A method for directly plating nickel on the surface of an aluminum substrate after copper activation

Abstract

The present invention belongs to the technical field of alloy nickel plating, and specifically relates to a method for nickel plating directly on the surface of an aluminum substrate after copper activation. The method of the present invention can successfully initiate a chemical nickel plating process on aluminum with copper through steps such as degreasing, alkaline etching, acid etching, and activation, thereby preparing a nanocrystalline nickel-phosphorus alloy with uniform composition and fine crystallization through copper activation. The present invention solves the problems of complicated process, environmental pollution, slow plating time, and poor coating bonding in the prior art, and can also reduce production costs compared with silver activation and palladium activation, and is suitable for large-scale industrial production.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy nickel plating, and in particular relates to a method for directly plating nickel on the surface of an aluminum substrate after copper activation. Background Art

[0002] Aluminum alloy has broad application prospects in aerospace, aviation, computer manufacturing, automobile, rail train manufacturing and other industries, and has become an increasingly important new material. When aluminum alloy is used as a structural part, hardness, wear resistance and corrosion resistance are its essential requirements, but due to the inherent characteristics of the aluminum phase in aluminum alloy, aluminum alloy has the problem of insufficient hardness, wear resistance and corrosion resistance, which limits the application of aluminum alloy.

[0003] Chemical nickel plating refers to a process that does not require power but relies on the catalytic reaction of the reducing agent in the solution to release electrons to reduce metal ions to metal atoms. It can be plated regardless of the shape of the product. Chemical plating is a process that uses chemical methods to plate an amorphous Ni-P alloy coating on a metal or non-metal surface. The nickel plating layer has the characteristics of excellent corrosion resistance and wear resistance, high hardness, uniform thickness, better bonding strength than the electroplating layer, and excellent finish. The performance of the coating is closely related to the pretreatment of the aluminum substrate. Different activation methods can obtain nickel coatings with different bonding strengths. It is well known that the initiation of chemical nickel plating on the surface of the substrate requires catalytic active centers. Some transition metals, such as Pd and Ni, can initiate the chemical nickel plating process because they have catalytic activity for sodium hypophosphite. Other metals, such as Zn, Fe and Al, have no catalytic activity themselves, but can reduce nickel ions to nickel elements, thereby playing the role of catalytic active centers. It is generally believed that copper is considered to have no catalytic activity in chemical nickel plating and cannot initiate chemical nickel plating.

[0004] At present, the main methods of aluminum alloy pretreatment are: secondary zinc immersion method, direct plating method, nickel immersion method, silver activation method and palladium activation method. The main disadvantage of the secondary zinc immersion method is that in a humid and corrosive environment, the zinc immersion layer is an anode relative to the nickel plating layer, and will be corroded laterally, eventually causing the nickel plating layer to peel off. Zinc ions will pollute the plating solution during plating. The pre-nickel plating method can overcome the shortcomings of the secondary zinc immersion method, but the molar ratio of the complexing agent to nickel ions and the pH value in this process are key process factors that require better control, and the degree of activation and the stability of the plating solution are not high. The direct plating method is difficult, similar to the nickel immersion method, and it is difficult to control the parameters. Silver and palladium are precious metals, and the cost of the silver activation method and the palladium activation method is high, which is not suitable for large-scale commercial applications.

[0005] In summary, although the above methods can be used to electrolessly plate nickel on aluminum alloy surfaces, various problems will occur during the pre-treatment process, and the process is complicated and the cost is high, which cannot meet the requirements of today's enterprises. Therefore, it has become a research difficulty in this field to develop an aluminum alloy surface electroless nickel plating technology that is suitable for industrial large-scale production, simple in process, environmentally friendly, and low in cost. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and to provide a method for preparing a nanocrystalline nickel-phosphorus alloy with uniform composition and fine crystallization by copper activation.

[0007] The present invention provides a method for directly plating nickel on the surface of an aluminum substrate after copper activation, such as Figure 1 As shown, the following steps are included:

[0008] Alkaline degreasing: remove dirt from the surface of the aluminum substrate, leaving the aluminum surface clean and pollution-free;

[0009] Alkali etching: removes the oxide film and various alloy elements and impurities on the surface of the aluminum substrate, further exposing the aluminum substrate and providing an aluminum-rich surface for subsequent plating.

[0010] Acid etching: After alkaline etching, the surface of the aluminum substrate is dark gray, which is the corrosion product produced by alkaline etching. This product is not conducive to the bonding of the coating, so it needs to be removed by acid etching. Acid etching makes the substrate surface bright, and can improve the bonding strength of the coating while activating the substrate.

[0011] Copper activation: After the above steps of pretreatment, the surface of the aluminum substrate has catalytic active centers. Redox reaction occurs in the copper activation solution, which can cause copper to be deposited on the surface of the aluminum substrate.

[0012] Chemical nickel plating: After the copper-activated aluminum alloy is placed in the nickel plating solution, the aluminum corrodes and releases electrons for nickel ion deposition. After the nickel is deposited, the chemical plating process continues according to the self-catalytic mechanism.

[0013] Among them, one or more deionized water washing steps are carried out between each step of degreasing, alkaline etching, pickling, copper activation and chemical nickel plating.

[0014] 1. Components of degreasing liquid: 25g / L sodium carbonate, 25g / L sodium phosphate, 2g / L sodium dodecylbenzene sulfonate.

[0015] 2. Components of alkaline etching solution: 50g / L sodium hydroxide, 2g / L sodium dodecylbenzene sulfonate.

[0016] 3. Acid etching solution composition: 10% nitric acid

[0017] 4. Components of copper activation solution: 15g / L copper sulfate, 15g / L sodium citrate, 20g / L disodium ethylenediaminetetraacetic acid, 30g / L sodium hypophosphite, 30mg / L anhydrous potassium ferrocyanide, 20mg / L 2,2'-bipyridine, 10g / L sodium chloride, 2g / L nickel sulfate, 0.8g / L cerium sulfate, 1.5g / L ferrous sulfate, 2g / L polyethylene glycol, 0.5g / L ammonium persulfate, 0.2g / L methyl blue, 0.2g / L thiourea, 30mg / L polyvinylpyrrolidone, pH 9.

[0018] The present invention provides a method for activating chemical nickel-copper plating on aluminum alloy, the method comprising the following steps:

[0019] (1) The cutting area is 2 x 2 cm 2 The 7075 aluminum substrate was placed in an alkaline degreasing solution for degreasing for 5 minutes at a temperature of 70°C, and then washed with deionized water.

[0020] (2) The degreased substrate was placed in an alkaline etching solution for 30 seconds at 25°C, and then washed with deionized water.

[0021] (3) The alkali-etched substrate was placed in an acid etching solution and etched for 30 seconds at a temperature of 25°C, and then washed with deionized water.

[0022] (4) The acid-etched substrate was placed in a copper activation solution at 70°C for 5 minutes, and then washed with deionized water.

[0023] (5) The copper-activated substrate was placed in a nickel plating solution for 60 minutes at a temperature of 85° C., and then taken out, washed with water, and dried.

[0024] In the present invention, after the aluminum alloy copper is activated, a layer of copper particles is deposited on the aluminum alloy surface. The copper particles do not completely cover the aluminum alloy surface, and there are microporous channels between the copper layer and the aluminum layer. After being placed in the nickel plating solution, copper and aluminum form a corrosion galvanic cell, copper acts as a cathode and aluminum acts as an anode. Aluminum corrosion releases electrons for nickel ion deposition. After nickel deposition, the chemical plating process continues according to the self-catalytic mechanism. Therefore, the scheme described in the present invention has the following advantages:

[0025] 1) The copper activation solution of the present invention is weakly alkaline with sodium hypophosphite as a reducing agent and a pH of 9. Under this pH condition, the aluminum substrate is not easily corroded. However, the pH value of the conventional chemical copper plating solution with formaldehyde as a reducing agent is greater than 12, and the concentration of sodium hydroxide is generally above 8g / L, which easily attacks the aluminum substrate and causes the coating to peel off laterally.

[0026] 2) The reducing agent used in the copper activation solution of the present invention is sodium hypophosphite. The traditional reducing agent for chemical copper plating is formaldehyde, which has a relatively weak reducing property and is a high-risk carcinogen, which is harmful to the environment and operators.

[0027] 3) The copper activation solution of the present invention uses sodium citrate and disodium ethylenediaminetetraacetic acid as complexing agents, and copper ions react easily with reducing agents to form cuprous ions under the action of the complexing agents. At the same time, ammonium persulfate is used as a surfactant to oxidize cuprous oxide particles into soluble copper ions, which not only improves the stability of the plating solution and prolongs the service life of the plating solution (more than one and a half years), but also allows the scrapped old solution to be reprocessed and reused, greatly reducing the production cost of the enterprise.

[0028] 4) The copper activation solution of the present invention can reduce the temperature of the plating solution, reduce energy consumption and reduce costs.

[0029] 5) The copper activation solution of the present invention contains a methyl blue corrosion inhibitor, which can further reduce the displacement attack of copper on aluminum. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the experimental process flow chart;

[0031] Figure 2 is the average hardness of aluminum substrate, zinc activation, Example 1, and Example 2;

[0032] Figure 3 Schematic diagram of the mechanism of electroless nickel plating on copper-activated aluminum;

[0033] Figure 4 The Tafel polarization curves of nickel coatings with two activation methods in 3.5% NaCl solution;

[0034] Figure 5 This is a SEM image of copper activated chemical nickel plating in Example 1;

[0035] Figure 6 This is a SEM image of copper activated chemical nickel plating in Example 2;

[0036] Figure 7 This is the SEM image of zinc-activated chemical nickel plating in Comparative Example 1. DETAILED DESCRIPTION

[0037] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0038] Example 1

[0039] Take a piece of 2x2cm 2The 7075 aluminum alloy was placed in a 70°C degreasing solution to remove surface oil for 5 minutes. After taking out the aluminum alloy, it was rinsed with deionized water several times. The degreased aluminum alloy was placed in an alkaline etching solution at room temperature for 30 seconds. After taking out the aluminum alloy, it was rinsed with deionized water several times and placed in an acid etching solution at room temperature for 15 seconds. After taking out, it was rinsed with deionized water several times and placed in a copper activation solution at 70°C for 2 minutes. After taking out, it was rinsed with deionized water several times and placed in a nickel plating solution for 60 minutes. A uniform, complete, and strongly bonded nickel-phosphorus coating can be obtained.

[0040] Example 2

[0041] Take a piece of 2x2cm 2 The 7075 aluminum alloy was placed in a 60°C degreasing solution to remove surface oil for 10 minutes. After taking out the aluminum alloy, it was rinsed with deionized water several times. The degreased aluminum alloy was placed in an alkaline etching solution at room temperature for 45 seconds. After taking out the aluminum alloy, it was rinsed with deionized water several times and placed in an acid etching solution at room temperature for 30 seconds. After taking it out, it was rinsed with deionized water several times and placed in a copper activation solution at 65°C for 5 minutes. After taking it out, it was rinsed with deionized water several times and placed in a nickel plating solution for 60 minutes. A uniform, complete, and strongly bonded nickel-phosphorus coating can also be obtained.

[0042] Comparative Example 1

[0043] In order to demonstrate the feasibility of copper activation, a set of comparative experiments was set up for Zn activation. Except for the different activation solutions, the other operating steps were exactly the same as in Example 1. The formula composition of Comparative Example 1 was: 2g / L zinc oxide, 45g / L sodium hydroxide, 40g / L ferric chloride, and 20g / L potassium tartrate. After 60 minutes of plating, a uniform, complete, and strongly bonded nickel-phosphorus coating was obtained.

[0044] In accordance with GB / T5270-2005 “A Review of Test Methods for Adhesion Strength of Electrodeposited and Chemically Deposited Metal Coatings on Metal Substrates”, the nickel-plated aluminum substrate obtained by the above process was subjected to bending test, adhesive tape scribing test, grid test three times and thermal shock test (the plated parts were heated to 220°C and kept warm for 1 hour, then taken out and immediately placed in room temperature water for rapid cooling). The results are shown in Appendix 1.

[0045] Table 1 Performance test of aluminum substrate after nickel plating in Examples 1 to 2

[0046]

[0047]

[0048] Since the hardness and wear resistance of aluminum alloy are relatively poor, the hardness of the nickel-plated layer can be greatly improved after chemical nickel-phosphorus plating. In order to prove this point, the following comparative experiment was conducted, and the hardness was measured at five points, including the top corners and the center of the aluminum substrate, zinc activation, Example 1, and Example 2. The results are as follows Figure 2 As shown, the hardness of the plated parts after copper activation is 2 to 2.5 times that of pure aluminum, which is slightly higher than the conventional zinc activation method.

[0049] Schematic diagram of the mechanism of electroless nickel plating on copper activated aluminum Figure 3 As shown, the aluminum substrate has been pre-treated by degreasing, alkali etching, acid etching, etc., and the surface of the aluminum substrate has catalytic active centers. After being placed in the copper activation solution, an oxidation-reduction reaction occurs, which can cause nickel and phosphorus to be deposited on the surface of the aluminum substrate.

[0050] The Tafel polarization curves of the nickel coatings with two activation methods in 3.5% NaCl solution are shown in Figure 4 As shown. The polarization curve represents the relationship between electrode potential and current density. In chemical plating, electrode potential is often used as the horizontal coordinate and current density is used as the vertical coordinate. The polarization curve is one of the basic methods to explain the basic laws of metal corrosion, reveal the mechanism of metal corrosion and explore the control of corrosion pathways. The more positive the electrode potential, the more corrosion-resistant the plated part is, and the smaller the corrosion current density, the slower the corrosion rate. Figure 4 It can be seen that the potential of the Cu activated coating is more positive and the current density is smaller than that of the Zn activated coating, which indicates that the Cu activated coating has better corrosion resistance.

[0051] The SEM images of chemical nickel plating in Example 1, Example 2 and Comparative Example 1 are shown as follows: Figure 5 , Figure 6 , Figure 7 As shown. The morphology of the coating determines the properties of the coating, such as hardness, wear resistance, corrosion resistance, etc. In addition, the process of chemical nickel plating on aluminum alloy is affected by the activation method and nucleation mode, and the morphology of the plated parts obtained by different activation methods is different. Therefore, it can be explained from the morphology that copper activation has better hardness and corrosion resistance than zinc activation.

[0052] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for nickel plating directly on the surface of an aluminum substrate after copper activation, characterized in that: The method comprises the following steps: The surface of the aluminum substrate is subjected to alkaline degreasing, alkaline etching, and acid etching in sequence, and then the aluminum substrate is placed in a copper activation solution for activation; the activation temperature is 70° C., and the activation time is 2 to 5 minutes; The copper activation solution comprises the following components: 15 g / L copper sulfate, 15 g / L sodium citrate, 20 g / L disodium ethylenediaminetetraacetic acid, 30 g / L sodium hypophosphite, 30 mg / L anhydrous potassium ferrocyanide, 20 mg / L 2,2'-bipyridine, 10 g / L sodium chloride, 2 g / L nickel sulfate, 0.8 g / L cerium sulfate, 1.5 g / L ferrous sulfate, 2 g / L polyethylene glycol, 0.5 g / L ammonium persulfate, 0.2 g / L methyl blue, 0.2 g / L thiourea, 30 mg / L polyvinyl pyrrolidone, and the pH value is 9; The nickel plating process involves placing the activated aluminum substrate in a nickel plating solution for 60 minutes at a temperature of 85°C.

2. The method according to claim 1, characterized in that: The components of the alkaline degreasing liquid include 25 g / L sodium carbonate, 25 g / L trisodium phosphate, and 2 g / L sodium dodecylbenzene sulfonate.

3. The method according to claim 1 or 2, characterized in that: The degreasing temperature is 70° C. and the degreasing time is 5 minutes.

4. The method according to claim 1, characterized in that: The alkaline etching solution used in the alkaline etching comprises 50 g / L sodium hydroxide and 2 g / L sodium dodecylbenzene sulfonate; The acid etching solution used in the acid etching includes 10% nitric acid.

5. The method according to claim 4, characterized in that: The alkali etching temperature is 25°C and the time is 30s; The acid etching temperature is 25° C. and the time is 30 seconds.

6. The method according to claim 1, characterized in that: One or more deionized water washing steps are performed between each of the steps of degreasing, alkali etching, acid etching, activation and nickel plating.

7. The method according to claim 1, characterized in that: The nickel plating process also includes taking out, washing and drying.

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