1 micro-arc oxidation ceramic layer
Microarc oxidation (Microarcoxidation, MAO), also known as micro-plasma oxidation (Microplasmaoxidation, MPO), is a combination of electrolyte and the corresponding electrical parameters, in the aluminum, magnesium, titanium and its alloy surfaces rely on the arc discharge generated by the transient high temperature and high pressure. The surface of aluminum, magnesium, titanium and their alloys is grown by the transient high temperature and high pressure generated by arc discharge. Due to the micro-arc oxidation process, chemical oxidation, electrochemical oxidation, plasma oxidation at the same time, the micro-arc oxidation process will be introduced into the high-pressure discharge area, greatly improving the comprehensive performance of the film layer. Micro-arc oxidation film layer and the substrate combined firmly, dense structure, high toughness, with good wear resistance, corrosion resistance, high temperature impact resistance and electrical insulation and other characteristics. The technology is simple to operate and easy to realize the function of the film layer adjustment, and the process is not complex, does not cause environmental pollution, is a new green environmentally friendly material surface treatment technology, in the aerospace, machinery, electronics, decorative and other fields have broad prospects for application.
The alloy elements Cu and Mg are favorable for micro-arc oxidation, while the Si element hinders micro-arc oxidation. Hou Zhaohui et al [1] on the silicon content of 8% ~ 12% of the ZL series of cast aluminum alloy of the micro-arc oxidation process conditions, film structure, and film formation process was studied. The results show that: cast aluminum alloy in the water-glass composite system for micro-arc oxidation, you can get a layer of fine, uniform, thicker, higher microhardness of the ceramic oxide film; micro-arc oxidation electrolyte system, the water-glass can make the cast aluminum alloy micro-arc oxidation smoothly; Na2WO4 and EDTA disodium complex can improve the hardness of the film layer; the process conditions for obtaining the micro-arc oxidation film of the ZL109 alloy under the conditions of this study The conditions were NaOH: 2~4g/L, water glass: 5~7mL/L, Na2WO4: 2~4g/L, disodium EDTA: 2~4g/L, microarc oxidation current density of 30~40A/dm2, solution temperature of 30~40℃, and strong stirring. In addition, Gong Jianfei et al [2] also studied the micro-arc oxidation of ZL109, and obtained a dense layer thickness of more than 76 μm, microhardness HV1600 uniform oxide ceramic film layer.
ADC12 die-cast aluminum alloy is widely used in automotive, motorcycle and instrumentation industries such as pistons, pulleys and other components and structural parts. Zhang Jinbin et al [3] studied the ADC12 aluminum alloy surface micro-plasma oxidation method for the preparation of black ceramic film of the electrolyte composition and electrical parameters, such as the impact on the performance of the film layer, the results show that the lower the concentration of sodium phosphate, the surface roughness, the concentration is too high easy to precipitate salts and the film layer chipping, the optimal concentration of 12 ~ 15g / L; additives M1 and M2 components of the metal element oxides in the film layer, the larger the specific gravity of the oxide K, the higher. The higher the black saturation of the film layer, the more stable it is, and the optimal concentration is 10.0-11.0g/L and 15.0-18.0g/L respectively; the optimal pH value to make the film layer black is 8.0-9.0; the optimal current density for the formation of saturated dark black is 3.0-4.0A/dm2; the thickness of the black film layer prepared with the optimal electrolyte formula is 20-30μm, the hardness is HV500-700, the black saturation is HV500-700, the black saturation is HV500-700, and the black saturation is HV500-700, and the black saturation is HV500-700. HV500～700, black saturation in 0.8～1.0.
Wang Zongren et al [4] applied the plasma-enhanced electrochemical surface ceramization (PECC technology) process to the surface strengthening treatment of Y112 die-cast aluminum alloy, which resulted in the generation of a ceramic film of α-Al2O3 and γ-Al2O3 phases on its surface. It is claimed that the performance of the film are superior to Teflon technology coating.
Jin Ling et al [5] carried out micro-arc oxidation on the surface of ZL109 alloy and SiCp/ZL109 composites, and found that both ZL109 alloy and SiCp/ZL109 composites can be surface micro-arc oxidized, and their micro-arc oxidized layer consists of two layers of structure, which are the sparse layer and the dense layer.The micro-arc oxidized layer of ZL109 alloy is mainly composed of the different structures of the Al2O3 phase, and the microarc oxidation layer of SiCp/ZL109 composite is composed of Al2O3 and MgAl13O40.
The integrated micro-arc oxidation-blackening treatment of aluminum alloy surface under constant-current condition of AC power supply [6] showed that vanadate has a decisive role in the blackening effect of micro-arc oxidation ceramic film; the black ceramic film is stable in color, has a high micro-hardness, and provides effective corrosion protection against the base metal; the main elemental compositions of the black ceramic film include O, Al, Si, V, and P, and the film The main elemental composition of the black ceramic film includes O, Al, Si, V and P. The compounds mainly exist in amorphous and/or microcrystalline forms, and only a small amount of γ-Al2O3 and ε-Al2O3 crystals are found; the black ceramic film is a relatively loose monolayer structure, and its surface is rough and uneven on the microscopic scale, and there are dense micropores of μm size, and obvious high-temperature sintering traces and micro-cracks; the microstructure of the black ceramic film is related to its formation mechanism.
ZL101 cast aluminum 2 silicon alloy micro-arc oxidation ceramic membrane [7] growth is divided into three stages, oxidation of the early stage, the current density is higher, but the membrane growth is slower. In the rapid growth stage of the membrane, the membrane growth rate reaches a great value; membrane growth into a stable period, basically remain constant, the external size of the sample is no longer increased, the membrane gradually turned to the internal growth of the substrate; the effect of alloying element silicon is mainly manifested in the early oxidation of the membrane growth of the hindering effect; casting aluminum alloy after micro-arc oxidation treatment, the corrosion current decreased dramatically, and the polarization resistance increased by a few orders of magnitude; a thinner Micro-arc oxidation film likewise substantially improve the corrosion resistance of aluminum - silicon alloys.
Neutral salt spray corrosion test method to study the high-strength casting aluminum alloy ZL205 micro-arc oxidation ceramic film [8] results show that the micro-arc oxidation treatment can significantly improve the corrosion resistance of ZL205, with the increase in the thickness of the ceramic film of corrosion resistance, but after the thickness reaches a certain value, the ceramic film of corrosion resistance is not obvious; with the increase in the thickness of the micro-arc oxidized film of the With the increase in thickness, the surface morphology and phase structure of the micro-arc oxidized film are changed, which leads to changes in the corrosion resistance of the micro-arc oxidized film.
2 electrodeposition
Electrodeposition (electrodeposition) is the process of electrochemical deposition of metals or alloys from their compounds in aqueous solution, non-aqueous solution or molten salt. Is the basis of metal electrolytic smelting, electrolytic refining, electroplating, electroforming process. These processes are carried out under certain electrolytes and operating conditions, and the ease of metal electrodeposition and the morphology of the deposits are related to the nature of the deposited metal, and also depend on the composition of the electrolyte, pH, temperature, current density and other factors. Wu Xiangqing et al [9] investigated the corrosion resistance of Ni2SiC composite coatings electrodeposited on the surface of ZL105 aluminum alloy using electrochemical methods. The results show that the surface morphology of Ni2SiC composite plating is very different from that of pure Ni plating, and the corrosion resistance is better than that of pure Ni plating, and the corrosion resistance is further improved after heat treatment at 300℃×2h.
3 multi-arc ion plating
Multi-arc ion plating is a vacuum chamber, the use of gas discharge or evaporated material partially dissociated, in the gas ions or evaporated material particles bombardment effect at the same time, will be evaporated or reactive material deposited on the substrate. Ion plating the glow discharge phenomenon, plasma technology and vacuum evaporation of the three organic combination, not only can significantly improve the quality of the film, but also to expand the scope of application of the film.
The advantages are strong adhesion of the film, good bypassing radiation, a wide range of film materials and so on. There are many types of ion plating, evaporation far heating methods such as resistance heating, electron beam heating, plasma electron beam heating, high-frequency induction heating. Multi-arc ion plating uses arc discharge, not the traditional ion plating glow discharge for deposition. Simply put, the principle of multi-arc ion plating is to use the cathode target as the evaporation source, and through the arc discharge between the target and the anode shell, the target material is evaporated, so as to form a plasma in the space, and the substrate is deposited. Multi-arc ion plating of Ti-Cr-N coating on the surface of ZL201 aluminum alloy and preparation of a lipid film on the Ti-Cr-N coating [10]. The results show that: the Cr in the Ti-Cr-N coating exists in the TiN crystals as a solid solution, and there is no formation of a separate CrN phase; the coating can effectively improve the resistance to salt spray corrosion of ZL201 aluminum alloy.
4 chemical composite plating
In the plating solution to add non-water-soluble solid particles, so that it and the main metal **** with the deposition of the formation of plating process is called composite plating. If the process of electroplating is called composite plating; if the process of chemical plating is called composite chemical plating. The resulting plated layer is called a composite plated layer. In principle, where the metal can be plated can be used as the main metal, but more research and application is nickel, chromium, cobalt, gold, silver, copper and other metals. As solid particles are mainly two types, one is to improve the wear resistance of the plating layer of high hardness, high melting point of the particles; one is to improve the plating layer of self-lubricating properties of the solid lubricant particles. In the cast aluminum surface preparation Ni-P-diamond chemical composite plating [11], the results show that high cerium sulfate can promote diamond particles into the plating layer, with the high cerium sulfate content increases the plating solution stability increased dramatically after the tendency to stabilize, Ni-P-diamond composite plating wear resistance is better than the Ni-P plating layer, after adding 2mg/L high cerium sulfate to further significantly improve the composite plating layer compared to the Ni-P coating Compared with Ni-P plating, the corrosion resistance of composite plating is poor, which is improved after adding high cerium sulfate.
5 chemical conversion film
Chemical conversion film is to make the metal in contact with a specific corrosive solution, under certain conditions of chemical reaction, the formation of a layer of good adhesion on the surface of the metal, insoluble generator film layer. These film layers, or can protect the base metal from the effects of water and other corrosive media, or can improve the organic coating adhesion and aging resistance, or can give the surface other properties. Chemical conversion films are generated as a result of the direct participation of the base metal in the film-forming reaction, and thus have a much greater bond to the substrate than electroplated and chemically coated layers. Almost all metals can be in the selected medium through the transformation process, to get different applications for the purpose of chemical conversion film, but the current industrial applications are more steel, aluminum, zinc, copper, magnesium and its alloys. Chemical conversion film with the metal on the other cover layer (such as metal electrodeposition layer) is not the same, its generation must have the direct participation of the base metal, and the medium anion generated by its own transformation products (MmAn), so it can also be said that the formation of chemical conversion film can actually be regarded as a controlled corrosion of the metal process. Chemical conversion membrane according to the membrane of the main components of the type is divided into: oxide film, phosphate film, chromate film, oxalate film, etc..
Aluminum alloys are susceptible to intergranular corrosion and damage under atmospheric conditions. The current application of high-strength casting aluminum alloy generally contains silicon, copper, magnesium and other elements, the addition of these elements increases the corrosion sensitivity of the alloy. Secondly, the surface hardness is low, easy to wear and tear, the appearance of gloss can not be maintained for a long time, so it requires a high degree of protection measures. Among them, the generation of chemical conversion film on the surface of aluminum alloys has the advantages of simple equipment, low cost, and provincial investment. Peng Liang et al [12] used chromate method to generate chemical conversion
film on Y112 alloy, the experimental results show that the conversion film has a high corrosion resistance, and has a beautiful golden yellow surface.
With manganate and zirconium salts as the main salts, the chemical oxidation of the chemical oxidation film obtained on the surface of aluminum alloy [13] has a corrosion potential of about 0.45V positive than that of the aluminum alloy specimen, and the density of corrosion current is only 0.286 μA/cm2; the value of impedance at the low-frequency end of the AC impedance spectrogram is one order of magnitude larger than that of the aluminum alloy specimen; and the chemical oxidation film of the aluminum alloy has a golden-yellow appearance, with a The appearance of the chemical oxide film of aluminum alloy is golden yellow with regularly arranged columnar growth structure.
Ge Shengsong et al [14] used a chromium-free chemical method to produce a black conversion film on the surface of cast aluminum alloy, and evaluated the corrosion resistance of the film using the dot-drop test. Scanning electron microscopy and electron microprobe were used to observe the morphology of the film, determine its constituent elements, and finally proposed the formation mechanism and corrosion resistance mechanism of the black film.
6 Conclusion
The surface corrosion resistance treatment of cast aluminum alloys can be improved by electrochemical methods. Existing research mostly stays on specimens, with fewer application studies. In practical application, alone with a process technology can improve the protective, decorative and functional casting aluminum alloy is relatively rare, there is a need for existing modification technology comprehensive consideration, to carry out a systematic study. Cast aluminum alloy surface corrosion resistance improvement and wear resistance improvement of the comprehensive study is more meaningful.