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Surface Treatment of Aluminum and Aluminum Alloy
- May 15, 2017 -

Aluminum and section aluminum usually require surface treatment to meet different needs.Common aluminum alloy surface treatments  are   electroplating, spraying, wire drawing, anodizing ,  abrasive blasting , passivation, polishing, oxide film processing etc.
1 Abrasive blasting

Abrasive blasting  is mainly  to clean up the surface .Before the blasting, this process  can increase the surface roughness, and to some degrees  improve adhesion. However the  effect is limited, not as good  as pretreatment of chemical coating .

2 Coloring 

There are two  aluminum coloring processes. One is the oxidation of aluminum , the other is  aluminum electrophoresis process.Coloring can  bring a variety of colors  on the oxide film,  meeting  certain technology requirements. These color include black color  in optical instruments  parts,golden colors  on medals etc.


3  Conductive oxidation

 Conductive oxidation (chromate conversion film) is used   for both protective and conductive occasions.


4.Chemical oxidation 

This oxide film is thin, porous,soft,and  with good adsorption. The thickness is  about 0.5 to 4 microns. It can be used as the bottom of the organic coating, but its wear resistance and corrosion resistance are not as good as the anodizing  film.

By  the nature of the solution ,aluminum and aluminum alloy chemical oxidation process  can be divided into alkaline oxidation and acid oxidation two categories.

 

5  Electrochemical oxidation

For electrochemical oxidation,the aluminum and aluminum alloy chemical oxidation treatment equipment of is simple , easy to operate, and with high production efficiency as well as less consumption of electricity.This process can be used  for a wide range, not limited by the size and shape of parts.

The thickness of electrochemical oxidation   film  is about 5 to 20 microns (hard anodize oxide film’s thickness can reach to 60 to 200 microns), with   higher hardness, good heat and insulation.Besides  its  corrosion resistance is higher than that t of  chemical oxide film. The  porous  oxidation film is of very good adsorption capacity.

6 Powder spraying 

Powder spraying process is mainly used for external protection, decoration of certain equipment. Before the powder spraying ,aluminum parts should receive pre-treatment to solid  the coating and the workpiece .There are three general methods: phosphating(phosphate), chroming(chrome-free), chemical oxidation.

7 Anodizing

Anodizing is the process of coating a thin layer of other kinds of  metals or metal alloy  on the surface  the metal.

Brush plating is for partial plating or partial  repair.Barrel   plating is  for small pieces, such as fasteners, washers, pins and so on. Through plating, you can not only get decorative products on the decorative and a variety of functional surface layer for mechanical products, but also repair wear and processing errors of the workpiece.There are acidic and  alkaline electroplating solution,  as well as  aand neutral solution  with chromium mixture. Using what kind of  plating method is  related with  the  plating tank ( between the  pending plating  and plating solution)and suspension system etc.


8 Chemical Polishing

Chemical polishing is   a process that takes  the advantage of  aluminum and aluminum alloy’s selective self - dissolution in acid or alkaline electrolyte to flatten the surface of the polishing system  reducing its surface roughness and PH values.The purity of aluminum and aluminum alloys has a great effect on the quality of chemical polishing. The higher the purity, the better the polishing quality is, and vice versa  .

The advantages of chemical polishing:

  • The equipment is simple.

  • Don’t use  electircf power 

  • No limitation of part’s size,

  • High throwing speed and low processing cost.



 9 Passivation

Passivation is a process that makes the metal surface  not easy to be oxidized , and  slows the corrosion rate of metal.An active metal or alloy, in which the chemical activity is greatly reduced, forming a state of precious metal is called passivation.

If metal's corrosion  products caused by the action of the  dielectric  have a dense structure, it can form  a layer of film (often invisible) that closely covers the surface of the metal, changing the surface state of the metal so that the the electrode potential of the metal jumps to the positive direction greatly, and become a passive state of corrosion.For instance, Fe → Fe + + , the standard electric potential   is -0.44V,after passivation ,it increases  to +0.5 ~ 1V, and shows the corrosion resistance that belongs to precious metals. The  film is  called passive film.