Surface treatment involves using mechanical and chemical methods to create a protective layer on a product’s surface, which serves to safeguard the body. This process allows the product to reach a stable state in nature, enhances its corrosion resistance, and improves its aesthetic appeal, ultimately increasing its value. When selecting surface treatment methods, it’s important to consider the product’s usage environment, expected lifespan, aesthetic appeal, and economic value.
The surface treatment process consists of pre-treatment, film formation, post-film treatment, packing, warehousing, and shipment. Pre-treatment comprises mechanical and chemical treatments.
Mechanical treatment involves processes such as blasting, shot blasting, grinding, polishing, and waxing. Its purpose is to eliminate surface unevenness and address other unwanted surface imperfections. Meanwhile, chemical treatment removes oil and rust from the product’s surface and creates a layer that allows film-forming substances to combine more effectively. This process also ensures that the coating attains a stable state, enhances the adhesion of the protective layer, and provides protective benefits to the product.
Aluminum surface treatment
Common chemical treatments for aluminum include processes such as chromization, painting, electroplating, anodizing, electrophoresis, and more. Mechanical treatments consist of wire drawing, polishing, spraying, grinding, and others.
1. Chromization
Chromization creates a chemical conversion film on the product’s surface, with a thickness ranging from 0.5 to 4 micrometers. This film has good adsorption properties and is primarily used as a coating layer. It can have a golden yellow, natural aluminum, or green appearance.
The resulting film has good conductivity, making it an excellent choice for electronic products such as conductive strips in mobile phone batteries and magnetoelectric devices. It is suitable for use on all aluminum and aluminum alloy products. However, the film is soft and not wear-resistant, so it is not ideal for use on external precision parts of the product.
Customization process:
Degreasing—> aluminic acid dehydration—> customization—> packaging—> warehousing
Chromization is suitable for aluminum and aluminum alloys, magnesium, and magnesium alloy products.
Quality Requirements:
1) The color is uniform, the film layer is fine, there can be no bruises, scratches, touch by hand, no roughness, ash and other phenomena.
2) The thickness of the film layer is 0.3-4um.
2. Anodizing
Anodizing: It can form a uniform and dense oxide layer on the surface of the product (Al2O3). 6H2O, commonly known as steel jade, this film can make the surface hardness of the product reach 200-300 HV. If the special product can undergo hard anodizing, the surface hardness can reach 400-1200 HV. Therefore, hard anodizing is an indispensable surface treatment process for cylinders and transmissions.
In addition, this product has very good wear resistance and can be used as a necessary process for aviation and aerospace-related products. The difference between anodizing and hard anodizing is that anodizing can be colored, and the decoration is much better than hard oxidation.
Construction points to consider: anodizing has strict requirements for materials. Different materials have different decorative effects on the surface. Commonly used materials are 6061, 6063, 7075, 2024, etc. Among them, 2024 has a relatively worse effect due to the different content of CU in the material. 7075 hard oxidation is yellow, 6061 and 6063 are brown. However, ordinary anodizing for 6061, 6063, and 7075 is not much different. 2024 is prone to a lot of gold spots.
1. Common process
Common anodizing processes include brushed matte natural color, brushed bright natural color, brushed bright surface dyeing, and matte brushed dyeing (which can be dyed into any color). Other options include polished glossy natural color, polished matte natural color, polished glossy dyeing, and polished matte dyeing. Additionally, there are spray noisy and bright surfaces, spray noisy foggy surfaces, and sandblasting dyeing. These plating options can be utilized in lighting equipment.
2. Anodizing process
Degreasing—> alkali erosion—> polishing—> neutralization—> lidi—> neutralization
Anodizing—> dyeing—> sealing—> hot water washing—> drying
3. Judgment of common quality abnormalities
A. Spots may appear on the surface due to inadequate quenching and tempering of the metal or poor material quality, and the suggested remedy is to perform re-heat treatment or change the material.
B. Rainbow colors appear on the surface, which is usually caused by an error in anode operation. The product may hang loosely, resulting in poor conductivity. It requires a specific treatment method and re-anodic treatment after power is restored.
C. The surface is bruised and severely scratched, which is generally caused by mishandling during transportation, processing, treatment, power withdrawal, grinding, or re-electrification.
D. White spots may appear on the surface during staining, typically caused by oil or other impurities in the water during anode operation.
4. Quality standards
1) The film thickness should be between 5-25 micrometers, with a hardness of over 200HV, and the color change rate of the sealing test should be less than 5%.
2) The salt spray test should last for more than 36 hours and must meet the CNS standard of level 9 or above.
3) The appearance must be free of bruises, scratches, colored clouds, and any other undesirable phenomena. There should be no hanging points or yellowing on the surface.
4) Die-cast aluminum, such as A380, A365, A382, etc., cannot be anodized.
3. Aluminum electroplating process
1. Advantages of aluminum and aluminum alloy materials:
Aluminum and aluminum alloy materials have various advantages, such as good electrical conductivity, fast heat transfer, light-specific gravity, and easy forming. However, they also have disadvantages, including low hardness, lack of wear resistance, susceptibility to intergranular corrosion, and difficulty in welding, which can limit their applications. To maximize their strengths and mitigate their weaknesses, modern industry often uses electroplating to address these challenges.
2. Advantages of aluminum electroplating
- improve decorativeness,
- Improves surface hardness and wear resistance
- Reduced coefficient of friction and improved lubricity.
- Improved surface conductivity.
- Improved corrosion resistance (including in combination with other metals)
- Easy to weld
- Improves adhesion to rubber when hot pressed.
- Increased reflectivity
- Repair dimensional tolerances
Aluminum is quite reactive, so the material used for electroplating needs to be more active than aluminum. This requires a chemical transformation before electroplating, such as zinc-immersion, zinc-iron alloy, and zinc-nickel alloy. The intermediate layer of zinc and zinc alloy has good adhesion to the middle layer of cyanide copper plating. Due to the loose structure of die-cast aluminum, the surface cannot be polished off during grinding. If this is done, it may lead to pinholes, acid-spitting, peeling, and other issues.
3. The process flow of aluminum electroplating is as follows:
Degreasing – > alkali etching – > activation – > zinc replacement – > activation – > plating (such as nickel, zinc, copper, etc.) – > chrome plating or passivation – > drying.
-1- Common aluminum electroplating types are:
Nickel plating (pearl nickel, sand nickel, black nickel), silver plating (bright silver, thick silver), gold plating, zinc plating (colored zinc, black zinc, blue zinc), copper plating (green copper, white tin copper, alkaline copper, electrolytic copper, acid copper), chrome plating (decorative chrome, hard chrome, black chrome), etc.
-2- The use of common plating seeds
- Black plating, such as black zinc and black nickel, is used in optical electronics and medical devices.
- Gold plating and silver are the best conductors for electronic products. Gold plating also enhances the decorative properties of products, but it is relatively expensive. It is generally used in the conductivity of electronic products, such as the electroplating of high-precision wire terminals.
- Copper, nickel, and chromium are the most popular hybrid plating materials in modern science and are widely used for decoration and corrosion resistance. They are cost-effective and can be used in sports equipment, lighting, and various electronic industries.
- White tin copper, developed in the seventies and eighties, is an environmentally friendly plating material with a bright white color. It is a popular choice in the jewelry industry. Bronze (made of lead, tin, and copper) can imitate gold, making it an attractive decorative plating option. However, copper has poor resistance to discoloration, so its development has been relatively slow.
- Zinc-based electroplating: The galvanized layer is blue-white and soluble in acids and alkalis. Since the standard potential of zinc is more negative than that of iron, it provides reliable electrochemical protection for steel. Zinc can be used as a protective layer for steel products used in industrial and marine atmospheres.
- Hard chrome, deposited under certain conditions, has high hardness and wear resistance. Its hardness reaches HV900-1200kg/mm, making it the hardest coating among commonly used coatings. This plating can improve the wear resistance of mechanical parts and prolong their service life, making it essential for cylinders, hydraulic pressure systems, and transmission systems.
-3- Common abnormalities and improvement measures
- Peeling: The zinc replacement is not optimal; the timing is either too long or too short. We need to revise the measures and re-determine the replacement time, bath temperature, bath concentration, and other operating parameters. Additionally, the activation process needs to be improved. We need to enhance the measures and alter the activation mode. Furthermore, the pretreatment is inadequate, leading to oil residue on the workpiece surface. We should improve the measures and intensify the pretreatment process.
- Surface roughness: The electroplating solution needs adjustment due to discomfort caused by the light agent, softener, and pinhole dose. The body surface is rough and requires re-polishing before electroplating.
- The surface is beginning to turn yellow, indicating a potential issue, and the mounting method has been modified. Add the appropriate amount of displacement agent.
- Surface fluffing teeth: The electroplating solution is too dirty, so strengthen filtration and make appropriate bath treatment.
-4- Quality requirements
- The product should not have any yellowing, pinholes, burrs, blistering, bruises, scratches, or any other undesired defects in its appearance.
- The film thickness should be at least 15 micrometers, and it should pass a 48-hour salt spray test, meeting or exceeding the US military standard of 9. Additionally, the potential difference should fall within the range of 130-150mV.
- The binding force should withstand a 60-degree bending test.
- Products intended for special environments should be customized accordingly.
-5- Precautions for aluminum and aluminum alloy plating operation
- Always use aluminum alloy as a hanger for electroplating of aluminum parts.
- Erode aluminum and aluminum alloys quickly and with as few intervals as possible to avoid re-oxidation.
- Ensure that the second immersion time is not too long to prevent excessive corrosion.
- Thoroughly clean with water during the washing process.
- It is important to prevent power outages during the plating process.
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Post time: Sep-11-2024