Aluminum surface treatment process introduction

October 22, 2020

Surface treatment

After it is processed by mechanical and chemical methods, it can form a protective layer on the surface of the product to protect the body. In nature, it can achieve a stable state, increase the body's corrosion resistance and increase the appearance of the product, thereby enhancing the value of the product. The choice of surface treatment type must first be based on the use environment, service life, and human appreciation. Of course, economic value is also the core of consideration.

The surface treatment process includes pretreatment, film formation, and post-film treatment. Packaging and storage. Shipping and other processes, including pre-treatment including mechanical treatment, chemical treatment.

Mechanical processing includes squeezing, blasting, grinding, polishing, and waxing. The purpose of the mechanical treatment is to eliminate unevenness on the surface of the product and to remedy other appearance defects on the surface. Chemical treatment removes rust on the surface of the product and forms a layer that allows the film-forming substance to better bond or form into an active metal body, ensuring a stable state of the coating and increasing the bond strength of the protective layer, thereby protecting the body. effect.

Aluminum surface treatment

The common chemical treatments of aluminum include chromizing, painting, electroplating, anodizing, and electrophoresis. Among them, mechanical processing includes drawing, polishing, squeezing, grinding and other processes.

Section 1 Chromation

Chromizing will form a layer of chemical conversion film on the surface of the product, with a film thickness of 0.5-4 μm. This layer of conversion film has good adsorption and is mainly used as a coating underlayer. The appearance of golden yellow, aluminum color, green and so on. This conversion film has good electrical conductivity and is the best choice for electronic products, such as conductive strips in mobile phone batteries, magnetoelectric devices, etc. The film is suitable for all aluminum and aluminum alloy products. However, the conversion film is soft and does not wear, so it is not conducive to the use of external components.

Chromizing process:

Degreasing -> Alumina Removal -> Chromating -> Packaging -> Storage

Chrome is suitable for aluminum and aluminum alloys, magnesium and magnesium alloy products.

Quality requirements:

1) The color is uniform, the film is fine, and there must be no bumps, scratches, and hands to touch. It can't be rough or gray.

2) Film thickness 0.3-4um.

Section 2 Anodizing

Anodizing: It can form a uniform and dense oxide layer on the surface of the product. (Al2O3. 6H2O common name steel jade) This film can make the surface hardness of the product reach (200-300HV), if the special product can do hard anodizing , The surface hardness of the product can reach 400-1200HV, so the hard anodizing is the cylinder, transmission, and indispensable surface treatment process.

In addition, the wear resistance of this product is very good, and it can be used as a necessary process for aviation and aerospace related products. Anodic oxidation differs from hard anodizing: Anodizing can be colored, and the decorative properties are much better than those of hard oxidation. Construction points: The material requirements for anodic oxidation are very strict. Different material surfaces have different decorative effects. Commonly used materials include 6061, 6063, 7075, 2024, etc. Among them, the relative effect of 2024 is worse, because the content of CU in the material is different. Therefore, the 7075 hard oxidation is yellow, 6061, 6063 is brown, but ordinary anodizing 6061,6063,7075 is not much difference, but 2024 is easy to appear many gold spots.

A common process

The common anodic oxidation processes include drawing matte color, drawing bright surface color, drawing bright surface dyeing, matte drawing dyeing (can be dyed into any color system). Polished bright surface color, polished matte color, polished bright surface dyeing, polished matte dyeing. Squeeze the bright surface color, spray mist matte color, sandblasting staining. Above plating can be used on lighting equipment.

Second, anodizing process

Degreasing -> Alkali -> Chemicals -> Neutralization -> Lidi -> Zhonghe

Anodizing -> Dyeing -> Sealing -> Hot Wash -> Drying

Third, the common quality of abnormal judgment

A spotted spots on the surface. This kind of anomaly is generally due to poor metal tempering or poor material quality, treatment methods, and re-heat treatment. Or replace the material.

B surface appears rainbow. This kind of abnormality is usually caused by mistakes in anode work. , Loose when hanging, causing poor product conductivity. , Treatment methods, and re-anodize.

C. The surface was bruised and severely scratched. This kind of anomaly is generally caused by the carelessness of the work during transportation or processing, handling methods, power backoff, grinding and recharging.

D, white spots appear on the surface when dyed. This kind of anomaly is generally caused by oil, or other impurities in the water during anode operation.


Fourth, quality standards

1) The film thickness is 5-25um, the hardness is above 200HV, and the color change rate of the sealing experiment is less than 5%

2) The salt spray experiment is over 36 hours and can reach the CNS standard above 9

3) The appearance can not be bumped, scratched, clouds and other phenomena. Surface can not have hanging points, yellow and other undesirable phenomena

Die-cast aluminum, such as A380, A365, A382, etc. cannot be anodized

Section III Aluminum Plating Process

First, the advantages of aluminum and aluminum alloy materials

It has the advantages of good conductivity, fast heat transfer, light weight, and easy molding. However, aluminum and aluminum alloys have low hardness, are not wearable, are prone to intergranular corrosion, and are not easy to be welded. Therefore, in order to avoid weaknesses, modern electroplating has solved this problem.

Second, the advantages of aluminum plating

1) Improve decorativeness,

2) Improve surface hardness and wear resistance

3) Reduce friction coefficient and improve lubricity.

4) Improve surface conductivity.

5) Improve corrosion resistance (combined with other metals)

6) Easy to weld

7) Improve the bonding force with the rubber thermocompression.

8) Improve the reflectance

9) Repairing dimensional tolerances

Since aluminum is a relatively active metal, the electroplated materials are more active than aluminum. Therefore, prior to electroplating, corresponding chemical transformations such as zinc immersion, zinc-iron alloys, and zinc-nickel alloys must be performed to make the zinc and zinc alloy substrates. An intermediate layer of cyanide copper plating with good adhesion. Due to the loose structure of the body of die-casting aluminum, the surface layer of the body cannot be polished off when grinding, otherwise pinholes, spit acid, and peeling occur.

Third, the aluminum plating process is

Degreasing -> alkaline corrosion -> activation -> zinc replacement -> activation -> plating (such as nickel, zinc, copper, etc.) -> chrome plating or passivation -> drying.

1, common aluminum plating plating species

Nickel-plated (Pearl Nickel, Sand Nickel, Black Nickel), Silver Plated (Surface Silver, Thick Silver), Gold Plated, Zinc Plated (Color Zinc, Black Zinc, Blue Zinc), Copper Plated (Green Drum Copper, White Tin Copper, Alkali Copper, copper coke, acid copper, chrome plated (white chrome, hard chrome, black chrome), etc.

2, the use of common plating species

1) Black plating, such as black zinc, black nickel, etc. are used in optical electronics and medical devices.

2) Gold plating, silver is the best conductor of electronic products, but also can improve the product's high decorative, but the price is relatively expensive, are generally used in electronic products conductive, such as high precision wire terminals and other plating.

3) Copper, nickel and chromium are the most popular mixed plating species in the modern scientific field. Regardless of their decorative or anti-corrosion properties, they are the most popular plating species in the world. Cheap, can be used in sports equipment, lighting, most of the electronics industry.

4) Among the copper plating seeds, white tin bronze was developed in the 1970s and 1980s for environmentally friendly plating and bright white, making it the first choice for the jewelry industry. Bronze (lead-tin-copper can be used as imitation gold, and therefore it is a relatively good decorative plating). However, the discoloration resistance of copper is relatively poor, and therefore it is slow to develop.

5) Zinc Plating: The galvanized layer is bluish white, soluble in acid and alkali. Since the standard potential of zinc is negative compared to iron, it provides reliable electrochemical protection for steel. Zinc can be selected as a protective layer for steel products used in industrial atmospheres and marine atmospheres.

6) Hard chrome, chromium deposit deposited under certain conditions has high hardness and abrasion resistance, hardness reaches HV900-1200kg/mm, chromium is the highest hardness coating in commonly used coatings, which can improve the wear resistance of parts. Extended service life. Such as work, model, quantity, fixtures, etc. Therefore, this kind of plating is an indispensable treatment method for cylinder, oil pressure, and transmission system.

3, common abnormalities and improvement measures

1) Peeling: a. The zinc replacement is not good, the time is too long or too short, the improvement measures, re-determine the replacement time, bath temperature, bath concentration and other operating parameters.

b, activation is not enough, improvement measures, change the way of activation.

c The pre-treatment is not complete, the surface of the workpiece has oil, improvement measures, and strengthen the pre-treatment.

2) Rough surface: a, electroplating liquid agent, softener, pinhole dose discomfort, re-adjust the amount of additives.

b The body surface itself is rough, re-grinding and then electroplating.

3) The surface is yellow and the low potential is not correct, change the mounting method. Add appropriate amount of positioning agent.

4) Surface Raising Teeth: The plating solution is too dirty to enhance filtration and proper bath treatment.

4, quality requirements

1) No yellow appearance, pinholes, burrs, blisters, bumps, scratches, etc.

2) The film thickness is 15um or more, the salt spray test is 48 hours, and the US military standard is 9 or more, and the potential difference is 130-150mv.

3) The binding force can pass the 60 degree bending test.

4) Products in special environments can be changed accordingly.

5, aluminum and aluminum alloy plating operation precautions

1) Aluminum alloy plating should use aluminum alloy as hanger;

2) After the aluminum and aluminum alloys have been etched, each process must be carried out quickly. The shorter the interval between the processes, the better, so as to avoid reoxidation;

3) The second zinc immersion time should not be too long to avoid excessive corrosion;

4) Wash must be washed;

5) During the electroplating process, it is necessary to prevent power cuts in the middle.

The fourth section of aluminum coating

The coating is divided into dip coating, spray coating, shower coating, roller coating and brush coating. Among them, dip coating and spray coating are the main methods, and dip coating is electrophoretic coating. It uses electrochemical method to deposit the rubber particles of organic resin on the workpiece. On the surface, transparent or various color organic coatings are formed. Among them, cathodic electrophoresis is a new process developed in the 1970s. It is also the main process in the dip coating industry. Advantages: Good corrosion resistance, strong anti-tarnishing properties, and good bonding strength. Spraying is divided into powder coating and liquid coating. Electrostatic coating is a popular technology in the industry.

First, the coating process

Due to the good coating corrosion resistance, low price, many colors (can do any different colors) It is suitable for all aluminum materials, but the material structure is too loose, such as sanding products, need shave operation, the relative quality is also poor .

1. How does electrostatic painting work?

Under the action of the pump, the paint enters the electric field and the paint particles are negatively charged. Under the effect of atomizing air pressure, the paint becomes finer and more uniform. Under the action of the working air pressure, the paint is evenly sprayed on the surface of the workpiece under the action of the screw cup or the nozzle. Since the distance between the spray gun and the workpiece is the same on the same vertical plane, the electric field strength of the same interface is the same, so the positive and negative charges attract and the surface of the workpiece is coated with a uniform layer of organic resin. The resin is then leveled and cured under high temperature baking conditions. Firmly inlaid on the surface of the workpiece to protect the workpiece.

2, aluminum coating process

Mechanical grinding->degreasing->in addition to oxide film->chromium->dusting or liquid->baking->under hanging->full inspection->packaging->storage

1) Common abnormalities and treatment methods for powder liquids

2) Quality Standard Reference

Section 5 Electroless Plating of Aluminum

First, the principle of electroless plating

Electroless plating is a method of redox that occurs when a current passes through (no external force) in the same solution with the aid of a reducing agent, so that the metal ions are reduced on the surface of the part. Therefore, electroless plating can be described as a controlled, autocatalytic chemical reduction process for the deposition of metals. The reaction has the following general formula:

Second, the characteristics of electroless plating

With the development of industry, electroless plating has become a promising process technology. Compared with other plating methods, electroless plating has the following characteristics:

1) Metals can be plated on parts made of various materials such as metals, semiconductors and non-conductors.

2) Regardless of the complex geometry of the part, uniform thicknesses are obtained wherever the solution is exposed.

3) For autocatalytic electroless plating, larger thicknesses can be obtained and even electroformed.

4) No current is needed.

5) The coating is dense and has less porosity.

6) Plating often has special chemical and mechanical properties.

Because electroless plating has many advantages, electroless plating is more and more developed in the industrial and electronic industries. The following is an example of electroless nickel plating as an example:

1) Electroless nickel plating Since sodium hypophosphite is used as a reducing agent, the generally obtained coating is a nickel-phosphorus alloy. According to different phosphorus content can be divided into low phosphorus (1% -4%) medium phosphorus (4% -10%) and high phosphorus (10% -12%). Plating solutions with different phosphorus content can be obtained from different pH values ​​of the plating solution. At PH = 4-5, phosphorus and high-phosphorus alloys can be obtained. Low-phosphorus and medium-phosphorus alloys can be obtained from PH=8-10 in weak alkaline solution. The NI-P alloy containing more than 8% of phosphorus has an amorphous coating and has excellent corrosion resistance due to its lack of grain boundaries. It can reach a hardness of HV=1150 when it is transformed into a mixture of amorphous and crystalline by heat treatment at 300-400°C. The hardness after electroless plating heat treatment is close to the hardness of hard chrome. It is an ideal coating instead of hard chromium layer, and it can also be used on aluminum. Plating good plating species (may be suitable for all aluminum and aluminum alloy products, including forging, die-casting, aluminum, etc.). Disadvantages: The coating is brittle, but the plasticity of the product after heat treatment is greatly improved. (In general, after 400 hours heat treatment above 400 degrees, the plasticity is much improved)

2) Electroless nickel plating process for aluminum:

Ultrasonic degreasing -> In addition to oxide film -> First zinc replacement -> Chemicals -> Second zinc replacement -> Electroless nickel -> Passivation -> Ultrasonic cleaning -> Baking -> Full inspection -> Packaging

3) Retirement of defective plating:

Chemical nickel reclaims such as steel, aluminum, copper, and plastic can all be removed with concentrated nitric acid. However, if the parts are dried and then put into the tank, the moisture will lead to excessive corrosion of the base metal. Working temperature is below 35 degrees.

4) Quality requirements:

a, film thickness in the range of 5-30um

b. The salt spray test reaches more than 24 hours and reaches CNS Class 9 or above.

c. Through the 60 degree bending test, no peeling phenomenon

d, appearance no bumps, scratches, water marks and other undesirable phenomena.

e, uniform film, can not have a fog, white spots, dew plating phenomenon

Third, common exception handling

Section 6 Classification and Selection of Coating Application Conditions

From the perspective of corrosion protection requirements, the design of surface treatment should be considered as follows:

1) Precious metals (gold, platinum), stainless steels containing more than 18% chromium, magnetic alloys, nickel-copper alloys, etc., generally do not require additional protective layers.

2) Parts made of carbon steel, low-alloy steel and cast iron are liable to be corroded in the atmosphere and a protective layer should be added.

3) Copper and copper alloy parts, according to different conditions of use, using bright pickling, passivation, plating or paint protection. Precise parts made of phosphor bronze or beryllium bronze may not be surface-treated.

4) Aluminium and aluminum alloy parts can be anodized and closed. Small parts that are not suitable for anodizing can be chemically oxidized. Cast aluminum alloys can be painted and protected.

5) Parts made of zinc alloys may be protected by phosphating, passivation, plating or painting.

Section 7 Electrophoresis of Aluminum

The color electrophoretic coating of aluminum is a novel surface treatment process. It uses electrochemical methods to deposit colloidal particles of organic resin on parts to form transparent or various colors of organic coating layers. According to the difference in electrification state of the resin particles in the electrophoretic paint after ionization, it can be divided into anodal electrophoresis (ionization of resin particles into negative ions) and cathodic electrophoresis (cationization of resin particles into positive ions).

The electrocoating coating has extremely good corrosion resistance (can pass the neutral salt spray test for more than 400 hours), strong discoloration resistance, good bonding force with the base metal, and can be processed by various kinds of mechanical processing; the coating layer is colorful, according to The user's requirements can be formulated into a variety of colors, common gold, brown, gun color, black, etc.; compared with the paint, construction performance, and environmental pollution and harm. Therefore applied in the car shell and various accessories, bicycle handlebars and accessories, all kinds of daily hardware accessories, furniture, crafts and so on. Due to the advanced cathodic electrophoresis, we will use cathodic electrophoresis as the main topic of our introduction.

Cathodic electrophoretic coating is a complex electrochemical and colloidal chemical process. The electrophoretic paint itself is a multi-component system of colloids and suspensions. There are two components: dispersed phase (resin, pigment particles) and continuous phase (water). Cathodic electrophoresis four processes:

1. Electrophoresis The positively charged water-soluble resin particles and their adsorbed pigments move toward the cathode.

2. Electrodeposited positively charged resin particles reach the discharge surface of the part (cathode) to form a water-insoluble sediment layer, which forms a paint film after baking.

3, electroosmotic water from the sedimentary layer dialysis out, when the water content dropped to 5% -15%, you can bake.

4. Electrolyzed water is electrolyzed by direct current and releases hydrogen and oxygen. Since the electrolysis leads to a decrease in the penetrating power, it affects the appearance of the paint film, reduces the adhesion of the paint film, and increases the power consumption. Therefore, it is necessary to weaken the electrolysis of water.

Electrophoresis process:

Degreasing -> Descale -> Chromation -> Electrophoresis -> Drying -> Packaging


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