Control of various factors in anodic oxidation process

Factor control in anodic oxidation process

How is the voltage controlled during anodic oxidation?

The voltage is regulated according to the temperature of the solution.When the solution temperature is low, the voltage at the upper limit shall be adopted, because the oxidation film obtained at the lower solution temperature is dense and the oxidation film resistance is large. To thicken the oxide film, a higher voltage must be adopted, otherwise it is difficult to obtain the normal quality of the oxide film.When the temperature of the solution is high, the voltage should be lowered, otherwise the film solution will be too fast due to the resulting loose oxide film, and it is difficult to obtain the ideal oxide film thickness.

For example, in units without cooling devices, the temperature of the summer solution will approach the limit temperature. If it still needs to continue working, the voltage shall not exceed 12V.In winter, when the solution temperature is lower than the lower limit temperature, the voltage should rise to a higher value, such as 18V.

Anodic oxidation is an exothermic reaction, when the workload is full, the temperature of the solution will gradually rise, so it is necessary to test at any time, as a basis for the regulation of voltage.If the temperature continues to rise, the voltage will be regulated below the quality assurance.Production should be stopped at this point.Take appropriate measures to cool down, to meet the technical requirements before processing.

How to control the current density during anodic oxidation?

Under normal temperature conditions (about 20℃), the current density of anodic oxidation of aluminum and its alloys is controlled between 1 -- 1.5A/dm2, except for special process formulations.

According to the temperature of the solution, the concentration of the solution, the shape of the parts and other relevant technological conditions to choose.

Under possible conditions, increasing the current density can accelerate the formation of the film, shorten the anodic oxidation time, increase the porosity of the film and improve the coloring effect.However, when the current density continues to increase, the influence of joule heat will increase during the anodic oxidation process, and the thermal effect in the membrane hole will increase, resulting in the local temperature rise, which will accelerate the dissolution rate of the oxide film, reduce the film formation speed, and lead to uneven current distribution in complex parts, which will affect the coloring effect.The surface of the product may also appear easy to wipe off the loose oxidation film, or the film layer brittle, cracking, or white marks, serious may cause ablative products.

The growth rate of the film can be accelerated by selecting the appropriate current density in a certain range, but the film formation speed decreases when the current density exceeds a certain value.

According to the law above, in order to ensure product quality and improve production efficiency, the following methods can be adopted.

When the cooling condition is good and the solution can meet the strong agitation, the upper limit of the current density can be used to improve the working efficiency.

Under the condition of neither cooling device nor strong agitation, the current density should be controlled properly, even though the temperature of the solution was moderate at that time, to prevent the quality problem caused by the excessive temperature rise in the process of anodic oxidation, which may cause the ablative corrosion of the parts in serious cases.The more effective method is to reduce the volume current density.

The correct estimation of the surface area of the anodized parts is also an important condition to control the current density reasonably.

The surface of the deep depression of the anodized part shall have the same current density as other surfaces.

Several main factors affecting the quality of anodic oxidation film

(1) current density: within a certain limit, the current density increases, the growth rate of the film increases, the oxidation time is shortened, the generated film has more pores, easy to stain, and the hardness and wear resistance increase;If the current density is too high, the surface of the part will be overheated and the local solution temperature will increase due to the effect of joule heat, and the dissolution rate of the film will increase, which may burn the part.If the current density is too low, the growth rate of the film is slow, but the generated film is relatively dense, and the hardness and wear resistance are reduced.

Oxidation time: the choice of oxidation time depends on the concentration of electrolyte, temperature, anode current density and required film thickness.Under the same conditions, when the current density is constant, the growth rate of the membrane is proportional to the oxidation time.However, when the film grows to a certain thickness, the film resistance increases, which affects the conductivity, and the dissolution rate of the film increases due to the temperature rise, so the growth rate of the film will gradually decrease and will not increase after the film grows thicker.

(3) sulfuric acid concentration: usually use 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 force is strong, the elasticity is rich, the dyeing property is good (easy to dye dark color), but the hardness, wear resistance is slightly poor;However, when the concentration of sulfuric acid is reduced, the growth rate of the oxide film is accelerated, the porosity of the film is less, the hardness is higher and the wear resistance is better.

(4) electrolyte temperature: electrolyte temperature has a great influence on the quality of oxide film.As the temperature increases, the dissolution rate of the film increases and the film thickness decreases.When the temperature is 22 -- 30℃, the obtained film is soft, good adsorption ability, but poor wear resistance;When the temperature is more than 30℃, the membrane becomes loose and uneven, sometimes even discontinuous, and the hardness is low, so it loses its use value.When the temperature is between 10 -- 20℃, the oxide film generated is porous, strong adsorption capacity, and elastic, suitable for dyeing, but the hardness of the film is low, 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 is strictly controlled during production.To make thick and hard oxide film, reduce the operating temperature, in the oxidation process using compressed air agitation and lower temperature, usually around zero for hard oxidation.

Stirring and moving: can promote the electrolyte convection, strengthen the cooling effect, to ensure the uniformity of the solution temperature, will not cause the metal local temperature rise and lead to the quality of oxide film decline.

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, can 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 specular white blocks often appear on the surface of the workpiece, and the adsorption performance of the membrane 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 membrane with brown powder.

Aluminum alloy composition: generally speaking, other elements in aluminum metal make the quality of the film decline, and the obtained oxide film is not as thick as the pure aluminum, hardness is also low, different components of aluminum alloy, in the anodic oxidation treatment should pay attention to the same tank.