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Surface Treatments Used for Aluminum Street Lamp Cover

Source: Junying   Author: Junying   Posted: 2016-05-20 10:59:49   Hits: 66
Electrophoresis and powder spraying are two surface treatments used for aluminum die casting street lamp cover, high improving corrosion resistance and service life. The technology that charged particles move to the electrode having contrary electric property under the action of electric field is called as electrophoresis (EP).

Electrophoresis is classified into anodic electrophoresis and cathode electrophoresis. If painting particles carry negative electricity, workpiece is anode. Painting particles depositing into film on workpiece is called as anodic electrophoresis. If painting particles carry positive electricity, workpiece is cathode. Painting particles depositing into film on workpiece is called as cathode electrophoresis. Anodic electrophoresis paint includes anode acrylic, mainly used for aluminum products such as aluminum die casting street lamp cover. Aluminum is protected by anodic electrophoresis paint after being oxidized, glossiness ranging between 50 and 90 degree and having thinner paint film. The technology is widely applied on aluminum. Cathode electrophoresis paint is classified into epoxy electrophoresis paint, acrylic electrophoresis paint and polyurethane electrophoresis paint. 

Features of anodic electrophoresis are that low raw materials cost (generally cheaper than cathode electrophoresis by 50%), simple equipment, less investments (lower than cathode electrophoresis by 30%), lower requirements for technology while coating has poorer corrosion resistance compared with cathode electrophoresis (about quarter of service life of cathode electrophoresis. The reasons that cathode electrophoresis coating has stronger corrosion resistance are that workpiece is cathode which cannot cause anodic dissolution easily. The surface and phosphate film cannot be broken. Electrophoresis paint (generally is nitrogenous resin) can protect metal, having high price and quality. 

Common processes of anodic electrophoresis include pretreatment of workpiece (unoil→hot washing→rust cleaning→cold washing→phosphorization→hot washing→passivation), anode elextrophoresis and aftertreatment of workpiece (washing→drying).

1. Unoil
The solution generally is hot alkaline chemical degreasing solution. The temperature is 60℃, heated by steam and the time is about 20 minutes. 

2. Hot washing
The temperature is 60℃, heated by steam and the time is about 20 minutes.

3. Rust cleaning
Use sulfuric acid or hydrochloric acid. For example, total acidity of hydrochloric acid is higher than 43 points, when using hydrochloric acid. Free acidity is higher than 41 points. Mass friction of cleaning agent is 1.5%. The workpiece is washed for 10~20 minutes under room temperature.

4. Cold washing
Workpiece is washed in flowing cold water for 1 minute. 

5. Phosphorization
Workpiece is phosphated in medium temperature.
Above process can be replaced by using abrasive blasting and washing. 

6. Passivation 
Use drugs matching with phosphating solution for 1~2 minutes under room temperature.

7. Anodic electrophoresis
Ingredient of electrolyte solution: H08-1 black electrophoresis paint
Mass friction of solid: 9%~12%.
Mass friction of distilled water: 88%~91%.
Voltage: 70±10V.
Time: 2~2.5 minutes.
Temperature of paint solution: 15~35℃.
PH value of paint solution: 8~8.5.
Note: Slot must be interrupted. The electric current declines gradually with the thickness of paint film increasing during electrophoresis.

8. Clean 
Clean workpiece in flowing cold water.

9. Drying
Workpiece is dried for 40~60 minutes in drying oven under temperature of 165±5℃.

Painting has high utilization, which can be as high as more than 95% or even 100%. For bath solution having low solid content and low viscosity, few coating is taken away by paint. Especially ultrafiltration technology applied, coating process is sealed and cyclic, having high recovery of coating. Pain film has strong corrosion resistance. Electrophoresis painting has even film under the action of electric field. Using electrophoresis can significantly enhance corrosion resistance on inner chamber, welded joint and edge of workpiece. 

Powder spraying is also called as powder coating, referring to that powder coating adhere to workpiece by using brush discharge. It is a newly-developed technology in recent dozens of years. Used raw materials are molding powder. A film is formed on the surface of workpiece through hot melting and solidification after spraying. 

Powder spray is firstly used for protection in order to improve service life of workpiece. The following is decoration for beautiful appearance. Then, it is special purpose for special features, such as sound insulation, thermal insulation and fire proof. Due to different objectives and requirements of painting, painting consists of several layers of coating, including primer, putty and paint finish.


Surface Treatments Used for Aluminum Street Lamp Cover