Method for electroplating nickel on the surface of a hot-pressed graphite piece

Abstract

The present application belongs to the technical field of object surface treatment, and in particular relates to a method for electroplating nickel on the surface of a hot-pressed graphite piece, which comprises the following steps: ultrasonic oil removal of the graphite piece in anhydrous ethanol; impregnation and hole sealing of the oil-removed graphite piece in a zinc-hard acid-anhydrous ethanol solution, and air drying after taking out; impregnation of the hole-sealed graphite piece in a dilute hydrochloric acid solution for activation, and water washing; electroplating of nickel in an electroplating solution after activation, and water washing after treatment is completed. The technology provided by the present application can realize uniform and rapid electroplating of nickel on the surface of a hot-pressed graphite piece without chemical activation of special chemicals, the plating layer has good adhesion with the substrate, absorption of corrosive chloride ions by the porous graphite substrate is avoided, and the technology is suitable for mass continuous production.

Description

Technical Field

[0001] This invention belongs to the field of surface treatment technology, specifically, it is a method for electroplating nickel on the surface of hot-pressed graphite parts. Background Technology

[0002] Hot-pressed graphite parts possess excellent electrical conductivity, lubricity, high-temperature resistance, and corrosion resistance, making them a preferred material for demanding service environments such as high-temperature sealing gaskets, conductive brushes, solution electrodes, smelting electrodes, and EDM electrodes. The world's natural graphite reserves are abundant, and the technology for synthesizing graphite is constantly improving; therefore, further enhancing the performance of graphite materials and expanding their applications is of great practical significance. However, in some high-end applications, such as motor brushes, impermeable gaskets, and microwave absorbing devices, graphite parts also have some significant drawbacks, such as easy surface weathering and peeling, low strength, low hardness, poor wear resistance, high porosity, and difficulties in material bonding.

[0003] Surface treatment can improve the shortcomings of graphite while maintaining its good properties, thus maximizing its application potential. Among them, nickel plating can metallize the graphite surface, improving its resistance to high-pressure penetration and giving it good brazing properties while maintaining good heat resistance, corrosion resistance, and thermal and electrical conductivity. Therefore, nickel plating on graphite surfaces has important application value in the fields of high-temperature impermeable washers, motor brushes, and commutator brushes, and conducting research in related fields is of great significance.

[0004] However, most existing research focuses on nickel plating on the surface of graphite powder and expanded graphite parts. The surface state of hot-pressed parts made from non-expanded graphite powder is quite different, and research on nickel plating processes is relatively scarce. It is currently known that nickel layers can be prepared by chemical plating, i.e., activation with palladium (Pd) followed by plating; nickel layers can also be prepared by activation with iron (Fe) followed by chemical plating; and nickel layers can also be prepared by activation with hydrogen peroxide and potassium permanganate followed by electrodeposition. However, the above methods all have some problems that urgently need to be solved: the pretreatment of electroless nickel plating is complicated and costly; the conductivity of electroless nickel plating layer is significantly different from that of electroplated nickel layer, making it unsuitable for high-end products that require excellent conductivity; the electrodeposition solution uses nickel sulfate as the main salt, has a high pH, ​​and does not use a Ni anode, so the Ni2+ concentration in the solution cannot be maintained stably, making it impossible to achieve large-scale, continuous, and automated production; graphite parts generally have high porosity, and during the pretreatment and nickel plating process, acid solution penetrates into the substrate, and corrosive ions, such as chloride ions, cannot be removed even after high-temperature baking, reducing the coating bonding strength and gradually corroding the coating, causing it to pulverize, which poses a serious threat to the safety of parts that have been in long-term service. Summary of the Invention

[0005] The purpose of this invention is to provide a method for electroplating nickel on the surface of hot-pressed graphite parts. This method has a simple process, is economical and practical, can obtain a pure nickel plating layer with excellent performance, has a simple plating solution formula, is easy to maintain, can achieve continuous production, and leaves no corrosive media residue.

[0006] The specific technical solution adopted in this invention is as follows:

[0007] A method for electroplating nickel onto the surface of a hot-pressed graphite part includes the following steps:

[0008] Step (1) Degreasing: Degrease the graphite parts by ultrasonic treatment in anhydrous ethanol and then air-dry them;

[0009] Step (2) Sealing: Immerse the parts in a zinc stearate-anhydrous ethanol solution to seal the holes, then remove and air dry;

[0010] Step (3) Activation: Immerse the parts in a dilute hydrochloric acid solution for activation, then wash with water;

[0011] Step (4) Nickel plating: Electroplating nickel in the electroplating solution and then rinsing with water.

[0012] In the above technical solution, in step (1), no heating is required during the cleaning process; during the treatment process, anhydrous ethanol must completely cover the carbon parts, and then ultrasonic waves are applied for cleaning for more than 5 minutes; after the ultrasonic waves are finished, the sample is taken out and left to air dry naturally with anhydrous ethanol or can be dried by warm air.

[0013] Surface degreasing is a necessary step in electroplating and chemical plating. Hot-pressed graphite parts, due to their forming temperature of 300-600℃ or higher, are less likely to have significant oil stains on their surface, and therefore can be cleaned with organic solvents to achieve a clean surface. On the other hand, hot-pressed graphite parts are not as strong as ceramic parts, and their surfaces have a large number of easily detachable carbon particles. By using ultrasonic action to remove weakly bonded particles from the surface in advance, it is beneficial to protect the plating solution and improve the adhesion of the plating layer.

[0014] In step (2), the impregnation solution is made by dissolving zinc stearate in anhydrous ethanol. The concentration of zinc stearate should be controlled at 5-15 g / L, and the temperature of the solution should be controlled at above 60°C to keep it clear and avoid gelation. The impregnation time should be controlled at 10-30 minutes.

[0015] This step aims to seal shallow pores, so the immersion time should not be too short or too long. Experiments have shown that immersion for 10-30 minutes can completely seal the shallow pores, preventing the intrusion of corrosive solutions during later activation and nickel plating. Since zinc stearate is insoluble in cold ethanol, the solution needs to be heated to maintain good wettability; however, the temperature should not be too high to avoid excessive ethanol evaporation. After immersion, the graphite part is removed. During natural cooling, the ethanol in the pores evaporates, leaving zinc stearate to achieve the sealing purpose. Generally speaking, for porous parts with large pores and hard wear resistance, such as powder metallurgy parts and porous ceramic bricks, the molten zinc stearate impregnation method can be used to seal the pores. However, although hot-pressed graphite parts have high porosity and small pore size, molten zinc stearate does not easily penetrate into them. Moreover, the molten impregnation method leaves a large amount of zinc stearate residue on the surface of the workpiece, which requires a combination of mechanical grinding and chemical degreasing to effectively remove. For graphite parts with relatively fragile surfaces, this process will inevitably cause damage, so it is not suitable.

[0016] In step (3), the dilute hydrochloric acid solution required for activation is prepared by 10ml-100ml / L concentrated hydrochloric acid and deionized water, soaked at room temperature for 3 minutes, and then washed with water 2-3 times; the water used for washing is also deionized water.

[0017] This step aims to surface activate the sealed graphite parts. In this step, a metathesis reaction removes the zinc stearate from the outermost layer of the graphite part, exposing a clean substrate and restoring good conductivity. Under the action of hydrochloric acid, the zinc stearate adsorbed on the outermost layer of the graphite part decomposes into stearic acid and zinc chloride, thus peeling off from the graphite surface. Therefore, in this step, the concentration of hydrochloric acid should not be too high, and the soaking time should not be too long, otherwise the sealing effect will be damaged; conversely, if the hydrochloric acid concentration is low or the soaking time is short, activation will not be sufficient, resulting in problems such as incomplete plating and low bonding strength.

[0018] In step (4), the electroplating nickel solution contains 50-100 g / L of nickel chloride hexahydrate. The pH of the plating solution is adjusted to between 2 and 4 using HCl and NaOH solutions. A current of 3-10 A / dm2 is applied, and nickel metal is used as the anode. After electroplating, the solution is rinsed with deionized water 2-3 times and then subjected to subsequent treatments such as sealing or tin plating.

[0019] In the plating solution formulation, nickel chloride provides nickel ions and chloride ions to activate the anode, and also gives the plating solution good conductivity. Hydrochloric acid is used to promote anode activation and adjust the pH of the plating solution. Experiments show that the pH of the plating solution determines the main reactions on the graphite part, cathode, and surface: when the pH is below 2, a large number of fine hydrogen bubbles are precipitated on the surface of the graphite part. At this time, hydrogen ion reduction is the main reaction, and only a small amount of nickel is deposited even with prolonged plating time; when the pH is above 2, a nickel layer can be gradually deposited on the graphite surface; however, when the pH is too high, the plating solution is unstable, the coating is rough, the anode activation efficiency is low, and the surface is easily covered by mud-like oxides. The nickel ion concentration and current directly affect the growth rate of the electroplated layer. Under the premise of ensuring the quality of the electroplated layer, for graphite parts, to prevent absorption of the plating solution, a slightly higher current should be used and the plating time should be shortened.

[0020] In summary, the method for electroplating nickel on the surface of hot-pressed graphite parts provided by this invention overcomes the difficulties in the nickel plating process of hot-pressed graphite parts. The process is simple, uses conventional chemicals throughout, is pollution-free, and produces rapid and uniform nickel plating, making it suitable for large-scale continuous production.

[0021] The beneficial effects of this invention are as follows:

[0022] (1) No special chemicals are required for chemical activation, which can achieve uniform and rapid electroplating of nickel on the surface of hot-pressed graphite parts. Compared with chemical nickel plating, it has good conductivity.

[0023] (2) The coating has good adhesion to the substrate;

[0024] (3) The nickel ions in the plating solution are replenished by consuming the nickel anode, which is suitable for large-scale continuous production;

[0025] (4) Pre-nickel plating is performed to seal the holes to prevent the substrate from absorbing corrosive chloride ions and to prevent corrosion of the plating layer. Attached Figure Description

[0026] Figure 1 This is a SEM image of the surface morphology of the graphite part after cleaning and degreasing in Embodiment 1 of the present invention.

[0027] Figure 2 This is the EDS analysis spectrum of the graphite part after cleaning and degreasing in Embodiment 1 of the present invention.

[0028] Figure 3 This is a SEM image of the surface morphology of the graphite part after nickel plating in Embodiment 1 of the present invention.

[0029] Figure 4 This is the EDS analysis spectrum of the graphite part after nickel plating in Embodiment 1 of the present invention.

[0030] Figure 5 This is the XRD analysis spectrum of the graphite part after nickel plating in Embodiment 1 of the present invention.

[0031] Figure 6 These are SEM images of the surface morphology of a graphite part after nickel plating without sealing.

[0032] Figure 7 This is an EDS analysis spectrum of a graphite part that has been nickel-plated without sealing treatment.

[0033] Figure 8 These are SEM images of the surface morphology of graphite parts after plating when the plating solution pH=1.5.

[0034] Figure 9 This is an EDS analysis spectrum of the graphite part surface after plating when the plating solution pH=1.5.

[0035] Specific implementation

[0036] To enhance understanding of the present invention, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. These embodiments are only used to explain the invention and do not limit the scope of protection of the invention.

[0037] Example 1: A method for electroplating nickel on the surface of a hot-pressed graphite part, comprising the following steps:

[0038] Step (1) Degreasing: At room temperature, place the hot-pressed graphite parts into anhydrous ethanol and turn on ultrasonic-assisted cleaning for 10 minutes. After cleaning, remove the parts and allow them to air dry quickly and naturally.

[0039] Step (2) Sealing: Weigh out zinc stearate at a concentration of 10 g / L, and slowly add it to anhydrous ethanol preheated to 60°C. Continue to maintain the temperature and stir constantly. The zinc stearate will gradually dissolve, forming a transparent zinc stearate-anhydrous ethanol solution. Immerse the degreased graphite parts in the above solution, maintaining a temperature of 60°C, for 20 minutes. After immersion, remove and allow to air dry naturally.

[0040] Step (3) Activation: Prepare a dilute hydrochloric acid solution with 37% concentrated hydrochloric acid and deionized water at a concentration of 20 ml / L. Immerse the sealed graphite part in the solution at room temperature, remove it after 1 minute, rinse it with deionized water 3 times, and then plate it with nickel.

[0041] Step (4) Nickel plating: Prepare the electroplating solution with the following concentrations: nickel chloride hexahydrate 80 g / L. Adjust the pH of the electroplating solution to 3 using concentrated hydrochloric acid and dilute sodium hydroxide solution. Using nickel metal as the anode and the graphite part to be plated as the cathode, apply a current density of 6 A / dm² and electroplat for 10 minutes. After electroplating, perform three consecutive deionization cleaning cycles, dry with warm air at 60°C, and then perform characterization and analysis.

[0042] The substrate after cleaning and degreasing was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 1As shown. Due to the poor flowability of the particles, the surface of the hot-pressed graphite parts already contains pits of varying sizes and shapes, as well as some poorly bonded particles. After ultrasonic cleaning, the poorly bonded graphite particles completely detached, the surface roughness increased, and the flake-like structure of graphite could be seen under high magnification. This indicates that ultrasonic cleaning effectively removes weakly bonded areas on the surface, increases the surface roughness of the graphite parts, and improves the bonding strength of the coating. Component analysis was performed using energy dispersive X-ray spectroscopy (EDS), such as... Figure 2 As shown, the results indicate that the matrix is ​​composed of a single element, C, indicating that the matrix has high purity and no impurities were introduced during the degreasing process.

[0043] During electroplating, the sample surface quickly turns white and bright after being energized. After nickel plating, a uniform and bright coating is visually inspected on the sample surface, with no missed plating areas. Figure 3 This is a SEM image of the surface morphology of a graphite substrate after nickel plating. It can be seen that the small pits and lamellar structures disappear, replaced by densely packed spherical crystals, which is a typical morphology of electroplated nickel coatings. During electroplating, nickel particles first deposit at favorable nucleation sites, forming small grains. Then, as the plating time increases, nickel particles gradually deposit at other locations on the substrate surface, accumulating into island-like crystals, and eventually evolving into a dense coating. Figure 4 As shown, EDS analysis indicates that the main component of the coating is Ni, with no chlorine (Cl) or other elements detected. The coating thickness was measured to be 2.3 micrometers using a film thickness gauge.

[0044] The graphite parts and the nickel-plated graphite parts were analyzed using X-ray diffraction (XRD), and the results are as follows: Figure 5 As shown, after nickel plating, obvious new diffraction peaks appeared in the spectrum near 44.5° and 51.8°. By comparing with the XRD database (PDF01-1260), it can be determined that the coating is nickel metal.

[0045] According to GB9286-98, the adhesion strength of the coating is evaluated using the cross-cut test. For general applications, a grade of 0-3 is sufficient. A 1mm x 1mm grid is drawn on the nickel plating using a cross-cut tester, ensuring the cut penetrates the plating to the substrate in one stroke. After cleaning with a soft brush, 3M tape is flattened and quickly peeled off. The extent of plating peeling is observed under a magnifying glass. Only a small amount of peeling was observed on both sides of the scratches, resulting in a grade of 1, indicating good adhesion between the coating and the graphite substrate.

[0046] Comparative Example 1

[0047] The basic steps of this comparative example are the same as those of the substrate in Example 1, except that ultrasonic assistance was not used during the cleaning and degreasing process. After electroplating, no unplated areas were observed on the workpiece, but SEM showed that the coating was rough, EDS analysis did not detect Cl, and the coating thickness was 2.0 micrometers. However, the cross-cut adhesion test result was grade 4, indicating poor adhesion between the coating and the substrate.

[0048] Example 2: The basic steps of this example are the same as those of the substrate in Example 1, except that the concentration of the zinc stearate-ethanol solution is reduced to 5 g / L during the sealing process. After electroplating, no unplated areas were visually inspected, SEM observation showed that the coating was dense and uniform, EDS analysis did not detect Cl, the coating thickness was 2.4 micrometers, and the cross-cut adhesion test result was grade 1, indicating good adhesion between the coating and the substrate.

[0049] Comparative Example 2

[0050] The basic steps of this comparative example are the same as those of the substrate in Example 1, except that the concentration of the zinc stearate-ethanol solution was increased to 20 g / L during the sealing process. After the sealing process, there was an obvious gel-like substance formed by the cooling of the zinc stearate-ethanol solution on the surface of the graphite part; after the activation treatment, some of this gel-like substance remained; after the electroplating was completed, obvious plating defects were visually observed.

[0051] Comparative Example 3

[0052] The basic steps of this comparative example are the same as those of the substrate in Example 1, except that the pores are not sealed. After electroplating, visual inspection revealed no missed plating areas, and the plating thickness was 2.6 micrometers. SEM observation showed that the plating was dense and uniform, but EDS analysis revealed significant Cl elements, such as... Figure 6 As shown. Figure 7 EDS analysis spectrum of unsealed graphite parts after nickel plating

[0053] Example 3: The basic steps of this example are the same as those of the substrate in Example 1, except that the activation time is extended to 3 minutes. After electroplating, no unplated areas were visually inspected. SEM observation showed that the coating was dense and uniform, EDS analysis showed no Cl element, the coating thickness was 2.4 micrometers, and the cross-cut adhesion test result was grade 1, indicating good adhesion between the coating and the substrate.

[0054] Comparative Example 4

[0055] The basic steps of this comparative example are the same as those of the matrix in Example 1, except that the concentration of the hydrochloric acid solution used for activation is increased to 80 ml / L.

[0056] After electroplating, no plating was found to be missed, and the plating thickness was 2.6 micrometers. SEM observation showed that the plating was dense and uniform, but EDS analysis revealed obvious Cl elements.

[0057] Example 4: The basic steps of this example are the same as those of the substrate in Example 1, except that the concentration of the activated nickel salt is reduced to 40 g / L. After electroplating, no unplated areas were found by visual inspection, SEM observation showed that the coating was dense and uniform, EDS analysis did not detect Cl element, the coating thickness was 1.9 micrometers, the cross-cut adhesion test result was grade 1, and the coating and substrate were well bonded.

[0058] Example 5: The basic steps of this example are the same as those of the substrate in Example 1, except that the pH is increased to 4. After electroplating, no unplated areas were found by visual inspection. SEM observation showed that the coating was dense and uniform, EDS analysis showed no Cl element, the coating thickness was 3.2 micrometers, and the cross-cut adhesion test result was grade 2, indicating good adhesion between the coating and the substrate.

[0059] Comparative Example 5

[0060] The basic steps of this embodiment are the same as those of the substrate in Example 1, except that the pH is increased to 5. During the electroplating process, mud-like substances are continuously generated at the anode, and the plating solution becomes turbid. Visual inspection shows no missed plating, but the plating layer is dull and not bright, with burrs on the edges. The plating thickness is 3.5 micrometers, and the cross-cut adhesion test result is only level 4, indicating that the plating layer is brittle and poorly bonded to the substrate.

[0061] Comparative Example 6

[0062] The basic steps of this comparative example are the same as those of the substrate in Example 1, except that the pH is reduced to 1.5. During the electroplating process, a large number of fine bubbles continuously emerged from the surface of the workpiece, and the graphite part did not quickly turn white and bright. After electroplating, most areas were not covered by the coating. SEM observation revealed that the surface of the graphite part only had sparse nickel particles and no complete coating had formed. Figure 8 As shown. EDS analysis reveals a strong C peak originating from the graphite matrix and a distinct Cl element peak, such as... Figure 9 As shown.

[0063] Example 6: The basic steps of this example are the same as those of the substrate in Example 1, except that the current is reduced to 3A / dm2. After electroplating, no unplated areas were found by visual inspection. SEM observation showed that the coating was dense and uniform. EDS analysis did not detect Cl element. The coating thickness was 1.8 micrometers, and the cross-cut adhesion test result was grade 2, indicating good adhesion between the coating and the substrate.

[0064] Comparative Example 7

[0065] The basic steps of this comparative example are the same as those of the substrate in Example 1, except that the current is increased to 12 A / dm². After 10 minutes of electroplating, visual inspection revealed a bright white coating on some areas of the graphite part, but it was noticeably rough, and some areas of the coating showed obvious burn marks, indicating that the current was too high.

[0066] Table 1. Summary of results from Examples 1-6 and Comparative Examples 1-7

[0067]

[0068] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions, and these changes and improvements shall fall within the scope of the present invention as claimed.

Claims

1. A method for electroplating nickel onto the surface of a hot-pressed graphite part, characterized in that, Includes the following steps: Step (1) Degreasing: Degrease the graphite parts by ultrasonic treatment in anhydrous ethanol and then air-dry them; Step (2) Sealing: Immerse the parts in a zinc stearate-anhydrous ethanol solution to seal the holes, then remove and air dry; Step (3) Activation: Immerse the parts in a dilute hydrochloric acid solution for activation, then wash with water; Step (4) Nickel plating: Electroplating nickel in the electroplating solution and rinsing with water; In step (1), the cleaning process is carried out in a normal temperature environment. During the process, anhydrous ethanol completely covers the hot-pressed graphite part, and then ultrasonic waves are applied. The cleaning time is more than 5 minutes. After the ultrasonic wave is finished, the sample is taken out and left to air dry naturally with anhydrous ethanol or dried by warm air. In step (2), the impregnation solution is made by dissolving zinc stearate in anhydrous ethanol, wherein the concentration of zinc stearate is controlled at 5-15 g / L, the temperature of the solution is controlled at above 60°C, and the impregnation time is controlled at 10-30 minutes. In step (3), the dilute hydrochloric acid solution required for activation is prepared by mixing 37% concentrated hydrochloric acid and deionized water at a concentration of 20 ml / L. Soak at room temperature for 3 minutes, then wash with water 2-3 times. The water used for washing is also deionized water. In step (4), the electroplating nickel solution contains 50-100 g / L of nickel chloride hexahydrate, the pH of the plating solution is adjusted to between 2 and 4 using HCl and NaOH solutions, and a current of 3-10 A / dm is applied. 2 Nickel metal is used as the anode. After electroplating, the product is rinsed with deionized water 2-3 times before further processing.

2. The method for electroplating nickel on the surface of hot-pressed graphite parts according to claim 1, characterized in that, This hot-pressed graphite part is formed by high-temperature hot pressing of pure graphite powder without adding binders or metal powder, and it does not belong to expanded graphite.

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