Preparation method of environment-friendly trivalent chromium-based multi-layer composite coating

Abstract

The invention provides a preparation method of an environment-friendly trivalent chromium-based multi-layer composite coating, belongs to the technical field of metal surface treatment, and aims to solve the technical problems that a trivalent chromium coating is low in hardness and easy to peel off. According to the multi-layer composite coating, a pulse electrodeposition technology is adopted, and a nickel base layer, a nickel-chromium alloy transition layer and a chromium-aluminum oxide nano composite surface layer are sequentially plated, so that a multi-layer gradient structure of a matrix / nickel layer / nickel-chromium transition layer / chromium-nano composite layer is constructed; through the synergistic effect of the multi-layer gradient structure design, the pulse electrodeposition process and nano-particle dispersion strengthening, the technical bottlenecks that a traditional trivalent chromium coating is insufficient in hardness, poor in abrasion resistance, high in internal stress, prone to generating microcracks, weak in binding force and the like are successfully solved. The hardness of the prepared coating can reach 500 HV or above, 1.9 * 1-5 mm < 3 > / (N.m), and the coating is suitable for industrial components with extremely high requirements for wear resistance and reliability and has great environmental protection and economic value.

Description

Technical Field

[0001] This invention belongs to the technical field of metal surface treatment, and particularly relates to a chromium-based coating. Background Technology

[0002] With increasingly stringent global environmental regulations, traditional hexavalent chromium electroplating processes are severely restricted due to their high toxicity (carcinogenicity, mutagenicity) and high environmental risks (water pollution, bioaccumulation). Trivalent chromium electroplating, as an environmentally friendly alternative, has broad application potential. However, the practical industrial application of environmentally friendly trivalent chromium electroplating technology still faces a series of key technical bottlenecks, making it difficult for its coating performance to fully reach or surpass that of hexavalent chromium coatings, thus limiting its application in high-end wear-resistant and protective fields. For example, while a single trivalent chromium coating is environmentally friendly, its hardness and wear resistance are generally lower than traditional hexavalent chromium hard chromium coatings. Existing Ni-Cr composite coatings can improve performance to some extent, but their strengthening mechanism is relatively simple, and the improvement in hardness and wear resistance still has an upper limit, failing to meet the growing demand for high performance. Furthermore, during trivalent chromium electroplating, especially when pursuing high thickness or high current density, the internal stress of the coating is high, easily generating network microcracks or even macrocracks. These defects not only reduce the corrosion resistance of the coating but also become stress concentration points, causing the coating to peel off under stress. Meanwhile, insufficient adhesion between the plating layer and the substrate, as well as between plating layers themselves, is also a major cause of plating peeling and flaking, severely affecting the service life and reliability of the workpiece. In summary, current environmentally friendly trivalent chromium electroplating technology still suffers from problems such as insufficient overall mechanical properties of the plating layer, high internal stress in the plating layer, susceptibility to microcracks, and poor adhesion between the plating layer and the substrate.

[0003] In 2020, Li et al. (Xu L.Y., Pi L., Dou Y.H., et al. Electroplating of thickhard chromium coating from a trivalent chromium bath containing a ternary complexing agent: A methodological and mechanistic study[J]. ACS Sustainable Chemistry & Engineering, 2020, 8(41): 15540-15549.) prepared PbO2, PbO2-SiO2, PbO2-ZrO2 and PbO2-SiO2-ZrO2 composite electrodes in a lead nitrate system by constant current electrodeposition and used them as anode materials for trivalent chromium electroplating. The results show that the PbO2-SiO2-ZrO2 electrode has good electrocatalytic activity and corrosion resistance. During the trivalent chromium electroplating process, the PbO2-SiO2-ZrO2 electrode has the lowest initial cell voltage and voltage rise, and the content of hexavalent chromium generated is significantly lower than that of the pure PbO2 electrode, which has obvious advantages in energy saving.

[0004] In 2021, Zhao Huan et al. (Zhao Huan. Study on the process and reduction mechanism of trivalent chromium thick chromium electroplating [D]. Shenyang: Northeastern University, 2021.) analyzed the root cause of the difficulty in increasing the thickness of trivalent chromium plating. Starting from the perspective of inhibiting the formation of chromium hydroxy bridge polymer, they designed and optimized additives, and achieved the preparation of trivalent chromium thick chromium plating with a thickness of more than 100 μm. The application was verified on 30CrMnSi high-strength steel for piston rods.

[0005] In 2024, Wang et al. (Wang S, Chen Y, Zhang J, et al. Electrodeposition of nanocrystalline chromium coatings from atrivalent chromium electrolyte containing formic acid[J]. Materials Chemistry and Physics, 2024, 258(5): 1238-1291.) successfully obtained trivalent chromium coatings with nanocrystalline chromium by adding formic acid to a trivalent chromium electrolyte. They analyzed the mechanism of formic acid on grain refinement and verified that the nanostructured coatings exhibited higher hardness and wear resistance.

[0006] In 2022, Guo R et al. (Guo R, Xu Y, Wang L, et al. High-speed electrodeposition of trivalent chromium coatings using pulsed current[J]. Process optimization and properties evaluation, 2022,439(6): 141-172.) used pulsed current for high-speed electrodeposition of trivalent chromium coatings and optimized the pulse parameters (frequency, duty cycle) to improve the deposition rate and coating uniformity.

[0007] In 2018, Yan Hui et al. (Yan Hui, Huang Shuaishuai, Yang Fangzu, et al. Deposition mechanism and coating characterization of trivalent chromium in sulfate system [J]. Electrochemistry, 2018, 24(01):20-27.) studied the electrodeposition process of trivalent chromium on the surface of a copper electrode in a thick chromium plating system using linear sweep voltammetry (LSV) and cyclic voltammetry (CV). The results showed that the deposition reaction of chromium ions in the thick chromium plating system of trivalent chromium proceeds in two steps (Cr... 3+ +e→Cr 2+ ,Cr 2+ +2e→Cr), the first step gains 1 electron, which is controlled by both electrochemical and diffusion processes; the second step gains 2 electrons, which is an irreversible process under diffusion control. The coating has a nodular nanocrystalline structure.

[0008] In 2024, Xia Shuang et al. (Xia Shuang, Wang Bin, Wang Yi, et al. Optimization of trivalent chromium electrodeposition process in chloride system using response surface methodology [J]. Electroplating & Finishing, 2024, 43(03):10-21.) used sodium formate, sodium oxalate, and urea as complexing agents for trivalent chromium electroplating in chloride system and studied the effects of additives, pH, temperature, current density, and deposition time on deposition rate, plating solution depth capability, and coating corrosion resistance. The results showed that the optimal formulation and process conditions for trivalent chromium electrodeposition were: 0.6 mol / L chromium trichloride, 0.8 mol / L sodium formate, 0.2 mol / L sodium oxalate, 0.3 mol / L urea, pH 1.8, temperature 30℃, and current density 150 mA / cm². 2 The deposition time was 30 min. Under these conditions, the resulting Cr coating exhibited an amorphous structure, a thickness exceeding 12 μm, and good corrosion resistance. Therefore, selecting a suitable ligand to suppress the hydroxyl bridging reaction of chromium during electrodeposition is an effective means to maintain a high plating rate and improve coating performance.

[0009] In 2017, Liu Haipeng et al. (Liu Haipeng, Zhang Zhitong, Wang Xinyue, et al. Effect of pH value on pulse electroplating of Ni-Cr-Mo alloy coating [J]. Hot Working Technology, 2017, 46(18):146-148.) prepared Ni-Cr-Mo alloy coating on Q235 steel surface by pulse electroplating. The effects of duty cycle on coating element content, deposition rate, surface morphology and corrosion resistance were studied. The results showed that with the increase of duty cycle, the nickel and molybdenum content of the coating increased, the chromium content decreased, and the deposition rate decreased.

[0010] In 2024, Yang Munan et al. (Yang Munan, Luo Sangen, Zou Yaru, et al. Process and corrosion resistance of Ni-Cr alloy coating on NdFeB magnet surface by bidirectional pulse electrodeposition [J]. Jiangxi Metallurgy, 2024, 44(04): 246-256.) studied the effects of different process parameters on the corrosion resistance and surface quality of Ni-Cr composite coating using bidirectional pulse electrodeposition technology. The results showed that at pH=4.5 and current density of 0.20 A / cm 2 Under the conditions of a positive duty cycle of 60% and a Cr salt content of 25 g / L, the prepared Ni-Cr alloy coating has excellent surface quality and good corrosion resistance.

[0011] Despite significant advancements in various fields of trivalent chromium electroplating, environmentally friendly trivalent chromium plating still faces challenges in high-end applications. For example, while adding formic acid to refine grains and preparing Ni-Cr alloy coatings have improved hardness and corrosion resistance to some extent, the hardness of these coatings is typically in the range of 200-400 HV, struggling to break the 500 HV barrier, and still unable to meet the extreme requirements for hardness and wear resistance in demanding conditions. Furthermore, the greater the thickness of the environmentally friendly trivalent chromium plating, the more easily internal stress is induced, leading to micro-cracks and porosity, seriously threatening the long-term service life and reliability of the coating in harsh environments. Additionally, the weak adhesion between the coating and the substrate makes it prone to peeling. Currently, most solutions employ composite coatings or double-layer structures; however, this "hard connection" method, lacking a transition between the substrate with vastly different properties and the hard coating, results in stress concentration at the interface, weak adhesion, and a tendency for interfacial peeling or overall detachment under impact or alternating loads. Summary of the Invention

[0012] To address the technical problems of low hardness and easy peeling of trivalent chromium plating, this invention proposes an environmentally friendly method for preparing a trivalent chromium-based multilayer composite plating. On the one hand, by optimizing pulse electrical parameters and utilizing a multilayer gradient structure to effectively release and redistribute stress, microcracks are fundamentally eliminated, ensuring the integrity and reliability of the plating under high loads. On the other hand, through the innovative design of a "multilayer gradient structure," the interlayer bonding force is maximized through a smooth transition of composition and properties (matrix → tough Ni layer → Ni-Cr transition layer → hard Cr-nanocomposite layer), solving the problems of plating peeling and flaking. Finally, by introducing the synergistic effect of "nanoparticle dispersion reinforcement" and "multilayer composite structure," the aim is to achieve a significant improvement in the comprehensive mechanical properties of the plating, such as hardness (>500 HV) and wear resistance.

[0013] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for preparing an environmentally friendly trivalent chromium multilayer composite coating includes the following steps: (1) Degreasing: Immerse the metal substrate in an aqueous solution containing 50-80 g / L of alkaline degreasing agent (main components are NaOH, Na3PO4, Na2CO3 and surfactant), and ultrasonically clean it at 60-70℃ for 10-15 minutes. After taking it out, rinse it thoroughly with deionized water to ensure that there is no oil residue on the surface.

[0014] (2) Pickling activation: Immerse the degreased substrate in an acid solution with a volume concentration of 8-12% for 30-60 seconds at room temperature until fine bubbles are uniformly precipitated on the surface of the substrate. Then remove it immediately, rinse it quickly with deionized water, and immediately transfer it to the electroplating tank to prevent the surface from oxidizing again.

[0015] (3) Pulse electroplating nickel undercoat deposits a dense and tough nickel layer on the substrate, serving as a solid foundation for subsequent plating layers and effectively relieving stress. The main components of the plating solution are as follows: plating solution I includes: nickel salt I with a concentration of 80-120 g / L, conductive agent I with a concentration of 10-15 g / L, complexing agent I with a concentration of 100-140 g / L, and buffer I with a concentration of 15-25 g / L; the pH of plating solution I is 6-7.5. The process parameters for pulse electroplating are: pulse frequency: 70-90 Hz, duty cycle: 35%-45%, average current density: 1.2-1.8 A / dm³. 2 The plating solution temperature is 32-38℃, the plating time is 12-15 minutes, and the anode is a nickel plate or an inert anode (such as a platinum-titanium mesh).

[0016] (4) Pulse electroplating of a nickel-chromium transition layer: depositing an alloy layer with a gradient change in composition and properties on the underlayer to achieve a smooth transition from the tough nickel layer to the hard chromium layer and avoid stress concentration at the interface. The plating solution formula is as follows: the electroplating solution II used in step (2) for electroplating the chromium-alumina nanocomposite layer includes: nickel salt II with a concentration of 50-80 g / L, chromium salt I with a concentration of 100-200 g / L, conductive agent I with a concentration of 50-100 g / L, complexing agent II with a concentration of 50-100 g / L, and buffer II with a concentration of 40-60 g / L; the pH of the electroplating solution II is 2.5-3.5. The process parameters of pulse electroplating are: pulse frequency: 70-90 Hz (consistent with the previous layer to ensure process stability), duty cycle: 35%-45%, average current density: 4.5-5.5 A / dm 2 Plating solution temperature: 30-35℃, electroplating time: 6-8 minutes, anode is graphite.

[0017] (5) Pulse electroplating of chromium-alumina nanocomposite layer. This is a functional surface layer. It achieves dispersion reinforcement by introducing nanoparticles, which is the key to obtaining ultra-high hardness and wear resistance. The plating solution formula is as follows: plating solution III includes: chromium salt II with a concentration of 300-400 g / L, nano-alumina particles with a concentration of 6-8 g / L, conductive agent III with a concentration of 150-200 g / L, complexing agent III with a concentration of 80-140 g / L, buffer II with a concentration of 50-70 g / L, and wetting agent with a concentration of 0.1-0.3 mL / L; the pH of the plating solution III is 2.5-3. The process parameters of pulse electroplating are: pulse frequency: 90-110 Hz (higher frequency helps to achieve uniform co-deposition of nanoparticles), duty cycle: 45%-55%, average current density: 7-9 A / dm³. 2 Plating solution temperature: 28-32 ℃, electroplating time: 6-8 minutes, anode is DSA or graphite.

[0018] (6) Post-treatment aims to eliminate internal stress in the coating, further enhance interlayer adhesion, and complete the final treatment. Washing and drying: Rinse the coated parts three times sequentially with deionized water and anhydrous ethanol, then place them in a forced-air drying oven at 80-100℃ for thorough drying. Heat treatment: Perform heat treatment on the dried coated parts under vacuum or inert atmosphere protection. The heat treatment parameters are: heat to 240-260℃ at a rate of 3-5℃ / min, hold for 1-1.5 hours, and then cool to room temperature with the furnace. This process effectively promotes inter-layer atomic diffusion, eliminates hydrogen embrittlement, and thus significantly improves the adhesion and toughness of the coating.

[0019] Preferably, nickel salt I and nickel salt II are independently selected from nickel sulfate (NiSO4·6H2O) and / or nickel chloride; conductive agent I, conductive agent II and conductive agent III are independently selected from any one or more of sodium chloride, potassium chloride, sodium sulfate, potassium sulfate and sodium aminosulfonate; complexing agent I, complexing agent II and complexing agent III are independently selected from any one or more of citrate (such as sodium citrate), sodium formate, sodium oxalate, sodium gluconate, aminosulfonic acid and EDTA-2Na; buffer agent I, buffer agent II and buffer agent III are all boric acid; chromium salt I and chromium salt II are independently selected from chromium chloride or basic chromium sulfate (Cr(OH)SO4); and the wetting agent is OP-10.

[0020] Preferably, the pH of electroplating solutions I, II, and III can be adjusted using dilute sulfuric acid or NaOH solution.

[0021] Preferably, the alumina nanoparticles have a particle size of 20-50 nm; the alumina nanoparticles are surface-modified with a silane coupling agent to improve their dispersion stability in the plating bath; the silane coupling agent is any one or more of KH-550, KH-560, KH-570 or KH-792.

[0022] Preferably, the method for surface modification treatment of the nano-alumina particles with silane coupling agent is as follows: the nano-alumina particles are placed in a certain amount of aqueous solution of silane coupling agent and ultrasonically treated for 1-2 hours, with a pH of 3-4.

[0023] The beneficial effects of this invention are: (1) This invention innovatively integrates "nano-dispersion strengthening" and "pulse fine grain strengthening" to achieve a coating hardness of ≥500HV in an all-environmentally friendly trivalent chromium electroplating system. Its wear resistance is significantly improved compared to the comparative example, and the wear rate is significantly reduced to 1.9×1 -5 mm 3 / (N·m), achieving a comprehensive improvement in hardness and wear resistance over traditional hexavalent chromium plating, enabling it to be truly applied to harsh working conditions with high wear.

[0024] (2) This invention utilizes a multi-layer gradient structure of “substrate → tough Ni layer → Ni-Cr transition layer → hard Cr-nano composite layer” to effectively release the stress caused by the difference in thermal expansion coefficient between the coating and the substrate by taking advantage of the smooth transition of the composition and mechanical properties of each layer, thus completely eliminating the stress concentration problem caused by the performance mismatch of traditional single-layer coatings; at the same time, a strong metallurgical bond is formed at the interlayer interface, which significantly improves the interlayer bonding force and fundamentally solves the technical problem of easy peeling and flaking of hard coatings.

[0025] (3) By precisely controlling parameters such as the frequency, duty cycle and current density of pulse electroplating, this invention not only refines the grain size of the coating and improves the density of the coating, but also significantly reduces the internal stress and hydrogen embrittlement tendency of the coating, avoids micro-cracks in the coating during processing and use, ensures that the coating does not crack after bending, assembly and other processes, and improves the product processing qualification rate and use safety.

[0026] (4) This invention achieves a dense coating with high adhesion, low internal stress, and no microcracks through the triple synergistic effect of "nano-dispersion reinforcement + multilayer composite structure + pulsed fine grains". This coating not only has excellent hardness and wear resistance, but also exhibits good corrosion resistance, and the reliability of the product is significantly improved under long-term dynamic load. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A schematic diagram of the preparation method of an environmentally friendly trivalent chromium-based multilayer composite coating.

[0029] Figure 2 This is a schematic diagram of the structure of a multilayer composite coating (substrate → tough Ni layer → Ni-Cr transition layer → hard Cr-nano composite layer, a multilayer gradient structure).

[0030] Figure 3 The images shown are SEM images of the electroplated layers. a is Example 2; b is Comparative Example 1; c is Comparative Example 2; d is Comparative Example 3.

[0031] Figure 4 This is a hardness comparison chart of Example 2 with Comparative Examples 1, 2, and 3.

[0032] Figure 5 This is a comparison chart of the wear rate of Example 2 with Comparative Examples 1, 2, and 3. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1 A method for preparing an environmentally friendly trivalent chromium-based multilayer composite coating, the preparation process is as follows: Figure 1 As shown, a multi-layer composite structure consisting of "substrate → tough Ni layer → Ni-Cr transition layer → hard Cr-nano composite layer" is constructed using pulse electroplating technology. A schematic diagram of the multi-layer composite plating structure is shown below. Figure 2 As shown, the specific steps include the following: (1) Degreasing: Immerse the 316 stainless steel substrate in an aqueous solution containing 70g / L of alkaline degreasing agent (main components are NaOH, Na3PO4, Na2CO3 and surfactant), and ultrasonically clean it at 65℃ for 10 minutes. After taking it out, rinse it thoroughly with deionized water to ensure that there is no oil residue on the surface.

[0035] (2) Pickling activation: Immerse the degreased substrate in a 10% sulfuric acid aqueous solution at room temperature for 50 seconds. Once fine bubbles are uniformly precipitated on the surface of the substrate, remove it immediately, rinse it quickly with deionized water, and immediately transfer it to the electroplating tank to prevent the surface from oxidizing again.

[0036] (3) Nickel plating undercoat: The main components of the plating solution are as follows: nickel sulfate (NiSO4·6H2O): 100 g / L, sodium citrate (Na3C5H5O7): 120 g / L, sodium chloride (NaCl): 10 g / L, boric acid (H3BO3): 20 g / L, and the pH value is adjusted to 6 with 10 wt% NaOH solution. In addition, the pulse electroplating parameters are as follows: pulse frequency: 70 Hz, duty cycle: 40%, average current density: 1.5 A / dm², plating solution temperature: 35℃, electroplating time: 15 minutes, and the anode is a high-purity electrolytic nickel plate.

[0037] (4) Electroplating of nickel-chromium transition layer: The plating solution formula is as follows: nickel sulfate: 70 g / L, basic chromium sulfate (Cr(OH)SO4): 150 g / L, complexing agent sodium formate: 70 g / L, conductive salt potassium sulfate: 70 g / L, boric acid: 50 g / L, and the pH value is adjusted to 3 with dilute sulfuric acid and NaOH solution. The pulse electroplating parameters are as follows: pulse frequency: 70 Hz, duty cycle: 40%, average current density: 5 A / dm², plating solution temperature: 32℃, electroplating time: 8 minutes, and the anode is graphite.

[0038] (5) Electroplated chromium-alumina nanocomposite layer: The plating solution formula is as follows: basic chromium sulfate: 350 g / L, KH-550 modified nano-alumina particles (particle size 20-50 nm): concentration 7 g / L, main complexing agent sodium formate: 80 g / L, auxiliary complexing agent urea: 20 g / L, conductive salt potassium sulfate: 150 g / L, boric acid: 60 g / L, environmentally friendly wetting agent OP-10: 0.2 mL / L, and the pH value is adjusted to 3.0 with dilute sulfuric acid. The pulse electroplating parameters are as follows: pulse frequency: 90 Hz, duty cycle: 50%, average current density: 8 A / dm², plating solution temperature: 30℃, electroplating time: 8 minutes, anode: DSA is used. During the electroplating process, mechanical stirring or air stirring is used at a speed of 300 rpm to ensure that the nanoparticles are uniformly suspended and to prevent sedimentation.

[0039] (6) Post-treatment: Rinse the plated parts three times with deionized water and anhydrous ethanol, then dry them thoroughly in a 90°C forced-air drying oven. After drying, heat-treat the plated parts under vacuum or inert atmosphere. The heat treatment parameters are: heat to 250°C at a rate of 5°C / min, hold for 1 hour, and then cool to room temperature in the furnace. This process effectively promotes inter-layer atomic diffusion, eliminates hydrogen embrittlement, and thus significantly improves the adhesion and toughness of the coating.

[0040] Example 2-11 Examples 2-11: Environmentally friendly trivalent chromium multilayer composite coatings were prepared according to the preparation steps of Example 1 above. The difference from Example 1 is the pulse electroplating parameters in step (5). The other steps are exactly the same. The specific process parameters are shown in Table 1. Table 1. Summary of process parameters in Examples 1-11 Comparative Example 1 All steps are the same as in Example 2, except that in step 5 of Example 2, plating is performed without the presence of a nano-alumina reinforcing phase, in order to demonstrate the dispersion strengthening effect of the nano-alumina particles.

[0041] Comparative Example 2 All steps are the same as in Example 2, but the "nickel-chromium transition layer" is omitted in Example 2 to demonstrate the importance of multilayer gradient structures in ensuring interlayer bonding.

[0042] Comparative Example 3 Using conventional direct current electroplating instead of pulse electroplating, the current density and temperature of the nickel plating layer were 1.5 A / dm². 2 At 35℃, the current density and temperature of the nickel-chromium composite coating were 8 A / dm³. 2 At 35℃ and 35℃, the current density and temperature of the chromium-alumina nanocomposite coating are 8 A / dm³, respectively.2 and 35℃.

[0043] To better illustrate the present invention, the properties of different structural coatings in Embodiment 2 and Comparative Examples 1, 2, and 3 are described in conjunction with the accompanying drawings.

[0044] Figure 3 SEM images of the electroplated surfaces of Examples 2 and Comparative Examples 1-3 are provided. Figure 3 It can be seen that the coating surface of Example 2 is the densest and most uniform, exhibiting a fine granular or cellular structure. The coating is continuous and intact, and no obvious micro-cracks, pores, or peeling were observed. Figure 3 (a) This indicates that the multi-layer gradient structure of "matrix → tough Ni layer → Ni-Cr transition layer → hard Cr-nanocomposite layer" adopted in this invention, combined with pulse electroplating and nano-alumina reinforcement, is successful. Pulse electroplating effectively refines the grains and reduces internal stress, while nano-alumina particles play a dispersive strengthening role, making the structure more compact. The multi-layer gradient design ensures good interlayer bonding and stress transition. This structure provides an ideal microstructure basis for achieving high hardness, high wear resistance, high bonding strength, and excellent corrosion resistance. Compared with Example 2, the surface grains of the coating in Comparative Example 1 are significantly coarser, and the structure is relatively loose. Figure 3 (b) This is because the absence of nano-alumina particles causes the coating to lose its crucial dispersion strengthening and grain refinement effects. Chromium grains, lacking the pinning resistance of nanoparticles during growth, grow freely, forming a coarse microstructure. In Comparative Example 2, the coating surface exhibits peeling, flaking, or cracking, with poor interfacial bonding between layers. This is due to the lack of a gradient transition in composition and properties; the significant difference in physical properties (such as thermal expansion coefficient and modulus) between the tough nickel layer and the hard chromium layer generates substantial interfacial stress. This stress is released during the deposition process or subsequent treatment, leading to adhesion failure and coating peeling. In Comparative Example 3, obvious network microcracks are observed on the coating surface, with uneven grain size and poor structural density. This is because the continuous current of DC electroplating depletes metal ions at the cathode interface, intensifying the hydrogen evolution reaction and resulting in extremely high internal stress in the coating. This high internal stress is released through the formation of microcracks, while hydrogen evolution also causes pinholes and hydrogen embrittlement. The above results show that the present invention employs the synergistic effect of "multi-layer gradient structure design", "pulse electrodeposition process" and "nanoparticle composite reinforcement" to obtain a dense and uniform electroplated coating, solving problems such as grain coarsening, interface peeling and network cracks that occur during trivalent chromium electroplating.

[0045] Figure 4A hardness comparison chart is provided between Example 2 and Comparative Examples 1, 2, and 3. Hardness testing was conducted according to GB5934-1986 "Metallic Coatings for Light Industrial Products - Hardness Test Method: Microhardness Method" using an HVS-1000 Vickers microhardness tester. The test conditions were a load of 10g and a holding time of 10s. As can be seen from the chart, Example 2 exhibits the highest hardness compared to Comparative Examples 1-3. This is because pulse electroplating provides a dense, low-stress substrate, preventing microcracks from weakening the hardness; the nickel-chromium transition layer ensures excellent adhesion, allowing the hard surface layer to adhere firmly; and the nano-alumina particles, as a hard phase, are uniformly dispersed in the chromium matrix, producing a strong dispersion strengthening and grain refinement effect.

[0046] Figure 5 A comparison chart of wear rates between Example 2 and Comparative Examples 1, 2, and 3 is provided. The tribological properties of the coating were analyzed using an HSR-2M high-speed reciprocating friction testing machine, employing a ball / plate point contact method. The sample size was 50mm × 50mm × 1mm. The tribological test conditions were: load 10N, wear time 5min, reciprocating speed 200mm / s, and displacement distance 5mm. As can be seen from the chart, Example 2 exhibits the lowest wear rate, i.e., the best wear resistance, compared to Comparative Examples 1-3. This is mainly due to the introduction of nano-alumina, which effectively resists the indentation and ploughing of abrasive particles through dispersion strengthening. The multi-layer gradient structure design, with a gradient transition layer, prevents coating peeling under cyclic stress. The dense, defect-free structure formed by electroplating eliminates weak points such as microcracks, preventing rapid material loss due to defect propagation during wear. Through the synergy of these three factors, this invention not only achieves high hardness but, more importantly, a unified combination of high hardness, high toughness, and strong bonding, thus realizing a leap in wear resistance. This gives the environmentally friendly trivalent chromium composite coating of the present invention great potential to replace traditional hexavalent chromium coatings under harsh working conditions of high wear and high load.

[0047] Example 12 A method for preparing an environmentally friendly trivalent chromium-based multilayer composite coating specifically includes the following steps: (1) Degreasing: Immerse the 316 stainless steel substrate in an aqueous solution containing 50g / L of alkaline degreasing agent (main components are NaOH, Na3PO4, Na2CO3 and surfactant), and ultrasonically clean it at 65℃ for 10 minutes. After taking it out, rinse it thoroughly with deionized water to ensure that there is no oil residue on the surface.

[0048] (2) Pickling activation: Immerse the degreased substrate in a 12% sulfuric acid aqueous solution at room temperature for 30 seconds. After fine bubbles are uniformly precipitated on the surface of the substrate, remove it immediately, rinse it quickly with deionized water, and immediately transfer it to the electroplating tank to prevent the surface from oxidizing again.

[0049] (3) Nickel plating undercoat: The main components of the plating solution are as follows: nickel sulfate (NiSO4·6H2O): 80 g / L, sodium citrate (Na3C5H5O7): 100 g / L, sodium chloride (NaCl): 15 g / L, boric acid (H3BO3): 25 g / L, and the pH value is adjusted to 7 with 10 wt% NaOH solution. In addition, the pulse electroplating parameters are as follows: pulse frequency: 80 Hz, duty cycle: 45%, average current density: 1.2A / dm², plating solution temperature: 38℃, electroplating time: 12 minutes, and the anode is a high-purity electrolytic nickel plate.

[0050] (4) Electroplating of nickel-chromium transition layer: The plating solution formula is as follows: nickel sulfate: 50 g / L, basic chromium sulfate (Cr(OH)SO4): 200 g / L, complexing agent sodium formate: 100 g / L, conductive salt potassium sulfate: 50 g / L, boric acid: 40 g / L, and the pH value is adjusted to 3.5 with dilute sulfuric acid and NaOH solution. The pulse electroplating parameters are as follows: pulse frequency: 80 Hz, duty cycle: 40%, average current density: 5 A / dm², plating solution temperature: 30℃, electroplating time: 6 minutes, and the anode is graphite.

[0051] (5) Electroplated chromium-alumina nanocomposite layer: The plating solution formula is as follows: basic chromium sulfate: 300 g / L, KH-550 modified nano-alumina particles (particle size 20-50 nm): concentration 6 g / L, main complexing agent sodium formate: 60 g / L, auxiliary complexing agent urea: 20 g / L, conductive salt potassium sulfate: 200 g / L, boric acid: 70 g / L, environmentally friendly wetting agent OP-10: 0.1 mL / L, and the pH value is adjusted to 2.5 with dilute sulfuric acid. The pulse electroplating parameters are as follows: pulse frequency: 90 Hz, duty cycle: 50%, average current density: 8 A / dm², plating solution temperature: 30℃, electroplating time: 8 minutes, anode: DSA is used. During the electroplating process, mechanical stirring or air stirring is used at a speed of 300 rpm to ensure that the nanoparticles are uniformly suspended and to prevent sedimentation.

[0052] (6) Post-treatment: Rinse the plated parts three times with deionized water and anhydrous ethanol, then dry them thoroughly in a 100°C oven. After drying, heat-treat the plated parts under vacuum or inert atmosphere. The heat treatment parameters are: heat to 240°C at a rate of 5°C / min, hold for 1.5 hours, and then cool to room temperature in the furnace. This process effectively promotes inter-layer atomic diffusion, eliminates hydrogen embrittlement, and thus significantly improves the adhesion and toughness of the coating.

[0053] Example 13 A method for preparing an environmentally friendly trivalent chromium-based multilayer composite coating specifically includes the following steps: (1) Degreasing: Immerse the 316 stainless steel substrate in an aqueous solution containing 80g / L of alkaline degreasing agent (main components are NaOH, Na3PO4, Na2CO3 and surfactant), and ultrasonically clean it at 65℃ for 10 minutes. After taking it out, rinse it thoroughly with deionized water to ensure that there is no oil residue on the surface.

[0054] (2) Pickling activation: Immerse the degreased substrate in an 8% sulfuric acid aqueous solution at room temperature for 60 seconds. After small bubbles are uniformly precipitated on the surface of the substrate, remove it immediately, rinse it quickly with deionized water, and immediately transfer it to the electroplating tank to prevent the surface from oxidizing again.

[0055] (3) Nickel plating undercoat: The main components of the plating solution are as follows: nickel chloride: 120 g / L, sodium oxalate: 140 g / L, potassium chloride (KCl): 10 g / L, boric acid (H3BO3): 15 g / L, and the pH value is adjusted to 7.5 with 10 wt% NaOH solution. In addition, the pulse plating parameters are as follows: pulse frequency: 90 Hz, duty cycle: 35%, average current density: 1.8 A / dm², plating solution temperature: 32℃, plating time: 12 minutes, and the anode is a high-purity electrolytic nickel plate.

[0056] (4) Electroplating of nickel-chromium transition layer: The plating solution formula is as follows: nickel sulfate: 80 g / L, chromium chloride: 100 g / L, complexing agent sodium formate: 50 g / L, conductive salt potassium sulfate: 100 g / L, boric acid: 60 g / L, and the pH value is adjusted to 2.5 with dilute sulfuric acid and NaOH solution. The pulse electroplating parameters are as follows: pulse frequency: 90 Hz, duty cycle: 45%, average current density: 5.5 A / dm², plating solution temperature: 35℃, electroplating time: 8 minutes, and the anode is graphite.

[0057] (5) Electroplated chromium-alumina nanocomposite layer: The plating solution formula is as follows: basic chromium sulfate: 400 g / L, KH-570 modified nano-alumina particles (particle size 20-50 nm): concentration 8 g / L, main complexing agent sodium formate: 100 g / L, auxiliary complexing agent urea: 40 g / L, conductive salt potassium sulfate: 150 g / L, boric acid: 50 g / L, environmentally friendly wetting agent OP-10: 0.3 mL / L, and the pH value is adjusted to 2.5 with dilute sulfuric acid. The pulse electroplating parameters are as follows: pulse frequency: 90 Hz, duty cycle: 50%, average current density: 8 A / dm², plating solution temperature: 30℃, electroplating time: 8 minutes, anode: DSA is used. During the electroplating process, mechanical stirring or air stirring is used at a speed of 300 rpm to ensure that the nanoparticles are uniformly suspended and to prevent sedimentation.

[0058] (6) Post-treatment: Rinse the plated parts three times with deionized water and anhydrous ethanol, then dry them thoroughly in a 90°C forced-air drying oven. After drying, heat-treat the plated parts under vacuum or inert atmosphere. The heat treatment parameters are: heat to 250°C at a rate of 5°C / min, hold for 1 hour, and then cool to room temperature in the furnace. This process effectively promotes inter-layer atomic diffusion, eliminates hydrogen embrittlement, and thus significantly improves the adhesion and toughness of the coating.

[0059] Example 14 A method for preparing an environmentally friendly trivalent chromium-based multilayer composite coating specifically includes the following steps: (1) Degreasing: Immerse the 304 stainless steel substrate in an aqueous solution containing 60g / L of alkaline degreasing agent (main components are NaOH, Na3PO4, Na2CO3 and surfactant), and ultrasonically clean it at 65℃ for 12 minutes. After taking it out, rinse it thoroughly with deionized water to ensure that there is no oil residue on the surface.

[0060] (2) Pickling activation: Immerse the degreased substrate in a 10% sulfuric acid aqueous solution at room temperature for 60 seconds. After fine bubbles are uniformly precipitated on the surface of the substrate, remove it immediately, rinse it quickly with deionized water, and immediately transfer it to the electroplating tank to prevent the surface from oxidizing again.

[0061] (3) Nickel plating undercoat: The main components of the plating solution are as follows: nickel chloride: 100 g / L, sodium oxalate: 130 g / L, potassium chloride (KCl): 12 g / L, boric acid (H3BO3): 20 g / L, and the pH value is adjusted to 7 with 10 wt% NaOH solution. In addition, the pulse plating parameters are as follows: pulse frequency: 90 Hz, duty cycle: 35%, average current density: 1.8 A / dm², plating solution temperature: 32℃, plating time: 12 minutes, and the anode is a high-purity electrolytic nickel plate.

[0062] (4) Electroplating of nickel-chromium transition layer: The plating solution formula is as follows: nickel sulfate: 70 g / L, chromium chloride: 140 g / L, complexing agent sodium formate: 70 g / L, conductive salt sodium sulfate: 90 g / L, boric acid: 50 g / L, and the pH value is adjusted to 2.5 with dilute sulfuric acid and NaOH solution. The pulse electroplating parameters are as follows: pulse frequency: 90 Hz, duty cycle: 45%, average current density: 5.5 A / dm², plating solution temperature: 35℃, electroplating time: 8 minutes, and the anode is graphite.

[0063] (5) Electroplated chromium-alumina nanocomposite layer: The plating solution formula is as follows: basic chromium sulfate: 350 g / L, KH-570 modified nano-alumina particles (particle size 20-50 nm): concentration 7 g / L, main complexing agent sodium formate: 100 g / L, auxiliary complexing agent urea: 20 g / L, conductive salt potassium sulfate: 170 g / L, boric acid: 60 g / L, environmentally friendly wetting agent OP-10: 0.3 mL / L, and the pH value is adjusted to 2.5 with dilute sulfuric acid. The pulse electroplating parameters are as follows: pulse frequency: 90 Hz, duty cycle: 50%, average current density: 8 A / dm², plating solution temperature: 30℃, electroplating time: 8 minutes, anode: DSA is used. During the electroplating process, mechanical stirring or air stirring is used at a speed of 300 rpm to ensure that the nanoparticles are uniformly suspended and to prevent sedimentation.

[0064] (6) Post-treatment: Rinse the plated parts three times with deionized water and anhydrous ethanol, then dry them thoroughly in a 90°C forced-air drying oven. After drying, heat-treat the plated parts under vacuum or inert atmosphere. The heat treatment parameters are: heat to 250°C at a rate of 5°C / min, hold for 1 hour, and then cool to room temperature in the furnace. This process effectively promotes inter-layer atomic diffusion, eliminates hydrogen embrittlement, and thus significantly improves the adhesion and toughness of the coating.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an environmentally friendly trivalent chromium-based multilayer composite coating, characterized in that, Includes the following steps: (1) A nickel layer is plated on the activated substrate surface using a pulse electroplating process; (2) A nickel-chromium transition layer is deposited on the surface of the nickel layer using a pulse electroplating process; (3) A chromium-alumina nanocomposite layer is deposited on the surface of the nickel-chromium transition layer using a pulse electroplating process; (4) Heat treatment is performed on the electroplated substrate to obtain a composite coating.

2. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to claim 1, characterized in that, The electroplating solution I used in step (1) for electroplating the nickel layer includes: nickel salt I with a concentration of 80-120 g / L, conductive agent I with a concentration of 10-15 g / L, complexing agent I with a concentration of 100-140 g / L, and buffer I with a concentration of 15-25 g / L; the pH of the electroplating solution I is 6-7.

5.

3. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to claim 2, characterized in that, The process parameters for pulse electroplating in step (1) are: pulse frequency: 70-90 Hz, duty cycle: 35%-45%, average current density: 1.2-1.8 A / dm³. 2 The plating solution temperature is 32-38℃, the plating time is 12-15 minutes, and the anode is a nickel plate or an inert anode.

4. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to claim 1, characterized in that, The electroplating solution II used in step (2) for electroplating the nickel-chromium transition layer includes: nickel salt II with a concentration of 50-80 g / L, chromium salt I with a concentration of 100-200 g / L, conductive agent I with a concentration of 50-100 g / L, complexing agent II with a concentration of 50-100 g / L, and buffer II with a concentration of 40-60 g / L; the pH of the electroplating solution II is 2.5-3.

5.

5. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to claim 4, characterized in that, The process parameters for pulse electroplating in step (2) are: pulse frequency: 70-90 Hz, duty cycle: 35%-45%, average current density: 4.5-5.5 A / dm³. 2 Plating solution temperature: 30-35℃, electroplating time: 6-8 minutes, anode is graphite.

6. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to claim 1, characterized in that, The electroplating solution III for the electroplating chromium-alumina nanocomposite layer in step (3) comprises: chromium salt II with a concentration of 300-400 g / L, nano-alumina particles with a concentration of 6-8 g / L, conductive agent III with a concentration of 150-200 g / L, complexing agent III with a concentration of 80-140 g / L, buffer II with a concentration of 50-70 g / L, and wetting agent with a concentration of 0.1-0.3 mL / L; the pH of the electroplating solution III is 2.5-3.

7. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to claim 6, characterized in that, The process parameters for pulse electroplating in step (3) are: pulse frequency: 90-110 Hz, duty cycle: 45%-55%, average current density: 7-9 A / dm³ 2 Plating solution temperature: 28-32 ℃, electroplating time: 6-8 minutes, anode is DSA or graphite.

8. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to claim 1, characterized in that, The alumina nanoparticles have a particle size of 20-50 nm; the alumina nanoparticles are surface modified with a silane coupling agent; the silane coupling agent is any one or more of KH-550, KH-560, KH-570 or KH-792.

9. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to any one of claims 2-8, characterized in that, The nickel salt I and nickel salt II are independently selected from nickel sulfate and / or nickel chloride; the conductive agent I, conductive agent II, and conductive agent III are independently selected from any one or more of sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, and sodium aminosulfonate; the complexing agent I, complexing agent II, and complexing agent III are independently selected from any one or more of citrate, sodium formate, sodium oxalate, sodium gluconate, aminosulfonic acid, and EDTA-2Na; the buffer agent I, buffer agent II, and buffer agent III are all boric acid; the chromium salt I and chromium salt II are independently selected from chromium chloride or basic chromium sulfate; the wetting agent is OP-10.

10. The method for preparing the environmentally friendly trivalent chromium-based multilayer composite coating according to claim 1, characterized in that, The heat treatment is performed at a temperature of 240-260℃ for 1-1.5 hours.

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