A magnesium alloy chemical oxidation process based on multi-step passivation film formation and a magnesium alloy structure

Abstract

The present application relates to a magnesium alloy chemical oxidation process based on multi-step passivation film forming and a magnesium alloy structure, the magnesium alloy structure comprising: a magnesium alloy base; a pre-passivation film layer formed on a target region of the magnesium alloy base; the target region comprising at least one of a sharp edge region, a microporous region and a slit region of the magnesium alloy base; a base passivation film layer formed on the surface of the magnesium alloy base and the pre-passivation film layer; a reinforced passivation film layer formed on the surface of the base passivation film layer; a self-repairing sealing layer filled in the micropores of each film layer and attached to the surface of the reinforced passivation film layer. The present application solves the defects of traditional magnesium alloy passivation film, such as easy peeling in a cooling liquid environment, swelling, local damage and protection failure after local damage, and poor film forming uniformity of complex parts.

Description

Technical Field

[0001] This application relates to the field of metal chemical oxidation technology, specifically to a magnesium alloy chemical oxidation process based on multi-step passivation film formation and a magnesium alloy structure. Background Technology

[0002] Magnesium alloys, as one of the lightest metallic structural materials currently used in engineering applications, possess advantages such as low density, high specific strength, good electromagnetic shielding performance, and ease of processing and forming, showing broad application prospects in aerospace, automotive, electronic communications, and medical devices. However, magnesium alloys are chemically reactive, with a low standard electrode potential. In humid environments, especially in coolant environments containing corrosive media such as chloride and sulfate ions, they are prone to electrochemical corrosion reactions, leading to pitting corrosion, uniform corrosion, and even stress corrosion cracking on the magnesium alloy surface. This severely affects the structural integrity and service life of the magnesium alloy matrix. Therefore, improving the corrosion resistance of magnesium alloys in complex corrosive environments has become a key technical issue in their engineering applications.

[0003] To improve the corrosion resistance of magnesium alloys, surface treatments are typically used to create a protective layer on their surface. Common surface treatment processes include chemical oxidation, anodizing, micro-arc oxidation, electroplating, electroless plating, and organic coatings. Compared to other processes, chemical oxidation is widely used in industry due to its relatively simple process, lower cost, and the self-healing ability of the film, especially chromate conversion coatings. However, chromate conversion coatings typically contain hexavalent chromium, which is highly toxic and environmentally polluting, and its use has been restricted or even banned by increasingly stringent environmental regulations. Against this backdrop, chromium-free passivation technology has gradually become an important development direction in the field of magnesium alloy chemical oxidation treatment.

[0004] Currently, existing chromium-free passivation systems mainly include phosphate-permanganate systems, molybdate systems, rare earth salt systems, silane systems, and fluorozirconate systems. While these chromium-free passivation methods can improve the corrosion resistance of magnesium alloys to some extent, in complex environments containing corrosive ions such as coolants, the passivation film formed by current passivation processes often contains micropores or microcracks, resulting in weak bonding with the magnesium alloy substrate. Under the action of coolants, it is prone to swelling, peeling, or chemical dissolution. Especially under alternating high and low temperatures or mechanical stress, the film is more likely to suffer local damage and detachment, leading to coolant penetration or diffusion into the magnesium alloy substrate. Therefore, it cannot provide long-term and reliable protection for the magnesium alloy substrate in a corrosive environment. Summary of the Invention

[0005] One objective of this invention is to provide a magnesium alloy structure based on a multi-step passivation film formation process using chemical oxidation, thereby addressing the shortcomings of existing magnesium alloy structures, such as easy peeling, swelling, and protective failure after localized damage in a coolant environment, as well as poor film uniformity in complex parts. A second objective is to provide a magnesium alloy chemical oxidation process based on multi-step passivation film formation, thereby solving the problems of uneven film thickness, micropores or microcracks, and weak interfacial adhesion between the existing oxidation process (which only involves one passivation step) and the magnesium alloy substrate.

[0006] To achieve the above objectives, the first aspect of this invention proposes a magnesium alloy structure based on a multi-step passivation film formation process using chemical oxidation, and the technical solution adopted is as follows: A magnesium alloy structure based on multi-step passivation film formation using a chemical oxidation process, comprising: Magnesium alloy matrix; A pre-passivation film is formed in a target area of ​​the magnesium alloy substrate; the target area includes at least one of the sharp edge area, microporous area and slit area of ​​the magnesium alloy substrate. A basic passivation film is formed on the surface of the magnesium alloy substrate and the pre-passivation film; A reinforced passivation film layer is formed on the surface of the base passivation film layer; A self-healing sealing layer fills the micropores of each membrane layer and adheres to the surface of the reinforced passivation membrane layer.

[0007] Furthermore, the pre-passivation film layer includes a rare earth conversion film layer obtained by passivation with a pre-passivation liquid used in the pre-passivation step, the basic passivation film layer includes a cerium-based oxide passivation base layer obtained by passivation with a first passivation liquid used in the first main passivation step, and the enhanced passivation film layer includes a silane-zirconium-based chelate layer obtained by passivation with a second passivation liquid used in the second main passivation step.

[0008] Furthermore, the rare earth conversion film layer comprises hydrated cerium dioxide, cerium hydroxide, and magnesium hydroxide; the cerium-based oxide passivation layer comprises cerium dioxide, hydrated cerium dioxide, cerium malate complex, high cerium hydroxide, magnesium oxide, and magnesium hydroxide; and the silane-zirconium-based chelate layer comprises zirconium dioxide, hydrated zirconium dioxide, and zirconium oxyfluoride. The pre-passivation solution comprises cerium nitrate, citric acid, anhydrous ethanol, and deionized water; the first passivation solution comprises cerium nitrate, potassium permanganate, malic acid, and deionized water; and the second passivation solution comprises ammonium fluorozirconate, γ-aminopropyltriethoxysilane, disodium ethylenediaminetetraacetate, and deionized water.

[0009] Furthermore, the self-healing sealing layer is filled into the micropores of each membrane layer and attached to the surface of the reinforced passivation membrane layer by the sealing liquid treatment in the sealing step; The self-healing sealing layer comprises nano-silica, hydrated cerium dioxide, and cerium hydroxide. The sealing liquid comprises cerium nitrate, nano silica sol, phytic acid, polyvinyl alcohol, and deionized water.

[0010] The second aspect of this invention provides a multi-step passivation film formation process for magnesium alloys, used to prepare magnesium alloy structures as provided in the first aspect of this invention, comprising the following steps: Pre-treatment of the magnesium alloy matrix; The pretreated magnesium alloy substrate is subjected to pre-passivation treatment to generate a pre-passivation film in the target area of ​​the magnesium alloy substrate. The magnesium alloy substrate with the pre-passivation film layer is subjected to a first-step main passivation treatment to generate a basic passivation film layer in situ on the surface of the magnesium alloy substrate and the pre-passivation film layer. The magnesium alloy substrate after the first main passivation treatment is subjected to a second main passivation treatment to generate a reinforced passivation film in situ on the surface of the basic passivation film, thereby obtaining a magnesium alloy with a gradient composite passivation film.

[0011] Furthermore, during the pre-passivation process, the pre-treated magnesium alloy substrate is immersed in a pre-passivation solution, and a directional airflow is applied to directionally purge the target area of ​​the magnesium alloy substrate so that the pre-passivation solution can fully penetrate into the target area of ​​the magnesium alloy substrate, thereby obtaining a rare earth conversion film layer on the magnesium alloy substrate. The target area includes at least one of the following: a sharp edge area, a microporous area, and a slit area.

[0012] Furthermore, the pre-passivation solution comprises: 3-5 g / L cerium nitrate, 1-2 g / L citric acid, 50-80 ml / L anhydrous ethanol, and the balance being deionized water.

[0013] Furthermore, in the first step of the main passivation process, the magnesium alloy substrate with the pre-passivation film layer is placed in the first passivation solution and oxidized to obtain a cerium-based oxide passivation base layer. In the second main passivation process, the magnesium alloy substrate after the first main passivation is directly transferred to the second passivation solution and low-frequency ultrasonic-assisted oxidation is applied to obtain a silane-zirconium chelate layer.

[0014] Furthermore, the first passivation solution comprises: 15-20 g / L cerium nitrate, 2-3 g / L potassium permanganate, 3-5 g / L malic acid, with the remainder being deionized water; The second passivation solution comprises: ammonium fluorozirconate 8-12 g / L, γ-aminopropyltriethoxysilane 5-8 g / L, disodium ethylenediaminetetraacetate 1-2 g / L, and the balance being deionized water.

[0015] Furthermore, after obtaining the gradient composite passivation film, the magnesium alloy substrate with the gradient composite passivation film is transferred to a sealing liquid for sealing treatment, thereby generating a self-healing sealing layer inside the micropores of each film layer and on the surface of the reinforced passivation film layer located on the outside.

[0016] Furthermore, the sealing liquid comprises: 2-3 g / L cerium nitrate, 5-8 g / L nano silica sol, 1-2 g / L phytic acid, 0.5-1 g / L polyvinyl alcohol, with the balance being deionized water.

[0017] Furthermore, the magnesium alloy substrate after the sealing treatment is carried out with post-treatment, which includes hot water cleaning, gradient drying and hanging treatment.

[0018] Furthermore, the preprocessing includes: The magnesium alloy substrate is cut at the edges and corners and then hung on. The magnesium alloy substrate is first degreased using a first degreasing solvent. The magnesium alloy substrate after the first degreasing is degreased a second time using a second degreasing solvent.

[0019] Furthermore, the pre-passivated magnesium alloy matrix is ​​subjected to activation treatment and alkaline etching treatment; wherein, The activation process includes: placing the pre-passivated magnesium alloy substrate in an activation solution for activation, and then performing room temperature ultrasonic water washing on the activated magnesium alloy substrate under oxygen-free conditions. The alkaline etching process includes placing the activated magnesium alloy substrate in an alkaline etching solution for alkaline etching.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: The magnesium alloy structure provided by this invention features a pre-passivation film preferentially distributed in stress concentration areas or areas prone to uneven film formation in magnesium alloy parts, avoiding the problems of local discontinuity and unevenness in film formation that occur at uneven interfaces in traditional magnesium alloys. The basic passivation film is formed in situ on the surface of the magnesium alloy substrate and the pre-passivation film, covering the entire surface of the magnesium alloy substrate and encapsulating the pre-passivation film. The reinforced passivation film is formed in situ on the surface of the basic passivation film, used to cover or seal micropores and defect channels in the basic passivation film, forming a dense and continuously transitioning basic-reinforced composite structural layer with strong interfacial bonding on the outside of the magnesium alloy substrate. As the main protective layer, the basic passivation film provides high structural stability and primary barrier function, thereby improving the stability of the film under thermal cycling or mechanical disturbance conditions. The reinforced passivation film further densifies micropores, microcracks, and other defect channels in the basic passivation film, improving overall barrier performance. Meanwhile, a gradual transition interface is formed between the base-reinforced composite passivation film layers, thereby improving the interlayer bonding strength. This effectively enhances the structural continuity, interfacial bonding force, and barrier capacity of the resulting film system, thus significantly improving the corrosion resistance and service reliability of magnesium alloys in coolant corrosive environments.

[0021] Compared to the traditional one-step passivation process, the process provided by this invention solves the industry problems of film cracking at sharp edges and uneven film formation in deep holes for complex magnesium alloy parts in engine cooling systems through pre-passivation modification. This allows for control of the film thickness uniformity in complex parts, improves the batch production qualification rate, and has extremely high engineering and large-scale application value. Then, a two-step gradient main passivation film formation process is used to construct a tightly integrated double-layer gradient passivation film structure, overcoming the defects of weak interfacial bonding and poor water resistance in traditional one-step passivation films. The basic passivation film layer, as the main protective layer, provides high structural stability and primary barrier function, thereby improving the stability of the film layer under thermal cycling or mechanical disturbance conditions. The reinforced passivation film layer can further densify the micropores, microcracks, and other defect channels in the basic passivation film layer, improving the overall barrier performance. Meanwhile, a gradual transition interface is formed between the base-reinforced composite passivation film layers, thereby improving the interlayer bonding strength. This effectively enhances the structural continuity, interfacial bonding force, and barrier capacity of the resulting film system, thus significantly improving the corrosion resistance and service reliability of magnesium alloys in coolant corrosive environments. Attached Figure Description

[0022] To more clearly illustrate the technical solution of this application, the drawings used in the description of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic cross-sectional view of a magnesium alloy chemical oxidation process based on multi-step passivation film formation. Figure 2 The flowchart shows the steps of a multi-step passivation film formation process for magnesium alloy chemical oxidation. Figure 3 This is a photograph of the bare magnesium alloy specimen. Figure 4 This is a photograph of the bare material of the magnesium alloy test piece bundle; Figure 5 This is a photograph of the passivated magnesium alloy sample bundle. Figure 6 Microstructure characterization of the passivation film; Figure 7 These are photographs of magnesium alloy specimens from the bare material comparison group and the passivation test group before the experiment. Figure 8 The images show the bare material comparison group and passivation test group of magnesium alloy test pieces before the test. Figure 9 The images show the physical bundles of magnesium alloy test pieces in the bare material comparison group and the passivation test group after the experiment. Figure 10 These are physical images of magnesium alloy specimens from the bare material comparison group and the passivation test group after the experiment. Figure 11 These are physical images of magnesium alloy test pieces from the bare material comparison group and the passivation test group after the test and cleaning.

[0024] 1. Magnesium alloy substrate; 2. Pre-passivation film layer; 3. Basic passivation film layer; 4. Reinforced passivation film layer; 5. Self-healing sealing layer. Detailed Implementation

[0025] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0026] It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0027] To better understand the technical solution of this application, the chemical oxidation treatment process for magnesium alloys will now be further explained: In existing magnesium alloy surface treatment technologies, chemical oxidation (or chemical conversion) treatment typically involves pretreatment, passivation film formation, and post-treatment. Pretreatment generally includes degreasing, descaling, and activation to remove surface contaminants and enhance reactivity. Subsequently, a passivation film is formed on the magnesium alloy surface through chemical oxidation or conversion. Finally, post-treatment such as washing or drying completes the construction of a corrosion-resistant protective layer on the magnesium alloy surface. In these common processes, each step is completed in a single operation, especially the passivation film formation process, which is uniformly completed in one step using a single reaction system—that is, the passivation film layer is formed through a single immersion or reaction. While the resulting protective layer can improve the corrosion resistance of the substrate to some extent in normal environments, its limitations gradually become apparent in practical applications, especially under the long-term action of complex corrosive media such as coolants.

[0028] On the one hand, the interfacial bonding between the film and the substrate mainly relies on transient reactions for formation. Its internal structure is often relatively loose, containing micropores, microcracks, or defect channels. Furthermore, the interfacial bonding with the magnesium alloy substrate 1 is weak, restricting the film's adhesion and service reliability. Especially under the influence of media containing corrosive ions, such as coolants, the corrosive medium can penetrate into the magnesium alloy substrate 1 along the defect channels, initiating localized corrosion that gradually expands, leading to swelling, localized detachment, or even overall failure of the film. Consequently, when subjected to temperature alternation or mechanical stress, or in long-term fluid scouring or thermal cycling environments, the weak interfacial bonding of this film makes it more prone to interfacial failure, further causing localized damage to rapidly evolve into overall failure. On the other hand, because the film is formed on the substrate surface in a short time during the film formation process in the one-time film formation method, the resulting film has non-uniformity in thickness, composition and structure. Especially in the corners, pores or local stress concentration areas of complex structural components (such as magnesium alloys with complex structures in engine cooling systems), it is difficult to be fully modified and passivated, which easily forms weak areas. These areas are prone to corrosion during subsequent service, thus affecting the overall protection performance.

[0029] For the reasons mentioned above, existing chemical oxidation treatment methods for magnesium alloys still struggle to simultaneously meet the requirements of film density, structural stability, interfacial adhesion, and film uniformity in complex structural components under coolant corrosive environments, making it difficult to achieve long-term stable protection. Therefore, it is necessary to improve existing chemical oxidation treatment methods to obtain a magnesium alloy structure that enhances film integrity, density, and interfacial adhesion, providing stable and durable protection in coolant corrosive environments, while also considering process cost and feasibility, to meet the corrosion resistance and engineering application reliability requirements of magnesium alloys in coolant corrosive environments.

[0030] To achieve the above objectives, the present invention provides a chemical oxidation treatment method for magnesium alloys. This method optimizes traditional chemical oxidation treatment by finely controlling the film structure through staged processing during the film formation process. Under the premise of ensuring process feasibility, the resulting film layer is effectively improved in terms of interface state, microstructure, and film uniformity, thereby forming a protective film layer with good structural integrity and interface stability on the surface of magnesium alloys, thus improving the overall protective performance.

[0031] Based on the above technical requirements, the technical solution of this application will be further described below with reference to specific embodiments.

[0032] Firstly, such as Figure 1 The diagram shows a cross-sectional view of a magnesium alloy structure formed by multi-step passivation film formation using a chemical oxidation process. One objective of this invention is to provide a magnesium alloy structure formed by multi-step passivation film formation using a chemical oxidation process, comprising: a magnesium alloy substrate 1; a pre-passivation film layer 2, formed on a target region of the magnesium alloy substrate 1; the target region includes at least one of a sharp edge region, a microporous region, and a slit region of the magnesium alloy substrate 1; a base passivation film layer 3, formed in situ on the surface of the magnesium alloy substrate 1 and the pre-passivation film layer 2; and a reinforcing passivation film layer 4, formed in situ on the surface of the base passivation film layer 3.

[0033] In one embodiment of the present invention, a magnesium alloy structure based on a multi-step passivation film formation process using chemical oxidation is provided. This structure includes at least a magnesium alloy substrate 1 and a multi-layer passivation film system sequentially formed on its surface. The magnesium alloy substrate 1 can be a common cast magnesium alloy or a wrought magnesium alloy. For example, vehicle components such as electric drive system housings, cooling water channels, water pump housings, motor end covers, electronic control housings, heat dissipation module structural components, and lightweight support components can be selected as the magnesium alloy substrate 1. Of course, the magnesium alloy substrate 1 can also be applied to other fields such as aircraft components, electronic components, military components, and medical device components. The surface of the magnesium alloy substrate 1 can be a machined surface or an activated surface after sandblasting or chemical pretreatment. Its surface morphology can include flat areas and geometrically abrupt areas with sharp edges, micropores, and slits.

[0034] The pre-passivation film 2 is formed in the target area of ​​the magnesium alloy substrate 1. The target area is a region on the surface of the magnesium alloy substrate 1 that is relatively unfavorable for uniform film formation or prone to defects during the film formation process, including but not limited to at least one of sharp edge regions, micropore regions, and slit regions. For example, sharp edge regions can be edges formed during processing, areas with insufficient chamfering, or edge regions with large local curvature; micropore regions can be pores, shrinkage cavities, or surface open holes formed during casting; slit regions can be assembly gaps, overlapping parts, or structural gaps, etc. Therefore, by pre-passivating the sharp edges, micropores, slits, and other areas of the part that are prone to stress concentration and uneven film formation, a pre-passivation film 2 is formed in the above-mentioned areas, which can avoid defects such as film cracking and insufficient film formation in deep holes after subsequent film formation, and significantly improve the film uniformity of complex structural parts.

[0035] In some embodiments, the pre-passivation film 2 can be a relatively thin pre-conversion film, preferably thinner than the base passivation film 3 or the reinforced passivation film 4 formed in the subsequent main passivation step. It is used to preferentially modify or fill the target area, allowing the pre-passivation film 2 to form a continuous or semi-continuous coverage structure in the target area, thereby improving the local surface condition and enhancing the overall film uniformity. Specifically, the pre-treated magnesium alloy parts are immersed in a pre-passivation modification solution. During the immersion process, directional airflow is used to purge target areas such as deep holes, slits, and complex internal cavities in the part structure to promote the renewal and flow of the modification solution within these target areas. This preferentially forms the pre-passivation film 2 in stress concentration areas or areas prone to uneven film formation in the magnesium alloy parts, thereby improving the uniformity and adhesion stability of the passivation film obtained in subsequent forming, and overall enhancing the film formation consistency and structural integrity of the complex magnesium alloy substrate 1.

[0036] In this embodiment, a gradient base-reinforcement composite passivation film can be formed on the magnesium alloy substrate 1. The base passivation film 3 is formed in situ on the surface of the magnesium alloy substrate 1 and the pre-passivation film 2. The base passivation film 3 can be a dense inner layer structure, and its thickness can be controlled according to the process conditions to achieve basic barrier protection for the magnesium alloy substrate 1. In the main passivation step, in areas not covered by the pre-passivation film 2, the base passivation film 3 directly undergoes an in-situ growth reaction on the surface of the magnesium alloy substrate 1; while in areas where the pre-passivation film 2 has been formed, the base passivation film 3 continues to undergo an in-situ growth reaction on the surface of the pre-passivation film 2, thereby achieving continuous overall coverage of the surface of the magnesium alloy substrate 1. Because it is bonded to the magnesium alloy substrate 1 through the pre-passivation film 2, the base passivation film 3 is improved in terms of overall coverage and uniformity, thus achieving the purpose of improving basic barrier protection.

[0037] The enhanced passivation film 4 is formed in situ on the surface of the base passivation film 3, and is used to further optimize the structure of the base passivation film 3. The thickness of the enhanced passivation film 4 can be greater than, less than or close to the thickness of the base passivation film 3, depending on the process parameter settings. This enhanced passivation film 4 can form a gradual transition interface with the base passivation film 3, so that the film layer as a whole exhibits gradient structural characteristics, making the film layer more compact, thereby reducing interface stress concentration and improving the overall structural stability.

[0038] The specific composition, thickness, and structural form of each of the above-mentioned film layers can be adjusted according to different application requirements. For example, this can be achieved by changing the composition of the passivation liquid system, reaction time, temperature, or processing sequence, thereby obtaining a multi-layer continuous protective structure for magnesium alloy surfaces with different performance focuses without departing from the core concept of this invention.

[0039] In terms of structure, the pre-passivation film 2 is preferentially distributed in the target area. The basic passivation film 3 is formed in situ on the surface of the magnesium alloy substrate 1 and the pre-passivation film 2, covering the entire surface of the magnesium alloy substrate 1 and encapsulating the pre-passivation film 2, thus forming a uniformly covered inorganic oxide protective layer. The reinforced passivation film 4 is formed in situ on the surface of the basic passivation film 3, used to cover or seal the micropores and defect channels in the basic passivation film 3, forming a dense and continuously transitioning composite structure layer with strong interfacial bonding on the outside of the magnesium alloy substrate 1. Overall, it is approximately formed as a multi-layer composite structure of "substrate-pre-passivation film 2-basic passivation film 3-reinforced passivation film 4" from the inside to the outside. Therefore, since the basic-reinforced composite passivation film is grown in situ on the magnesium alloy substrate 1 after the pre-passivation film 2 is modified, the problems of local discontinuity and unevenness in film formation that occur at the uneven interface of traditional magnesium alloys are avoided. The basic passivation film layer 3, serving as the main protective layer, provides high structural stability and primary barrier properties, thereby enhancing the stability of the film layer under thermal cycling or mechanical disturbance conditions. The reinforced passivation film layer 4 is formed in situ on the surface of the basic passivation film layer 3, creating a basic-reinforced composite passivation film layer. This further passivates micropores, microcracks, and other defect channels in the basic passivation film layer 3, significantly reducing the penetration path of corrosive media and improving overall barrier performance. Simultaneously, a gradual transition interface is formed between the basic and reinforced composite passivation film layers, improving interlayer bonding strength. This effectively enhances the structural continuity, interfacial bonding strength, and barrier capability of the resulting film system, significantly improving the corrosion resistance and service reliability of the magnesium alloy in coolant corrosive environments.

[0040] The enhanced passivation film layer 4 can also be further combined with an external sealing layer, such as forming an organic / inorganic composite sealing layer or a functional coating, to further improve the barrier performance or environmental adaptability of the film layer.

[0041] In a preferred embodiment, the pre-passivation film layer 2 comprises a rare earth conversion film layer obtained by passivation with the pre-passivation solution used in the pre-passivation step; the base passivation film layer 3 comprises a cerium-based oxide passivation base layer obtained by passivation with the first passivation solution used in the first main passivation step; and the reinforced passivation film layer 4 comprises a silane-zirconium-based chelate layer obtained by passivation with the second passivation solution used in the second main passivation step. The rare earth conversion film layer comprises hydrated cerium dioxide, cerium hydroxide, and magnesium hydroxide; the cerium-based oxide passivation base layer comprises cerium dioxide, hydrated cerium dioxide, cerium malate complex, high cerium hydroxide, magnesium oxide, and magnesium hydroxide; and the silane-zirconium-based chelate layer comprises zirconium dioxide, hydrated zirconium dioxide, and zirconium oxyfluoride.

[0042] In this embodiment, the pre-passivation film layer 2, the basic passivation film layer 3, and the enhanced passivation film layer 4 are formed in situ on the magnesium alloy substrate 1 through different passivation steps in the chemical oxidation process, thereby constructing a composite protective film layer system with gradient structure characteristics. The pre-passivation treatment, the first main passivation treatment, and the second main passivation treatment all belong to the chemical oxidation process, and their basic reaction mechanisms and passivation processes are roughly the same, so they will not be elaborated further. However, the three processes differ in reaction conditions and system composition, resulting in composite films that complement each other's functions and are combined in pairs.

[0043] Specifically, the pre-passivation film 2 is a rare earth conversion film formed on the surface of the magnesium alloy substrate 1 through a pre-passivation step. This rare earth conversion film serves as an initial interface control layer and may include rare earth conversion products generated by the reaction of rare earth ions with the magnesium alloy surface. The main components of the rare earth conversion film include cerium-based compounds, magnesium oxides, or hydroxides, with the cerium-based compounds existing in the form of cerium oxides or cerium hydroxides. For example, the rare earth conversion film may include hydrated cerium dioxide, cerium hydroxide, and magnesium hydroxide. Correspondingly, the pre-passivation solution used in the pre-passivation step may include cerium nitrate, citric acid, anhydrous ethanol, and deionized water. Through the synergistic effect of these components, the pre-passivation film 2 can preferentially form an ultra-thin and uniform rare earth conversion film in the non-uniform target area on the magnesium alloy surface.

[0044] Specifically, the basic passivation film layer 3 is a cerium-based oxide passivation base layer formed in situ through the first main passivation step. This cerium-based oxide passivation base layer serves as a continuous and dense basic film layer, playing the most important protective role. For example, the main components of the basic passivation film layer 3 include cerium dioxide, hydrated cerium dioxide, cerium malate complex, cerium hydroxide, magnesium oxide, and magnesium hydroxide. Correspondingly, the first passivation solution used in the first main passivation step preferably includes cerium nitrate, potassium permanganate, malic acid, and deionized water. Through the synergistic effect of the above components, the basic passivation film layer 3 grows in situ on the surface of the pre-passivation film layer 2 and the magnesium alloy substrate 1, forming a continuous, uniform, and dense cerium-based oxide structural layer.

[0045] Specifically, the enhanced passivation film layer 4 is a silane-zirconium-based chelate layer that has undergone structural densification and interface sealing on the existing basic passivation film layer 3, in order to further improve the barrier performance of the film layer. For example, the main components of the silane-zirconium-based chelate layer include zirconium dioxide, hydrated zirconium dioxide, and zirconium oxyfluoride. Correspondingly, the second passivation solution used in the second main passivation step preferably includes ammonium fluorozirconate, γ-aminopropyltriethoxysilane, disodium ethylenediaminetetraacetate, and deionized water. Through the action of the above components, a silane-zirconium chelate layer is formed on the surface of the cerium-based passivation base layer in this step. Since the enhanced passivation film layer 4 is a further in-situ reaction on the surface of the basic passivation film layer 3, a dual-gradient passivation film structure combining the cerium-based dense base layer and the silane-zirconium chelate layer is formed. Compared with the traditional single-layer passivation film, on the one hand, the chemical bonding between the base layer and the subsequent sealing layer is achieved through the bridging effect of the silane coupling agent, forming a cross-interface bridge between the substrate, passivation film and sealing layer. The bonding force between the film layer and the substrate is increased by more than 40%. After immersion in 88℃ coolant for 336 hours, there is no swelling or interface peeling, which greatly improves the interfacial bonding force of the film layer and solves the core pain point of traditional passivation films being prone to failure in coolant. On the other hand, the zirconium-based component can further seal the micropores of the basic passivation film layer 3, while the hydrophobic silane groups can significantly improve the water resistance of the film layer and inhibit the penetration and swelling of coolant in the film layer.

[0046] In one embodiment of the present invention, a self-healing seal is formed in situ on the surface and inside of the film layer after treatment with a sealing liquid. The self-healing sealing layer 5 extends and fills the micropores and defect channels inside the pre-passivated film layer 2, the basic passivated film layer 3, and the reinforced passivated film layer 4. The main components of the self-healing sealing layer 5 include nano-silica, hydrated cerium dioxide, and cerium hydroxide. Correspondingly, the sealing liquid used for the sealing treatment includes cerium nitrate, nano-silica sol, phytic acid, polyvinyl alcohol, and deionized water. These components seal the magnesium alloy substrate 1 with the composite film layer through physical filling, chemical bonding, or coordination, thereby further improving the overall barrier performance. The nano-silica sol can quickly fill the micropores of the passivation film, achieving physical densification. Phytic acid can simultaneously form multi-toothed chelates with the magnesium substrate and the rare earth passivation film, further strengthening the interfacial bonding force of the film layer. It can also chelate chloride and sulfate ions in the coolant, achieving targeted fixation of the corrosive medium and inhibiting pitting corrosion. Rare earth cerium salts can rapidly deposit rare earth oxide protective films upon encountering water and hydroxide ions in the coolant when the film layer suffers mechanical damage, achieving rapid self-repair of the damaged area and preventing accelerated local corrosion. Therefore, through the synergistic effect of the above-mentioned multi-components, the triple action of phytic acid's targeted chelation of corrosion ions, nano-silica's pore filling, and rare earth cerium salt's self-repair mechanism, a comprehensive performance enhancement of the passivation film is achieved. Moreover, the film layer can complete self-repair within 24 hours in the coolant environment after damage, significantly enhancing its long-term protective performance.

[0047] Correspondingly, in a second aspect, the present invention provides a multi-step passivation film formation process for magnesium alloys, which can be used to prepare the magnesium alloy structure based on the multi-step passivation film formation process of chemical oxidation provided in the first aspect of the present invention, so as to improve its corrosion resistance and structural stability in corrosive environments, such as... Figure 2 The flowchart shown illustrates the steps of a multi-step passivation film formation process for magnesium alloy chemical oxidation, which includes at least the following steps: S1. Pre-treat the magnesium alloy matrix 1.

[0048] In this embodiment, the pretreatment of the magnesium alloy substrate 1 specifically includes: S11. Surface quality inspection: The sharp edges and corners of the selected magnesium alloy substrate 1 are rounded with a rounding radius of not less than 0.5 mm to eliminate stress concentration in sharp areas and prevent subsequent film layers from cracking and falling off at the edges and corners.

[0049] S12. Stainless steel coated hangers are used for clamping. The clamping position of the hangers is strictly limited to the position specified in the process design or the position where the process allowance is reserved for the part, so as to avoid contact with the working surface of the part, while ensuring that the part can be evenly contacted with each treatment liquid in subsequent processing. S13. Two-stage degreasing treatment: The first degreasing treatment is performed on the clamped parts using a first degreasing solvent. The magnesium alloy substrate 1 is immersed in the first degreasing solvent to remove minor surface oil, dust, and metal debris. The first degreasing solvent can include organic solvents or water-based cleaning agents. Acetone or anhydrous ethanol can be selected as organic solvents, and the immersion time is 3-5 minutes. Then, a second degreasing treatment is performed. The parts after the first degreasing are placed in a second degreasing solvent and immersed at 45-55℃ for 5-10 minutes. A magnesium alloy-specific degreasing agent with a concentration of 80g / L-100g / L (internal control concentration 85g / L-95g / L) can be prepared as the second degreasing solvent. The degreasing qualification standard is that after the second degreasing, the water film on the surface of the magnesium alloy substrate 1 remains continuous within 30 seconds, without cracking or water droplet coagulation.

[0050] S2. The pretreated magnesium alloy substrate 1 is subjected to pre-passivation treatment to generate a pre-passivation film layer 2 in the target area of ​​the magnesium alloy substrate 1.

[0051] The pretreated magnesium alloy substrate 1 is placed in a pre-passivation system for pre-passivation treatment to form a pre-passivation film layer 2 in the target area of ​​the magnesium alloy substrate 1. In some embodiments, the target area includes areas on the surface of the magnesium alloy substrate 1 that are prone to stress concentration and uneven film formation, such as sharp edge areas, micropore areas, and slit areas. By pre-passivating this target area, an ultra-thin rare earth conversion film is formed in advance, avoiding defects such as film cracking at the corners and insufficient film formation in deep holes caused by excessively rapid film formation during the subsequent main passivation process, thus significantly improving the film uniformity of complex structural parts.

[0052] Specifically, during the pre-passivation process, the pre-treated magnesium alloy substrate 1 is immersed in a pre-passivation solution to ensure full contact between the solution and the surface of the substrate. Simultaneously, during immersion, a directional airflow is applied to the substrate to directionally purge the target area, promoting the flow and deep penetration of the pre-passivation solution within this area. This improves the uniformity of the pre-passivation solution's wetting in micropores, slits, and complex structural areas. Through this method, the pre-passivation solution can penetrate more fully into the local structure of the target area of ​​the magnesium alloy substrate 1, where a preferential interfacial reaction occurs, pre-forming the pre-passivation film 2 on the surface of the substrate. The thickness of this pre-passivation film 2 can be controlled to be relatively thin, primarily used to adjust the interfacial uniformity in areas prone to uneven film formation, ensuring a more uniform subsequent main passivation reaction on the overall surface. This avoids film cracking in corner areas due to excessively rapid reaction, and also improves the problem of film defects caused by insufficient reaction in deep pores and slits.

[0053] In some embodiments, the application of directional airflow can be achieved through a controllable gas delivery device, whose airflow parameters include at least one or a combination of blowing velocity, injection direction, and air flow rate. For example, the blowing velocity can be adjusted according to the configuration of the magnesium alloy substrate 1 and the wetting state of the pre-passivation liquid to promote the directional flow and full filling of the pre-passivation liquid in complex areas, thereby improving the passivation film stability and uniformity of the target area.

[0054] It should be explained that in this embodiment, during the pre-passivation process, the pre-passivation film 2 is formed on the entire surface of the magnesium alloy substrate 1, exhibiting preferential growth characteristics in the target area. The magnesium alloy substrate 1 is immersed in the pre-passivation solution. Under the action of the pre-passivation solution, the entire surface of the magnesium alloy substrate 1 undergoes an interfacial reaction, thereby forming a continuous or nearly continuous pre-passivation film 2. Simultaneously, with the assistance of directional airflow, the target areas (such as sharp edge areas, microporous areas, and slit areas) have more sufficient solution contact conditions, resulting in a relatively higher growth rate of the pre-passivation film 2 in the target areas, thus making the film thickness in these areas greater than in other areas. This leads to the formation of a continuous and uniformly covering pre-passivation film 2 on the entire surface of the non-uniform and continuous magnesium alloy substrate 1, ensuring overall film continuity while improving the interfacial state of complex structural areas.

[0055] In some alternative embodiments, the pre-passivation film 2 can be selectively formed in the target area by locally controlling the application method of the pre-passivation solution. For example, methods such as local spraying, directional airflow carrying the solution, local wetting, or masking treatment can be used to directionally form the pre-passivation film 2 in the target area, while only a very thin film layer or no obvious film layer is formed in non-target areas.

[0056] In this step, the concentration of the prepassivation solution, reaction time, temperature, solution pH value, and stirring or airflow parameters can be controlled to control the film characteristics such as the thickness, coverage, distribution, and uniformity of the prepassivation film 2. This embodiment does not make specific limitations on these aspects.

[0057] S3. Perform the first step of main passivation treatment on the magnesium alloy substrate 1 with the pre-passivation film layer 2 to generate the basic passivation film layer 3 in situ on the surface of the magnesium alloy substrate 1 and the pre-passivation film layer 2.

[0058] The magnesium alloy substrate 1 with the pre-passivated film layer 2 is subjected to a first-step main passivation treatment to further generate a basic passivation film layer 3 on the surface of both the magnesium alloy substrate 1 and the pre-passivated film layer 2. In this step, the basic passivation film layer 3 grows and forms on a more uniform interface. Combined with the pre-passivation treatment step S2, for magnesium alloy substrate 1 selectively pre-passivated in the target area, it can be directly generated on the surface of the magnesium alloy substrate 1 not covered by the pre-passivated film layer 2, and simultaneously generated on the surface of the pre-passivated film layer 2, thereby achieving continuous overall coverage of the magnesium alloy substrate 1. For magnesium alloy substrate 1 preferentially pre-passivated in the target area, the surface of the pre-passivated film layer 2 is directly generated with continuous and uniform coverage, transforming the film layer from interface-controlled to a dense structure, achieving basic barrier protection for the substrate.

[0059] S4. The magnesium alloy substrate 1 after the first main passivation treatment is further subjected to the second main passivation treatment to generate a reinforced passivation film 4 in situ on the surface of the basic passivation film 3, so as to obtain a magnesium alloy with a gradient composite passivation film.

[0060] The magnesium alloy substrate 1, after the first main passivation treatment, undergoes a second main passivation treatment to generate a reinforced passivation film 4 in situ on the surface of the basic passivation film layer 3, ultimately resulting in a magnesium alloy structure with a gradient composite passivation film layer. In this step, the reinforced passivation film layer 4, as the outermost structure, can seal and strengthen the internal film structure, while simultaneously achieving chemical bonding between the film layer and the subsequent sealing layer.

[0061] In the first and second main passivation steps, the density and thickness uniformity of the basic passivation film layer 3 and the enhanced passivation film layer 4 can be controlled by adjusting the concentration of the main passivation solution, the reaction temperature, the reaction time, and the pH value of the solution, thereby controlling the interfacial bonding strength between the magnesium alloy substrate 1 and the film layer.

[0062] Through the above multi-step processing technology, the final passivation film system exhibits the characteristics of a multi-layer composite film structure consisting of a pre-passivation film layer 2, a basic passivation film layer 3, and a reinforced passivation film layer 4. Specifically, a continuous transitional basic-reinforcement gradient composite film layer is formed on the pre-passivation film layer 2, thereby significantly improving the interfacial bonding force, structural integrity, and film uniformity.

[0063] Therefore, compared to the traditional one-step passivation process, this invention addresses the industry challenges of film cracking at sharp edges and uneven film formation in deep holes for complex magnesium alloy parts in engine cooling systems through pre-passivation modification. This allows for control of film thickness uniformity in complex parts, improves batch production yield, and demonstrates high engineering and large-scale application value. Then, a two-step gradient main passivation film formation process is used to construct a tightly integrated double-layer gradient passivation film structure, overcoming the shortcomings of weak interfacial bonding and poor water resistance in traditional one-step passivation films.

[0064] In one embodiment of the present invention, the pre-passivation solution includes cerium nitrate, citric acid, an organic solvent, and deionized water. Cerium nitrate, as a rare earth ion source, is used to form a rare earth conversion film on the magnesium alloy surface; citric acid, as a complexing agent, is used to regulate the release rate of cerium ions and stabilize the solution system, thereby improving the film uniformity; anhydrous ethanol is used to reduce the surface tension of the solution and improve the wetting ability on the magnesium alloy surface, promoting the penetration of the pre-passivation solution into micropores and slits. Preferably, when preparing the pre-passivation modification solution, the mass concentration or volume concentration of each component in the pre-passivation solution is: cerium nitrate 3-5 g / L, citric acid 1-2 g / L, anhydrous ethanol 50-80 ml / L, and the balance is deionized water. The pretreated magnesium alloy substrate 1 is placed in the pre-passivation solution, the pH value of the solution is controlled at 4.0-4.5, and the wettation treatment is carried out at room temperature and low pressure for 1-2 minutes. During the wettation process, directional airflow is applied to the deep holes and slits of the part to ensure that the modification solution fully penetrates into the micropores. This step uses a low-concentration rare earth modification solution to pre-modify areas of the part that are prone to stress concentration and uneven film formation, such as sharp edges, micropores, and slits. An ultra-thin rare earth conversion film is pre-formed in these areas, avoiding defects such as film cracking at the corners and insufficient film formation in deep holes caused by excessively rapid film formation during the subsequent main passivation process, and significantly improving the film uniformity of complex structure parts.

[0065] Furthermore, step S3 includes: S31. In the first step of the main passivation treatment, the magnesium alloy substrate 1 with the pre-passivated film layer 2 is placed in the first passivation solution for chemical oxidation reaction, and a cerium-based oxide passivation base layer is generated in situ on the surface of the magnesium alloy substrate 1 and the surface of the pre-passivated film layer 2. The first passivation solution is prepared with the following mass concentration or volume concentration of each component: cerium nitrate 15-20 g / L, potassium permanganate 2-3 g / L, malic acid 3-5 g / L, and the balance being deionized water. Among them, cerium nitrate acts as a cerium source to participate in the construction of the oxide network structure; potassium permanganate acts as an oxidant to promote the oxidation reaction on the magnesium alloy surface and increase the film formation rate; malic acid acts as an organic complexing agent to regulate the release behavior of metal ions and stabilize the reaction process, thereby improving the uniformity and density of the film layer. The pre-passivated magnesium alloy substrate 1 is placed in the first passivation solution, and the pH value of the first passivation solution is controlled to be less than 3.5. It is immersed at room temperature for 1-2 minutes to grow a continuous and dense cerium-based oxide passivation base layer in situ on the surface of the magnesium alloy substrate 1, thereby achieving basic barrier protection for the substrate.

[0066] In some preferred embodiments, before step S31, the pre-passivated magnesium alloy substrate 1 can be activated and alkali-etched, and then the alkali-etched magnesium alloy substrate 1 can be placed in the first passivation solution.

[0067] Specifically, the activation treatment includes: Prepare an activation solution (phosphate-based activation system) with a concentration of 180-220 ml / L. Place the pre-passivated magnesium alloy substrate 1 into the activation solution and immerse it at room temperature for 3-5 minutes. Then, place the activated magnesium alloy substrate 1 into room temperature oxygen-free deionized water for ultrasonic washing for 2 minutes. The residual activator and reaction byproducts adhering to the surface of the workpiece are removed by ultrasonic cavitation. Nitrogen gas is introduced throughout the washing process to isolate oxygen and avoid secondary oxidation of the magnesium substrate between processes, thus ensuring the interfacial adhesion of the subsequent film formation.

[0068] Specifically, alkaline etching treatment includes: Prepare an alkaline etching solution with a concentration of 180-220 ml / L. Place the activated magnesium alloy substrate 1, which has been ultrasonically washed, into the alkaline etching solution and immerse it at a temperature of 75-85℃ for 1-2 minutes. The alkaline etching treatment further refines the microstructure of the magnesium alloy surface, removes surface oxide scale and micro-impurities, and works synergistically with the activation treatment to provide a uniform and active substrate for the subsequent growth of the gradient passivation film, while enhancing the adhesion between the film layer and the substrate.

[0069] Furthermore, step S4 includes: S41. In the second main passivation process, the magnesium alloy substrate 1 after the first main passivation is directly transferred to the second passivation solution, and low-frequency ultrasonic-assisted oxidation is applied to obtain a silane-zirconium-based chelate layer. The composition of the second passivation solution includes: ammonium fluorozirconate 8-12 g / L, γ-aminopropyltriethoxysilane 5-8 g / L, disodium ethylenediaminetetraacetate 1-2 g / L, and the balance is deionized water. The magnesium alloy substrate 1 after the first main passivation does not need to be washed with water and is directly transferred into the second passivation solution. The pH value of the second passivation solution is controlled at 5.0-6.0, and it is immersed at room temperature for 0.5-1 min. During the immersion process, 20-30 kHz low-frequency ultrasonic assistance is applied to generate a silane-zirconium-based chelate layer in situ on the surface of the basic passivation film layer 3, resulting in a double-layer gradient passivation film structure of "cerium-based dense base layer + silane-zirconium-based chelate layer". In this embodiment, on the one hand, the bridging effect of the silane coupling agent enables chemical bonding between the passivation film and the subsequent sealing layer, significantly improving the interfacial adhesion of the film and preventing interfacial peeling after immersion in coolant; on the other hand, the zirconium-based component can further seal the micropores of the substrate, while the hydrophobic silane groups can significantly improve the water resistance of the film and inhibit the penetration and swelling of coolant in the film.

[0070] Therefore, by performing a two-step gradient passivation film formation process on a magnesium alloy substrate 1 with a continuous and uniform interface using specific first and second passivation solutions, the defects of weak interfacial bonding and poor water resistance of traditional one-step passivation films are overcome. Through the chemical bonding effect of silane coupling agent, cross-interfacial bridging between the substrate, passivation film, and sealing layer is achieved, and the bonding force between the film layer and the substrate is increased by more than 40%. After immersion in 88℃ coolant for 336 hours, there is no swelling or interfacial peeling, which solves the core pain point of traditional passivation films being prone to failure in coolant.

[0071] Correspondingly, in some embodiments, after step S4, the following is further included: S5. The magnesium alloy substrate 1 with gradient composite passivation film is transferred to a sealing liquid for sealing treatment, and a self-healing sealing layer 5 is generated inside the micropores of each film layer and on the surface of the reinforced passivation film 4 located on the outside.

[0072] After completing the multi-step passivation treatment, the magnesium alloy substrate 1 with the gradient composite passivation film is transferred to a sealing liquid for sealing treatment, so as to further construct a self-healing sealing layer 5 inside and on the surface of each film structure. Specifically, the sealing treatment can be carried out by immersion, so that the sealing liquid is in full contact with the magnesium alloy substrate 1 and enters the micropores, microcracks and defect channels inside each passivation film under the action of capillary action and interfacial penetration, thereby achieving filling and repair inside the film. By strengthening the bridging effect of the silane coupling agent structure contained in the passivation film 4, the sealing layer simultaneously forms a continuous chemical bond layer on the surface of the strengthened passivation film 4, thereby significantly improving the interfacial bonding strength and overall structural stability of the entire film system (passivation film and sealing film). Thus, under long-term coolant immersion or fluid scouring conditions, the interfaces can still maintain high interfacial stability, effectively avoiding delamination, peeling or local failure of the interfaces (substrate and film interface, passivation film and sealing film interface) caused by insufficient interfacial bonding, thereby improving the overall corrosion resistance and service life.

[0073] Another improvement in this embodiment lies in the formation of the sealing layer. Step S5 can further establish a targeted sealing-self-healing sealing treatment, specifically including: preparing a targeted sealing-self-healing sealing solution, the composition of which is: 2-3 g / L cerium nitrate, 5-8 g / L nano silica sol, 1-2 g / L phytic acid, 0.5-1 g / L polyvinyl alcohol, with the balance being deionized water. The particle size of the nano silica sol can be selected between 20-50 nm, and the pH value of the sealing solution is controlled to be 6.0-7.5; the gradient passivated magnesium alloy substrate 1 is placed in the sealing solution and immersed at room temperature for 30-60 seconds to obtain the self-healing sealing layer 5. Unlike traditional physical filling sealing processes, in this embodiment, during the formation of the sealing layer, nano-silica sol can quickly fill the micropores of the passivation film, achieving physical densification; phytic acid can simultaneously form multi-toothed chelates with the magnesium substrate and rare earth passivation film, further strengthening the interfacial bonding of the film layer, and can also chelate chloride and sulfate ions in the coolant, achieving targeted fixation of the corrosive medium and inhibiting pitting corrosion; when the film layer is mechanically damaged, rare earth cerium salt can rapidly deposit upon contact with water and hydroxide ions in the coolant to form a rare earth oxide protective film, achieving rapid self-repair of the damaged area and preventing accelerated local corrosion.

[0074] Therefore, through the triple action of phytic acid's targeted ion chelation, nano-silica's pore filling, and rare earth cerium salt's self-repair mechanism, a comprehensive performance enhancement of the passivation film is achieved. Sealing can be completed at room temperature in 30-60 seconds, significantly improving production efficiency, and eliminating the need for high-temperature treatment, making it suitable for continuous batch production. Furthermore, the film can self-repair within 24 hours in a coolant environment after damage, significantly enhancing its long-term protective performance.

[0075] Following step S5, the following further includes: S6. The magnesium alloy substrate 1 after the sealing treatment is completed is subjected to post-treatment, which includes hot water cleaning, gradient drying and hanging treatment.

[0076] Furthermore, the post-processing of the magnesium alloy substrate 1 in step S6 specifically includes: 1. Hot water cleaning: Immerse the sealed magnesium alloy substrate 1 in oxygen-free hot water at 75-85℃ for 50-80 seconds to remove residual sealing liquid from the surface and avoid residual components affecting the appearance and performance of the film.

[0077] 2. Gradient drying: Place the parts washed with hot water into a tank-type insulated drying oven, pre-dry at 60-70℃ for 5-10 minutes, and then heat to 90-110℃ for 10-15 minutes. Gradient drying avoids the formation of micro-cracks caused by rapid evaporation of moisture in the film layer, thus ensuring the integrity of the film layer structure.

[0078] 3. Hanging and full inspection: After drying, operators wearing clean gloves conduct an initial inspection of the parts' appearance. After confirming that there are no defects such as film peeling, discoloration, or spots, the parts are hung up and handled gently to avoid damaging the film. After hanging, the parts are sorted and placed on a clean workbench for a comprehensive quality inspection. Only those that pass the inspection can be put into storage.

[0079] It should be noted that, for the sake of simplicity, the method embodiments are all described as a series of actions. However, those skilled in the art should understand that the embodiments of this application are not limited to the described order of actions, because according to the embodiments of this application, some steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also understand that the embodiments described in the specification are all preferred embodiments, and the actions involved are not necessarily required by the embodiments of this application.

[0080] Since the method already has the above-mentioned technical effects, the magnesium alloy structure prepared by this process should also have similar technical effects, so it will not be described in detail in the method embodiments.

[0081] The following description, in conjunction with the above embodiments and accompanying drawings, provides a detailed explanation of a specific technical solution of the present invention.

[0082] This embodiment provides a multi-step passivation film formation process for magnesium alloy chemical oxidation. Through pre-passivation modification, two-step gradient passivation film formation, and targeted sealing-self-healing treatment, a gradient composite film layer with strong interfacial adhesion, dense structure, and self-healing ability is formed on the magnesium alloy surface, improving its corrosion resistance and engineering adaptability in coolant environments. The specific implementation process is as follows: S101. Pretreatment of Magnesium Alloy Parts: Surface quality inspection of magnesium alloy parts is performed. Sharp edges and corners are rounded with a radius of 0.5 mm. The surface is wiped with anhydrous ethanol using a cotton cloth to remove metal debris. Stainless steel coated hangers are used for clamping, with the clamping position at the pre-reserved process hole of the test piece. Magnesium alloy parts are degreased twice. The first degreasing involves soaking in acetone solution at room temperature for 4 minutes to remove slight surface oil stains. The second degreasing involves preparing a 90 g / L magnesium alloy-specific degreasing agent, immersing the magnesium alloy parts in the solution, and soaking at 50°C for 8 minutes. After degreasing, the water film is verified to be continuous without breakage, thus completing the degreasing process.

[0083] S102, Pre-passivation modification of sharp edges / micropores of magnesium alloy parts; Preparation of pre-passivation solution: 4g / L cerium nitrate, 1.5g / L citric acid, 60ml / L anhydrous ethanol, with the remainder being deionized water, and pH adjusted to 4.2; Place the pretreated magnesium alloy parts into the pre-passivation solution and immerse them at room temperature for 1.5min, then apply directional airflow to the deep holes to ensure that the modification solution fully enters the holes.

[0084] S103. Activate the magnesium alloy parts and perform an oxygen-free ultrasonic water wash; prepare a 200ml / L phosphate-based activator solution, immerse the pre-passivated magnesium alloy parts in the activator solution, and soak at room temperature for 4 minutes; then transfer them to room temperature oxygen-free deionized water for ultrasonic water wash for 2 minutes. Nitrogen gas is introduced throughout the water wash process to control the dissolved oxygen in the water to ≤0.5mg / L.

[0085] S104. Perform alkaline etching treatment on magnesium alloys; prepare an alkaline etching solution of 200 ml / L, immerse the washed magnesium alloy parts in the alkaline etching solution, and soak them at 80°C for 1.5 min to further refine the surface microstructure and remove residual oxide scale.

[0086] S105. A two-step gradient passivation treatment is performed on the magnesium alloy to form a gradient composite passivation film in situ on the magnesium alloy surface. The first passivation solution is prepared with the following composition: 18 g / L cerium nitrate, 2.5 g / L potassium permanganate, 4 g / L malic acid, and the remainder being deionized water, with the pH adjusted to 3.0. The alkaline-etched magnesium alloy parts are immersed in the first passivation solution at room temperature for 1.5 min to grow a cerium-based passivation base layer in situ. The second passivation solution is prepared with the following composition: 10 g / L ammonium fluorozirconate, 6 g / L γ-aminopropyltriethoxysilane, 1.5 g / L disodium ethylenediaminetetraacetate, and the remainder being deionized water, with the pH adjusted to 5.5. The magnesium alloy parts with the base passivated layer are directly transferred to the second passivation solution and immersed at room temperature for 1 min. During the immersion process, 25 kHz low-frequency ultrasound is applied to assist in the preparation of the gradient passivation film.

[0087] S106. Use a targeted sealing-self-healing sealing solution to seal the passivation film on the magnesium alloy surface; prepare the sealing solution with the following composition: 2.5 g / L cerium nitrate, 6 g / L nano silica sol (particle size 30 nm), 1.5 g / L phytic acid, 0.8 g / L polyvinyl alcohol, and the balance being deionized water, with the pH value adjusted to 7.0; immerse the magnesium alloy parts after the two-step passivation in the sealing solution and soak them at room temperature for 45 seconds to fill the pores of the passivation film and impart self-healing ability to the film layer; S107, Post-processing of magnesium alloy parts; Hot water cleaning: Place the sealed sample in 80℃ oxygen-free hot water for 60s to remove residual sealing liquid from the surface; Gradient drying: First pre-dry at 65℃ for 8min, then heat to 100℃ for 12min to ensure the film layer is completely dry; Hanging inspection: After the initial inspection shows no defects, hang the sample and complete the full inspection to obtain the finished product sample.

[0088] To enable those skilled in the art to better understand the present invention, a comparative description of the magnesium alloy chemical oxidation process based on multi-step passivation film formation of the present invention is provided below.

[0089] Example: First, the typical magnesium alloy material used in engine cooling systems is processed into magnesium alloy test pieces. These test pieces are 50×25×7mm square plates with a 7mm diameter hole at the center. Surface quality is inspected, and sharp edges and corners are rounded. The surface of the magnesium alloy test piece bundle is wiped clean with an alcohol-soaked cotton cloth. The test pieces are then clamped, weighed, and connected using steel bolts and sleeves to form a bundle. See the attached image for a photograph of the bare magnesium alloy test pieces and bundle. Figure 3 and attached Figure 4 As shown.

[0090] Then, the magnesium alloy sample bundle is subjected to a series of treatments using steps S101-S107 above, forming a dense and stable gradient composite passivation film on its surface. This protects the magnesium alloy substrate 1, resisting corrosion from the engine coolant and thus improving the engine's service life. A photograph of the bare magnesium alloy sample bundle after passivation and sealing treatment is attached. Figure 5 As shown. During the film preparation process, the microstructure of the magnesium alloy sample bundles (samples after step S106) after multi-step passivation treatment and sealing treatment were characterized respectively. The characterization results are shown in the attached figure. Figure 6 As shown, by Figure 6 It can be seen that the passivation film on the surface of the magnesium alloy sample bundle formed by passivation is dense and continuous, with a few micropores; the composite film layer on the surface of the magnesium alloy sample bundle after sealing treatment is even denser, and the sealing liquid molecules fill the micropores of the passivation film.

[0091] Comparative example: In this comparative example, a set of magnesium alloy test specimens was reprocessed and prepared. This set of magnesium alloy test specimens underwent a one-step passivation process, i.e., step S102 and the second passivation liquid treatment in step S105 were omitted, while the remaining steps were the same. The magnesium alloy test specimens obtained by this treatment were used as the bare material comparison group. The magnesium alloy test specimens with a dense and stable passivation film on the surface prepared in the above examples were used as the passivation detection group. In this comparative example, the bare material comparison group and the passivation detection group were simultaneously subjected to coolant corrosion tests, and weighed before and after the coolant corrosion tests. The degree of corrosion was evaluated by the change in mass of the bare material comparison group and the passivation detection group before and after the coolant corrosion tests. Specifically: Before the test, the physical images of the magnesium alloy test pieces in the bare material comparison group and the passivation test group are attached. Figure 7 As shown, the magnesium alloy specimens in the bare material comparison group and the passivation test group were weighed and recorded before the test. After weighing, the magnesium alloy specimens were connected into magnesium alloy specimen bundles. See the attached diagram for actual images of the magnesium alloy specimen bundles in the bare material comparison group and the passivation test group. Figure 8As shown; then prepare two beakers, pour coolant into each beaker, and heat the coolant to 88°C. Immerse the magnesium alloy sample bundles from the bare material comparison group and the passivation test group into the two beakers respectively, and maintain for 336 hours; after 336 hours, remove the magnesium alloy sample bundles from the bare material comparison group and the passivation test group respectively. See attached figures for actual images of the magnesium alloy sample bundles from the bare material comparison group and the passivation test group after the test. Figure 9 As shown, the two sets of magnesium alloy test specimen bundles were disassembled separately. The actual images of the bare material comparison group and the passivation test group after disassembly are attached. Figure 10 As shown, magnesium alloy specimens were cleaned using a magnesium alloy-specific rust remover and ultrasonic vibration in a dispersed state. Photos of the cleaned bare material comparison group and the passivation test group of magnesium alloy specimens are attached. Figure 11 As shown, the surface condition of each magnesium alloy specimen was observed, and the weight was recorded after the test. The weight records of the bare material comparison group and the passivation test group during the above test process are shown in Table 1 below.

[0092] Table 1:

[0093] As shown in Table 1, the magnesium alloy specimens in the passivation test group, under the protection of a dense and stable multilayer composite passivation film, exhibited a reduced corrosion rate and improved resistance to coolant corrosion under long-term coolant corrosion.

[0094] Therefore, the above experiments demonstrate that the magnesium alloy prepared by the multi-step passivation film-forming magnesium alloy chemical oxidation process provided by this invention overcomes the defects of existing magnesium alloy parts, such as easy peeling, swelling, and failure of protection after local damage in the coolant environment, as well as poor film uniformity of complex parts.

[0095] The above embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention.

[0096] It should also be noted that, in this document, the terms "upper," "lower," "left," "right," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations, nor should they be construed as indicating or implying relative importance. Moreover, the term "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements, but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device.

Claims

1. A magnesium alloy structure based on multi-step passivation film formation via chemical oxidation process, characterized in that, include: Magnesium alloy matrix; A pre-passivation film is formed in the target area of ​​the magnesium alloy substrate; The target region includes at least one of the sharp edge region, microporous region, and slit region of the magnesium alloy substrate; A basic passivation film is formed on the surface of the magnesium alloy substrate and the pre-passivation film; A reinforced passivation film layer is formed on the surface of the base passivation film layer; A self-healing sealing layer fills the micropores of each membrane layer and adheres to the surface of the reinforced passivation membrane layer.

2. The magnesium alloy structure based on multi-step passivation film formation by chemical oxidation process according to claim 1, characterized in that, The pre-passivation film layer includes a rare earth conversion film layer, the basic passivation film layer includes a cerium-based oxide passivation base layer, and the enhanced passivation film layer includes a silane-zirconium-based chelate layer.

3. The magnesium alloy structure based on multi-step passivation film formation by chemical oxidation process according to claim 2, characterized in that, The rare earth conversion film layer comprises hydrated cerium dioxide, cerium hydroxide, and magnesium hydroxide; the cerium-based oxide passivation base layer comprises cerium dioxide, hydrated cerium dioxide, cerium malate complex, high cerium hydroxide, magnesium oxide, and magnesium hydroxide; the silane-zirconium-based chelate layer comprises zirconium dioxide, hydrated zirconium dioxide, and zirconium oxyfluoride.

4. The magnesium alloy structure based on multi-step passivation film formation by chemical oxidation process according to claim 1, characterized in that, The self-healing sealing layer is composed of nano-silica, hydrated cerium dioxide, and cerium hydroxide.

5. A chemical oxidation process for magnesium alloys based on multi-step passivation film formation, characterized in that, Includes the following steps: Pre-treatment of the magnesium alloy matrix; The pretreated magnesium alloy substrate is subjected to pre-passivation treatment to generate a pre-passivation film in the target area of ​​the magnesium alloy substrate. The magnesium alloy substrate with the pre-passivation film layer is subjected to a first-step main passivation treatment to generate a basic passivation film layer in situ on the surface of the magnesium alloy substrate and the pre-passivation film layer. The magnesium alloy substrate after the first main passivation treatment is subjected to a second main passivation treatment to generate a reinforced passivation film in situ on the surface of the basic passivation film, thereby obtaining a magnesium alloy with a gradient composite passivation film.

6. The magnesium alloy chemical oxidation process based on multi-step passivation film formation according to claim 5, characterized in that, During the pre-passivation process, the pre-treated magnesium alloy substrate is immersed in a pre-passivation solution, and a directional airflow is applied to directionally purge the target area of ​​the magnesium alloy substrate so that the pre-passivation solution can fully penetrate into the target area of ​​the magnesium alloy substrate, thereby obtaining a rare earth conversion film layer on the magnesium alloy substrate. The target area includes at least one of the following: a sharp edge area, a microporous area, and a slit area.

7. The magnesium alloy chemical oxidation process based on multi-step passivation film formation according to claim 6, characterized in that, The prepassivation solution comprises: 3-5 g / L cerium nitrate, 1-2 g / L citric acid, 50-80 ml / L anhydrous ethanol, and the remainder is deionized water.

8. The magnesium alloy chemical oxidation process based on multi-step passivation film formation according to claim 5, characterized in that, In the first step of the main passivation process, the magnesium alloy substrate with the pre-passivation film layer is placed in the first passivation solution and oxidized to obtain a cerium-based oxide passivation base layer. In the second main passivation process, the magnesium alloy substrate after the first main passivation is directly transferred to the second passivation solution and low-frequency ultrasonic-assisted oxidation is applied to obtain a silane-zirconium chelate layer.

9. A magnesium alloy chemical oxidation process based on multi-step passivation film formation according to claim 8, characterized in that, The first passivation solution comprises: 15-20 g / L cerium nitrate, 2-3 g / L potassium permanganate, 3-5 g / L malic acid, with the remainder being deionized water; The second passivation solution comprises: ammonium fluorozirconate 8-12 g / L, γ-aminopropyltriethoxysilane 5-8 g / L, disodium ethylenediaminetetraacetate 1-2 g / L, and the balance being deionized water.

10. A magnesium alloy chemical oxidation process based on multi-step passivation film formation according to claim 5, characterized in that, After obtaining the gradient composite passivation film, the magnesium alloy substrate with the gradient composite passivation film is transferred to a sealing liquid for sealing treatment, and a self-healing sealing layer is generated inside the micropores of each film layer and on the surface of the reinforced passivation film layer located on the outside.

11. The magnesium alloy chemical oxidation process based on multi-step passivation film formation according to claim 10, characterized in that, The sealing solution comprises: 2-3 g / L cerium nitrate, 5-8 g / L nano silica sol, 1-2 g / L phytic acid, 0.5-1 g / L polyvinyl alcohol, with the balance being deionized water.

12. The magnesium alloy chemical oxidation process based on multi-step passivation film formation according to claim 10, characterized in that, The magnesium alloy substrate after sealing is subjected to post-treatment, which includes hot water cleaning, gradient drying and hanging treatment.

13. A magnesium alloy chemical oxidation process based on multi-step passivation film formation according to any one of claims 5-12, characterized in that, The preprocessing includes: The magnesium alloy substrate is hung after the edges and corners are cut. The magnesium alloy substrate is first degreased using a first degreasing solvent. The magnesium alloy substrate after the first degreasing is degreased a second time using a second degreasing solvent.

14. The magnesium alloy chemical oxidation process based on multi-step passivation film formation according to claim 13, characterized in that, The pre-passivated magnesium alloy matrix is ​​then subjected to activation and alkaline etching treatment; wherein, The activation process includes: placing the pre-passivated magnesium alloy substrate in an activation solution for activation, and performing room temperature ultrasonic water washing on the activated magnesium alloy substrate under oxygen-free conditions. The alkaline etching process includes: placing the activated magnesium alloy substrate in an alkaline etching solution for alkaline etching.

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