Abstract: According to the characteristics of aluminum alloys, an inorganic acid treatment process is used to pretreat the aluminum alloy, and then electroless nickel plating is performed directly. The effects of main salt, reducing agent, complexing agent, pH and other factors on the reaction deposition rate of electroless nickel plating were discussed, and the optimum process recipe was obtained. The Ni P alloy coating obtained by the verification test has uniform surface, good corrosion resistance and strong binding force.
Key words: aluminum alloy; electroless nickel plating; deposition rate CLC number: TG146.21; TG174.41 Document code: A Article ID: 1007-7235 (2006) 03-0028-04
Due to its excellent performance, aluminum and its alloys have been widely used in various aspects such as construction, aerospace, electrical appliances, electronic products and daily necessities, and its output and use have become the second largest metal material after steel. With the development of modern science and technology, the requirements for the surface properties of aluminum alloys are getting higher and higher, such as corrosion resistance, wear resistance, and decorativeness, etc., and the surface treatment technology of aluminum alloys is generated and continuously developed under this background. [1 3].
The surface treatment methods for aluminum alloys include electroplating, electroless plating, chemical oxidation, anodization, and micro-arc oxidation, among which the application of electroless nickel plating is common. Electroless nickel plating of aluminum alloys can improve its corrosion resistance and wear resistance, making it brazeable. However, the aluminum oxide film formed in the atmosphere does not bond well with the metal plating. In order to obtain a good plating layer, the surface oxide film must be completely removed and no longer formed before electroless nickel plating. This is an aluminum alloy electroless nickel plating. One of the difficulties, aluminum alloy is also considered as one of the "difficult to plate" substrate. At present, the process of electroless nickel plating on difficult-to-plate substrates still needs improvement[4]. This article adopts a special method to pretreat the aluminum alloy and determine the electroless nickel plating process conditions of the aluminum alloy through orthogonal test. The coating was tested for deposition rate, adhesion, corrosion resistance, and porosity.
1 Electroless nickel plating test 1.1 Optimization of process conditions The test is carried out in a constant temperature water bath, using a vertical mixer to stir, with a rotation speed of 250 rpm to 500 rpm. The pH of the bath is accurately determined with a pHS 25 acidity meter. The test material was an aluminum alloy bar having a diameter of 3 mm.
The test temperature is controlled between 80°C and 85°C, and then the nickel salt, reducing agent and complexing agent content and plating solution pH value are the main factors, and four factors and three levels of orthogonal test are performed on the basis of single factor test [10 11 ], The inspection index is the plating deposition rate. The sample is then processed by a preferred process and tested for performance.
1.2 Processes Each test is carried out according to the following process: chemical degreasing → cold water washing → alkali etching → cold water washing → special treatment → cold water washing → distilled water washing → electroless nickel plating → cold water washing → hot water washing → drying → heat treatment.
Special treatment liquid consists of inorganic acid and oxidant, inorganic acid: 150mL L; oxidant: 75mL L; treatment at room temperature for 20min.
1.3 Performance Test At the end of the test, the cross section of the coating was observed under a scanning electron microscope and the thickness of the coating was measured. The thickness of the coating was divided by the plating time to obtain the reaction deposition rate, which is the average plating speed. According to the relevant provisions of GB-T13913-92, the thermal shock test was used to check the bond strength of the coating, and the corrosion resistance of the test sample was tested with concentrated nitric acid test and immersion test [12].
2 Results and discussion 2.1 Single factor test 2.1.1 Determination of nickel sulfate concentration Nickel sulfate concentration is a major factor affecting the deposition rate of the coating. With the increase of nickel sulfate concentration, the deposition rate will increase, but this trend is not obvious in the electroless nickel plating solution of lactic acid system. According to tests, when the content of nickel sulfate reaches 30 g L, the surface of the coating is rough, the self-decomposition phenomenon of the plating solution is serious, and nickel deposits are formed on the container wall. In addition, the increase in nickel sulfate concentration, coating adhesion and corrosion resistance increases, therefore, the nickel sulfate content in the bath is controlled at 20g L ~ 30g L.
2.1.2 Determination of sodium hypophosphite concentration The concentration of sodium hypophosphite mainly affects the deposition rate and corrosion resistance of the coating. In a certain range of pH values, the concentration of sodium hypophosphite increases and the deposition rate of the coating increases. However, if the content of sodium hypophosphite is too high during the test, the plating solution is unstable and the coating may be roughened or a dark plating may be formed. The content of sodium hypophosphite is controlled between 20g L and 30g L.
2.1.3 Determination of Lactic Acid Concentration After the addition of lactic acid, the plating solution is usually stable because the two ligands in the nickel ion and lactic acid form coordination bonds, so that the complex ion has a cyclic structure, ie, the internal complex is generated [ 11]. When lactic acid is present in the plating solution, the plating rate increases as the lactic acid concentration increases. After reaching a large value, the lactic acid concentration continues to increase and the plating rate decreases. The addition of lactic acid can also improve the appearance of the coating and increase the working pH of the bath [7]. Through the test, the amount of lactic acid was determined to be 15 mL L to 25 mL L.
2.1.4 Determination of pH The pH value also affects the deposition rate of the coating. When the pH value is lower than 3, the electroless nickel plating reaction hardly occurs; when the pH value is higher than 6, the self-decomposition of the electroplating solution is serious, the pH value drops rapidly during electroplating, and a large number of pores appear on the surface of the electroplating layer. When the pH value is controlled between 4.5 and 5.5, the plating solution is stable and the plating layer is smooth and flat.
2.2 Orthogonal test and results The primary and secondary order of influential factors are: A> B (C)> D. The preferred process conditions are A3B3C1D2, namely: nickel sulfate 30g L, sodium hypophosphite 30g L, lactic acid 15mL L, pH 5.0.
2.3 Verification test results 2.3.1 Effect of special treatment on the surface state of the coating After the special treatment, the sample surface is formed with fine and evenly distributed honeycombs. These honeycombs can be used as deposition points for the plating layer to make the coating and the substrate more tightly bonded. With the prolongation of processing time, the honeycomb distribution is more uniform, but after the processing time exceeds 30 min, the amount of substrate corrosion increases. Visually inspect the coating for complete, bright, surface defects such as pinholes, bubbles, and yin and yang. Observe under scanning electron microscope that the surface of the coating is a uniform spherical cell, smooth and dense, indicating that the holes left after special treatment will not affect the coating. Surface Quality.
2.3.2 Plating Thickness and Deposition Velocity Figure 2 shows the scanning electron microscope image of the cross-section of the coating after heat treatment. After 2 hours of plating, the thickness of the coating is 25.3 μm. The plating thickness is divided by the plating time to obtain the deposition rate. Therefore, the deposition rate is 12.6 μm h.
2.3.3 Bonding test results There was no gap between the coating and the substrate, indicating a good bonding force between the two. When the plating is heat-treated, atomic interdiffusion occurs, resulting in the recrystallization of the amorphous and microcrystals, the formation of metallic nickel cells and intermetallic compounds. These intermetallic compounds cause the coating to harden; as the heat treatment continues, The mutual diffusion between the coating and the substrate improves the bonding force between the two, and the low-temperature heat treatment can also improve the corrosion resistance of the coating and eliminate some internal stresses [13-14].
2.3.4 Determination of corrosion resistance In the immersion test, the corrosion weight loss of the coating and the time are shown in Figure 3.
In the initial stage, the corrosion rate of the sample was slower, and white haze appeared around the sample after 48 hours. After 72 hours, traces of the penetration of the etching solution could be observed at the corners of the sample, and the corrosion rate accelerated. In the nickel plating process, there is an edge effect. The growth direction of the plating is to advance along the “stepâ€. If there is a sharp corner in the substrate, the plating will preferentially grow here; and the thickening of the plating is achieved by the overlap of the layer and the layer [15]. With the increase in the number of overlaps of the plating layer and the uneven arrangement between the layers, a layered structure is formed on the edge of the sample, and it is easily etched preferentially. After immersion for 96 hours, there was no blistering or peeling on the main surface of the coating, indicating good chemical resistance. When tested with the nitric acid drop method, the color change time of the surface of the sample was 47 seconds, which met the requirement of corrosion resistance.
3 Conclusions (1) The pretreatment process adopted in this experiment is simple to operate, which overcomes the disadvantages of the traditional “secondary zincation†method with complex process flow, satisfies the surface quality and binding force requirements of the coating, and the treatment solution can be used repeatedly, and it will not polluted environment.
(2) The experiment selected the electroless nickel plating process for aluminum alloy with high deposition rate and good plating performance: nickel sulfate, 30 g L; sodium hypophosphite, 30 g L; lactic acid, 15 mL L; pH, 5.0.
(3) The surface of the coating obtained by the preferred process is bright, uniform, and has good bonding strength and corrosion resistance.
references:
[1] Yang Yuqin. Surface treatment of aluminum and its alloys [J]. Guangdong Nonferrous Metals, 1993, (3): 74-79.
[2] Lu Guimin, Wang Zhaoguo, Li Bing. Aluminum alloy corrosion and surface treatment [M]. Shenyang: Northeastern University Press, 2000.
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