Factors influencing the quality of oxide film during the oxidation of aluminum products

The main factors affecting the quality of oxide film are:

Sulphuric acid concentration: usually 15% ~ 20%.

As the concentration increases, the dissolution rate of the film increases, the growth rate of the film decreases, the porosity of the film is high, the adsorption capacity is strong, the membrane is elastic, the dyeing property is good (easy to dye the dark color), but the hardness and wear resistance are slightly poor.

With the decrease of sulfuric acid concentration, the oxidation film grows faster, has fewer pores, higher hardness and better wear resistance.

Therefore, for protection, decoration and pure decoration processing, the upper limit of allowable concentration, namely 20% of the concentration of sulfuric acid as electrolyte.

(2) Electrolyte temperature: electrolyte temperature has a great influence on the quality of oxide film.

As the temperature increases, the dissolution rate increases and the film thickness decreases.

When the temperature is 22 ~ 30℃, the resulting film is soft, good adsorption capacity, but poor wear resistance;

When the temperature is more than 30℃, the film will become loose and uneven, sometimes even discontinuous, and low hardness, thus losing its use value.

When the temperature is between 10 ℃ and 20℃, the generated oxide film is porous, with strong adsorption capacity and elasticity, suitable for dyeing, but the hardness of the film is low and the wear resistance is poor.

When the temperature is lower than 10℃, the thickness of the oxide film increases, the hardness is high, the wear resistance is good, but the porosity is low.

Therefore, the temperature of electrolyte must be strictly controlled during production.

In order to make thick and hard oxide film, the operating temperature must be lowered. In the process of oxidation, the compressed air is used to stir and the temperature is relatively low.

(3) Current density: within a certain limit, the current density increases, the film growth rate increases, the oxidation time shortens, the pores of the film are more, easy to color, and the hardness and wear resistance increase;

If the current density is too high, the surface of the part will overheat and the temperature of local solution will increase due to joule heat, and the dissolution rate of the film will increase, and the part may be burned out.

If the current density is too low, the film grows slowly, but the resulting film is denser, with lower hardness and wear resistance.

(4) Oxidation time: the choice of oxidation time depends on electrolyte concentration, temperature, anode current density and required film thickness.

Under the same conditions, when the current density is constant, the film growth rate is proportional to the oxidation time.

However, when the film grows to a certain thickness, the conductivity will be affected due to the increase of film resistance, and the dissolution rate of the film will increase due to the temperature rise, so the growth rate of the film will gradually decrease, and finally no longer increase.

(5) Agitation and movement: can promote the electrolyte convection, enhance the cooling effect, ensure the uniformity of solution temperature, will not cause the metal local temperature and lead to the quality of the oxide film.

6. The impurities in the electrolyte: in aluminum electrolyte used in anodic oxidation of the impurities have Cl ˉ, F ˉ, NO3 ˉ, Cu2 +, Al3 +, Fe2 +, etc.

The Cl ˉ ˉ F, NO3 ˉ increase the porosity of membrane, the surface is rough and loose.

If its content is more than limit value, and even make the parts occur corrosion perforation (Cl ˉ should be less than 0.05 g/L, F ˉ should be less than 0.01 g/L);

When the content of Al3+ in the electrolyte exceeds a certain value, white spots or spotted white blocks often appear on the surface of the workpiece, and the adsorption performance of the film decreases, making it difficult to dye (Al3+ should be less than 20g/L);

When the content of Cu2+ reaches 0.02g/L, dark streaks or black spots will appear on the oxide film.

Si2+ often exists in the electrolyte in a suspended state, making the electrolyte slightly cloudy and adsorbed on the film with brown powder.

7 aluminum alloy composition: in general, other elements in aluminum metal make the quality of the film down, and the oxide film is not pure aluminum get thick, low hardness, different components of aluminum alloy, in the anodic oxidation treatment should pay attention to not with the slot.