New Process For Aluminum Alloy Plating

- Oct 09, 2019-

 1 Introduction


With the rapid development of science and technology, the application range of aluminum alloy is expanding, and it has been widely used in the aircraft, automobile, motorcycle, instrumentation and film machinery industries. The aluminum alloy not only has excellent strength and rigidity, but also the die-casting of complex geometrical parts can be formed at one time, achieving no cutting, simple process and high production efficiency. We use aluminum alloy die-casting parts to illuminate bright nickel first, and then apply a layer of gun black plating process, which not only saves cost, but also obtains a high decorative surface coating with more characteristics and improves the competitiveness of products in the international market. There is a certain difference between aluminum alloy electroplating and ordinary electroplating process. Since aluminum alloy is a relatively active metal, the reduction and replacement ability is strong, which brings many difficulties to the electroplating process. Generally, the zinc dip method is used for pretreatment. In recent years, many domestic and foreign electroplating technicians have developed a number of new aluminum alloy plating processes. On this basis, we have developed a new type of aluminum alloy surface pretreatment liquid, H·S·F liquid, which is more electroplated. Simple, the bonding strength of the coating is greatly improved, thereby ensuring the black plating quality of the aluminum alloy die-casting part.


2 Aluminum alloy die casting pretreatment


Aluminum alloy die-casting parts have high silicon content, and often have small pores and gaps on the surface. In order to obtain a high decorative appearance, mechanical polishing is required. Due to the low hardness of the aluminum alloy, the mechanical polishing wheel should be soft and flexible to avoid deformation of the corners of the parts during mechanical polishing. The pretreatment mainly includes organic solvent degreasing, alkali etching, acid etching, and immersion of H·S·F liquid.


2.1 Organic solvent degreasing


It is generally degreased with an organic solvent such as gasoline or trichloroethylene to dissolve minerals and polishing paste, and can also be scrubbed with a detergent solution.


2.2 Alkali etching


In order to remove the slight grease and Al2O3 film on the surface of the part, corrosion is performed in the weak alkali to expose the aluminum alloy substrate and produce micro-roughness. However, the alkaline solution should not be too strong. At the same time, the temperature and alkali etching time of the alkali etching solution should be strictly controlled to avoid the phenomenon of over-corrosion. The alkali etching process conditions are as follows:


Na2CO3 30g/L


Na3PO4 30g/L


Additive 2~4g/L


OP-10 emulsifier 0.5~1mL/L


Temperature 75~85°C


Time 30~60s


2.3 Acid etching (ash removal)


When the aluminum alloy die-casting part is corroded in the hot alkali etching solution, due to the chemical dissolution of aluminum and the insolubilization of the alloying element Si, a layer of attached black film remains on the surface of the part. In order to completely remove the film, it must be mixed below. Acid treatment:


HNO 33 parts


HF 1 part


Water


Temperature


Time 20~40s


2.4 immersion H·S·F liquid


H·S·F liquid is an improvement of zinc immersion solution. It is a solution specially prepared for surface pretreatment of aluminum parts. The obtained multi-alloy layer has compact structure, fine crystal structure, small pores, good bonding force and dipping. After the H·S·F solution, the nickel plating can be directly brightened, which simplifies the electroplating process. The process specifications are as follows:


    H·S·F concentrate 500mL/L


Water balance


Temperature 15~30°C


Time 30~40s


3 The middle layer of the plating intermediate layer is generally formulated with ordinary bright nickel plating solution and process conditions:


Nickel sulfate (NiSO4·7H2O) 250g/L


Nickel chloride (NiCl2·6H2O) 60g/L


Boric acid (H3BO3) 40g/L


Sodium lauryl sulfate 0.05~01g/L


Brightener


pH 4~45


Temperature 52~55°C


Cathode current density 25~4A/dm2


Time 12 to 15 minutes


Cathode movement


When electroplating bright nickel, it is best to charge the tank into the tank, and then galvanize it for 1 to 2 minutes with a double current, and then nickel plating as usual.


4 aluminum alloy die casting electroplating gun black process


Aluminum alloy die casting blank → blank inspection → mechanical polishing → gasoline or trichloroethylene degreasing → cold dry → upper fixture → chemical degreasing and alkali corrosion → warm water cleaning → cold water washing → running water cleaning → acid etching → water washing → running water Cleaning→Immersion H·S·F solution→Washing→Flowing water cleaning→Glossy nickel plating (preferably charged into the tank)→Washing→Running in flowing water→Activation in 5%H2SO4 solution→Washing→Flowing in water→Plasting gun black→Washing → Flowing water cleaning → Chemical passivation → Water washing → Flowing water cleaning → Drying (5 to 10 minutes) → Lower fixture → Inspection → Dipping or painting. Domestic gun black plating processes are mostly tin-nickel alloy coatings, but also tin-cobalt alloy coatings. There are three types of plating solution: fluoride type, cyanide type, pyrophosphate type. From the perspective of environmental protection safety, we choose the pyrophosphate type gun black plating process.


4.1 Plating solution formula and process conditions


SnCl2·2H2O 13~15g/L


NiCl2·6H2O 55~60g/L


K4P2O7·3H2O 230~250g/L


H·S·F-2 additive 5~15g/L


Ammonia or ethylenediamine 5~10mL/L


pH 8~9


T 45~55°C


t 1~3'


Dk 0.5~1.5A/dm2


Anode nickel plate


Cathode movement needs


4.2 plating solution preparation


1 The calculated amount of stannous chloride, nickel chloride and potassium pyrophosphate are dissolved in hot water of 50 to 60 ° C, respectively.


2 Slowly add the dissolved tin salt and nickel salt solution to the potassium pyrophosphate solution with stirring, and then stir for about 15 minutes. If there is turbidity, continue to stir and stir until all clear.


3 Add the calculated amount of additive, add a small amount of NaOH when dissolved in water.


4 Add the calculated amount of ammonia water, add water to the required volume, and mix well.


5 Measure and adjust the pH to 8 ~ 9, and warm to 45 ~ 55 ° C, while electrolysis and try plating.


4.3 The role of various components of the bath


4.3.1 Stannous chloride


It is the main salt that provides tin ions. The content of stannous chloride is increased, and the content of tin in the tin-nickel alloy plating layer is increased. The change in the stannous chloride content over a wide range has no significant effect on the color of the coating. When the content of stannous chloride is too high, the color of the coating becomes lighter; when the content is too low, the coating is brown. The plating solution is not allowed to add an oxidizing agent such as hydrogen peroxide, nor is it allowed to be stirred by air, and only the cathode can be moved.


4.3.2 Nickel chloride


It is the main salt that provides nickel ions. The nickel chloride content increases, and the nickel content in the tin-nickel alloy coating increases slightly. When the nickel chloride content is too low, the coating is light in color. Chloride ions facilitate the activation and dissolution of the nickel plate anode.


4.3.3 Potassium pyrophosphate


It is a complexing agent for nickel ions and tin ions. In addition to complexing nickel and tin, potassium pyrophosphate must have a certain amount of free potassium pyrophosphate. When the potassium pyrophosphate content is low, the plating layer is rough and the color is uneven. When the content is high, the cathode current density decreases and the deposition rate slows down.


4.3.4 Ammonia or Ethylenediamine


The addition of ammonia water can reduce the internal stress of the coating and make the coating uniform in color. Due to the heavy smell of ammonia, the evaporation of the plating solution is more serious after heating, so ethylenediamine is used instead.


4.3.5 Additives


H·S·F-2 additive, also known as blackening agent, is a complexing agent for tin and nickel ions, and is an indispensable component for gun black plating. Light, medium and deep iron grey and brown appearance can be obtained due to the type and content of the blackening agent.


5 post-plating treatment


After the aluminum alloy die-casting gun is black plated, it must be washed immediately, passivated and dried. Passivation can improve the corrosion resistance of the coating. The drying process in the oven is the process of coating the membrane. This process is indispensable.


5.1 Chemical passivation


Chromic anhydride 40~60g/L


Acetic acid 1~2mL/L


Temperature


Time 30~60s


5.2 Hard film


Aging the coating to improve the corrosion resistance of the coating. After the plating layer is passivated, it is washed with water and dried in an oven at 100 ° C for 15 to 20 minutes.


5.3 Painting


The purpose of painting is to extend the life of the black coating of the gun. According to the quality level of the product. Some low-grade products are immersed in oil to improve the corrosion resistance. If the product quality is high, it must be painted.