A method for enhancing corrosion resistance of magnesium-lithium alloy by heat treatment induced secondary phase reconfiguration and solute redistribution

Abstract

This invention relates to a method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, belonging to the field of metallic materials and their corrosion protection technology. The invention involves sample encapsulation and protection, thermal homogenization, and high-temperature heat treatment of a magnesium-lithium alloy containing multiple secondary phases. The heat treatment induces secondary phase reconstruction and solute redistribution, optimizing the microstructure and local electrochemical differences of the magnesium-lithium alloy containing multiple secondary phases. This results in the formation of a continuously distributed network of (Ca,Y)₂Mg₆Zn₃ phases and other uniformly distributed granular secondary phases within the matrix, significantly hindering the propagation of filamentous corrosion. This invention achieves solute redistribution and secondary phase continuity by controlling the partial dissolution of the high-potential secondary phase and its interaction with other secondary phases, thereby improving overall corrosion resistance. The process is highly applicable, simple and adjustable, cost-controllable, and easy to industrialize.

Description

Technical Field

[0001] This invention relates to the field of metallic materials and their corrosion resistance technology, specifically to a method for improving the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment. This method is applicable to magnesium-lithium alloys containing multiple secondary phases and can be used in the field of structural materials where lightweighting and corrosion resistance are critical. Background Technology

[0002] Magnesium-lithium alloy (Mg-Li alloy) is one of the structural metals with the lowest density, reaching as low as 1.3 g / cm³. 3 -1.6g / cm 3 Magnesium-lithium alloys possess high specific strength and good ductility, making them valuable for applications in aerospace, transportation, and biomedicine. However, they are prone to corrosion in actual service environments. This is primarily due to the low standard electrode potentials of magnesium and lithium, which makes the α-Mg matrix susceptible to anodic dissolution in aqueous environments. Furthermore, a significant potential difference typically exists between the matrix and secondary phases in magnesium-lithium alloys, making them prone to microgalvanic corrosion under corrosive media. This induces pitting and filamentary corrosion, severely limiting their engineering applications.

[0003] Currently, common methods to improve the corrosion resistance of magnesium-lithium alloys include alloying, heat treatment, and surface treatment. Adding Zn, Ca, and rare earth elements can promote the formation of phases such as Ca2Mg6Zn3, thereby reducing the potential difference between this phase and the α-Mg matrix, which helps to mitigate localized corrosion. However, in actual as-cast microstructures, secondary phases such as Ca2Mg6Zn3 are usually distributed discontinuously in strip or granular forms, making it difficult to form a continuous "barrier network." This not only fails to effectively suppress microgalvanic corrosion but may also accelerate corrosion propagation due to localized cathodic effects.

[0004] Furthermore, in systems such as Mg-Li-Zn-Y, the presence of high-potential secondary phases such as the quasi-crystalline I phase (Mg3Zn6Y) further increases the risk of microgalvanic corrosion. Although the I phase may have a positive impact on corrosion resistance when forming a continuous network structure, under traditional casting conditions, the I phase usually precipitates as discontinuous particles, which instead become local cathodic points, exacerbating corrosion. At the same time, the coexistence of high-potential intermetallic compounds such as the W phase (Mg3Zn3Y2) and LiMgZn phase makes the electrochemical differences in the multiphase system more complex, and the corrosion behavior more difficult to control.

[0005] Although existing studies have improved the corrosion performance of magnesium-lithium alloys through composition optimization and heat treatment, systematic research on heat treatment-induced secondary phase morphology reconstruction, solute redistribution and their effects on electrochemical performance and corrosion product layer formation is still insufficient, especially in complex systems with multiple secondary phases coexisting, where clear control strategies are still lacking.

[0006] Therefore, there is an urgent need for a method to improve the corrosion resistance of magnesium-lithium alloys by reconstructing the morphology and continuity of the secondary phase and regulating the solute distribution through heat treatment, thereby reducing the microcouple potential difference and improving the density of the corrosion product film, so as to meet the needs of practical engineering applications. Summary of the Invention

[0007] This invention provides a method for improving the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment. This method significantly improves the corrosion resistance of magnesium-lithium alloys by optimizing their microstructure and local electrochemical state, and is particularly suitable for magnesium-lithium alloys containing multiple secondary phases. Unlike traditional methods of removing high-potential cathode phases, this invention reduces the potential difference between the matrix and the secondary phases by controlling the morphology, continuity, and elemental distribution of the secondary phases, thereby suppressing microgalvanic corrosion and enhancing the alloy's corrosion resistance.

[0008] To achieve the above objectives, the technical solution of the present invention includes the following:

[0009] This invention provides a method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, comprising the following steps:

[0010] S1. Alloy material selection: A magnesium-lithium alloy containing multiple secondary phases is selected; the alloy contains, by mass percentage, Li 2wt.%-5wt.%, Zn 3wt.%-8wt.%, Y 0.1wt.%-1.5wt.%, Ca 0.1wt.%-2.0wt.%, and Mg balance; the multiple precipitated phases include α-Mg matrix, LiMgZn phase, quasi-crystalline I phase (Mg3Zn6Y), W phase (Mg3Zn3Y2), Mg2Ca phase, and Ca2Mg6Zn3 phase;

[0011] S2. Sample packaging and protection:

[0012] The magnesium-lithium alloy sample is surface-cleaned and dried, then coated with metal foil. The coated sample is placed in a thermally conductive metal container, and graphite powder is filled around the sample for coverage and isolation. After being coated with metal foil and filled with graphite powder, the sample is not in contact with the external gas environment and is in a relatively closed environment with low oxygen partial pressure. The metal foil is aluminum foil and / or copper foil. The sample is coated with multiple layers of metal foil.

[0013] During the sample encapsulation and protection process, before placing the coated sample into a thermally conductive metal container, the coated sample can be first placed in a quartz tube for vacuum sealing. Subsequently, the sealed sample is placed in the thermally conductive metal container. This method further enhances sample protection, providing better sealing and a lower oxygen partial pressure environment, thereby effectively preventing oxidation or other undesirable reactions.

[0014] S3. Thermal homogenization treatment:

[0015] The encapsulated and protected sample is heated to 250℃-350℃ and held at that temperature for 1h-7h; preferably, the sample is heated to 330℃ and held at that temperature for 4h.

[0016] S4. High-temperature heat treatment:

[0017] The sample after heat homogenization is further heated and held at a certain temperature to form (Ca,Y)2Mg6Zn3 phase or Y-enriched Ca2Mg6Zn3 phase; a corrosion product film rich in MgO and CaCO3 is formed on the surface of the magnesium-lithium alloy, and the corrosion product film is dense and without obvious cracks; wherein, the heating temperature is 380℃-440℃ and the holding time is 1h~8h.

[0018] The Ca2Mg6Zn3 phase forms a continuously distributed network structure, and a uniformly distributed granular secondary phase is formed inside the matrix.

[0019] The technical advantages of this invention are: by inducing secondary phase reconstruction and solute redistribution through heat treatment, the microstructure and local electrochemical differences of magnesium-lithium alloys containing multiple secondary phases are optimized, and the corrosion resistance can be significantly improved without eliminating the high-potential cathode phase, thus having wider applicability and engineering application prospects.

[0020] The design concept of this invention is:

[0021] This invention improves the corrosion resistance of magnesium-lithium alloys by controlling the morphology, distribution, and elemental distribution of secondary phases through heat treatment. Unlike traditional methods, this invention does not rely on eliminating specific high-potential secondary phases. Instead, it optimizes the continuity and morphology of secondary phases by inducing partial dissolution of high-potential phases and achieving solute redistribution, thereby reducing the potential difference between the matrix and the secondary phases and inhibiting microgalvanic corrosion. Simultaneously, secondary phases such as Ca2Mg6Zn3 transform from discontinuous strip-like or granular structures into continuous network structures, forming uniformly distributed particles within the matrix, thus significantly hindering the propagation of filamentary corrosion. During heat treatment, if the homogenization temperature is below 250℃ or homogenization is not performed, low-melting-point secondary phases (such as the LiMgZn phase) cannot dissolve sufficiently, leading to overheating during high-temperature heat treatment. Undissolved phases may cause localized melting, affecting the uniformity of the alloy structure and its corrosion resistance. High-temperature heat treatment temperatures exceeding 500℃ may lead to grain growth, excessive dissolution of secondary phases, and oxidation of the alloy surface, significantly reducing corrosion resistance. Therefore, the homogenization temperature and high-temperature treatment temperature must be strictly controlled to ensure the best performance of the alloy.

[0022] In this invention, the sample encapsulation involves covering the sample with metal foil or further sealing it with a quartz tube, then covering it with graphite powder for protection. After the sample is covered with metal foil (such as aluminum foil and / or copper foil), the sample is not in contact with the external gas environment of the graphite powder, effectively reducing oxygen permeation, maintaining a low oxygen partial pressure environment, and preventing oxidation and volatilization. The quartz tube seal further provides better atmosphere isolation, ensuring that the sample is not disturbed by external gases during heat treatment. The graphite powder coating helps improve thermal conductivity and gas isolation, promotes uniform heating of the sample, and reduces thermal stress. This invention, through the synergistic optimization of microstructure and elemental distribution, promotes the formation of a dense corrosion product film (rich in MgO and CaCO3), fundamentally improving the corrosion resistance of magnesium-lithium alloys, avoiding the limitations of traditional methods, and showing promising prospects for industrial applications.

[0023] The advantages and beneficial effects of this invention are:

[0024] (1) Significantly improve corrosion resistance: This invention reduces the potential difference between the matrix and the secondary phase by inducing secondary phase reconstruction and solute redistribution through heat treatment, thereby inhibiting microgalvanic corrosion and significantly improving the corrosion resistance of magnesium-lithium alloy in aqueous environment.

[0025] (2) No need to completely eliminate high-potential secondary phases: Unlike traditional methods, this invention does not rely on completely eliminating specific high-potential secondary phases (such as quasi-crystalline I phases). Instead, it achieves solute redistribution and secondary phase continuity by controlling the partial dissolution of high-potential secondary phases and their interaction with other secondary phases, thereby improving the overall corrosion performance.

[0026] (3) Optimization of secondary phase morphology and distribution: Through heat treatment, secondary phases such as Ca2Mg6Zn3 are transformed from discontinuous strip or granular structures into continuous network structures and form uniformly distributed particles inside the grains, which enhances the protective effect of secondary phases on the matrix and effectively inhibits the spread of pitting and filiform corrosion.

[0027] (4) Improve the structure of corrosion product layer: The corrosion product layer formed after heat treatment is rich in MgO and CaCO3, dense and without obvious cracks, which can effectively prevent the intrusion of corrosive media, thereby improving the service stability of the alloy in corrosive environment.

[0028] (5) Strong applicability and adjustable process: This method is applicable to magnesium-lithium alloys containing multiple secondary phases, has a wide process window, and can adjust heat treatment parameters according to different alloy systems and application requirements, which is convenient for industrial promotion.

[0029] (6) Simple process and controllable cost: The present invention can significantly improve corrosion resistance through heat treatment, without the need for additional complex surface treatment processes, simplifying the process flow and reducing production costs. Attached Figure Description

[0030] Figure 1 The XRD patterns of the alloy under different heat treatment conditions are shown.

[0031] Figure 2 SEM images of the alloy under different conditions: (a) as-cast state, (b) HT-1, (c) HT-2; (a2-c2) are magnified views of the rectangular labeled areas in (a1-c1).

[0032] Figure 3 The following are the scanning transmission electron microscopy (STEM) bright-field imaging (BF-STEM) characterization results of the alloy: (a) as-cast sample, (b) HT-2 sample; (a1,b1) bright-field (BF) image, (a2,b2) selected area electron diffraction (SAED) pattern, (a3,b3) corresponding EDS elemental distribution map.

[0033] Figure 4 The following are in-situ observations of filamentous corrosion propagation during immersion in 0.1 mol / L NaCl solution: (a1-a3) as-cast samples, (b1-b3) HT-2 samples.

[0034] Figure 5 The cross-sectional morphology of the samples after soaking in 0.1 mol / L NaCl solution for 50 h was observed: (a) as-cast state; (b) HT-1; (c) HT-2.

[0035] Figure 6 XPS spectral analysis results for the as-cast and HT-2 samples: (a) As-cast; (b) HT-2.

[0036] Figure 7 Surface potential distribution of SKPFM: (a) Ca2Mg6Zn3 phase in the as-cast sample; (b) (Ca,Y)2Mg6Zn3 phase in the HT-2 sample. Detailed Implementation

[0037] This invention provides a method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, comprising the following steps:

[0038] 1. Alloy material selection:

[0039] The magnesium-lithium alloy used in this invention is a Mg-based alloy containing Li, Zn, Y, and Ca, wherein the Li content is 2wt.%-5wt.%, the Zn content is 3wt.%-8wt.%, the Y content is 0.1wt.%-1.5wt.%, the Ca content is 0.1wt.%-2.0wt.%, and the balance is Mg, preferably Mg-(3-4)Li-(5-7)Zn-(0.5-1.5)Y-1Ca (wt.%). This alloy comprises an α-Mg matrix, a LiMgZn phase, a quasicrystalline I phase (Mg3Zn6Y), a W phase (Mg3Zn3Y2), a Mg2Ca phase, and a Ca2Mg6Zn3 phase.

[0040] 2. Sample packaging and protection before heat treatment:

[0041] Before thermal homogenization, the magnesium-lithium alloy sample to be treated was surface-cleaned and dried. Then, the sample was coated with multiple layers of metal foil (aluminum foil and / or copper foil). The coated sample was placed in a thermally conductive metal container, and graphite powder was used to cover and isolate the sample. The metal foil and graphite powder coverage prevented contact between the sample and the external gas environment, reducing oxygen penetration and thus decreasing the volatilization and oxidation tendency of Li during heat treatment. This also slowed down surface overheating and oxide scale formation, and helped maintain the stability of the alloy surface composition. Furthermore, the good thermal conductivity and heat buffering properties of graphite powder promoted uniform heating of the sample, reducing thermal stress and localized overheating.

[0042] 3. Heat homogenization treatment:

[0043] The as-cast alloy after encapsulation and protection is heated to 250℃-350℃ and held for 1h-7h to promote the dissolution and homogenization of various elements in the alloy, reduce component segregation, and dissolve some of the Li-Zn enriched phases.

[0044] 4. High-temperature heat treatment:

[0045] Based on homogenization treatment, the alloy is heated to 380℃-440℃ and held for 1-8 hours to partially dissolve the high-potential I phase and Mg2Ca phase, releasing Zn and Y elements. The dissolved Y element preferentially dissolves into the Ca2Mg6Zn3 phase, forming the (Ca,Y)2Mg6Zn3 phase or a Y-enriched Ca2Mg6Zn3 phase, while Zn partially dissolves into the α-Mg matrix, thereby reducing the potential difference between the secondary phase and the matrix to suppress micro-galvanic corrosion. A dense corrosion product film rich in MgO and CaCO3, without obvious cracking, is formed on the surface of the magnesium-lithium alloy. Among them, the Ca2Mg6Zn3 phase forms a continuously distributed network structure, and uniformly distributed granular secondary phases are formed inside the matrix.

[0046] To further enhance the protection of the sample, this invention also provides an optional encapsulation method: in addition to metal foil coating, the sample is vacuum-sealed using a quartz tube. The sealed sample continues to be placed in a thermally conductive metal container and covered and isolated with graphite powder. This encapsulation method can further reduce the risk of oxidation and prevent interference from external gases, while improving the airtightness of the sample and ensuring more stable environmental conditions during heat treatment.

[0047] In this invention, during heat treatment, the Ca2Mg6Zn3 phase gradually transforms from a discontinuous strip-like or granular structure into a continuous network structure, forming uniformly distributed spherical particles within the matrix. This structural reconstruction effectively inhibits the propagation of filamentous corrosion. Simultaneously, solute redistribution reduces the potential difference between the matrix and the secondary phase, suppressing microgalvanic corrosion and promoting the formation of a dense corrosion product layer rich in MgO. Furthermore, CaCO3 fills the defects in the product layer, improving the density of the corrosion product film.

[0048] After the above heat treatment, the corrosion rate of the alloy is reduced by 30% to 70% compared with the cast alloy, effectively suppressing local corrosion phenomena such as pitting corrosion and filiform corrosion, thereby significantly improving the corrosion resistance of magnesium-lithium alloy.

[0049] The method provided by this invention is applicable to magnesium-lithium alloys containing multiple secondary phases. Without eliminating specific high-potential secondary phases, the corrosion resistance can be improved by controlling the morphology of the secondary phases and the distribution of solutes.

[0050] In its specific implementation, this invention selects a magnesium-lithium alloy containing Li, Zn, Y, and Ca as the research object. By setting different heat treatment paths, the dissolution behavior of the high-potential secondary phase, the redistribution of solute elements, and the morphological evolution of the secondary phase in the alloy are controlled to reduce the potential difference between the matrix and the secondary phase, suppress microgalvanic corrosion, and improve the structure of the corrosion product film. The invention is further illustrated below with specific embodiments. It is worth noting that the provided embodiments are only for illustrating the implementation of the invention and do not constitute a limitation on the invention. The scope of protection of this invention is not limited to the specific content shown in the following embodiments.

[0051] Example 1:

[0052] This embodiment provides a method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, including the following steps:

[0053] S1. Alloy Material Selection: A cast magnesium-lithium alloy with a microstructure primarily comprising an α-Mg matrix, LiMgZn phase, Mg3Zn6Y quasicrystalline I phase, Mg3Zn3Y2 (W phase), Ca2Mg6Zn3 phase, and Mg2Ca phase was selected. The alloy contained, by mass percentage, 4 wt.% Li, 6 wt.% Zn, 1 wt.% Y, 1 wt.% Ca, with the balance being Mg. For example... Figure 1 As shown, the XRD pattern of the as-cast sample shows the diffraction peaks of the aforementioned phases, indicating the presence of multiple secondary phases in the alloy. The absence of a LiMgZn phase peak is likely due to its small size and low content. SEM observations are as follows... Figure 2 As shown in (a), the Ca2Mg6Zn3 phase is mainly distributed in discontinuous rod-like shapes, while a small number of spherical particles can be observed inside the matrix; the Mg2Ca phase often exists adjacent to the Ca2Mg6Zn3 phase. In addition, a non-uniform region of LiMgZn phase can also be observed in the matrix.

[0054] S2. Sample packaging and protection:

[0055] The selected magnesium-lithium alloy was cleaned and dried. Then, the alloy sample was wrapped with two layers of aluminum foil. The wrapped sample was placed in a thermally conductive stainless steel container, and graphite powder was filled around the sample for coverage and isolation. The sample was in a relatively closed environment with low oxygen partial pressure.

[0056] S3. Thermal homogenization treatment:

[0057] The encapsulated and protected sample was heated to 330℃ and kept at that temperature for 4 hours.

[0058] S4. High-temperature heat treatment:

[0059] The sample after heat homogenization was further heated to 415℃ and held for 2 hours. The resulting alloy sample was designated as HT-2 sample.

[0060] like Figure 1 As shown, the diffraction peaks of the quasicrystalline I phase and the Mg2Ca phase in the HT-2 sample were significantly weakened, indicating that these phases partially or completely dissolved under high-temperature conditions. SEM observation results are as follows... Figure 2 As shown in (c), the Ca2Mg6Zn3 phase transforms from a discontinuous rod-like structure into a continuous network structure, while simultaneously forming uniformly distributed fine particles within the matrix. BF-STEM and EDS analysis results are as follows: Figure 3 As shown, the network phase in the HT-2 sample is (Ca,Y)2Mg6Zn3. Compared with the as-cast sample, the Y element is significantly enriched in this phase, indicating that the high-temperature heat treatment induced solute redistribution and secondary phase reconstruction.

[0061] The HT-2 sample was subjected to corrosion testing in a 0.1 mol / L NaCl solution. The in-situ corrosion morphology is as follows: Figure 4 As shown in (b1-b3), due to the presence of the continuous network structure, the propagation of filamentous corrosion is significantly inhibited, and the size and number of corrosion pits are greatly reduced. Figure 5 Furthermore, the resulting corrosion product layer has a dense structure without obvious cracks, effectively preventing the intrusion of corrosive media and providing excellent protection for the substrate. Figure 5 ).

[0062] In addition, XPS analysis results such as Figure 6 As shown, the corrosion product film of the HT-2 sample is rich in MgO and CaCO3, further demonstrating that its corrosion product layer not only has high density, but its composition also contributes to improving the film's corrosion resistance. The presence of MgO and CaCO3 can enhance the film's density and erosion resistance. The SKPFM surface potential test results are shown below. Figure 7 As shown, the potential difference between the (Ca,Y)2Mg6Zn3 phase and the α-Mg matrix in the HT-2 sample was significantly reduced, thereby effectively weakening the microgalvanic corrosion effect.

[0063] Example 2:

[0064] This embodiment provides a method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, including the following steps:

[0065] S1. Alloy material selection: By mass percentage, a magnesium-lithium alloy containing 4 wt.% Li, 6 wt.% Zn, 1 wt.% Y, 1 wt.% Ca, and the balance Mg is selected. The magnesium-lithium alloy contains an α-Mg matrix, a LiMgZn phase, a quasi-crystalline I phase (Mg3Zn6Y), a W phase (Mg3Zn3Y2), a Mg2Ca phase, and a Ca2Mg6Zn3 phase.

[0066] S2. Sample packaging and protection:

[0067] The magnesium-lithium alloy was surface cleaned and dried. Then, the sample was wrapped with copper foil. The sample with three layers of coating was placed in a thermally conductive stainless steel container. After filling the sample with graphite powder for covering and isolation, the sample was in a relatively closed environment with low oxygen partial pressure.

[0068] S3. Thermal homogenization treatment:

[0069] The encapsulated and protected sample was heated to 330℃ and kept at that temperature for 4 hours.

[0070] S4. High-temperature heat treatment:

[0071] The samples after heat homogenization were further heated to 440℃ and held for 2 hours to form a (Ca,Y)₂Mg₆Zn₃ phase or a Y-enriched Ca₂Mg₆Zn₃ phase. A corrosion product film rich in MgO and CaCO₃ was formed on the surface of the treated magnesium-lithium alloy. The Ca₂Mg₆Zn₃ phase formed a continuously distributed network structure, and uniformly distributed granular secondary phases were formed within the matrix. The corrosion current density was 18 μA / cm². 2 .

[0072] Example 3:

[0073] This embodiment provides a method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, including the following steps:

[0074] S1. Alloy material selection: By mass percentage, a magnesium-lithium alloy containing 4 wt.% Li, 6 wt.% Zn, 1 wt.% Y, 1 wt.% Ca, and the balance Mg is selected. The magnesium-lithium alloy contains an α-Mg matrix, a LiMgZn phase, a quasi-crystalline I phase (Mg3Zn6Y), a W phase (Mg3Zn3Y2), a Mg2Ca phase, and a Ca2Mg6Zn3 phase.

[0075] S2. Sample packaging and protection:

[0076] The magnesium-lithium alloy was surface cleaned and dried. Then, the sample was wrapped with aluminum foil. The sample with three layers of foil was placed in a quartz tube and vacuum sealed. The vacuum-sealed sample was placed in a thermally conductive stainless steel container and covered and isolated with graphite powder to keep the sample in a closed environment with low oxygen partial pressure.

[0077] S3. Thermal homogenization treatment:

[0078] The encapsulated and protected sample was heated to 330℃ and kept at that temperature for 4 hours.

[0079] S4. High-temperature heat treatment:

[0080] The heat-homogenized sample was further heated to 440℃ and held for 1 hour to form a (Ca,Y)₂Mg₆Zn₃ phase or a Y-enriched Ca₂Mg₆Zn₃ phase. A corrosion product film rich in MgO and CaCO₃ was formed on the surface of the treated magnesium-lithium alloy. The Ca₂Mg₆Zn₃ phase formed a continuously distributed network structure, and uniformly distributed granular secondary phases were formed within the matrix. The corrosion current density was 12 μA / cm². 2 .

[0081] Example 4:

[0082] S1. Alloy material selection: By mass percentage, a magnesium-lithium alloy containing 4 wt.% Li, 6 wt.% Zn, 1 wt.% Y, 1 wt.% Ca, and the balance Mg is selected. The magnesium-lithium alloy contains an α-Mg matrix, a LiMgZn phase, a quasi-crystalline I phase (Mg3Zn6Y), a W phase, a Mg2Ca phase, and a Ca2Mg6Zn3 phase.

[0083] S2. Sample packaging and protection:

[0084] The magnesium-lithium alloy was surface cleaned and dried. Then, the sample was wrapped with aluminum foil. The sample with three layers of foil was placed in a thermally conductive stainless steel container. Graphite powder was filled around the sample for coverage and isolation. The sample was then placed in a relatively closed environment with low oxygen partial pressure.

[0085] S3. Thermal homogenization treatment:

[0086] The encapsulated and protected sample was heated to 330℃ and kept at that temperature for 2 hours.

[0087] S4. High-temperature heat treatment:

[0088] The sample after heat homogenization was further heated to 400℃ and held for 1 hour to form a (Ca,Y)₂Mg₆Zn₃ phase or a Y-enriched Ca₂Mg₆Zn₃ phase. A corrosion product film rich in MgO and CaCO₃ was formed on the surface of the treated magnesium-lithium alloy. The Ca₂Mg₆Zn₃ phase formed a continuously distributed network structure, and uniformly distributed granular secondary phases were formed within the matrix. The corrosion current density was 9 μA / cm². 2 .

[0089] Comparative Example 1:

[0090] The as-cast magnesium-lithium alloy selected in this comparative example is the same as in Example 1. This as-cast magnesium-lithium alloy was not treated and was directly subjected to corrosion testing in a 0.1 mol / L NaCl solution. The corrosion test results show that the hydrogen evolution rate and corrosion rate of the as-cast sample are both high. The test results of corrosion current density, hydrogen evolution amount over 50 hours, corrosion rate, and typical corrosion pit depth are shown in Table 1. The in-situ corrosion morphology observation results are as follows: Figure 4 As shown in (a1-a3), obvious filamentous corrosion and pitting corrosion are observed on the sample surface. Due to the discontinuous distribution of the secondary phase in the alloy, the filamentous corrosion spreads rapidly after initiation, and the corrosion process lacks effective barriers, resulting in severe corrosion. The formed corrosion product layer has a loose structure and severe cracking, providing limited protection to the substrate. Figure 5 ).

[0091] Comparative Example 2:

[0092] The as-cast magnesium-lithium alloy selected in this comparative example is the same as that in Example 1, except that the magnesium-lithium alloy is subjected to only thermal homogenization treatment, including the following steps:

[0093] S1. Alloy material selection: The selected as-cast magnesium-lithium alloy mainly includes α-Mg matrix, LiMgZn phase, Mg3Zn6Y quasicrystalline I phase, Mg3Zn3Y2 (W phase), Ca2Mg6Zn3 phase and Mg2Ca phase, containing Li 4wt.%, Zn 6wt.%, Y 1wt.%, Ca 1wt.%, and the balance being Mg by mass percentage.

[0094] S2. Sample packaging and protection:

[0095] The selected magnesium-lithium alloy was cleaned and dried. Then, the alloy sample was wrapped with two layers of aluminum foil. The wrapped sample was placed in a thermally conductive metal container and covered and isolated with graphite powder. The sample was in a relatively closed environment with low oxygen partial pressure.

[0096] S3. Thermal homogenization treatment:

[0097] The encapsulated and protected sample was heated to 330℃ and held for 4 hours. The resulting alloy sample was designated as HT-1.

[0098] like Figure 1 As shown, the XRD pattern of the HT-1 sample is basically consistent with that of the as-cast sample. SEM observation results are as follows... Figure 2 As shown in (b), the distribution of the I and W phases did not change significantly. The Ca2Mg6Zn3 phase was still distributed in a discontinuous rod-shaped or granular form. Only in local areas was the dissolution of some LiMgZn phase observed, and the overall uniformity of the structure was improved.

[0099] Corrosion test results show that the hydrogen evolution rate and corrosion rate of the HT-1 sample are lower than those of the as-cast sample, and its performance parameters are shown in Table 1. Furthermore, severe localized corrosion is still observed on the sample surface, and the density of the corrosion product layer shows limited improvement, with obvious cracking still present. Figure 5 ).

[0100] Comparative Example 3:

[0101] This comparative example provides a method for enhancing the corrosion resistance of magnesium-lithium alloys, comprising the following steps:

[0102] S1. Alloy material selection: By mass percentage, a cast magnesium-lithium alloy containing 4 wt.% Li, 6 wt.% Zn, 1 wt.% Y, 0.2 wt.% Ca, and the balance Mg is selected. The magnesium-lithium alloy contains an α-Mg matrix, LiMgZn phase, quasi-crystalline I phase (Mg3Zn6Y), W phase (Mg3Zn3Y2), and a small amount of Ca2Mg6Zn3 phase.

[0103] S2. Sample packaging and protection:

[0104] The magnesium-lithium alloy was surface cleaned and dried. Then, the sample was wrapped with aluminum foil. The sample with three layers of foil was placed in a thermally conductive stainless steel container. Graphite powder was filled around the sample for coverage and isolation. The sample was then placed in a relatively closed environment with low oxygen partial pressure.

[0105] S3. Thermal homogenization treatment:

[0106] The encapsulated and protected sample was heated to 330℃ and kept at that temperature for 2 hours.

[0107] S4. High-temperature heat treatment:

[0108] The heat-homogenized sample was further heated to 400℃ and held for 2 hours, forming a small amount of (Ca,Y)₂Mg₆Zn₃ phase or a Y-enriched Ca₂Mg₆Zn₃ phase. A corrosion product film rich in MgO and Mg(OH)₂ was formed on the surface of the treated magnesium-lithium alloy. The Ca₂Mg₆Zn₃ phase content was very low, failing to form a continuously distributed network structure, resulting in poor corrosion resistance of the alloy (corrosion current density of 27 μA / cm²). 2 ).

[0109] Comparative Example 4:

[0110] This comparative example provides a method for enhancing the corrosion resistance of magnesium-lithium alloys, comprising the following steps:

[0111] S1. Alloy material selection: By mass percentage, select a magnesium-lithium alloy containing 4wt.% Li, 6wt.% Zn, 1wt.% Y, 0wt.% Ca, and the balance Mg. The magnesium-lithium alloy contains an α-Mg matrix, a LiMgZn phase, a quasi-crystalline I phase (Mg3Zn6Y), and a W phase (Mg3Zn3Y2).

[0112] S2. Sample packaging and protection:

[0113] The magnesium-lithium alloy was surface cleaned and dried. Then, the sample was wrapped with aluminum foil. The sample with three layers of foil was placed in a thermally conductive nickel-based alloy container. After filling the sample with graphite powder for coverage and isolation, the sample was placed in a relatively closed environment with low oxygen partial pressure.

[0114] S3. Thermal homogenization treatment:

[0115] The encapsulated and protected sample was heated to 330℃ and kept at that temperature for 2 hours.

[0116] S4. High-temperature heat treatment:

[0117] The sample, after heat homogenization, was further heated to 400℃ and held for 2 hours. No (Ca,Y)₂Mg₆Zn₃ phase or Y-enriched Ca₂Mg₆Zn₃ phase could be formed. A corrosion product film rich in MgO and Mg(OH)₂ formed on the surface of the treated magnesium-lithium alloy. The sample exhibited poor corrosion resistance (corrosion current density of 55 μA / cm²). 2 ).

[0118] Comparative Example 5:

[0119] This comparative example provides a method for enhancing the corrosion resistance of magnesium-lithium alloys, comprising the following steps:

[0120] S1. Alloy material selection: By mass percentage, a magnesium-lithium alloy containing 4 wt.% Li, 6 wt.% Zn, 1 wt.% Y, 1 wt.% Ca, and the balance Mg is selected. The magnesium-lithium alloy contains an α-Mg matrix, a LiMgZn phase, a quasi-crystalline I phase (Mg3Zn6Y), a W phase (Mg3Zn3Y2), a Mg2Ca phase, and a Ca2Mg6Zn3 phase.

[0121] S2. Sample packaging and protection:

[0122] The magnesium-lithium alloy was surface cleaned and dried. Then, the sample was wrapped with aluminum foil. The sample with two layers of foil was placed in a thermally conductive graphite container. Graphite powder was filled around the sample for coverage and isolation. The sample was then placed in a relatively closed environment with low oxygen partial pressure.

[0123] S3. High-temperature heat treatment:

[0124] The unhomogenized sample was directly heated to 440℃ and held at that temperature for 8 hours. Due to the lack of homogenization and the presence of a low-melting-point phase, the sample overheated. A significant reduction in corrosion resistance was expected.

[0125] Comparative Example 6:

[0126] This comparative example provides a method for enhancing the corrosion resistance of magnesium-lithium alloys, comprising the following steps:

[0127] S1. Alloy material selection: By mass percentage, a magnesium-lithium alloy containing 4 wt.% Li, 6 wt.% Zn, 1 wt.% Y, 1 wt.% Ca, and the balance Mg is selected. The magnesium-lithium alloy contains an α-Mg matrix, a LiMgZn phase, a quasi-crystalline I phase (Mg3Zn6Y), a W phase (Mg3Zn3Y2), a Mg2Ca phase, and a Ca2Mg6Zn3 phase.

[0128] S2. Sample packaging and protection:

[0129] The magnesium-lithium alloy was surface cleaned and dried. Then, the sample was wrapped with aluminum foil. The sample with two layers of foil was placed in a thermally conductive stainless steel container. Graphite powder was filled around the sample for coverage and isolation. The sample was then placed in a relatively closed environment with low oxygen partial pressure.

[0130] S3. Thermal homogenization treatment:

[0131] The encapsulated and protected sample was heated to 330℃ and kept at that temperature for 2 hours.

[0132] S4. High-temperature heat treatment:

[0133] The sample after heat homogenization was further heated to 500℃ and held for 18 hours; the treated magnesium-lithium alloy was overheated and the surface was severely oxidized, and the corrosion resistance of the alloy was expected to decrease significantly.

[0134] The corrosion current density, hydrogen evolution amount over 50 hours, corrosion rate, and typical corrosion pit depth over 50 hours of the alloy samples prepared in Example 1 and Comparative Examples 1 and 2 were measured, and the results are shown in Table 1:

[0135] Table 1. Test results of corrosion behavior of alloys obtained in Example 1 and Comparative Examples 1 and 2;

[0136] ;

[0137] As can be seen from the corrosion behavior test results in Table 1, the corrosion current density, hydrogen evolution amount over 50 hours, corrosion rate, and typical corrosion pit depth of the HT-2 sample are significantly lower than those of the as-cast and HT-1 samples.

Claims

1. A method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, characterized in that, Includes the following steps: S1. Alloy material selection: Select a magnesium-lithium alloy containing multiple precipitated phases; The various precipitated phases include α-Mg matrix, LiMgZn phase, quasi-crystalline I phase Mg3Zn6Y, W phase Mg3Zn3Y2, Mg2Ca phase and Ca2Mg6Zn3 phase; S2. Sample encapsulation and protection: The magnesium-lithium alloy sample is surface treated, and after being wrapped with metal foil, it is placed in a thermally conductive metal container. Graphite powder is then filled around the sample for coverage and isolation. S3. Thermal homogenization treatment: The sample after encapsulation and protection is heated and kept at a certain temperature to perform homogenization treatment; S4. High-temperature heat treatment: The sample after heat homogenization is further heated and kept at a certain temperature to form (Ca,Y)2Mg6Zn3 phase or Y-enriched Ca2Mg6Zn3 phase, and a corrosion product film is formed on the surface of the magnesium-lithium alloy.

2. The method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment according to claim 1, characterized in that, In S4, the heating temperature is 380℃~440℃, and the holding time is 1h~8h.

3. The method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment according to claim 2, characterized in that, In S4, the heating temperature is 415℃ and the holding time is 2 hours.

4. The method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment according to claim 2, characterized in that, In S3, the heat homogenization treatment temperature is 250℃~350℃, and the holding time is 1h~7h.

5. The method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment according to claim 4, characterized in that, In S3, the heat homogenization treatment temperature is 330℃ and the holding time is 4h.

6. A method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, as described in claim 2 or 4, characterized in that... The magnesium-lithium alloy contains, by mass percentage, 2wt.%-5wt.% Li, 3wt.%-8wt.% Zn, 0.1wt.%-1.5wt.% Y, 0.1wt.%-2.0wt.% Ca, with the balance being Mg.

7. A method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, as described in claim 2 or 5, characterized in that... The metal foil is aluminum foil and / or copper foil; After the sample is covered with metal foil and filled with graphite powder, the sample has no contact with the external gas environment of the graphite powder.

8. A method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, as described in claim 2 or 5, characterized in that... In S2, during the sample encapsulation and protection process, before placing the encapsulated sample into a thermally conductive metal container, the encapsulated sample is first placed in a quartz tube for vacuum sealing, and then the vacuum-sealed sample is placed in the thermally conductive metal container.

9. A method for enhancing the corrosion resistance of magnesium-lithium alloys by inducing secondary phase reconstruction and solute redistribution through heat treatment, as described in claim 2 or 5, characterized in that... The Ca2Mg6Zn3 phase forms a continuously distributed network structure, and a uniformly distributed granular secondary phase is formed inside the matrix.

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