Corrosion resistance technology of aluminum alloy forged plates
Although aluminum and aluminum alloys can naturally form an oxide film in the atmosphere, the film is thin (40-50A), loose and porous, amorphous, uneven and discontinuous, and cannot be used as a protective decorative film Floor. With the development of the aluminum product processing industry from time to time, anodizing or chemical oxidation is used in the industry to form an oxide film on the surface of aluminum and aluminum alloy parts to achieve the purpose of protection and decoration. The thickness of the oxide film obtained by chemical oxidation Du Li is generally 0.3 ~ 4um, and its softness, wear resistance and corrosion resistance are all lower than that of an anodic oxide film. Therefore, it is rarely used alone except for its usefulness. But it has better adsorption ability, and then paint on its surface, which can improve the corrosion resistance and decoration of aluminum products.
1. Using anodizing
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 60-2500m. It has high corrosion resistance. This is due to the high chemical stability of the anodic oxide film. The difference between the corrosion resistance characteristics of aluminum and aluminum alloy: After testing, the anodic oxide film of pure aluminum has better corrosion resistance than the anodic oxide film of aluminum alloy. This is because the alloy components or the constituent metal compounds cannot be oxidized or dissolved, which makes the oxide film discontinuous or produces voids, which greatly reduces the corrosion resistance of the oxide film. Therefore, the film obtained after ordinary anodic oxidation must be blocked to improve its corrosion resistance. Therefore, the corrosion resistance of aluminum after being treated by anodizing method is stronger than that of aluminum alloy.
- 1 Aluminum and aluminum alloys can form an extremely thin oxide film in the natural environment.
- 2 Aluminum has better corrosion resistance than aluminum alloy.
- 3 The higher the purity of aluminum, the higher the corrosion resistance.
- 4 There are many types of aluminum alloys, and the corrosion resistance of different aluminum alloys varies greatly. Aluminum-magnesium alloys have the best protection performance, and aluminum-copper alloys and cast aluminum alloys have poor corrosion resistance.
- 5 The anti-corrosion performance of aluminum and aluminum alloys that have undergone anodization or chemical oxidation will be greatly improved, but the comparison of protective capabilities remains unchanged, that is, the original good is still good, and the original poor is still poor.
Two, electric accumulation
Electrodeposition is the process of electrochemical accumulation of metal or alloy from its compound aqueous solution, non-aqueous solution or molten salt. It is the foundation of metal electrolytic smelting, electrolytic refining, electroplating, and electroforming. These processes are stopped under certain electrolyte and operating conditions. The difficulty level of metal electrical accumulation and the shape of the accumulation are related to the properties of the accumulated metal, and also depend on the composition of the electrolyte, pH, temperature, current density and other factors. Wu Xiangqing and others used electrochemical methods to study the corrosion resistance of the Ni2SiC composite coating on the surface of the ZL105 aluminum alloy. The results show that the appearance of the Ni2SiC composite coating is completely different from that of the pure Ni coating, and the corrosion resistance is better than that of the pure Ni coating. After 300℃×2h heat treatment, the corrosion resistance has been further improved.
3. Multi-arc ion plating
Multi-arc ion plating is a vacuum chamber where gas discharge or partial ionization of evaporated substances is used. While gas ions or particles of evaporated substances are bombarded, the evaporated substances or reaction substances are deposited on the substrate. Ion plating organically separates the glow discharge phenomenon, plasma technology and vacuum evaporation, which not only significantly improves the film quality, but also expands the application range of the film.
Its advantages are strong film adhesion, good diffraction, and film materials. There are many types of ion plating, and the evaporation remote heating methods include resistance heating, electron beam heating, plasma electron beam heating, high frequency induction heating and so on. Multi-arc ion plating uses arc discharge instead of glow discharge in traditional ion plating to stop accumulation. To put it simply, the principle of multi-arc ion plating is to use the cathode target as the evaporation source to evaporate the target through the arc discharge between the target and the anode casing, thereby forming a plasma in the space and stopping the accumulation of the substrate. Multi-arc ion plating Ti-Cr-N coating on the surface of ZL201 aluminum alloy, and a lipid film is prepared on the Ti-Cr-N coating . The results indicate that the Cr in the Ti-Cr-N coating exists in the TiN crystal as a solid solution, and does not constitute a separate CrN phase; the coating can effectively improve the salt spray corrosion resistance of the ZL201 aluminum alloy.
Four, chemical composite coating
The process of adding water-insoluble solid particles in the plating solution to form a plating layer together with the main metal 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 resulting coating is called a composite coating. On the guideline, any metal that can be plated can be used as the main metal, but nickel, chromium, cobalt, gold, silver, copper and other metals are more researched and applied. There are two main types of solid particles. One is the particles with high hardness and high melting point that improve the wear resistance of the coating; the other is the solid smoothing agent particles that improve the self-smoothing characteristics of the coating. A Ni-P-diamond chemical composite coating was prepared on the surface of cast aluminum . The results showed that cerium sulfate can promote diamond particles to enter the coating. With the increase of cerium sulfate content, the stability of the plating solution will increase significantly, and the Ni-P will stabilize. -The wear resistance of the diamond composite coating is better than that of the Ni-P coating. The addition of 2mg/L cerium sulfate further improves significantly. Compared with the Ni-P coating, the composite coating has poor corrosion resistance, which is improved after the addition of cerium sulfate.
Five, chemical conversion coating
The chemical conversion coating is to make the metal come into contact with a specific corrosive liquid phase, and a chemical reaction occurs under certain conditions, forming a layer of good adhesion and insoluble product film on the surface of the metal. These films can either protect the base metal from the influence of water and other corrosive media, or can improve the adhesion and aging resistance of the organic coating film, or can impart other properties to the surface. The chemical conversion coating is generated by the base metal directly participating in the film-forming reaction, so the separation force from the base is much greater than that of the electroplating layer and the electroless plating layer. Metals can be transformed and processed in selected media to obtain chemical conversion coatings for different application purposes. However, the most industrial applications are steel, aluminum, zinc, copper, magnesium and their alloys. The chemical conversion coating is different from other masking layers on metals (such as the electro-deposited layer of metals). Its formation involves the direct participation of the base metal and generates its own conversion products (MmAn) with the anions in the medium, so it can also be said that chemical conversion In practice, the formation of the film can be regarded as a controlled metal corrosion process. Chemical conversion coatings are classified into oxide films, phosphate films, chromate films, oxalate films, etc. according to the types of the main components of the film.
Aluminum alloys are easily destroyed by intergranular corrosion in the atmospheric environment. The currently used high-strength cast aluminum alloys generally contain elements such as silicon, copper, and magnesium. 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 cannot be maintained for a long time, so higher maintenance measures are requested. 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.  used the chromate method to form a chemical conversion coating on the Y112 alloy. The experimental results showed that the conversion coating has high corrosion resistance and a beautiful golden yellow surface.
With manganate and zirconium salt as the main salt, the corrosion potential of the chemical oxide film  obtained by chemical oxidation on the aluminum alloy surface is about 0.45V higher than that of the aluminum alloy sample, and the corrosion current density is only 0.286μA/cm2; The impedance value at the low frequency end of the AC impedance spectrum is an order of magnitude larger than that of the aluminum alloy sample; the aluminum alloy chemical oxide film is golden yellow in appearance and has a regularly arranged columnar growth structure.
Ge Shengsong et al. used a chromium-free chemical method to prepare a black conversion coating on the surface of cast aluminum alloys, and applied spot experiments to evaluate the corrosion resistance of the coating. Scanning electron microscopy and electron probe were used to observe the morphology of the film and determine its constituent elements. Finally, the composition mechanism and corrosion resistance mechanism of the black film were proposed.
The surface corrosion resistance of cast aluminum alloys can be improved by electrochemical methods. The existing research mostly stays on the sample, and the application research is less. In practical applications, it is relatively rare to use a single process technology to improve the protective, decorative and functional issues of cast aluminum alloys. Considering the existing modification technology comprehensively, we will start a systematic discussion on this. The comprehensive study on the corrosion resistance of cast aluminum alloys is meaningful.