Method for improving corrosion resistance of SA508Gr.4N steel for nuclear pressure vessel

Abstract

The invention discloses a method for improving corrosion resistance of SA508Gr.4N steel for a nuclear pressure vessel, and belongs to the technical field of nuclear power key structural materials. According to the method, Zn < 2 + > is injected into a loop water solution of a simulated pressurized water reactor nuclear power station, so that the surface rust layer of the SA508Gr.4N steel is modified, and the corrosion resistance of the SA508Gr.4N steel is improved. The modified rust layer structure is more stable, the protection performance on a matrix is improved, and the corrosion behavior of the SA508Gr.4N steel for the nuclear pressure vessel can be effectively improved. Compared with an existing nuclear pressure vessel protection means, the nuclear pressure vessel protection device is simpler in operation and more remarkable in effect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nuclear power key structure materials, and particularly relates to a method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels. BACKGROUND

[0002] In the field of nuclear power equipment, the nuclear pressure vessel is a core pressure-bearing component for ensuring the safe operation of the reactor, and the material performance thereof directly determines the service life and safety level of the nuclear power plant. Compared with the SA508Gr.3 steel widely used in the existing nuclear pressure vessels, the SA508Gr.4N steel has more excellent comprehensive mechanical properties and theoretical corrosion resistance potential by optimizing the composition design, i.e., increasing the Cr and Ni element contents based on the SA508Gr.3 steel, and has been listed as a core candidate material for the fourth-generation nuclear pressure vessel, and has important application value in the development of advanced nuclear power technology.

[0003] However, the nuclear pressure vessel material itself does not have the ability to resist corrosion for a long time. The current industry generally adopts a protective scheme of welding austenitic stainless steel or nickel-based alloy on the inner wall of the nuclear pressure vessel to isolate the direct contact between the borate aqueous solution in the primary loop and the base material, so as to avoid corrosion failure. However, the design service period of the nuclear pressure vessel is as long as 60-80 years, and under the coupling effect of long-term high temperature, high pressure and complex water chemical environment, the inner wall of the nuclear pressure vessel is prone to aging degradation and corrosion damage: on the one hand, the intergranular corrosion and stress corrosion of the welded layer will gradually intensify, resulting in the failure of the protective performance, so that the borate aqueous solution in the primary loop breaks through the protective layer and contacts the nuclear pressure vessel base material, causing base corrosion; on the other hand, the alloy tube penetrator at the top of the nuclear pressure vessel is a weak structural part, which is prone to stress corrosion cracking to produce a leakage channel, and the primary loop water leaks into the containment outside the nuclear pressure vessel, which will directly contact the nuclear pressure vessel base material, and further cause the primary loop water environment to directly invade the base material. At this time, the external environment is close to normal temperature and pressure, which is significantly different from the high temperature and high pressure service conditions (350℃, 17MPa) in the primary loop pipeline. In this case, the corrosion process of the nuclear pressure vessel may be accelerated, the structural integrity of the nuclear pressure vessel is weakened, and nuclear safety risks are more likely to occur. Therefore, the research and development of corrosion improvement technology for the steel for nuclear pressure vessels are of great practical significance for prolonging the overall service life of the nuclear power plant and ensuring its long-term safe and stable operation.

[0004] Although the SA508Gr.4N steel improves the theoretical corrosion resistance by optimizing the composition, existing research shows that the steel still has obvious corrosion safety hazards in the severe service environment of the nuclear reactor primary loop, and its actual corrosion resistance performance cannot fully meet the long-term protection needs of the fourth-generation nuclear pressure vessel. Therefore, in-depth research on the improvement of the corrosion behavior of the SA508Gr.4N steel is a key link for breaking through the application bottleneck of advanced nuclear pressure vessel materials and ensuring the safe service of nuclear power plants.

[0005] Zinc injection technology, as an efficient and economical method for controlling water chemical corrosion, has been widely used in pressurized water reactor nuclear power plants abroad, effectively improving the corrosion resistance of key structural materials in the nuclear island. However, in China, due to insufficient basic research and weak core technology support, related applied research is still in its early stages. Existing research mainly focuses on 304 stainless steel, 316L stainless steel, and other steels used in nuclear power plant pipelines, investigating the effects of zinc injection under high temperature and high pressure environments. 2+ The impact of zinc injection on corrosion behavior. However, these studies all focus on the corrosion behavior of materials under high temperature and pressure, mainly studying austenitic stainless steel, without addressing the corrosion protection of SA508Gr.4 steel for nuclear pressure vessels at ambient temperature and pressure after primary circuit leakage. A zinc injection protection technology system applicable to domestic nuclear pressure vessel materials (especially SA508Gr.4N steel) has not yet been established. Therefore, it is urgent to systematically study and absorb advanced international zinc injection technologies, establish a zinc injection corrosion control theory for SA508Gr.4N steel, accumulate key process parameters and performance data, and ultimately form a domestically produced zinc injection protection scheme that can be widely applied. This is not only an urgent need to solve the current problem of improving the corrosion resistance of domestic nuclear pressure vessel materials, but also an important technical support for promoting the localization of nuclear power equipment and ensuring the high-quality development of the nuclear power industry in my country. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a method for improving the corrosion resistance of SA508Gr.4N steel used in nuclear pressure vessels. The aim is to explore the mechanism by which zinc injection technology improves the corrosion resistance of SA508Gr.4N steel under ambient temperature and pressure, and to determine the optimal zinc injection concentration. The method employed in this invention involves immersing SA508Gr.4N steel samples in water infused with different amounts of Zn. 2+ The rust layer on the sample surface was modified by immersing it in a simulated pressurized water reactor primary loop aqueous solution of a certain concentration, thereby improving its corrosion resistance.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] One objective of this invention is to provide a method for improving the corrosion resistance of SA508Gr.4N steel used in nuclear pressure vessels, comprising the following steps:

[0009] (1) Sample preparation: Select SA508Gr.4N steel with uniform composition and cut it into block samples of 10mm×10mm×5mm, and make a hole with a diameter of 2mm on the side.

[0010] (2) Polishing and cleaning: Polish the above sample from 600# to 3000# and then mechanically polish it until the surface is smooth and free of scratches. After polishing, clean the sample with anhydrous ethanol and then dry it with cold air for later use.

[0011] (3) The cleaned sample is hung in a beaker by using corrosion-resistant rubber thread, and different concentrations of Zn 2+ solution of simulated pressurized water reactor primary loop water is injected into the beaker, the beaker is placed in a constant temperature (25℃) water bath and is kept for 168 hours, so that a modified rust layer is formed on the surface of the sample;

[0012] (4) After the soaking is completed, the sample is taken out, the surface of the sample is washed by deionized water and anhydrous ethanol, and is dried by cold air, and is placed in a dry and sealed sample box for standby.

[0013] The method for improving the corrosion behavior of SA508Gr.4N steel for nuclear pressure vessels by pressurized water reactor primary loop zinc injection is adopted in the application, after zinc injection under standard conditions (25℃, 101.3kPa), no ZnB2O4 spinel structure occurs in the modified rust layer, but a phase change process of γ-FeOOH→α-FeOOH→FeCr2O4 occurs. The stability of α-FeOOH and FeCr2O4 is higher than that of γ-FeOOH, so that the corrosion resistance of the rust layer is improved after zinc injection. The application of the technology not only can save materials and reduce process steps, but also can provide a new idea for improving the corrosion resistance of nuclear pressure vessel materials in the future, and has important theoretical value and practical application value.

[0014] Further, in step (1), the SA508Gr.4N steel sample is a sample with uniform composition after heat treatment, and the chemical composition is: C 0.18wt.%, Si 0.15wt.%, Mn 0.35wt.%, Cr 1.70wt.%, Ni 3.68wt.%, Mo 0.51wt.%, and the rest is Fe.

[0015] Further, in step (3), the simulated pressurized water reactor primary loop water solution is made by mixing 4800ppm boric acid and 8ppm lithium hydroxide. The Zn 2+ is provided in the form of zinc acetate, and the concentration is 0ppb, 30ppb, 50ppb and 70ppb, and the concentration of Zn 2+ is preferably 50ppb.

[0016] Further, in step (3), when the concentration of Zn 2+ is 0ppb, the modified rust layer is mainly composed of γ-FeOOH; and when the concentration of Zn 2+ is not 0ppb, the modified rust layer is mainly composed of α-FeOOH and FeCr2O4.

[0017] The second object of the application is to provide a modified SA508Gr.4N steel prepared by the method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels.

[0018] The third object of the present application is to provide a method for evaluating corrosion resistance, comprising the following steps:

[0019] S1, embedding SA508Gr.4N steel in epoxy resin, and polishing the embedded sample;

[0020] S2, testing by using a three-electrode electrochemical workstation, the auxiliary electrode is metal platinum, the reference electrode is saturated calomel electrode, and the working electrode is the sample polished in step S1 with an exposed area of 1cm 2 , immersing the working electrode into simulated primary loop water solution containing different concentrations (0ppb, 30ppb, 50ppb and 70ppb) of Zn 2+ for 20 minutes, and testing the polarization curve after the open circuit potential is stable.

[0021] Further, the scanning range of the electrochemical workstation is-1V~0.6V, and the scanning rate is 1mV / s.

[0022] Compared with the prior art, the present application has the following advantages and technical effects:

[0023] The method adopted by the present application is to inject Zn 2+ into the simulated primary loop water solution of pressurized water reactor nuclear power plant with different concentrations, so as to modify the rust layer on the surface of SA508Gr.4N steel and improve its corrosion resistance. 2+ However, unlike the existing research on zinc injection in high temperature and high pressure environment, the research environment of the present application is normal temperature and pressure (25℃, 101.3kPa), which is closer to the actual contact conditions of nuclear pressure vessel under primary loop water leakage conditions. The experimental results show that after injecting Zn 2+ under normal temperature and pressure, no ZnFe2O4 spinel structure commonly formed by zinc injection in high temperature and high pressure environment is generated in the rust layer, but a phase change process of γ-FeOOH→α-FeOOH→FeCr2O4 is experienced. This phase change makes the rust layer change from loose γ-FeOOH to FeCr2O4 structure with higher stability and better compactness, thereby significantly improving the corrosion resistance of SA508Gr.4 steel. Compared with the existing protection means of nuclear pressure vessel, the operation of the present application is simpler and the effect is more significant.

[0024] Compared with the prior art, the present application has significant innovations in research objects, environmental conditions and action mechanisms. Most of the existing researches focus on the zinc injection protection of 304 stainless steel and other pipeline materials in the primary loop high temperature and high pressure environment, while the present application first applies the zinc injection technology to SA508Gr.4 steel for nuclear pressure vessel and realizes the improvement of its corrosion resistance under normal temperature and pressure conditions. In addition, the present application proposes a ZnThe mechanism of inducing the phase transformation from γ-FeOOH to α-FeOOH to FeCr2O4 breaks through the limitation that traditional zinc injection is only applicable to high-temperature and high-pressure environments. It provides a low-cost protection strategy for nuclear pressure vessels in the event of primary circuit water leakage or abnormal operating conditions. It fills the gap in the current research on zinc injection technology for nuclear pressure vessel material SA508Gr.4N steel, and provides a new protection approach for its future application in pressurized water reactor nuclear power plants. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0026] Figure 1 Zn in Example 1 2+ SEM and TEM images of the rust layer morphology on the surface of SA508Gr.4N steel sample at a concentration of 0 ppb, and TEM diffraction spot calibration results. Among them, (a) is SEM, (b) is TEM image, (c) is high-resolution image of the area in the red box in Figure (b), and (d) is diffraction spot and calibration results of the area in the yellow box in Figure (c).

[0027] Figure 2 Zn in Example 2 2+ SEM and TEM images of the rust layer morphology on the surface of SA508Gr.4N steel sample at a concentration of 30 ppb, and TEM diffraction spot calibration results. Among them, (a) and (b) are SEM images, (c) and (d) are TEM images of (b), (e) and (f) are high-resolution images of the purple box area in (c) and (d) respectively, and (g) and (h) are the diffraction spots and calibration results of the yellow box area in (e) and (f) respectively.

[0028] Figure 3 Zn in Example 3 2+ SEM and TEM images of the rust layer morphology on the surface of SA508Gr.4N steel sample at a concentration of 50 ppb, and TEM diffraction spot calibration results. Among them, (a) and (b) are SEM images, (c) is a TEM image, and (d) is the polycrystalline diffraction ring and calibration results of the red elliptical region in Figure (c).

[0029] Figure 4 Zn in Example 4 2+ SEM and TEM images of the rust layer morphology on the surface of SA508Gr.4N steel sample at a concentration of 70 ppb, and TEM diffraction spot calibration results. Among them, (a) and (b) are SEM images, (c) and (d) are TEM images, and (e) and (f) are diffraction spots and calibration results of the red elliptical region in (c) and (d), respectively.

[0030] Figure 5 Potentiodynamic polarization curves of SA508Gr.4N steel samples prepared for Examples 1-4 in different zinc injection concentration solutions;

[0031] Figure 6 Nyquist plots of SA508Gr.4N steel samples prepared for Examples 1-4 in different zinc injection concentration solutions;

[0032] Figure 7 Bode modulus plots of SA508Gr.4N steel samples prepared for Examples 1-4 in different zinc injection concentration solutions;

[0033] Figure 8 Bode phase plots of SA508Gr.4N steel samples prepared for Examples 1-4 in different zinc injection concentration solutions;

[0034] Figure 9 Mechanism diagram of phase transition process of γ-FeOOH→α-FeOOH→FeCr2O4 in the present application. DETAILED DESCRIPTION

[0035] The various illustrative embodiments of the present application will now be described in detail in various examples, which should be considered illustrative of the principles of the application, but not in limitation of the same. It is to be understood that the terminology used herein is for the purpose of describing the particular illustrative embodiments only and is not intended to be limiting.

[0036] It should be understood that the terms used herein are merely descriptive, but that the application should not be construed as being limited thereto. In addition, with respect to numerical ranges in the present application, it should be understood that each intervening value, to the upper and lower limits of the ranges stated for the context are also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is also encompassed. The upper and lower limits of these smaller ranges can independently be included or excluded in the range.

[0037] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All publications mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. In case of conflict, the content of the present specification will control.

[0038] Many modifications and variations to the illustrative embodiments described herein will be apparent to those of ordinary skill in the art from the foregoing description. Such modifications and variations are intended to fall within the scope of the present application. The description herein is illustrative and is not intended to limit the present application.

[0039] As used herein, the terms "comprise", "comprising", "include", "including", "have", "having" or the like are open-ended and allow for elements, components, etc., not expressly listed to be present.

[0040] The "normal temperature and pressure" described in the present application, if not specifically stated, refers to "25℃, 101.3kPa".

[0041] The raw materials used in the present application are all commercially available.

[0042] The technical solutions of the present application are further illustrated by the following examples.

[0043] The SA508Gr.4N steel samples described in the following examples of the present application are samples with uniform composition after heat treatment, the chemical composition of which is shown in Table 1, and the size is 10mm×10mm×5mm.

[0044] Table 1. Chemical composition of SA508Gr.4N steel (wt.%)

[0045] Ingredients C Si Mn Cr Ni Mo Fe SA508 Gr. 4N 0.18 0.15 0.35 1.70 3.68 0.51 Balance

[0046] The rust layer preparation method is as follows: after polishing, polishing and anhydrous ethanol cleaning, the sample is suspended and soaked in a simulated pressurized water reactor primary loop aqueous solution containing Zn 2+ (Zn 2+ in the form of zinc acetate), the soaking temperature is 25℃, and the soaking time is 168h.

[0047] Figure 9 It is a mechanism diagram of the phase transition process of γ-FeOOH→α-FeOOH→FeCr2O4 in the present application.

[0048] Example 1

[0049] A method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels, comprising the following steps:

[0050] (1) Prepare the sample: select a SA508Gr.4N steel with uniform composition and cut it into a block sample with a size of 10mm×10mm×5mm, and open a hole with a diameter of 2mm on the side surface;

[0051] (2) Polishing and cleaning treatment: the sample is polished from 600# to 3000# and then mechanically polished to be smooth and scratch-free, and then the polished sample is cleaned with anhydrous ethanol and dried with cold air for standby;

[0052] (3) The cleaned sample is hung in a beaker by using corrosion-resistant rubber threads, and the sample is immersed in a simulated pressurized water reactor primary loop aqueous solution (prepared by mixing 4800 ppm boric acid and 8 ppm lithium hydroxide) without zinc at 25°C for 168 hours to form a modified rust layer on the surface of the sample;

[0053] (4) After the immersion, the sample is taken out, the surface of the sample is washed with deionized water and anhydrous ethanol, and then dried with cold air and placed in a dry and sealed sample box for standby.

[0054] The method for evaluating corrosion resistance comprises the following steps:

[0055] S1, the SA508Gr.4N steel sample obtained after the polishing treatment in step (2) is embedded in epoxy resin;

[0056] S2, a three-electrode electrochemical workstation is used for testing, a metal platinum is used as an auxiliary electrode, a saturated calomel electrode is used as a reference electrode, and the embedded sample with an exposed area of 1 cm 2 is used as a working electrode, the working electrode is immersed in a simulated primary loop aqueous solution (prepared by mixing 4800 ppm boric acid and 8 ppm lithium hydroxide) without Zn 2+ (concentration 0 ppb) for 20 minutes, and after the open circuit potential is stable, a polarization curve is tested, wherein the scanning range of the electrochemical workstation is-1 V-0.6 V, and the scanning rate is 1 mV / s.

[0057] Figure 1 The SEM and TEM images and TEM diffraction spot calibration results of the rust layer on the surface of the SA508Gr.4N steel sample when the Zn 2+ concentration is 0 ppb in Example 1 are shown in (a), (b), (c) and (d) of FIG. 6, wherein (a) is an SEM image, (b) is a TEM image, (c) is a high-resolution image of the red square region in (b), and (d) is a diffraction spot and calibration result of the yellow square region in (c). Figure 1 As can be seen from (a) and (b) of FIG. 6, the rust layer obtained in Example 1 is mainly irregular lamellar structure. Figure 1 As can be seen from (d) of FIG. 6, the TEM diffraction spot is calibrated as γ-FeOOH. This is because Fe in the matrix dissolves in the corrosion solution to produce Fe 2+ , then unstable Fe(OH) + , and finally γ-FeOOH, and the chemical reactions in the process are shown as formulas (1)-(3).

[0058] (1);

[0059] (2);

[0060] (3);

[0061] Embodiment 2

[0062] A method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels, comprising the following steps:

[0063] (1) Preparing the sample: selecting a SA508Gr.4N steel with uniform composition and cutting it into a block-shaped sample of 10mm x 10mm x 5mm, and opening a hole with a diameter of 2mm on the side;

[0064] (2) Polishing and cleaning treatment: polishing the above-mentioned sample from 600# to 3000# and then mechanically polishing it to make the surface smooth and scratch-free, washing the polished sample with anhydrous ethanol and drying it with cold air for standby;

[0065] (3) Using corrosion-resistant rubber fine wire to hang the cleaned sample in a beaker, immersing the test sample in a simulated pressurized water reactor primary loop water solution (the simulated pressurized water reactor primary loop water solution is made by mixing 4800ppm boric acid and 8ppm lithium hydroxide) with a Zn 2+ concentration of 30ppb for 168h, so that a modified rust layer is formed on the surface of the test sample;

[0066] (4) After the immersion is completed, the test sample is taken out, the surface of the test sample is washed with deionized water and anhydrous ethanol, and then dried with cold air and placed in a dry and sealed test sample box for standby.

[0067] A method for evaluating corrosion resistance, comprising the following steps:

[0068] S1, embedding the SA508Gr.4N steel sample obtained after the polishing treatment of step (2) above in epoxy resin;

[0069] S2, testing by using a three-electrode electrochemical workstation, the auxiliary electrode is metal platinum, the reference electrode is a saturated calomel electrode, and the working electrode is the sample with an exposed area of 1cm 2 after embedding, immersing the working electrode in a simulated primary loop water solution (the simulated pressurized water reactor primary loop water solution is made by mixing 4800ppm boric acid and 8ppm lithium hydroxide) containing Zn 2+ (concentration 30ppb) for 20 minutes, and then testing the polarization curve after the open circuit potential is stable, wherein the scanning range of the electrochemical workstation is-1V~0.6V, and the scanning rate is 1mV / s.

[0070] Figure 2Zn in Example 2 2+ SEM and TEM images of the rust layer morphology on the surface of SA508Gr.4N steel samples at a concentration of 30 ppb, and TEM diffraction spot calibration results. (a) and (b) are SEM images; (c) and (d) are TEM images of (b); (e) and (f) are high-resolution images of the purple-boxed areas in (c) and (d), respectively; and (g) and (h) are the diffraction spots and calibration results of the yellow-boxed areas in (e) and (f), respectively. Figure 2 As can be seen from (a) and (b) in the figure, the rust layer obtained in Example 2 is mainly composed of regular rod-shaped structures. Figure 2 The tissue enclosed in the blue oval) and sheet-like tissue ( Figure 2 The tissue enclosed in the yellow oval (in the middle) is composed of Figure 2 As seen in (g) and (h), TEM diffraction patterns indicate that the lamellar structure is α-FeOOH and the rod-like structure is FeCr2O4. This is due to the injection of Zn. 2+ After that, Zn 2+ It promotes the phase transformation process of γ-FeOOH→α-FeOOH, and FeCr2O4 is Cr dissolved in the rust layer. 3+ Partial Fe substitution in α-FeOOH 3 + The chemical reaction formulas formed by the position are shown in formulas (4) to (5).

[0071] (4);

[0072] (5);

[0073] Figure 5 The figure shows the potentiodynamic polarization curve of the SA508Gr.4N steel sample prepared in Example 2. It can be seen from the figure that injecting 30 ppb of Zn into the primary loop aqueous solution of the simulated pressurized water reactor... 2+ Subsequently, the self-corrosion current was 1.35 × 10⁻⁶ for those without zinc injection. - 6 A / cm 2 Reduced to 6.23×10 -7 A / cm 2 .

[0074] Figure 6 The image shows the Nyquist plot of the SA508Gr.4N steel sample prepared in Example 2. The plot shows that injecting 30 ppb of Zn into the primary loop aqueous solution of the simulated pressurized water reactor... 2+ Afterwards, the diameter of the capacitor arc is much larger than that before zinc injection, and the larger the diameter of the capacitor arc, the better the corrosion resistance of the rust layer.

[0075] Figure 7The Bode modulus diagram of the SA508Gr.4N steel sample prepared in Example 2 is shown. It can be seen from the diagram that, in the low-frequency range, 30 ppb of Zn injected into the primary loop aqueous solution of a simulated pressurized water reactor... 2+ The impedance modulus after zinc injection is greater than that before zinc injection. In the low-frequency range, the higher the impedance modulus value, the better the corrosion resistance of the rust layer.

[0076] Figure 8 The Bode phase diagram of the SA508Gr.4N steel sample prepared in Example 2 shows that, in the high-frequency region (f>10kHz), 30ppb of Zn was injected into the primary loop aqueous solution simulating a pressurized water reactor. 2+ The phase angle after zinc injection increased from 6.6° to 11.5°. In the high-frequency region, the closer the phase angle is to 90°, the more complete the rust layer.

[0077] The above results all indicate that injecting 30 ppb of Zn into the primary loop aqueous solution of the simulated pressurized water reactor... 2+ The corrosion resistance of the post-corrosion rust layer is improved. This is because after zinc injection, γ-FeOOH transforms into α-FeOOH and FeCr2O4. γ-FeOOH, as an electrochemically active substance, is the initial product of the corrosion process and has poor stability. When Zn is injected... 2+ After that, Zn 2+ Cr is attracted by electrostatic adsorption 3+ It occupies oxygen vacancies in γ-FeOOH, promoting its transformation into α-FeOOH, while simultaneously dissolving Cr in the rust layer. 3+ It reacts with α-FeOOH to form FeCr2O4, which has higher stability.

[0078] The results of comparing the corrosion behavior of nuclear power materials under high temperature and high pressure conditions with existing research on zinc injection technology show that the present invention can effectively address the corrosion behavior of nuclear power materials under normal temperature and pressure conditions using Zn. 2+ The mechanisms of action are completely different, indicating that environmental factors affect Zn. 2+ The induced phase transformation of corrosion products has a decisive influence.

[0079] Example 3

[0080] A method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels includes the following steps:

[0081] (1) Sample preparation: Select SA508Gr.4N steel with uniform composition and cut it into block samples of 10mm×10mm×5mm, and make a hole with a diameter of 2mm on the side.

[0082] (2) Polishing and cleaning: Polish the above sample from 600# to 3000# and then mechanically polish it until the surface is smooth and free of scratches. After polishing, clean the sample with anhydrous ethanol and then dry it with cold air for later use.

[0083] (3) The cleaned sample is hung in a beaker by using corrosion-resistant rubber thread, and the sample is hung in a simulated pressurized water reactor primary loop water solution (the simulated pressurized water reactor primary loop water solution is prepared by mixing 4800 ppm boric acid and 8 ppm lithium hydroxide) with a Zn 2+ concentration of 50 ppb at 25°C, and is soaked for 168 h, so that a modified rust layer is formed on the surface of the sample;

[0084] (4) After the soaking is completed, the sample is taken out, the surface of the sample is washed with deionized water and anhydrous ethanol, and the sample is dried by cold air and placed in a dry and sealed sample box for standby.

[0085] The method for evaluating corrosion resistance performance comprises the following steps:

[0086] S1, embedding the SA508Gr.4N steel sample obtained after the polishing treatment in step (2) in epoxy resin;

[0087] S2, testing by using a three-electrode electrochemical workstation, the auxiliary electrode is metal platinum, the reference electrode is a saturated calomel electrode, and the working electrode is the sample with an exposed area of 1 cm 2 after embedding, immersing the working electrode in a simulated primary loop water solution (the simulated pressurized water reactor primary loop water solution is prepared by mixing 4800 ppm boric acid and 8 ppm lithium hydroxide) containing Zn 2+ (the concentration is 50 ppb) for 20 minutes, and after the open circuit potential is stable, testing the polarization curve, wherein the scanning range of the electrochemical workstation is -1 V to 0.6 V, and the scanning rate is 1 mV / s.

[0088] Figure 3 The SEM and TEM images and TEM diffraction spot calibration results of the rust layer on the surface of the SA508Gr.4N steel sample when the Zn 2+ concentration is 50 ppb in Example 3 are shown in FIG. 6, wherein (a) and (b) are SEM images, (c) is a TEM image, and (d) is the polycrystalline diffraction ring in the red oval area in (c) and the calibration results. Figure 3 As can be seen from (a) and (b) in FIG. 6, the rust layer obtained in Example 3 is a regular rod-shaped structure. Figure 3 As can be seen from (d) in FIG. 6, the TEM diffraction ring calibrates the rod-shaped structure as FeCr2O4. This is because the process of γ-FeOOH→α-FeOOH→FeCr2O4 is accelerated with the increase of the zinc injection concentration, and Cr 3+ replaces more Fe 3+ in α-FeOOH to form FeCr2O4.

[0089] Figure 5The figure shows the potentiodynamic polarization curve of the SA508Gr.4N steel sample prepared in Example 3. It can be seen from the figure that injecting 50 ppb of Zn into the primary loop aqueous solution of the simulated pressurized water reactor... 2+ Subsequently, the self-corrosion current was injected from 30 ppb of Zn. 2+ 6.23×10 -7 A / cm 2 Reduced to 5.02×10 -7 A / cm 2 .

[0090] Figure 6 The image shows the Nyquist plot of the SA508Gr.4N steel sample prepared in Example 3. The plot shows that injecting 50 ppb of Zn into the primary loop aqueous solution of the simulated pressurized water reactor... 2+ The diameter of the capacitor arc after injection is larger than that of 30ppb of injected Zn. 2+ At that time, the larger the diameter of the capacitor arc, the better the corrosion resistance of the rust layer.

[0091] Figure 7 The Bode modulus diagram of the SA508Gr.4N steel sample prepared in Example 3 is shown. It can be seen from the diagram that, in the low-frequency range, 50 ppb of Zn injected into the primary loop aqueous solution of a simulated pressurized water reactor... 2+ The impedance modulus after implantation is greater than that of 30 ppb Zn. 2+ At low frequencies, the higher the impedance modulus, the better the corrosion resistance of the rust layer.

[0092] Figure 8 The Bode phase diagram of the SA508Gr.4N steel sample prepared in Example 3 shows that 50 ppb of Zn was injected into the primary loop aqueous solution of a simulated pressurized water reactor in the high-frequency region (f>10kHz). 2+ The phase angle after injection of 30 ppb Zn 2+ The phase angle increased from 11.5° to 17.9°. In the high-frequency region, the closer the phase angle is to 90°, the more complete the rust layer.

[0093] The above results all indicate that the corrosion resistance of the rust layer improves after the zinc concentration is increased from 30 ppb to 50 ppb. This is because the phase transformation process of γ-FeOOH→α-FeOOH→FeCr2O4 is accelerated after the zinc concentration is increased from 30 ppb to 50 ppb, resulting in the formation of more stable FeCr2O4 (see [reaction mechanism]). Figure 9 ).

[0094] Example 4

[0095] A method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels includes the following steps:

[0096] (1) Sample preparation: Select SA508Gr.4N steel with uniform composition and cut it into block samples of 10mm×10mm×5mm, and make a hole with a diameter of 2mm on the side.

[0097] (2) Polishing and cleaning: Polish the above sample from 600# to 3000# and then mechanically polish it until the surface is smooth and free of scratches. After polishing, clean the sample with anhydrous ethanol and then dry it with cold air for later use.

[0098] (3) Use corrosion-resistant rubber thread to suspend the cleaned sample in a beaker, and suspend the sample at 25℃, Zn 2+ The sample was immersed in a simulated pressurized water reactor primary loop aqueous solution with a concentration of 70 ppb (the simulated pressurized water reactor primary loop aqueous solution was prepared by mixing 4800 ppm boric acid and 8 ppm lithium hydroxide) for 168 h to form a modified rust layer on the sample surface.

[0099] (4) After soaking, take out the sample, rinse the sample surface with deionized water and anhydrous ethanol, dry it with cold air, and place it in a dry and sealed sample box for later use.

[0100] The method for evaluating corrosion resistance includes the following steps:

[0101] S1. The SA508Gr.4N steel sample obtained after polishing in step (2) above is embedded in epoxy resin;

[0102] S2. The test was conducted using a three-electrode electrochemical workstation. The auxiliary electrode was a platinum electrode, the reference electrode was a saturated calomel electrode, and the working electrode was an electrode with an exposed area of ​​1 cm² after embedding. 2 The working electrode of the sample was immersed in a Zn-containing solution. 2+ The polarization curve was tested after the open circuit potential stabilized in a simulated primary loop aqueous solution (70 ppb concentration, which was prepared by mixing 4800 ppm boric acid and 8 ppm lithium hydroxide) for 20 minutes. The scanning range of the electrochemical workstation was -1V to 0.6V, and the scanning rate was 1mV / s.

[0103] Figure 4 Zn in Example 4 2+ SEM and TEM images of the rust layer morphology on the surface of SA508Gr.4N steel samples at a concentration of 70 ppb, and TEM diffraction spot calibration results. (a) and (b) are SEM images, (c) and (d) are TEM images, and (e) and (f) are the diffraction spots and calibration results of the red elliptical regions in (c) and (d), respectively. Figure 4 As can be seen from (a) and (b) in the figure, the rust layer obtained in Example 4 is similar to that in Example 3, also exhibiting a regular rod-shaped structure. Figure 4(b) in FIG. 2, but compared with Example 3, the rust layer obtained in Example 4 also has a small amount of irregular flaky structure (enclosed by the blue ellipse in (b) in FIG. 2). Figure 4 (b) in FIG. 2, but compared with Example 3, the rust layer obtained in Example 4 also has a small amount of irregular flaky structure (enclosed by the blue ellipse in (b) in FIG. 2). 3+ substituted Fe 3+ position is reduced to γ-FeOOH in the corrosion solution, and the chemical reaction formula is formula (6). Since FeCr2O4 has high stability, it can exist stably in the corrosion solution.

[0104] (6);

[0105] Figure 5 The potentiodynamic polarization curve of the SA508Gr.4N steel sample prepared in Example 4 is shown in FIG. 6. As can be seen from the figure, after 70ppb of Zn 2+ is injected into the simulated pressurized water reactor primary loop water solution, the self-corrosion current is 5.25x10 -7 A / cm 2 .

[0106] Figure 6 The Nyquist plot of the SA508Gr.4N steel sample prepared in Example 4 is shown in FIG. 7. As can be seen from the figure, after 70ppb of Zn 2+ is injected into the simulated pressurized water reactor primary loop water solution, the capacitance arc diameter is basically the same as that when 50ppb of Zn 2+ is injected.

[0107] Figure 7 The Bode modulus plot of the SA508Gr.4N steel sample prepared in Example 4 is shown in FIG. 8. As can be seen from the figure, in the low frequency range, after 70ppb of Zn 2+ is injected into the simulated pressurized water reactor primary loop water solution, the impedance modulus is basically the same as that when 50ppb of Zn 2 + is injected.

[0108] Figure 8 The Bode phase plot of the SA508Gr.4N steel sample prepared in Example 4 is shown in FIG. 9. As can be seen from the figure, in the high frequency region (f>10kHz), after 70ppb of Zn 2+ is injected into the simulated pressurized water reactor primary loop water solution, the phase angle is 17.3°.

[0109] The above results all show that the corrosion resistance of the rust layer does not significantly improve after the zinc injection concentration increases from 50 ppb to 70 ppb, indicating that the phase transition process of γ-FeOOH→α-FeOOH→FeCr2O4 reaches saturation when the zinc injection concentration is 50 ppb, and the rust layer formed at this time has the best corrosion resistance.

[0110] The above, only for the preferred specific embodiments of the present application, but the scope of protection of the present application is not limited to this, any skilled in the technical field of the technical personnel in the technical range disclosed by the present application, can easily think of changes or replacement, should be covered within the scope of protection of the present application. Therefore, the scope of protection of the present application should be subject to the scope of protection of the claims.

Claims

1. A method for improving the corrosion resistance of SA508 Gr. 4N steel used for nuclear pressure vessels, characterized in that, The SA508Gr.4N steel sample was immersed in a simulated pressurized water reactor primary loop water solution containing Zn 2+ at normal temperature and pressure to form a modified rust layer on the surface.

2. The method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels according to claim 1, characterized in that, The method comprises the following steps: (1) preparing a SA508Gr.4N steel sample; (2) polishing, polishing and cleaning the sample; (3) The sample is hung in a simulated pressurized water reactor primary loop water solution containing Zn 2+ for 168 hours at normal temperature and pressure to form a modified rust layer on the surface of the sample. (4) taking out the sample and cleaning and drying.

3. The method for improving the corrosion resistance of SA508 Gr. 4N steel for nuclear pressure vessels according to claim 1, characterized in that, The chemical composition of the SA508Gr.4N steel sample is: C 0.18wt.%, Si 0.15wt.%, Mn 0.35wt.%, Cr 1.70wt.%, Ni 3.68wt.%, Mo 0.51wt.%, and the rest is Fe.

4. The method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels according to claim 2, characterized in that, The specific operation steps of polishing, polishing and cleaning the sample are: the sample is polished with 600# to 3000# sandpaper in turn, and then mechanically polished to make the sample surface smooth and scratch-free; the polished sample is cleaned with anhydrous ethanol, and then dried with cold air.

5. The method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels according to claim 1, characterized in that, The simulated pressurized water reactor primary circuit aqueous solution is prepared by mixing 4800ppm boric acid and 8ppm lithium hydroxide.

6. The method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels according to claim 1, characterized in that, The Zn 2+ is provided in the form of zinc acetate at a concentration of 30 ppb - 70 ppb.

7. The method for improving the corrosion resistance of SA508Gr.4N steel for nuclear pressure vessels according to claim 6, characterized in that, The concentration of said Zn 2+ is 50 ppb.

8. A modified SA508Gr.4N steel prepared by the method of any one of claims 1-7.

9. A method of evaluating corrosion resistance, characterized by, The method comprises the following steps: S1, embedding the SA508Gr.4N steel in the epoxy resin, and polishing the embedded sample; S2, using a three-electrode electrochemical workstation, the auxiliary electrode is a metal platinum, the reference electrode is a saturated calomel electrode, and the working electrode is the sample polished in step S1 with an exposed area of 1 cm 2 S2, using a three-electrode electrochemical workstation, the auxiliary electrode is a metal platinum, the reference electrode is a saturated calomel electrode, and the working electrode is the sample polished in step S1 with an exposed area of 1 cm 2+ S2, using a three-electrode electrochemical workstation, the auxiliary electrode is a metal platinum, the reference electrode is a saturated calomel electrode, and the working electrode is the sample polished in step S1 with an exposed area of 1 cm 10. The method of evaluating corrosion resistance according to claim 9, characterized in that, The scan range of the electrochemical workstation is -1 V~0.6 V, the scan rate is 1 mV / s, and the Zn 2+ The concentration is 30 ppb-70 ppb.

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