Although aluminum and its alloy can naturally form an oxide film in the atmosphere, the film is thin (40- 50A) and porous, which is amorphous, uneven and discontinuous, and cannot be used as a protective and decorative film. With the development of aluminum products processing industry from time to time, anodic oxidation or chemical oxidation is adopted in industry to form an oxide film on the surface of aluminum and its alloy products to achieve the purpose of protection and decoration. The oxide film obtained by chemical oxidation is usually 0.3～4um thick, and its softness, wear resistance and corrosion resistance are lower than those of anodic oxide film. Therefore, it is rarely used alone except for its usefulness. However, it has good adsorption ability, and the corrosion resistance and decoration of aluminum products can be improved by repainting its surface.

1, adopting anodic oxidation

The thickness of the oxide film obtained by anodic oxidation treatment is generally 5-20v m, and the thickness of the hard anodic oxide film can reach 60-2500 m. It has high corrosion resistance, which is due to the high chemical stability of the anodic oxide film. The corrosion resistance characteristics of aluminum and aluminum alloy are different: after testing, the anodic oxide film of pure aluminum is better than that of aluminum alloy, which is due to the inclusion of alloy components or the formation of metal compounds that cannot be oxidized or dissolved. However, the oxide film is discontinuous or has gaps, which greatly reduces the corrosion resistance of the oxide film. Therefore, the film obtained by ordinary anodic oxidation must be stopped to improve its corrosion resistance. Therefore, the corrosion resistance of aluminum after anodic oxidation is stronger than that of aluminum alloy.

Aluminum and aluminum alloys can form an extremely thin oxide film in natural environment.

Aluminum has better corrosion resistance than aluminum alloy.

The higher the purity of aluminum, the higher the corrosion resistance.

There are many kinds of aluminum alloys, and the corrosion resistance of different aluminum alloys is very different. Al-Mg alloy has the best protection performance, while Al-Cu alloy and cast aluminum alloy have poor corrosion resistance.

After anodic oxidation or chemical oxidation, the corrosion resistance of aluminum and aluminum alloys will be greatly improved, but the comparison relationship of protection ability will not change, that is, the original good is still good, and the original poor is still poor.

Second, the electric accumulation

Electro-deposition is a process of electrochemical deposition of metals or alloys from aqueous solution, non-aqueous solution or molten salt of their compounds. It is the foundation of metal electrolytic smelting, electrolytic refining, electroplating and electroforming. These processes stop under certain electrolyte and operating conditions. The difficulty level of metal electrodeposition and the shape of the deposit are related to the properties of the deposited metal, as well as the composition of the electrolyte, pH value, temperature, current density and other factors. Wu Xiangqing and others stopped discussing the corrosion resistance of electrodeposited Ni2SiC composite coating on ZL105 aluminum alloy by electrochemical method. The results show that the appearance of Ni2SiC composite coating is completely different from that of pure Ni coating, and its corrosion resistance is better than that of pure Ni coating. After heat treatment at 300℃×2h, the corrosion resistance is further improved.

III. Multi-arc ion plating

Multi-arc ion plating is the application of gas discharge or local ionization of evaporated substances in vacuum chamber, and the evaporation or reaction is accumulated on the substrate while gas ions or evaporated substance particles bombard. Ion plating organically separates glow discharge, plasma technology and vacuum evaporation, which not only obviously improves the film quality, but also expands the application scope of the film.

Its advantages are strong film adhesion, good diffraction, film material and so on. There are many kinds of ion plating, such as resistance heating, electron beam heating, plasma electron beam heating and high frequency induction heating. Multi-arc ion plating uses arc discharge instead of glow discharge of traditional ion plating to stop accumulation. Simply put, the principle of multi-arc ion plating is that the cathode target is used as the evaporation source, and the target is evaporated by arc discharge between the target and the anode shell, thus forming plasma in space and stopping the accumulation of the substrate. Ti-Cr-N coating was deposited on the surface of ZL201 aluminum alloy by multi-arc ion plating, and a lipid film was prepared on the Ti-Cr-N coating [10]. The results show that Cr in Ti-Cr-N coating exists in TiN crystal in the form of solid solution and does not form a separate CrN phase. Coating can effectively improve the salt spray corrosion resistance of ZL201 aluminum alloy.

Fourth, chemical composite coating

The process of adding water-insoluble solid particles into the plating solution to make them pile up with the main metal to form a coating is called composite plating. If the electroplating process is adopted, it is called composite electroplating; If the electroless plating process is adopted, it is called composite electroless plating. The obtained coating is called composite coating. According to the criterion, any metal that can be plated can be used as the main metal, but several metals such as nickel, chromium, cobalt, gold, silver and copper are widely studied and applied. There are two main types of solid particles, one is high hardness and high melting point particles that improve the wear resistance of coatings; One kind is solid lubricant particles that improve the self-smoothing characteristics of coatings. Ni-P- diamond chemical composite coating was prepared on the surface of cast aluminum [11]. The results showed that ceric sulfate could promote diamond particles to enter the coating, and the stability of the plating solution improved greatly with the increase of ceric sulfate content, and then stabilized. The wear resistance of Ni-P- diamond composite coating was better than that of Ni-P coating, and it was further improved after adding 2mg/L ceric sulfate. Compared with Ni-P coating, the corrosion resistance of the composite coating was poor, and it was improved after adding ceric sulfate.

V. Chemical conversion film

Chemical conversion film is to make the metal contact with a specific corrosive liquid, and under certain conditions, a chemical reaction will occur, forming a layer of insoluble product film with good adhesion on the metal surface. These films can either protect the base metal from water and other corrosive media, or improve the adhesion and aging resistance of organic coatings, or give other appearance properties. Because the chemical conversion film is generated by the direct participation of the base metal in the film-forming reaction, the separation force from the base is much greater than that of the electroplated layer and the electroless plating layer. Metals can be transformed in the selected medium to obtain chemical conversion films with different application purposes, but steel, aluminum, zinc, copper, magnesium and their alloys are widely used in industry at present. The chemical conversion film is different from other masking layers on metals (such as electro-deposited layers of metals). Its formation is directly involved by the base metal, and it is transformed with anions in the medium (MmAn), so it can be said that the composition of chemical conversion film can be regarded as a controlled metal corrosion process in practice. Chemical conversion film can be divided into oxide film, phosphate film, chromate film, oxalate film, etc.

Aluminum alloy is easy to be destroyed by intergranular corrosion in atmospheric environment. At present, the high-strength cast aluminum alloy commonly contains elements such as silicon, copper and magnesium, and the participation of these elements increases the corrosion sensitivity of the alloy. Secondly, the surface hardness is low, it is easy to wear, and the surface gloss can't last long, so higher maintenance measures are required. Among them, the formation of chemical conversion film on the surface of aluminum alloy has the advantages of simple equipment, low cost and low investment. Peng Liang et al. [12] used chromate method to form chemical conversion film on Y112 alloy. The experimental results show that the conversion film has high corrosion resistance and beautiful golden yellow surface.

The corrosion potential of chemical oxide film [13] obtained by chemical oxidation on the surface of aluminum alloy with manganate and zirconium salt as main salts is about 0.45V higher than that of aluminum alloy samples, and the corrosion current density is only 0.286 μ a/cm2. The impedance value at the low frequency end of AC impedance spectrum is one order of magnitude larger than that of aluminum alloy sample. The appearance of chemical oxide film on aluminum alloy is golden yellow with regular columnar growth structure.

Ge Shengsong et al. prepared black conversion film on the surface of cast aluminum alloy by chromium-free chemical method, and evaluated the corrosion resistance of the film by drop experiment. Scanning electron microscope and electron probe were used to observe the morphology of the film and determine its composition elements. Finally, the composition mechanism and corrosion resistance mechanism of the black film were put forward.

VI. Conclusion

The surface corrosion resistance of cast aluminum alloy can be improved by electrochemical method. The existing research mostly stays on the sample, and the application research is less. In practical application, it is rare to improve the protection, decoration and function of cast aluminum alloy by using a single process technology. The existing modification technology is comprehensively considered and systematically discussed. It is meaningful to comprehensively study the corrosion resistance of cast aluminum alloy.

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