What is the relationship between the surface treatment process conditions of aluminum and the quality of the anodized film

The mastery of anodizing process conditions is closely related to the quality of the oxide film because the process conditions are derived from a series of experiments based on different process recipes. Before performing anodizing, one should be fully aware of the specified process conditions and strictly follow the process requirements during operation. The most fundamental and quality-sensitive factors include: control ranges of solution temperature, voltage, and current density; anodizing time; solution agitation method; current density of the solution volume; and the ratio of solution volume to anodized area, among others. Any deviation from these process conditions will significantly affect the quality of the anodized film, and if the deviation is too large, it may even lead to scrapping of the parts, causing economic loss.

How is the voltage controlled during anodizing?

The voltage should be adjusted according to the solution temperature. When the solution temperature is low, the upper limit of the specified voltage should be used because at lower temperatures, the resulting oxide film is dense, and its resistance is high. To thicken the oxide film, a higher voltage is necessary; otherwise, it is difficult to achieve the desired quality of the oxide film. Conversely, when the solution temperature is high, the voltage should be reduced, otherwise the resulting oxide film may be loose, causing the film to dissolve in the solution too quickly and making it difficult to obtain the ideal thickness of the oxide film.

Anodizing is an exothermic reaction. When the workload is relatively high, the temperature of the solution will gradually rise, so it is necessary to test it at any time as a basis for adjusting the voltage. If the temperature continues to rise, the quality cannot be guaranteed even if the voltage is kept below the standard. At this point, production should be stopped. Appropriate measures should be taken to cool it down, and processing should resume only when the conditions meet the process requirements.

How is the current density controlled during anodizing?

Under normal temperature conditions (around 20°C), except for special process formulas, the current density for anodizing general aluminum and its alloys is generally controlled between 1 and 1.5 A/dm².

Select according to the solution's temperature, concentration, the shape of the workpiece, and other relevant process conditions.

Under possible conditions, appropriately increasing the current density is beneficial for accelerating the formation of the film, shortening anodizing time, increasing the porosity of the film, and improving the coloring effect. However, when the current density continues to increase, the anodizing process will be more affected by Joule heat, causing a greater thermal effect inside the film pores, significant local temperature rise, which accelerates the dissolution of the oxide film and slows down the film formation rate. For complex parts, it may also cause uneven current distribution, affecting the coloring effect. On the surface of the workpiece, it may also result in a loosely adherent oxide film that is easy to rub off, film brittleness, cracking, or the appearance of white marks; in severe cases, it may even lead to erosion of the workpiece.

Selecting an appropriate current density within a certain range can accelerate the growth rate of the film, but when it exceeds a certain value, the film formation rate actually decreases.

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

Under good cooling conditions and when the solution can withstand vigorous stirring, the upper limit of current density can be used to improve operational efficiency.

Under conditions without cooling devices and without intense stirring, although the temperature of the solution at the time is moderate, the current density still needs to be properly controlled to prevent quality issues during the anodic oxidation process caused by rapid temperature rise, which in severe cases may even lead to burning of the workpiece. At this time, the most effective method is to reduce the volumetric current density.

Correct estimation of the surface area of anodized parts is an important condition for reasonably controlling the current density and should be given due attention.

The surfaces of the deep recessed areas of the anodized parts should be supplied with the same current density as the other surfaces.

Major Factors Affecting the Quality of Anodized Film

① Current Density: Within a certain range, an increase in current density accelerates the membrane growth rate, shortens oxidation time, produces a more porous membrane which is easy to color, and improves hardness and wear resistance. If the current density is too high, the Joule heating effect can cause the part surface to overheat and local solution temperature to rise, increasing the dissolution rate of the membrane and potentially damaging the parts. If the current density is too low, membrane growth is slow, but the resulting membrane is denser, with reduced hardness and wear resistance.

Aluminum oxidation, used for protection, decoration, and pure decorative processing, often employs the maximum allowable concentration, that is, a 20% concentration of sulfuric acid as the electrolyte.

② Oxidation Time: The choice of oxidation time depends on the electrolyte concentration, temperature, anode current density, and the desired film thickness. Under the same conditions, when the current density is constant, the growth rate of the film is proportional to the oxidation time. However, when the film reaches a certain thickness, the increase in film resistance affects its conductivity, and due to the temperature rise, the dissolution rate of the film increases. Therefore, the growth rate of the film gradually decreases and eventually stops increasing.

③ Sulfuric Acid Concentration: Typically 15%–20% is used. 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, adhesion is strong, it is elastic, and it has good dyeing properties (easy to dye dark colors), but the hardness and wear resistance are slightly reduced. Conversely, reducing the sulfuric acid concentration accelerates the growth of the oxide film, resulting in fewer pores, higher hardness, and better wear resistance.

④ Electrolyte temperature: The temperature of the electrolyte greatly affects the quality of the oxide film. As the temperature rises, the dissolution rate of the film increases, and the film thickness decreases. When the temperature is between 22–30°C, the resulting film is soft, has good adsorption capacity, but relatively poor wear resistance; when the temperature exceeds 30°C, the film becomes loose and uneven, sometimes even discontinuous, with low hardness, thus losing its practical value; when the temperature is between 10–20°C, the resulting oxide film is porous, has strong adsorption capacity, and is elastic, suitable for dyeing, but the film's hardness is low and wear resistance is poor; when the temperature falls below 10°C, the oxide film becomes thicker, harder, and wear-resistant, but the porosity is low. Therefore, the electrolyte temperature must be strictly controlled during production. To obtain a thick and hard oxide film, the operating temperature must be lowered, using compressed air stirring during the oxidation process at relatively low temperatures, usually around zero degrees, to achieve hard anodizing.

⑤ Stirring and movement: They can promote convection of the electrolyte, enhance cooling efficiency, ensure uniform solution temperature, and prevent local heating of the metal from causing a decline in the quality of the oxide film.

⑥ Impurities in the electrolyte: Possible impurities in the electrolyte used for aluminum anodizing include Clˉ, Fˉ, NO3ˉ, Cu2+, Al3+, Fe2+, etc. Among them, Clˉ, Fˉ, and NO3ˉ increase the porosity of the film, making the surface rough and loose. If their content exceeds the limit, it can even cause corrosion perforation of the workpiece (Clˉ should be less than 0.05 g/L, Fˉ should be less than 0.01 g/L); when the Al3+ content in the electrolyte exceeds a certain value, white spots or patchy white areas often appear on the surface of the workpiece, and the film's adsorption performance decreases, making dyeing difficult (Al3+ should be less than 20 g/L); when the Cu2+ content reaches 0.02 g/L, dark streaks or black spots appear on the oxide film; Si2+ usually exists in a suspended state in the electrolyte, causing slight cloudiness and adsorbing onto the film as brown powdery deposits.

⑦ Aluminum alloy composition: Generally speaking, other elements in aluminum reduce the quality of the film, and the resulting oxide film is not as thick or hard as that obtained on pure aluminum. For aluminum alloys of different compositions, care must be taken not to anodize them in the same bath.