Abstract:
In the plating for electrodeposition of aluminum using a non-aqueous solution, a plating bath is prepared to comprise a low-melting plating bath, which comprises a molten mixture comprising an aluminum halide and a dialkyl-and/or trialkylpyridinium halide represented by the following formula: ##STR1## wherein R 1  represents an alkyl group having 1 to 12 carbon atoms, R 2  represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, R 3  represents an alkyl group having 1 to 6 carbon atoms, and X represents a halogen atom; and said alkyl groups each refer to a straight-chain hydrocarbon group, a branched hydrocarbon group, an alicyclic hydrocarbon group, or any of these further partly containing an aromatic hydrocarbon group, 
     by which the bath life, operability in handling, conductivity, and current efficiency can be improved.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a plating bath for electrodeposition of aluminum, having a long bath life, also having a good operability in handling, and yet having a high current efficiency and conductivity, and a plating process making use of the bath. 
     Plating for electrodeposition of aluminum can be carried out with difficulty using a plating bath of an aqueous solution type, because of a large affinity of aluminum for oxygen and a lower potential thereof than hydrogen. For this reason, the plating for electrodeposition of aluminum has been hitherto carried out using a plating bath of a non-aqueous solution type, in particular, a plating bath of an organic solvent type. 
     This plating bath of an organic solvent type typically includes baths comprising AlCl 3  and LiAlH 4  or LiH dissolved in ether, and those comprising AlCl 3  and LiAlH 4  dissolved in tetrahydrofuran (see, for example, D.E. Couch et al. Electrochem., Vol. 99, (6), p.234). 
     All of these plating baths, however, contain very active LiAlH 4  or LiH in the bath, and hence it may react with oxygen or water which may exist therein, to decompose, resulting in a lowering of current efficiency and also a shortened bath life. They also have the problem that they use organic solvents having so a low boiling point that they are highly in danger of explosion or burning. As other plating baths of an organic solvent type, a plating bath has been proposed, comprising triethylaluminum and NaF dissolved in toluene (R. Suchentrunk, X. Werkstofftech., Vol. 12, p.190). In this instance also, however, there is a problem in handling the triethylaluminum, which is highly dangerous, and it is considered difficult to put the bath into practical use in an industrial scale. 
     As discussed in the above, the conventional techniques have succeeded for the time being in settling the technical subject that the plating for electrodeposition of aluminum is carried out, but, because of employment of chemical substances which are difficult to handle, they all have disadvantages in the bath life and operability, leaving problems in widely and commonly using them as techniques for practical use. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide a plating bath for electrodeposition of aluminum, that has a long bath life and promises the safety in handling, and a plating process making use of the bath. 
     Another object of the present invention is to provide a plating bath for electrodeposition of aluminum, that has a high current efficiency and conductivity. 
     The present inventors made intensive studies to develop a new plating bath for electrodeposition of aluminum and plating process making use of the bath, that can solve the problems in the prior art, and as a result found that the problems can be solved by using a plating bath comprising a molten mixture comprising an aluminum halide and dialkyl- and/or trialkylpyridinium halide(s) (or at least one of a dialkylpyridinium halide and a trialkylpyridinium halide). 
     The aluminum halide and the dialkyl- and/or trialkylpyridinium halide(s), when mixed and melted, are made into a low-melting molten salt bath that turns into a liquid even at room temperature under a wide compositional range. This plating bath contains no chemical substances which are chemically active, so that it can have a long bath life and can be free from dangers such as explosion and burning, resulting in good operability in handling. The bath also undergoes electrolytic dissociation into di(tri)alkylpyridinium cations of quaternary ammonium ions and Al complex anions in a low-temperature molten state, showing a reasonably high ionic conductivity, so that the plating can be carried out in a high current efficiency even at a high current density of 50 A/dm 2 . 
     Here, the dialkyl- and/or trialkylpyridinium halide(s) used in the plating bath is a compound represented by the following general formula: ##STR2## wherein R 1  represents an alkyl group having 1 to 12 carbon atoms, R 2  represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, R 3  represents an alkyl group having 1 to 6 carbon atoms, and X represents a halogen atom; and said alkyl groups each refer to a straight-chain hydrocarbon group, a branched hydrocarbon group, an alicyclic hydrocarbon group, or any of these further partly containing an aromatic hydrocarbon group. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To show specific examples of these di(tri)alkylpyridinium salts, they include 1,2-dimethylpyridinium chloride, 1-ethyl-2-methylpyridinium chloride, 1-ethyl-2-methylpyridinium bromide, 1-ethyl-2-methylpyridinium iodide, 1-ethyl-2-methylpyridinium fluoride, 1-n-butyl-2-methylpyridinium chloride, 1-isobutyl-2-methylpyridinium chloride, 1-n-octyl-2-methylpyridinium chloride, 1-benzyl-2-methylpyridinium chloride, 1-ethyl-3-methylpyridinium chloride, 1-ethyl-3-methylpyridinium bromide, 1-cyclohexyl-3-methylpyridinium bromide, 1-ethyl-2-ethylpyridinium chloride, 1-butyl-2-ethylpyridinium chloride, 1-ethyl-3-methylpyridinium bromide, 1-ethyl-3-phenylpyridinium bromide, 1-ethyl 2,4-dimethylpyridinium chloride, 1-ethyl-2,6-dimethylpyridinium chloride, and 1-n-butyl-2,4-dimethylpyridinium chloride. 
     The aluminum halide refers to a compound represented by the general formula: AlX 3  (X is halogen), and specifically includes AlF 3 , AlCl 3 , AlBr 3  and AlI 3 . 
     These aluminum halide and dialkyl- and/or trialkylpyridinium halide(s) may be mixed in the proportion of from 20 to 80 mol % of the aluminum halide and from 20 to 80 mol % of the dialkyl- and/or trialkylpyridinium halide(s), thereby being made into a low-melting plating solution. For example, in a plating solution comprising a mixture of aluminum chloride with a 1-ethyl-3-methylpyridinium halide, it can be liquid at room temperature over the whole range of from 20 to 80 mol % of the aluminum chloride, showing a reasonably low viscosity. 
     However, to efficiently carry out the plating for electrodeposition of aluminum, the aluminum halide may be mixed in the proportion of from 50 to 75 mol %, and more preferably from 55 to 70 mol %; and the dialkyl- and/or trialkylpyridinium halide(s), from 25 to 50 mol %, and more preferably from 30 to 45 mol %. In a system containing the aluminum halide in an excessively small proportion, a reaction presumed to be decomposition of dialkyl- and/or trialkylpyridinium cations may take place in carrying out the electroplating, and, in a system containing the aluminum halide in an excessively large proportion, the viscosity of the plating bath tends to increase, undesirably. 
     The plating bath of the present invention is prepared by mixing and melting the aluminum halide and an N-alkylpolyalkylpyridinium halide, and in this instance can be prepared through the two steps described below. 
     First step: 
     An alkyl halide and a polyalkylpyridine are charged together with a reaction solvent in an autoclave provided with a stirrer, and heated to 30 to 200° C., and more preferably 50 to 150° C., to carry out reaction to form them into a quaternary ammonium salt. After the reaction, the solvent and unreacted matters are removed to prepare an Nalkylpolyalkylpyridinium halide. The reaction solvent that can be used here includes hydrocarbons such as benzene, toluene and hexane, and polar solvents such as water, methanol, ethanol, tetrahydrofuran, dimethylformamide and dimethyl sulfoxide. 
     Second step: 
     The N-alkylpolyalkylpyridinium halide prepared in the first step and an aluminum halide are mixed in the given proportion and then heated in the atmosphere of an inert gas, or mixed under heating in a state that both are suspended in a suitable solvent, followed by removal of solvent, thus making a plating solution. In either instance, considerable heat generation accompanies the mixing of them, and hence it is necessary to take care not to cause reckless increase in temperature. 
     The plating for electrodeposition of aluminum using the plating bath of the present invention is carried out in a dry, oxygen-free atmosphere from the viewpoints of maintaining the stability of the plating bath and improving plating quality. As for plating conditions, the plating may be carried out using direct current or pulse current at a high temperature of from 0 to 300° C. and at a current density of from 0.01 to 50 A/dm 2 , so that the plating can be carried out uniformly with a good current efficiency. An excessively low bath temperature enables no uniform plating, and an excessively high bath temperature may cause the decomposition of quaternary ammonium cations, non-uniform coatings, and further a lowering of current efficiency, undesirably. 
     In instances in which a strip or the like is continuously plated, it is required to supply Al ions in a bath so that the Al ion concentration in the bath may be kept in a given range. In such instances, however, if the anode is comprised of a soluble anode made of aluminum, the Al ions can be automatically supplied in accordance with the amount of electrification, so that the Al ion concentration can be kept in a given range without supplying the aluminum halide, and also the bath composition may not loss its balance. 
     In instances in which the plating is carried out efficiently at low temperatures, a method is effective in which an organic solvent is added to lower the viscosity of the plating bath. In such instances, an inert solvent such as toluene, xylene, benzene or chlorobenzene may preferably be added as the organic solvent, and may be added in an amount of usually from 5 to 100 vol. %, so that the plating efficiency can be improved. 
     For the purpose of increasing the conductivity of the plating bath or making coatings uniform, it is also effective to add an alkali metal and/or alkaline earth metal halide(s). The alkali metal halide and alkaline earth metal halide in this instance may include LiCl, NaCl, NaF and CaCl 2 , and these compounds are added in the plating bath in an amount of usually from 0.1 to 30 mol %. 
     EXAMPLES 
     Example 1 
     In an autoclave made of stainless steel, 1.0 mol (93.1 g) of 3-picoline and 1.1 mol (119.9 g) of ethyl bromide, and 100 g of methanol as a solvent were charged, and then reacted at 100° C. for 8 hours with stirring. Using a rotary evaporator, the solvent and unreacted matters were evaporated from the reaction mixture to obtain 200.1 g of a solid product. This solid product was found to be 1-ethyl-3-methylpyridinium bromide, with the reaction yield of 99 mol % based on 3-picoline. 
     Next, 0.10 mol (20.2 g) of the 1-ethyl-3-methylpyridinium bromide obtained was put in a reaction vessel made of glass in a nitrogen atmosphere, into which 0.20 mol (26.67 g) of aluminum chloride was fed little by little. The feeding thereof caused reaction with the 1-ethyl-3-methylpyridinium bromide at their solid interfaces, and fusion gradually proceeded, accompanied, however, with considerable heat generation at the initial stage of the reaction, and accordingly the aluminum chloride was fed in its whole amount while taking care so as for the reaction temperature not to exceed 70° C. The resulting mixture was liquid at room temperature, and had a conductivity of 10.3 mS/cm (25° C). 
     In this system, the mixing molar ratio (A/B) of aluminum chloride (A) to 1-ethyl-3-methylpyridinium bromide (B) was varied from 1 to 2, but the mixture was kept liquid at room temperature within the whole molar ratio, and showed a high conductivity at a low temperature as shown by the conductivities in Table 1. Taking account of these, the mixture can be deemed to be excellent as the plating bath for electrodeposition of aluminum. 
     
                       TABLE 1______________________________________Relationship between molar ratio andconductivity (mS/cm, 25° C.).Temper-     Molar ratioature (°C.)       1.0          1.5    2.0______________________________________25          14.5         11.4   10.330          16.6         13.3   11.740          21.1         17.0   14.850          26.9         21.7   18.660          33.2         26.5   22.670          39.6         31.8   26.8______________________________________ 
    
     Examples 2 to 8 
     By the same reaction procedures as Example 1, 1-ethyl-3-methylpyridinium chloride was synthesized from 3-picoline and ethyl chloride (Example 2); 1-n-butyl-3-methylpyridinium chloride, from 3-picoline and n-butyl chloride (Example 3); 1-benzyl-3-methylpyridinium chloride, from 3-picoline and benzyl chloride (Example 4); 1-ethyl-3,5-dimethylpyridinium bromide, from 3,5-lutidine and ethyl bromide (Example 5); 1-ethyl-3,5-dimethylpyridinium chloride, from 3,5-lutidine and ethyl chloride (Example 6); respectively. 
     These di(tri)alkylpyridinium salts were mixed with aluminum chloride in the same manner as Example 1 to make plating baths for electrodeposition of aluminum, in which the molar ratio of aluminum chloride to di(tri)alkylpyridinium salts was 2. Also, using aluminum bromide in place of aluminum chloride, prepared were plating baths for electrodeposition of aluminum, in which the molar ratio of 1-ethyl-3-methylpyridinium bromide of Example 1 or 1-ethyl-3,5-dimethylpyridinium bromide of Example 5 to the aluminum bromide was 2 (Examples 7 and 8). 
     The plating baths thus prepared had the conductivity as shown in Table 2. 
     
                       TABLE 2______________________________________Conductivity of each plating bathEx-am-  Alumi-                           Conduc-ple  num                        Temp. tivityNo.  halide  Di(tri)alkylpyridinium halide                           (°C.)                                 (mS/cm)______________________________________2    AlCl.sub.3        1-Ethyl-3-methylpyridinium                           25    11.0        chloride           50    19.83    AlCl.sub.3        1-n-Butyl-3-methylpyridinium                           25    6.7        chloride4    AlCl.sub.3        1-Benzyl-3-methylpyridinium                           50    5.8        chloride5    AlCl.sub.3        1-Ethyl-3,5-dimethylpyridinium                           25    7.5        bromide            50    13.66    AlCl.sub.3        1-Ethyl-3,5-dimethylpyridium                           25    8.1        chloride           50    14.57    AlBr.sub.3        1-Ethyl-3-methylpyridinium                           50    10.1        bromide8    AlBr.sub.3        1-Ethyl-3,5-dimethylpyridinium                           50    7.3        bromide______________________________________ Note: The molar ratio of the aluminum halide to the di(tri)alkylpyridinium salt was 2 in each example. 
    
     Examples 9 to 18 
     A cold rolled sheet with a sheet thickness of 0.5 mm was subjected to solvent vapor cleaning, alkali degreasing, pickling, and so forth in conventional manners, followed by drying, and the sheet thus treated was immediately immersed in the plating bath of Examples 1 to 8 each, previously kept in an N 2  atmosphere. Thereafter, the cold rolled sheet was plated with aluminum using a direct current, setting the cold rolled sheet serving as the cathode, and an aluminum sheet (purity: 99.99 %; sheet thickness: 1 mm) as the anode. The relationship between the plating bath composition, electrolysis conditions, and the resulting aluminum-plated steel sheets is shown in Table 3. 
     
                       TABLE 3______________________________________Ex-am-       Plating bath compositionple       AluminumNo.       halide     Di(tri)alkylpyridinium halide______________________________________9         AlCl.sub.3 1-Ethyl-3-methylpyridinium bromide     66.7 mol % 33.3 mol %10        AlCl.sub.3 1-Ethyl-3-methylpyridinium bromide     60.0 mol % 40.0 mol %11        AlCl.sub.3 1-Ethyl-3-methylpyridinium chloride     66.7 mol % 33.3 mol %12        AlCl.sub.3 1-Ethyl-3-methylpyridinium chloride     66.7 mol % 33.3 mol %13        AlCl.sub.3 1-n-Butyl-3-methylpyridinium chloride     66.7 mol % 33.3 mol %14        AlCl.sub.3 1-Benzyl-3-methylpyridinium chloride     66.7 mol % 33.3 mol %15        AlCl.sub.3 1-Ethyl-3,5-dimethylpyridinium bromide     66.7 mol % 33.3 mol %16        AlCl.sub.3 1-Ethyl-3,5-dimethylpyridinium chloride     66.7 mol % 33.3 mol %17        AlBr.sub.3 1-Ethyl-3-methylpyridinium bromide     66.7 mol % 33.3 mol %18        AlBr.sub.3 1-Ethyl-3,5-dimethylpyridinium bromide     66.7 mol % 33.3 mol %______________________________________ 
    
     
                       TABLE 3______________________________________(Cont&#39; d laterally)Electrolysis conditions        Cur-    Elec- Cur-Ex-          rent    trol- rent  Coatingsam-  Bath    density ysis  effi- Thick-                                  Stateple  temp.   (A/     time  ciency                            ness  of    Work-No.  (°C.)        dm.sup.2)                (min) (%)   (μm)                                  crystal                                        ability______________________________________9    25      10      10    97    20    Dense Good10   50      30      3     96    18    Dense Good11   25      10      5     97    10    Dense Good12   50      20      2.5   95    10    Dense Good13   25      5       20    99    20    Dense Good14   50      10      10    98    20    Dense Good15   25      1       50    100   10    Dense Good16   25      1       50    99    10    Dense Good17   50      5       20    98    20    Dense Good18   50      5       20    98    20    Dense Good______________________________________ 
    
     Example 19 
     In the plating bath comprising aluminum chloride and 1-ethyl-3-methylpyridinium bromide (molar ratio: 2:1), prepared in Example 1, a toluene solvent was added in the proportion of 1:1 (volume ratio) to prepare a plating bath having a lowered viscosity. This plating bath had a conductivity of 17.8 mS/cm (25° C.), which was made 70 % larger than the conductivity of the plating bath of Example 1. 
     Next, using this plating bath, a cold rolled sheet was electroplated with aluminum according to the plating procedures in Examples 9 to 18 under conditions of a bath temperature of 25° C., a current density of 20 A/dm 2  and an electrolysis time of 2.5 minutes. As a result, a very dense coating of about 10 μm thick was formed on the surface of the cold rolled sheet, with a current efficiency of 99 %. 
     Example 20 
     In the plating bath comprising aluminum chloride and 1-ethyl-3-methylpyridinium chloride (molar ratio: 2:1), prepared in Example 3, NaCl was added in an amount of 5 mol % based on the amount of 1-ethyl-3-methylpyridinium chloride to prepare a plating bath. Using this plating bath, a cold rolled sheet was electroplated with aluminum according to the plating procedures in Examples 9 to 18 under conditions of a bath temperature of 50° C., a current density of 10 A/cm 2  and an electrolysis time of 5 minutes. As a result, a very dense glossy coating of about 10 μm thick was formed on the cold rolled sheet, with a current efficiency of 98 %.