Due to its small specific gravity, high specific strength, excellent ductility, good conductivity and corrosion resistance, easy forming and other excellent physical and chemical properties, aluminum alloy has become the second largest class of metal materials after steel. Aluminum alloy under natural conditions will spontaneously form a layer of dense oxide film, its thickness is generally below 5nm. Although the natural oxide film on the surface of aluminum alloy can be automatically repaired immediately after it is destroyed, its corrosion resistance and wear resistance are limited because of its thin thickness. In order to meet the requirements of modern industry, aluminum alloy must be properly surface treated, and anodic oxidation is the most commonly used surface treatment method for aluminum and aluminum alloys.

Anodic oxidation of aluminum and its alloys refers to the aluminum sample placed in a specific electrolyte, with aluminum sample as the anode, stainless steel or lead as the cathode, after it is energized on the surface of the aluminum sample will generate a layer of oxide film method.

According to the final use of aluminum, it can be divided into aluminum anodizing for construction, aluminum anodizing for decoration, aluminum anodizing for corrosion protection, anodizing for electrical insulation and aluminum alloy oxidation for engineering (such as hard anodizing), etc. According to the characteristics of the power waveform, it can be divided: direct current (DC) anodic oxidation, alternating current (AC) anodic oxidation, AC/DC superimposed (DC/AC) anodic oxidation, pulse (PC) anodic oxidation and periodic commutation (PR) anodic oxidation, etc. According to electrolyte, there are: sulfuric acid anodic oxidation, oxalic acid anodic oxidation, chromic acid anodic oxidation, phosphoric acid anodic oxidation and mixed acid anodic oxidation. According to the function of oxide film, it can be divided: wear-resistant film layer, corrosion-resistant film layer, adhesive film layer, insulating film layer, porcelain film layer, decorative film layer, etc.

Aluminum anodic oxide film has two categories: barrier type anodic oxide film and porous anodic oxide film. Barrier-type anodic oxide film is a dense non-porous thin anodic oxide film close to the metal surface. Its thickness depends on the applied anodic oxidation voltage, but it is generally very thin, usually less than 1μm, and is mainly used to make electrolytic capacitors. The porous anodic oxide film is composed of two layers of oxide film: the bottom layer is a dense non-porous thin oxide layer with the same barrier film structure, called a barrier layer, and its thickness is only related to the applied anodic oxidation voltage; The main part is a porous structure, and its thickness depends on the amount of electricity passed.

The film-forming research of aluminum anodic oxidation started from the barrier film of aluminum at the end of the 19th century, and many aspects such as its formation law and mechanism have been relatively complete and clear, and the mathematical formula for the growth of barrier-type anodic oxide film was Bernard established by the middle of the 20th century, and the research was more in-depth. At present, the research of barrier film has been extended to the synergistic effect of several oxidation processes, such as hydration oxidation or thermal oxidation plus anodic oxidation, etc. The research background is to improve the performance of electrolytic capacitors.

For the structure of porous anodic oxide films, the earliest model was proposed by Kellar in 1953. Kellar believes that the anodic oxide film consists of two layers, of which the film layer combined with the substrate is a barrier layer, which is non-porous and dense; the film layer above the barrier layer is called a porous layer, which is composed of many hexagonal columnar structural units, as shown in Figure 1. The star-shaped hole morphology of Keller's model has now been modified to be circular, but their view that the structural unit is a hexagonal cylinder is still of reference value today.

In the early stage of the reaction, the voltage increases sharply with time to the maximum value, and a dense oxide film is formed on the aluminum surface, which is called a barrier layer. The barrier layer has a high resistance and hinders the progress of the reaction. The thicker the barrier layer, the greater the film resistance. The main reactions occurring at the anode at this time are divided into membrane formation reactions (2Al 3H2O-6e-→ Al2O3 6H) and membrane dissolution reactions (2Al 6H → Al3 3H2 and Al2O3 6H → 2Al 3H2O). At this time, the formation rate of the film is much greater than the dissolution rate of the film, so the thickness of the barrier layer gradually increases. Since the current is constant and the resistance per unit thickness of the barrier layer is constant, the formation voltage gradually increases.

Due to the uneven distribution of the barrier layer during the electrolysis process, the electric field is not evenly distributed and local overheating is caused. In these areas, the electrolyte dissolves the oxide film layer quickly, so that regularly arranged pore nuclei are formed on the surface of the oxide film. At the core of the hole, the distance between the electrolyte and the base metal decreases, causing a tip discharge and a corresponding decrease in voltage.

At this time, the formation rate of the anodic oxide film and the dissolution rate of the anodic oxide film reach a dynamic balance, and the thickness of the barrier layer remains unchanged, and continues to move towards the aluminum substrate. At the same time, the alumina film is also dissolved at the interface between the outside of the porous layer and the electrolyte, but it is only a general chemical dissolution, and the dissolution rate is very slow, so the porous layer is continuously thickened.

Aluminum alloy anodizing commonly used processes are: sulfuric acid anodizing process, chromic acid anodizing process, oxalic acid anodizing process and phosphoric acid anodizing process. When different electrolytes are used, the resulting oxide films differ greatly in appearance, properties, etc. In actual production, it is necessary to select the appropriate anodic oxidation process according to the purpose of use.

At present, the anodic oxidation process widely used at home and abroad is sulfuric acid anodic oxidation. Sulfuric acid anodic oxidation has the advantages of low cost, simple process, short time, easy production operation, high film transparency, candle resistance and good wear resistance. Compared with other acid anodic oxidation, it has obvious advantages in various aspects. Because the current density of sulfuric acid AC anodic oxidation is low, the quality of the oxide film is poor, so most of the current domestic and foreign use of DC sulfuric acid anodic oxidation. The process of sulfuric acid anodic oxidation is as follows: mechanical polishing → degreasing → twice cleaning → chemical polishing or electrolytic polishing → twice cleaning → anodic oxidation → twice cleaning → dyeing preparation.

The chromic acid anodizing process was first developed by Bengough and Staurt in 1923 (referred to as the B- S method). The film obtained by chromic acid anodic oxidation is thin, generally only 2-5μm thick, which can maintain the original accuracy and surface roughness of the workpiece. The film is soft, the wear resistance is not as good as the sulfuric acid oxide film, but the elasticity is good. In addition, the film is opaque, the porosity is low, it is difficult to dye, and it can be used directly without sealing treatment. The solubility of chromic acid solution to aluminum alloy is low, so that the residual solution in pinholes and crevices has little effect on the corrosion of components. It is used in surface treatment such as castings, riveting parts and machining. This process is also used in military equipment.

The oxalic acid anodic oxidation process was widely used in Japan and Germany as early as 1938. Because the solubility of oxalic acid to aluminum and aluminum alloy is small, the porosity of the oxide film is low, so the corrosion resistance, wear resistance and electrical insulation of the film are better than the sulfuric acid film. But the oxalic acid anodic oxidation cost is high, generally 3-5 times of sulfuric acid anodic oxidation; and the color of the oxalic acid oxide film is easy to change with the process conditions, so that the product has a color difference, so the process is subject to certain restrictions in the application, generally only in the case of special requirements, such as the production of electrical insulation layer. 6.4 phosphoric acid anodic oxidation

Phosphoric acid anodic oxidation was first used as a pretreatment process for aluminum plating. Since the oxide film dissolves more in the phosphoric acid electrolyte than sulfuric acid, the phosphoric acid film is thin (about 3 μm in thickness) and has a large pore size. Because the phosphoric acid film has strong water resistance, it can prevent the adhesive from aging due to hydration so that the bonding force of the adhesive is better, so it is mainly used for the surface treatment of printed metal plates and the pretreatment of aluminum workpiece bonding.

Anodizing gives aluminum and aluminum alloy better corrosion resistance, wear resistance and decorative and electrical insulation, is currently the most widely used aluminum and aluminum alloy surface treatment technology. Due to the porosity of the anodic oxide film of aluminum alloy, more and more research focuses on the functional characteristics of the porous anodic oxide film, such as the preparation of new functional materials, nanowires, nanotubes, etc. by depositing various substances with different properties, such as metals, semiconductors, polymer materials, etc. in the nano-scale pores, and the development of new ultra-precision separation membranes by using the porous structure with uniform pore size and narrow distribution. With the further deepening of understanding, the application of aluminum alloy anodic oxidation technology in functional devices will be more extensive.