Introduction to metal anodic oxidation process
By jett NG | 26 April 2021 | 0 Comments

Introduction to metal anodic oxidation process

Introduction to metal anodic oxidation process

1.2 Anodizing of aluminum and aluminum

A. The chemical properties of aluminum are active. In the dry air, the surface of aluminum immediately forms A dense oxide film about 5μm thick, so that aluminum will not be further oxidized and can resist water.

Aluminum is amphoteric and soluble in both strong bases and dilute acids.

C. The aluminum and its alloys are placed in an appropriate electrolyte as an anode for electrifying treatment, which is called anodizing.

D. After anodizing, the surface of aluminum can produce a porous honeycomb oxide film with a thickness of several to several hundred microns.

2. Anodic oxidation pretreatment

2.1 Degreasing and degreasing


The adhesion of the oil film on the surface of the aluminum parts is reduced or even left the surface, so that the next step in the alkali corrosion of the surface oil can be easily removed, even alkali corrosion.

Degreasing agent composition: acid degreasing agent 2%-3%, the tank liquid main component is sulfuric acid (H2S04) or phosphoric acid (H2P04)

Tank temperature: room temperature


Degreasing time: 3~5min.


2.2 alkali corrosion


Reaction with aluminum parts, remove surface dirt, remove natural oxide film process, the purpose is to activate the surface, but also to remove extrusion fringe, obtain different reflective surface function.

Alkali corrosion tank solution: sodium hydroxide (Na0H) solution, the concentration of 40~60g/L

Tank temperature: 40~80℃

Alkali corrosion time: 3~10s

2.3 and 1


Remove the dirt hanging ash on the surface after alkali corrosion, so as to obtain a relatively clean surface. At the same time, it can also neutralize the effect of residual alkaline solution on the surface of aluminum parts.


Composition of tank liquid: nitric acid (HNO3) solution, concentration of 120~150g/L


Tank temperature: room temperature


Neutralization time: 5~15s


2.4 Electrochemical polishing


Using the action of the current, the aluminum alloy electrochemical reaction, in the aluminum alloy surface uneven part of the deep solution of different degrees, so that the surface of the aluminum parts have a smooth mirror effect. After anodic oxidation treatment, most of the luster of the electrolytically polished aluminum parts can still be maintained. High purity aluminum sheets (99.99%) are electrolytically polished to achieve a mirror effect of nearly 100% reflectivity. The higher the purity of the aluminum sheet, the higher the reflectivity.

Polishing fluid composition: phosphoric acid - chromic acid type, phosphoric acid - sulfuric acid, chromic acid type, phosphoric acid - sulfuric acid - glycerol type


Tank temperature: room temperature to 90℃,


Current density: 10~20A/dm2


Polishing time: 10~30s


2.5 Chemical polishing

Through the chemical ion reaction between aluminum and chemical polishing fluid, the deep solution of different degrees occurs on the uneven part of the aluminum surface, and the gloss effect is produced on the surface of the aluminum parts.


Composition of polishing bath: 75% phosphoric acid (volume fraction), 15% nitric acid (volume fraction), 10% sulfuric acid (volume fraction)


Tank temperature: 90~110℃


Polishing time: 5~15s


2.6 and 2

Remove stains and ashes from the surface after polishing with acid solution to obtain a cleaner surface. At the same time can also neutralize the residual acid solution and aluminum surface chemical reaction.

Composition of tank liquid: metal salt solution (CuSO4), concentration of 100~150g/L

Tank temperature: room temperature

Neutralization time: 5~20s

3 Anodic Oxidation


With aluminum or aluminum alloy products as the anode placed in the electrolyte solution, the use of electrolysis to make the surface of alumina film. When the current passes through, hydrogen gas is released from the negative stage. Anode on oxygen precipitation is not only the state of molecular oxygen, oxygen and oxygen atoms and ions, usually the reaction with molecular oxygen, said as the anodic aluminum oxide by precipitation of oxygen, form of anhydrous aluminum oxide film, the porosity of the oxide film diameter is about 0.01 0.03 um, oxygen is not generated with aluminum, all part of the precipitation in the form of gas. Anodic oxidation mainly has three ways: sulfuric acid anodic oxidation, hard anodic oxidation, porcelain anodic oxidation, the main use of sulfuric acid anodic oxidation process.


3.1 Anodic oxidation of sulfuric acid

The composition of the bath liquid: the main component is sulfuric acid solution (H2S04), 150-180g /L Al ion :5~15g/L


Tank temperature: 18~22

Oxidation voltage: 14~18V

Current density: 0.6~3 A/dm2

Oxidation time: 10~60 min, specific oxidation time according to different products and colors

Thickness of oxide film: according to different colors and process requirements, the thickness of oxide film can be 5-50um. If an appropriate amount of oxalic acid solution is added into the solution,

The toughness of the oxide film will be appropriately optimized.

3.2 Hard anodic oxidation


Hard anodic oxidation is a process for forming a thick and hard oxide film on the surface of aluminum and aluminum alloys.


The maximum thickness of hard film can be up to 250μm, the micro hardness of the film formed on pure aluminum is 12000-15000MPa, the alloy is generally 4000-6000MPa, and the hard chromium coating is almost the same, they are in low compliance with the wear resistance is excellent, hard film porosity is about 20%, lower than the conventional sulfuric acid film.




3.3 Anodic oxidation of porcelain


Porcelain anodic oxidation is refers to the aluminum and aluminum alloy titanium in oxalic acid, citric acid and boric acid salt, zirconium salt or thorium anodic oxidation in salt solution, salt in the solution of metal hydroxide into oxidation membrane pore, so that the products showed dense and opaque and the surface of the enamel or similar with special gloss appearance of plastic processing. The process of anodic oxidation of porcelain is basically the same as that of conventional anodic oxidation of sulfuric acid. The difference is that the anodic oxidation of porcelain is carried out under the conditions of high DC voltage (115-125V), higher solution temperature (50-60 degrees), constant stirring of electrolyte and constant adjustment of pH value to make it within the range of 1.6-2.


3.4 the dyeing


The oxide film on the surface of anodized aluminum parts is placed in the dyeing solution so that the color molecules are adsorbed in the pores of the anodized aluminum parts. The pore diameter of the anodized film is about 0.01-0.03um, and the dye is separated from the single molecule in water with a diameter of about 0.0015-0.0030um.


Dyeing mode: divided into hot dyeing and cold dyeing. Hot dyeing is carried out under the condition that the dyeing solution is heated. All oxide films adsorb the dyeing molecules quickly, but it is difficult to control the high time accuracy, so cold dyeing is usually chosen.


Tank composition: organic dye solution, pH value according to different brands of dye


Tank temperature: normal temperature


Oxidation time: 5-50 min, specific dyeing time according to different products and colors




3.5 hole sealing


Fresh anodic oxidation film in boiling water or close to the boiling point of hot water treatment for a certain time, no longer reactive, no dye adsorption, the color is not easy to fade, this process is to seal the hole.


Sealing mode: high temperature sealing, medium temperature sealing, low temperature sealing, considering the cost and technical difficulty and other factors, low temperature sealing is mainly used now.


Composition of tank liquid: low temperature nickel sealing agent solution, Ni ion: 0.8~1.2g/L, F ion: 0.35~0.6g/L, pH value :5.6~6.5


Tank temperature: 20~30℃


Sealing time: 10~30 min, according to different products and colors to determine the specific sealing time

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