Aluminum Alloy Material and Processing Anodizing Technology Decryption

Aluminum alloy material by anodic oxidation coloring treatment, after treatment, high hardness, wear resistance, corrosion resistance and electrical insulation is good, through chemical pretreatment can also be electroplating, electrophoresis, spraying, etc. to further improve the decorative and protective aluminum. Beijing Jingke Xingye Technology Development Co., Ltd. has unique advantages of anodic oxidation technology for its own processing and production of products to provide quality assurance.

Definition of Anodizing

Anodizing is an electrolytic oxidation in which the surface of the aluminum alloy is usually converted into an oxide film, which has protective, decorative and other functions.

Classification of anodic oxide film

Oxide film is divided into two categories: barrier type oxide film and porous oxide film. Barrier type oxide film is a dense non-porous thin oxide film close to the metal surface. The thickness depends on the applied voltage and generally does not exceed 0.1um. The porous oxide film consists of a barrier layer and a porous layer. The thickness of the barrier layer is related to the applied voltage, and the thickness of the porous layer depends on the amount of electricity passed. We are most commonly used porous oxide film.

Characteristics of anodic oxide film

A. oxide membrane structure is porous honeycomb junction, the porous membrane has a good adsorption capacity, can be used as the coating layer of the bottom layer can also be dyed, improve the decorative effect of metal.

B. The hardness of the oxide film is high, and the hardness of the anodic oxide film is very high. Its hardness is about 196-490HV, because the high hardness determines the wear resistance of the oxide film.

C. corrosion resistance of oxide film, aluminum oxide film in the air, soil are very stable, with the matrix of the binding force is also very strong, under normal circumstances after oxidation will be dyeing sealing or spraying treatment, so that its corrosion resistance is further enhanced.

D. the bonding force of the oxide film, the bonding force of the oxide film to the base metal is very strong, and it is difficult to separate them mechanically. even if the film layer bends with the metal, the film layer still maintains good bonding with the base metal, but the oxide film has small plasticity and large brittleness. when the film layer is subjected to large impact load and bending deformation, it will crack, so this oxide film is not easy to be used under mechanical action, can be used as the bottom layer of the paint layer.

E. the insulation of the oxide film, the anodic oxide film of aluminum has high impedance, low thermal conductivity, thermal stability up to 1500 degrees, and thermal conductivity 0.419W/(m • K)-1.26W/(m • K). It can be used as a dielectric layer of an electrolytic capacitor or an insulating layer of an electrical product.

Aluminum alloy oxide film formation process

1. First stage of anodizing

In the formation stage of the non-porous layer, in section AB, the voltage increases sharply during the power-on start-off time (a few seconds to tens of seconds) and reaches the critical voltage (the maximum value of the voltage), which indicates that a continuous, non-porous thin film layer is formed on the anode surface at this time. The non-porous layer has a large resistance, which hinders the continuous thickening of the film. The thickness of the non-porous layer is proportional to the formation voltage, and the dissolution rate of the oxide film in the electrolyte is inversely proportional. The thickness is about 0.01~0.1 microns.

2. Second stage of anodic oxidation

In the stage of porous layer formation, bc section, holes will be dissolved first at the thinnest part of the membrane, and the electrolyte can reach the fresh surface of aluminum through these holes. The electrochemical reaction can continue, the resistance decreases, the voltage decreases (the decrease range is 10-15% of the highest value), and a porous layer appears on the membrane.

3. The third stage of anodic oxidation

The porous layer thickens, the cd section, then the voltage rises steadily and slowly, then the non-porous layer is continuously dissolved into a porous layer, and new non-porous layer friends are growing, so the porous layer is continuously thickening, when the generation speed and the dissolution speed reach a dynamic balance, the thickness of the membrane will no longer increase, then the reaction should stop.

Anodic Oxidation Process of Aluminum Alloy

1. Common process of anodic oxidation

Aluminum alloy anodizing commonly used processes are: sulfuric acid anodizing process, chromic acid anodizing process, oxalic acid anodizing process and phosphoric acid anodizing process. The most commonly used is sulfuric acid anodizing.

2. Sulfuric acid anodic oxidation

At present, the anodic oxidation process widely used at home and abroad is sulfuric acid anodic oxidation. Compared with other methods, it has great advantages in production cost, oxide film characteristics and performance. It has low cost, good film transparency, corrosion resistance and friction resistance. Good, easy coloring and so on. It is based on dilute sulfuric acid as electrolyte, anodic oxidation of the product, the thickness of the film can reach 5um-20um, the film has good adsorption, colorless and transparent, simple process and convenient operation.

3. chromic acid anodic oxidation

The film obtained by chromic acid anodic oxidation is relatively thin, only 2-5um, which can maintain the original accuracy and surface roughness of the workpiece. The porosity is low and difficult to dye, and it can be used without sealing. The film is soft, with poor wear resistance but good elasticity. Strong corrosion resistance and small solubility of chromium to aluminum make the residual liquid in pinholes and crevices less corrosive to components, which is suitable for structural parts such as castings, the process is used more in the military. At the same time, the quality of the parts can be inspected, and the brown electrolyte will flow out at the crack, which is obvious.

4. Oxalic acid anodic oxidation

Oxalic acid has small solubility in aluminum oxide film, so the porosity of oxide film is low, and the wear resistance and electrical insulation of the film are better than sulfuric acid film. However, the oxidation cost of oxalic acid is 3~5 times higher than that of sulfuric acid. At the same time, oxalic acid will be reacted at both cathode and anode, resulting in poor electrolyte stability. The color of oxalic acid oxide film is easy to change with process conditions, resulting in color difference, so the application. However, oxalic acid can be used as sulfuric acid oxidation additive.

5. Phosphoric acid anodic oxidation

The oxide film dissolves more in the phosphoric acid electrolyte than sulfuric acid, so the oxide film is thin (only 3um) and the pore size is large. Because the phosphoric acid film has strong water resistance, it can prevent the adhesive from aging due to hydration so that the bonding force of the adhesive is better, so it is mainly used for the surface treatment of printed metal plates and the pretreatment of aluminum workpiece bonding.

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