A Zn-Ni / Cr(III) sandwich structure composite film for carbon steel surface and its preparation method

Abstract

This invention relates to the field of metal surface electroplating and passivation technology, specifically to a Zn-Ni / Cr(III) sandwich structure composite film for carbon steel surfaces and its preparation method. First, the carbon steel substrate is pretreated, including grinding, polishing, and activation. Then, a Zn-Ni alloy is deposited. The electroplating solution is composed of zinc chloride, nickel chloride, sodium hydroxide, triethanolamine, and polyethyleneimine, and the alloy layer performance is optimized by adjusting the electroplating temperature, electroplating time, and current density. Subsequently, the coating undergoes Cr(III)-based passivation treatment, forming a dense Zn-Ni / Cr(III) sandwich structure composite film on the carbon steel substrate surface. Under alkaline conditions, this film has uniform thickness and extremely low corrosion tendency, providing long-term stable protection for the metal substrate. Furthermore, the preparation method is simple, with mild and controllable conditions, and has good prospects for industrial application.

Description

Technical Field

[0001] This invention relates to the field of surface treatment of metallic materials, specifically to a Zn-Ni / Cr(III) sandwich structure composite film for carbon steel surface and its preparation method. Background Technology

[0002] Zn-Ni alloy electroplating is widely used as a protective coating for steel materials, especially high-strength steel, due to its excellent corrosion resistance and low hydrogen embrittlement. Furthermore, this coating also possesses good heat resistance and weldability, maintaining stable protective effects even in high-temperature, high-humidity, or marine atmospheric environments, making it a mainstream technology to replace traditional toxic Cd coatings and ordinary Zn coatings. However, existing Zn-Ni alloy electroplating technology still faces many challenges in practical applications. On the one hand, Zn-Ni alloys belong to a typical anomalous co-deposition system, and the coating composition is extremely sensitive to current density distribution. Currently used sulfate-based electroplating solutions (such as CN102312238A, CN102260888A, etc.) have poor conductivity, leading to Ni… 2+ Uneven Ni content distribution affects localized protective performance. On the other hand, the Ni content in the plating bath... 2+ It is easy to form hydroxide precipitates, and a large amount of complexing agents are needed to maintain stability (such as CN119265651A, CN114075684B, etc.). The plating solution has a complex composition and is difficult to maintain during long-term production, resulting in high production costs.

[0003] Zn-Ni alloy coatings have relatively inert surface chemicals, making it difficult to form a uniform and dense conversion film using conventional passivation treatments. Generally, Zn and Zn alloy surfaces can be treated with an environmentally friendly dense Cr(III) passivation process, which can form a conversion film on pure Zn surfaces, improving the corrosion resistance of the coating to some extent. However, with increasingly stringent industrial protection standards, the combination of a single Zn coating and Cr(III) passivation is showing insufficient performance in terms of salt spray resistance and resistance to localized corrosion, failing to meet the stringent long-term protection requirements of high-end fields such as automotive and marine engineering. In contrast, Zn-Ni alloy coatings, due to their higher chemical stability and superior corrosion-resistant substrate, provide better film-forming conditions for Cr(III) passivation. Existing technologies (such as CN116180063A) show that special Cr(III) passivation modification on the Zn-Ni alloy surface can obtain a more uniform and dense conversion film, significantly improving the adhesion and corrosion resistance life of the passivation layer. However, the inherent high chemical inertness of Zn-Ni alloys presents a natural technical bottleneck in their passivation process: the coating is difficult to undergo sufficient anodic dissolution in the passivation solution, and the Ni element on the alloy surface hinders the continuous and dense growth of the passivation film. This results in a thin conversion film formed by conventional processes, often accompanied by micropore defects, making it difficult to fully utilize the synergistic anti-corrosion advantages of the "alloy coating + conversion film," and placing extremely high demands on the supporting pretreatment and passivation systems. In addition, patent CN111945200A proposes an environmentally friendly passivation and stripping method for zinc-nickel alloy coatings. The passivation solution of this scheme has a high concentration of chromium nitrate as the main salt, reaching 70-90 g / L, resulting in extremely high raw material costs. Furthermore, its passivation solution uses a multi-component complexing agent system composed of sodium oxalate, malonic acid, maleic anhydride, and fluoroboric acid, which has a complex composition and is difficult to maintain during production. The patent with publication number CN103741127A proposes a passivation solution and passivation method for zinc-nickel alloy coating. The passivation solution of this scheme uses chromium oxide of hexavalent chromium as the main salt. Hexavalent chromium is a highly toxic and carcinogenic substance, which poses serious environmental risks and cannot solve the problems of uneven composition and poor stability of the plating solution in the preparation process of Zn-Ni alloy coating.

[0004] Existing technologies either focus on optimizing a single electroplating or passivation process, failing to achieve synergistic matching of processes; or employ high-concentration chromium salts and complex multi-component complexing agent systems, resulting in high production costs and significant environmental risks; or the process parameter design cannot balance the corrosion resistance and passivation film-forming performance of the Zn-Ni alloy coating, failing to overcome the film-forming bottleneck caused by the high inertness of the Zn-Ni alloy, ultimately leading to composite films that cannot meet the long-term protection requirements of high-end applications. To address these issues, this invention provides a Zn-Ni / Cr(III) sandwich structure composite film for carbon steel surfaces and its preparation method by systematically optimizing the carbon steel substrate pretreatment process, the composition and electroplating conditions of the Zn-Ni alloy electroplating solution, and the composition and passivation conditions of the Cr(III) passivation solution. This significantly improves the overall protective performance and service life of the composite layer. Summary of the Invention

[0005] The purpose of this invention is to provide a Zn-Ni / Cr(III) sandwich structure composite film for carbon steel surfaces and its preparation method, to solve problems such as weak uniformity and poor corrosion resistance of single film layers. This method improves the corrosion resistance and service life of the composite film by optimizing the carbon steel pretreatment process, electroplating solution formulation and electroplating parameters, and passivation solution formulation and passivation parameters.

[0006] The technical solution of the present invention is as follows:

[0007] A method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface includes the following steps:

[0008] (a) Pretreatment of carbon steel;

[0009] (b) Preparation of Zn-Ni alloys;

[0010] (c) Cr(III) group passivation treatment;

[0011] (d) Curing of the film;

[0012] In step (b), the electroplating solution used for preparing the Zn-Ni alloy contains the following components: zinc chloride 7–10 g / L, nickel chloride 1–3 g / L, sodium hydroxide 100–130 g / L, triethanolamine 1–2.5 ml / L, polyethyleneimine 95–110 ml / L, and the balance being deionized water; in the electroplating solution, Ni 2+ The content is 0.3–1.2 g / L;

[0013] In step (c), the passivation solution used for Cr(III)-based passivation treatment contains the following components: 7-10 g / L chromium nitrate, 1-2 g / L sodium nitrate, 2-3 g / L cobalt nitrate, 15-25 g / L boric acid, 9-15 g / L oxalic acid, and the balance is deionized water.

[0014] The method for preparing Zn-Ni / Cr(III) sandwich structure composite film on carbon steel surface, wherein the carbon steel pretreatment in step (a) includes edge chamfering, wire connection, insulating resin encapsulation, substrate grinding and polishing, and substrate activation treatment; the treatment solution for substrate activation treatment is selected from one or more of sulfuric acid-ammonium sulfate system, hydrogen peroxide-acid system, and sodium nitrate-sodium chloride system, wherein the volume ratio of the two components in each system is 5:1, and the activation time is 40-60s.

[0015] The method for preparing Zn-Ni / Cr(III) sandwich structure composite film on carbon steel surface, in step (b), the preparation steps of the electroplating solution are as follows: dissolve nickel chloride and zinc chloride in part of deionized water, add sodium hydroxide to dissolve fully, and when the temperature is below 70°C, add triethanolamine and polyethyleneimine in sequence, and finally add the remaining deionized water, and stir for 1 to 2 hours to obtain the film.

[0016] In the method for preparing the Zn-Ni / Cr(III) sandwich structure composite film on the surface of carbon steel, step (b) involves the following process conditions for Zn-Ni alloy electroplating: temperature 25–50°C, current density 1–2 A / dm³. 2 Electroplating time: 25-40 minutes.

[0017] In the method for preparing the Zn-Ni / Cr(III) sandwich structure composite film on the surface of carbon steel, in step (b), after the Zn-Ni alloy is electroplated, it is placed in 0.5-1.5wt% dilute nitric acid for 3-10s for brightening, and then taken out and washed with water to obtain the Zn-Ni alloy layer.

[0018] The method for preparing the Zn-Ni / Cr(III) sandwich structure composite film on the surface of carbon steel, in step (c), the preparation steps of the passivation solution are as follows: first, dissolve sodium nitrate and cobalt nitrate in part of deionized water, then add boric acid and chromium nitrate in sequence, stir until fully dissolved, then add oxalic acid, and finally add the remaining deionized water, let stand for 2 to 4 hours, and take the supernatant to obtain the product.

[0019] In the method for preparing the Zn-Ni / Cr(III) sandwich structure composite film on the surface of carbon steel, in step (c) of the Cr(III)-based passivation treatment, the passivation temperature is 30-60℃, the passivation time is 130-180s, and the pH value of the passivation solution is controlled within the range of 1.2-2.5.

[0020] The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface, in step (c), includes the following steps for using the Cr(III)-based passivation solution:

[0021] (1) pH adjustment: Use 5-15 wt% sodium hydroxide aqueous solution or 5-15 wt% nitric acid solution to adjust the pH of the passivation solution to 1.2-2.5;

[0022] (2) Passivation treatment: Immerse the Zn-Ni coated workpiece to be treated in the passivation solution, control the height of the workpiece surface to be passivated from the liquid surface to be 1-2 cm, and passivate at 30-60℃ for 130-180 s;

[0023] (3) Post-treatment: After passivation, the workpiece is taken out and rinsed with deionized water and ethanol in sequence, and then dried with cold air.

[0024] In the method for preparing Zn-Ni / Cr(III) sandwich structure composite film on carbon steel surface, the curing temperature in step (d) is 80-95℃ and the curing time is 25-40min.

[0025] A Zn-Ni / Cr(III) sandwich structure composite film for carbon steel surfaces is disclosed. The composite film has a three-layer sandwich structure consisting of a carbon steel substrate, a Zn-Ni alloy intermediate layer, and a Cr(III) passivation top layer. The thickness of the Zn-Ni alloy intermediate layer is 8–15 μm, the thickness of the Cr(III) passivation top layer is 0.3–1.0 μm, and the self-corrosion current density of the composite film is 3.0 × 10⁻⁶. -4 ~3.0×10 -6 A•cm -2 .

[0026] The design concept of this invention is:

[0027] Cr(III)-based passivation technology for Zn-Ni alloys balances the dual requirements of environmental protection and corrosion resistance, making it valuable for applications in metal corrosion protection. However, existing film preparation processes and corrosion resistance still have certain shortcomings, making it difficult to meet increasingly stringent industrial application requirements. Existing Zn-Ni alloy sulfate plating solutions have poor conductivity and Ni... 2+ Uneven Ni content distribution leads to insufficient local protection in the coating; at the same time, existing plating solutions rely on a large amount of complexing agents to maintain Ni content. 2+ The process of preparing a Zn-Ni / Cr(III) sandwich-structured composite film is characterized by instability, complex plating bath composition, accumulation of decomposition products over long-term production, high maintenance difficulty, and high production costs. To address these issues, this invention focuses on optimizing the preparation process of the Zn-Ni / Cr(III) sandwich-structured composite film. Through systematic improvements to carbon steel pretreatment, electroplating, and passivation processes, a composite film with high corrosion resistance is ultimately obtained. In the preparation of the Zn-Ni alloy electroplating layer, this invention uses chlorides with superior conductivity as the main salt system, which improves the conductivity and current efficiency of the plating bath, ensuring a uniform Ni content distribution in the coating from the source. Simultaneously, it innovatively employs triethanolamine and polyethyleneimine as a dual complexing agent system to effectively complex Ni. 2+To prevent the formation of hydroxide precipitates, significantly improve the long-term production stability of the plating solution, reduce the accumulation of various complex decomposition products, improve coating leveling and reduce brittleness through polyethyleneimine, and obtain a more finely crystalline Zn-Ni alloy coating; simultaneously, by controlling parameters such as electroplating temperature, the stability of coating quality is ensured, and the Ni content in the plating solution is controlled. 2+ With a content of 0.3–1.2 g / L, the coating's own corrosion resistance and subsequent passivation film formation performance are balanced. This avoids the dual defects of poor corrosion resistance caused by too low a Ni content, which results in the coating being mixed with pure zinc or nickel-poor phases, while also avoiding the inert electrochemical behavior of the coating caused by too high a Ni content, which makes it difficult to dissolve and form a film during passivation treatment.

[0028] Furthermore, regarding Cr(III)-based passivation treatment, this invention addresses the challenges of film formation in highly inert Zn-Ni alloy coatings by optimizing the passivation solution composition and ratio. Using low-concentration chromium nitrate as the main salt, the synergistic catalytic effect of sodium nitrate and cobalt nitrate accelerates the anodic dissolution of the Zn-Ni alloy coating in the passivation solution, overcoming the obstacle of Ni element to the continuous growth of the passivation film and providing a foundation for dense passivation film growth. Boric acid is introduced into the passivation solution to stabilize the system, while expensive additives are removed to reduce production costs. By controlling parameters such as pH value and passivation time (e.g., pH 1.2–2.5, temperature 30–60℃, time 130–180s), the stability of the passivation solution composition is ensured, promoting uniform and dense passivation film growth. Through the synergistic optimization of the above process design, the Zn-Ni / Cr(III) sandwich structure composite film finally prepared by this invention possesses both excellent corrosion resistance and good environmental characteristics, better meeting the needs of industrial applications.

[0029] The advantages and beneficial effects of this invention are:

[0030] 1. The present invention performs pretreatment before electroplating, which can reduce electrochemical testing errors, make the electroplated layer tightly bonded to the substrate, and avoid the problems of scorching and uneven thickness of the plating layer caused by tip discharge during the electroplating process.

[0031] 2. The Zn-Ni alloy plating solution of this invention uses triethanolamine and polyethyleneimine as a complexing agent system, which can reduce the formation of various organic complex decomposition products, improve the dispersion ability and long-term production stability of the plating solution; the addition of polyethyleneimine helps to obtain a bright coating, improves leveling, and reduces brittleness. This complexing system can improve the dispersion ability of the plating solution, resulting in a Zn-Ni alloy coating with finer crystals and uniform Ni content distribution. Through multiple optimizations of electroplating parameters, the stability of the coating is further improved.

[0032] 3. The Cr(III)-based passivation solution of this invention contains cobalt nitrate, sodium nitrate, and boric acid, which can catalyze and accelerate the dissolution of the Zn-Ni coating and the deposition of the chromium film, stabilize the pH value during the passivation process, and maintain the uniformity of the film. By optimizing the passivation process, the problem of film formation difficulties caused by the high inertness of zinc-nickel alloys is effectively overcome, resulting in a denser film and ultimately forming a uniform, dense trivalent chromium passivation film without obvious unevenness, cracks, or pores.

[0033] 4. This invention, through optimization of the electroplating solution, passivation solution, and electroplating and passivation processes, prepares a Zn-Ni / Cr(III) sandwich-structured composite film that reduces brittleness, maintains uniform and dense film layers, and significantly improves the overall flatness and interlayer bonding of the film layer through the synergistic effect of the Zn-Ni alloy intermediate layer and the Cr(III) passivation top layer, thereby greatly enhancing corrosion resistance. The self-corrosion current density of the composite film is 3.0 × 10⁻⁶. -4 ~3.0×10 -6 A•cm -2 Within the specified range, in an alkaline environment containing chloride ions with a pH of 9–10, the film layer did not develop obvious cracks or defects after corrosion, and the surface structure remained intact. It can form an effective and stable protection for carbon steel substrates, significantly delaying the corrosion process and exhibiting excellent long-term protective performance.

[0034] 5. The passivation solution of the present invention does not contain highly toxic hexavalent chromium, and is an environmentally friendly product with good prospects for green application. At the same time, the passivation solution eliminates expensive additives, the electroplating solution system has a simple composition and low maintenance difficulty, effectively reducing the production and preparation cost. The overall process is simple, the conditions are mild and controllable, and it has good prospects for industrial application. Attached Figure Description

[0035] Figure 1 Surface and cross-sectional views of the Zn-Ni / Cr(III) sandwich structure composite film: (a) Surface view; (b) Cross-sectional view.

[0036] Figure 2 Scanning electron microscope images of the Zn-Ni / Cr(III) sandwich composite film: (a) before corrosion; (b) after corrosion.

[0037] Figure 3 The results are polarization curves of Zn-Ni / Cr(III) sandwich composite films in an alkaline environment of pH 9–10.

[0038] Figure 4 for Figure 3 Enlarged view of point A in the image. Detailed Implementation

[0039] In its specific implementation, this invention proposes a method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface, comprising the following steps: (a) pretreatment of the carbon steel substrate, including grinding, polishing, and activation; (b) deposition of the Zn-Ni alloy, wherein the electroplating solution is composed of zinc chloride, nickel chloride, sodium hydroxide, triethanolamine, and polyethyleneimine, and the alloy layer performance is optimized by adjusting the electroplating temperature, electroplating time, and current density; (c) Cr(III)-based passivation treatment of the coating, wherein the passivation solution contains chromium nitrate, sodium nitrate, cobalt nitrate, boric acid, and oxalic acid, and the passivation conditions are: temperature 30–60℃, time 130–180s, and pH value 1.2–2.5; (d) curing at 80–95℃ for 25–40 min, thereby forming a dense Zn-Ni / Cr(III) sandwich structure composite film on the carbon steel substrate surface. The sandwich structure is a three-layer sandwich structure consisting of a carbon steel substrate, a Zn-Ni alloy intermediate layer, and a Cr(III) passivation top layer. The thickness of the Zn-Ni alloy layer is controlled within the range of 8–15 μm, and the thickness of the Cr(III) passivation film is controlled within the range of 0.3–1.0 μm.

[0040] The electroplating solution for Zn-Ni alloys contains the following components: zinc chloride 7–10 g / L, nickel chloride 1–3 g / L, sodium hydroxide 100–130 g / L, triethanolamine 1–2.5 ml / L, polyethyleneimine 95–110 ml / L, with the balance being deionized water. The Ni content in the plating solution... 2+ The Ni content is 0.3–1.2 g / L. Studies have shown (Zhang Shujuan, et al., New Technology and New Process, 2024) that when the Ni content in the coating is insufficient, it may contain pure zinc phase or nickel-poor phase, resulting in poor corrosion resistance; when the Ni content in the coating is too high, the surface electrochemical behavior tends to be inert, making it difficult to dissolve during subsequent passivation treatment and thus unable to form a dense and complete passivation film. When the Ni content in the plating solution is too high... 2+ Maintaining the content within the range of 0.3 to 1.2 g / L can effectively improve the overall quality of Zn-Ni coatings.

[0041] The preparation steps of the electroplating solution are as follows: Dissolve nickel chloride and zinc chloride in part of deionized water; then, add sodium hydroxide to dissolve it completely. When the temperature is below 70°C, add triethanolamine and polyethyleneimine in sequence to avoid the organic complexing agent from decomposing or becoming ineffective due to high temperature; finally, add the remaining deionized water and stir for 1-2 hours to obtain the solution.

[0042] The passivation solution for Cr(III)-based passivation treatment contains the following components: 7-10 g / L chromium nitrate, 1-2 g / L sodium nitrate, 2-3 g / L cobalt nitrate, 15-25 g / L boric acid, 9-15 g / L oxalic acid, and the balance is deionized water.

[0043] Passivation treatment was carried out at a temperature of 30–60℃, with the passivation time controlled between 130–180 seconds. According to the research of Wen Quan et al. (Electroplating and Environmental Protection, 2017), the pH value of the passivation solution has a significant impact on the performance of the passivation film. When the pH value is too low, the dissolution rate of the passivation film is too fast, resulting in an indistinct microcrack structure, which is detrimental to improving the film's corrosion resistance. Conversely, when the pH value is too high, the activity of the passivation solution decreases, inhibiting the normal growth of the passivation film. Therefore, sodium hydroxide or nitric acid is used to control the pH value of the passivation solution within the range of 1.2–2.5 to ensure that the passivation film has good structure and corrosion resistance.

[0044] The present invention will be further described below with reference to specific embodiments and accompanying drawings, but the embodiments described herein do not constitute a limitation of the present invention.

[0045] Example 1

[0046] This embodiment provides a specific method for preparing a Zn-Ni / Cr(III) sandwich structure composite film, using Q235 steel as the substrate. The preparation process includes, in sequence, substrate pretreatment, Zn-Ni alloy electroplating, passivation solution preparation, passivation treatment, and curing, as detailed below:

[0047] (1) Substrate pretreatment

[0048] To ensure a uniform coating on the substrate surface, Q235 steel is pretreated according to the following steps:

[0049] (a) Edge chamfering: Use 400# to grind the rust on the sample surface and the edges around the working surface on a metallographic grinding machine to prevent the coating from burning or uneven thickness due to the tip discharge effect during the electroplating process.

[0050] (b) Exposing the working surface: The non-working area is encapsulated with insulating resin to precisely expose the working surface with a regular geometric shape (10mm×10mm in this embodiment) to ensure that the working area is constant and calculable during subsequent electroplating and testing.

[0051] (c) Wire connection: Connect copper wires on the back of the working surface by soldering or spot welding to ensure conductivity.

[0052] (d) Resin encapsulation: Place the sample face down in the mold and fill it with insulating resin for encapsulation. After the resin dries and cures for 24±2 hours, remove the sample and polish it in sequence with 400 to 2000# wet sandpaper (using 400#, 800#, 1500#, and 2000# sandpaper in sequence) to remove excess resin from the surface. Then, wash it with 80℃ hot water to remove the sealing wax.

[0053] (e) Ultrasonic cleaning: Place the packaged sample in alcohol and ultrasonically vibrate for 6 minutes to remove dust and grease. Finally, rinse with deionized water and dry with cold air for later use.

[0054] (f) Polishing before electroplating: Place the surface to be electroplated in a mixed solution of sodium nitrate and sodium chloride with a volume ratio of 5:1 and polish for 50 seconds. After removing it, rinse it with clean water.

[0055] (2) Preparation of Zn-Ni alloy electroplating layer

[0056] (a) Preparation of the electroplating solution: Under magnetic stirring, 1.3g of nickel chloride and 8g of zinc chloride were dissolved in a portion of deionized water; 125g of sodium hydroxide was added, and after complete dissolution and cooling to below 70°C, 1.5ml of triethanolamine and 100ml of polyethyleneimine were added sequentially; finally, the remaining deionized water was added to bring the volume to 1L, and stirring was continued for 1.5h to obtain the Zn-Ni alloy electroplating solution. The Ni content in the plating solution... 2+ The content is 0.32g / L.

[0057] (b) Electroplating treatment: 150 ml of the above electroplating solution was injected into the electroplating apparatus, the main structure of which was made of glass. The pre-treated and polished sample was placed inside. A platinum sheet electrode was used as the anode, and the temperature was 50°C with a current density of 1.95–2.05 A / dm³. 2 DC electroplating was performed under the specified conditions for 30 minutes. After electroplating, the sample was removed, rinsed with deionized water and ethanol in sequence, and then dried with cold air.

[0058] (c) Polishing of electroplated surface: Place the passivated surface of the electroplated sample in 1.0wt% dilute nitric acid for 5s for polishing, and then take it out and clean it with deionized water.

[0059] (3) Preparation of Cr(III)-based passivation film

[0060] (a) Preparation of passivation solution:

[0061] Under magnetic stirring conditions, 1.7 g of sodium nitrate and 2.9105 g of cobalt nitrate were dissolved in a portion of deionized water; 18.549 g of boric acid and 8.003 g of chromium nitrate were added sequentially, and the mixture was stirred until fully dissolved; then 12.604 g of oxalic acid was added; finally, the remaining deionized water was added to bring the volume to 1 L, and the pH of the passivation solution was adjusted to 2.0 with a 10 wt% sodium hydroxide aqueous solution and a 10 wt% nitric acid solution. After standing for 2 hours, the supernatant was collected to obtain the Cr(III) passivation solution.

[0062] (b) Passivation treatment: Pour 100 ml of the above passivation solution into a 100 ml beaker, place the Zn-Ni plated substrate in the center of the passivation solution, control the height of the surface to be passivated from the liquid surface to be 1.5 cm, and passivate at 50 °C for 2 min. After passivation, remove the sample, rinse it with deionized water and ethanol in sequence, and dry it with cold air.

[0063] (c) Curing treatment: The sample with the passivation film was placed on a heating stage and dried at 90°C for 30 min to obtain the Zn-Ni / Cr(III) sandwich structure composite film. In this embodiment, the sandwich structure is a three-layer sandwich structure consisting of a carbon steel substrate, a Zn-Ni alloy intermediate layer, and a Cr(III) passivation top layer. The thickness of the Zn-Ni alloy layer is 10 μm, and the thickness of the Cr(III) passivation film is 0.6 μm.

[0064] like Figure 1 As shown, the obtained Zn-Ni / Cr(III) sandwich structure composite film was scanned using an OLYMPU Z61 stereomicroscope and a ZEISS EVO1 scanning electron microscope. From... Figure 1 (a) As can be clearly observed in the stereomicroscopic surface image, the macroscopic surface of the obtained film is smooth and flat, without obvious defects, cracks or holes. Figure 1 (b) Microscopic cross-sectional images show that the film achieves uniform coverage on the substrate surface and has good thickness uniformity. In contrast, traditional single metal coatings or ordinary composite films often have unavoidable defects such as microcracks, pinholes or uneven thickness due to internal stress concentration (Ren Yanping, et al., Material Protection, 2007). The Zn-Ni / Cr(III) sandwich structure composite film prepared in this invention effectively overcomes the above problems.

[0065] Example 2

[0066] This embodiment characterizes the surface morphology of the Zn-Ni / Cr(III) sandwich composite film prepared in Example 1 before and after corrosion. The sample obtained in Example 1 was scanned using a ZEISS EVO1 scanning electron microscope. The film morphology before corrosion is shown in the figure below. Figure 2 As shown in (a). The sample obtained in Example 1 was immersed in a boric acid buffer solution with a pH of 9.5 (with additional NaCl added to make the molar concentration 0.01M) for 18 hours. After removal, the etched film was scanned, and its morphology is shown in Figure 1. Figure 2 As shown in (b). Comparison Figure 2 (b) and Figure 2(a) As can be seen, the film layer did not develop obvious cracks or defects after corrosion, and its surface structure remained relatively intact. In contrast, traditional ordinary composite films often exhibit obvious cracks, pitting, or peeling due to localized corrosion after immersion in a weakly alkaline environment containing chloride ions, making it difficult to maintain structural integrity (Oleg Kozaderov, et al, Journal of Solid State Electrochemistry, 2021). This indicates that the Zn-Ni / Cr(III) sandwich structure composite film layer has good structural stability in a weakly alkaline environment containing chloride ions, can effectively protect the substrate, and exhibits good corrosion resistance.

[0067] Example 3

[0068] In this embodiment, the electrochemical corrosion performance test of the Zn-Ni / Cr(III) sandwich composite film was conducted using a three-electrode system: the sample obtained in Example 1 was used as the working electrode, the counter electrode was a 20mm×20mm×0.5mm platinum sheet (with a φ0.5mm platinum wire welded on it and sealed and insulated with a polytetrafluoroethylene tube), and the reference electrode was a saturated KCl calomel electrode (SCE). Using a CHI660E electrochemical workstation (manufactured by Shanghai Chenhua Instrument Co., Ltd.), the sample obtained in Example 1 was immersed in a boric acid buffer solution (with NaCl added to make its molar concentration 0.01M) at a temperature of 298.15K and a pH of 9.5 for 1 hour, and then polarization curves were tested to obtain the polarization curve diagram. Figures 3-4 ).

[0069] like Figures 3-4 As shown, the polarization curves indicate that the film exhibits excellent corrosion resistance over a wide pH range. The self-corrosion current density measured under this pH condition is 3.0 × 10⁻⁶. -4 ~3.0×10 -6 A•cm -2 Within this range, the membrane maintains good stability in alkaline environments, exhibiting excellent protective performance. The lowest points of all curves are concentrated around 10. -6 A•cm -2 The magnitude indicates that the Zn-Ni / Cr(III) sandwich structure composite film of this invention can maintain extremely low corrosion current density in a wide range of alkaline chlorine-containing environments with pH 9 to 10, exhibiting stable and excellent corrosion resistance and no obvious environmental sensitivity.

[0070] Generally, a material's corrosion resistance is inversely proportional to its self-corrosion current density and directly proportional to its corrosion resistance. In this embodiment, the film obtained exhibits a self-corrosion current density as low as 10 under pH 9.5 conditions. -6 A•cm-2 Order of magnitude (6.5233 × 10) -6 A•cm -2 The corrosion resistance reaches 23.617 kΩ•cm. 2 Furthermore, it maintains a low corrosion current density within the pH range of 9–10. In contrast, traditional composite films typically exhibit a self-corrosion current density of 10 in alkaline, chlorine-containing environments. -4 ~10 -5 A·cm -2 Orders of magnitude, corrosion resistance is generally below 10 kΩ•cm 2 The corrosion resistance varies significantly with pH fluctuations (Yan Han, et al., Chinese Journal of Corrosion and Protection, 2017). This fully demonstrates that the Zn-Ni / Cr(III) sandwich composite film can form an effective and stable protection for the substrate in an alkaline environment containing chloride ions, significantly delaying the corrosion process and showing good application potential.

[0071] Example 4

[0072] This embodiment provides a method for preparing a Zn-Ni / Cr(III) sandwich structure composite film. The substrate to be treated is Q235 steel, and the preparation process includes the following steps in sequence: substrate pretreatment, Zn-Ni alloy electroplating, passivation solution preparation, passivation treatment, and curing, as detailed below:

[0073] (1) Substrate pretreatment

[0074] To ensure a uniform coating on the substrate surface, Q235 steel is pretreated according to the following steps:

[0075] (a) Edge chamfering: Use 400# sandpaper to polish the rust on the sample surface and the edges around the working surface on a metallographic grinding machine to avoid the coating being scorched or uneven in thickness due to tip discharge during the electroplating process.

[0076] (b) Exposed working surface: The non-working area is encapsulated with insulating resin to precisely expose a regular working surface of 10mm×10mm, ensuring that the working area is constant and calculable during subsequent electroplating and testing.

[0077] (c) Wire connection: Copper wires are spot welded to the back of the working surface to ensure conductivity.

[0078] (d) Resin encapsulation: Place the sample face down in the mold, pour in insulating resin for encapsulation, and after the resin dries and cures for 24±2 hours, take out the sample and polish it in sequence with 400#, 800#, 1500# and 2000# wet sandpaper to remove excess resin from the surface. Then wash it with 60℃ hot water to remove the sealing wax.

[0079] (e) Ultrasonic cleaning: Place the packaged sample in alcohol and ultrasonically vibrate for 8 minutes to remove dust and grease. Finally, rinse with deionized water and dry with cold air for later use.

[0080] (f) Polishing and activation before electroplating: Place the surface to be electroplated in a sulfuric acid-ammonium sulfate mixed solution with a volume ratio of 5:1 for 40 seconds, then take it out and rinse it with water.

[0081] (2) Preparation of Zn-Ni alloy electroplating layer

[0082] (a) Preparation of the electroplating solution: Under magnetic stirring, 1.45 g of nickel chloride and 7 g of zinc chloride were dissolved in a portion of deionized water; 100 g of sodium hydroxide was added, and after complete dissolution and cooling to below 70°C, 1.0 ml of triethanolamine and 95 ml of polyethyleneimine were added sequentially; finally, the remaining deionized water was added to bring the volume to 1 L, and stirring was continued for 1 h to obtain the Zn-Ni alloy electroplating solution. The Ni content in the plating solution... 2+ The content is 0.36g / L.

[0083] (b) Electroplating treatment: 120 ml of the above electroplating solution was injected into the electroplating apparatus. The main structure of the electroplating apparatus was made of polytetrafluoroethylene (PTFE). The pretreated and activated sample was placed in it, with a platinum sheet electrode as the anode, at a temperature of 25°C and a current density of 1.0 A / dm³. 2 DC electroplating was performed under the specified conditions for 40 minutes. After electroplating, the sample was removed, rinsed with deionized water and ethanol in sequence, and then dried with cold air.

[0084] (c) Polishing of electroplated surface: Place the passivated surface of the electroplated sample in 0.8wt% dilute nitric acid for 8s for polishing, and then take it out and clean it with deionized water.

[0085] (3) Preparation of Cr(III)-based passivation film

[0086] (a) Preparation of passivation solution: Under the stirring condition of magnetic stirrer, 1.0 g sodium nitrate and 2.0 g cobalt nitrate were dissolved in part of deionized water; 15 g boric acid and 7 g chromium nitrate were added in sequence and stirred until fully dissolved; then 9 g oxalic acid was added; finally, the remaining deionized water was added to make up to 1 L, and the pH value of the passivation solution was adjusted to 1.2 with a 5 wt% sodium hydroxide aqueous solution and a 5 wt% nitric acid solution. After standing for 4 h, the supernatant was taken to obtain the Cr(III) passivation solution.

[0087] (b) Passivation treatment: Pour 90 ml of the above passivation solution into a 90 ml beaker, place the Zn-Ni plated substrate in the center of the passivation solution, control the height of the surface to be passivated from the liquid surface to be 1 cm, and passivate at 30 °C for 180 s. After passivation, remove the sample, rinse it with deionized water and ethanol in sequence, and dry it with cold air.

[0088] (c) Curing treatment: The sample with the passivation film was placed on a heating stage and dried at 80°C for 40 min to obtain the Zn-Ni / Cr(III) sandwich structure composite film. In this embodiment, the thickness of the Zn-Ni alloy layer is 12 μm and the thickness of the Cr(III) passivation film is 0.4 μm.

[0089] The composite film obtained in this embodiment was characterized using scanning electron microscopy. The film surface was smooth and flat, without obvious cracks or pores, with uniform cross-sectional thickness and tight interlayer bonding. Polarization curve testing was performed in a borate buffer solution containing 0.01M NaCl at pH 9.0, and its self-corrosion current density was measured to be 2.1 × 10⁻⁶. -6 A•cm -2 The corrosion resistance reaches 20.3 kΩ•cm. 2 After being immersed in the same medium for 18 hours, the film surface showed no obvious corrosion defects and the structure remained intact, demonstrating excellent corrosion resistance.

[0090] Example 5

[0091] This embodiment provides a method for preparing a Zn-Ni / Cr(III) sandwich structure composite film. The substrate to be treated is 30CrMnSi high-strength steel. The preparation process includes the following steps in sequence: substrate pretreatment, Zn-Ni alloy electroplating, passivation solution preparation, passivation treatment, and curing, as detailed below:

[0092] (1) Substrate pretreatment

[0093] To ensure a uniform coating on the substrate surface, the high-strength steel is pretreated according to the following steps:

[0094] (a) Edge chamfering: Use 400# sandpaper to polish the rust on the sample surface and the edges around the working surface on a metallographic grinding machine to prevent the coating from burning or uneven thickness due to the tip discharge effect during the electroplating process.

[0095] (b) Exposed working surface: The non-working area is encapsulated with insulating resin to precisely expose a regular working surface of 10mm×10mm, ensuring that the working area is constant and calculable during subsequent electroplating and testing.

[0096] (c) Wire connection: Copper wires are connected by solder on the back of the working surface to ensure conductivity.

[0097] (d) Resin encapsulation: Place the sample face down in the mold, pour in insulating resin for encapsulation, and after the resin dries and cures for 24±2 hours, take out the sample and polish it in sequence with 400#, 800#, 1500# and 2000# wet sandpaper to remove excess resin from the surface. Then clean it with 90℃ hot water to remove the sealing wax.

[0098] (e) Ultrasonic cleaning: Place the packaged sample in alcohol and ultrasonically vibrate for 10 minutes to remove dust and grease. Finally, rinse with deionized water and dry with cold air for later use.

[0099] (f) Polishing and activation before electroplating: Place the surface to be electroplated in a mixed solution of hydrogen peroxide and acid with a volume ratio of 5:1 for 60 seconds, then remove and rinse with water.

[0100] (2) Preparation of Zn-Ni alloy electroplating layer

[0101] (a) Preparation of the electroplating solution: Under magnetic stirring, 3.0 g of nickel chloride and 10 g of zinc chloride were dissolved in a portion of deionized water; 130 g of sodium hydroxide was added, and after complete dissolution and cooling to below 70°C, 2.5 ml of triethanolamine and 110 ml of polyethyleneimine were added sequentially; finally, the remaining deionized water was added to bring the volume to 1 L, and stirring was continued for 2 hours to obtain the Zn-Ni alloy electroplating solution. The Ni content in the plating solution... 2+ The content is 0.74 g / L.

[0102] (b) Electroplating treatment: 170 ml of the above electroplating solution was injected into the electroplating apparatus. The main structure of the electroplating apparatus was made of glass. The pre-treated and activated sample was placed in it, with a platinum sheet electrode as the anode, at a temperature of 35°C and a current density of 1.5 A / dm³. 2 DC electroplating was performed under the specified conditions for 35 minutes. After electroplating, the sample was removed, rinsed with deionized water and ethanol in sequence, and then dried with cold air.

[0103] (c) Polishing of electroplated surface: Place the passivated surface of the electroplated sample in 1.2wt% dilute nitric acid for polishing for 10s, and then take it out and clean it with deionized water.

[0104] (3) Preparation of Cr(III)-based passivation film

[0105] (a) Preparation of passivation solution: Under the stirring condition of magnetic stirrer, 2.0g sodium nitrate and 3.0g cobalt nitrate were dissolved in part of deionized water; 25g boric acid and 10g chromium nitrate were added in sequence and stirred until fully dissolved; then 15g oxalic acid was added; finally, the remaining deionized water was added to make up to 1L, and the pH value of the passivation solution was adjusted to 2.5 with a 15wt% sodium hydroxide aqueous solution and a 15wt% nitric acid solution. After standing for 3h, the supernatant was taken to obtain the Cr(III) passivation solution.

[0106] (b) Passivation treatment: Pour 110 ml of the above passivation solution into a 110 ml beaker, place the Zn-Ni plated substrate in the center of the passivation solution, control the height of the surface to be passivated from the liquid surface to be 2 cm, and passivate at 60 °C for 130 s. After passivation, remove the sample, rinse it with deionized water and ethanol in sequence, and dry it with cold air.

[0107] (c) Curing treatment: The sample with the passivation film was placed on a heating stage and dried at a constant temperature of 95°C for 25 min to obtain the Zn-Ni / Cr(III) sandwich structure composite film. In this embodiment, the thickness of the Zn-Ni alloy layer is 15 μm and the thickness of the Cr(III) passivation film is 0.8 μm.

[0108] The composite film obtained in this embodiment was characterized using scanning electron microscopy. The film surface was dense and smooth, free of defects such as pinholes and microcracks. The layers in the cross-section were tightly bonded without delamination. Polarization curves were tested in a borate buffer solution containing 0.01 M NaCl at pH 10.0, and the self-corrosion current density was measured to be 5.8 × 10⁻⁶. -6 A•cm -2 The corrosion resistance reaches 18.7 kΩ•cm. 2 After being immersed in the same medium for 18 hours, the film layer showed no obvious corrosion marks and good structural integrity. At the same time, in the neutral salt spray test, the film layer withstood 360 hours without white rust formation, demonstrating excellent corrosion resistance and environmental adaptability, which can meet the long-term protection requirements of high-strength steel in harsh environments.

[0109] The results show that the Zn-Ni / Cr(III) sandwich structure composite film prepared by this invention has a dense film structure and good corrosion resistance. The self-corrosion current density of the Zn-Ni / Cr(III) sandwich structure composite film is 3.0 × 10⁻⁶ in a chloride-containing alkaline environment with a pH of 9.0–10.0. -6 ~3.0×10 -4 A•cm -2 The corrosion resistance is 18.0–24.0 kΩ•cm. 2 After immersion in the same medium for 18 hours, the film layer showed no obvious cracks, pitting, or peeling, exhibiting a complete and dense structure. Under alkaline conditions, the film layer showed uniform thickness and extremely low corrosion tendency, providing long-term stable protection for the metal substrate. Furthermore, the preparation method is simple, with mild and controllable conditions, and has good prospects for industrial application.

Claims

1. A method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface, characterized in that, Includes the following steps: (a) Pretreatment of carbon steel; (b) Preparation of Zn-Ni alloys; (c) Cr(III) group passivation treatment; (d) Curing of the film; In step (b), the electroplating solution used for preparing the Zn-Ni alloy contains the following components: zinc chloride 7–10 g / L, nickel chloride 1–3 g / L, sodium hydroxide 100–130 g / L, triethanolamine 1–2.5 ml / L, polyethyleneimine 95–110 ml / L, and the balance being deionized water; in the electroplating solution, Ni 2+ The content is 0.3–1.2 g / L; In step (c), the passivation solution used for Cr(III)-based passivation treatment contains the following components: 7-10 g / L chromium nitrate, 1-2 g / L sodium nitrate, 2-3 g / L cobalt nitrate, 15-25 g / L boric acid, 9-15 g / L oxalic acid, and the balance is deionized water.

2. The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface according to claim 1, characterized in that, The carbon steel pretreatment in step (a) includes edge chamfering, wire connection, insulating resin encapsulation, substrate grinding and polishing, and substrate activation treatment. The treatment solution for substrate activation treatment is selected from one or more of the following systems: sulfuric acid-ammonium sulfate system, hydrogen peroxide-acid system, and sodium nitrate-sodium chloride system. The volume ratio of the two components in each system is 5:1, and the activation time is 40-60s.

3. The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface according to claim 1, characterized in that, In step (b), the preparation steps of the electroplating solution are as follows: dissolve nickel chloride and zinc chloride in part of deionized water, add sodium hydroxide to dissolve fully, and when the temperature is below 70°C, add triethanolamine and polyethyleneimine in sequence, and finally add the remaining deionized water. Stir for 1 to 2 hours to obtain the solution.

4. The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface according to claim 1, characterized in that, In step (b), the electroplating process conditions for Zn-Ni alloy are: temperature 25–50℃, current density 1–2 A / dm³. 2 Electroplating time: 25-40 minutes.

5. The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface according to claim 4, characterized in that, In step (b), after the Zn-Ni alloy electroplating is completed, it is placed in 0.5-1.5wt% dilute nitric acid for 3-10 seconds for brightening, and then rinsed with water to obtain the Zn-Ni alloy layer.

6. The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface according to claim 1, characterized in that, In step (c), the preparation steps of the passivation solution are as follows: First, dissolve sodium nitrate and cobalt nitrate in part of deionized water, then add boric acid and chromium nitrate in sequence, stir until fully dissolved, then add oxalic acid, and finally add the remaining deionized water. Let it stand for 2 to 4 hours, and take the supernatant to obtain the solution.

7. The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface according to claim 1, characterized in that, In step (c) of the Cr(III)-based passivation treatment, the passivation temperature is 30–60 °C, the passivation time is 130–180 s, and the pH value of the passivation solution is controlled within the range of 1.2–2.

5.

8. The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface according to claim 1, characterized in that, In step (c), the method of using the Cr(III)-based passivation solution includes the following steps: (1) pH adjustment: Use 5-15 wt% sodium hydroxide aqueous solution or 5-15 wt% nitric acid solution to adjust the pH of the passivation solution to 1.2-2.5; (2) Passivation treatment: Immerse the Zn-Ni coated workpiece to be treated in the passivation solution, control the height of the workpiece surface to be passivated from the liquid surface to be 1-2 cm, and passivate at 30-60℃ for 130-180 s; (3) Post-treatment: After passivation, the workpiece is taken out and rinsed with deionized water and ethanol in sequence, and then dried with cold air.

9. The method for preparing a Zn-Ni / Cr(III) sandwich structure composite film on a carbon steel surface according to claim 1, characterized in that, In step (d), the curing temperature is 80-95℃ and the curing time is 25-40 min.

10. A Zn-Ni / Cr(III) sandwich structure composite film for carbon steel surface prepared by the method according to any one of claims 1-9, characterized in that, The composite film has a three-layer sandwich structure consisting of a carbon steel substrate, a Zn-Ni alloy intermediate layer, and a Cr(III) passivation top layer. The thickness of the Zn-Ni alloy intermediate layer is 8–15 μm, the thickness of the Cr(III) passivation top layer is 0.3–1.0 μm, and the self-corrosion current density of the composite film is 3.0 × 10⁻⁶. -4 ~3.0×10 -6 A•cm -2 .

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