Nanometer injection molding steel-magnesium composite workpiece and preparation method thereof

Abstract

This invention relates to the field of steel-magnesium composite workpiece technology, and discloses a nano-injection molded steel-magnesium composite workpiece and its preparation method; including the following operation steps: Step 1: Degrease and wash the steel-magnesium composite workpiece with water, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 10~20s to obtain steel-magnesium composite workpiece A; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in an electrolyte, take it out, wash it with water, and dry it to obtain steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water, activate it in a silane coupling agent solution, and perform nano-injection molding to obtain a nano-injection molded steel-magnesium composite workpiece. In this scheme, the potential difference on the surface of the steel-magnesium composite workpiece is improved and the surface roughness difference is reduced by the electrolyte to improve the interfacial bonding stability.

Description

Technical Field

[0001] This invention relates to the field of steel-magnesium composite workpiece technology, specifically a nano-injection molded steel-magnesium composite workpiece and its preparation method. Background Technology

[0002] Steel-magnesium composites combine the structural strength and corrosion resistance of stainless steel with the extreme lightweight advantages of magnesium alloys, making them valuable for applications in precision instruments, medical devices, and high-end consumer electronics. However, steel and magnesium have significantly different chemical properties: 1. Stainless steel has a dense passivation film (Cr2O3) on its surface, which has high chemical stability and is difficult to effectively roughen with traditional acidic corrosion solutions; 2. Magnesium reacts violently in most acidic environments, easily leading to over-corrosion and dimensional instability; 3. There is a significant potential difference between steel and magnesium (stainless steel is about -0.5V, while magnesium is about -2.37V), and the current distribution is extremely uneven during electrolytic treatment; 4. Traditional processing methods are difficult to effectively activate the surface of stainless steel without damaging magnesium.

[0003] This invention aims to solve the above-mentioned technical problems and is of great significance for preparing a nano-injection molded steel-magnesium composite workpiece. Summary of the Invention

[0004] The purpose of this invention is to provide a nano-injection molded steel-magnesium composite workpiece and its preparation method, thereby solving the problems raised in the prior art. The core innovation of this process lies in utilizing the selective corrosion characteristics of an alkaline environment to effectively roughen the steel surface while leaving magnesium almost unaffected. Then, the magnesium is precisely corroded through a weak acid system, ultimately improving the interfacial bonding stability by reducing the potential difference and the difference in surface roughness.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a nano-injection molded steel-magnesium composite workpiece includes the following steps: Step 1: Degrease and wash the steel-magnesium composite workpiece, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 10-20 seconds to obtain steel-magnesium composite workpiece A; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in the electrolyte, take it out, wash it with water, and dry it to obtain the steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water, activate it in the silane coupling agent solution, and perform nano-injection molding to obtain the nano-injection molded steel-magnesium composite workpiece.

[0006] In a more optimized form, the raw materials for the alkaline solution include the following components: 20-40 g / L sodium hydroxide, 15-30 g / L persulfate, 1-5 g / L sodium silicate, and pH > 10.5; The acidic solution comprises the following components: 20-30 g / L ferric nitrate, 0.1-0.3 g / L hydrochloric acid, and 3.5 g / L sodium chloride. <pH≤5.5; The raw materials for the chromate-nitric acid solution include the following components: 5~10g / L chromic anhydride and 30~50mL / L nitric acid.

[0007] The solution first removes processing oil, fingerprints, polishing paste residue, and trace oxides from the workpiece surface through degreasing and water washing, providing an absolutely clean surface for subsequent corrosion.

[0008] The solution employs a highly alkaline and oxidizing formulation with a pH > 10.5. This alkaline environment maintains the stability of the Mg(OH)2 protective film on the magnesium surface, while the oxidant specifically attacks the Cr2O3 passivation film on the stainless steel surface, achieving selective corrosion and increasing the specific surface area and mechanical anchoring points. When the pH > 10.5, a dense Mg(OH)2 film rapidly forms on the magnesium surface, providing protection for the magnesium alloy.

[0009] A three-stage pH gradient washing process using citric acid and sodium citrate is employed to achieve a smooth transition from strong alkali to weak acid corrosion, avoiding sudden pH changes that could lead to localized corrosion, pitting, or over-corrosion on the magnesium alloy surface.

[0010] The solution removes loose corrosion products from the magnesium surface in a short time, exposing a fresh active surface, while avoiding the formation of a thick passivation film, thus ensuring the effectiveness of subsequent electrolytic film formation.

[0011] In this invention, the magnesium alloy in the steel-magnesium composite workpiece is selected from one of AZ31B, AZ61, AZ91D, and AM60, preferably AZ91D magnesium alloy; the stainless steel is selected from one of 304 stainless steel, 316L stainless steel, 430 stainless steel, and 201 stainless steel, preferably 304 stainless steel.

[0012] A more optimized process is as follows: using a citric acid-sodium citrate buffer solution, performing three-stage gradient washing, with the temperature controlled at 25~35℃, and each stage of washing lasting 3~8 minutes; the first stage of washing uses a buffer solution with pH 8.5~9.5, the second stage uses a buffer solution with pH 6.5~7.5, and the third stage uses a buffer solution with pH 5.5~6.0. After each stage of washing, the surface residual liquid is rinsed off with pure water before proceeding to the next stage of washing.

[0013] In a more optimized form, the electrolyte comprises the following components: 3-8 g / L potassium fluoride, 2-5 g / L sodium silicate, 1-3 g / L phytic acid, 0.2-0.5 g / L thiourea, 4-8 g / L glycerol, 0.8-1.5 g / L modified PEDOT, and 1-3 g / L nano-cerium oxide (average particle size 30 nm).

[0014] The optimized electrolytic treatment process conditions are: voltage of 3.5~6V, temperature of 25~35℃, and time of 15~25 minutes.

[0015] A more optimized method for preparing the modified PEDOT is as follows: by weight, (1) carboxylated EDOT is added to the mixture and mixed to obtain an EDOT mixed solution; carboxylated β-cyclodextrin is added to MES buffer at pH 4-6, EDOT mixed solution, EDC, and NHS are added and stirred in an ice-water bath for 30-60 minutes, polydopamine is added and stirred for 6-10 hours to obtain modified EDOT; (2) EDOT and modified EDOT are added to deionized water and mixed evenly, PSS is added and stirred evenly, ammonium persulfate and ferric sulfate heptahydrate are added and stirred at room temperature for 8-10 hours, centrifuged, washed, and freeze-dried to obtain modified PEDOT.

[0016] In the scheme, the preparation method of carboxylated EDOT is as follows: (1) Under nitrogen protection, 3 parts of 3,4-dimethoxythiophene are dissolved in 80 parts of toluene, and then 7.2 parts of methyl glycerate, 0.3 parts of p-toluenesulfonic acid monohydrate and 0.12 parts of butylated hydroxytoluene are added. The mixture is heated at 100°C for 48 hours. The resulting blue mixture is filtered, toluene is removed under reduced pressure, and rapid column chromatography is performed to obtain EDOT-methyl ester; (2) 3 parts of EDOT-methyl ester, 150 parts of tetrahydrofuran and 60 parts of deionized water are mixed evenly, 1.6 parts of hydrochloric acid are added and stirred for 2 days. The mixture is extracted three times with ethyl acetate, the organic phases are combined, washed with saturated sodium chloride, dried with anhydrous sodium sulfate, filtered, and the solvent is removed to obtain carboxylated EDOT.

[0017] The preparation method of carboxylated β-cyclodextrin is as follows: 1.2 parts of β-cyclodextrin are added to 15 parts of solvent (N,N-dimethylformamide) and mixed evenly. 0.3 parts of succinic anhydride and 0.2 parts of triethylamine are added and stirred at 100°C for 7 hours. After cooling to room temperature, chloroform is added to precipitate the product. The product is then centrifuged, washed, and dried to obtain carboxylated β-cyclodextrin.

[0018] In this process, if EDOT and PSS are directly added to the electrolyte for electrolysis, it is difficult to deposit PEDOT in situ under a certain voltage. Therefore, PEDOT is prepared and added to the electrolyte. However, PEDOT has poor dispersibility, resulting in uneven deposition on the steel-magnesium composite workpiece. This leads to a greater difference in roughness between the stainless steel and magnesium alloy surfaces, resulting in increased peel strength. To solve this problem, the proposed solution prepares modified PEDOT by modifying it with carboxylated EDOT, carboxylated β-cyclodextrin, and polydopamine. Therefore, when placed in the electrolyte, some of the modified PEDOT in the electrolyte preferentially adsorbs onto the surface of the steel-magnesium composite workpiece without affecting electrolysis.

[0019] Among them, carboxylated EDOT provides polarity and reaction sites, enabling PEDOT to have hydrophilic adsorption properties and regulate conductivity; carboxylated β-cyclodextrin inhibits PEDOT aggregation and improves dispersibility and film uniformity through covalent grafting and steric hindrance effects; polydopamine significantly enhances the adsorption strength of modified PEDOT on steel and magnesium bimetallic surfaces through strong interfacial coordination and covalent crosslinking.

[0020] In a more optimized form, the raw materials for the modified EDOT include the following components: by mass, 2-4 parts carboxylated EDOT, 1-2 parts carboxylated β-cyclodextrin, 3-6 parts EDC (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, CAS No. 25952-53-8), 2-3 parts NHS (N-hydroxysuccinimide, CAS No. 6066-82-6), and 3-6 parts polydopamine; the raw materials for the modified PEDOT include the following components: by mass, 4 parts EDOT (3,4-ethylenedioxythiophene, CAS No. 126213-50-1), 0.8-1.3 parts modified EDOT, 10-12 parts PSS (sodium polystyrene sulfonate, CAS No. 9080-79-9), 0.9-1.5 parts ammonium persulfate, and 0.01-0.015 parts ferric sulfate heptahydrate.

[0021] More optimally, the roughness of the stainless steel surface of the steel-magnesium composite workpiece B is 1.5~3.0μm; the roughness of the magnesium alloy surface of the steel-magnesium composite workpiece B is 2.0~4.0μm.

[0022] In a more optimized manner, the washing temperature in the hot water is 60~70℃, and the time is 3~5 minutes; the concentration of the silane coupling agent solution is 0.5~2wt%, and the activation is carried out at room temperature for 1~3 minutes.

[0023] In this design, the nano-injection molding temperature is 70~90℃, the nano-injection molding speed is 200~600mm / s, and the injection molding resin is selected from one or more of polyphenylene sulfide resin, polybutylene terephthalate, polyamide resin, polyo-phenylenediamine, polyhydroxy ether epoxy resin, and polyhydroxy acrylic resin. Compared with the prior art, the beneficial effects of the present invention are: 1. Selective corrosion of stainless steel in alkaline environment: Using a strong alkaline oxidizing corrosion solution, this system can effectively destroy the chromium oxide film on the surface of stainless steel and achieve uniform corrosion. However, since magnesium forms a protective Mg(OH)2 film under alkaline conditions, it achieves "automatic protection" of the magnesium matrix without the need for additional protective agents.

[0024] 2. pH gradient conversion corrosion: directly converts from alkaline corrosion (for steel) to weakly acidic corrosion (for magnesium), achieving a smooth transition of the surface chemical state through intermediate conversion steps, avoiding surface damage caused by sudden pH changes.

[0025] 3. Potential equalization and synergistic film growth: KF forms a high-resistivity film on the magnesium surface; glycerol reduces solution fluidity and ion migration rate, weakening the galvanic effect; modified PEDOT is uniformly adsorbed on both steel and magnesium surfaces, balancing the electrochemical potential of the two phases and reducing the potential difference; combined with phytic acid, thiourea, and nano-cerium oxide, simultaneous and uniform film formation on steel and magnesium is achieved, reducing the roughness difference on both steel and magnesium surfaces, significantly improving the interfacial bonding stability, and ensuring balanced bonding strength between the steel and magnesium surfaces and the resin, reducing the risk of internal stress, debonding, and cracking. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] In the following embodiments; The preparation method of modified PEDOT is as follows: by weight, (1) 4 parts of carboxylated EDOT are added to the mixture to obtain EDOT mixed solution; 2 parts of carboxylated β-cyclodextrin are added to MES buffer at pH 5.2, EDOT mixed solution, 6 parts of EDC, and 3 parts of NHS are added and stirred in an ice water bath for 30-60 minutes, 6 parts of polydopamine are added and stirred for 6-10 hours to obtain modified EDOT; (2) 4 parts of EDOT and 1 part of modified EDOT are added to deionized water and mixed evenly, 12 parts of PSS are added and stirred evenly, 0.92 parts of ammonium persulfate and 0.015 parts of ferric sulfate heptahydrate are added and stirred at room temperature for 8 hours, centrifuged, washed, and freeze-dried to obtain modified PEDOT. Example 1

[0028] Step 1: Degrease and wash the steel-magnesium composite workpiece, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 15 seconds to obtain steel-magnesium composite workpiece A; The raw materials for the alkaline solution include the following components: 35 g / L sodium hydroxide, 20 g / L persulfate, and 5 g / L sodium silicate. The pH gradient washing process conditions are as follows: using citric acid-sodium citrate buffer, washing in three stages, with the temperature controlled at 30℃. The first stage wash uses a buffer with pH 9.3 for 5 minutes, the second stage wash uses a buffer with pH 7.0 for 3 minutes, and the third stage wash uses a buffer with pH 5.8 for 5 minutes. After each stage wash, the surface residual liquid is rinsed off with pure water before proceeding to the next stage wash. The raw materials for the acidic solution include the following components: 25 g / L ferric nitrate and 0.3 g / L hydrochloric acid; The raw materials for the chromate-nitric acid solution include the following components: 10 g / L chromic anhydride and 40 mL / L nitric acid; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in the electrolyte, take it out, wash it with water, and dry it to obtain the steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water (temperature is 60℃) for 3 minutes, activate it in a silane coupling agent (γ-glycidyl oxypropyltrimethoxysilane, CAS number is 2530-83-8) solution (concentration is 1.2wt%) for 3 minutes, and then perform nano-injection molding (the resin used for injection molding is polyphenylene sulfide resin) to obtain the nano-injection molded steel-magnesium composite workpiece; The electrolyte raw materials include the following components: 8 g / L potassium fluoride, 5 g / L sodium silicate, 3 g / L phytic acid, 0.4 g / L thiourea, 6 g / L glycerol, 0.8 g / L modified PEDOT, and 1 g / L nano cerium oxide.

[0029] The electrolytic treatment process conditions are: voltage 4.3V, temperature 30℃, and time 20 minutes.

[0030] Performance testing: In Example 1, the steel-magnesium composite workpiece A was subjected to a single electrolytic treatment in an electrolyte solution to form a conductive network layer. The potential difference between the stainless steel surface and the magnesium alloy surface was measured, and the effective potential difference was reduced by 78%. In Example 1, the roughness of the stainless steel surface of the steel-magnesium composite workpiece B is 2.0±0.2μm; the roughness of the magnesium alloy surface of the steel-magnesium composite workpiece B is 2.4±0.2μm.

[0031] Example 2 is based on Example 1, except that the amount of modified PEDOT added is 1.2 g / L; Step 1: Degrease and wash the steel-magnesium composite workpiece, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 15 seconds to obtain steel-magnesium composite workpiece A; The raw materials for the alkaline solution include the following components: 35 g / L sodium hydroxide, 20 g / L persulfate, and 5 g / L sodium silicate. The pH gradient washing process conditions are as follows: using citric acid-sodium citrate buffer, washing in three stages, with the temperature controlled at 30℃. The first stage wash uses a buffer with pH 9.3 for 5 minutes, the second stage wash uses a buffer with pH 7.0 for 3 minutes, and the third stage wash uses a buffer with pH 5.8 for 5 minutes. After each stage wash, the surface residual liquid is rinsed off with pure water before proceeding to the next stage wash. The raw materials for the acidic solution include the following components: 25 g / L ferric nitrate and 0.3 g / L hydrochloric acid; The raw materials for the chromate-nitric acid solution include the following components: 10 g / L chromic anhydride and 40 mL / L nitric acid; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in the electrolyte, take it out, wash it with water, and dry it to obtain the steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water (temperature is 60℃) for 3 minutes, activate it in a silane coupling agent (γ-glycidyl oxypropyltrimethoxysilane, CAS number is 2530-83-8) solution (concentration is 1.2wt%) for 3 minutes, and then perform nano-injection molding (the resin used for injection molding is polyphenylene sulfide resin) to obtain the nano-injection molded steel-magnesium composite workpiece; The electrolyte raw materials include the following components: 8 g / L potassium fluoride, 5 g / L sodium silicate, 3 g / L phytic acid, 0.4 g / L thiourea, 6 g / L glycerol, 1.2 g / L modified PEDOT, and 1 g / L nano cerium oxide.

[0032] The electrolytic treatment process conditions are: voltage 4.3V, temperature 30℃, and time 20 minutes.

[0033] Performance testing: In Example 2, the steel-magnesium composite workpiece A was subjected to a single electrolytic treatment in an electrolyte solution to form a conductive network layer. The potential difference between the stainless steel surface and the magnesium alloy surface was measured, and the effective potential difference was reduced by 81%. In Example 2, the roughness of the stainless steel surface of the steel-magnesium composite workpiece B is 2.7±0.2μm; the roughness of the magnesium alloy surface of the steel-magnesium composite workpiece B is 2.3±0.2μm.

[0034] Example 3 is based on Example 2, except that the amount of nano-cerium oxide added is 2 g / L; Step 1: Degrease and wash the steel-magnesium composite workpiece, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 10-20 seconds to obtain steel-magnesium composite workpiece A; The raw materials for the alkaline solution include the following components: 35 g / L sodium hydroxide, 20 g / L persulfate, and 5 g / L sodium silicate. The pH gradient washing process conditions are as follows: using citric acid-sodium citrate buffer, washing in three stages, with the temperature controlled at 30℃. The first stage wash uses a buffer with pH 9.3 for 5 minutes, the second stage wash uses a buffer with pH 7.0 for 3 minutes, and the third stage wash uses a buffer with pH 5.8 for 5 minutes. After each stage wash, the surface residual liquid is rinsed off with pure water before proceeding to the next stage wash. The raw materials for the acidic solution include the following components: 25 g / L ferric nitrate and 0.3 g / L hydrochloric acid; The raw materials for the chromate-nitric acid solution include the following components: 10 g / L chromic anhydride and 40 mL / L nitric acid; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in the electrolyte, take it out, wash it with water, and dry it to obtain the steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water (temperature is 60℃) for 3 minutes, activate it in a silane coupling agent (γ-glycidyl oxypropyltrimethoxysilane, CAS number is 2530-83-8) solution (concentration is 1.2wt%) for 3 minutes, and then perform nano-injection molding (the resin used for injection molding is polyphenylene sulfide resin) to obtain the nano-injection molded steel-magnesium composite workpiece; The electrolyte raw materials include the following components: 8 g / L potassium fluoride, 5 g / L sodium silicate, 3 g / L phytic acid, 0.4 g / L thiourea, 6 g / L glycerol, 1.2 g / L modified PEDOT, and 2 g / L nano cerium oxide.

[0035] The electrolytic treatment process conditions are: voltage 4.3V, temperature 30℃, and time 20 minutes.

[0036] Performance testing: In Example 3, the steel-magnesium composite workpiece A was subjected to a single electrolytic treatment in an electrolyte solution to form a conductive network layer. The potential difference between the stainless steel surface and the magnesium alloy surface was measured to be reduced by 85%.

[0037] In Example 3, the roughness of the stainless steel surface of the steel-magnesium composite workpiece B is 2.2±0.2μm; the roughness of the magnesium alloy surface of the steel-magnesium composite workpiece B is 1.9±0.2μm.

[0038] Comparative Example 1 is based on Example 3, with EDOT monomer and PSS added to the electrolyte; the remaining operation steps are the same. Step 1: Degrease and wash the steel-magnesium composite workpiece, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 15 seconds to obtain steel-magnesium composite workpiece A; The raw materials for the alkaline solution include the following components: 35 g / L sodium hydroxide, 20 g / L persulfate, and 5 g / L sodium silicate. The pH gradient washing process conditions are as follows: using citric acid-sodium citrate buffer, washing in three stages, with the temperature controlled at 30℃. The first stage wash uses a buffer with pH 9.3 for 5 minutes, the second stage wash uses a buffer with pH 7.0 for 3 minutes, and the third stage wash uses a buffer with pH 5.8 for 5 minutes. After each stage wash, the surface residual liquid is rinsed off with pure water before proceeding to the next stage wash. The raw materials for the acidic solution include the following components: 25 g / L ferric nitrate and 0.3 g / L hydrochloric acid; The raw materials for the chromate-nitric acid solution include the following components: 10 g / L chromic anhydride and 40 mL / L nitric acid; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in the electrolyte, take it out, wash it with water and dry it to obtain the steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water, activate it in the silane coupling agent solution and nano-injection mold it to obtain the nano-injection molded steel-magnesium composite workpiece. The electrolyte raw materials include the following components: 8 g / L potassium fluoride, 5 g / L sodium silicate, 3 g / L phytic acid, 0.4 g / L thiourea, 6 g / L glycerol, 1.2 g / LEDOT, 2.2 g / LPSS, and 2 g / L nano cerium oxide.

[0039] Performance testing: In Comparative Example 1, steel-magnesium composite workpiece A was subjected to a single electrolytic treatment in an electrolyte solution. After forming a conductive network layer, the potential difference between the stainless steel surface and the magnesium alloy surface was measured, and the effective potential difference was reduced by 73%. In Comparative Example 1, the roughness of the stainless steel surface of the steel-magnesium composite workpiece B is 3.3 ± 0.3 μm; the roughness of the magnesium alloy surface of the steel-magnesium composite workpiece B is 4.0 ± 0.3 μm.

[0040] Comparative Example 2 is based on Example 3, but without the addition of modified PEDOT or nano-cerium oxide; the remaining operating steps are the same. Step 1: Degrease and wash the steel-magnesium composite workpiece, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 15 seconds to obtain steel-magnesium composite workpiece A; The raw materials for the alkaline solution include the following components: 35 g / L sodium hydroxide, 20 g / L persulfate, and 5 g / L sodium silicate. The pH gradient washing process conditions are as follows: using citric acid-sodium citrate buffer, washing in three stages, with the temperature controlled at 30℃. The first stage wash uses a buffer with pH 9.3 for 5 minutes, the second stage wash uses a buffer with pH 7.0 for 3 minutes, and the third stage wash uses a buffer with pH 5.8 for 5 minutes. After each stage wash, the surface residual liquid is rinsed off with pure water before proceeding to the next stage wash. The raw materials for the acidic solution include the following components: 25 g / L ferric nitrate and 0.3 g / L hydrochloric acid; The raw materials for the chromate-nitric acid solution include the following components: 10 g / L chromic anhydride and 40 mL / L nitric acid; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in the electrolyte, take it out, wash it with water and dry it to obtain the steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water, activate it in the silane coupling agent solution and nano-injection mold it to obtain the nano-injection molded steel-magnesium composite workpiece. The electrolyte contains the following components: 8 g / L potassium fluoride, 5 g / L sodium silicate, 3 g / L phytic acid, 0.4 g / L thiourea, and 6 g / L glycerol.

[0041] The electrolytic treatment process conditions are: voltage 4.3V, temperature 30℃, and time 20 minutes.

[0042] Performance testing: In Comparative Example 2, steel-magnesium composite workpiece A was subjected to a single electrolytic treatment in an electrolyte solution. After forming a conductive network layer, the potential difference between the stainless steel surface and the magnesium alloy surface was measured, and the effective potential difference was reduced by 68%.

[0043] In Comparative Example 2, the roughness of the stainless steel surface of the steel-magnesium composite workpiece B was 2.5 ± 0.2 μm; the roughness of the magnesium alloy surface of the steel-magnesium composite workpiece B was 3.8 ± 0.3 μm.

[0044] Comparative Example 3 is based on Example 3, except that the modified PEDOT is replaced with conventional PEDOT; the remaining operating steps are the same. Step 1: Degrease and wash the steel-magnesium composite workpiece, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 15 seconds to obtain steel-magnesium composite workpiece A; The raw materials for the alkaline solution include the following components: 35 g / L sodium hydroxide, 20 g / L persulfate, and 5 g / L sodium silicate. The pH gradient washing process conditions are as follows: using citric acid-sodium citrate buffer, washing in three stages, with the temperature controlled at 30℃. The first stage wash uses a buffer with pH 9.3 for 5 minutes, the second stage wash uses a buffer with pH 7.0 for 3 minutes, and the third stage wash uses a buffer with pH 5.8 for 5 minutes. After each stage wash, the surface residual liquid is rinsed off with pure water before proceeding to the next stage wash. The raw materials for the acidic solution include the following components: 25 g / L ferric nitrate and 0.3 g / L hydrochloric acid; The raw materials for the chromate-nitric acid solution include the following components: 10 g / L chromic anhydride and 40 mL / L nitric acid; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in the electrolyte, take it out, wash it with water and dry it to obtain the steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water, activate it in the silane coupling agent solution and nano-injection mold it to obtain the nano-injection molded steel-magnesium composite workpiece. The electrolyte raw materials include the following components: 8 g / L potassium fluoride, 5 g / L sodium silicate, 3 g / L phytic acid, 0.4 g / L thiourea, 6 g / L glycerol, 1.2 g / L LPEDOT, 2.2 g / L LPSS, and 2 g / L nano cerium oxide.

[0045] The electrolytic treatment process conditions are: voltage 4.3V, temperature 30℃, and time 20 minutes.

[0046] Performance testing: In comparative example 3, steel-magnesium composite workpiece A was placed in an electrolyte for a single electrolytic treatment. After forming a conductive network layer, the potential difference between the stainless steel surface and the magnesium alloy surface was measured, and the effective potential difference was reduced by 71%.

[0047] In Comparative Example 3, the roughness of the stainless steel surface of the steel-magnesium composite workpiece B was 2.4 ± 0.2 μm; the roughness of the magnesium alloy surface of the steel-magnesium composite workpiece B was 3.5 ± 0.3 μm.

[0048] Results and Discussion: Experimental results show that as the amount of modified PEDOT and nano-cerium oxide added to the electrolyte increases, the effective potential difference between the stainless steel and magnesium alloy surfaces decreases more significantly. The potential difference reductions in Examples 1-3 reached 78%, 81%, and 85%, respectively, all significantly better than the comparative examples, indicating that the combination of modified PEDOT and nano-cerium oxide can effectively construct a uniform conductive network and balance the interfacial potential between the steel and magnesium phases. Regarding surface roughness, in Comparative Examples 1-3, due to the absence of modified PEDOT or the use of only monomers and ordinary PEDOT, the uniformity of film deposition decreased, the surface roughness difference between stainless steel and magnesium alloy was large, and the overall roughness was high, which is detrimental to the stability of the bonding force in subsequent nano-injection molding. In Example 3, the surface roughness of stainless steel and magnesium alloy were 2.2±0.2μm and 1.9±0.2μm, respectively, with more similar roughness between the two phases and optimal interfacial uniformity.

[0049] Comparative Example 1, using EDOT monomer and PSS directly, failed to form a stable and continuous PEDOT conductive film, resulting in inferior potential control and surface uniformity compared to the examples. Comparative Example 2, without modified PEDOT or nano-cerium oxide, lacked conductive and corrosion-inhibiting film-forming components at the interface, leading to a potential difference reduction of only 68%. Comparative Example 3, using ordinary PEDOT, exhibited poor film formation and dispersibility on the steel-magnesium bimetallic surface, with weaker potential difference reduction and roughness control compared to the modified PEDOT system.

[0050] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing a nano-injection molded steel-magnesium composite workpiece, characterized in that: The following steps are included: Step 1: Degrease and wash the steel-magnesium composite workpiece, immerse it in an alkaline solution, wash it with water, perform pH gradient washing, immerse it in an acidic solution, wash it with water, and sonicate it in a chromate-nitric acid solution for 10-20 seconds to obtain steel-magnesium composite workpiece A; Step 2: (1) Electrolyze the steel-magnesium composite workpiece A in the electrolyte, take it out, wash it with water, and dry it to obtain the steel-magnesium composite workpiece B; (2) Wash the steel-magnesium composite workpiece B in hot water, activate it in the silane coupling agent solution, and perform nano-injection molding to obtain the nano-injection molded steel-magnesium composite workpiece.

2. The method for preparing a nano-injection molded steel-magnesium composite workpiece according to claim 1, characterized in that: The raw materials for the alkaline solution include the following components: 20-40 g / L sodium hydroxide, 15-30 g / L persulfate, 1-5 g / L sodium silicate, and pH > 10.5; The acidic solution comprises the following components: 20-30 g / L ferric nitrate, 0.1-0.3 g / L hydrochloric acid, and 3.5 g / L sodium chloride. <pH≤5.5; The raw materials for the chromate-nitric acid solution include the following components: 5~10g / L chromic anhydride and 30~50mL / L nitric acid.

3. The method for preparing a nano-injection molded steel-magnesium composite workpiece according to claim 1, characterized in that: The pH gradient washing process conditions are as follows: a citric acid-sodium citrate buffer solution is used, and the washing is performed in three stages. The temperature is controlled at 25~35℃, and the washing time for each stage is 3~8 minutes. The first stage of washing uses a buffer solution with pH 8.5~9.5, the second stage uses a buffer solution with pH 6.5~7.5, and the third stage uses a buffer solution with pH 5.5~6.

0. After each stage of washing, the surface residual liquid is rinsed off with pure water before proceeding to the next stage of washing.

4. The method for preparing a nano-injection molded steel-magnesium composite workpiece according to claim 1, characterized in that: The electrolyte comprises the following components: 3-8 g / L potassium fluoride, 2-5 g / L sodium silicate, 1-3 g / L phytic acid, 0.2-0.5 g / L thiourea, 4-8 parts glycerol, 0.8-1.5 parts modified PEDOT, and 1-3 parts nano-cerium oxide.

5. The method for preparing a nano-injection molded steel-magnesium composite workpiece according to claim 1, characterized in that: The electrolytic treatment process conditions are: voltage of 3.5~6V, temperature of 25~35℃, and time of 15~25 minutes.

6. The method for preparing a nano-injection molded steel-magnesium composite workpiece according to claim 4, characterized in that: The preparation method of the modified PEDOT is as follows: by weight, (1) carboxylated EDOT is added to the mixture and mixed to obtain EDOT mixed solution; carboxylated β-cyclodextrin is added to MES buffer at pH 4~6, EDOT mixed solution, EDC, NHS are added and stirred in an ice water bath for 30~60 minutes, polydopamine is added and stirred for 6~10 hours to obtain modified EDOT; (2) EDOT and modified EDOT are added to deionized water and mixed evenly, PSS is added and stirred evenly, ammonium persulfate and ferric sulfate heptahydrate are added and stirred at room temperature for 8~10 hours, centrifuged, washed, and freeze-dried to obtain modified PEDOT.

7. The method for preparing a nano-injection molded steel-magnesium composite workpiece according to claim 6, characterized in that: The modified EDOT raw materials include the following components: by mass, 2-4 parts carboxylated EDOT, 1-2 parts carboxylated β-cyclodextrin, 3-6 parts EDC, 2-3 parts NHS, and 3-6 parts polydopamine; the modified PEDOT raw materials include the following components: by mass, 4 parts EDOT, 0.8-1.3 parts modified EDOT, 10-12 parts PSS, 0.9-1.5 parts ammonium persulfate, and 0.01-0.015 parts ferric sulfate heptahydrate.

8. The method for preparing a nano-injection molded steel-magnesium composite workpiece according to claim 1, characterized in that: The roughness of the stainless steel surface of the steel-magnesium composite workpiece B is 1.5~3.0μm; the roughness of the magnesium alloy surface of the steel-magnesium composite workpiece B is 2.0~4.0μm.

9. The method for preparing a nano-injection molded steel-magnesium composite workpiece according to claim 1, characterized in that: The washing temperature in the hot water is 60~70℃, and the time is 3~5 minutes; the concentration of the silane coupling agent solution is 0.5~2wt%, and the activation is carried out at room temperature for 1~3 minutes.

10. The nano-injection molded steel-magnesium composite workpiece prepared by the method for preparing a nano-injection molded steel-magnesium composite workpiece according to any one of claims 1 to 9.