High-corrosion-resistance alloy composite coating for fastener and manufacturing method of high-corrosion-resistance alloy composite coating

Abstract

The invention relates to a high-corrosion-resistance alloy composite coating of a fastener and a manufacturing method of the high-corrosion-resistance alloy composite coating, and belongs to the technical field of metal coatings of fasteners. The problems that an existing fastener surface coating cannot meet the requirement for the service life of a zinc-aluminum-magnesium alloy coating steel plate workpiece engineering structure after fixing and connecting, and a hot dipping process suitable for a fastener zinc-aluminum-magnesium coating lacks are solved. A high-corrosion-resistance alloy composite coating of a fastener comprises an inner layer and an outer layer, the inner layer is an electrogalvanizing layer which is tightly attached to the outer surface of a fastener base body and has the thickness of 2-5 micrometers, and the outer layer is a zinc-aluminum-magnesium alloy coating tightly attached to the outer surface of the inner layer. The novel composite coating which is excellent in corrosion resistance and completely matched with the engineering structure of the zinc-aluminum-magnesium alloy coated steel plate workpiece can be prepared without changing the existing fastener hot galvanizing process, process and equipment.

Description

Technical Field

[0001] This invention relates to the field of fastener metal coating technology, and in particular to a high corrosion-resistant alloy composite coating for fasteners and its manufacturing method. Background Technology

[0002] In recent years, the development of zinc-aluminum-magnesium alloy coatings in my country has progressed very rapidly. These highly corrosion-resistant zinc-aluminum-magnesium alloy coated steel sheets have quickly been applied in industries such as photovoltaic engineering, livestock breeding engineering, transportation, and construction.

[0003] In the application scenarios of zinc-aluminum-magnesium coated steel sheets, a large number of fasteners are needed to fix and connect the zinc-aluminum-magnesium coated steel sheet components. However, currently, except for some major projects and a small number of stainless steel fasteners, the vast majority of fasteners used are hot-dip galvanized, Dacromet, electro-galvanized, or mechanically galvanized.

[0004] Stainless steel fasteners are very expensive and often suffer from galvanic corrosion with the connected zinc-aluminum-magnesium coated steel sheet components, or pitting corrosion in marine environments, leading to premature failure of the connection structure. Electro-galvanized coatings are thin, and mechanically galvanized coatings are not dense, with limited corrosion resistance that cannot meet service requirements in most environments. Dacromet coatings lack sufficient surface hardness, are easily damaged during assembly, and do not provide sacrificial anode protection. They also suffer from galvanic corrosion with the connected zinc-aluminum-magnesium coated steel sheet components, thus failing to meet service requirements. Hot-dip galvanized fasteners have dense coatings and sufficient thickness, but their corrosion resistance still cannot maintain the same service life as the main zinc-aluminum-magnesium coated steel sheet structure.

[0005] Therefore, fasteners made of stainless steel and treated with hot-dip galvanizing, Dacromet, electro-galvanizing, and mechanical galvanizing in commercial applications often become weak links in engineering projects, making it difficult for the overall engineering structure to meet the designed service life requirements. Therefore, it is essential to develop a manufacturing method for fasteners with high corrosion-resistant alloy composite coatings that match the current large-scale engineering applications of zinc-aluminum-magnesium alloy coated steel plates and to achieve large-scale commercialization.

[0006] Because zinc-aluminum-magnesium alloy coatings contain more than 1% aluminum and magnesium, they are typically only suitable for gas reduction processes and cannot be used in the commonly employed solvent-based hot-dip galvanizing process. Existing solvent-based hot-dip galvanizing processes cannot solve the problem of incomplete coating in zinc-aluminum-magnesium alloy coatings, resulting in poor surface quality and making them unsuitable for hot-dip galvanizing steel components and fasteners. In other words, existing hot-dip galvanizing processes and fluxing treatments cannot directly produce zinc-aluminum-magnesium alloy coatings. Summary of the Invention

[0007] Based on the above analysis, the present invention aims to provide a high corrosion-resistant alloy composite coating for fasteners and its manufacturing method, in order to solve at least one of the following problems: the existing fasteners used to connect zinc-aluminum-magnesium coated steel plate components do not match the corrosion resistance and service life of the main components; the existing coating process cannot solve the problem of zinc-aluminum-magnesium alloy coating leakage; it cannot produce fastener zinc-aluminum-magnesium alloy coatings with qualified surface quality; and the hot-dip galvanizing process of fastener zinc-aluminum-magnesium alloy coating is not compatible with the existing production line.

[0008] On one hand, embodiments of the present invention provide a high corrosion-resistant alloy composite coating for fasteners, comprising an inner layer and an outer layer. The inner layer is an electroplated zinc layer with a thickness of 2 to 5 micrometers that is closely attached to the outer surface of the fastener substrate, and the outer layer is a zinc-aluminum-magnesium alloy coating that is closely attached to the outer surface of the inner layer.

[0009] Furthermore, the zinc-aluminum-magnesium alloy coating has a thickness of 20–70 micrometers.

[0010] Specifically, the inner layer volume ratio accounts for no more than 10% of the total composite coating.

[0011] Furthermore, the zinc-aluminum-magnesium alloy coating has the following main components by mass percentage: Al: 5-23%, Mg: 2.0-4.0%, total impurities such as Pb and Cd <0.05%, and the balance being Zn.

[0012] On the other hand, embodiments of the present invention provide a method for preparing a high corrosion-resistant alloy composite coating for fasteners, which is used to prepare the high corrosion-resistant alloy composite coating for fasteners.

[0013] Specifically, the method for preparing the composite coating includes the following steps:

[0014] (1) Fastener pretreatment;

[0015] (2) Electroplating pure zinc layer: The pretreated fasteners are placed in a zinc salt plating solution for electroplating, and the thickness of the pure zinc plating layer is controlled to be 2 to 5 micrometers.

[0016] (3) Medium-temperature centrifugal hot-dip plating: The fasteners after electroplating in step (2) are placed in the zinc-aluminum-magnesium alloy plating solution for medium-temperature centrifugal hot-dip plating, and the thickness of the zinc-aluminum-magnesium alloy plating layer is controlled to be 20-70 micrometers.

[0017] For example, the fastener pretreatment process in step (1) is: alkaline washing to remove oil - water washing - acid washing - water washing.

[0018] Specifically, the zinc salts in the plating solution in step (2) include ammonium salts, chloride salts, sulfates, and zincates.

[0019] It should be noted that the medium-temperature centrifugal hot-dip galvanizing temperature mentioned in step (3) is 470-510℃.

[0020] Specifically, the zinc-aluminum-magnesium coating structure obtained in step (3) consists of a first-precipitated aluminum-rich phase, a binary eutectic phase composed of the aluminum-rich phase and MgZn2, and a ternary eutectic phase composed of the aluminum phase, zinc phase and MgZn2.

[0021] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0022] 1. This invention, through the design of a composite coating combining an inner electroplated zinc layer and an outer zinc-aluminum-magnesium coating, obtains a novel composite coating with excellent corrosion resistance and a service life that can perfectly match the fixing and connection of engineering structures of zinc-aluminum-magnesium alloy coated steel plate parts. By controlling the thickness of the electroplated zinc layer to 2-5 micrometers, the quality of the composite coating is significantly improved. It ensures that there will be no incomplete plating of the zinc-aluminum-magnesium alloy without the need for flux in the subsequent hot-dip galvanizing process for preparing the zinc-aluminum-magnesium alloy coating, and that the electroplated zinc layer and the zinc-aluminum-magnesium coating form a complete metallurgical bond, ensuring that the composite coating will not peel off when the workpiece is under stress during installation.

[0023] 2. By controlling the proportion of the inner layer volume of the composite coating to no more than 10% of the total composite coating volume, this invention improves the corrosion resistance of the composite coating while ensuring the quality and adhesion of the composite coating.

[0024] 3. This invention employs a two-step method combining electroplating zinc and medium-temperature hot-dip galvanizing magnesium-aluminum alloy plating to prepare a highly corrosion-resistant composite coating on the surface of fasteners. This achieves a novel highly corrosion-resistant composite coating with 3-10 times higher corrosion resistance than traditional zinc coatings without altering the existing electroplating and centrifugal hot-dip galvanizing processes, techniques, and equipment for fasteners. The electroplating zinc process allows for precise control of the zinc coating, while the hot-dip galvanizing magnesium-aluminum alloy plating process is carried out in the medium-temperature range of 470-510℃. The combination of electroplating zinc and medium-temperature hot-dip galvanizing magnesium-aluminum alloy plating avoids the need for high-temperature hot-dip galvanizing processes at 520-560℃ to prepare the zinc coating, thus significantly saving energy.

[0025] 4. This invention employs a two-step preparation process, strictly controlling the thickness of the electroplated zinc layer to be between 2 and 5 micrometers. This allows for the elimination of the fluxing treatment in the hot-dip galvanizing process of zinc-aluminum-magnesium layers, while still achieving a composite coating with corrosion resistance more than 3 times higher than that of traditional zinc plating layers, greatly improving the environment of hot-dip galvanizing.

[0026] 5. This invention significantly improves the corrosion resistance of fasteners by controlling the aluminum content (14%–23%) and magnesium content (2.0–4.0%) of the zinc-aluminum-magnesium alloy coating in the outer layer.

[0027] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0028] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0029] Figure 1 This is a photograph of a fastener coated with the high corrosion-resistant alloy composite coating of this invention. Detailed Implementation

[0030] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0031] In large-scale engineering applications of high corrosion-resistant zinc-aluminum-magnesium alloy coated steel sheet components, such as photovoltaic projects and building construction, metal fasteners such as bolts, nuts, and washers are required to fix and connect these components. Fasteners with only a zinc coating have a short service life, which cannot match the service life of the zinc-aluminum-magnesium alloy coated steel sheets. Currently, domestic fastener manufacturers typically have production lines for electroplating, hot-dip galvanizing, and other surface treatments that cannot directly produce zinc-aluminum-magnesium alloy coatings. This often makes fasteners a bottleneck in the entire project, making it difficult for the overall structure to reach its designed service life. Therefore, developing a high corrosion-resistant alloy composite coating for fasteners and its preparation method, matching the fixing and connection of zinc-aluminum-magnesium alloy coated steel sheet components, and meeting the service life requirements of various engineering structures using zinc-aluminum-magnesium alloy coated steel sheets, is an urgent need in the field of metal coating technology.

[0032] The fastener composite coating of this invention includes an inner zinc layer and an outer zinc-aluminum-magnesium alloy coating. The thickness and quality of the zinc coating have a crucial impact on the quality and adhesion of the composite coating. Common zinc plating processes include electroplating and hot-dip galvanizing.

[0033] For the hot-dip galvanizing process, generally, a low-temperature hot-dip galvanizing process with a temperature of about 450 °C is used for the galvanized layer of large steel structures. However, the hot-dip galvanizing of fasteners is different from that of general large steel structures. This is because fasteners usually use carbon steels with low carbon (C% ≤ 0.25%), medium carbon (0.25% < C% ≤ 0.45%), high carbon (C% > 0.45%) and steel materials containing alloying elements such as high Si, Mn, Cr, and Mo according to the requirements of grades, and it is necessary to control the coating thickness to ensure the requirement of thread accuracy. First, due to the possible tempering hardness during the hot-dip galvanizing process, the hot-dip galvanizing process is not applicable to high-strength components made of alloy steel materials, such as components of grade 12.9. On the other hand, the hot-dip galvanizing process for large steel structures is to place the workpiece in molten zinc and take it out after reacting for a certain time. The obtained galvanized layer has a large thickness, usually between 50 and 200 microns, and it is difficult to accurately control. Therefore, if the hot-dip galvanizing of fasteners is to accurately control a thinner coating thickness, centrifugal hot-dip galvanizing needs to be used, and the excess zinc liquid on the workpiece is centrifugally thrown off by a centrifuge in a short time, and this can only be achieved when the temperature of the zinc liquid is relatively high, that is, centrifugal hot-dip galvanizing needs to be carried out in a high-temperature zone of 520 - 560 °C to control the coating thickness.

[0034] Therefore, there are problems such as high energy consumption, poor hot-dip galvanizing environment, difficult control of coating thickness, and unstable coating quality when using the hot-dip galvanizing process to prepare the zinc layer of fasteners; while there are problems such as high cost, low efficiency, insufficient coating hardness and damage resistance performance, and inability to fully utilize the existing production line when both the zinc layer and the zinc-aluminum-magnesium layer use electroplating. In addition, in the case of requiring a thin coating thickness, how to meet the requirements of coating quality, coating adhesion, and corrosion resistance is a difficult problem to be solved.

[0035] On the one hand, a specific embodiment of the present invention discloses a high-corrosion-resistant alloy composite coating for fasteners, including an inner layer and an outer layer. The inner layer is an electro-galvanized layer with a thickness of 2 - 5 microns closely adhering to the outer surface of the fastener substrate, and the outer layer is a zinc-aluminum-magnesium alloy coating closely adhering to the outer surface of the inner layer.

[0036] Further, the thickness of the zinc-aluminum-magnesium alloy coating is 20 - 70 microns.

[0037] Specifically, the volume ratio of the inner layer in the overall composite coating does not exceed 10%. If the proportion of the electroplated pure zinc inner layer is greater than 10%, it means that the proportion of the outer zinc-aluminum-magnesium alloy coating for improving corrosion resistance decreases, which is not conducive to improving the service life of the fasteners. <正确的文本应为

[0038] ,但你提供的文本

[0038] 为空,无法翻译。请确认文本内容后重新提供。Further, the main components of the zinc-aluminum-magnesium alloy coating are as follows by mass percentage: Al: 5 - 23%, Mg: 2.0 - 4.0%, the total content of impurities such as Pb and Cd < 0.05%, and the balance is Zn.

[0039] Preferably, the zinc-aluminum-magnesium alloy coating has the following main components by mass percentage: Al: 14-23%, Mg: 3.5-4.0%, total impurities such as Pb and Cd <0.05%, and the balance being Zn.

[0040] The specific functions of each component in the highly corrosion-resistant composite coating are as follows:

[0041] (a) The inner electroplated pure zinc layer as the bottom layer can solve the technical bottleneck of missing plating and poor surface quality in the commonly used solvent-assisted plating method for hot-dip zinc-aluminum-magnesium alloy coatings.

[0042] (b) The aluminum (Al) in the zinc-aluminum-magnesium alloy coating in the outer layer can improve the fluidity of the coating bath, improve the coating quality and improve the corrosion resistance of the coating. A high aluminum content can improve corrosion resistance, but if the aluminum content is higher than 23%, the effect of improving corrosion resistance will no longer be obvious and will have an adverse effect on the surface quality. Therefore, the aluminum content should be controlled between 5% and 23%.

[0043] The addition of magnesium (Mg) to the outer zinc-aluminum-magnesium alloy coating significantly improves the corrosion resistance of the zinc-based alloy coating. This is because magnesium and zinc can form intermetallic compounds such as MgZn2 and Mg2Zn. 11 These compounds have lower anodic dissolution currents than pure zinc phases, and the formation of these intermetallic compounds results in zinc-magnesium alloys having higher corrosion resistance than pure zinc coatings. On the other hand, the addition of magnesium can make the corrosion products of the coating more compact. When the magnesium content is less than 13%, the higher the magnesium content, the more beneficial it is to improve corrosion resistance. However, the increase in magnesium content will lead to a higher alloy melting point and surface oxidation, resulting in poor surface quality of the workpiece. Therefore, the magnesium content is controlled between 2.0% and 4.0%.

[0044] On the other hand, a specific embodiment of the present invention discloses a method for preparing a high corrosion-resistant alloy composite coating for fasteners, which is used to prepare the high corrosion-resistant alloy composite coating for fasteners.

[0045] Specifically, the method for preparing the composite coating includes the following steps:

[0046] (1) Fastener pretreatment;

[0047] (2) Electroplating pure zinc layer: The pretreated fasteners are placed in a zinc salt plating solution for electroplating, and the thickness of the pure zinc plating layer is controlled to be 2 to 5 micrometers.

[0048] (3) Medium-temperature centrifugal hot-dip plating: The fasteners after electroplating in step (2) are placed in the zinc-aluminum-magnesium alloy plating solution for medium-temperature centrifugal hot-dip plating, and the thickness of the zinc-aluminum-magnesium alloy plating layer is controlled to be 20-70 micrometers.

[0049] The pretreatment process of fasteners before electroplating in step (1) is: alkaline washing to remove oil, water washing, acid washing, and water washing, so that the surface quality of the fasteners meets the process requirements of conventional electroplating.

[0050] The electroplating of zinc in step (2) can be any of the current electroplating zinc processes, such as ammonium salt electroplating, chloride salt electroplating or sulfate electroplating.

[0051] It should be noted that the thickness of the electroplated zinc layer is strictly controlled between 2 and 5 micrometers. If the thickness of the electroplated zinc layer is less than 2 micrometers, the zinc-aluminum-magnesium alloy will be under-plated in the next hot-dip galvanizing process without flux. If the thickness of the electroplated zinc layer is greater than 5 micrometers, the zinc layer will not be able to melt completely during the next hot-dip galvanizing process and will be unable to form a complete metallurgical bond with the zinc-aluminum-magnesium coating, resulting in the final composite coating peeling off when the workpiece is under stress during installation.

[0052] Compared with high-temperature hot-dip galvanizing, electro-galvanizing is more conducive to precise control of coating thickness, laying the foundation for ensuring the proportion of the outer zinc-aluminum-magnesium alloy coating and the corrosion resistance of fasteners.

[0053] The control of the electroplating layer thickness is related to the size and shape of the workpiece, the loading amount, the electroplating voltage and current density, and the electroplating additives. During the production process, the above variables are controlled according to the actual situation, and the process range of electroplating time and coating thickness is determined. Preferably, for small parts such as fasteners, the zinc layer is electroplated by barrel plating. During the barrel plating process, the current density on the surface of the part changes continuously within a certain range.

[0054] The relationship between coating thickness and electroplating time conforms to the following formula:

[0055] δ=(K*J*t*η) / ρ

[0056] K - Electrochemical equivalent (g / C);

[0057] J - Cathode current density (A / dm) 2 );

[0058] t - electroplating time (s);

[0059] η - Cathode current efficiency (%);

[0060] ρ - Density of the plated metal (g / dm³) 3 );

[0061] δ - Coating thickness (dm).

[0062] During barrel plating, the current is generally given using the "per barrel" method. The current is given to each barrel according to the type of plating, the specifications and types of parts to be plated, and the load of parts to be plated. The given current shall not exceed the upper limit of the allowable current. The upper limit of the current is the current used when the plating layer is obviously rough or burnt and produces "barrel eye marks". Under the premise of not exceeding the upper limit, the larger the current is used as much as possible to obtain a fine and qualified plating layer. The barrel plating time is closely related to the size of the plating tank and the size of the barrel plating drum of the manufacturer, and the thickness of the electroplated zinc layer is controlled between 2 and 5 μm.

[0063] In one possible design, zinc is electroplated using a chloride salt barrel plating method. The plating bath composition is: KCl: 210-240 g / L, ZnCl2: 55-75 g / L, H2BO3: 25-30 g / L, additives: 12-18 g / L, pH: 5.4-6.2. The electroplating process is as follows: plating bath temperature 25-30℃, current density 0.2-0.5 A / dm³. 2 Electroplating time: 1–40 min.

[0064] It should be noted that the medium-temperature centrifugal hot-dip galvanizing temperature in step (3) is 470-510℃; the main components of the zinc-aluminum-magnesium alloy plating solution are as follows by mass percentage: Al: 5-23%, Mg: 2.0-4.0%, total impurity content of Pb, Cd and other impurities <0.05%, and the balance is Zn; which is beneficial to improving the corrosion resistance of the composite coating.

[0065] Centrifugal hot-dip galvanizing is a type of hot-dip galvanizing process. The workpiece is placed in a stainless steel basket and immersed in a molten zinc bath. The zinc bath temperature and immersion time are determined according to the workpiece material, size, and loading amount to optimize the process. Then, the basket containing the workpiece is lifted out of the zinc bath using the cantilever of the centrifugal galvanizing machine and moved to a centrifuge for centrifugal treatment. By controlling the speed of the centrifuge and the centrifugal spinning time, excess zinc on the surface is removed, which can improve production efficiency, reduce material consumption, and improve the uniformity of the coating.

[0066] Fasteners that have undergone electroplating zinc plating do not require fluxing treatment and can be directly subjected to centrifugal hot-dip galvanizing in a frame. Conventional centrifugal hot-dip galvanizing of fasteners uses a high-temperature galvanizing process of 520-560℃ and often employs a ceramic zinc pot, which can meet the requirements of the zinc-aluminum-magnesium alloy galvanizing pot of this invention. There is no need to replace existing production equipment. The hot-dip galvanizing bath temperature of this invention is in the medium-temperature range of 470-510℃.

[0067] Specifically, in step (3), the fastener is lifted out of the plating solution and the excess plating solution is removed by centrifuge. The centrifuge speed is 3200-3800 rpm and the centrifugation time is 0.5-2 min. The required plating thickness is controlled to be 20-70 micrometers by controlling the hot-dip plating process and the centrifugation time. After air cooling for 10-15 seconds, the workpiece is water-cooled in a water bath at a water temperature of 60-85℃ to obtain a composite plating fastener sample.

[0068] Pre-cooling with air can reduce stress and deformation in the coating, and improve the uniformity and adhesion of the coating; subsequent water cooling can promote the formation of crystal nuclei, obtain finer grains, and help improve the density and corrosion resistance of the coating.

[0069] Specifically, the zinc-aluminum-magnesium coating obtained in step (3) consists of a first-precipitated aluminum-rich phase, a binary eutectic phase composed of the aluminum-rich phase and MgZn2, and a ternary eutectic phase composed of the aluminum phase, zinc phase and MgZn2. It has a lower anodic dissolution current than the pure zinc phase, which improves the corrosion resistance of the composite coating.

[0070] In summary, this invention, through a composite coating design combining an inner electroplated zinc layer and an outer zinc-aluminum-magnesium coating, achieves a novel composite coating with excellent corrosion resistance and a service life perfectly matched to the fixing and connection of zinc-aluminum-magnesium alloy coated steel plate components. Furthermore, by controlling the thickness of the electroplated zinc layer to 2–5 micrometers, the quality of the composite coating is significantly improved. In the hot-dip galvanizing process without fluxing, there is no incomplete plating of the zinc-aluminum-magnesium alloy, and a complete metallurgical bond is ensured between the electroplated zinc layer and the zinc-aluminum-magnesium coating. This ensures that the composite coating will not peel off under stress during workpiece installation. The coating adhesion is qualitatively evaluated by using a file test on fastener samples with the composite coating; no coating peeling during the test indicates acceptable adhesion.

[0071] The composite coating of the present invention and its preparation method are described below with reference to specific embodiments. In the embodiments and comparative examples, M8X35 standard fasteners are used as samples.

[0072] Example 1

[0073] like Figure 1 As shown, this embodiment provides a fastener coated with a composite coating of the following specifications:

[0074] The inner zinc plating layer has a thickness of 2.0 micrometers, and the total thickness of the composite plating layer is 42.2 micrometers. The chemical composition of the outer zinc-aluminum-magnesium alloy plating layer, by mass percentage, is: aluminum (Al): 6%, magnesium (Mg): 2.0%, total impurities such as Pb and Cd <0.05%, and the balance is zinc (Zn).

[0075] The preparation method of the above fasteners is as follows:

[0076] (1) Fastener pretreatment: Fasteners are degreased with a general-purpose alkaline solution for electroplating and then washed with water. After degreasing, fasteners are pickled with a general-purpose hydrochloric acid solution for electroplating and then washed with water.

[0077] (2) Zinc plating by barrel plating: Zinc plating is performed using chloride salts. The plating solution composition is: KCl: 210-240g / l, ZnCl2: 55-75g / l, H2BO3: 25-30g / l, additives: 12-18g / l, pH: 5.4-6.2.

[0078] The electroplating process is as follows: plating bath temperature 25-30℃, current density 0.2-0.5A / dm³. 2 Electroplating time: 1–40 min.

[0079] (3) Medium-temperature centrifugal hot-dip plating:

[0080] The alloy plating ingot with the above alloy composition is heated and melted, and the temperature of the plating solution is controlled at 480°C.

[0081] Dry the electroplated fasteners in step (2) and then put them into a stainless steel basket for hot-dip galvanizing.

[0082] A stainless steel basket fitted with fasteners is placed into a centrifugal hot-dip galvanizing machine containing a zinc-aluminum-magnesium alloy plating solution for hot-dip galvanizing. The solution temperature is 480℃ and the hot-dip time is approximately 4 minutes.

[0083] Lift the fastener out of the plating solution and use a centrifuge to remove excess plating solution. The centrifuge speed is 3200-3800 rpm and the centrifugation time is 0.5-2 min.

[0084] After air cooling for 10-15 seconds, the workpiece is water-cooled in a water bath at a temperature of 60-85℃ to obtain a composite coating fastener sample.

[0085] Pre-cooling with air can reduce stress and deformation in the coating, and improve the uniformity and adhesion of the coating; subsequent water cooling can promote the formation of crystal nuclei, obtain finer grains, and help improve the density and corrosion resistance of the coating.

[0086] The evaluation was conducted according to the "GB / T 13912-2020 Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components with Metallic Coatings". A magnetic thickness gauge was used to measure the coating thickness of the fasteners. The surface quality of the workpiece after hot-dip galvanizing (e.g., aluminum-magnesium) was excellent. Figure 1 As shown in the figure, the adhesion test was conducted using the file method in Appendix C.6 of the standard, and the coating showed good adhesion.

[0087] The coating adhesion in this embodiment is rated as qualified, indicating that zinc-aluminum-magnesium alloys will not have incomplete plating when no flux is used in the hot-dip galvanizing process.

[0088] The samples were subjected to a neutral salt spray test according to the national standard (GB / T10125-1997) for 120 hours. The corrosion resistance of the workpieces was evaluated by the corrosion weight loss before and after the neutral salt spray test. The results are shown in Table 1.

[0089] Example 2

[0090] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0091] The inner zinc plating layer has a thickness of 3.5 micrometers, and the total thickness of the composite plating layer is 42.8 micrometers; the outer zinc-aluminum-magnesium alloy plating layer has the same chemical composition as in Example 1.

[0092] The preparation method of the above fasteners is the same as that in Example 1.

[0093] The evaluation was conducted in accordance with the requirements of GB / T 13912-2020 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components" and ISO 1461 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components". The coating thickness of the fasteners was measured using a magnetic thickness gauge. The surface quality of the workpiece after hot-dip galvanizing with aluminum-magnesium alloy was excellent. The adhesion test was conducted using the file method in Appendix C.6 of the standard, and the coating adhesion was good.

[0094] The samples were subjected to a neutral salt spray test according to the national standard (GB / T10125-1997) for 120 hours. The corrosion resistance of the workpieces was evaluated by the corrosion weight loss before and after the neutral salt spray test. The results are shown in Table 1.

[0095] Example 3

[0096] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0097] The inner zinc plating layer has a thickness of 5.0 micrometers, and the total thickness of the composite plating layer is 43.3 micrometers; the outer zinc-aluminum-magnesium alloy plating layer has the same chemical composition as in Example 1.

[0098] The preparation method of the above fasteners is the same as that in Example 1.

[0099] The evaluation was conducted in accordance with the requirements of GB / T 13912-2020 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components" and ISO 1461 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components". The coating thickness of the fasteners was measured using a magnetic thickness gauge. The surface quality of the workpiece after hot-dip galvanizing with aluminum-magnesium alloy was excellent. The adhesion test was conducted using the file method in Appendix C.6 of the standard, and the coating adhesion was good.

[0100] The samples were subjected to a neutral salt spray test according to the national standard (GB / T10125-1997) for 120 hours. The corrosion resistance of the workpieces was evaluated by the corrosion weight loss before and after the neutral salt spray test. The results are shown in Table 1.

[0101] Example 4

[0102] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0103] The inner zinc plating layer has a thickness of 3.5 micrometers, and the total thickness of the composite plating layer is 25.1 micrometers; the outer zinc-aluminum-magnesium alloy plating layer has the same chemical composition as in Example 1.

[0104] The preparation method of the above fasteners is the same as that in Example 1.

[0105] The evaluation was conducted in accordance with the requirements of GB / T 13912-2020 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components" and ISO 1461 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components". The coating thickness of the fasteners was measured using a magnetic thickness gauge. The surface quality of the workpiece after hot-dip galvanizing with aluminum-magnesium alloy was excellent. The adhesion test was conducted using the file method in Appendix C.6 of the standard, and the coating adhesion was good.

[0106] The samples were subjected to a neutral salt spray test according to the national standard (GB / T10125-1997) for 120 hours. The corrosion resistance of the workpieces was evaluated by the corrosion weight loss before and after the neutral salt spray test. The results are shown in Table 1.

[0107] Example 5

[0108] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0109] The inner zinc plating layer has a thickness of 3.5 micrometers, and the total thickness of the composite plating layer is 38.1 micrometers; the outer zinc-aluminum-magnesium alloy plating layer has the same chemical composition as in Example 1.

[0110] The preparation method of the above fasteners is the same as that in Example 1, except that the temperature of the plating solution for centrifugal hot dipping is set to 500°C.

[0111] The evaluation was conducted in accordance with the requirements of GB / T 13912-2020 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components" and ISO 1461 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components". The coating thickness of the fasteners was measured using a magnetic thickness gauge. The surface quality of the workpiece after hot-dip galvanizing with aluminum-magnesium alloy was excellent. The adhesion test was conducted using the file method in Appendix C.6 of the standard, and the coating adhesion was good.

[0112] The samples were subjected to a neutral salt spray test according to the national standard (GB / T10125-1997) for 120 hours. The corrosion resistance of the workpieces was evaluated by the corrosion weight loss before and after the neutral salt spray test. The results are shown in Table 1.

[0113] Example 6

[0114] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0115] The inner zinc plating layer is 3.5 micrometers thick, and the total thickness of the composite plating layer is 40.5 micrometers. The chemical composition of the outer zinc-aluminum-magnesium alloy plating layer, by mass percentage, is: aluminum (Al): 11%, magnesium (Mg): 3.0%, total impurities such as Pb and Cd <0.05%, and the balance is zinc (Zn).

[0116] The preparation method of the above fasteners is the same as that in Example 1, except that the temperature of the plating solution for centrifugal hot dipping is set to 490°C.

[0117] The evaluation was conducted in accordance with the requirements of GB / T 13912-2020 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components" and ISO 1461 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components". The coating thickness of the fasteners was measured using a magnetic thickness gauge. The surface quality of the workpiece after hot-dip galvanizing with aluminum-magnesium alloy was excellent. The adhesion test was conducted using the file method in Appendix C.6 of the standard, and the coating adhesion was good.

[0118] The samples were subjected to a neutral salt spray test according to the national standard (GB / T10125-1997) for 120 hours. The corrosion resistance of the workpieces was evaluated by the corrosion weight loss before and after the neutral salt spray test. The results are shown in Table 1.

[0119] Examples 7 to 9

[0120] The chemical composition of the outer zinc-aluminum-magnesium alloy coating, by mass percentage, is as follows: aluminum (Al): 11%, magnesium (Mg): 3.0%, total impurities such as Pb and Cd <0.05%, and the balance is zinc (Zn).

[0121] The sample preparation method is the same as in Implementation 1. The specific parameter settings and test results are detailed in Table 1.

[0122] Examples 10 to 12

[0123] The chemical composition of the outer zinc-aluminum-magnesium alloy coating, by mass percentage, is as follows: aluminum (Al): 5%, magnesium (Mg): 2.0%, total impurities such as Pb and Cd <0.05%, and the balance is zinc (Zn).

[0124] The sample preparation method is the same as in Implementation 1. The specific parameter settings and test results are detailed in Table 1.

[0125] Example 13

[0126] The chemical composition of the outer zinc-aluminum-magnesium alloy coating, by mass percentage, is as follows: aluminum (Al): 20%, magnesium (Mg): 2.0%, total impurities such as Pb and Cd <0.05%, and the balance is zinc (Zn).

[0127] The sample preparation method is the same as in Implementation 1. The specific parameter settings and test results are detailed in Table 1.

[0128] Example 14

[0129] The chemical composition of the outer zinc-aluminum-magnesium alloy coating, by mass percentage, is as follows: aluminum (Al): 20%, magnesium (Mg): 3.0%, total impurities such as Pb and Cd <0.05%, and the balance is zinc (Zn).

[0130] The sample preparation method is the same as in Implementation 1. The specific parameter settings and test results are detailed in Table 1.

[0131] Example 15

[0132] The chemical composition of the outer zinc-aluminum-magnesium alloy coating, by mass percentage, is as follows: aluminum (Al): 20%, magnesium (Mg): 4.0%, total impurities such as Pb and Cd <0.05%, and the balance is zinc (Zn).

[0133] The sample preparation method is the same as in Implementation 1. The specific parameter settings and test results are detailed in Table 1.

[0134] Comparative Example 1

[0135] A pure zinc-plated fastener is provided, which is available in the existing commercial market using a commonly used high-temperature hot-dip pure zinc plating sample with a plating thickness of 45.5 micrometers.

[0136] The high-temperature hot-dip plating process used for the samples obtained in the aforementioned commercial market was as follows: degreasing - pickling - water washing - fluxing - drying - loading - hot-dip plating + centrifugal spinning - water cooling - drying; wherein the hot-dip plating temperature was 520-560℃.

[0137] The fluxing agent is: ammonium chloride 180-200 g / L, zinc chloride 35-55 g / L, the pH is adjusted to 3.5-4.5 with ammonia water, and iron ions are removed with hydrogen peroxide; the fluxing temperature is 70-85℃.

[0138] Comparative Example 2

[0139] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0140] The inner zinc plating layer has a thickness of 1.5 micrometers, and the total thickness of the composite plating layer is 41.8 micrometers; the outer zinc-aluminum-magnesium alloy plating layer has the same chemical composition as in Example 1.

[0141] The preparation method of the above fasteners is the same as that in Example 1.

[0142] In Comparative Example 2, the workpiece surface after hot-dip zinc-aluminum-magnesium plating showed unplated spots, but the coating adhesion was good.

[0143] The samples were subjected to a neutral salt spray test according to the national standard (GB / T10125-1997) for 120 hours. The corrosion resistance of the workpieces was evaluated by the corrosion weight loss before and after the neutral salt spray test. The results are shown in Table 1.

[0144] Comparative Example 3

[0145] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0146] The inner zinc plating layer has a thickness of 6 micrometers, and the total thickness of the composite plating layer is 44.5 micrometers; the outer zinc-aluminum-magnesium alloy plating layer has the same chemical composition as in Example 1.

[0147] The preparation method of the above fasteners is the same as that in Example 1.

[0148] The surface quality of the workpiece after hot-dip zinc-aluminum-magnesium plating of the sample in Comparative Example 3 was excellent, but the adhesion of the coating was poor.

[0149] The samples were subjected to a neutral salt spray test according to the national standard (GB / T10125-1997) for 120 hours. The corrosion resistance of the workpieces was evaluated by the corrosion weight loss before and after the neutral salt spray test. The results are shown in Table 1.

[0150] Comparative Example 4

[0151] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0152] The inner zinc plating layer has a thickness of 1.5 micrometers, and the total thickness of the composite plating layer is 42.3 micrometers; the outer zinc-aluminum-magnesium alloy plating layer has the same chemical composition as in Example 6.

[0153] The preparation method of the above fasteners is the same as that of Example 6. The specific parameter settings and evaluation results are detailed in Table 1.

[0154] Comparative Example 5

[0155] This embodiment provides a fastener coated with a composite coating of the following specifications:

[0156] The inner zinc plating layer has a thickness of 6 micrometers, and the total thickness of the composite plating layer is 45.2 micrometers; the outer zinc-aluminum-magnesium alloy plating layer has the same chemical composition as in Example 6.

[0157] The preparation method of the above fasteners is the same as that of Example 6. The specific parameter settings and evaluation results are detailed in Table 1.

[0158] Table 1 Evaluation of the composite coatings of the examples and comparative samples

[0159]

[0160] Note: The coating adhesion strength evaluation is based on the file method in Appendix C.6 of GB / T 13912-2020 Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings of Steel Components.

[0161] As can be seen from the coating evaluation results in Table 1, the corrosion resistance (corrosion weight loss) of Examples 1-15 of the present invention is 3-10 times higher than that of the fasteners in Comparative Example 1 which only have a hot-dip galvanized pure zinc coating. The thickness of the electroplated zinc layer in Examples 1-15 is strictly controlled within 2-5 micrometers, and the coating appearance quality is good and the coating adhesion is qualified. However, Comparative Example 2 and Comparative Example 4, where the electroplated zinc layer thickness is less than 2 micrometers, show cases of outer layer incomplete coating. Comparative Example 3 and Comparative Example 5, where the electroplated zinc layer thickness is more than 5 micrometers, show unqualified coating adhesion. As can be seen from the corrosion weight loss of Examples 1-5, 6-12, and 13-15, the higher the Al content in the outer zinc-aluminum-magnesium coating, the better the corrosion resistance of the fastener samples. Among them, the Al mass fraction in Examples 13-15 is 20%, and the corrosion resistance of the fasteners is more than 3 times higher than that in Examples 1-5 (Al mass fraction 6%).

[0162] In summary, this invention, through a composite coating design combining an inner electroplated zinc layer and an outer zinc-aluminum-magnesium coating, achieves a novel composite coating with excellent corrosion resistance and a service life perfectly matched to the fixing and connection of zinc-aluminum-magnesium alloy coated steel sheet components. Furthermore, by controlling the thickness of the electroplated zinc layer to 2–5 micrometers, the quality of the composite coating is significantly improved. This prevents incomplete plating of the zinc-aluminum-magnesium alloy in the hot-dip galvanizing process without the need for fluxing agents, and ensures a complete metallurgical bond between the electroplated zinc layer and the zinc-aluminum-magnesium coating, guaranteeing that the composite coating will not peel off under stress during workpiece installation. It also greatly improves the hot-dip galvanizing environment and reduces the hot-dip galvanizing temperature to 470–510°C, saving energy. Additionally, by controlling the aluminum content of the outer zinc-aluminum-magnesium alloy coating to a high level of 14%–23%, the corrosion resistance of the fasteners is significantly improved.

[0163] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A high corrosion-resistant alloy composite coating for fasteners, characterized in that, It includes an inner layer and an outer layer. The inner layer is an electroplated zinc layer with a thickness of 2 to 5 micrometers that is closely attached to the outer surface of the fastener substrate. The outer layer is a zinc-aluminum-magnesium alloy plating layer that is closely attached to the outer surface of the inner layer.

2. The composite coating according to claim 1, characterized in that, The zinc-aluminum-magnesium alloy coating has a thickness of 20–70 micrometers.

3. The composite coating according to claim 1, characterized in that, The volume ratio of the inner layer to the overall composite coating is no more than 10%.

4. The composite coating according to claim 2, characterized in that, The main components of the zinc-aluminum-magnesium alloy coating, by mass percentage, are: Al: 5-23%, Mg: 2.0-4.0%, with the balance being Zn and impurities.

5. A method for preparing a high corrosion-resistant alloy composite coating for fasteners, characterized in that, Used to prepare the high corrosion-resistant alloy composite coating for fasteners as described in claims 1 to 4.

6. The preparation method according to claim 5, characterized in that, The method for preparing the composite coating includes the following steps: (1) Fastener pretreatment; (2) Electroplating pure zinc layer: The pretreated fasteners are placed in a zinc salt plating solution for electroplating, and the thickness of the pure zinc plating layer is controlled to be 2 to 5 micrometers. (3) Medium-temperature centrifugal hot-dip plating: The fasteners after electroplating in step (2) are placed in the zinc-aluminum-magnesium alloy plating solution for medium-temperature centrifugal hot-dip plating, and the thickness of the zinc-aluminum-magnesium alloy plating layer is controlled to be 20-70 micrometers.

7. The preparation method according to claim 6, characterized in that, The fastener pretreatment process in step (1) is: alkaline washing to remove oil - water washing - acid washing - water washing.

8. The preparation method according to claim 6, characterized in that, The zinc salts in the plating solution mentioned in step (2) include ammonium salts, chloride salts, sulfates, and zincates.

9. The preparation method according to claim 6, characterized in that, The medium-temperature centrifugal hot-dip galvanizing temperature mentioned in step (3) is 470-510℃.

10. The preparation method according to claim 6, characterized in that, The zinc-aluminum-magnesium coating structure obtained in step (3) includes the first precipitated aluminum-rich phase, the binary eutectic phase composed of the aluminum-rich phase and MgZn2, and the ternary eutectic phase composed of the aluminum phase, zinc phase and MgZn2.

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