Aluminum is a metal element with a high content in the earth's crust, accounting for 7.5% (mass fraction). It not only has metal luster, corrosion resistance and other properties, but also light, non-toxic, heat conductive, conductive, made of aluminum or aluminum alloy surface coating of various metal materials, can be corrosion-resistant, beautiful, and has excellent mechanical properties Composite material. However, aluminum is a very active metal. Its standard electrode potential is −1.66 V, which is also negative compared to hydrogen. Therefore, electrodeposition of aluminum cannot be achieved in an aqueous solution of aluminum salt, but only in a non-aqueous solution. At home and abroad, a large number of non-aqueous solution electroplating aluminum has been studied, and has developed two basic systems: organic solvent system and molten salt system, molten salt system is divided into inorganic molten salt system and organic molten salt system. This article will review these two systems.
1 Organic solvent system
The research of electroplating aluminum was carried out in organic solvent system earlier. The application of this system was earlier and the process was relatively mature. However, the disadvantages were that the preparation of the electroplating solution was complicated, the performance was unstable, and the organic solvent was volatile, flammable and pungent. And toxicity.
1.1 Several organic solvent systems
Representative electroplating aluminum organic solvent formulations have been developed: aluminum chloride-LiH-ether, triethylaluminum (TEA)-NaF-toluene, aluminum chloride-lithium aluminum tetrahydroaluminate (LiAlH4)-tetrahydrofuran (THF) , aluminum chloride - n-butylamine - diethyl ether, AlBr3-alkyl benzene solvents (such as toluene, ethylbenzene, xylene).
In the aluminum chloride-LiH-ether system [1], the ideal formulation consists of 1L of ether solvent plating solution AlCl3: 2mol~3mol, LiH: 0.5mol~l.0mol, operating temperature is room temperature. When the current density is 2 A/dm2 to 5 A/dm2, a plating layer of 0.5 mm to 0.75 mm can be obtained at a deposition rate of 25 μm/h to 50 μm/h. The role of LiH as an additive in this set of formulations helps to improve the conductivity of the bath.
DRDotzer [2] studied the triethylaluminum-NaF-toluene system with a chemical composition of 2 mol of triethylaluminum, 1 mol of NaF and 3.35 mol of toluene. At an average current density of 0.5 A/dm 2 to 5 A/dm 2 at 80° C. to 95° C., the average deposition rate of the aluminum plating layer is 10 μm/h to 20 μm/h. The specific conductivity of the electrolyte has a great influence on the quality of the coating, and its value mainly depends on the ratio of NaF to TEM, the amount of toluene and the temperature.
T. Daenen et al. [3] obtained a dense, good luster, and good adhesion aluminum layer at room temperature using a plating solution composed of aluminum chloride-lithium tetrahydroaluminum-tetrahydrofuran. The corresponding deposition rate at a current density of 1 A/dm2 was 12.4 μm/h. During the plating process, two compounds, LiA1C14 and A1HCl2, play a key role. The larger the ratio, the better the quality of the resulting coating and the greater the deposition rate.
IAMercies and DBSalt at a current density of 3.54 A/dm2 at a temperature of 20°C were electrolyzed from aluminum chloride-n-butylamine diethyl ether (aluminum chloride: 36.7 wt%, n-butylamine: 12.28 wt%, diethyl ether 51.02 wt The electrolyte of the composition of %) obtains an aluminum coating of at least 0.004 mm thickness. The key to good plating is the use of high purity anhydrous AlCl3.
For the A1Br3-alkylbenzene solvent system [5-7], the better effect is A1Br3-toluene-ethylbenzene, a typical formulation with a mass ratio of 2:1:1. Using this formula, GACapuano et al. At 4A/dm2, bright colors, strong adhesion, and dense aluminum were obtained.
1.2 Reaction Mechanism of Organic Solvent System
The organic solvent system forms a cyclic mechanism during the electroplating process, depositing aluminum later. Taking the widely used aluminum chloride-lithium tetrahydroaluminate (LiAlH4)-tetrahydrofuran (THF) as an example, the following reaction occurs during the formation of this system:
4AlCl3+LiAlH4→4AlHCl2+LiAlCl4
The electrode reaction is the discharge of AlCl2 on the cathode to deposit aluminum:
AlHCl2+3e→H-+2C1-+Al
At the same time, the free H- ions react with AlCl3 and regenerate AlHCl2. Therefore, electroplating aluminum is performed according to this "circulation mechanism". That is 〔3〕:
AlHCl2+3e→H-+2C1-+Al
↑ ↓
3AlCl4-+AlHCl2â†-H-+2C1-+AlCl3
The use of organic solvent aluminum plating, can be operated at temperatures below 100 °C, will not affect the mechanical properties of the matrix material, electrodeposition process will not produce hydrogen and will not produce corrosive products, high current efficiency. However, with the continuous development of aluminum electroplating research, people found that the electroplating of aluminum in organic solvents has a lot of operational inconvenience, and the quality of the obtained aluminum coating is unstable, so he began to seek another non-water system - molten salt plating aluminum.
2 molten salt system
2.1 Inorganic molten salt system
(1) NaCl-KCl system
Aluminum plating with NaC1-KC1 molten salt is characterized by high temperatures. The melting point of NaC1 is 801°C, the melting point of KC1 is 776°C, the melting point of the molten salt after mixing is 750°C, and aluminum plating is generally performed at 900°C.
The ideal molten salt composition of this system is that the molar fraction of NaCl and KCl is 1:1. Many domestic and foreign workers have studied this system. Godshall [9] used an equimolar fraction of the NaC1-KC1 molten salt system to achieve electrolytical aluminizing of nickel-based superalloys at 800°C. The results of the study show that the quality of the aluminized layer can be improved only by adding a small amount of fluoride to the molten salt system.
Shi Shengtai [10] also used the same molten salt system to study the process of electrolytic aluminum infiltration on industrial pure iron. The molar fraction ratio of NaC1 to KC1 in the mixed molten salt is 1:1, and a small amount of cryolite is added, and electrolytic aluminizing is performed at a current density of 900° C. and 1.5 A/dm 2 to 2 A/dm 2 . Experiments show that the weight gain of the aluminized layer is determined by the current density and time and is independent of temperature.
Deng Zhiping and Wu Shansu [11] studied the electrolytic aluminizing of Ni-base superalloys in the temperature range of 800°C to 900°C with an equimolar fraction of NaC1-KC1 molten salt containing cryolite. The results show that diffusion of aluminum atoms into the interior of the matrix is ​​a speed control step throughout the process. In the electrolytic aluminizing process, the diffusion current is inversely proportional to the square root of the electrolysis time.
Du Daobin [12] studied the initial deposition, growth and phase structure of aluminum during electrolysis of aluminum in industrial pure iron at 900 °C and current density of 1.5 A/dm2. The results show that the selective segregation of aluminum occurs at the initial stage of deposition. With the prolongation of the time of aluminizing, aluminum continues to deposit, and it aggregates and expands. Its morphology is spherical.
(2) AlCl3-NaCl
Molten salt system A1C13-NaC1 molten salt system is a very active branch of molten salt theory and technology research in recent years. The molten salt system has a low melting point and its eutectic temperature is 175° C. (AlCl 3 : 80 wt %, NaCl: 20 wt %).
Li Qingfeng et al. [13] studied the electroplating of aluminum on the glassy carbon electrode with A1C13-NaC1 molten salt. The electroplating molten salt composition used was: A1C 1350.1 mol%, NaC 149.9 mol%, and a temperature of 175C. It was found that when the current density is lower than 0.07 A/dm2, a floccular aluminum coating is obtained; when the current density is 0.2 A/dm2 to 1 A/dm2, a smooth aluminum coating is obtained; and when the current density is higher than 1.5 A/dm2, Dendritic or porous aluminum deposits.
B. Nayak et al. [14] conducted aluminum electroplating tests on brass with A1C13-NaC1 molten salt. In the molten salt, the mass fractions were 80% A1C13 and 20% NaC1, respectively. In the temperature range of 135°C to 195°C, a smooth, dense, silver-white aluminum coating is obtained at different current densities by using a rotating cathode method. They also studied aluminum electroplating of mild steel using the same molten salt system [15]. It was found that the rotating cathode significantly improves the quality of the aluminum coating and increases the critical current density of dendritic growth.
Niu Hongjun et al. [16] conducted a preliminary study on the microscopic morphology and corrosion resistance of the aluminum coating of the A1C13-NaC1 molten salt system using brass as the matrix material. The mass fraction of A1C13 in the molten salt ranged from 70% to 80%, the plating temperature range was from 180°C to 200°C, the time of electrolytic aluminizing was 30 minutes, and the cathode current density was 1.39 A/dm2. The results show that the cathode current density has a great influence on the morphology of the electroplated aluminum layer. When the current density is 1.39 A/dm2, the aluminum coating is uniform and fine equiaxed grains. With the increase of current density, the grain size becomes larger and larger, and it becomes more and more irregular and more and more inhomogeneous. When the current density reaches 4.17 A/dm2, the crystal grains grow and have a certain directionality. When the current density is as high as 5.56 A/dm2, a distinct dendritic structure appears.
(3) AlCl3-NaCl-KCl
Molten Salt System The ternary A1C13-NaC1-KC1 molten salt system is an inorganic molten salt system that has been studied in recent years. Compared with A1C13-NaC1 molten salt system, the eutectic point of A1C13-NaC1-KC1 molten salt system is lower, which is 98°C (A1C13, 60mol%; NaC1, 28mol%; KC1, 12mol%).
Feng Qiuyuan et al. studied the possibility of electroplating of Q235 steel in A1C13-NaC1-KC1 molten salt and the effect of electroplating on the microstructure of the electroplated aluminum layer by molten salt electroplating. The results show that Q235 steel can be electroplated in molten salt. X-ray diffraction analysis showed that the phase structure of the coating was single-phase aluminum. The thickness of the plating increases with the increase of the current density and the plating time, and has a linear relationship with the square root of the plating time.
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