The quality of the plating obtained by electroplating of inorganic molten salt is excellent, but generally it needs a temperature of 100° C. or more and is controlled by the atmosphere. This increases the difficulty of plating, and thus has certain limitations and is not easily converted to industrial production. So people began to research and develop the lower temperature molten salt - organic molten salt for electroplating aluminum.
2.2 Organic Molten Salt System
Among the studied A1C13-organic molten salt systems, the most studied and representative organic salts are: (1) Halogenated alkyl pyridines such as ethyl bromide (EPB), n-butyl pyridine chloride ( BPC); (2) Halogenated alkyl imidazolines: such as 1-methyl-3-ethylimidazoline chloride (MEIC), 1,2-1,2-methyl-3-propyl imidazoline chloride (DMPIC) (3) Alkylammonium chlorides such as trimethylphenylammonium chloride (TMPAC).
Among these A1C13-organic salt systems, the earliest found to maintain a stable molten state at room temperature was a 2:1 (molar ratio) A1C13-EPB mixture. WHSafranek et al. [21, 22] were obtained at room temperature using an electrolyte consisting of 32% mixture of A1C13-EPB (molar ratio 2:1), 67% toluene, and 1% methyl-t-butyl-ether. Thick aluminum coating of 0.1mm to 1mm. However, when the mixture is exposed to light, it is easily decomposed, and a slight change in the ratio causes a sharp increase in the melting point.
At the end of the 1970s, J. Robsion et al. [23] reported that A1C13-BPC can maintain its liquid state at room temperature in the entire range of 2:1 to 0.75:1 (molar ratio), and it has been widely used. Among them, Takahashi Noriko et al. [24] obtained a bright 99.99% pure aluminum coating with the developed A1C13-BPC molten salt. However, since the pyridine cations are prone to reduction reactions, the cathode stability is limited, and it is far from being able to meet the needs of electrochemical research electrolytes.
In 1982, J.S.Wilkes [25] and others reported the molten salt systems of halogenated alkyl imidazolines and A1C13, broadening the composition range of room temperature molten salts and electrochemical window. Among them, the molten salt of A1C13-MEIC is the most studied. R.T. Carlin et al. [26] obtained a dense, flat, corrosion-resistant aluminum coating at room temperature using a 2:1A1C13-MEIC molten salt. However, such salts have difficulties in preparation, particularly in the preparation of the molten salt A1C13-MEIC, and if a reaction rate is not controlled in advance, a strong exothermic reaction of MEIC and A1C13 causes decomposition of the MEIC.
In 1989, S.D. Jone [27] proposed a new molten salt system of A1C13-aryl ammonium chloride which is easily available and simple in preparation. Among them, the chemical stability of molten salt of A1C13-TMPAC is the same as that of A1C13-MEIC, and its electrical conductivity is comparable to that of A1C13-BPC, and it is easily available, inexpensive, and high in purity. It is very suitable as an electrolyte for electrochemical research. Zhao Yuguan et al. [28] studied the room-temperature aluminum electrodeposition of a molten Al1C13-TMPAC salt at a tungsten electrode of 2:1 (molar ratio). The deposition of aluminum was found to be three-dimensional instantaneous ridged and hemispherical diffusion-controlled growth, and the coating consisted of several monoatomic layers.
The research of organic aluminum molten salt has opened up a new way for aluminum electroplating, which has significant advantages compared with inorganic molten salt aluminum plating and organic solvent aluminum plating, so that electroplating aluminum can be carried out at room temperature.
2.3 Reaction Mechanism of Molten Salt System
(1) Reaction mechanism of NaC1-KC1 molten salt system
Anode reaction: Al-3e=Al3+
Cathodic reaction: Al3++3e=A1
After the aluminum of the anode melts, it enters the molten salt to form aluminum ions. Under the effect of high temperature and anode current, the molten aluminum reacts at the interface of the molten salt to form an aluminum-molten salt reaction. The generated aluminum ions enter the molten salt and move toward the cathode. . Due to the presence of Al3+ concentration difference and electric field in the molten salt, Al3+ is further promoted to migrate to the surface of the cathode, and aluminum atoms that obtain electrons to become active on the cathode substrate crystallize. The active material when electroplating aluminum at high temperature molten salt is mainly Al3+. Du Daobin [12] thinks that the cathode reaction is: AlCl4-+3e=A1+4C1-. The active aluminum atom A1 is deposited on the cathode substrate and forms an aluminized layer by diffusion at a high temperature.
(2) Reaction mechanism of molten salt system of A1C13-NaCl and A1C13-NaC1-KC1
The main ionic particles present in A1C13-NaCl and A1C13-NaC1-KC1 systems are Na+, AlCl4-Al2Cl7-, and Cl-. The following equilibriums exist between the anion particles:
2AlCl4-Al2Cl7-+Cl-
In this type of system, A1C13 content of 50 mol% of the melt is neutral, A1C13 content of less than 50 mol% of the melt is basic, aluminum-containing ions are mainly in the form of AlCl4-, and A1C13 of greater than 50 mol% of melt. The body is acidic, and the aluminum-containing ions exist mainly in the form of Al2Cl7-.
The reaction mechanism of this type of system:
Alkaline melts:
Anode reaction: Al-3e=Al3+
Cathodic reaction: A1Cl4-+3e Al+4Cl-
In acidic melts:
Anode reaction: Al-3e=Al3+
Cathodic reaction: 4A12Cl7-+3e Al+7AlCl-4
(3) Organic Molten Salt System Reaction Mechanism
The following balances also exist in the A1C13-organic salt molten salt system:
2AlCl4-Al2Cl7-+Cl-
In the melt of A1C13: organic salt (molar ratio) of 1:1, AlCl4- is predominantly present, and very few Al2Cl7-, and AlCl3 is added to the 1:1 melt to form Al2Cl7-complex anion with AlCl4-. A1C13: Organic salt is 2:1 Melt is mainly Al2Cl7-complex anion.
The reaction mechanism of this type of system:
A1C13: When the organic salt (molar ratio) is ≤ 1:
Anode reaction: Al-3e=Al3+
Since in all organic molten salts, the organic anion is also negative than the reduction potential of AlCl4-, the cathodic reaction of this type of system does not result in the deposition of aluminum, but the reduction of organic cations.
A1C13: When organic salt (molar ratio)>1:
Anode reaction: Al-3e=Al3+
Cathodic reaction: 4A12Cl7-+3e Al+7AlCl-4
Therefore, in the A1C13-organic salt molten salt system, aluminum can only be deposited by electrolysis in an acidic system, while the alkaline system cannot.
3 Conclusion
Aluminum electroplating research has been carried out for many years, and many breakthroughs have been made in organic solvent systems and molten salt systems. However, because of the special electrochemical properties of aluminum, the entire operation of electroplating aluminum must be in the sealing device. In the protective gas protection, waterless environment, and various components of the plating solution is also difficult to obtain, so the process of electroplating aluminum is very complex, difficult to operate, and high cost, so that the aluminum plating It has not been widely used in industry and daily life, and the focus of future research should be on the further development of a simple low-temperature organic molten salt system for raw material synthesis and simple preparation so that electroplating aluminum can be applied more widely.
2.2 Organic Molten Salt System
Among the studied A1C13-organic molten salt systems, the most studied and representative organic salts are: (1) Halogenated alkyl pyridines such as ethyl bromide (EPB), n-butyl pyridine chloride ( BPC); (2) Halogenated alkyl imidazolines: such as 1-methyl-3-ethylimidazoline chloride (MEIC), 1,2-1,2-methyl-3-propyl imidazoline chloride (DMPIC) (3) Alkylammonium chlorides such as trimethylphenylammonium chloride (TMPAC).
Among these A1C13-organic salt systems, the earliest found to maintain a stable molten state at room temperature was a 2:1 (molar ratio) A1C13-EPB mixture. WHSafranek et al. [21, 22] were obtained at room temperature using an electrolyte consisting of 32% mixture of A1C13-EPB (molar ratio 2:1), 67% toluene, and 1% methyl-t-butyl-ether. Thick aluminum coating of 0.1mm to 1mm. However, when the mixture is exposed to light, it is easily decomposed, and a slight change in the ratio causes a sharp increase in the melting point.
At the end of the 1970s, J. Robsion et al. [23] reported that A1C13-BPC can maintain its liquid state at room temperature in the entire range of 2:1 to 0.75:1 (molar ratio), and it has been widely used. Among them, Takahashi Noriko et al. [24] obtained a bright 99.99% pure aluminum coating with the developed A1C13-BPC molten salt. However, since the pyridine cations are prone to reduction reactions, the cathode stability is limited, and it is far from being able to meet the needs of electrochemical research electrolytes.
In 1982, J.S.Wilkes [25] and others reported the molten salt systems of halogenated alkyl imidazolines and A1C13, broadening the composition range of room temperature molten salts and electrochemical window. Among them, the molten salt of A1C13-MEIC is the most studied. R.T. Carlin et al. [26] obtained a dense, flat, corrosion-resistant aluminum coating at room temperature using a 2:1A1C13-MEIC molten salt. However, such salts have difficulties in preparation, particularly in the preparation of the molten salt A1C13-MEIC, and if a reaction rate is not controlled in advance, a strong exothermic reaction of MEIC and A1C13 causes decomposition of the MEIC.
In 1989, S.D. Jone [27] proposed a new molten salt system of A1C13-aryl ammonium chloride which is easily available and simple in preparation. Among them, the chemical stability of molten salt of A1C13-TMPAC is the same as that of A1C13-MEIC, and its electrical conductivity is comparable to that of A1C13-BPC, and it is easily available, inexpensive, and high in purity. It is very suitable as an electrolyte for electrochemical research. Zhao Yuguan et al. [28] studied the room-temperature aluminum electrodeposition of a molten Al1C13-TMPAC salt at a tungsten electrode of 2:1 (molar ratio). The deposition of aluminum was found to be three-dimensional instantaneous ridged and hemispherical diffusion-controlled growth, and the coating consisted of several monoatomic layers.
The research of organic aluminum molten salt has opened up a new way for aluminum electroplating, which has significant advantages compared with inorganic molten salt aluminum plating and organic solvent aluminum plating, so that electroplating aluminum can be carried out at room temperature.
2.3 Reaction Mechanism of Molten Salt System
(1) Reaction mechanism of NaC1-KC1 molten salt system
Anode reaction: Al-3e=Al3+
Cathodic reaction: Al3++3e=A1
After the aluminum of the anode melts, it enters the molten salt to form aluminum ions. Under the effect of high temperature and anode current, the molten aluminum reacts at the interface of the molten salt to form an aluminum-molten salt reaction. The generated aluminum ions enter the molten salt and move toward the cathode. . Due to the presence of Al3+ concentration difference and electric field in the molten salt, Al3+ is further promoted to migrate to the surface of the cathode, and aluminum atoms that obtain electrons to become active on the cathode substrate crystallize. The active material when electroplating aluminum at high temperature molten salt is mainly Al3+. Du Daobin [12] thinks that the cathode reaction is: AlCl4-+3e=A1+4C1-. The active aluminum atom A1 is deposited on the cathode substrate and forms an aluminized layer by diffusion at a high temperature.
(2) Reaction mechanism of molten salt system of A1C13-NaCl and A1C13-NaC1-KC1
The main ionic particles present in A1C13-NaCl and A1C13-NaC1-KC1 systems are Na+, AlCl4-Al2Cl7-, and Cl-. The following equilibriums exist between the anion particles:
2AlCl4-Al2Cl7-+Cl-
In this type of system, A1C13 content of 50 mol% of the melt is neutral, A1C13 content of less than 50 mol% of the melt is basic, aluminum-containing ions are mainly in the form of AlCl4-, and A1C13 of greater than 50 mol% of melt. The body is acidic, and the aluminum-containing ions exist mainly in the form of Al2Cl7-.
The reaction mechanism of this type of system:
Alkaline melts:
Anode reaction: Al-3e=Al3+
Cathodic reaction: A1Cl4-+3e Al+4Cl-
In acidic melts:
Anode reaction: Al-3e=Al3+
Cathodic reaction: 4A12Cl7-+3e Al+7AlCl-4
(3) Organic Molten Salt System Reaction Mechanism
The following balances also exist in the A1C13-organic salt molten salt system:
2AlCl4-Al2Cl7-+Cl-
In the melt of A1C13: organic salt (molar ratio) of 1:1, AlCl4- is predominantly present, and very few Al2Cl7-, and AlCl3 is added to the 1:1 melt to form Al2Cl7-complex anion with AlCl4-. A1C13: Organic salt is 2:1 Melt is mainly Al2Cl7-complex anion.
The reaction mechanism of this type of system:
A1C13: When the organic salt (molar ratio) is ≤ 1:
Anode reaction: Al-3e=Al3+
Since in all organic molten salts, the organic anion is also negative than the reduction potential of AlCl4-, the cathodic reaction of this type of system does not result in the deposition of aluminum, but the reduction of organic cations.
A1C13: When organic salt (molar ratio)>1:
Anode reaction: Al-3e=Al3+
Cathodic reaction: 4A12Cl7-+3e Al+7AlCl-4
Therefore, in the A1C13-organic salt molten salt system, aluminum can only be deposited by electrolysis in an acidic system, while the alkaline system cannot.
3 Conclusion
Aluminum electroplating research has been carried out for many years, and many breakthroughs have been made in organic solvent systems and molten salt systems. However, because of the special electrochemical properties of aluminum, the entire operation of electroplating aluminum must be in the sealing device. In the protective gas protection, waterless environment, and various components of the plating solution is also difficult to obtain, so the process of electroplating aluminum is very complex, difficult to operate, and high cost, so that the aluminum plating It has not been widely used in industry and daily life, and the focus of future research should be on the further development of a simple low-temperature organic molten salt system for raw material synthesis and simple preparation so that electroplating aluminum can be applied more widely.
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