CA1324105C - Process for electrodeposition of aluminum on metal sheet - Google Patents

Abstract

ABSTRACT

In electroplating aluminum on a metal sheet with use of a molten salt bath comprised of from 40 to 80 mol % of an aluminum halide and from 20 to 60 mol % of an N-alkylpyridinium halide, or a molten salt bath obtained by mixing an organic solvent in said first-mentioned bath, said metal sheet is activated to improve deposit adhesion by carrying out, before plating, electrolysis on said metal sheet, bringing it to serve as an anode and with use of an activating bath having the same composition as said molten salt bath. Also, the bath is purified by leaving metallic aluminum immersed in the bath, or by carrying out preliminary electrolysis under a current density of 0.5 A/dm2 or less with immersion of an anode and a cathode both made of metallic aluminum in the molten salt bath.

Description

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S P E C I F I C A T I O N

TITLE OF THE INVENTION
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Process for electrodeposition of aluminum on metal she~t ;

TECHNICAL FIEI,D -~

This invention relates to a process enabling i improvement of the adhesion of electrodeposits and purity of coatings in a process for the electrodeposition of aluminum on a metal sheet by use of a molten salt bath. ~

BACKGROUND OF THE ART ~;

Electroplating of aluminum can be carried out with difficulty using a plating bath of an aqueous solution type, since aluminum has a large affinity for oxygen and shows a lower potential than hydrogen. For ~his reason, the electroplating of aluminum has hitherto been carried ~-~
out using a plating bath of a non-aqueous solution type, in particular, a plating hath of an organic solvent type.
In regard ~o the plating bath of this organic 15 ~ ~ solvent type, available as a plating ba~h that secures safety in operations is a molten salt bath comprised of an -;
aluminum h~lide and an N-alkylpyridinium halide. It 1~ includes, for example, a molten salt bath comprised of an ^
aluminum halide and an N-ethylpyridinium halide or a bath ob~ained by mixing an organic solvent in this bath (USP

2,446,33l; 2,446,349; and 2,446,350), and a molten salt bath comprised of an aluminum halide and an N-bu~ylpyridinium halide, or a bath obtained by mixing an ;~
organic solvent, capable o~ obtaining products having ~ be~ter appearance than those obtained by using the above ~'' ' ~ .
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plating baths even if the plating is carried out in a high current density (Japanese Unexamined Patent Publications No.
70592/1987 and No. 70593/1987, to Nisshin Steel Co. Ltd., both of which were laid open for public inspection on April 1, 1987.
This molten salt bath, though sometimes bringing about coatings partly having a problem in appearance, comes to a liquid near room temperature and makes it possible to carry out electroplating in the temperature range of approximately from 0 to 150C when a compound comprising a halogen atom selected from C1, Br or I is used as the aluminum halide and a compound whose N-substituted alkyl group has l to 5 carbon atoms is used as the N-alkylpyridinium halide in amounts of from 40 to 80 mol for the aluminum halide and from 20 to 60 mol ~ for the N-alkylpyridinium halide.
USP 2,446,349 discloses that the above molten salt bath dislikes water and oxygen, and presence of these results in I oxidation of the bath to lower the quality of plating, thus ¦ xequiring to bring its atmosphere into a dry and oxygen-free atmosphere. For thi~ reason, also when a metal strip is ~ 20 continuously plated, it is necessary to take care that the l~ water adhered on the strip in the course of a pretreatment of ;~
the plating may not be carried in the bath.
Since, however, when oxides exist on the strip, the oxides cannot be removed unless an inorganic acid is used/ the ~-; 25 pretreatment must be carried out using a treatment solution of an aqueous solution type. This pretreatment is commonly carxied out through khe steps of degreasing, washing with Z~ ~ water, pickling using an inorganic acid, washing with water, .~ and, as occasion demands, strike plating, but drying is required before plating to prevent the water from being carried in the ~ath. This drying, because of an activated state on the ~; surface of the strip, must be carried out in the dry and o~ygen-free atmosphere before a good deposit adhesion can . ~
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be obtained.
- A generally available method for giving the dry and o~ygen-free atmosphere is a method in which nitrogen gas or argon gas is used. However, this method, though not requiring to use gas in a large quantity in carried out the plating in an experimental scale since there is less evaporation of water from the strip, requires to use gas in a large quantity in carrying out the continuous plating since there occurs the evaporation of water in a large quantity. Hence, in the case of the continuous plating, a step for carrying out drying in an atmosphere has hitherto ;
been provided between the washing after pickling and the -~
plating in a bath. This method, however, have had the problem that because of the activated state on the surface lS of the strip an oxide coating is formed thereon even if the drying is carried out in a short time, resulting in a lowering of the deposit adhesion. -:
Also, in recent years, high-puri~y aluminized metal ;
sheets have attracted notices as electronic parts such as IC lead frames and materials for magnetic discs, but plating on such products requires high purity of the bath.
However, the aluminum halide to be used contains a trance amount of impurities such as Fe, Pb and H2O even if it is ~ of high purity, and hence has been inevitably accompanied ;~ 25 with inclusion of impurities. It has hence been difficult to attain a 99.9 % or more Al purity of coatings, and , ~ moreover, the impurities may give a greater influence if the plating is carried out at a bath temperature of 40C
or more, resulting in no denseness of the coatings.
This have caused the problems that employment ~ thereof in the IC lead frames may result in a poor bonding J~. ~ property ~o aluminum wires and working thereof into the -~i magnetic discs may result in no capability of precise bore I working by anodizing treatment.
A first objec-t of this invention is to provide a process for the electrodeposition of aluminum, that can ., .
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activate the surface by an inexpensive means even if the drying is carried out in an atmosphere after the pretreatment with use of a treatment solution of the aqueous solution type, when aluminum is continuously plated on the metal strip.
A second object of this invention is to provide a process for the electrodeposition of aluminum, that enables aluminum plating with a 99.9% or more purity.
A third object of this invention is to further provide a process for the electrodeposition of aluminum, that enables aluminum plating with a 99.9% or more purity and with denseness, even when the plating is carried out at a bath temperature o~ 400C or more.

DISCLOSURE OF THE INVENTION

This invenkion provides a process ~or the electrodeposition of aluminum, comprising electroplating aluminum on a metal sheet with use of a molten salt bath comprised of from 40 to 80 mol % of an aluminum halide represented by the formula AlX3~ wherein X is Cl, Br or I, and from 20 to 60 mol ~ of an N-alkylpyridinium halide represented by the formula CsH5N-RX, wherein R is an alkyl group having 1 to 5 carbon atoms and X is a halogen atom, or a molten salt bath obtained by mixing an organic solvent in said first-mentioned bath, wherein, before plating, said metal sheet is activated by carrying out el~ctrolysis on said metal sheet, bringing it to serve as an anode and with use of an activating bath having the same composition as said molten salt bath. Also, the molten salt bath is preferably purified by leaving metallic aluminum immersed in the bath be~ore plating, or by carrying out preliminary electrolysis under a current density of 0.5 A/dm2 or less with immersion of an anode and a cathode both made of metallic aluminum.
The above molten salt bath is so large in the ~

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corrosive power tha~ the materials that can resist it are - almost limited to fluorine resins and composite ma;terials with the resins, and has a corrosive power to dissolve almost all metals. Hence, carrying out electrolysis by bringing a metal sheet to serve as an anode and with use of an activating ba~h having the same composition as the molten salt bath enables ready removal of such a thin oxide coating that has been formed as a result of the drying in an atmosphere after pickling, thus bringing about activation of the surface.
Also, immersing of metallic aluminum in the molten salt bath enables substitution of impurities Fe, Pb, etc.
with A1 owing to potential difference, which impurities are deposited on the surface of the metallic aluminum and removed. This substitution deposition can be further promoted when the electrolysis is carried out by using - ;~-metallic aluminum for both electrodes.
', ¦~ BEST MODE FOR WORKING THE INVENTION

~; The activating bath may preferably be comprised of ~, 20 from SO to 75 mol % of the aluminum halide and from 25 to SO mol % of the N-alkylpyridinium halide. The aluminum halide concentration otherwise lower than 50 mol % results in a high concentration of N-alkylpyridinium cations to readily cause reduction of the cations at the anode, thus bringing about loss of the balanc of the bath composition and also easy adhesion of organic matters on the anode.
On the other hand, the aluminum halide c,therwise more than 75 mol % may result in a lowering of the electrical ~g~ conductivity of the bath. -;
Adding from 25 to 75 mol % of an organic solvent to ~-the activating bath enables increase in the electrical conductivity and makes it possible to achieve activation at a high speed. This organic solvent may preferably include solvents of aromatic group types.
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The activa~ing bath is disposed in the same chamber as the molten salt bath for use in plating, and kept in the dry and oxygen-free atmosphere so that the strip may not be oxidized while it is transferred from the activating bath into the molten salt bath. The electrolysis, when it comprises the bath to which no organic solvent is added, may be carried out under 1 x 10 3 to 1 A/dm2 using a direct current or a pulse current, -~
so that the bath can be activated with good efficiency.
The bath temperature may preferably be set to 0 to 150C.
The temperature lower than 0C makes it difficult to achieve uniform activation because of a high viscosity, and the temperature made higher than 150C tends to cause side reaction or adhesion of organic mat~ers in the case - of a high current density, resulting in difficulty in the activation of the metal sheet surface.
Carrying out the electrolysis under the above ~- current density can bring about the activation with high ~ efficiency also when the organic solvent has been added in -~
¦ 20 the mol-ten salt bath. However, a bath temperature made lower than 10C may sometimes cause coagulation, and a ' bath temperature made higher than 80C may result in ¦ ~ excessive evaporation of the solvent. AGcsrdingly, the I temperature may preferably be set to 10 to 80C.
l~- 25 The electrolysis is carried out for the time ranging from 5 seconds to 1 hour. The time otherwise shorter than 5 second~ may bring about poorness of deposit adhesion because of insufficient achievement of the activation, resuIting in easy peeling when a coating is peeled by applying thereon an adhesive tape. The aGtivation, however, can be promised if the electrolysis is carried out for 1 hour even by electrolysis under a low current denslty.
In carrying out the electrolysis, the anode ; 35 comprised of Al enables the impurities having been eluted from the metal sheet into the bath, to be deposited on the ' ' '.~, ~ W . ' . .
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anode owing to the potential difference without their accumulation in the bath, thus making it possible to keep always clean the activating bath.
After the activation, there is no problem if the solution in the activating bath is carried in the molten salt bath, because the activating bath and the molten salt bath for use in plating are constituted of the same components, so that the strip can be transferred into the molten salt bath only by simply removing the activating solution. If the bath composition of the molten salt bath is desired not to be changed, both the baths may be made to have the same composition.
The metallic aluminum used for purifying the molten `
salt bath may preferably take the form of some~hing that can be readily immersed in and lifted out of the plating -bath, as exemplified by a wire.
In the instance where the purification is carried out by immersing, the metallic aluminum may be immersed for 5 hours or mora while controlling the bath temperature at from 20 to 150C. The bath temperature otherwise less than 20C makes the substitution-deposition reaction to take place with difficulty to require much time for the purification, and the temperature otherwise more than 150C may bring about decomposition of the N-alkylpyridinium halide.
In the instance where the purification i5 carried out by electrolysis, it can be carried out by effecting electrolysis for 1 hour or more in usual cases and under a current density of 0.5 A/dm2 or less. The current density otherwise made more than 0.5 A/dm2 may result in predominan~ deposition of A1, may result in a greater proportion of the deposition of Al with respect to the ~ deposition of impurities, and also may result in a large ¦~ waste of Al.
In this electrolysis, the anode is made to comprise metallic aluminum for the purpose of preventing a `
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variation of bath composition that may be caused if the anode is constituted of an insoluble pole, when Al in the bath is deposited during the preliminary electrolysis under the above current density. The anode constituted of metallic aluminum enables Al to be dissolved and ed from the anode in proportion to the amount o~ applied electric currents, so that the plating bath composition can be kept in the state of initial make-up of an electrolytic bath.
Purifying the molten salt bath according to such procedures, which contains 0.01 to 0.1 % of Fe, 0.005 to 0.03 % of Pb and 0.01 to 0.1 % of H20 at the time of the initial make-up of an electrolytic bath, can bring the Fe content to 0.003 % or less, the Pb content to no -detection, and the H20 content to 0.005 % or less after the purification, and carryiny out aluminum plating with use of this bath can bring about a coating having a high -purity of 99.9 % or more.
Carrying out the purification in the manner as described above makes the coating dense and yet not to have any powdery dendrite texture, even when the plating on the metal sheet is carried out at bath temperatures of from 0 to 150C and also under c:urrent densities of from ~ -0.1 to 30 A/dm2. The bath temperature o~hsrwise made lower than 0C may result in difficulty in carrying out the platin~ under a high current density. Also, the bath temperature otherwise made higher than 150C and the current density made higher ~han 30 A/dm2 makes the coating grayish, resulting in poor appearance and also a ,~ lowering of workability on the coating.
The plating is carried out in the dry and oxygen-free atmosphere as conventionally done (for example, in dry N2 or Ar) for the purpose of preventing oxidation of the hath. As for the electric current, any o~ direct ~, current and pulse current may be used, but the pulse !~ 35 current can make crystals finer and can make workability ~
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. ., To perform uniform plating in the continuous plating, it is necessary to supply Al ions to the molten salt bath to control the Al ions in the bath to a constant amount. In this occasion, however, the anode may be constituted of a soluble anode made of Al, thereby enabling automatical supply of Al ions in accordance with the amount of electric current, advantageously.
To supply Al ions when the continuous plating is carried out by using as the anode an insoluble anode of Ti-Pt type or the like, halides such as AlC13, AlBr3 and AlI3 may be supplied. In this occasion, the halides to be supplied requires purification, which purification may be carried out by providing a halide-supply tank outside a plating tank for the molten salt bath, and providing a purification tank between this supply tank and the plating tank, thus being carried out in this purification tank according to the method previously described. How~ver, when the continuous plating is performed by using the ~-insoluble anode, reac~ion takes place to generate halogen gas at the anode interface during the electrolysis. ~;
Example 1 ~A cold rolled sheet, a SUS430 sheet and a SUS316 sheet, all having a sheet thickness of 0.1 mm were subjected to pretreatments for electrol~tic degreasing by use of a treatment solution o~ an aqueous solution type, washing with water, pickling by inorganic acid, and -washing with water, followed by drying in an atmosphere, and ~hen immersed i~ an activating bath of an aluminum chloride/N-butylpyridinium chloride (PBC) type (mixing molar ratio: 2:1~ in an N2 atmosphere to carry out electrolysis in an Al plate serving as the anode and using ~ a direct current.
I Thereafter, the steel sheets were each trans~erred in a molten salt bath having the same composition as the ~;~ 35 above activating bath, and an A1 plate (purity: 99.99 %;
sheet thickness: 5 mm) was used as the anode, thus .

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applying electroplating of aluminum for 15 minutes using a direct curren-t under a current density of 1 A/dm . Table 1 shows the relationship between the activating conditions and the deposit adhesion.

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Table 1 Conditions for electrolysis activation treatment Bath composition Current Depos-MetalOrganic density it ad- . .
No. sheet AlCl3 BPC solvent (A/dm2) Time hesion :
Examples~
1 CRS* 60 40 - 0.1 3 min Good 2 " " " - 0.0510 min " .

3 " 67 33 - 0.5 30 sec "

4 " " " - 0.0150 min "
" "" Benzene 0.1 3 min " :
6 SUS430 5545 - 1.0 15 sec 7 " "" - 0.1 3 min " ~ ~
8 " 6238 - 0.5 30 sec 1l .. ~. :' 9 " "" - 0.05 10 min "
" "" Toluene 0.1 3 min "
11 SUS~16 5743 - 0.5 30 sec " :
12 " "" - 0.05 10 min "
13 " 6535 - 0.1 3 min 14 " "" - 0.01 50 min ; 15 " "" Ben/l'ol 0.1 3 min " :~ ~
;: Compatative Examples: ~-I CRS* ~ - Peeled ~, : . " . .
: 2 SUS430 * Cold ralled ~heet Notes: :~
: In the bath composition, AlC13 and BPC were added in terms of mol %; and the organic solvents, in an amount of 50 vol ~ based on the total o~ AlCl3 and BPC. Ben/Tol indicates a mixed sol~ent of benzene -~
~ with toluene in equal amount.

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132~5 Example 2 - Example 1 was repeated to carry out the activation treatment and the electroplating but using an activating .
bath and a molten sal~ bath in which aluminurn bromide or :
alkylpyridinium iodide was used as the aluminum halide (AlX3) and N-alkylpyridinium bromide or N-alkylpyridinium iodide was used as the N-alkylpyridiniurn halide (RPX). In each bath, AlX3 and PRX were mixed in molar ratio of 2:1.
Table 2 shows the relationship between the activating lQ conditions and the deposit adhesion.

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Table 2 Conditions ~or ~.
electrolysis activation treatment Bath composition Current Depos-MetalOrganic density it ad-No. sheet AlX3 RPX solvent (A/dm2) Time hesion Examples:
1CRS* AlBr BPBr - 0.1 3 min Good (60~ (40) 2 " " " - 0.0510 min "
(67) (333 3 " AlI EPI - 0.530 sec "
(6~) (40) 4 " " " - 0.0150 rnin (67) (33) "AlBr MPBr Benzena 0.1 3 min ~' ;
(60~ (40) 6SUS430 " BPBr - 1.015 sec " ::
(55) (45) 7 " " EPBr - 0.530 sec " :.
(~2) (38) 8 " AlI BPI - 0~0510 min "
(5~) (45) -:
9 " " EPI - 0.13 min "
(~2) (38) "AlBr BPBr Toluene 0.1 3 min "
(65~ (35) 11SUS316 " " - 0.530 sec (57) 143) :
12 " " MPBr - 0.0510 min "
(65) (35) 13 " AlI BPI - 0.13 min "
(5~) (43) -14 " " EPI - 0~0150 min "
(65) (35) : 15 "AlBr EPBr Ben/Tol 0.1 3 min (~2~ (38) _ _ * Cold rolled shee~ :~
No~es: In the bath composition, AlX3 and RPX were added in terms of mol %; and the organic solvents, in an amount of 50 vol % based on the ~otal of AlX3 an~ RPX. Ben/Tol indicates a mixed solvent of benzene with toluene in equal amount. In RPX, MPBr is methylpyridinium bromide; EPBr, ethylpyridinium bromide; BPBr, butylpyridinium bromide; EPI, ethyl-pyridinium iodide; and BPI, butylpyridinium iodide. :~

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Example 3 In a molten salt bath comprised of 60 mol % of AlCl3 and 40 mol % of N-butylpyridinium chloride and kept in an N2 atmosphere, an Al wire (purity: 99.99 % or more) was immersed and left for 10 hours at 100C to effect purification of the bath Next, the temperature of this plating bath was lowered to 40C, and a cold rolled sheet (sheet thicknees:
0.5 mm) having been pretreated as in No. 1 in Table 1 of Example 1 was immersed to carry out the electroplating of aluminum under the same electrolysiis conditions ai in Example 1.
A coating on the resulting aluminized sheet had a purity of 99.97 %, showed a uniform thickness, assumed a ~hite color, and comprised den~e crystals. Also, no crack.ing or peeling occurred even when bending was -repeatedly applied on the steel sheet, showing both good workability and adhesion.
Example 4 Aluminum plates were used to constitute the both electrodes in a molten salt bath comprised of 67 mol % of AlC13 and 33 mol % of N-butylpyri.dinium chloride and kept -~
in an N2 atmosphere, and preliminary electrolysis was carried out for 3 hours under 0.1 A/dm to effect purification. Thereafter, using thisi bath, the electroplating of aluminum was carried out on a cold .
rolled sheet having been pretreated as in No. 1 in Table 1 of Example 1, bringing an aluminum plate (the same as in .
Example 1) to serve as the anode, or 2 minutes at a bath temperature of 60C, using a direct current under a .:
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current density of 10 A/dm .
A coating on the resulting aluminized sheet had a purity o~ 99.99 %, and showed the same appearance, crystal .~
~ state and workability as in the case of Example 2.
Example 5 : ,.
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~ ~ 2 .~ 5 A molten salt bath comprised of 60 mol % of AlBr3 and 40 mol % of N-butylpyridinium bromide and kept in an N2 atmosphere was purified in the same Al wire immersing :
method as in Exarnple 2, except that the bath was kept to a temperature of 60C and left for immersion for 20 hours.
Subsequently, the temperature of this plating bath was raised to 80C, and the electroplating of aluminum was carried out on a cold rolled sheet having been activated for an electrolysis time of 30 seconds under a current density of 0.5 A/dm with use of an activating bath :~
comprised of 60 mol % of AlBr3 and 40 mol % of N- -butylpyridinium bromide, bringing an aluminum plate ~-the same as in Example 1) to serve as the anode, for 1 minute using a direct current under a current density of 20 A/dm2-A coating on the resulting aluminized sheet had a purity of 99.99 %, showed good appearance, crystal state and workability.
Example 6 In Examples 2 to 4, plating was carried out by replacing the electric current with a pulse current at the time of the electroplating of aluminum. In each instance, the plating was carried ou~ using the pulse current having a duty ratio of from 1/10 to 1/100 and an average current density of from 0.1 to 30 A/dm . As a result, coatings showed the same properties as in the case when the plating was carried out using the direct current, and the coatings all had a purity of 99.98 % or more. - -Comparative Example Using an unpurified molten salt bath comprised of 67 mol % of AlC13 and 33 mol % of N-butylpyridinium chloride ~.
and kep-t in an N2 atmosphere, aluminum plating was carried ::
out under the same electrolysis conditions as in E~Pmple 3 ~:
on the cold rolled sheet having been pretreated as No. 1 in Table 1 of Example 1. The resulting plated sheet had a grayish appearance, and comprised coarse crystals. A

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coating thereof had a purity of 99.0 %.

POSSIBILITY OF INDUSTRIAL UTILIZATION

The aluminized metal sheet obtained by this invention has a superior adhesion and also has a high purity of coatings, and hence can be utilized in the -manufacture of IC lead frames and magnetic discs.

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Claims

CLAIMS:
(1) A process for the electrodeposition of aluminum on a metal sheet, comprising electroplating aluminum with use of a molten salt bath comprised of from 40 to 80 mol % of an aluminum halide represented by the formula AlX3, wherein X is Cl, Br or I, and from 20 to 60 mol % of an N-alkylpyridinium halide represented by the formula C5H5N-RX, wherein R is an alkyl group having 1 to 5 carbon atoms and X is a halogen atom, or a molten salt bath obtained by mixing an organic solvent in said first-mentioned bath, wherein, before plating, electrolysis is carried out on said metal sheet, bringing it to serve as an anode and with use of an activating bath having the same composition as said molten salt bath.
(2) The process for the electrodeposition of aluminum on a metal sheet according to Claim 1, wherein the cathode comprises aluminum.
(3) The process for the electrodeposition of aluminum on a metal sheet according to Claim 1, wherein the electrolysis is carried out under a current density of from 1 x 10 3 to 1 A/dm2.
(4) A process for the electrodeposition of aluminum on a metal sheet, comprising electroplating aluminum with use of a molten salt bath comprised of from 40 to 80 mol % of an aluminum halide represented by the formula AlX3, wherein X is C1, Br or I, and from 20 to 60 mol % of an N-alkylpyridinium halide represented by the formula C5H5N-RX, wherein R is an alkyl group having 1 to 5 carbon atoms and X is a halogen atom, or a molten salt bath obtained by mixing an organic solvent in said first-mentioned bath, wherein, before plating, said metal sheet is activated by carrying out electrolysis on said metal sheet, bringing it to serve as an anode and with use of an activating bath having the same composition as said molten salt bath, and the molten salt bath is purified by leaving metallic aluminum immersed in the bath.
(5) A process for the electrodeposition of aluminum on a metal sheet, comprising electroplating aluminum with use of a molten salt bath comprised of from 40 to 80 mol % of an aluminum halide represented by the formula AlX3, wherein X is Cl, Br or I, and from 20 to 60 mol % of an N-alkylpyridinium halide represented by the formula C5H5N-RX, wherein R is an alkyl group having 1 to 5 carbon atoms and X is a halogen atom, or a molten salt bath obtained by mixing an organic solvent in said first-mentioned bath, wherein, before plating, said metal sheet is activated by carrying out electrolysis on said metal sheet, bringing it to serve as an anode and with use of an activating bath having the same composition as said molten salt bath, and preliminary electrolysis is carried out under a current density of 0.5 A/dm2 or less with immersion of an anode and a cathode both made of metallic aluminum in the molten salt bath.