What is aluminum hard oxidation?

The main purpose of hard anodizing treatment of aluminum alloy is to improve various properties of aluminum and aluminum alloy, including corrosion resistance, wear resistance, weather resistance, insulation and adsorption. It is suitable for both wrought aluminum alloys and possibly die-cast aluminum alloy parts.

Hard Oxidation vs Normal Oxidation

50% of the oxide film of hard oxidation penetrates into the interior of the aluminum alloy and 50% adheres to the surface of the aluminum alloy. Therefore, after hard oxidation, the external size of the product becomes larger and the inner hole becomes smaller, while after ordinary oxidation, the outer size becomes smaller and the inner hole becomes bigger.

The formation process of the hard oxide film can still be analyzed through the voltage-time characteristic curve. The growth of hardness film has the same law as that of ordinary film, but it has different characteristics. In the formation of the barrier layer and the generation of film holes, the rules are the same, the difference is that the formation voltage of the hard film is high and the barrier layer is thicker. It is different in the thickening stage of the porous layer. At this time, the voltage curve rises faster, indicating that the porosity is not large when the porous layer is thickened. With the thickening of the film layer, the resistance increases faster and the voltage also rises significantly. The longer this period is, the longer it takes for the growth rate and dissolution rate of the film to reach equilibrium, and the thicker the film layer will be. After the voltage rises to a certain value, the oxygen evolution in the membrane pores is accelerated and the diffusion is difficult, which increases the resistance, the voltage rises suddenly, and the heat in the membrane pores causes gas discharge and sparks, resulting in the destruction of the membrane layer.

Hard oxidation color

Hard oxidation is generally done black and the original color of the oxide film (natural color, the original color is determined by the composition of the aluminum alloy material, the same type of aluminum profile (6061-T6), but different manufacturers, the color of the oxide film is different, if the same The materials of the same type of aluminum alloy manufacturers may also be different. Therefore, the natural color of the oxide film is generally used for the internal parts of the product.

     Hard oxide film

1) The thickness of the film layer can reach 250-300 um, and the appearance is gray, brown to black.

2) The hardness of the oxide film is very high, generally HV300-600, which is related to the alloy grade and processing technology, and there is a hardness gradient. The hardness near the base is higher, while the hardness of the outer layer is lower.

3) Since the oxide film has tiny pores, it can absorb lubricant, so it can improve the anti-wear ability. The wear resistance of the hard oxide film is excellent at low loads. Tests have shown that it is superior to quench hardened steel and hard chrome coatings; in practical applications, the wear amount of the hard film is roughly the same as that of nitrided steel. equal.

4) Under industrial atmospheric and marine climate conditions, as well as in salt spray tests and humidity chamber tests, the hard film has good corrosion resistance, which is generally better than ordinary oxide films.

5) The film layer has high electrical insulation. When the film thickness is 100um, the breakdown voltage is 1850V, and it can reach 2000V after dipping in insulating varnish.

6) The melting point of the film is as high as 2050, and the heat transfer coefficient is very low, only 67kW / m2.K. It is an excellent heat-resistant material and can withstand high temperatures of 1500-2000 in a short time. The thicker the film layer, the longer the flame shock resistance time.

Principle of Hard Oxidation

There is no essential difference between the simple sulfuric acid type aluminum alloy hard anodic oxidation and ordinary anodic oxidation. If it is a mixed acid type hard anodic oxidation, there will be some side reactions.

1. Cathode reaction: 4H++4e=2H2

2. Anode reaction: 4OH－－4e=2H2O+O2

3. Aluminum oxidation: Oxygen precipitated on the anode is in the atomic state, which is more active than molecular oxygen and more likely to react with aluminum: 2Al+3OAl2O3

4. The dynamic balance of oxidation and dissolution of the anode film: the oxide film thickens as the current increases as the energization time increases. At the same time, due to the dual nature of the chemical properties of (Al2O3), that is, it is an alkaline oxide in an acidic solution and an acidic oxide in an alkaline solution. Undoubtedly, the oxide film liquid dissolves in the sulfuric acid solution. Only when the formation rate of the oxide film is greater than its dissolution rate, can the oxide film thicken. When the dissolution rate is equal to the formation rate, the oxide film will no longer thicken. When the oxidation rate is too much faster than the dissolution rate, the surface of aluminum and aluminum alloy parts is easy to form a powdery oxide film.

In summary, hard oxidation is required for wear-resistant parts that require high hardness (such as pistons, cylinders, bearings, guide rails, etc.), parts that require insulation, parts that are resistant to airflow erosion, and parts that are subjected to high temperatures instantaneously.