultrasonic cleaning.

[0015] Preferably, in step (1), the FeCrAl alloy material comprises, by mass percentage: Cr 10.0-12.0%, Al 4.3-5.5%, with the balance being iron and unavoidable impurities.

[0016] Preferably, in step (1), the FeCrAl alloy material further includes one or more of Mo, Nb, Si, Ta, Zr, and Y; wherein, by mass percentage, Mo is 1.8-2.6%, Nb is 1.0-1.5%, Si is 0.1-0.3%, Ta is 0.1-0.2%, Zr is 0.1-0.2%, and Y is 0.05-0.1%.

[0017] Preferably, in step (1), the total mass percentage of Cr, Al and Si elements in the FeCrAl alloy material is ≥14.5%, and the total mass percentage of Zr and Y elements is ≥0.25%.

[0018] This invention imposes strict limits on the content of each metallic element in the FeCrAl-based alloy. The total mass percentage of Cr, Al, and Si elements is ≥14.5% to maintain good room-temperature mechanical strength of the FeCrAl alloy. Simultaneously, to prevent the FeCrAl alloy from exhibiting increased hardening and irradiation embrittlement tendencies, which could lead to brittle fracture during reactor operation and processing, the Cr and Al contents should be strictly controlled and reduced while ensuring good resistance to high-temperature steam oxidation. In this invention, the mass percentage of Cr is ≤12.0%. Furthermore, the total mass percentage of Zr and Y elements is ≥0.25% to synergistically enhance the alloy's resistance to high-temperature oxidation.

[0019] Preferably, in step (1), the FeCrAl alloy material comprises the following components by mass percentage: Cr 10.0-12.0%, Al 4.3-5.5%, Mo 1.8-2.6%, Nb 1.0-1.5%, Si 0.1-0.3%, Ta 0.1-0.2%, Zr 0.1-0.3%, Y 0.05-0.1%, C≤0.008%, N≤0.005%, O≤0.003%, with the balance being iron and unavoidable impurities.

[0020] More preferably, the preparation method of FeCrAl alloy material is as follows:

[0021] Each alloying element is weighed according to its mass percentage to form a steel billet. The steel billet is hot-rolled in multiple passes to obtain a hot-rolled plate. The hot-rolled plate is then subjected to high-temperature solution treatment, low-temperature rolling, and two annealing treatments in sequence. After being taken out and cooled to room temperature, the FeCrAl alloy material is obtained.

[0022] More preferably, in step (1), the preparation method of the FeCrAl alloy material is as follows:

[0023] Steel billets are prepared by weighing each alloying element according to mass percentage. The billets are then hot-forged and hot-rolled, with an initial forging temperature of not less than 1000℃ and a final forging temperature of not less than 900℃. After forging, the billets are hot-rolled at 800℃ with a rolling ratio of 40%. Subsequently, high-temperature homogenization annealing and warm rolling are performed. The hot-rolled steel plate is heated to 1150-1250℃ and solution-treated for 1.5-3.0 hours. It is then rolled at 420℃ to a set thickness with a rolling ratio of 65%. After rolling, it is air-cooled and then cold-rolled and straightened before undergoing two-pass annealing. The first pass is a high-temperature annealing at 1130-1170℃ for 15-20 seconds, followed by water cooling. The second pass is a low-temperature annealing at 600-650℃ for 30-60 minutes. The material was then removed and air-cooled to room temperature to prevent the growth of Laves second-phase particles during processing and heat treatment. This resulted in fine second-phase particles densely precipitated at the subgrain boundaries, ensuring both room temperature and high temperature strengthening effects of the alloy. At the same time, the large number of subgrain boundaries facilitated element diffusion during the early oxide film formation stage of corrosion, making it easier for a uniform and dense protective oxide layer to form on the alloy surface, which helped improve its corrosion resistance.

[0024] Preferably, in step (2), the conditions of the high temperature and high pressure water environment are: 350~370℃, 17~20MPa, dissolved oxygen content of 5~30ppb, and dissolved hydrogen content of 25-30cc / kg STP.

[0025] The temperatures and pressures mentioned above are slightly higher than those of a pressurized water reactor under normal operating conditions, which is intended to accelerate the experiment without changing the film formation mechanism. The dissolved oxygen content range covers the actual range of a pressurized water reactor from normal operation to abnormal conditions, and the dissolved hydrogen content is the numerical range under standard temperature and pressure.

[0026] More preferably, in step (2), the conditions of the high temperature and high pressure water environment are: 360℃, 18.6MPa, dissolved oxygen content of 20ppb, and dissolved hydrogen content of 25cc / kg STP.

[0027] Preferably, in step (2), the time for the preliminary uniform etching treatment is 1000-2000h.

[0028] Preferably, in step (2), the double oxide film includes an inner layer and an outer layer, wherein the inner layer is a chromium-rich oxide and the outer layer is a mixed oxide layer formed by iron oxide particles.

[0029] The promoting effect of pre-conduction uniform corrosion in nuclear environments on the high-temperature steam oxidation performance of FeCrAl alloys stems from a complex synergistic mechanism. Under nuclear conditions, a bilayer oxide film structure forms on the FeCrAl alloy surface, consisting of an inner layer of chromium-rich oxides and an outer layer of iron oxide particles and spinel. This bilayer formation process is accompanied by the enrichment of Cr at the oxide film / matrix interface, altering the local chemical environment and establishing a chemical driving force favorable for subsequent selective oxidation. The formation of the oxide film significantly alters the residual stress distribution on the material surface, improving its thermodynamic stability by reducing the elastic strain energy during the formation of the protective α-Al₂O₃ film.

[0030] The defect structures introduced on the material surface by the pre-corrosion process provide preferential channels for the rapid outward diffusion of Al. These defects, together with the oxide particles formed by the pre-corrosion, serve as heterogeneous nucleation sites for α-Al₂O₃, effectively reducing the nucleation energy barrier. When the material operates in a high-temperature steam environment, the iron oxide particles and spinel formed by the pre-corrosion undergo a phase transformation due to element diffusion. This phase transformation not only releases interfacial stress but, more importantly, further promotes the selective oxidation of Al by altering the local oxygen activity.

[0031] Therefore, the mechanisms of double oxide film structure construction, element enrichment, stress regulation, defect engineering, nucleation promotion and phase transformation are coupled with each other, which together achieve a significant improvement in the high-temperature steam oxidation performance of FeCrAl alloy materials.

[0032] The key to the promoting effect of pre-corrosion uniformity on the high-temperature steam oxidation performance of FeCrAl alloys in nuclear environments lies in the hierarchical transformation mechanism of oxides. Under nuclear environment conditions (325℃, 15.5MPa high-temperature and high-pressure water environment), a double-layer oxide film structure is formed on the surface of FeCrAl alloys, consisting of an inner layer of chromium-rich oxide particles and an outer layer of iron oxide particles and spinel. When the material is used in a high-temperature steam environment (1200℃), the surface oxide particles formed by pre-corrosion undergo a crucial phase transformation, gradually transforming into a thermodynamically stable iron-aluminum spinel layer (FeAl2O4). This iron-aluminum spinel layer formation process consumes some Fe ions and releases Al ions, creating a local chemical environment conducive to the selective oxidation of Al. More importantly, the iron-aluminum spinel layer, as an intermediate transition layer, effectively regulates the oxygen activity gradient inside the oxide film, providing ideal thermodynamic and kinetic conditions for the formation of the inner stable α-Al2O3 protective layer. During the pre-corrosion process, the enrichment of Cr at the oxide film / matrix interface further lowers the formation energy barrier of α-Al₂O₃. Simultaneously, the defect structure introduced by pre-corrosion provides a preferential channel for the rapid outward diffusion of Al, accelerating the nucleation and growth of the α-Al₂O₃ protective layer. Therefore, the transformation of the surface oxide to iron-aluminum spinel and the preferential formation of the inner α-Al₂O₃ protective layer constitute a synergistic oxide transformation system. This hierarchical oxide structure reorganization is the fundamental mechanism by which pre-corrosion significantly improves the high-temperature steam oxidation performance of FeCrAl alloys.

[0033] Preferably, the method further includes an antioxidant performance testing step, specifically, placing the FeCrAl alloy material with the double oxide film in a high-temperature water vapor environment to test its antioxidant performance.

[0034] Preferably, the conditions for the high-temperature water vapor environment are: temperature 1000-1400℃, liquid water supply flow rate 80-120μL / min, corrosion time 1-3h, and argon carrier gas flow rate 40-60mL / min.

[0035] More preferably, the conditions of the high-temperature water vapor environment are: temperature 1200℃, liquid water supply flow rate 100μL / min, corrosion time 2h, and argon carrier gas flow rate 50mL / min.

[0036] Preferably, this method is applicable to the preparation of fuel cladding materials for nuclear power plants.

[0037] Therefore, the present invention provides a method for improving the high-temperature water vapor oxidation resistance of FeCrAl alloys based on pre-corrosion using the above-described structure, which has the following beneficial effects:

[0038] (1) This invention is the first to discover and utilize the promoting effect of the pre-uniform corrosion oxide film formed under normal operating conditions (360℃, 18.6MPa) of accelerated nuclear reactor on the high-temperature steam oxidation performance of FeCrAl alloy under accident conditions (1200℃).

[0039] (2) This invention reveals the regulatory mechanism of dissolved oxygen (DO) and dissolved hydrogen (DH) content on the structure and composition of the double oxide film in the preceding corrosion environment, and establishes the intrinsic relationship between water chemical parameters and subsequent high-temperature oxidation resistance. This discovery enables FeCrAl alloy materials with low Cr content (10-12%) to exhibit oxidation resistance comparable to or even better than those with high Cr content (>13%) in a 1200℃ high-temperature steam environment, thus effectively avoiding the radiation embrittlement problem caused by high Cr content.

[0040] (3) The key to the method of this invention lies in the special microstructure and compositional transformation of the double-layer oxide film structure formed by the pre-corrosion uniform corrosion in a high-temperature steam environment. This transformation process significantly improves the material's oxidation resistance and anti-scraping properties. More importantly, the method of this invention does not require additional surface treatment equipment and processes, and fully utilizes the naturally formed corrosion oxide film during nuclear reactor operation to improve the material's high-temperature oxidation resistance. From the perspective of oxidation kinetics, the thickness of the corrosion layer of the alloy after pre-corrosion treatment is about 40% of that of the untreated alloy, the corrosion weight gain rate is reduced by more than 70%, and the hot corrosion resistance is significantly enhanced. This provides strong technical support for the safe application of low-Cr content FeCrAl alloys under nuclear accident conditions.

[0041] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0042] Figure 1 The oxidation kinetics curves of the materials of Example 1, Comparative Example 1, Comparative Example 3 and Comparative Example 5 in a high-temperature water vapor environment at 1200°C (or 1400°C);

[0043] Figure 2 From left to right, the macroscopic morphology of Example 1 after pre-corrosion, and the macroscopic morphology of the materials of Comparative Example 3 and Comparative Example 5 after pre-corrosion followed by high-temperature steam oxidation are shown.

[0044] Figure 3 The surface morphology of Example 2 and Comparative Example 3 after undergoing pre-corrosion in low-DO and high-DO environments;

[0045] Figure 4 The results are TEM characterizations of the cross-section of Example 2 after prior corrosion in a low DO, low DH environment.

[0046] Figure 5 The results are TEM characterizations of the cross-section of Comparative Example 3 after prior corrosion in a high DO environment.

[0047] Figure 6 The high-resolution and selected area electron diffraction results of α-Al2O3 and FeAl2O4 phases after high-temperature steam oxidation are shown in Example 2.

[0048] Figure 7 The TKD characterization results of Comparative Example 1 and Example 1 after undergoing high-temperature steam oxidation are shown. Detailed Implementation

[0049] The present invention will be further described below. It should be noted that this embodiment is based on the present technical solution and provides detailed implementation methods and specific operation processes, but the present invention is not limited to this embodiment.

[0050] Example 1

[0051] A method for improving the resistance of FeCrAl alloys to high-temperature water vapor oxidation based on pre-corrosion includes the following steps:

[0052] (1) The chemical composition design of FeCrAl alloy material is shown in Table 1.

[0053] Table 1. Chemical composition of the alloy (by mass percentage)

[0054]

[0055] (2) Weigh each component according to the mass percentage of each metal element in Table 1 and make steel billets. After hot forging, the steel billets are hot rolled. The initial forging temperature is 1000℃, the final forging temperature is 900℃, the hot rolling temperature is 800℃, and the rolling ratio is 40%. Then, high-temperature homogenization annealing and low-temperature rolling are carried out. The hot-rolled plate is heated to 1200℃, solution-treated for 2.0h, and rolled at 420℃ with a rolling ratio of 65%, finally forming a plate with a thickness of 3mm. After rolling, the plate is air-cooled and then cold-rolled and straightened before two-pass annealing. The first pass high-temperature annealing temperature is 1150℃ and the annealing time is 18s. Then, it is water-cooled. Then, the second pass low-temperature annealing is carried out at a temperature of 625℃ and the annealing time is 40min. Then, it is taken out and air-cooled to room temperature to obtain FeCrAl alloy material.

[0056] (3) The FeCrAl alloy material in step (2) is pretreated. Specifically, the FeCrAl alloy material is processed into a 15mm*10mm*3mm hot corrosion sample using a wire cutting machine and a grinding machine. The sample is mechanically polished with 400#, 800#, 2000# and 7000# sandpaper in sequence, and then ultrasonically cleaned with acetone and alcohol for 10 minutes each, and dried with nitrogen.

[0057] (4) The pretreated sample was placed in an environment simulating the working conditions of an accelerated nuclear reactor (high temperature and high pressure water environment conditions: 360℃, 18.6MPa, dissolved oxygen DO of 10ppb, dissolved hydrogen DH of 25cc / kg STP) and kept for 2000 hours to form a uniformly distributed double oxide film.

[0058] (5) High-temperature water vapor oxidation test: The sample that has undergone prior corrosion treatment and the control sample that has not undergone prior corrosion treatment are placed in a high-temperature water vapor atmosphere for oxidation test. The conditions of the high-temperature water vapor environment are: temperature 1200℃, liquid water supply flow rate 100μL / min, oxidation corrosion time 2h, and argon carrier gas flow rate 50mL / min.

[0059] (6) Testing and characterization:

[0060] Oxidation kinetics: The mass gain of the sample after different oxidation times was measured in situ by TGA, and the oxidation weight gain curve was plotted.

[0061] Microstructure analysis: Scanning electron microscopy (SEM) was used to observe the cross-sectional morphology of the oxide film after the pre-etching treatment and the cross-sectional morphology of the sample after high-temperature steam oxidation.

[0062] Compositional analysis: The chemical composition and phase structure of the oxide layer were analyzed using energy dispersive spectroscopy (EDS).

[0063] Peel resistance test: The anti-peel performance of the oxide layer under thermal cycling conditions is evaluated through thermal cycling test.

[0064] Example 2 (Slight variations in alloy composition and pre-corrosion time)

[0065] The difference between this embodiment and Embodiment 1 is as follows: By mass percentage, the alloy material comprises 11.6% Cr, 5.1% Al, 2.0% Mo, 1.0% Nb, 0.2% Si, 0.15% Ta, 0.2% Zr, 0.05% Y, C≤0.008%, N≤0.005%, and O≤0.003%; the environmental differences simulating the working conditions of an accelerated nuclear reactor are addressed by maintaining the same temperature and pressure as in Embodiment 1, with dissolved oxygen (DO) at 20 ppb, dissolved hydrogen (DH) at 30 cc / kg STP, and a pre-corrosion time of 1000 h. All other steps are the same as in Embodiment 1.

[0066] Example 3 (Slightly varying DO and DH conditions)

[0067] The difference between this embodiment and Embodiment 1 is that the environmental conditions simulating the working conditions of an accelerated nuclear reactor are different. The temperature and pressure are kept the same as in Embodiment 1, the dissolved oxygen (DO) is 15 ppb, the dissolved hydrogen (DH) is 25 cc / kg STP, and the pre-corrosion time is 1500 h. Other steps are the same as in Embodiment 1.

[0068] Comparative Example 1 (No Pre-etching Time)

[0069] The difference between this embodiment and Embodiment 1 is that the environmental pre-corrosion time for simulating the working conditions of an accelerated nuclear reactor is 0 hours, while the other steps are the same as in Embodiment 1.

[0070] Comparative Example 2 (Shortening Pre-etching Time)

[0071] The difference between this embodiment and Embodiment 1 is that the environmental conditions simulating the working conditions of an accelerated nuclear reactor are different. The temperature and pressure are kept the same as in Embodiment 1, the dissolved oxygen (DO) is 20 ppb, the dissolved hydrogen (DH) is 25 cc / kg STP, and the pre-corrosion time is 50 h. Other steps are the same as in Embodiment 1.

[0072] Comparative Example 3 (variable dissolved oxygen conditions, extremely high dissolved oxygen concentration)

[0073] The difference between this embodiment and Embodiment 1 is that the environmental conditions simulating the working conditions of an accelerated nuclear reactor are different. The temperature and pressure are kept the same as in Embodiment 1, the dissolved oxygen (DO) is 200 ppb, the dissolved hydrogen (DH) is 25 cc / kg STP, and the pre-corrosion time is 2000 h. Other steps are the same as in Embodiment 1.

[0074] Comparative Example 4 (varied dissolved hydrogen conditions, extremely high concentration of dissolved hydrogen)

[0075] The difference between this embodiment and Embodiment 1 is that the environmental conditions simulating the working conditions of an accelerated nuclear reactor are different. The temperature and pressure are kept the same as in Embodiment 1, the dissolved oxygen (DO) is 20 ppb, the dissolved hydrogen (DH) is 80 cc / kg STP, and the pre-corrosion time is 2000 h. Other steps are the same as in Embodiment 1.

[0076] Comparative Example 5 (Increasing Oxidation Temperature)

[0077] The difference between this embodiment and Embodiment 1 is that the environmental conditions simulating the working conditions of an accelerated nuclear reactor are different. The temperature and pressure are kept the same as in Embodiment 1, the dissolved oxygen (DO) is 200 ppb, the dissolved hydrogen (DH) is 30 cc / kg STP, the pre-corrosion time is 1500 h, and the oxidation temperature in the high-temperature steam environment is 1400℃. Other steps are the same as in Embodiment 1.

[0078] Comparative Example 6 (Variable Alloy Composition Ratio)

[0079] The difference between this embodiment and Embodiment 1 is that the environment simulating the working conditions of an accelerated nuclear reactor is the same as in Embodiment 1, but the alloy composition used is Cr 16.8%, Al 4.7%, Mo 1.8%, Nb 1.0%, Si 0.15%, V 0.15%, Hf 0.15%, Ga 0.1%, Ni 0.15%, La 0.05%, C≤0.008%, N≤0.005%, O≤0.003%, with the balance being iron.

[0080] Comparative Example 7 (Zirconium alloy as a comparison)

[0081] The difference between this comparative example and Example 1 is that the pre-oxidized FeCrAl alloy material was replaced with a commercial Zr-4 alloy. The composition of this commercial Zr-4 alloy is Fe 0.21%, Cr 0.12%, Al 0.0033%, Si 0.009%, C≤0.008%, N≤0.018%, O 0.13%, with the remainder being Zr elements and other impurities that conform to the standards of commercial Zr-4 alloys.

[0082] Comparative Example 8 (compared to materials in other patents)

[0083] The difference between this comparative example and Example 1 is that the Cr content in the FeCrAl alloy material is changed, and no pre-oxidation treatment is performed. Specifically, the low-Cr and high-strength-toughness alloy prepared in Example 4 of Chinese Patent CN117187705B is used. The composition ratio of the above alloy is Cr 12.8%, Al 4.3%, Mo 2.0%, Nb 1.0%, Si 0.15%, V 0.15%, Hf 0.15%, Ga 0.1%, Ni 0.15%, La 0.05%, C≤0.008%, N≤0.005%, O≤0.003%, with the balance being iron. The impurity content meets the current standards for commercial industrial pure iron and ferritic stainless steel. The two-pass annealing process is 1100℃-30s-550℃-120min.

[0084] Test data

[0085] High-temperature steam oxidation experiments were conducted on the FeCrAl alloy materials prepared in Examples 1-3 and the alloys in Comparative Examples 1-8 using a TGA device. The specific test procedures are as follows:

[0086] The alloy materials obtained in Examples 1-3 and Comparative Examples 1-8 were subjected to high-temperature water vapor corrosion tests on a TGA device at 1200-1400°C. The parameters for the high-temperature water vapor corrosion tests were as follows: liquid water flow rate of 100 μL / min, corrosion time of 2 h, and argon carrier gas flow rate of 50 mL / min.

[0087] The resistance to high-temperature water vapor oxidation of Examples 1-3 and Comparative Examples 1-8 was determined by sampling and weighing at intervals and by in-situ high-temperature water vapor corrosion test. The average thickness of the inner / outer oxide layer of the oxide film was obtained by TEM image measurement. The test results are shown in Table 2.

[0088] Table 2. Effects of different pre-corrosion conditions on the high-temperature steam oxidation performance of 10Cr-5Al FeCrAl type.

[0089]

[0090]

[0091] Note: α-Al2O3 continuity rating: A-Excellent, B-Good, C-Average, D-Poor, E-Very Poor; Anti-stripping performance rating: A-Excellent, B-Good, C-Average, D-Poor, E-Very Poor.

[0092] Oxide film continuity rating criteria:

[0093] Continuity rating is based on the number of pores in the oxide film and the uniformity of the film layer. Grade A (Excellent) requires a completely continuous oxide film without pores, with a uniform thickness variation of <5% and a dense microstructure; Grade B (Good) allows for micropores with an area of ​​<1% and a thickness variation of 5-10%; Grade C (Average) has scattered micropores with an area of ​​1-5% and a thickness variation of 10-20%; Grade D (Poor) has pore defects with an area of ​​5-15% and a thickness variation of 20-35%, with a loose microstructure and cracks; Grade E (Very Poor) has a defect area >15%, a thickness variation >35%, an extremely loose microstructure, and essentially loses its protective function.

[0094] Anti-stripping performance rating standard:

[0095] The peel resistance rating is based on the area of ​​the oxide film peeled off after high-temperature steam oxidation. Grade A (Excellent): No peeling after high-temperature steam oxidation; the oxide film remains intact. Grade B (Good): Slight peeling; peeling area <5%. Grade C (Average): Minor peeling; peeling area 5-15%. Grade D (Poor): Significant peeling; peeling area 15-40%. Grade E (Very Poor): Large-area peeling; peeling area >40%; most of the oxide film has detached and failed. A larger peeling area indicates poorer peel resistance and lower stability of the oxide film under high-temperature steam conditions.

[0096] Rating test method:

[0097] The rating is determined by methods such as SEM observation of microstructure and film thickness measurement, providing a basis for the quality control and application selection of α-Al2O3 films.

[0098] Figure 1The figures show the oxidation kinetics curves of the materials from Examples 1, 1, 3, and 5 in a high-temperature steam environment at 1200°C (or 1400°C). As can be seen from the figures, in the high-temperature steam oxidation experiment at 1200°C, the weight gain of the FeCrAl alloy after pre-corrosion treatment was significantly lower than that of the unoxidized group. Furthermore, the dissolved oxygen (DO) and hydrogen hydride (DH) in the pre-corrosion water environment should be controlled within a reasonable range; excessively high DO or DH can worsen the resistance to high-temperature steam oxidation. When the oxidation temperature is 1400°C, the sample subjected to uniform corrosion in an ultra-high DO water environment undergoes destructive oxidation, with the weight gain curve showing a linear increase, further illustrating the importance of controlling the pre-corrosion water environment.

[0099] Figure 2 From left to right, the macroscopic morphology of Example 1 after pre-corrosion, the macroscopic morphology of the materials of Comparative Example 3 and Comparative Example 5 after pre-corrosion and high-temperature steam oxidation are shown. It can be seen that after pre-corrosion, the sample remains intact and smooth. However, the sample that underwent pre-corrosion at 1400°C in a high DO environment underwent destructive oxidation, and a large amount of the surface oxide film peeled off.

[0100] Figure 3 The images show the surface morphology of Examples 2 and 3 after pre-corrosion in low-DO and high-DO environments, respectively. It is evident that when the DO and DH content of the pre-corrosion is controlled at low levels, the pre-corrosion only produces discrete oxide particles on the surface, providing "seeds" for the formation of the α-Al2O3 film during subsequent high-temperature steam oxidation, thus lowering the energy barrier for forming a uniform and dense oxide film. However, when the DO content is too high, a large number of oxide particles are generated on the sample surface. These particles will capture a large amount of Al element during the subsequent formation of the FeAl2O4 film, exacerbating the Al depletion effect during the formation of the α-Al2O3 film. Under ultra-high temperature conditions (e.g., 1400℃), this can lead to the formation of numerous pores in the α-Al2O3 film, destroying its structural integrity and severely degrading its protective properties.

[0101] Figure 4 This is the TEM characterization result of the cross-section of Example 2 after undergoing pre-corrosion in a low DO, low DH environment. From... Figure 4 As can be seen, under this environment, the surface of the sample after pre-corrosion only contains diffuse oxide particles, and the whole can be divided into two layers: an outer iron oxide layer and an inner spinel layer.

[0102] Figure 5 This is the TEM characterization result of the cross-section of Comparative Example 3 after prior corrosion in a high DO environment. From... Figure 5 As can be seen, samples pre-etched under high DO conditions will form a continuous oxide film on the surface, which is generally divided into two layers: an outer iron oxide layer and an inner spinel layer.

[0103] Figure 6 This is the high-resolution and selected-area electron diffraction result of the α-Al₂O₃ and FeAl₂O₄ phases after high-temperature steam oxidation in Example 2. Figure 6 As can be seen, the oxide film structure of the sample that has undergone prior corrosion and then high-temperature steam oxidation is more complex than that of the fresh sample that has directly undergone high-temperature steam oxidation, and the influencing mechanism needs to be further investigated.

[0104] Figure 7 The figures show the TKD characterization results of Comparative Example 1 and Example 1 after high-temperature steam oxidation. As can be seen from the figures, the sample that did not undergo prior corrosion (Comparative Example 1, ...) Figure 7 The two images above show that after high-temperature steam oxidation, the oxide film exhibits a single-layer structure, with its main structure being the α-Al2O3 phase; while the sample that underwent prior corrosion (Example 1) shows a different structure. Figure 7 (The next two images) After oxidation by high-temperature water vapor, the oxide film exhibits a bilayer structure, with the outer layer being the FeAl2O4 phase and the inner layer being the dense α-Al2O3 phase. There are many pores between the two layers, but the inner layer structure is relatively dense.

[0105] This invention, based on the oxide hierarchical transformation mechanism, presents a method for improving the high-temperature steam oxidation resistance of FeCrAl alloys, which has broad application prospects. This method is particularly suitable for performance enhancement of key components such as fuel cladding tubes in nuclear power plants. By utilizing the naturally formed pre-corrosion oxide film under normal reactor operating conditions and its transformation behavior under high-temperature accident conditions, it significantly improves the accident tolerance of FeCrAl alloys, greatly reducing the risk of hydrogen release due to material oxidation during accident conditions. A deeper understanding of this oxide transformation mechanism opens up new technical pathways for the development of low-Cr content FeCrAl alloy materials, enabling 10–12% Cr FeCrAl alloys to exhibit oxidation resistance comparable to or even better than high-Cr content alloys in high-temperature steam environments, while effectively avoiding the radiation embrittlement and processing performance problems associated with high Cr content. Mastering this oxide transformation law further provides a material guarantee foundation for the long-term operation of nuclear reactors. By predicting and evaluating the oxide film transformation behavior of FeCrAl alloy materials during long-term reactor operation, the service status and safety margin of the materials can be assessed more accurately.

[0106] More importantly, the regulatory mechanism of water chemical parameters such as dissolved oxygen and dissolved hydrogen on the oxide transformation process revealed by this invention provides a scientific basis for optimizing the water chemical control strategy of nuclear power plants. By precisely controlling the water chemical environment to adjust the structure of the oxide film in the preceding corrosion, the safety margin under accident conditions can be optimized while ensuring the long-term service performance of the material.

[0107] Therefore, the method of this invention not only achieves a breakthrough in material performance in terms of technology, but also avoids additional surface treatment costs in terms of economy. In terms of safety, it provides new material technology support for nuclear power plant accident prevention and control, and is of great significance for promoting the engineering application of next-generation accident-tolerant nuclear materials and improving the overall safety level of nuclear power plants. The method of this invention can significantly improve the safety performance of FeCrAl alloy as a nuclear fuel cladding material under accident conditions without increasing material costs and manufacturing complexity, resulting in significant economic and social benefits.

[0108] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for improving the high temperature water vapor oxidation resistance of FeCrAl alloys based on pre-etching, characterized in that: The method comprises the following steps: (1) pretreating FeCrAl alloy material to remove surface impurities, wherein the FeCrAl alloy material comprises, by mass percentage, Cr 10.0-12.0%, Al 4.3-5.5%, and the balance being iron and inevitable impurities; (2) placing the pretreated FeCrAl alloy material in a high-temperature and high-pressure water environment for preliminary uniform corrosion treatment to form a double-layer oxide film, wherein the high-temperature and high-pressure water environment has the conditions of 350-370 DEG C, 17-20 MPa, a dissolved oxygen content of 5-30 ppb, and a dissolved hydrogen content of 25-30 cc / kg STP, the preliminary uniform corrosion treatment has a time of 1000-2000 h, and the double-layer oxide film comprises an inner layer and an outer layer, the inner layer is a chromium-rich oxide, and the outer layer is a mixed oxide layer formed by iron oxide particles.

2. The method for improving the high-temperature water vapor oxidation resistance of FeCrAl alloy based on pre-etching according to claim 1, characterized in that: In step (1), the pretreatment process comprises mechanical polishing, chemical cleaning, and ultrasonic cleaning.

3. The method for improving the high-temperature water vapor oxidation resistance of FeCrAl alloy based on pre-etching according to claim 1, characterized in that: In step (1), the FeCrAl alloy material further comprises one or more of Mo, Nb, Si, Ta, Zr, and Y, wherein the Mo is 1.8-2.6% by mass percentage, the Nb is 1.0-1.5% by mass percentage, the Si is 0.1-0.3% by mass percentage, the Ta is 0.1-0.2% by mass percentage, the Zr is 0.1-0.2% by mass percentage, and the Y is 0.05-0.1% by mass percentage. The method further comprises an antioxidation performance detection step, specifically placing the FeCrAl alloy material forming the double-layer oxide film in a high-temperature water vapor environment to test the antioxidation performance.

4. The method for improving the high-temperature water vapor oxidation resistance of FeCrAl alloy based on pre-etching according to claim 1, characterized in that: The high-temperature water vapor environment has the conditions of a temperature of 1000-1400 DEG C, a liquid water feed flow rate of 80-120 muL / min, a corrosion time of 1-3 h, and an argon carrier gas flow rate of 40-60 mL / min.

5. The method for improving the high-temperature water vapor oxidation resistance of FeCrAl alloy based on pre-etching according to claim 4, characterized in that: The method is used for preparing a nuclear power plant fuel cladding material.

6. The method for improving the high-temperature water vapor oxidation resistance of FeCrAl alloy based on pre-etching according to any one of claims 1-5, characterized in that: ​

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