CA1311710C - Plating bath for electrodeposition of aluminum and process for the same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In a plating bath for electrodeposition of aluminium, comprising a nonaqueous electrolyte using a eutectic mixture fusible at room temperature, the plating bath comprises a mixture of about 40 to 80 mol % of an aluminum halide and about 20 to 60 mol % of butyl pyridinium halide, or further comprises an organic solvent mixed in said mixture. This plating bath may not cause any color change of a coating into gray or black to assure white and uniform appearance, even when current density is increased to 30 A/dm2 or varied in the range from 0.1 to 30 A/dm2.

Description

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TITL~ OF TH~ INV~NTION

Plating bath for electrodeposition of aluminum and process ~or the ~ame BACKGROUND OF TH~ INV~NTION

This invention relates to a plating bath for electrodepo~ition of aluminum and a process for the ~ame.
U.S. Patents No. 2,446,331, Na. 2,446,349 and No.

2,446,350 disclose a process for electrodeposition of aluminum by use of a nonaqueous electrolyte, whlch is a process carried out by using a platiny bath co~prising a mixture (a eutectic mixture) o~ an aluminum halide and a N-alkyl p~ridinium halide. However, it is stated in the above patents that a good aluminum coating can be obtained when the aluminu~ halide comprises aluminum chloride and the N-substituent for the N-alkyl pyridinium halide ~omprises an eth~l group, and there is no disclo~ure as to the e~amples where the N-~ubstituent for N-alkyl pyri~inium halide e~mprises other alkyl groups. -The N-ethyl pyridinium halide used when a plating bath is prepared is not commercially available since there i5 no demand th~re~or even as a reagent. For this r~a~on, it must be syntheslzed when u~ed ~or preparing a plating bath. SyntheQis therefor can be achieved by reactin~ an ethyl halide with pyridine. HQwever, the ethyl hallde, which is ga~eous, has a disadvantage that it i5 in dan~er of explosion during it~ synthesi~.
Al~o, in carrying out the electrod~posi~ion of aluminum in an indu~trlal scale, it has been necessary to carry out platlng under high current deneity in order to . .
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make high the plating speed, and also to control appearance to be uni~orm even with change in t~le current density. llowever, platin~ under high current density with use of a plating bath comprising the mixture of aluminum chlori~e and N-ethyl pyridinium halide, the appearance may suIfer color-changed and constarlt appearance may be as~ured only within a very narrow range of the current density. For example, in the instance of the plating bath disclosed in U.S. Patent No. 2,446,33l, the current density at which a coating havirlg appearance of aluminum can be ob-tained is 0.5 A/dm2, and the density lower than that may result in a cloudy appearance. On the other hand, the current density higher than that may resul-t in a grayish appearance, and the density excessively hicgher that that may result in generatioIl of black spots. ~lso, in the instance of the plating bath disclosed in U.S.
Patent No. 2,446,349, an optimum current density is 0.~ to l.o A/dm2, and the denslty higher than l.O A/dm2 may result in generation o~ blown colored streaks.
Accordingly, these plating baths can be suited for batch plating for the partæ (for example, screw thread, rivets, frames, etc. for aircrafts), but, irl the continuous plating where the plating i5 carried out while a metallic band is moved at constant speed, the plating can be achieved taking an over-ly long time to seriously lower the productivity. It is also difficult to make uniform the appearanae as a whole of the metallic band.

SUMMARY OF TEIE INVENTION

This invention aims at providing a plating bath, and a plat1ng process, that can safely synthesiæe the N-alkyl pyridinium halide to be used, and may not cause any change in appearance even at the current dens1ty irlcreased to 30 A/dm and the current derlsity varied in the ran~e ~rom O.l to 30 A/dm , 13~17 LO

According to this invention, there are provided a plating bath for electrodeposition of aluminum, comprising a mixture of about 40 to 80 mol % o~ an aluminum halide (AlX3, wherein X represents Cl, Br or I) and about 20 to 60 mol ~ of butyl pyridinium halide (C5H5N-(C4H9)X, wherein X represents a halogen atom), and a plating bath further comprising an organic solvent mixed in the above plating bath. With use of any of these plating baths, the electrodeposition is effected on a metal in a dried and oxygen free atmosphere using a direct current or pulse current, at a bath temperature of 0 to 150C and at a current density of 0.1 to 30 A/dm2.

DESCRIPTION OF THE PREFERRED ~MBODIMENTS

The mixture of about 40 to 80 mol % of an aluminum halide and about 20 to 60 mol % of butyl pyridinium halide is a eùtectic mixture fusible at room te~perature Accordingly, the electrodeposition of aluminum can be achieved by flowing a current with use of a metal on the cathode.
The aluminum halide may include AlCl3, AlBr3 and AlI3, which can be used alone or as a mixture of any of these. The concentration thereof in a bath may be controlled to he about 40 to 80 mole %. The concentration less than 40 mol % may result in higher concentration of butyl pyridinium ions and simultaneous progress of the reduction of the butyl pyridinium ions, causing the deterioration of the bath, a lowering of the current efficiency and the poorness in the surface appearance of a coating. On the other hand, the concentration more than 80 mol ~ may result in a lowering of conductivity and also result in an increased vapor pressure.
Controlling the concentration of the aluminum halide to the value as mentioned above, the remaining is comprised of the butyl pyridinium halide, and it follo~s ' ~. :
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:'' ' 13:1~7~ o that the latter is in conceJ1tration of 20 to 60 mol %.
Thi~ compound may preferably include those in which the halogen atom is Cl, Br or I.
The plating bath may be prepared by mixing the aluminum halide and the butyl pyridinium halide according to the procedures as di~closed in U.S. Patent No.
2,446,349. In the mlxing, the butyl pyrldinium hallde, which is not commercially available, must be synthesized beforehand. The synthesis thereof may be carried out by reacting a haloyenated butyl with pyridinium. Since the halogenated butyl is a solid, there is no danger of explosion.
The plating bath having the above compo~ition remains somewhat viscous at room temperature, and, in contlnuous plating, a member to be plated may exces~ively carry away the plating ~olution, whereby the plating solution may be consumed in greater proportion. Also, appearance of good mirror gloss can not~be achieved. This problem can be solved by mixirl~ an orgaIIic ~olvent in the plating bath. The organic solvent, preferable in view of the solubility, may include aromatlc solvents, for example, toluene, ~enzene, xylene, etc. It may be mixed in an amount of 10 to 75 vol. % based on the total amount of the aluminum haIide and butyl pyridinium halide IAccordinyly, the total amount of both the halides i~ 25 to 90 vol. %). The amount of less than lO vol. % may result in small effect of the mixing, and the amount more than 75 vol. % may result in a lowerin~ of the concentration of the aluminum halide, thu~ extremely lowering the current e~ficiency at the time of the plating.
The plating Oath can be kept safe even wherl it comes into contact with oxygeh or water, but, in order to prevent the oxldatlon of alumirium~complex lons, the plating l~ carried out in a dried and oxygen free atmosphere (for example, in a dried N2 gas or Ar gas~.

l3~7~a Electrolysis may be carried out :in the dried and oxygen free atmosphere using a direct current or pulse current, at a bath temperature of 0 to 150C and at a current density of O.1 to 30 A/dm2. The bath t0mperature lower than 0C may cause the coagulation of a plating bath to make it difficult to carry out the plating at a high current den~ity. The bath temperature higher than 150C
an~ the current density higher than 30 may result in the formation of a grayish coating and also the ~ormation of 10 coarse dendritic crystals to make poor the appearance and the processability. When the electrolysi~ is carried out at a current density near to 30 A/dm2, it is preferably to control the bath temperature to 100C or more. In the plating bath using the ethyl pyridinium halide, U.S.
Patent No. 2,446,331 also discloses that the bath temperature is controlled to 40 to 150C. However, a color change may occur when the current density i5 higher than 0 . 5 A/dm . As to the current, application of a pulse current, rather than a direct current, can make crystals finer and can achieve better formability.
In order to always apply a uniform coating in continuously carrying out the platlng in an industrial scale, it is required to supply A1 ions to a platiny bath to control the content of Al ions in the bat~ to a predetermined range. Here, aluminum may be used as a soluble anode, whereby aluminum can be supplied fro~ the anode to make substantially constant the Al ion concentration in the bath. If, however, an insoluble anode ~for example, a Ti-Pt alloy) is used, Al ions are consumed. In such an instance/ A1 ions may be supplied by adding AlC13, AlBr3, AlI3 or the like. However, employment of such an insoluble anode as the anode may cause a reaction to generate a halogen gas at the surface of ~he electrode to decrease the halogen ~omponents in the bath. Therefore, the bath composition ~ay vary to shorten the life of a bath.

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As mentioned above, a coating may suPfer a color change at l.0 A/dm2 when ethyl pyridinium hallde is used as the alkyl pyridinium halide. However, the plating bath of this invention does not cause any color change in the coating even at the current density increased to 30 A/dm2, as mentioned above. This is presumably because, in the ca~e of the ethyl pyridinium halide, the N-substituent (an ethyl group) is so ~mall and thl_ steric hindrance is so small that the ethyl pyridinium halide itselP may undergo reaction at an increa~ed current density, but, in contraæt thereto, in the case oP the butyl pyridinium halide, the N-substituent (a butyl group) is :Larger than the ethyl group, resulting in the steric hindrance to cause no reaction. As an example substantiating the reasonability of this, a coating turns black with much greater color change regardless of the current density, when a meth~l pyridinium halide whose N-æubstituent is a methyl group smaller than the ethyl group.

~XAMPLES

Cold-rolled steel ~heets (thickness: 0.5 mm) were subjected to solvent vapor cleaning, alkali~e degreasing, acid pickling and so Porth to clean their surfaces, Pollowed by drying, and immediately therea~ter dipped in a fused salt bath of N2 atmosphere and comprising a mixture of an aluminum h~lide and a butyI pyridlnium halide.
~lectroplating was carried out using a steel ~heet as a cathode and an aluminum plate Ipurity: 9Q.993 %;
thickness: 5 mm) a~ an anode. Table la and Table lb show the composition oP the plating bath, the conditions for electrolysis, appearance af~er plating, and adherence of a coating.
The plating uæing the pul~e current was carried out at a duty Pactor oP l~lO to l/lO0, and the coating adherence was examined by bending.

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, Table la . ~
Compo~ition o plating bath AlX3 C5H5N-(C4Hg)X Organic No. ---- _ solvent Kind mol%Kind mol~ Kind mol%
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1 AlCl3 60 C5H5N-~C4H~)Cl 40 2 AlCl3 60 CSH5N-~C4Hg)Cl 40B~nzene ~0 3 AlCl3 60 5 5 ( 4H9)Cl 40Benzerle 60 4 AlBr3 67 5 5 ( 4H~)Cl 33Toluene 60 AlCl3 67 5 5 ( 4 g)Cl 33 6 AlBr3 60 C5H5N-(c4H9)cl 40 7 AlCl3 60 C5H5N-~c4H9)cl 4~T~luene 50 8 AlBr3 60 5 5 ~ 4 g)Br 40 9 AlI3 60 C5}I5N-(C4Hg)I 40 - --AlCl3 67 C5H5N-(C4Hg)Cl 33 11 AlCl3 67 C5H5N-(C4H9)C1 33 12 ~lC13 67 5 5 ~ 4 9) l 33Benzene ~o 13 AlC13 67 C5H5N-~C4H9)Cl 33Toluene 40 BenzeneJ
14 AlCl3 67 C5H5N-(c4H9)cl 33toluene 40 mixture AlCl3 67 C5H5N-~C4H9)Cl 33 16 AlCl3 6~ C5H5N-(C4H~)C1 33 17 AlG13 67 C5H5N-(C4Hg)Cl 33 18 AlBr3 C5H5N (C4H9~Br 33 19 AlI3 67 5H5N ~C4~9)I 33 ,-~
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Table lb Conditions for electrolysis ~lec- Coat-troly- ing Bath Current 5iS adher~
No- temp. Current dens~ time Appearance ence (C) (A/dm ) (min) 1 20 Direct 1 15 White Good 2 20 " 5 5 White, glossy Good 3 20 Pulse 10 5 White, glossy Good 4 20 ~irect 0.5 30 White, glossy Good " 10 2 White Good 6 80 " 20 1 White Good 7 50 " 0.5 30 Glossy Good 8 30 " 5 5 White Good ~ 40 " 3 8 White Good 0 " 1 15 White Good 11150 " 30 1 White Good 12 30 " 10 3 White, gloss~ Good 13 50 " 0.5 30 Glo~sy Good 14 30 " 10 3 Glossy Good 15 -10 " 1 15 Gra~ Poor 16180 " 30 1 Dendritic Poor 1'1120 " 40 I Gray, black Poor 18 80 " 10 3 White Good 19lO0 " 10 3 White Good ._ _ . :

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Claims (6)

1. A plating bath for electrodeposition of aluminum, comprising a mixture of about 40 to 80 mol % of an aluminum halide (AlX3, wherein X represents Cl, Br or I) and about 20 to 60 mol % of butyl pyridinium halide (C5H5N-(C4H9)X, wherein X represents a halogen atom).

2. A process for electrodeposition of aluminum, comprising carrying out electroplating with use of a plating bath comprising a mixture of about 40 to 80 mol %
of an aluminum halide (AlX3, wherein X represents Cl, Br or I) and about 20 to 60 mol % of butyl pyridinium halide (C5H5N-(C4H9)X, wherein X represents a halogen atom), on a metal in a dried and oxygen free atmosphere using a direct current or pulse current, at a bath temperature of 0 to 150°C and at a current density of 0.1 to 30 A/dm2.

3. A plating bath for electrodeposition of aluminum, comprising a mixture of about 40 to 80 mol % of an aluminum halide (AlX3, wherein X represents Cl, Br or I) and about 20 to 60 mol % of butyl pyridinium halide (C5H5N-(C4H9)X, wherein X represents a halogen atom), and an organic solvent mixed into said mixture.

4. The plating bath for electrodeposition of aluminum according to Claim 3, wherein said organic solvent is of an aromatic type.

5. The plating bath for electrodeposition of aluminum according to Claim 3, wherein said organic solvent is mixed in the proportion of 10 to 75 vol. %.

6. A process for electrodeposition of aluminum, comprising carrying out electroplating with use of a plating bath comprising a mixture of about 40 to 80 mol %
of aluminum halide (AlX3, wherein X represents Cl, Br or I) and about 20 to 60 mol % of butyl pyridinium halide (C5H5N-(C4H9)X, wherein X represents a halogen atom), and an organic solvent mixed into said mixture, on a metal in a dried and oxygen free atmosphere using a direct current or pulse current, at a bath temperature of 0 to 150°C and at a current density of 0.1 to 30 A/dm2.