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What is the principle of anodizing for aluminum profiles?

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What is the principle of anodizing for aluminum profiles?


  • Mechanism of Aluminum Anodizing Process

The process of using aluminum and aluminum alloy products as anodes and placing them in an electrolyte solution and using the electrolyte to form an oxide film on the surface is called anodizing treatment of aluminum and aluminum alloy products.

The growth process of the oxide film includes two aspects that complement each other: the electrochemical formation process of the film; the chemical dissolution process of the film.

Both are indispensable, and the formation speed of the film must be always greater than the dissolution speed, so that a thicker oxide film can be obtained. Therefore, the selected electrolyte properties and process specifications must meet these conditions.


During the anodic oxidation process, electroosmotic flow occurs between the charged particles relative to the solid wall due to the action of the potential difference. The existence of electroosmotic flow is a necessary condition for the growth of the anodic oxide film. When aluminum and aluminum alloys are anodized in sulfuric acid solution, the mechanism of oxide film formation is as follows:


After the electricity is turned on, the reaction of the following formula occurs on the anode and the cathode:

2H++2e-=H2

40H-+4e- =2H 2O+O2

2A1³+3O²=Al2O3+ Heat quantity


Then, the aluminum used as the anode or the aluminum element in the aluminum alloy is oxidized by the oxygen generated by the anode reaction to form an aluminum oxide film (it should be pointed out that the oxidation here refers not only to molecular (O2), but also to atomic oxygen (O2). ) and ionic oxygen (0²-)).


Usually, molecular oxygen is represented in the reaction formula. Not all of the oxygen generated on the anode reacts with aluminum to form an oxide film, and some of it escapes from the anode in the form of gas. After a thin and dense oxide film is formed on the aluminum surface according to the reaction formula (this process can actually be considered to be completed within a few seconds of electrification), a part of the film is dissolved due to the reaction with sulfuric acid:


Al2O3+3H2SO4    Al2(SO4)3+3H2O


Thus, the dense oxide film becomes porous. Then the electrolyte penetrates into the voids (pinholes) and acts with the exposed aluminum to form a new oxide film, so that the entire oxide film seems to have been repaired and becomes complete again. Then, the new complete oxide film dissolves again, new voids (pinholes) appear, and the exposed metal aluminum is oxidized into aluminum oxide by the electrolytic solution to be "repaired".


In this cycle, new oxide films are continuously generated on the metal surface, and porous outer films are continuously created. As a result, an oxide film composed of a thick and porous outer layer and a thin and dense inner layer is formed.



During the oxidation process, the thickness of the inner layer film (the complete film is also called the barrier layer, active layer, and dielectric layer) almost changes with time, and is always maintained at 14-50 μm, while the thickness of the porous outer layer film is at a certain level. The processing time is thickened with time.

The growth of the anodic oxide film is summarized as follows:


1) The new film is grown directly on the metal surface under the old film.

2) Pores are the growth centers of oxide films, and the existence of pores is of decisive significance to the growth of oxide films.

3)  In a series of isolated pores, the newly formed oxide film expands in all directions and merges with each other, and finally a uniform thickness covering layer is formed.


Electrolysis of Aluminum

 Electrolysis of Aluminum




  • Structure of anodized film


The anodic oxide film consists of two layers, a thick outer porous layer is grown on a dense inner layer with dielectric properties, which is called the barrier layer (also called the active layer).


Observation with electron microscope shows that almost all the longitudinal and transverse planes of the film have tubular holes perpendicular to the metal surface, which penetrate through the outer layer of the film to the barrier layer at the interface between the oxide film and the metal.


With each pore as the main axis, there is a honeycomb hexagonal body composed of dense alumina, called unit cell, and the entire membrane layer is composed of countless such unit cells.

The barrier layer is composed of anhydrous alumina, which is thin and dense, and has high hardness and prevents the passage of current.

The porous outer layer of the oxide film is mainly composed of amorphous alumina Al2O3 and a small amount of hydrated alumina y-Al2O3·H2O.


In addition, it also contains the cations of the electrolyte. When the electrolyte is sulfuric acid, the sulfate content in the membrane layer is 13%~17% under normal circumstances.


Most of the excellent characteristics of oxide films are determined by the thickness and porosity of the porous outer layer, which are closely related to the anodization conditions. The thickness of the porous layer structure depends on the amount of electricity that is energized. The unit cell of the oxide film layer is a dense hexagonal honeycomb structure with pinholes as the center. The diameter of the pinhole (pore) is 10-50nm, and an oxide film cylinder formed by a hole can be called a "cell".


With the continuous change (growth) of the oxide film column, the resistance also gradually increases. When the film growth rate is equal to the film dissolution rate, the film thickness will remain unchanged. The maximum film thickness will depend on the electrolyte composition used and the electrolysis process conditions.


The structure size of the oxide film is shown in the following table:


                                                                                  Structure size table of porous oxide film

Electrolyte


Concentration /%

Temperature /℃

Barrier Thickness /nm·v-1

Hole wall thickness /nm·v-1

Aperture /mm

Phosphoric acid

4

24

1.19

1.1

33

Oxalic acid

2

24

1.18

0.97

17

Chromic acid

3

38

1.25

1.09

24

Sulfuric acid

15

10

1.00

0.8

12


The active layer was formed within about 25 s from the beginning of the electrolysis. The thickness of the active layer and the wall thickness of the honeycomb hexagonal body mainly depend on the voltage (under a certain electrolyte condition). That is, its thickness is only related to the anodization voltage. According to the determination, the thickness of the active layer is 1nm/V and the wall thickness of the hexagonal prism is 0.8nm/V under the sulfuric acid electrolysis condition. For example, sulfuric acid anodization is performed with a voltage of 15V, and the thickness of the active layer=1nm/Vx15V=15nm.


The diameter and number of pinholes (pores) in the porous layer depend on the nature and concentration of the electrolyte, which also determines the porosity of the oxide film. The total porosity generated in sulfuric acid, oxalic acid and chromic acid solution is in the range of 12%~30% (the upper limit of sulfuric acid).

Generally, the density of pinholes is 400 million to 500 million pieces/mm. The porous film also penetrates the anions of the solution, and the measurement results of the composition of the anodic oxide film of sulfuric acid are as follows:


                               Sulfuric acid oxide film composition table (%)

Composition

Unsealed oxide film

Water and Sealed Oxide Film

Al2O3

78.9

61.7

Al2O3.H2O

0.5

7.6

Al2(so4)3

20.2

17.9

H2O

0.4

2.8



Anodizing for aluminum extrusion parts

Anodizing for aluminum extrusion parts


  • Properties of Anodized Films

The properties of the anodic oxide film mainly include:


1)The anodic oxide film is a transparent amorphous oxide with a refractive index of 1.6~1.65.

2)The oxide film is a non-conductor, and its specific resistance is about 10MΩ2•cm at 1000V.

3)The hardness HV of the anodized film is about 230.

4) In an aqueous solution above 70 °C, the oxide film absorbs crystal water, expands in volume, and generates 1 to 3 parts of crystal water oxides.


Anodizing for aluminum parts

Anodizing for aluminum parts




  • Effect of Alloy Composition on Oxidation Process

The anodic oxide film of different aluminum alloys has different colors. The film on pure aluminum is colorless and transparent, so that the luster of the metal is completely maintained. When high purity adds a small amount of magnesium, the film color will not change due to the prolongation of oxidation time; when the magnesium content exceeds 2%, the magnesium becomes dark and cloudy.


When the aluminum-silicon alloy is anodized, the silicon will not be oxidized or dissolved, and part of it enters the film layer to make the film appear dark gray; when the silicon content is large, soak it with hydrofluoric acid before anodizing, and the film color will improve; generally containing Alloys with more than 5% silicon are not suitable for bright coloring products, and it is difficult to anodize when the content reaches 13%. For alloys containing copper, when the content is small, the film is green; with the increase of copper content, the film is thin and the color tone is dark.

Hanging for aluminum anodizing products

Hanging for aluminum anodizing products


 

 For video of anodizing process for aluminum heat sink and aluminum extrusion parts, please go to browse and watch here.


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