New aluminum alloy plating process
[China Aluminum Industry Network] 1 Introduction With the rapid development of science and technology, the application of aluminum alloy is expanding, and it has been widely used in aircraft, automobiles, motorcycles, instrumentation and film machinery industries. The aluminum alloy not only has excellent strength and rigidity, but also the die-casting of parts with complex geometric shapes can be formed at one time, realizing no cutting processing, simple process, and high production efficiency. We use aluminum alloy die-casting to first plate bright nickel and then plate a layer of gun black plating technology, which not only saves costs, but also obtains a highly decorative surface coating with more features and improves the competitiveness of products in the international market. Aluminum alloy electroplating and general electroplating process have certain differences, because aluminum alloy is a more active metal, reduction, replacement ability, give the electroplating process to bring a lot of difficulties, generally use zincation method for pretreatment. In recent years, electroplating science and technology workers at home and abroad have developed many new electroplating processes for aluminum alloys. On this basis, we have developed a new type of aluminum alloy pretreatment liquid—H·S·F liquid, and aluminum alloy electroplating process For simplicity, the bond strength of the coating is greatly improved, so as to ensure the black plating quality of the aluminum alloy die-casting gun. Metal Roofing Machine,Roof Panel Roll Forming Machine,Roof Roll Forming Machine,Metal Roofing Xinxiang Tianfeng Machinery Manufacture Co., Ltd. , http://www.tfrollforming.com
2 aluminum alloy die casting pre-treatment aluminum alloy die castings containing high silicon, the surface often small holes and gaps exist, in order to obtain a high decorative appearance, mechanical polishing. Due to the low hardness of the aluminum alloy, the mechanical polishing wheel should be soft and have a certain degree of elasticity so as to avoid distortion of the corners of the parts during mechanical polishing. The pretreatment mainly includes organic solvent degreasing, alkaline etching, acid etching, and immersing in H.S.F liquid.
2.1 Degreasing of Organic Solvents Degreasing is generally carried out using organic solvents such as gasoline and trichloroethylene to dissolve minerals and polishing pastes. Scrubbing with detergent solutions is also possible.
2.2 Alkaline erosion In order to remove light grease and Al2O3 films on the surface of parts, corrosion is performed in a weak lye to expose the aluminum alloy substrate and produce microscopic roughness. However, the alkalinity of the solution should not be too strong. At the same time, the temperature and alkali etching time of the alkali etching solution must be strictly controlled so as to avoid excessive corrosion. The alkaline etching process conditions are as follows:
Na2CO330g/L
Na3PO430g/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 is etched in the hot alkali etching solution, due to the chemical dissolution of aluminum and the insolubility of the alloying element Si, a layer of adhered black film remains on the surface of the part. In order to completely remove the film, the following must be mixed. Acid treatment:
HNO33 parts HF1 water a small amount of temperature room temperature time 20 ~ 40s
2.4 Dipping H·S·F liquid H·S·F liquid is an improvement of dipping zinc solution. It is a solution specially developed for aluminum surface preparation developed by ourselves. The obtained multi-component alloy layer is compact in structure, crystallized in detail, and has a relatively fine porosity. Small, good bonding force, and can directly bright nickel plating after leaching H · S · F solution, simplifying the electroplating process, the process specification is as follows:
H·S·F concentrate 500mL/L
Water residual temperature 15 ~ 30 °C
Time 30～40s
3 Plating Intermediate Layer The general use of ordinary bright nickel plating solution formulations 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 suitable pH 4 ~ 45
Temperature 52~55°C
Cathode current density 25 ~ 4A/dm2
From 12 to 15 minutes, when the cathode is to be electroplated with bright nickel, it is better to charge into the bath, and it is plated with a current twice as large as 1 to 2 minutes, and then it is conventionally plated with nickel.
4 Aluminum Alloy Die Castings Electroplating Gun Black Process Aluminum Alloy Die Casting Blanks → Blank Inspection → Mechanical Polishing → Gasoline or Trichloroethylene Degreasing → Cool Dry → Upper Fixture → Chemical Degreasing and Alkaline Corrosion → Warm Water Cleaning → Cold Wash → Flowing Water Cleaning → Etching → Washing → Washing with running water → Dipping H·S·F Solution → Washing → Washing with flowing water → Bright nickel plating (preferably charged into the tank) → Washing → Purging with running water → Activated in 5% H2SO4 solution → Washing → Cleaning in running water → Plating gun black → Washing → Washing in running water → Chemical passivation → Washing → Washing in running water → Drying (5 to 10 minutes) → Lower fixture → Inspection → Impregnation or painting. Domestic gun black plating process is mostly tin-nickel alloy coating, there are tin-cobalt alloy coating. There are three types of plating solutions: fluoride type, cyanide type, and pyrophosphate type. From the viewpoint of environmental safety, we chose the pyrophosphate type gun black plating process.
4.1 bath formulation and process conditions SnCl2 · 2H2O13 ~ 15g/L
NiCl2·6H2O55～60g/L
K4P2O7·3H2O230～250g/L
H·S·F-2 additive 5～15g/L
Ammonia or ethylenediamine 5 ~ 10mL/L
pH8～9
T45～55°C
T1～3′
Dk0.5~1.5A/dm2
Anode Nickel Plate Needs 4.2 Cathode Mobility Preparation A calculated amount of stannous chloride, nickel chloride, and potassium pyrophosphate are dissolved in 50-60°C hot water, respectively.
2 The dissolved tin salt and nickel salt solution should be slowly added to the potassium pyrophosphate solution under constant stirring, and then stirred for about 15 minutes. If there is turbidity, continue stirring with warming until all are clear.
3 Add the calculated amount of additive and 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, warm to 45-55°C, and electroplate with electrolysis.
4.3 The role of the bath components 4.3.1 Stannous chloride It is the main salt to provide tin ions. The content of stannous chloride increases, and the tin content in tin-nickel alloy plating increases. Stannous chloride content changes in a wide range have 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 becomes brown. The plating bath is not allowed to add oxidants such as hydrogen peroxide, nor is it allowed to stir with air, and only the cathode can be used.
4.3.2 Nickel chloride It is the main salt that provides nickel ions. As the content of nickel chloride increases, the nickel content in the tin-nickel alloy coating slightly increases. When the nickel chloride content is too low, the coating is light in color. Chloride ion facilitates anode plate activation and dissolution of nickel plates.
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 coating 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 water or ethylenediamine added to the ammonia can reduce the internal stress of the coating and make the coating uniform in color. Due to the heavy odor of the ammonia, the solution will be more volatile after being heated, so the ethylenediamine is used instead.
4.3.5 Additives H·S·F-2 additive, also known as blackening agent, is a complexing agent of tin and nickel ions and is an indispensable component for gun black plating. Due to the difference in the type and content of blackening agent, a gray, brown and brown appearance of light, medium and deep iron can be obtained.
5 plating after processing aluminum alloy die casting gun black plating, must be washed immediately, and passivation, drying. Passivation can improve the corrosion resistance of the coating. The drying process in the oven is the process of coating the film. This process is indispensable.
5.1 Chemical passivation chromic anhydride 40 ~ 60g/L
Acetic acid 1~2mL/L
Temperature: room temperature 30 ~ 60s
5.2 Hardened coating, improve the corrosion resistance of the coating. After the plating passivation, it is washed with water and placed in an oven for 15-20 minutes at 100°C.
5.3 The purpose of paint painting is to extend the service life of the gun black coating. According to the level of product quality may be. Some low-grade oil-impregnated products improve the anti-corrosion performance. Products with high quality requirements must be painted.