How to Carry Out the Aluminum Surface Treatment Process?

Surface treatment: A protective layer can be formed on the product’s surface to protect the body after treatment by mechanical and chemical methods. It can achieve a stable state in nature, increase the body’s corrosion resistance and increase the product’s beauty, thus enhancing its value. The type of surface treatment selection should first be based on the use of the environment, service life, human appreciation, and of course, the economic value is also the core of consideration.

The process of surface treatment includes pre-treatment, film formation, and post-treatment. Packaging, warehousing. Shipment and other processes, including mechanical treatment and chemical treatment.

Mechanical treatment includes spraying noise, shot blasting, grinding, polishing, waxing and other processes. The purpose of mechanical treatment is to remove unevenness from the surface of the product and to remedy other appearance defects on the surface. The chemical treatment makes the surface of the product remove the oil and rust and form a layer of film-forming substances to combine better or harmonize the active metal body, to ensure that the plating has a stable state, increase the bonding force of the protective layer, to achieve the role of protecting the body.

Aluminium surface treatment

The common chemical treatments for aluminium include chroming, painting, electroplating, anodizing, Electrophoresis and other processes. Mechanical treatment includes brushing, polishing, spraying, polishing and other processes. (We recommend you to pay attention to the “mechanical engineer” public number, the first time to master the dry knowledge, industry information)

Chroming

Chroming will form a layer of chemical conversion film on the product’s surface; the thickness of the film is 0.5-4um; this layer of conversion film is good adsorption, mainly as a coating base layer. The appearance is golden yellow, aluminium colour, green, etc. This good electrical conductivity conversion film is the best option for electronic products, such as conductive strips in cell phone batteries, magnetic devices, etc. The film is suitable for all aluminium and aluminium alloy products. However, the conversion film is soft and not wear-resistant, so it is not conducive to using external parts of products.

Chromatin process flow

Degreasing->aluminum acid degreasing->chromating->packaging->warehousing

Chromatin is suitable for aluminium and aluminium alloy, magnesium and magnesium alloy products.

Quality requirements

1 ) Uniform colour, meticulous film layer, no bruising, scratching, touching by hand, no roughness, dropping ash and other phenomena.

2) Film thickness 0.3-4um.

Anodic oxidation

Anodizing: can make the product surface form a uniform, dense oxide layer (Al2O3. 6H2O, commonly known as steel jade). This film can make the product surface hardness (200-300HV) if special products can do hard anodizing; product surface hardness can reach 400-1200HV; thus, hard anodizing is the cylinder, transmission, and necessary surface treatment process.

In addition, the wear resistance of this product is very good so it can be used for aviation and aerospace-related products. The difference between anodizing and hard anodizing: anodizing can be coloured, and the decorative properties are much better than hard anodizing. Construction points: anodizing material requirements are very strict, different materials have different decorative effects on the surface, commonly used materials are 6061, 6063, 7075, 2024, etc., of which, 2024 relative effect is worse, due to the different content of CU in the material, so 7075 hard oxidation is yellow, 6061, 6063 is brown, but ordinary anodizing 6061, 6063, 7075 is not much. 6063 and 7075 are not much different, but 2024 is easy to appear with many gold spots.

A common process

Common anodizing processes are brushed matte natural colour, brushed bright natural color, brushed bright color, and matte brushed color (can be dyed into any color). Polished glossy natural color, polished matte natural color, polished glossy dyeing, polished matte dyeing. Spray noisy, bright surface natural color, spray noisy matte surface natural color, sand spray dyeing. All the above plating types can be used in lighting equipment.

Anodic oxidation process

Degreasing->Alkali etching->Chemical polishing->Neutralization->Li Di->Neutralization

Anodizing->Dyeing->Sealing->Hot water washing->Drying

Common quality abnormal judgment

A.The surface appears with flower spots. This abnormality is generally due to poor metal tempering or the material itself being too poor to deal with reheat treatment. Or replace the material.

B. The surface appears rainbow color. An anode operation error generally causes this kind of abnormality. The product will not conduct well if the anode is loose when hanging. The treatment method is to anodize the product.

C. the surface is bruised and scratched seriously. This abnormality is generally due to careless operation during transportation or processing, treatment method, decommissioning, polishing and re-anodizing.

D. white spots appear on the surface when dyeing. This abnormality is generally due to the anode operation, the water having oil, or other impurities caused.

Quality standards

1) Film thickness 5-25um, hardness above 200HV, seal hole experiment colour change rate less than 5%

2) Salt spray test above 36 hours and can reach more than 9 levels of CNS standard

3) The appearance can not have bruises, scratches, color clouds and other phenomena. The surface can not have a hanging point, yellowing and other undesirable phenomena

4)die-casting aluminium, such as A380, A365, A382, etc., can not do anodic oxidation

Aluminium plating process

The advantages of aluminum and aluminum alloy materials

It has the advantages of good electrical conductivity, fast heat transfer, light-specific gravity, ease of shape, etc. However, aluminum and aluminum alloy have the disadvantages of low hardness, no wear resistance, easy to intergranular corrosion, not easy to weld, etc., which affect the scope of use. Therefore, to build on the strengths and avoid the weaknesses, modern industry has solved this problem using electroplating.

The advantages of aluminum plating

1)Improve the decorative properties

2)Improve surface hardness and wear resistance

3) Reduce friction coefficient and improve lubricity

4 ) Improve surface conductivity.

5 ) Improve corrosion resistance (including combination with other metals)

6 ) Easy to weld

7 ) Improve the bonding force when hot pressing with a rubber

8 ) Improve reflectivity

9 ) Fix the dimensional tolerance

As aluminum is a relatively active metal, the plated material is more active than aluminum; therefore, before plating, must do the corresponding chemical transformation, such as zinc dip, zinc-iron alloy, zinc-nickel alloy, and make the zinc and zinc alloy base layer has good bonding of the intermediate layer of cyanide copper plating layer. Due to the loose structure of the body, Die-casting aluminium grinding can not polish off the surface part of the body. Otherwise, pinhole, spit acid, peeling and other phenomena.

The process flow of aluminum plating is

Degreasing -> alkali etching -> activation -> zinc replacement -> activation -> plating (such as nickel, zinc, copper, etc.) -> chromium plating or passivation -> drying.

Common aluminum plating types are

Nickel plating (pearl nickel, sand nickel, black nickel), silver plating (surface silver, thick silver), gold plating, zinc plating (colour zinc, black zinc, blue zinc), copper plating (green drum copper, white tin copper, alkaline copper, coke copper, acid copper), chrome plating (white chrome, hard chrome, black chrome), etc.

Uses of common plating

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

2) Gold plating, silver is the best conductor of electronic products. Still, it also can improve the product’s high decoration but is more expensive, generally used in electronic products’ conductive aspects, such as high precision wire terminals and another plating.

3) Copper, nickel, and chromium, the most popular mixed plating in the modern scientific field, regardless of the decorative corrosion resistance, are currently the most popular plating in the world. They are inexpensive and can be used in sports equipment, lighting, and most electronic industries.

4) Copper plating, white tin copper, is developed in the ’70s and ’80s for environmentally friendly plating; bright white is the first choice for the jewellery industry. Bronze (lead-tin copper can be used to imitate gold, so it is a better decorative plating), but the anti-tarnish property of copper is relatively poor, so the development is slow. (We recommend you pay attention to the “mechanical engineer” public number the first time to grasp the dry knowledge and industry information)

5) Zinc plating: The galvanized layer is greenish-white and easily soluble in acids and alkalis. Since the standard potential of zinc is more negative than iron’s, it provides reliable electrochemical protection for steel. Zinc can be chosen as a protective layer for steel products used in industrial and marine atmospheres.

6 ) Hard chromium, under certain conditions, deposited chromium coating has high hardness and wear resistance, hardness reaches HV900-1200kg/mm; chromium is the hardest of the commonly used coatings, can improve the wear resistance of the parts and extend the service life. Such as work, mold, measurement, clamps, etc… Therefore, plating is a necessary treatment for cylinder, hydraulic and transmission systems.

Common abnormalities and improvement measures

1) Peeling: a) zinc replacement is not good, the time is too long or too short, improvement measures, redefine the replacement time, tank temperature, tank concentration and other operating parameters.

b)not enough activation, improve measures, change the activation method.

c)Incomplete pre-treatment, oil on the surface of the workpiece, improve measures, strengthen pre-treatment.

2 ) Surface roughness: a) plating solution light agent, soft agent, or pinhole dose is inappropriate; readjust the number of additives.

b)The body’s surface is rough, re-polishing it and then plating.

3 ) Surface yellowing is low potential alignment; change the way of hanging. Add the appropriate amount of alignment agent.

4 ) Surface hairy teeth: If the plating solution is too dirty, strengthen the filtration and make appropriate tank liquid treatment.

Quality requirements

1) No yellowing, pinhole, burr, blistering, bruising, scratching or other undesirable phenomena.

2) Film thickness of 15um or more, salt spray test for 48 hours, up to the U.S. military standard 9 levels or more, the potential difference of 130-150mv range.

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

4 ) For products of a unique environment, corresponding changes can be made.

Aluminum and aluminium alloy plating operation precautions

1) Aluminum parts plating should use aluminium alloy as a hanging tool.

2) Aluminum and aluminium alloy must be carried out quickly in each process after etching, and the shorter the interval between processes, the better to avoid re-oxidation.

3) the second zinc dip time should not be too long so as not to cause excessive corrosion.

4 ) The water must be washed cleanly.

5 ) The plating process should be prevented power failure in the middle of the process.

Coating of aluminum

The coating is divided into dip coating, spraying, drenching, rolling and brushing, etc. Among them, dip coating and spraying are mainly used. Dip coating is also known as electrophoretic coating, which uses electrochemical methods to deposit organic resin particles on the surface of the workpiece to form a transparent or organic coating of various colors. Cathodic Electrophoresis is a new process developed in the seventies and is the main process in the dip coating industry. The advantages of good corrosion resistance, strong resistance to discolouration, and good bonding characteristics. Spraying is divided into powder and liquid coating, of which electrostatic coating is the more popular process in the industry. (We recommend you pay attention to the “mechanical engineer” public number the first time to grasp the dry knowledge and industry information)

Scope of the coating process

It is suitable for all aluminum parts, but the material structure is too loose, such as sanding products, which requires ash scraping, and the relative quality is also poor.

The working principle of electrostatic coating

Under the action of the pump, the paint enters the electric field, and the paint particles are negatively charged. Under the action of atomization air pressure, the paint becomes finer and more uniform than under the action of working air pressure; the paint is evenly sprayed on the surface of the workpiece under the action of a spinning cup or nozzle. Since the distance between the spray gun and the workpiece is equal in the same vertical plane and the electric field strength is the same at the same interface, the positive and negative charges absorb each other, and the surface of the workpiece is coated with a uniform layer of organic resin. Then, under high-temperature baking, the resin reaches the state of levelling and curing. It is firmly set on the surface of the workpiece and achieves the role of protecting the workpiece.

The process of aluminum coating

Mechanical polishing->Degreasing->Delete oxide film->Chroming->Powder or liquid spraying->Baking->Hanging down->Full inspection ->Packaging->warehousing

1)Common abnormality of powder liquid operation and treatment

phenomenon

Cause

Correct preventive measures

Remark

pericarpium citri

1, Powder spray is too thick; 2, the oven temperature does not match; 3, powder quality is too poor

1、Adjust the gun spitting volume 2、Adjust the temperature 3、Change the quality of powder

Liquid spraying out of the skin 1, paint performance with 2, work when the splash to work 3, paint viscosity

painting peel off

1, the pre-treatment is too poor 2, the temperature is not enough, not baked dry 3, the powder itself is not well combined

1、Strengthen the pretreatment 2、Adjust the temperature 3、Adjust the quality of the powder

Liquid paint loss 1, improper pre-treatment 2, improper temperature 3, primer and finish paint difference value is too large.

betray a confidence

1、Powder sprayed too thin or not sprayed to 2、Mechanical pre-treatment is not complete

Change the way of homework

ditto mark

 chromatic aberration

1.The powder itself 2. The operation mode is not standard

Change the way of homework

Liquid time; 1. Primer and finish paint itself.2. Paint viscosity and spraying thickness.

sagging

1, powder quality is not good 2, powder spray too thick

Change the way of homework

Adjust the gun range, adjust the output amount.

pinhole

1、Bad pre-treatment, oil on the workpiece 2、Spraying too thin

1、Adjust the processing time of each pre-treatment process, assay the content of each liquid before treatment is standard 2、Replace the water of the relevant process

Liquid is the same

  

2) Quality standard reference

project

purity rubric

Test basis

dyestuff

Take the template as the standard, the color difference is less than 1.5E

Subject to customer requirements

hardness

More than 2H

GB6739-86

impact resistance value

50kg/cm2

GB/T1732

Bend the experiment

Ffold 45 degrees in half

ASTMD522

adhesive force

Bagel

ASTMD3002

Salt fog experiment

500 Hours

Anti-corrosion for 10 years 

Cold and hot experiment

Cold, hot 168 times each

Transgender for 10 years

surface

According to customer requirements

According to customer requirements

film thickness

Powder body 80-120um, liquid 30-40um

Anti-discoloration for 10 years

Boiling water

Boiling-resistant treatment for 1.5h

GB/T17748

Chemical plating of aluminum

chemical plating principle

Chemical plating is a method in which metal ions are deposited on the surface of parts using redox in the same solution when no current is passed (no external power). Therefore, chemical plating can be described as a controlled, autocatalytic chemical reduction process for depositing metals with the following reaction equation.

The characteristics of chemical plating

With the development of industry, chemical plating has become a promising technology; compared with other plating methods; chemical plating has the following characteristics.

1) It is possible to coat metal on parts made of various materials such as metals, semiconductors and non-conductors.

2) Regardless of the complexity of the part’s geometry, a uniform coating thickness can be obtained wherever the solution is accessible.

3 ) For autocatalytic chemical plating, obtaining large thicknesses of plating and even electroforming is possible.

4 ) No current is required.

5 ) Dense plating with few pores.

6 ) Plating often has special chemical and mechanical properties.

Since chemical plating has many advantages, chemical plating is developing faster and faster in the industrial and electronic industries. The following chemical nickel plating is an example to do the following introduction.

(1) Chemical nickel plating is generally obtained using sodium hypophosphite as the reducing agent, so the coating is a nickel-phosphorus alloy. Depending on the amount of phosphorus content can be divided into low phosphorus (1%-4%), medium phosphorus (4%-10%) and high phosphorus (10%-12%). Plating layers with different phosphorus content can be obtained from different pH values of the plating solution; at pH=4-5, medium and high phosphorus alloys can be obtained. Low and medium phosphorus alloys can be obtained from weakly alkaline solutions with PH=8-10. NI-P alloys containing 8% or more phosphorus are amorphous coatings with excellent corrosion resistance due to the absence of grain boundaries. The hardness can reach HV=1150 when it becomes a mixture of amorphous and crystalline states through heat treatment at 300-400 degrees. The hardness is close to that of hard chromium after chemical plating heat treatment, which is an ideal coating to replace the hard chromium layer and a good plating for aluminum (suitable for all aluminum and aluminum alloy products, including forging, die-casting, aluminum profiles, etc.). Disadvantage: the plating is brittle, but the plasticity of the product after heat treatment is greatly improved. (Generally, by 400 degrees above the heat treatment for four hours, its plasticity improved much)

2)Aluminium chemical nickel plating process.

Ultrasonic oil removal->Oxidation film removal->First zinc replacement->and chemical->Second zinc replacement->Nickel chemical plating->Passivation->Ultrasonic Cleaning->Baking->Full inspection->Packaging

3 ) Disqualification of unqualified plating.

Steel, aluminum, copper, plastic, etc., chemical nickel layer can be used to remove concentrated nitric acid. But the parts should be dried in the tank. If brought into the moisture will lead to the base metal corrosion. The operating temperature is below 35 degrees.

4) Quality requirements.

a film thickness in the range of 5-30um

b, salt spray test to achieve more than 24 hours, up to CNS standard 9 levels or more.

c, through 60 degrees bending test, no peeling phenomenon

d、No bruises, scratches, water stains and other undesirable phenomena in appearance.

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

 treatment

The common abnormal

abnormal phenomena

Possible causes and corrective methods

Slow reaction and low deposition rate

1) Low PH value, adjusted with NH4OH 2) the temperature is too low 3) the sodium phosphate content is insufficient, analysis and supplement

The plating solution is highly reactive and boiling, with dark gray nickel powder (a symbol of natural decomposition of the plating solution)

1) Excessive loading capacity 2) Too high temperature or the local overheating coating solution drops into metal chips or solid particles 3) the secondary phosphate content is too high 4) The PH value is too high

Metal nickel is deposited at the groove wall and bottom

1) Local temperature is too high 2) The tank lining is damaged, expose the substrate metal, and replace the tank

Nickel phosphic precipitation did not reach the rated content

1) The PH value is too high. 2) If the complexation agent is insufficient, it should be supplemented

The coating is dark

The coating solution is contaminated with zinc, copper and other metal impurities, which can be treated with low current density, or supplemented sodium hypophosphate with waste parts until the coating is normal

The plated layer has a hemp point

Hydrogen stays on the part, can stir the tank liquid or shake the workpiece to avoid

The coating is easy to peel off

1) Intreatment before plating 2) Temperature fluctuation is too large

Classification and selection of plating conditions of use

From the corrosion prevention requirements, the surface treatment design should be made as follows.

1) precious metals (gold, platinum), stainless steel containing more than 18% chromium, magnetic alloy materials, nickel-copper alloy, etc., generally do not need to add a protective layer.

2) carbon steel, low-alloy steel and cast iron manufactured in the atmosphere are prone to corrosion and should add a protective layer.

3) copper and copper alloy parts, according to the different conditions of use, using bright pickling, passivation, electroplating or paint protection, etc. Precision parts made of phosphor bronze or beryllium bronze can be made without surface treatment.

4) aluminum and aluminum alloy parts can be anodized and closed treatment. Not suitable for anodic oxidation of small parts, chemical oxidation treatment can be used. Cast aluminum alloy can be used to paint protection. (We recommend that you pay attention to the “mechanical engineer” public number the first time to master the dry knowledge and industry information)

5) Zinc alloy parts can use phosphating, passivation, electroplating or paint protection.

Electrophoretic treatment of aluminum

Color electrophoretic coating of aluminum is a novel surface treatment process. It uses electrochemical methods to deposit colloidal particles of organic resin on the parts, forming a transparent or variously colored organic coating layer. The ionization of resin particles in electrophoretic paint can be divided into anodic Electrophoresis (ionization of resin particles into negative ions) and cathodic Electrophoresis (ionization of resin particles into positive ions).

The corrosion resistance of the electrophoretic coating layer is extremely good (can pass the neutral salt spray test for more than 400 hours), and the resistance to discolouration is strong; the bonding force with the base metal is good, and various mechanical processing can be carried out; the coating layer has bright colors, and can be formulated into various colors according to the user’s requirements, such as gold, coffee, gunmetal, black, etc.; compared with paint, the construction performance is good, and the pollution and harm to the environment is small. Therefore, it is used in automobile shells and various accessories, bicycle handlebars and accessories, daily-use small hardware accessories, furniture, handicrafts, etc. Since cathodic Electrophoresis is more advanced, we take cathodic Electrophoresis as the main subject of our introduction below.

Cathodic electrophoresis coating is a complex electrochemical and colloidal chemical process. Electrophoretic paint is a multi-component system of colloids and suspensions with two components: the diffuse phase (resin, pigment particles) and the continuous phase (water). The four processes of cathodic Electrophoresis.

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

2、Electrodeposition The positively charged resin particles reach the part’s surface (cathode) and discharge, forming an insoluble deposition layer forming a paint film after baking.

3, electro-osmosis of water from the deposition layer; you can bake when the water content drops to 5%-15%.

4、Electrolysis water is electrolyzed by direct current, releasing hydrogen and oxygen. As electrolysis leads to a decrease in permeability, affects the paint film’s appearance, reduces the paint film’s adhesion and increases electricity consumption, it is necessary to weaken the electrolysis of water.

Process flow of Electrophoresis.

Degreasing->Oxidation removal->Chromatization->Electrophoresis->Drying->Packaging

phenomenon

Causes

Corrective method

Orange peel or a rough surface

High voltage too high solution temperature too high solid divided too high pole too close baking heating too fast PH value too high

Reduce voltage reduce temperature dilution solution increase the extreme distance compressed air drying before baking with organic acid adjustment

Needle hole or hemp point

Solid is divided too low PH value is too low cleaning water unclean solution conductivity is too high film thick too thin electroplating surface pinhole baking temperature is too fast

Adjust to the process range to reduce the acidity of the solution cleaning water ultrafiltration remove impurities ions increase electrophoretic voltage or time to eliminate the pinhole parts into the tank compressed air blow drying before baking

Crater or oil point

The workpiece is attached with the oil stain tank liquid surface with oil stain particles with high pollution voltage and thick film layer

Strengthen the oil removal process to prevent the tank liquid from being contaminated by the oil. Strengthen the circulation filtration and ultrafiltration to reduce the electrophoretic voltage and time

rainbow

The membrane layer is too thin

Increase the electrophoretic voltage

The color does not match

Membrane layer is too thick or too thin deployment of coating, the color ratio of the wrong solvent is too much, the permeability is poor, the membrane layer thickness is uneven

Select the appropriate working voltage and time to strictly prepare the electrophoretic paint to adjust the solvent content, so that it conforms to the process specification

irregular figure

The preprocessing is not complete

Strengthen the pre-processing process

Hard enough

Short baking time or low temperature

Strict accordance with the process specifications

Point or sheet color difference of coating surface

Parts have pinholes or sand eye color material emulsified mixing uneven on the surface of the water droplets are not dry, that is, the washing before baking electrophoresis is not thorough

Prevent the unqualified workpiece lower tank strengthening electrophoretic paint mixing with compressed air blow drying with compressed air blow drying to strengthen the cleaning of the parts into the tank

Quality standards

1)No color difference, orange peel, pinhole, rainbow, bruises, scratches and other abnormalities in appearance

(2) Film thickness of 40-60um, salt spray test more than 400 hours, more than 9 levels of CNS standard

3) The film can pass the 60-degree bending test with no peeling phenomenon

4) Hundred grid test and ball drop test can reach the national standard

 

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