Note: Descriptions are shown in the official language in which they were submitted.
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A Device for the Electrodeposition of Aluminum or Aluminum Alloys
from Organometallic Electrolytes Containing Alkylaluminum
The invention relates to a device for the electrodeposition of aluminum or
alumi-
num alloys from organometallic electrolytes containing alkylaluminum, said de-
vice consisting of a supporting frame with a stand and transportation
bearings, at
least one plating barrel, at least one drive unit for the plating barrel, and
one or
more holding arms for the plating barrel.
Electroplating of small parts and bulk material in aqueous solution, such as
nickel-plating or zinc-plating, is usually effected in rotating, perforated
barrels
made of polyethylene or polypropylene. These barrels are driven by electric
motors arranged in a plastic housing in the supporting rack. Current transfer
to
the goods mostly is effected by means of flexible copper cords arranged
laterally
on the barrels and enveloped with a plasticized PVC tube to prevent
undesirable
epitaxial growth of metal.
Electrodeposition of aluminum or aluminum alloys from aqueous solutions is not
possible due to the very low position of the potential of aluminum.
Consequently,
electrodeposition must be effected from non-aqueous organic systems. In par-
ticular, electrolytes containing alkylaluminum are used to this end, with
organic
solvents normally being employed. Therefore, deposition of finely crystalline
aluminum and layers of aluminum alloys is achieved in an excellent fashion
from
anhydrous alkylorganoaluminum electrolyte systems, the alkylaluminum com-
plexes being dissolved in aromatic hydrocarbons such as toluene.
However, the plating barrels employed in aqueous electroplating cannot be used
in organic electrolyte systems. This is connected with the organic solvents
being
used and with the operating temperatures of from 90 to 100°C where such
elec-
troplating is carried out. At such temperatures and in the corresponding
organic
solvents, conventional barrels for aqueous systems are not stable, undergoing
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decomposition or dissolution, and thus may contaminate the electrolyte. Fur-
thermore, there is a risk of distortion of the barrels to such an extent that
me-
chanical stability is no longer guaranteed.
Also, electroplating systems for bulk material which are used in organic
media,
particularly for the deposition of aluminum, are known from the prior art. How-
ever, these systems failed to gain general acceptance in practice.
This also includes the state of the art described in EP 0 042 503 A1. Therein,
a
device for the electrodeposition of aluminum from organic electrolytes has
been
described. The aim of the above invention is to create a device where the
plating
barrel is not required to be removed from the plating trough for loading and
un-
loading. The above state of the art describes the use of a conveyor means for
the parts to be coated, which is used to fill the plating barrel and extends
via a
gate into the interior of the plating trough, terminating above a closable
opening
of the plating barrel. The barrel can be opened and closed from outside, and,
in
order to empty the barrel, a discharge container exposable to inert gas and
inert
fluid is provided, which is arranged beneath the plating trough and is
connected
with same via a tube-shaped connector element which can be shut off.
The above state of the art represents a highly complex construction of a
plating
barrel which failed to gain general acceptance in practice as yet.
The invention is based on the object of providing a device for the
electrodeposi-
tion of aluminum from organic electrolyte systems, in which device the plating
barrel is modified in such a way that the plating barrel is stable in the
media em-
ployed and at the temperatures applied, has a safe drive in flammable media,
and nonetheless allows high-quality coating with aluminum or alloys thereof.
Said object is accomplished by means of a device wherein the drive unit 3 is
ar-
ranged in an encapsulated gas-tight housing, the plating barrel 13 has a perfo-
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rated inner tube 15 arranged along the longitudinal axis thereof and open at
its
side, the lateral openings being arranged directly opposite the electrolyte
feed in
the electrolyte container, and the plating barrel 13 consisting of a material
which
is stable both in aqueous and organometallic electrolytes at temperatures up
to
110°C.
By encapsulating the drive unit in a gas-tight housing, driving the barrel in
flam-
mable liquids is made much safer. The case preferably consists of stainless
steel, and the drive shaft for the barrel is conducted through the housing
wall by
means of a gas-tight shaft guide with a sealing, preferably one made of poly-
tetrafluoroethylene.
To protect the drive motor, and as an additional safeguard against penetrating
flammable organic solvents, the housing case is flooded with an inert gas such
as nitrogen or argon and provided with an overpressure of preferably 0.1 to
0.3
bars. The housing is also equipped with a feed valve and an overpressure blow-
off valve with a nonreturn flap.
In each loadinglunloading procedure, inert gas at a pressure of about 0.1 to
0.2
bars above the set value of the blow-off valve is automatically fed via the
feed
valve into the drive unit housing on the station. Following each coating
process,
the inert gas atmosphere in the drive unit housing is purged, and the overpres-
sure in the housing is reset after each cycle. The purging time or the amount
of
inert purge gas is set via the plant controls.
Another problem of the plating barrels known from the prior art is the
stability of
the barrel material. In the long run, conventional barrel materials such as
poly-
ethylene and polypropylene are not stable in the organic solvents used in
alumi-
num coating.
This problem is solved ~by using suitable plastics insoluble in organic
solvents
and reinforced with fiberglass. In a preferred fashion, the plating barrels
are pro-
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duced from at least glass-fiber reinforced polyphenylene sulfide including a
pro-
portion of glass fiber of at least 40%. This ensures chemical stability of the
plat-
ing barrels at operating temperatures in the electrolyte of up to
110°C, as well as
abrasion resistance.
In a preferred embodiment, the drive gearwheels are made of the same material.
Another advantage is that this material is also stable in dilute acids and
bases,
so that pretreatment and secondary treatment of the parts to be electroplated
can be effected in aqueous systems such as acids andlor bases in the same
barrel without transferring.
Providing the plating barrel with a perforated inner tube results in an
improve-
ment of electrolyte circulation. In electrodeposition of metals from organic
elec-
trolytes, the electrolyte circulation plays an exceptionally important role
because,
as a result of the limited solubility of organometallic complexes, depletion
of
metal ions in the liquid boundary layer near the product may rapidly occur in
case of insufficient electrolyte circulation. This results in quality losses
in the
coating of the materials, especially in burning of the materials to be coated,
in
rough and uneven layers, and possibly even in electrolyte decomposition. In
par-
ticular, this problem arises in alloy deposition of aluminum, but is also
observed
in pure aluminum deposition. To avoid this problem, the inventive device in
the
barrel is equipped with a perforated inner tube which is arranged along the
lon-
gitudinal axis of the plating barrel and has lateral openings facing the
container
wall of the electrolyte container. When placing the device of the invention in
the
electrolytic bath, the lateral openings of the inner tube are situated
directly oppo-
site the electrolyte feed lines in the container wall. In this way, pumping of
fresh
electrolyte at high speed through the inner tube and directly to the substrate
is
accomplished during coating, so that good exchange is ensured and the draw-
backs described above are prevented from occurring. In a preferred embodi-
ment, it is also possible to arrange an additional auxiliary anode in the
inner
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tube, thereby further augmenting the local concentration of metal ions and in-
creasing the coating rate.
According to the prior art, the holding arms of conventional barrels are
mostly
rubber-coated and thus unstable in organic electrolytic baths. This also
applies
to the conventional PVC jackets of electric conductor tracks for the
electrolyte
current. When using such an arrangement, epitaxial growth of metal on the
power supply bars is therefore to be expected. According to the invention,
this
problem is solved in that the holding arm in the form of a hollow body
consists of
steel and has a core of polyphenylene sulfide. Arranged in this core of
insulating
material is the power supply bar for the power supply of electrolysis. It is
only in-
side the barrel where connection between the power supply bar and the contact
bulb in the product is made in the bearing of the holding arm. Owing to this
type
of construction, additional protection of the power supply bar against
undesirable
epitaxial growth of metal is no longer necessary. The holding arm itself has
no
electric potential applied thereto and is additionally protected on the
outside by a
plastic layer coated thereon, preferably one made of PVDF (polyvinylidene fluo-
ride) or of thermoplastic fluorocarbons based on ethylene and
chlorotrifluoroeth-
ylene.
The invention will be illustrated in more detail with reference to Figure 1
below.
The numeral 1 designates the supporting frame with stand, which includes the
single elements of the device, the plating barrel, the drive unit, and the
holding
arms. The supporting frame has transportation bearings 4 arranged thereon
which are used to lower or lift the device into or out of the respective
electrolyte
or rinsing baths.
The supporting frame has the drive motor 3 encapsulated therein, which is sus-
pended so as to be electrically insulated and has a gas-tight shaft guide 5. A
drive gearwheel 6 preferably made of polyphenylene sulfide is arranged at the
end of the shaft. The drive gearwheels drive the plating barrel 13 which
prefera-
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bly consists of glass-fiber reinforced polyphenylene sulfide. The plating
barrel 13
is connected with the supporting frame 1 via the holding arms 11. The holding
arms 11 are preferably made of stainless steel, they are hollow and coated
with
fluoropolymers on the outside thereof. The hollow space of the holding arms 11
includes an insulating material wherein the power supply bars 9, 10 for the
elec-
trolysis power supply are arranged. The numeral 12 designates the bearing
block for the plating barrel. The plating barrel has perforated side walls 14
and a
perforated inner tube 15 which is open at its side. An inner auxiliary anode
17
can be introduced into the barrel through this tube so as to achieve higher
elec-
trolyte concentrations near the material to be coated. The numeral 18
designates
the pick-up contacts arranged in the barrel, which preferably consist of
copper.
Furthermore, flexible current transfer contacts 16 are situated inside the
plating
barrel.
The numeral 9 designates the power supply line far the material to be coated,
which line is insulated inside the barrel holders. The numeral 7 designates
the
inert gas vent of the drive unit housing, including a nonreturn flap.
Using the device according to the invention, it is possible to produce high-
quality
coatings of aluminum or aluminum alloys. Coatings of magnesium and magne-
sium alloys are also possible, in which case the corresponding alkymagnesium-
containing electrolytes are employed. The device of the invention is hard-
wearing and can also be used in aqueous systems, e.g. in rinsing procedures.
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Reference list
1 Stand
2 Power supply bar
3 Encapsulated drive motor with electrically insulated suspension
4 Transportation bearings
Gas-tight shaft guide
6 Drive gearwheels
7 Inert gas vent of motor housing with nonreturn flap
8 Inert gas purge valve
9 Power supply line for material to be coated, insulated inside barrel holder
Insulating material inside holder
11 Barrel holder made of stainless steel, coated with PVDFIHaIar on its out-
side
12 Bearing block for barrel
13 Barrel made of glass-fiber reinforced PPS
14 Perforated side walls
Perforated inner tube
16 Flexible current transfer contacts
17 Inner auxiliary anode
18 Pick-up contacts
