Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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An Electrolyte for the Electrodeposition of Aluminum
s The invention relates to an electrolyte for elec-
trodepositing aluminum, containing an organometallic alu-
minum complex, and to the use of said electrolyte in the
production of decorative, corrosion-resistant aluminum
coatings.
~o
Aluminum can be ele:ctrodeposited from a variety of
electrolytes. These electrolytes include, e.g., fused-salt
electrolytes and electrolyi~es containing aluminum halides
or alkyl aluminum complexes>. For the deposition of alumi-.
is num on an industrial scale,, however, it has been exclusi-
vely electrolyte systems based on alkyl aluminum complexes
which gained technical importance in recent years. The
electrolytic deposition of: aluminum from alkyl aluminum
complexes has already been described in the fifties by
zo Ziegler and Lehmkuhl. In general, these alkyl aluminum
complexes consist of an alkali metal halide salt and a
corresponding alkyl aluminum compound at a molar ratio of
1:1. However, they have relatively poor electric conducti-
vity.
zs
It has later been found that complexes having a
molar ratio of 1:2 are formed as well, which complexes
contain two moles of alkyl aluminum compound and only one
mole of alkali metal halide and have a considerably higher
3o electric conductivity. Thi:> discovery was particularly im-
portant for uses involving electrodeposition of aluminum,
because a certain minimum conductivity must be present for
economic performance of the electrodeposition process.
s5 Initially, electrolyte solutions containing the
NaF~~~l:t3 complex dissolved in aromatic hydrocarbons such
as toluene or xylene haves exclusively been used in the
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electrodeposition of aluminum. The above compound has a
vAr,i 1cw melting point, whi <:h is ad-~antageous ~~rhen ~issol-
vin, r_ he complex in a solvent. An electrolyte of such a
composition involves the major drawback of an exceedingly
s poor throwing power. As a result, uniform and complete
coating of large parts having complicated shapes with an
gles and corners can only be achieved with high input and
the additional use of auxiliary anodes. It is a costly
process with a high technical input which does not allow
io economic operation.
However, other alkyl aluminum complexes have also
been used. Amongst these are, e.g., electrolytes including
aluminum complexes such as lKF~2AlEt3 or KF~2A1Me3. As a re-
~s sult of changing the alkali metal cation to potassium,
these complexes have improved electrical conductivity and
a substantially improved throwing power comparable to
other depositions of metal in aqueous electrolytes.
zo However, these compounds involve a severe drawback
in that their melting points are about 127-129~C for
KF~2AlEt3 and 1S1-152~C for KF~2A1Me3, respectively. Also,
the solubility of these complexes in aromatic hydrocarbons
is very low. Thus, a 4 M toluene solution of KF~2AlEt3 un-
zs dergoes crystallization even at 60-6S~C. As a consequence,
partial crystallization of: the active aluminum complex
compound occurs during storage of such solutions, rende-
ring them unserviceable even after a short period of time.
Also, when using the electrolyte, blocking in pipelines,
3o pumps and filters tends to occur as a result of such cry-
stallization, so that these solutions cannot be put to
reasonable use in an industrial-technical application,
e.g. in a production coating plant. Similarly, the measure
of diluting the electrolyte solutions and increasing the
5~ percentage of solvent is L.rdesirable because the ~.racti-
cable current densities are dramatically decreasa~i in this
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way, thereby substantially increasing the duration of de-
ccs_ .ion.
To scive this problem, the prior art or EP-A 0 402
s 761 and US 4,417,954 suggest=s using complexes of other al-
kyl aluminum compounds such as triisobutyl aluminum or
trimethyl aluminum in addition to the above-mentioned po-
tassium-containing triethyl aluminum complexes. These mi-
xed aluminum complexes have lower melting points compared
to triethyl aluminum complexes and higher solubility in
aromatic hydrocarbons.
The EP-0 402 760 A1 describes electrolytes for the
deposition of aluminum which, in addition to compounds of
~s formula MF~2A1R3, contain non-complexed A1R3 and wherein
an aromatic hydrocarbon anal a glycol ether are used as
solvents.
As yet, the prior art has attempted to overcome
Zo these problems of poor throwing power and poor electric
conductivity on the one hand, and the problems related to
the solubility of the alkyl aluminum complexes on the
other hand. by using mixed alkyl aluminum complexes. Thus,
complexes having good electric conductivity and throwing
zs power, but poor solubility, have been mixed with complexes
having good solubility and poor electric conductivity. On
an average, a composition had been achieved which was ac-
ceptable for industrial processes in terms of its electric
conductivity and solubility, as well as its throwing po-
3o wer. Even today, deposition of aluminum on an industrial-
technical scale is performed using these mixtures. Howe-
ver, these electrolyte solutions still involve consider-
able drawbacks.
~Ahile compounds having the composition KF
~A1(i-3u)3 have a lower melting point and thereFore are
usacui as an additive to improve solubility, larger con-
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centrations of this compound in the electrolyte rapidly
gi-m rise tc gray depositions. Moreover, the curr:.~_t :en
sity resistance of these complexes is low and may rapi:ll
result in co-deposition of potassium which is most undesi
s rable in aluminum deposition.
Also, the thermal ;stability of these triisobutyl
complexes is inferior comp<~red to triethyl aluminum com-
plexes.
io The composition of an electroplating bath in ope-
ration is subject to continuous changes, and the mixing
ratio and concentration of the individual aluminum comple-
xes must be maintained con:ctant. For this reason, another
disadvantage of these multi.-component systems is the com-
as placated control and maini~enance of the composition at
constant, as well as the more extensive analytics.
Furthermore, some aluminum complexes such as trimethyl
aluminum are so costly that avoiding the use of such a
complex compound is desirable merely for economic reasons.
zo
It is therefore the technical object of the inven
tion to bring about an improvement of the prior art elec
trolyte solutions, which would allow a more economic use
of the electrolytes and would not necessarily require a
zs multi-component system.
Said technical object is accomplished by using an
electrolyte in the electrodeposition of aluminum which
contains an organometallic: aluminum complex of formula
30 (I)
MF~2A1R3 ( I ) ,
wherein M = Vila, K, Rb, Cs, and R = a Ci-C4 alkyl g=oup,
in a solvent mixture of an aromatic or aliphatic hydrocar-
bon and a Lewis base.
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'when using prior art electrolyte solutions, aro-
mavi: hjdrocarbons such as toluene or x~llene are used al-
most exclusively as solvents. Surprisingly, it has now be-
er~ found that partial substitution of these aromatic hy-
s drocarbons by organic Lewis bases results in a substantial
improvement regarding the solubility and other properties
of the alkyl aluminum complexes, so that the use of multi-
component systems is no longer necessary.
This was a11 the more surprising because when
using Lewis bases, a person. skilled in the art should have
expected that the aluminum complex, as a result of the
high affinity of Lewis basE:s to the A1 atom, would be de-
stroyed either completely or partially with elimination of
~s alkali metal halide and addition of the corresponding Le-
wis base to the aluminum atom. As one might expect, this
should have resulted in such a decrease in conductivity of
the aluminum complex that it could no longer be used in an
electrolysis.
Amazingly, however,, no such effect was observed
when using Lewis bases. Rather, the conductivity of the
electrolyte solution remained almost unchanged although,
as is known, Lewis bases have a higher affinity to alkyl
zs aluminum compounds compared to some alkali metal halides.
In a preferred embodiment, the ratio of employed
hydrocarbon solvent to Lewis bases is from 4:1 to 1:2. It
is also preferred that M in formula (I) be potassium, ru-
3o bidium or cesium. Triethyl aluminum is preferably used as
A183 aluminum compound. As Lewis base, it is preferred to
use an aliphatic, cycloaliphatic or aromatic ether compo-
und or an amine. It is particularly preferred to use an
ether of formula R1-0-R2 wherein Rl and R~ are C1-C4 al-
s~ kyl. nor examplz, these ethers in~.lude compounds such as
methy'_ ethers, ethyl er_he;=s, n-propyl ethers, isopropyl
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ethers, tert-butyl ethers, n-butyl ethers, and isobutyl
orhors.
~dhen using the electrolyte solution according to
s the invention, the previous multi-component systems can be
avoided. It is particularly preferred to use KF~2AlEt3 be-
cause this compound genera:Lly is one of the most readily
available and inexpensive alkyl aluminum complexes. Pre-
viously occurring problems related to the solubility of
such complexes can be overcome in a simple fashion by ad-
ding a Lewis base to the toluene-containing solution, and
in a particularly preferred fashion, diisopropyl ether or
an n-propyl ether.
is A 4 M solution of KF~2AlEt3 in toluene normally
crystallizes even at room temperature. This is not obser-
ved with the electrolyte of the invention in a solvent
mixture with Lewis bases. The electric conductivity of the
electrolytes according to t:he invention is somewhat lower
zo compared to electrolytes in pure toluene, and the decrease
in electric conductivity is far less than expected by a
person skilled in the art f:or the case of partial or com-
plete degradation of the complex. Furthermore, the slight-
ly lower conductivity is compensated by higher solubility
zs and higher resistance of such an electrolyte solution.
When using the electrolyte solution of the inven-
tion, it has also been found that the coating on parts of
complicated shape spreads further into crevices and bo-
3o rings than is the case with electrolytes with no Lewis ba-
ses added. Also, the tendency to dendritic growth or bur-
nings is prevented by the inhibiting effect of the added
Lewis base. The layers obtained are matte to semi-glossing
and low in pores and are produced at current densities of
up to ~ A/dm~ . The electro~.ytes may be operated bot~~. with
DC and AC.
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''
rJsing the electrolyte solution of the invention,
i' ~s ocssib_e for the fi:~st time to replace the multi-
ccmpcr:ent sls=enis used so f:ar which are expensi-Te and re-
quire a high input for maintenance, and at the same time,
s obtain improved aluminum coatings.
During the use of an electrolyte solution in elec-
troplating, the composition of said solution will be sub-
ject to continuous changes. For this reason, in multi-com-
ponent systems used so far, the individual components of
alkyl aluminum complexes had to be monitored continuously
during the electrodeposition of aluminum, and replenished,
if necessary. In doing so, care must be taken that the ra-
tio of the individual alkyl aluminum compounds employed is
~s held as constant as possible, so as not to alter the
properties of the bath in an undesirable fashion.
Such considerable and cost-intensive input can be
avoided completely by using the electrolyte solution ac-
Zo cording to the invention where only one single alkyl alu-
minum compound must be employed, the content of which may
easily be monitored. If required, only one single substan-
ce must be supplied to the bath without the necessity of
paying attention to the ratio between the individual alkyl
is aluminum components. The electrolyte of the invention also
allows deposition at high current intensities, thereby en-
abling more rapid deposition of aluminum and thus, enhan-
cing the economic efficiency of the aluminum electroplat-
ing process.
The electrolyte solution of the invention is pro-
duced in a conventional f<ishion by initially adding the
metal fluoride to the solvent mixture of hydrocarbon and a
Lewis base. Then, the amount of alkyl aluminum compound
calculatad for complex formation is added slowly in small
oortior.s. EolLowing addition, this is heated and stirred
until a1L the components are completely dissolved. The so-
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lution is then cooled down to room temperature and may be
stored 'or any period of time without crystallization of
the solution occurring.
s The electrolyte solution of the invention is pref
erably used in manufacturing decorative and corrosion-re
sistant aluminum coatings. Using the electrolyte solution
according to the invention, aluminum layers of high purity
and quality may be coated in a simple and highly economic
to fashion.
The following Examples are intended to illustrate
the invention in more detai:L.
Examples
Example 1
Zo Preparation of the electrolyte solution
In a heatable stirred vessel, an electrolyte ha-
ving the composition KF~2A1:Et3 dissolved in 4 mol of sol-
vent mixture per mol of complex was prepared under argon.
Zs The molar ratio of the so:Lvents toluene and diisopropyl
ether was 3:1.
To this end, the calculated amount of solvent mix-
ture was initially placed into the argon-flooded stirred
3o vessel. Thereafter, potassium fluoride which had been
dried at 120~C was added with intense stirring. Subse-
quently, the calculated amount of triethyl aluminum was
slowly added in small portions, and the solution underwent
heating to about 80~C. The solution was then heated to 100
~C and stirred For 2 hours. The soluti~:n had a conductivi-
ty of 19 mS/cm. Thereafter, the solution was cooled to 18~
C without stirring. The solution was completely fluid the-
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_<~_
reafter. Following pouring into a storage vessel, the so
'_e'_ior. eras stored for 2 weeks without motion at 15-18~r.
~nen a~'_~r a 2 weeks storage, the solution still was com
,,
p~ete?I ~luid.
s
Example 2
Coating of stepped angle metal sheets using AC
Coating was performed using the electrolyte of Ex-
ample 1. In a coating cell of about 6 1 capacity flooded
with argon and equipped with a supply lock system, two
stepped angle metal sheets having a step width of 20 mm
were coated in a rack panel of about 140 x 140 mm at a
~s current density of 1 A/dm2 and 100~C using AC. The anodes
were arranged parallel to i:he rack panel, and the deposi-
tion period was 60 minutes.
A finely crystalline, smooth, silken-matte alumi-
num layer had been produced., with no burnings or dendritic
growth on edges and tips. 'the cathode yield is 99.8. The
distribution was about 38~.
Example 3
zs
Coating of stepped angle metal sheets using DC
The same experiment as in Example 2 was carried
out using 1 A/dm2 DC instead of AC. A finely crystalline,
3o smooth, matte aluminum layer had been produced, with no
burnings or dendritic growth on edges and tips. Similarly,
the cathode yield was virtually 100. The layer-thickness
distribution on the metal sheet was identical to that of
Example
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Example 4
Coating of a slit cathode ~;J metal sheet)
s A slit cathode (J metal sheet) 50 mm in width, ha-
ving a slit of 2 mm, was coated for 30 min in the electro-
lyte of Example 1 using a current density of 1 A/dm2, with
the anodes in parallel arrangement to the flat side of the
cathode. Subsequent to bending up the metal sheet, it was
found that the coating had spread up to 7 mm from the ed-
ge, with a fluid run-out of the coating up to about 16 mm
from the edge. About 18 nun in the center of the metal
sheet remained uncoated.
is Comparative Example 1
Coating of a slit cathode (J metal sheet) according to EP
0 402 761
zo For comparison, an identical metal sheet was pla-
ted under the same conditions as in Example 4 in an
electrolyte having only to:Luene as solvent with no diiso-
propyl ether added (cf., EP 0 402 761 Al) . In this case,
the electrolyte spread only by about 4.5 mm from the edge
is into the split, the coating coming to an abrupt halt.
About 41 mm in the center of the metal sheet remained un-
coated.
