A passivation solution for improving the compactness of a passivation layer on a magnesium alloy surface and a treatment process for forming a passivation layer on a magnesium alloy surface

Abstract

This application discloses a passivation solution for improving the density of a passivation layer on a magnesium alloy surface and a processing method for forming a passivation layer on the magnesium alloy surface. The passivation solution for improving the density of the passivation layer is a phosphate-manganese salt aqueous solution, containing phosphate film-forming components and manganese salt film-forming components, and also containing a densifying additive. The amount of the additive does not exceed 10% of the amount of phosphate or manganese salt added. The passivation solution can form a passivation layer with a thickness of 5μm to 10μm in situ on the magnesium alloy surface. This application solves the problems of insufficient corrosion resistance and insufficient density of the passivation layer in magnesium alloys.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy surface protection technology, specifically to a passivation liquid for improving the density of the passivation layer on the surface of magnesium alloys and a process for forming a passivation layer on the surface of magnesium alloys. Background Technology

[0002] Magnesium alloys are lightweight metallic structural materials with a density that is only 2 / 3 that of aluminum alloys and 1 / 4 that of steel.

[0003] In addition, magnesium alloys have advantages such as excellent die-casting performance, high specific strength and specific stiffness, good damping, excellent shielding and thermal conductivity, good formability and easy recycling. They are known as "green engineering materials of the 21st century" and have extremely broad application prospects in aerospace, automotive industry, 3C electronics, rail transportation, medical devices and other fields.

[0004] Magnesium has an extremely low standard electrode potential (-2.37V) and is highly chemically reactive. It is prone to electrochemical corrosion in natural environments and service conditions. The oxide film that naturally forms on its surface is loose, porous, and incomplete in structure, and cannot form a continuous and effective barrier protection for the substrate. This poor corrosion resistance has become a key bottleneck restricting the large-scale engineering application of magnesium alloys.

[0005] This limits the scale of its application.

[0006] To address the corrosion protection problem of magnesium alloys, the industry has developed various surface protection technologies, which can be mainly divided into two categories: one is to directly generate a dense chemical conversion protective coating on the surface of the magnesium alloy substrate through chemical reaction. According to the composition of the film, it can be divided into chromate conversion film, phosphate conversion film, molybdate conversion film, etc.; the other is to directly coat the magnesium alloy surface with an organic coating through the cross-linking reaction of organic materials. After the coating cures, a relatively thick organic coating is formed directly on the magnesium alloy surface. Currently, commonly used organic coatings include epoxy resin, etc.

[0007] Compared with organic coatings, inorganic chemical conversion membranes have the advantages of low raw material cost, simple preparation process, strong adhesion between membrane and substrate, customizable adjustment, reusable bath solution, and stable anti-corrosion effect, and have been widely used in the industrial field.

[0008] Among the various preparation processes of chemical conversion films, compared with micro-arc oxidation, anodic oxidation, electroplating and other technologies, passivation process has low requirements for production equipment, short process flow, strong adaptability to mass production and low overall cost. It is currently the most widely used core process in the pretreatment of magnesium alloy surface corrosion protection.

[0009] Among various passivation systems, chromate passivation films once dominated the market due to their excellent corrosion resistance and mature technology. However, hexavalent chromium is highly carcinogenic and is a strictly controlled persistent environmental pollutant. Its production and application have been strictly restricted by environmental regulations in many countries around the world, and the cost of waste liquid treatment remains high, so it is gradually being phased out by the industry.

[0010] Against this backdrop, phosphate passivation systems have become one of the core research and development directions to replace chromate systems due to their environmental friendliness, good anti-corrosion effect, and readily available raw materials. Among them, manganese phosphate passivation films have the advantages of good insulation, high hardness, and strong adhesion to the substrate, showing outstanding application potential in the field of magnesium alloy protection.

[0011] However, existing conventional manganese phosphate passivation technology still has obvious technical defects: during the film formation process of traditional phosphate conversion membranes, the magnesium alloy substrate will undergo dissolution and hydrogen evolution reaction in the acidic passivation solution, resulting in drastic fluctuations in the pH value at the interface between the substrate and the solution. It is impossible to achieve precise control and dynamic balance between the dissolution rate of magnesium alloy and the deposition rate of manganese phosphate. The resulting passivation film generally has a large number of microcracks, pinholes and pores, and the film density is seriously insufficient.

[0012] Corrosive media can easily penetrate into the magnesium alloy substrate through the defective channels of the film, causing the film to peel off and the substrate to corrode. This results in a significant decrease in the long-term corrosion resistance of the passivation film, which cannot meet the protection requirements of magnesium alloys in complex service environments.

[0013] To improve the density and corrosion resistance of phosphate passivation films and reduce conversion film cracking, invention patent application CN103205741 discloses a magnesium alloy passivation solution and a magnesium alloy surface treatment method, wherein the core film-forming component of the passivation solution is Cr. 3+ Co 2+ and SiO3 2- Although it reduces the porosity of the membrane to a certain extent, the effect of improving corrosion resistance is limited. It still needs to be followed up with sealing or electrophoresis treatment to meet the usage requirements. At the same time, the introduction of a large amount of heavy metals in the treatment solution still poses the problem of environmental pollution and ecological harm, and cannot achieve truly green and environmentally friendly production.

[0014] In addition, some modification schemes regulate the film formation process by adding rare earth metal salts, which can optimize the film structure, but the high price and poor supply stability of rare earth raw materials significantly increase production costs and are not conducive to large-scale industrial application.

[0015] Therefore, developing a passivation liquid additive and surface treatment method that is simple in formulation, easy to obtain raw materials, low in overall cost, environmentally friendly, and can improve the density of the passivation layer of magnesium alloy from the root of the film formation mechanism, while significantly improving the corrosion resistance of the film layer, is of vital practical significance and industrial value for breaking through the bottleneck of magnesium alloy anti-corrosion technology and promoting the large-scale engineering application of magnesium alloy materials. Summary of the Invention

[0016] In order to overcome the shortcomings of the prior art, the purpose of this application is to provide a passivation liquid that improves the density of the passivation layer on the surface of magnesium alloys and solves the problems of insufficient corrosion resistance and insufficient density of the passivation layer in magnesium alloys.

[0017] To solve the above problems, the technical solution adopted in this application is as follows: This application provides a passivation solution for improving the density of the passivation layer on the surface of a magnesium alloy. It is a phosphate-manganese salt aqueous solution containing phosphate film-forming components and manganese salt film-forming components, and also contains a densifying additive. The amount of the densifying additive added does not exceed 10% of the sum of the amounts of phosphate and manganese salt added. The passivation solution can form a passivation layer with a thickness of 5μm to 10μm on the surface of the magnesium alloy in situ.

[0018] As a further preferred embodiment, the passivation solution described in this application comprises the following components, based on mass concentration: Phosphate 5g / L~50g / L Manganese salt 5g / L~50g / L Densification additives 0.1g / L~5g / L The remainder is purified water; When using the passivation solution, the pH value should be adjusted to 2.8~3.5 and the temperature to 50~70℃.

[0019] As a further preferred embodiment, the phosphate in the embodiments of this application is one or a mixture of two or more of tripotassium phosphate, disodium hydrogen phosphate, and ammonium dihydrogen phosphate; the manganese salt is one or a mixture of two or more of manganese chloride, manganese sulfate, manganese acetate, and manganese formate.

[0020] As a further preferred embodiment, the densifying additive described in the embodiments of this application is a weakly basic amine or a complex of a weakly basic amine and a weakly acidic salt; the weakly basic amine is one or a mixture of two or more of monoethanolamine, diethanolamine, and triethanolamine, and the weakly acidic salt is one or a mixture of two of molybdate and stannate.

[0021] As a further preferred embodiment, in this application, phosphoric acid or sodium hydroxide is used to adjust the pH value of the passivation solution.

[0022] This application also provides a process for forming a passivation layer on the surface of a magnesium alloy, including the following steps. Degreasing: Immerse the magnesium alloy to be treated in an alkaline degreasing solution for 5-10 minutes to remove oil, dust and mold release agent impurities from the surface of the magnesium alloy. Pickling: Immerse the degreased magnesium alloy in the pickling solution for 30-90 seconds to remove the original oxide layer on the surface of the magnesium alloy and activate the substrate surface; Surface conditioning: Immerse the pickled magnesium alloy in the surface conditioning solution for 2-5 minutes to homogenize the surface condition of the magnesium alloy. Passivation: The surface-conditioned magnesium alloy is immersed in the passivation solution described in any one of claims 1-5 and soaked at 50-70°C for 5-10 minutes to form a passivation layer in situ on the surface of the magnesium alloy. Drying: Place the passivated magnesium alloy in an environment of 30~100℃ and dry it with hot air for 5~15 minutes to complete the preparation of the passivation layer.

[0023] As a further preferred embodiment, the pickling solution described in this application is an aqueous solution prepared by mixing one or more of acetic acid, nitric acid, and lactic acid, with a total acid concentration of 10~20g / L.

[0024] As a further preferred embodiment, the surface conditioning liquid described in this application is an aqueous solution of sodium hydroxide with a sodium hydroxide concentration of 20~50g / L.

[0025] As a further preferred embodiment, the thickness of the passivation layer in this application embodiment is 5μm~10μm; the passivation film includes a surface film formed mainly by crystalline hydrated manganese orthophosphate, an intermediate host film formed by a mixed phase of hydrated manganese orthophosphate and manganese hydrogen phosphate and / or ammonium manganese phosphate, and an interface layer formed by manganese oxide and magnesium phosphate in a definite form, wherein the thickness of the surface film, the intermediate host film, and the interface layer accounts for 12%~16%, 75%~85%, and 4%~8% of the total thickness of the passivation film, respectively.

[0026] As a further preferred embodiment, in the degreasing, pickling, surface conditioning and passivation steps described in the embodiments of this application, each step involves rinsing the residual solution on the magnesium alloy surface with water before proceeding to the next step.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The passivation solution for improving the density of the passivation layer on the surface of magnesium alloy described in this application is based on a phosphate-manganese salt aqueous phase system. It is improved by using a low dose of densifying additives. By controlling the amount of densifying additives added, in-situ controllable film formation of a high-density passivation layer of 5μm~10μm is achieved.

[0028] 2. In the passivation process described in this application to improve the density of the passivation layer on the surface of magnesium alloy, the densification additive, with an addition amount not exceeding 10%, does not interfere with the normal progress of the main film-forming reaction and can achieve densification modification through a dual effect: on the one hand, through the interfacial pH buffering effect, it stabilizes the micro-region pH of the solid-liquid interface of the magnesium alloy, precisely matches the dynamic balance between the dissolution rate of the magnesium matrix and the deposition rate of manganese phosphate, and avoids the porosity, pores, and microcracks caused by the explosive deposition of manganese phosphate, thereby reducing the generation of defects from the source of film formation; on the other hand, through the complexation regulation effect, it achieves directional self-repair of weak points, pinholes, and microcracks in the passivation film, completely sealing the penetration channels of the corrosive medium. The final passivation layer has uniform and densely arranged grains without penetrating defects, and the density is improved by an order of magnitude compared with the traditional unmodified phosphate-manganese salt system.

[0029] 3. The passivation solution described in the embodiments of this application, which improves the density of the passivation layer on the magnesium alloy surface, forms a highly dense film structure, which greatly enhances the passivation layer's resistance to media penetration and electrochemical corrosion; the passivation layer's neutral salt spray corrosion resistance can be improved from pitting corrosion that first appears in 24-48 hours to significant corrosion that only appears in 96 hours.

[0030] 4. The passivation solution for improving the density of the passivation layer on the surface of magnesium alloy described in the embodiments of this application is a chromium-free and fluorine-free cleaning system. The low-dose additives greatly reduce the pressure of waste treatment. The passivation system is a mild aqueous system with no volatile toxic gases, low corrosivity, and high operational safety.

[0031] 5. This application addresses the deficiency of insufficient density in traditional passivation films from the root cause by synergistic effects of pH buffer dynamic balance regulation and complexation-oriented defect self-repair.

[0032] On the one hand, the buffering effect of weak acidic salts or organic amines stabilizes the local pH of the magnesium alloy surface, avoids drastic pH fluctuations at the interface during film formation, and precisely controls the dynamic balance between the dissolution rate of the magnesium alloy matrix and the deposition rate of manganese phosphate, thereby reducing the generation of microcracks and pinholes in the film layer from the source and significantly improving the uniformity of film formation. On the other hand, organic amines form complexes with manganese salts, which regulate the pH threshold for manganese phosphate deposition, allowing the passivation film to preferentially deposit at weak or defective areas of the already formed film layer, achieving self-repair of defects during the film formation process and further improving the density of the passivation layer.

[0033] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 The images show a comparison of the surface density of Example 1 and Comparative Example 1 of this application. (a) is Comparative Example 1, and (b) is Example 1.

[0036] Figure 2 The results of salt spray tests for Example 1 and Comparative Example 1 of this application are shown in (a) for Comparative Example 1 and (b) for Example 1. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, 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 some embodiments of this application, but not all embodiments.

[0038] Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0039] The term "comprising" and other equivalent descriptive terms used in the specification and claims of this application are intended to cover a non-exclusive inclusion, which includes both the contents explicitly described in the specification and claims and steps or units that are not described in the specification and claims but are inherent in the product, method or structure.

[0040] This application provides a passivation solution to improve the density of the passivation layer on the surface of magnesium alloys. It uses a phosphate-manganese salt aqueous solution as the film-forming basis and system framework of the passivation layer, and includes phosphate film-forming components and manganese salt film-forming components. In an acidic aqueous solution environment, phosphate and manganese salt can be stably dissolved and uniformly dispersed. After the magnesium alloy is immersed, the substrate undergoes surface dissolution under acidic conditions, resulting in a significant increase in the local pH of the interface. Phosphate and manganese salt precipitate synchronously with the increase in pH, and a manganese phosphate passivation layer with O, P and Mn as the core components is generated in situ on the surface of the magnesium alloy, thereby achieving full coverage protection of the substrate.

[0041] In the embodiments, the phosphate-manganese salt aqueous solution contains 5 g / L to 50 g / L of phosphate and 5 g / L to 50 g / L of manganese salt. Within this concentration range, it ensures that the system has sufficient precursors and driving force, avoiding discontinuous film layers, incomplete coverage, and insufficient thickness due to insufficient film-forming raw materials; it also avoids spontaneous precipitation of the bulk solution due to phosphate supersaturation, preventing rapid aging and failure of the bath solution, while avoiding coarse grains and loose, cracked film layers caused by excessively fast film formation rate; and it also ensures the availability of Mn. 2+ It forms a sufficient complex with organic amines to achieve self-repair of defects.

[0042] This application introduces a densifying additive into the system. The densifying additive is a weakly basic organic amine or a mixture of a weakly basic organic amine and a weakly acidic salt. The core function of the densifying additive is to regulate the passivation layer growth process from the root of the film-forming reaction, target and fill micro-defects, significantly improve the density of the passivation layer, and simultaneously optimize the corrosion resistance, adhesion and protective stability of the magnesium alloy passivation layer.

[0043] The core of the passivation film formation of magnesium alloy with manganese phosphate is the dynamic process of phosphate-manganese salt precipitation and deposition caused by the acidic dissolution of the magnesium alloy substrate leading to a local pH increase at the interface. The interface pH fluctuates drastically, resulting in a severe imbalance between the dissolution rate of the magnesium alloy and the deposition rate of manganese phosphate. The resulting passivation layer is prone to numerous cracks and pores, and has extremely poor density. In the embodiments of this application, weakly alkaline organic amines can precisely buffer and stabilize the local pH of the magnesium alloy interface, avoiding drastic pH changes during film formation. This allows the dissolution rate of the magnesium alloy and the deposition rate of manganese phosphate to remain in a dynamic balance for a long time, reducing the generation of cracks and pores from the source of film formation, resulting in a more uniform film growth and a denser structure. Organic amine densifying additives can form stable complexes with manganese salts, precisely controlling the pH threshold required for manganese phosphate deposition and significantly improving the sensitivity of the film-forming reaction to the surface state of the passivation film. In areas where a complete passivation film has been formed, substrate dissolution is blocked, the interface pH changes are small, and the complexed manganese salts will not dissociate and deposit, avoiding roughness and internal stress cracking caused by excessive film growth. Moreover, in weak areas, pores, and microcracks of the passivation film, the magnesium alloy substrate will still dissolve, and the local pH will be higher. The complexed manganese salts will preferentially dissociate and deposit in these locations, actively filling micro-defects, achieving targeted defect repair, completely blocking the penetration channels of corrosive media, and further improving the density and protective integrity of the passivation layer.

[0044] Densification additives are added at concentrations of 0.1 g / L to 5 g / L, with the addition amount not exceeding 10% of the amount of phosphate or manganese salts added. Organic amine additives can precisely buffer and stabilize the local pH of the magnesium alloy interface, avoiding drastic pH fluctuations during film formation. This allows for precise control of the dynamic balance between the dissolution rate of the magnesium alloy and the deposition rate of manganese phosphate, reducing crack and pore formation from the source of film formation and improving film density. Organic amine additives can form complexes with manganese salts, regulating the pH threshold for manganese phosphate deposition and increasing the sensitivity of the film formation reaction to the surface state of the interface. This allows manganese phosphate to preferentially deposit at weak points, pores, or cracks in the already formed passivated film, actively filling microscopic defects in the film and further improving density. A concentration range of 0.1 to 5 g / L and a proportion not exceeding 10% of the main film-forming component can prevent excessive additives from disrupting the balance of the main film-forming reaction, preventing excessive complexation from inhibiting normal manganese phosphate deposition, or excessive buffering from hindering the normal progress of the film formation reaction.

[0045] In this scheme, the passivation solution is used with a pH value adjusted to 2.8~3.5 and a temperature adjusted to 50~70℃.

[0046] Within this pH range, phosphates and manganese salts can be stably dissolved in aqueous solution without premature precipitation, ensuring the stability of the passivation solution during storage and use. At the same time, it provides a suitable pH reference range for the pH buffering and complexation regulation of densification additives, ensuring that the optimization effect of the additives is fully realized.

[0047] A temperature of 50~70℃ is necessary to ensure the formation of a passivation layer with the required thickness and complete structure at the appropriate time, meeting the requirements of the process.

[0048] The passivation solution of the densifying additive can form a passivation layer with a thickness of 5μm to 10μm in situ on the magnesium alloy surface; this thickness of passivation layer can simultaneously achieve excellent corrosion resistance, insulation, and excellent adhesion to the substrate, without obvious cracks and pores.

[0049] In the embodiments of this application, the selected phosphate is one or a mixture of two or more of tripotassium phosphate, disodium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0050] Tripotassium phosphate is a strongly alkaline phosphate with excellent water solubility, rapid dissolution rate at room temperature, no precipitate residue, simple preparation of passivation solution, and good stability of the bath solution. Tripotassium phosphate has a strong pH buffering capacity in the working range of pH 2.8~3.5, which can stabilize the pH of the passivation solution and reduce drastic pH fluctuations during the film formation process.

[0051] Disodium hydrogen phosphate is an amphoteric buffer phosphate with high film-forming activity of hydrogen phosphate ions and strong coordination binding ability with manganese ions. It can promote uniform nucleation of manganese phosphate and inhibit grain coarsening, resulting in a passivation film with finer grains and naturally higher density. Disodium hydrogen phosphate has a perfect match between buffering performance and film-forming mechanism in the pH range of 2.8 to 3.5. It can stabilize the pH of the passivation solution without excessively inhibiting the local pH rise at the magnesium alloy interface. It precisely matches the dynamic film-forming balance of "magnesium alloy dissolution-manganese phosphate deposition", and the film-forming rate is uniform and controllable, reducing film cracks and porosity from the source.

[0052] Ammonium dihydrogen phosphate is a weakly acidic phosphate. The ammonium ion has a weak complexing effect and can form a weak complex with manganese ions, thereby regulating the deposition rate of manganese phosphate, refining the grains, and improving the uniformity of the film. It can precisely maintain the weakly acidic working environment of the passivation solution at pH 2.8~3.5, avoiding crystallization of the solution due to excessive pH increase. The passivation solution has extremely strong stability during long-term storage and use.

[0053] The manganese salts used in the embodiments of this application are selected from, but are not limited to, one or a mixture of two or more of manganese chloride, manganese sulfate, manganese acetate, and manganese formate.

[0054] Manganese chloride is highly water-soluble and dissolves quickly. Chloride ions have a slight activating effect, which can etch the surface of magnesium alloys, promote the initial nucleation of film formation, and enhance the basic adhesion between the film and the substrate.

[0055] Manganese sulfate has stable sulfate properties, does not decompose or undergo side reactions within the pH range of 2.8 to 3.5, introduces no harmful impurities, and has a long service life for the passivation solution. The release rate of manganese ions is stable, the film formation process is smooth and controllable, the generated passivation film has uniform thickness, stable basic properties, and good compatibility with all three types of phosphates.

[0056] Manganese acetate is an organic weak acid manganese salt. Its aqueous solution can undergo slight hydrolysis, which helps to buffer the pH of the passivation solution and stabilize the dynamic equilibrium of film formation. The acetate ion has a weak complexing ability and can form a complex with manganese ions, thereby regulating the release rate of manganese ions, inhibiting the coarsening of manganese phosphate grains, refining the grains, and improving the density of the film. It contains no highly corrosive anions and will not cause over-corrosion of magnesium alloys. There are also no corrosive ion residues, and the corrosion resistance of the film is significantly better than that of manganese chloride.

[0057] Manganese formate is an organic weak acid manganese salt with stronger hydrolysis buffering capacity than manganese acetate. It can precisely control the local pH gradient at the magnesium alloy interface, perfectly matching the pH window for manganese phosphate deposition, allowing the dissolution of magnesium alloy and the deposition of manganese phosphate to reach the optimal dynamic balance, reducing film cracks and porosity from the root, and achieving optimal density. The formate complexing ability has excellent adaptability. The complex formed with manganese ions can both control the film formation rate and refine the grains, without excessively inhibiting manganese phosphate deposition. The resulting passivation film has the finest grains, the densest structure, no penetrating pores, and the best physical shielding effect. During the film formation process, formate decomposes into CO2 and H2O, leaving no corrosive anion residues, completely avoiding the long-term pitting corrosion risk of inorganic manganese salts, and achieving the best long-term corrosion resistance of the film. The synergistic effect between manganese formate and organic amine densifying additives is the strongest.

[0058] In some preferred embodiments, a system combining disodium hydrogen phosphate and manganese formate is preferred. The combination of these two substances, along with the amphoteric buffering properties of disodium hydrogen phosphate and the hydrolytic buffering capacity of manganese formate, forms a strong synergy. This not only stabilizes the pH of the passivation solution within the working range of 2.8 to 3.5, but also precisely controls the local pH gradient at the magnesium alloy interface, allowing the dynamic balance between magnesium dissolution and manganese phosphate deposition to reach the optimal state. The film formation rate is uniform and controllable, maximizing the density from the source of film formation. The basic formulation can achieve no significant pitting corrosion in neutral salt spray for 48 hours, with stable basic performance. When combined with organic amine densifying additives, the synergistic effect is strongest, doubling the salt spray resistance time to 96 hours.

[0059] Alternatively, systems composed of disodium hydrogen phosphate and manganese sulfate, or systems composed of ammonium dihydrogen phosphate and manganese formate, can be selected.

[0060] As a further preferred embodiment, the densifying additive described in this application is a weakly basic amine or a complex of a weakly basic amine and a weakly acidic salt; the weakly basic amine is one or a mixture of two or more of monoethanolamine, diethanolamine, and triethanolamine.

[0061] Monoethanolamine (MEA) is a weakly basic small-molecule organic amine that forms a stable buffer pair in a working system with a pH of 2.8 to 3.5. This precisely stabilizes the local pH at the magnesium alloy interface, preventing drastic pH fluctuations during film formation and balancing the dissolution rate of the magnesium alloy with the deposition rate of manganese phosphate. This reduces crack and pore formation from the source of film formation. The hydroxyl and amino groups in the molecule can form moderately stable complexes with manganese ions, regulating the pH threshold for manganese phosphate deposition. This allows manganese phosphate to preferentially deposit at weak points and pores in the passivation film, actively repairing microscopic defects. This small molecule structure has extremely strong permeability and dispersibility in the passivation solution, allowing it to act uniformly on all areas of the magnesium alloy surface. The film uniformity is excellent, with no mottling or incomplete deposition issues. Furthermore, its complexing ability is moderate, neither excessively inhibiting the normal deposition of manganese phosphate nor causing insufficient complexation, resulting in controllable film thickness.

[0062] Diethanolamine (DEA) is a dihydroxy organic amine. Its dihydroxy structure gives it a larger buffering capacity and a stronger effect on stabilizing the local pH at the magnesium alloy interface. It can more precisely lock the dynamic balance between magnesium dissolution and manganese phosphate deposition, minimizing the width and depth of film cracks at the source, resulting in optimal densification. The molecular structure of the dihydroxy and amino groups has excellent compatibility with the complexes formed by manganese ions, allowing for precise control of the pH threshold for manganese phosphate deposition. This results in stronger targeted repair capabilities at weak points in the passivation film, completely blocking the penetration channels of corrosive media without interfering with the main film-forming reaction. Diethanolamine possesses both pH buffering and selective deposition functions, and its buffering and complexing capacity ratio is perfectly suited to this P-Mn passivation system, making it the most effective additive.

[0063] Triethanolamine (TEA) is a trihydroxy organic amine. Its trihydroxy structure makes it more weakly basic and has a larger buffering capacity. It has a strong effect on stabilizing the pH of the passivation solution and the interface, and can effectively suppress drastic pH fluctuations during film formation. The trihydroxy and amino structure has the strongest complexing ability with manganese ions, which can form a highly stable complex. It has outstanding effects on targeted filling of micro-defects in the passivation film and grain refinement. Triethanolamine has dual functions of pH buffering and preferential deposition through complexation. Among the three organic amines, its complexing ability is the strongest and its buffering capacity is the largest.

[0064] In some preferred embodiments, diethanolamine is used as a densifying additive, with a preferred addition amount of 0.5~3 g / L. Diethanolamine alone can meet the requirements of this application for buffering capacity and complexing capacity, reduce cracks and porosity from the source of film formation, and target and repair micro-defects. Its densification enhancement effect is far superior to other additives, and it has a synergistic effect with the optimal main film-forming system (disodium hydrogen phosphate and manganese formate).

[0065] In other preferred embodiments, a combination of diethanolamine and monoethanolamine can be used, with a mass ratio of (2~3):1 and a total addition amount controlled at 0.5~4g / L. These two compounding methods can complement each other's performance advantages and avoid the performance boundaries of single components. Diethanolamine plays a leading role in pH stabilization and complexation regulation, and undertakes the core densification and corrosion resistance enhancement functions. Monoethanolamine, with its strong permeability due to its small molecules, preferentially nucleates in the micro-depressions and gaps of magnesium alloys, further improving the uniformity of film formation and avoiding appearance defects such as corner plating defects and spots. The pH working window of the buffer system after compounding is wider, the tolerance to pH and concentration fluctuations of the bath solution is higher, and the service life of the bath solution is longer, which is especially suitable for large-scale continuous production lines.

[0066] In some embodiments, diethanolamine and triethanolamine are compounded in a mass ratio of (3~4):1, with the total addition amount controlled at 0.5~4g / L. This compounding method combines the optimal corrosion resistance of diethanolamine with the strong complexing and buffering capacity of triethanolamine. By controlling the low proportion of triethanolamine, the problem of excessive complexing is avoided. At the same time, its strong complexing capacity can be used to inhibit crystallization of the bath solution, extend the service life of the bath solution, further refine the grains, and improve the smoothness of the film layer.

[0067] In these embodiments, adding a small amount of sodium molybdate or sodium stannate to the densifying additive can fill the tiny gaps on the surface of the passivation layer and improve the density of the passivation layer.

[0068] Furthermore, in a preferred embodiment of this application, phosphoric acid and / or sodium hydroxide are used to adjust the pH value of the passivation solution.

[0069] Since the passivation solution used in this application is a phosphate and manganese salt system, the phosphate dissociation only generates H+. + Phosphate groups, including the phosphate group itself, are the core film-forming material for manganese phosphate passivation films. Phosphate does not introduce corrosive or interfering anions such as chloride, sulfate, or nitrate ions. Phosphate is a tribasic weak acid that can form a stable phosphate-dihydrogen phosphate buffer pair with phosphates in the system. This buffer has a large buffer capacity and can effectively counteract the H+ consumed by the acidic dissolution of magnesium alloy during film formation. + The resulting drastic pH fluctuations can be addressed by maintaining a stable pH environment between the passivation solution and the magnesium alloy interface over a long period. This can balance the dissolution rate of the magnesium alloy and the deposition rate of manganese phosphate, thereby reducing cracks and porosity and improving density from the source of film formation.

[0070] Sodium hydroxide is the best choice for adjusting the alkalinity of the passivation solution in this application, as sodium hydroxide dissociates to produce only Na+. + and OH - OH - By neutralizing H + To achieve pH upregulation, Na + As an inert cation, it will not form precipitates with phosphate or manganese ions, and will not interfere with the main film-forming reaction of manganese phosphate. Simultaneously, it will not compete with or chemically react with the patented core organic amine densifying additive, and will not affect the additive's core functions of pH buffering and preferential deposition of manganese ions through complexation. The reaction of sodium hydroxide with phosphoric acid and phosphates in the system can optimize the ratio of the phosphate-dihydrogen phosphate-hydrogen phosphate buffer pair, further enhancing the system's pH buffering capacity and maintaining the long-term stability of the passivation solution. Furthermore, the system itself contains sodium salt components such as disodium hydrogen phosphate, introducing Na... + It is fully compatible with the original components of the system and will not compromise the solubility and stability of the bath solution.

[0071] When phosphoric acid and sodium hydroxide are used together, a phosphate-based buffer system is formed, which can stably maintain the passivation solution pH in the optimal working range of 2.8 to 3.5 for a long period of time.

[0072] This application also provides a process for forming a passivation layer on the surface of a magnesium alloy, including the following steps. Degreasing: Immerse the magnesium alloy to be treated in an alkaline degreasing solution for 5-10 minutes to remove oil, dust, and mold release agent impurities from the magnesium alloy surface. The time balances degreasing effect and substrate protection. The alkaline degreasing solution can decompose animal and vegetable oils through saponification, disperse and remove mineral oil and mold release agent through emulsification, and remove solid impurities such as dust by sedimentation. This provides a clean, completely hydrophilic substrate surface without hydrophobic obstructions for all subsequent processes. After degreasing, rinse with tap water for 10-30 seconds to thoroughly remove any residual alkaline degreasing solution from the surface. This prevents the alkaline solution from being carried into the subsequent pickling tank and neutralizing the pickling solution, which would cause the pickling effect to fail. It also prevents alkaline residue from causing uneven film formation in the subsequent process. Pickling: The degreased magnesium alloy is immersed in a pickling solution for 30-90 seconds to remove the original oxide layer on the magnesium alloy surface and activate the substrate surface. Specifically, chemical etching can be performed using a weak or medium-strong acid pickling solution composed of acetic acid, nitric acid, and lactic acid to completely dissolve and remove the original oxide layer and alkaline etching residue. Simultaneously, through microscopic uniform etching, a nanoscale uneven morphology is formed on the magnesium alloy surface, significantly increasing the specific surface area of ​​the substrate. This provides a large number of uniform active sites for the subsequent nucleation of the manganese phosphate passivation film, allowing the passivation film to nucleate synchronously and grow uniformly across the entire surface. Reduce porosity and cracks at the film formation source; the total acid concentration is 10~20g / L; the immersion time of 30~90s can avoid incomplete oxide layer removal and insufficient substrate activation due to too short a time, and excessive corrosion of the substrate due to too long a time, resulting in pitting and intergranular corrosion, which leads to a large number of defects in the subsequent passivation film, and a significant decrease in density and corrosion resistance; after pickling, rinse with tap water for 10~30s to remove residual acid and magnesium salt impurities generated by the pickling reaction, avoid acidic substances being brought into the surface conditioning tank and surface conditioning solution, and at the same time prevent acid residue from being brought into the passivation tank; Surface conditioning: Immerse the pickled magnesium alloy in a 20-50 g / L sodium hydroxide aqueous solution for 2-5 minutes to homogenize the surface condition of the magnesium alloy. This process serves two purposes: first, the weakly alkaline environment quickly neutralizes any residual acid on the surface, thoroughly removing pickling products and preventing impurities from interfering with the subsequent passivation reaction; second, controlled, slight alkaline etching smooths out the microscopic uneven etching caused by pickling, eliminating activity differences between different crystal planes and ensuring a highly uniform activity across the entire workpiece surface. This guarantees that manganese phosphate can effectively react throughout the entire surface during the subsequent passivation process. Surface-synchronous nucleation and uniform growth reduce defects from the film formation source, significantly improving the density and appearance consistency of the passivation layer; the 2-5 minute soaking time not only completes homogenization and neutralization, but also avoids excessive alkaline etching, forming a loose alkaline etching layer; after surface conditioning, rinsing with tap water for 10-30 seconds removes residual surface conditioning solution and alkaline etching products from the surface, preventing alkaline substances from being introduced into the passivation tank, causing a sudden increase in the local pH of the passivation solution, which would lead to premature precipitation of phosphates and manganese salts in the solution, completely destroying the stability of the passivation solution, and at the same time avoiding rough and uneven film formation; Passivation: The surface-treated magnesium alloy is immersed in the passivation solution described in any one of claims 1-5, and soaked at 50-70°C for 5-10 minutes to form a passivation layer in situ on the magnesium alloy surface; in the weakly acidic (pH 2.8-3.5) passivation solution at 50-70°C, the magnesium alloy matrix undergoes controllable slight dissolution, resulting in a continuous increase in the local pH at the solid-liquid interface, triggering the in-situ precipitation and deposition of phosphate and manganese salts in the passivation solution on the matrix surface, gradually forming a continuous manganese phosphate passivation film; at the same time, the densification additive has a dual effect: firstly, it buffers the pH fluctuations at the interface, precisely balancing the dissolution rate of the magnesium alloy and the deposition rate of manganese phosphate. The first step is to reduce cracks and porosity at the source of film formation. The second step is to complex with manganese ions to regulate the preferential deposition of manganese phosphate in weak and porous areas of the existing film, actively repairing micro-defects and ultimately forming a passivation layer with excellent density and corrosion resistance. The film formation reaction kinetics are precisely controlled within a temperature range of 50-70℃. Combined with a soaking time of 5-10 minutes, the passivation layer thickness is precisely controlled within the patented optimal range of 5-10μm. After passivation, the surface is rinsed with tap water for 10-30 seconds to thoroughly remove residual passivation solution and loose deposits, avoiding residual salts and ions from causing subsequent corrosion on the film surface and ensuring the long-term stability of the passivation layer. Drying: The passivated magnesium alloy is placed in an environment of 30~100℃ and dried with hot air for 5~15 minutes to complete the preparation of the passivation layer. The hot air convection quickly and evenly removes the residual moisture on the surface and in the pores, completely eliminating early corrosion caused by residual moisture. The gentle heating environment can promote the further solidification and crystallization of the passivation layer, improve the density of the passivation layer and the adhesion to the substrate, and at the same time avoid the violent boiling of moisture and the sudden increase of internal stress in the film layer caused by high temperature and rapid drying, which can lead to pinholes and cracks.

[0073] The passivation layer described in this application embodiment has a thickness of 5 μm to 10 μm; the passivation film includes a surface film formed mainly by crystalline hydrated manganese orthophosphate, an intermediate host film formed by a mixed phase of hydrated manganese orthophosphate and manganese hydrogen phosphate and / or ammonium manganese phosphate, and an interface layer formed by manganese oxide and magnesium phosphate in a definite form, wherein the thickness of the surface film, the intermediate host film, and the interface layer accounts for 12% to 16%, 75% to 85%, and 4% to 8% of the total thickness of the passivation film, respectively.

[0074] The following are preferred embodiments of the invention.

[0075] In all the experiments in the examples, AZ91D magnesium alloy die-cast plates were uniformly processed into standard samples of 50mm×100mm×2mm. Before the experiment, the samples were dry-ground stepwise with 600# to 1000# wet sandpaper to remove surface machining marks and burrs. Then, they were ultrasonically cleaned with anhydrous ethanol for 5 minutes, dried with cold air, and placed in a desiccator for later use.

[0076] All chemical reagents were of analytical grade, and solutions were prepared using deionized water. The pH of the passivation solution was uniformly adjusted using the patented phosphoric acid and sodium hydroxide, without any other acid-base adjusters.

[0077] Example 1 In this embodiment, a passivation layer is formed on the surface of a magnesium alloy. The specific steps are as follows. Degreasing: Immerse the prepared sample in 30g / L alkaline degreasing solution and soak at 55℃ for 5min. After taking it out, confirm that a continuous and uniform water film is formed on the surface of the sample, without any water gaps or beading phenomena. Water washing: Rinse the degreased sample with running tap water for 20 seconds to thoroughly remove any residual alkaline degreasing solution from the surface; Pickling: The sample after water washing is immediately immersed in a composite pickling solution of 15g / L acetic acid + 5g / L lactic acid for 60 seconds at room temperature to completely remove the original oxide layer and alkaline corrosion residue on the surface. Water washing: Rinse the sample with running tap water for 20 seconds after acid washing to remove residual acid and acid washing reaction products from the surface; Surface conditioning: Immediately after washing, the sample is immersed in 30 g / L sodium hydroxide surface conditioning solution and soaked at room temperature for 4 minutes. After taking it out, it is confirmed that the surface of the sample is uniformly milky white. Water washing: Rinse the surface-conditioned sample with running tap water for 20 seconds to thoroughly remove any residual surface-conditioning solution. Passivation: The sample after washing with water is immediately immersed in the passivation solution and soaked at a constant temperature of 60℃ for 10 minutes; the passivation solution is 35g / L disodium hydrogen phosphate, 45g / L manganese sulfate, 2g / L monoethanolamine, and deionized water to a final volume of 1L; the pH of the passivation solution is adjusted to 3.0 with phosphoric acid before use; Water washing: Immediately after passivation, rinse the sample with running tap water for 20 seconds to thoroughly remove any residual passivation solution and loose deposits from the surface; Drying: The washed sample was immediately placed in a forced-air drying oven and dried with hot air at a constant temperature of 60°C for 10 minutes. After that, it was taken out and placed in a desiccator to cool.

[0078] Example 2 In this embodiment, a passivation layer is formed on the surface of a magnesium alloy. The specific steps are as follows. The steps and parameters for degreasing, surface conditioning, washing, and drying were exactly the same as those in Comparative Example 1. Passivation process: The sample is immersed in the passivation solution and soaked at a constant temperature of 60℃ for 10 minutes; the passivation solution formula is: disodium hydrogen phosphate 45g / L, manganese formate 35g / L, diethanolamine 3g / L, and deionized water to a final volume of 1L; the pH of the passivation solution is adjusted to 3.0 with phosphoric acid before use.

[0079] Example 3 In this embodiment, a passivation layer is formed on the surface of a magnesium alloy. The specific steps are as follows. The steps and parameters for degreasing, surface conditioning, washing, and drying were exactly the same as those in Comparative Example 1. Passivation: The sample is immersed in the passivation solution and soaked at a constant temperature of 60℃ for 10 minutes. The passivation solution formula is: 35 g / L ammonium dihydrogen phosphate, 40 g / L manganese formate, 2 g / L triethanolamine, and deionized water to a final volume of 1 L. The pH of the passivation solution is adjusted to 3.0 with phosphoric acid before use.

[0080] Example 4 In this embodiment, a passivation layer is formed on the surface of a magnesium alloy. The specific steps are as follows. The steps and parameters for degreasing, surface conditioning, washing, and drying were exactly the same as those in Comparative Example 1. Passivation: The sample is immersed in the passivation solution and soaked at a constant temperature of 60℃ for 10 min; the passivation solution formula is: disodium hydrogen phosphate 45 g / L, manganese formate 35 g / L, diethanolamine 0.5 g / L, and deionized water to a final volume of 1 L; the pH of the passivation solution is adjusted to 3.0 with phosphoric acid before use.

[0081] Example 5 In this embodiment, a passivation layer is formed on the surface of a magnesium alloy. The specific steps are as follows. The steps and parameters for degreasing, surface conditioning, washing, and drying were exactly the same as those in Comparative Example 1. Passivation: The sample is immersed in the passivation solution and soaked at a constant temperature of 60℃ for 10 minutes; the passivation solution formula is: disodium hydrogen phosphate 45g / L, manganese formate 35g / L, diethanolamine 4.5g / L, and deionized water to a final volume of 1L; the pH of the passivation solution is adjusted to 3.0 with phosphoric acid before use.

[0082] Example 6 In this embodiment, a passivation layer is formed on the surface of a magnesium alloy. The specific steps are as follows. Degreasing: Immerse the prepared sample in 30g / L alkaline degreasing solution and soak at 55℃ for 5min. After taking it out, confirm that a continuous and uniform water film is formed on the surface of the sample, without any water gaps or beading phenomena. Water washing: Rinse the degreased sample with running tap water for 20 seconds to thoroughly remove any residual alkaline degreasing solution from the surface; Pickling: The sample after water washing is immediately immersed in a composite pickling solution of 15g / L acetic acid + 5g / L lactic acid for 60 seconds at room temperature to completely remove the original oxide layer and alkaline corrosion residue on the surface. Water washing: Rinse the sample with running tap water for 20 seconds after acid washing to remove residual acid and acid washing reaction products from the surface; Surface conditioning: Immediately after washing, the sample is immersed in 30 g / L sodium hydroxide surface conditioning solution and soaked at room temperature for 4 minutes. After taking it out, it is confirmed that the surface of the sample is uniformly milky white. Water washing: Rinse the surface-conditioned sample with running tap water for 20 seconds to thoroughly remove any residual surface-conditioning solution. Passivation: The washed sample was immediately immersed in the passivation solution and soaked at a constant temperature of 65℃ for 8 minutes. The passivation solution formula was: 45 g / L disodium hydrogen phosphate, 35 g / L manganese formate, 3 g / L diethanolamine, and deionized water to a final volume of 1 L. The pH of the passivation solution was adjusted to 3.2 with phosphoric acid before use. Water washing: Immediately after passivation, rinse the sample with running tap water for 20 seconds to thoroughly remove any residual passivation solution and loose deposits from the surface; Drying: The washed sample was immediately placed in a forced-air drying oven and dried with hot air at a constant temperature of 60°C for 10 minutes. After that, it was taken out and placed in a desiccator to cool.

[0083] Comparative Example 1 This comparative example demonstrates the formation of a passivation layer on the surface of a magnesium alloy. The specific steps are as follows. The steps and parameters for degreasing, surface conditioning, washing, and drying were exactly the same as those in Comparative Example 1. Passivation: Immerse the magnesium alloy in a passivation solution for 10 minutes. The passivation solution is an aqueous solution of 30 g / L tripotassium phosphate and 35 g / L manganese sulfate.

[0084] Comparative Example 2 This comparative example demonstrates the formation of a passivation layer on the surface of a magnesium alloy. The specific steps are as follows. The steps and parameters for degreasing, surface conditioning, washing, and drying were exactly the same as those in Comparative Example 1. Passivation: Immerse the magnesium alloy in a passivation solution for 10 minutes. The passivation solution consists of 40 g / L ammonium dihydrogen phosphate and 25 g / L manganese acetate.

[0085] Comparative Example 3 This comparative example demonstrates the formation of a passivation layer on the surface of a magnesium alloy. The specific steps are as follows. The steps and parameters for degreasing, surface conditioning, washing, and drying were exactly the same as those in Comparative Example 1. Passivation: The magnesium alloy was immersed in a passivation solution for 10 minutes. The passivation solution consisted of 25 g / L disodium hydrogen phosphate, 35 g / L manganese formate, and 3 g / L sodium molybdate.

[0086] Performance testing The magnesium alloy samples obtained in Examples 1-6 and Comparative Examples 1-3 were subjected to performance testing. Among them, the salt spray corrosion resistance was tested using a neutral salt spray test (NSS) according to GB / T10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test". The passivated samples were placed in a 35°C, 5wt.% NaCl solution atomization environment. The time of appearance of corrosion products and the overall corrosion of the passivation layer were observed. The time of corrosion at the scratches was recorded (salt spray resistance time).

[0087] The test results were graded and evaluated in accordance with GB / T6461-2002 "Rating of specimens and test pieces of metal and other inorganic coatings on metal substrates after corrosion test".

[0088] The test results are shown in Table 1 and Figure 1 and Figure 2 .

[0089] Table 1: Performance Test Results The results in Table 1 show that the traditional manganese phosphate passivation system without densifying additives can only achieve 48 hours of neutral salt spray protection, and the film layer has obvious microcracks and pores, with inherent bottlenecks in densification and corrosion resistance. Monoethanolamine, diethanolamine, and triethanolamine organic amine densifying additives can significantly eliminate micro-defects in the film layer and greatly improve the density of the passivation layer, doubling the salt spray resistance time from 48 hours to 96 hours. Among them, diethanolamine has the best overall optimization effect, with the corresponding passivation film having no obvious cracks and pores and a corrosion rating of 10. Diethanolamine has a protective improvement effect in the range of 0.5~4.5 g / L, with 3 g / L being the optimal addition amount. Too low an addition amount results in insufficient optimization effect, while too high an addition amount will result in a slight performance decline. However, the comprehensive scheme with synergistic optimization of all parameters of formulation, pH, passivation temperature and time can form a three-layer gradient passivation film in situ, and the salt spray resistance time can be increased to 120 hours, achieving an order of magnitude leap in the density and corrosion resistance of the magnesium alloy manganese phosphate passivation layer.

[0090] Figure 1 and Figure 2The results of the surface densification comparison and salt spray test show that: the passivation layer surface of Comparative Example 1 without the addition of organic amine densification additive has obvious micro-defects such as cracks and pores, while the passivation layer surface with the addition of diethanolamine densification additive is smooth and uniform, without obvious cracks and pores. This directly confirms the core effect of the organic amine densification additive of this patent, which can significantly eliminate micro-defects in the passivation layer and greatly improve the density of the passivation layer on the magnesium alloy surface. Comparative Example 1 showed obvious corrosion after only 24 hours of salt spray testing, while Example 1 still showed no obvious corrosion damage after 96 hours of salt spray testing. This intuitively verifies that while improving the density of the passivation layer, the densification additive can achieve an order-of-magnitude leap in the salt spray corrosion resistance of the magnesium alloy passivation layer.

[0091] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A passivation solution for improving the density of the passivation layer on the surface of magnesium alloys, characterized in that, The passivation solution contains phosphate film-forming components and manganese salt film-forming components, and also contains densifying additives, wherein the amount of densifying additives added does not exceed 10% of the sum of the amounts of phosphate and manganese salts added; the passivation solution can form a passivation layer with a thickness of 5μm to 10μm in situ on the surface of magnesium alloy.

2. The passivation liquid for improving the density of the passivation layer on the surface of magnesium alloys according to claim 1, characterized in that, The passivation solution comprises the following components, based on mass concentration: Phosphate 5g / L~50g / L Manganese salt 5g / L~50g / L Densification additives 0.1g / L~5g / L The remainder is purified water; When using the passivation solution, the pH value should be adjusted to 2.8~3.5 and the temperature to 50~70℃.

3. The passivation liquid for improving the density of the passivation layer on the surface of magnesium alloys according to claim 2, characterized in that, The phosphate is one or a mixture of two or more of tripotassium phosphate, disodium hydrogen phosphate, and ammonium dihydrogen phosphate; the manganese salt is one or a mixture of two or more of manganese chloride, manganese sulfate, manganese acetate, and manganese formate.

4. The passivation liquid for improving the density of the passivation layer on the surface of magnesium alloys according to claim 2, characterized in that, The densifying additive is a weakly basic amine or a complex of a weakly basic amine and a weakly acidic salt; the weakly basic amine is one or a mixture of two or more of monoethanolamine, diethanolamine, and triethanolamine, and the weakly acidic salt is one or a mixture of two of molybdate and stannate.

5. The passivation solution for improving the density of the passivation layer on the surface of magnesium alloys according to any one of claims 1-4, characterized in that, The pH of the passivation solution is adjusted using phosphoric acid or sodium hydroxide.

6. A process for forming a passivation layer on the surface of a magnesium alloy, characterized in that, Includes the following steps, Degreasing: Immerse the magnesium alloy to be treated in an alkaline degreasing solution for 5-10 minutes to remove oil, dust and mold release agent impurities from the surface of the magnesium alloy. Pickling: Immerse the degreased magnesium alloy in the pickling solution for 30-90 seconds to remove the original oxide layer on the surface of the magnesium alloy and activate the substrate surface; Surface conditioning: Immerse the pickled magnesium alloy in the surface conditioning solution for 2-5 minutes to homogenize the surface condition of the magnesium alloy. Passivation: The surface-conditioned magnesium alloy is immersed in the passivation solution described in any one of claims 1-5 and soaked at 50-70°C for 5-10 minutes to form a passivation layer in situ on the surface of the magnesium alloy. Drying: Place the passivated magnesium alloy in an environment of 30~100℃ and dry it with hot air for 5~15 minutes to complete the preparation of the passivation layer.

7. The surface treatment process for forming a passivation layer on a magnesium alloy surface according to claim 6, characterized in that, The pickling solution is an aqueous solution prepared by mixing one or more of acetic acid, nitric acid, and lactic acid, with a total acid concentration of 10-20 g / L.

8. The surface treatment process for forming a passivation layer on a magnesium alloy surface according to claim 6, characterized in that, The surface conditioning solution is an aqueous solution of sodium hydroxide with a sodium hydroxide concentration of 20~50 g / L.

9. The surface treatment process for forming a passivation layer on a magnesium alloy surface according to claim 6, characterized in that, The passivation layer has a thickness of 5 μm to 10 μm; the passivation film includes a surface film formed mainly by crystalline hydrated manganese orthophosphate, an intermediate host film formed by a mixed phase of hydrated manganese orthophosphate and manganese hydrogen phosphate and / or ammonium manganese phosphate, and an interface layer formed by manganese oxide and magnesium phosphate in a definite form, wherein the thickness of the surface film, the intermediate host film, and the interface layer accounts for 12% to 16%, 75% to 85%, and 4% to 8% of the total thickness of the passivation film, respectively.

10. The surface treatment process for forming a passivation layer on a magnesium alloy surface according to any one of claims 6-8, characterized in that, In the degreasing, pickling, surface conditioning, and passivation steps, each step involves rinsing the magnesium alloy surface with water to remove any residual solution before proceeding to the next step.

Images