Principles of Aluminum Anodization
Aluminum is a relatively active metal with a standard potential of -1.66v. It can naturally form an oxide film with a thickness of about 0.01 to 0.1 microns in the air. This oxide film is amorphous, thin and porous, and has poor corrosion resistance. However, if aluminum and its alloys are placed in an appropriate electrolyte, the aluminum product is used as the anode, and an oxide film is formed on the surface under the action of an external current. This method is called anodic oxidation.
By selecting different types of electrolytes with different concentrations, and controlling the process conditions during oxidation, anodized films with different properties and thicknesses of about tens to hundreds of microns can be obtained, and their corrosion resistance, wear resistance and decoration properties Etc. have been significantly improved and improved. The electrolyte used in the anodization of aluminum and aluminum alloys is generally an acidic solution with a medium dissolving power, and lead or aluminum is used as a cathode, which only conducts electricity. When aluminum and its alloys undergo anodization, the following reactions occur at the anode:
2Al ---> 6e- + 2Al3+
The following reactions occur at the cathode:
6H2O +6e- ---> 3H2 + 6OH-
At the same time, the acid chemically dissolves the aluminum and the formed oxide film, and the reaction is:
2Al + 6H+ ---> 2Al3+ +3H2
Al2O3 + 6H+ ---> 2Al3+ + 3H2O
The growth process of the oxide film is the process of continuous formation and continuous dissolution of the oxide film.
The first section a (curve section ab): non-porous layer formation. Within a few seconds to tens of seconds at the beginning of electrification, a dense, high-insulating oxide film is formed on the aluminum surface, with a thickness of about 0.01 to 0.1 microns, which is a continuous, non-porous film layer. As a non-porous layer or barrier layer, the appearance of this film hinders the passage of current and the continued thickening of the film. The thickness of the non-porous layer is directly proportional to the forming voltage and inversely proportional to the dissolution rate of the oxide film in the electrolyte. Therefore, the voltage of the ab segment of the curve shows a sharp increase from zero to the maximum value.
The second section b (curve bc section): Porous layer formation. With the formation of the oxide film, the dissolution of the electrolyte on the film begins. Because the formed oxide film is not uniform, holes will be dissolved first in the thinnest part of the film, and the electrolyte can reach the fresh surface of aluminum through these holes, the electrochemical reaction can continue, the resistance decreases, and the voltage Following the decrease (the decrease is 10-15% of the highest value), a porous layer appears on the membrane.
The third section c (curve cd section): thickening of the porous layer. After anodizing for about 20s, the voltage enters a relatively steady and slow rising stage. It shows that while the non-porous layer is continuously being dissolved to form a porous layer, a new non-porous layer is growing again. That is to say, the rate of formation and dissolution of the non-porous layer in the oxide film are basically balanced, so the non-porous layer is The thickness no longer increases, and the voltage change is small. However, at this time, the formation and dissolution of the oxide film at the bottom of the hole did not stop, they were still going on, as a result, the bottom of the hole gradually moved to the inside of the metal matrix. As the oxidation time continues, the holes deepen to form pores, and the film layer with pores gradually thickens. When the film formation rate and the dissolution rate reach a dynamic balance, even if the oxidation time is extended, the thickness of the oxide film will not increase anymore, and the anodic oxidation process should be stopped at this time. The anodic oxidation characteristic curve and the oxide film growth process are shown in the figure below. Aluminium and its alloys are anodized by direct current and alternating current in the dilute sulfuric acid electrolyte to obtain a colorless and transparent oxide film with a thickness of 5-20 microns and good adsorption.
The sulfuric acid anodizing process is simple, the solution is stable, the operation is convenient, the allowable range of impurity content is wide, the power consumption is low, the cost is low, and it can almost be applied to the processing of aluminum and various aluminum alloys, so it has been widely used in China.
The following table shows a typical anodic oxidation process: formula and process conditions DC method
Sulfuric acid (g/L) 160～180
Aluminum ion Al3+ (g/L) <25
Temperature (℃) 18～22
Anode current density (A/dm2) 1.2～1.5
Voltage (V) 16～20
Time (min) 20～40
Stirring, compressed air, tank liquid circulation
Cathode area/anode area 1.5:1 The main factors that affect the quality of the oxide film are:
①Sulfuric acid concentration: usually 15% to 20%. As the concentration increases, the dissolution rate of the film increases, and the growth rate of the film decreases. The film has a high porosity, strong adsorption, strong elasticity, and good dyeability (easy to dye dark colors), but the hardness and abrasion resistance are slightly worse; Decrease the concentration of sulfuric acid, the growth rate of the oxide film is accelerated, the film has less pores, high hardness and good wear resistance.
Therefore, when used for protection, decoration and pure decoration processing, the upper limit of the allowable concentration, that is, 20% sulfuric acid is used as the electrolyte.
② Electrolyte temperature: The electrolyte temperature has a great influence on the quality of the 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 and has good adsorption capacity, but the abrasion resistance is quite poor; when the temperature is higher than 30℃, the film becomes loose and uneven, sometimes even discontinuous, and The hardness is low, so it loses its use value; when the temperature is between 10 and 20 ℃, the formed oxide film is porous, has strong adsorption capacity, and is elastic, suitable for dyeing, but the film has low hardness and poor wear resistance;
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 the electrolyte must be strictly controlled during production. To prepare a thick and hard oxide film, the operating temperature must be lowered. In the oxidation process, compressed air stirring and a relatively low temperature are used, and the hard oxidation is usually carried out at around zero.
③Current density: Within a certain limit, the current density increases, the film growth rate increases, the oxidation time is shortened, the resulting film has more pores, is easy to color, and the hardness and wear resistance increase; if the current density is too high, it will be caused by The influence of Joule heat makes the surface of parts overheat and the temperature of the local solution increases, the dissolution rate of the film increases, and there is a possibility of burning the parts; if the current density is too low, the film growth rate is slow, but the resulting film is denser and harder. The wear resistance is reduced.
④Oxidation time: The choice of oxidation time depends on the electrolyte concentration, temperature, anode current density and the required film thickness. Under the same conditions, when the current density is constant, the growth rate of the film is proportional to the oxidation time; but when the film grows to a certain thickness, the conductivity of the film is increased due to the increase in the film resistance, 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 in the end.
⑤Stirring and moving: it can promote the electrolyte convection, strengthen the cooling effect, ensure the uniformity of the solution temperature, and will not cause the quality of the oxide film to be reduced due to the local heating of the metal.
⑥ Impurities in the electrolyte: Impurities that may exist in the electrolyte used for aluminum anodization include Clˉ, Fˉ, NO3ˉ, Cu2+, Al3+, Fe2+, etc. Among them, Clˉ, Fˉ, NO3ˉ increase the porosity of the membrane, and the surface is rough and loose. If its content exceeds the limit value, it will even cause corrosion and perforation of the parts (Clˉ should be less than 0.05g/L, Fˉ should be less than 0.01g/L); when in the electrolyte
When the content of Al3+ exceeds a certain value, white spots or speckled white blocks often appear on the surface of the workpiece, and the adsorption performance of the film is reduced, and it is difficult to dye (Al3+ should be less than 20g/L); when the content of Cu2+ reaches 0.02g/L, the oxide film Dark streaks or black spots will appear on the surface; Si2+ often exists in the electrolyte in a suspended state, making the electrolyte slightly turbid and adsorbed on the membrane as a brown powder.
⑦ Aluminum alloy composition: Generally speaking, other elements in aluminum metal reduce the quality of the film, and the obtained oxide film is not as thick as pure aluminum, and the hardness is also low. Aluminum alloys with different compositions are used for anodizing. Be careful not to do it in the same slot.
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