U.S. patent application number 12/919697 was filed with the patent office on 2011-01-06 for coating method for a workpiece.
This patent application is currently assigned to EWALD DOERKEN AG. Invention is credited to Sandra Boehm, Thomas Kruse, Gerhard Reusmann.
Application Number | 20110000793 12/919697 |
Document ID | / |
Family ID | 40723703 |
Filed Date | 2011-01-06 |
United States Patent
Application |
20110000793 |
Kind Code |
A1 |
Kruse; Thomas ; et
al. |
January 6, 2011 |
COATING METHOD FOR A WORKPIECE
Abstract
The invention relates to a coating method for a workpiece,
including the following method steps: a) applying a coating liquid
to the workpiece, wherein the coating liquid comprises an ionic
liquid containing ions of at least one element, b)
electrochemically depositing a layer of the at least one element
from the coating liquid on the workpiece, c) removing the workpiece
from the coating liquid, d) removing excess coating liquid from the
workpiece. In order to suggest an industrially suitable coating
process, particularly for workpieces having at least a partial
metal surface, using stable, durable baths, it is provided that the
temperature of the workpiece is set such that the temperature of
the coating liquid deviates by no more than 10.degree. C. from a
predetermined set temperature during the coating process
Inventors: |
Kruse; Thomas; (Dortmund,
DE) ; Reusmann; Gerhard; (Essen, DE) ; Boehm;
Sandra; (Ennepetal, DE) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET, SUITE 1201
NEW HAVEN
CT
06510
US
|
Assignee: |
EWALD DOERKEN AG
Herdecke
DE
|
Family ID: |
40723703 |
Appl. No.: |
12/919697 |
Filed: |
February 20, 2009 |
PCT Filed: |
February 20, 2009 |
PCT NO: |
PCT/EP2009/001243 |
371 Date: |
August 26, 2010 |
Current U.S.
Class: |
205/50 ; 204/274;
205/143; 205/191; 205/198; 205/205; 205/206; 205/209; 205/220 |
Current CPC
Class: |
C25D 5/34 20130101; C25D
17/28 20130101; C25D 21/18 20130101; C25D 5/48 20130101; C25D 3/665
20130101; C25D 5/08 20130101 |
Class at
Publication: |
205/50 ; 205/220;
205/209; 205/205; 205/143; 205/206; 205/191; 205/198; 204/274 |
International
Class: |
C25D 5/48 20060101
C25D005/48; C25D 5/34 20060101 C25D005/34; C25D 5/04 20060101
C25D005/04; C23C 28/00 20060101 C23C028/00; C25D 17/00 20060101
C25D017/00; C25D 7/00 20060101 C25D007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2008 |
DE |
10 2008 011 204.6 |
Claims
1-15. (canceled)
16. A coating method for a workpiece, comprising the following
method steps: a) applying a coating liquid on the workpiece,
wherein the coating liquid comprises an ionic liquid containing
ions of at least one element; b) electrochemically depositing a
layer of the at least one element from the coating liquid on the
workpiece; c) removing the workpiece from the coating liquid; d)
removing excess coating liquid from the workpiece, wherein the
temperature of the workpiece is adjusted in such a way that the
temperature of the coating liquid deviates by no more than
10.degree. C. from a predetermined set temperature during the
coating method.
17. The method according to claim 16, wherein the temperature of
the workpiece is adjusted in such a way that the temperature of the
coating liquid deviates by no more than 5.degree. C. from the
predetermined set temperature during the coating method.
18. The method according to claim 16, wherein the surface
temperature of the workpiece is adjusted in such a way that it
deviates by no more than 10.degree. C., from the predetermined set
temperature during the method steps a)-d).
19. The method according to claim 16, wherein the workpiece is
heated prior to and/or during the method steps a)-d) at least on
its surface by means of hot air, infrared irradiation, by blasting
the surface, by contact with a hot bath or in an inductive manner,
and/or is cooled by means of cold air, by contact with a cold bath
or by means of evaporation.
20. The method according to claim 18, wherein the workpiece is
heated and/or cooled a) exclusively prior to, or b) exclusively
during, or c) prior to and during at least one of method steps
a)-d), so that its surface temperature deviates by no more than
10.degree. C., from the predetermined set temperature during the
method steps a)-d).
21. The method according to claim 16, wherein a layer of a metal or
an alloy, preferably comprising at least one of the elements
aluminum, magnesium, zinc, nickel, chromium, tantalum, titanium,
copper, silver or gold, or a layer of a semiconductor is
deposited.
22. The method according to claim 16, wherein the said method steps
are carried out in an inert gas atmosphere.
23. The method according to claim 16, wherein the coating liquid is
mixed prior to and/or during at least one of method steps
a)-d).
24. The method according to claim 16, wherein the workpiece is
moved during coating within an apparatus, wherein the apparatus
preferably works according to the principle of a conveyer screw, a
rotating drum, or a G-type drum.
25. The method according to claim 16, wherein the coating liquid
removed from the workpiece is at least partially recycled into the
bath.
26. The method according to claim 16, wherein the workpiece is
cleaned and/or dried and/or electrolytically polished prior to
method step a).
27. The method according to claim 16, wherein the workpiece is
rinsed after method step d), and dried after the rinse and/or
rinsed residues of the coating liquid are recovered and recycled
into the bath.
28. The method according to claim 16, wherein a top coat is applied
after coating.
29. An apparatus for coating a workpiece by depositing a layer of
at least one element from a coating liquid onto the workpiece,
wherein the coating liquid comprises an ionic liquid containing
ions of the at least one element, characterized by means for
temperature measurement, by means of which a deviation of the
temperature of the coating liquid and/or the temperature of the
workpiece by 10.degree. C., from a predetermined set temperature is
determinable, and means for heating and/or cooling the
workpiece.
30. A workpiece coated using a method according to claim 16.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a coating method for a workpiece,
and to an apparatus for coating a workpiece.
[0002] Since the useful life of metallic or metal-plated workpieces
can be substantially limited by corrosion, the provision of
reliable corrosion protection is crucial, in particular for
workpieces exposed to the weather.
[0003] From the state of the art, coating methods are known for
this purpose where a liquid coating agent is used based on a
binder, it is also known to cover a workpiece with a metal coat
that directly chemically or physically adheres to the substrate. A
widely used approach for this is the electrochemical deposition of
metal from a coating liquid in which a salt of this metal is
dissolved. The workpiece to be coated is dipped into a bath of the
coating liquid. The workpiece in most cases acts as a cathode on
which the metal ions are reduced. For this purpose, a voltage is
applied to the workpiece resulting in a predetermined, usually
negative, potential on the workpiece with respect to a reference
electrode.
[0004] This method allows excellently adhering, closed,
anti-corrosive metal coatings to be produced, as the case may be
after suitable pre-treatment. With the method described above,
semi-metals and semiconductors, such as silicon, can be deposited
as well as metals. It is also possible to electrochemically coat
non-metallic workpieces, as long as their surfaces have been made
conductive by a pre-treatment.
[0005] It must be noted that only certain specific metals can be
deposited with this technique depending on the solvent used. If,
for example, an aqueous solution is used, it is not possible to
deposit elements having a deposition potential lying in the
decomposition range of water. Instead, hydrogen gas accumulates at
the cathode. Within certain limits, the problem mentioned may be
solved by increasing the salt concentration, however, if the
standard potential is too low, deposition of the element in
question is impossible from an aqueous solution. This applies to
aluminum, for example, which plays an important role in corrosion
protection as it forms a strong, passivated oxide layer when
exposed to ambient air, which prevents oxidation of the underlying
metal, and also serves as a sacrificial anode for the underlying
steel substrate in the case of damage to the coating. There is thus
great interest in aluminum coatings in parts of the automotive
industry, for example.
[0006] If the ions lost by deposition are to be replaced by means
of dissolution from a sacrificial anode, a potential range of about
2V is defined by the deposition processes of oxygen on the one hand
and hydrogen on the other, within which the elements must be
present that are to be used for coating in aqueous solution. It is
known from research that this drawback can also be overcome by the
use of a different solvent. Ionic liquids are of growing importance
in this context. They are salts having a melting temperature of
below 100.degree. C. The corresponding salt melt can also serve as
a solvent for a salt of the desired coating metal. By using ionic
liquids, according to the state of the art, the accessible
potential range with respect to water is extended from about 2V to
up to 6V.
[0007] As the case may be, the anions of the ionic liquid can be
identical to those of the dissolved salt. US 2004/0238352 thus
discloses a method wherein aluminum chloride is dissolved in a melt
of 1-butyl-3-methylimidazolium chloride and aluminum is deposited
from this solution.
SUMMARY OF THE INVENTION
[0008] Considering this, it is the object of the present invention
to suggest an industrially useful coating process, in particular
for workpieces having at least a partial metallic surface, with
stable, durable baths.
[0009] The object is achieved by a coating method for a workpiece,
by an apparatus and by a workpiece as disclosed herein. The method
comprises the following method steps: [0010] a) applying a coating
liquid on the workpiece, wherein the coating liquid comprises an
ionic liquid containing ions of at least one element; [0011] b)
electrochemically depositing a layer of the at least one element
from the coating liquid on the workpiece; [0012] c) removing the
workpiece from the coating liquid; [0013] d) removing the excess
coating liquid from the workpiece.
DETAILED DESCRIPTION
[0014] According to the present invention the temperature of the
workpiece is adjusted in such a way that the temperature of the
coating liquid only deviates from a predetermined set temperature
by no more than 10.degree. C. during the coating process.
[0015] The essential idea underlying the present invention is that
the useful life of an ionic liquid used as a coating liquid is
particularly stable and durable, and thus industrially useful, if
it is maintained within a range of a predetermined temperature, the
set temperature, that is within tight limits, i.e. deviating by no
more than 10.degree. C. from the set temperature. The term set
temperature refers to a precisely defined temperature that is
deemed optimal.
[0016] The temperature of the coating liquid--and thus the quality
of the coating--is determined in the industrial method in
particular by the fact that usually a great number of workpieces,
or a large workpiece, is coated in the individual coating process.
This is how the great mass of the workpiece(s) absorbs a lot of
heat from the coating liquid (or introduces it, as the case may
be). This applies all the more since metallic workpieces usually
have excellent heat conductivity and high specific heat capacity.
Moreover, industrial facilities usually work in a continuous
operation, i.e. coating processes of the individual batches follow
each other without major pauses. Keeping the temperature of the
coating liquid as constant as possible has thus been shown to
considerably extend the useful life of the ionic liquid for
coating, and is advantageous for the quality of the coating when
used in industrial coating methods.
[0017] This is why adjusting the temperature of the coating liquid
is of particular importance in the method of the present invention.
According to the present invention it is suggested that the
workpiece to be coated is heated and/or cooled, either as a sole
process or in combination with heating or temperature controlling
the coating liquid. Heating or temperature controlling the
workpiece has the advantage that there is no undesired cooling or
heating effect on the coating liquid by the workpiece at precisely
the point where the deposition takes place. By these means, an
excellent quality of the deposited layer is ensured. The terms
"heating" and "warming" will be used as synonyms in the following,
i.e. they are not to be construed as identifying different
strengths of heat flows or different temperature ranges.
[0018] It is advantageous that a deterioration of the conductivity
of the coating liquid due to temperature variations is prevented by
the method according to the present invention. By these means
coating is made possible under uniform conditions resulting in a
high-quality metal layer being produced which also fulfils premium
requirements, as for example in the automotive industry.
[0019] Furthermore, one and the same coating bath can be used over
long periods of time, i.e. there is less ionic liquid that has
become unusable, which must be disposed of. This means on the one
hand that costs can be saved and on the other hand that there is
less impact on the environment.
[0020] With reference to the constant set temperature according to
the present invention, preferably all components of the coating
liquid are explicitly included, i.e. also excess components, still
adhering to the workpiece until they are removed, or even beyond
that, as the case may be, if removal is incomplete, and which are
preferably recycled to be used for renewed coating of
workpieces.
[0021] In the method according to the present invention, workpieces
with a metallic surface are preferably coated, i.e. those that are
either completely of metal, or an alloy, as the case may be, or are
provided with a metallic coating. This metallic coating can also
include non-metallic components. However, workpieces with a
non-metallic surface can also be used, such as plastic parts that
are prepared for electrochemical coating by means of activation,
i.e. having a surface that has already been made conductive by
means of pre-treatment, for example, with a suitable varnish, or
that is already conductive due to the presence of additives in the
plastic material.
[0022] Preferably the ions of the at least one element to be
deposited are metal ions. Particularly preferred are ions of at
least one of the elements aluminum, zinc, magnesium, nickel,
chromium, tantalum, titanium, copper, silver and/or gold. In
particular, it is possible to deposit a layer of an alloy of these
metals from an ionic liquid containing ions of a plurality of
metals. Ions of semimetals or semiconductors, in particular silicon
and/or germanium ions, are also preferred, however. Again, it is
possible to deposit a layer constituting a mixed phase of the
respective elements from an ionic liquid containing ions of a
plurality of semiconductors or semimetals. The deposition of mixed
phases of at least one metal and at least one semiconductor or
semimetal (e.g. of aluminum and silicon) is also conceivable in the
context of the present invention. In the following "metal layers"
will be used as a simplified expression, but the explanations will
always refer to all layers of at least one element.
[0023] The coating liquid according to method step a) is usually
applied by dipping the workpiece into the coating liquid. It is
also conceivable to apply the coating liquid by different means,
e.g. by means of casting devices.
[0024] To remove the excess liquid in method step d) various
methods are possible, such as draining, spinning-off or
blow-removing by means of an air flow (temperature controlled, as
the case may be). All the methods mentioned can also be
combined.
[0025] In a further development of the inventive method, for those
cases in which maintaining the set temperature is particularly
important, the temperature of the workpiece is adjusted in such a
manner that the temperature of the coating liquid deviates during
the coating process by no more than 5.degree. C. from the
predetermined set temperature.
[0026] As already explained, the temperature of the coating liquid
is critically influenced by its contact with the workpiece. One way
of preventing any deleterious effects by the workpiece is to
maintain its surface temperature within the range of the set
temperature to a sufficient degree. This is why in a preferred
embodiment of the invention, care is taken that the surface
temperature of the workpiece does not deviate from the set
temperature by more than 10.degree. C. during the method steps a)
to d). If this temperature range is adhered to, a substantial
interfering factor in the deposition of metal layers from ionic
liquids is eliminated. In this context, surface means all surfaces
of the workpiece with which the coating liquid can come into
contact.
[0027] To ensure the above-described controlled temperature,
various measures are conceivable. Preferably, the workpiece is
heated for this purpose prior to and/or during the method steps
a)-d) at least on its surface by means of hot air, infrared
irradiation, by blasting the surface, by contact with a heat bath
or in an inductive manner. Whether advance heating is sufficient
can depend, for example, on the size or geometry of the workpiece.
A large workpiece with a relatively small surface area loses its
initial temperature more slowly than a small workpiece with a
relatively large surface area.
[0028] Heating with hot air has the advantage that all exposed
surfaces of a plurality of workpieces, as the case may be, can be
treated. Infrared irradiation is advantageous due to the more
efficient heat transfer. Inductive heating has the advantage that
the workpiece is not only heated on the surface but also in its
interior. This method is also particularly efficient. Heating by
means of blasting (e.g. sand blasting) the surface has the
advantage that heating is accompanied by a pre-treatment of the
surface. Heating in a heat bath in which the workpiece is dipped,
can also be combined, as the case may be, with degreasing or the
like of the workpiece in the same bath. Furthermore, heating in a
bath can be realized with relatively little technical overhead, and
good heat transfer to the workpiece is ensured.
[0029] If, for example, a workpiece heated too much due to
pre-treatment, is to be coated, it is preferable to cool the
workpiece prior to and/or during the method steps a)-d) at least on
its surface by means of cold air, by contact with a cold bath, or
by means of evaporation. The cold air can either be stagnant or
flow around the workpiece as an air flow. A cold bath can either be
a solid body or a liquid, which are cooler than the workpiece.
Herein, even a holding or receiving device can serve as a cold
bath. For example, a gripping arm, guiding the workpiece, or a
basket in which the workpiece resides, can be cooled in turn, in
order to cool the workpiece. A particularly effective method of
cooling is by means of evaporation. Herein, a liquid is evaporated
on the surface of the workpiece, whereby a particularly large
amount of heat is removed from the workpiece. If the temperature of
the workpiece is below the boiling temperature of the liquid,
evaporation can be forced by an air flow or a reduction of the air
pressure.
[0030] To ensure a sufficiently constant surface temperature of the
workpiece, a plurality of different temporal sequences are suitable
for heating. With reference to each individual method step, it is
conceivable to heat prior to and/or during said step in such a
manner that, during method steps a)-d), the surface temperature of
the workpiece deviates from the predetermined set temperature by no
more than 10.degree. C. It is thus possible to heat the workpiece,
for example, prior to being dipped into a coating bath, stop
heating during coating, and resume it subsequently prior to and
during spinning-off of the liquid residue. Alternatively, it is
possible to dispense with heating prior to and during coating, but
to heat the workpiece during spinning-off of excess liquid to
prevent the spun-off ionic liquid from cooling off too much. Which
temporal sequence is chosen in each individual case depends, for
example, on the duration of the individual method steps and the
nature of the workpiece (size, surface area, material etc.).
[0031] It is recommendable to estimate or compute for coating a
batch of workpieces, the amount of heat that is removed from the
coating liquid during coating and then to ensure that this amount
of heat is delivered to the workpieces in such a way that
undesirable cooling of the coating liquid is prevented. A suitable
guideline for estimating the required heat amount is the mass to be
coated. If the mass of the workpieces to be coated is known, the
required heat amount can still vary, e.g. depending on the heat
conductance of the workpieces and on the surface area of the
workpieces. If the coating liquid is temperature controlled, the
amount of heat introduced by temperature controlling must also be
considered in the estimation. Taking all these factors into
account, the amount of heat necessary for heating or temperature
controlling the workpieces can usually be estimated or calculated
with sufficient precision.
[0032] To prevent any deleterious effects on the coating liquid due
to unwanted chemical reactions with the ambient air, in a further
development of the method, the above-mentioned method steps are
carried out in an inert gas atmosphere. This precaution is
necessary for a number of ionic liquids to ensure durable quality,
typically with those ionic liquids that are strongly hygroscopic.
It must be taken care of, in particular, that in the method steps
c) and d), liquid is exposed to the atmosphere in film or drop
form, which means with a relatively large surface area. If the
components of the liquid in question are to be recuperated,
protection by inert gas is particularly important.
[0033] Almost all coating liquids show sedimenting behavior. It is
thus advantageous if the coating liquid is mixed prior to and/or
during the method steps a)-d). This can be done by means of a
mechanical stirrer that is driven by a motor and a shaft. Magnetic
stirrers are also advantageous, however, since they do not need an
additional opening in a container wall for mechanical coupling.
Mixing by means of ultrasonic waves is particularly advantageous
since no additional parts are needed inside of the respective
container at all. The respective means for mixing can be
periodically or continuously operated. In addition to homogenizing
the coating liquid, mixing also contributes to the uniform
temperature control of the coating liquid.
[0034] For the electrochemical coating of a workpiece, various
apparatuses are known. It has been found that these apparatuses are
also suitable for coating with ionic liquids. In a preferred
embodiment of the method the workpiece is moved during coating with
ionic liquid within an apparatus according to the principle of a
conveyor screw, a rotating drum or a G-type drum (patent
application filed with the official file no. DE 10 2007 018 887.2).
The above-mentioned apparatuses have the advantage that seamless
coating in a single coating process can be guaranteed also with
small bulk pieces, such as bolts, nuts, washers, rivets etc. This
is because the workpieces are rolled over which means that there
are no fixed points of contact which would otherwise prevent
overall treatment by the coating substance. Particularly preferable
are apparatuses that work according to the conveyor screw
principle. Such an apparatus is known, for example, from DE 42 05
672.
[0035] Due to the high investment needed for ionic liquids, all
losses of the coating liquid should be avoided. This is why in a
further embodiment of the method, the excess coating liquid removed
from the workpiece is at least partially recycled into the bath.
Recycling can be passive, by draining back, or active, by means of
pumping or the like. By these means, the losses due to removal of
liquid together with the workpiece can be substantially reduced,
which leads to high cost-savings in particular when a great number
of coating processes are involved.
[0036] In many cases it makes sense to appropriately prepare the
workpiece for coating. The workpiece is thus preferably cleaned
and/or dried for introduction into the coating bath. Various
methods are suitable for cleaning. This can be done mechanically,
such as by sand blasting, metal blasting, glass bead blasting or
soda blasting of the workpiece. Chemical cleaning steps are also of
particular importance, such as etching, pickling or degreasing of
the workpiece. Degreasing substances, in addition to organic
solvents, are, in particular, aqueous solutions, in particular
alkaline solutions or those to which additives, such as tensides,
have been added. Degreasing can be done by spraying under pressure
or in a dip bath, wherein, in the latter case, the degreasing
process is substantially improved by the use of ultrasonic waves.
The efficiency can also be increased by higher temperatures, such
as with hot alkaline degreasing. Drying the workpiece can be
carried out by means of cold or hot air, by irradiation with
infrared or microwaves and/or by means of negative pressure.
According to a preferred embodiment of the invention, drying of the
cleaned workpieces can be used for heating or cooling the
workpiece. The above described measures for preparing the workpiece
are of particular importance for working with ionic liquids, since
they are often sensitive with respect to contamination of any kind,
and in particular with respect to introduced moisture.
[0037] To ensure a particularly clean surface of the workpiece, in
a preferred embodiment of the method, the workpiece is
electrolytically polished prior to the deposition process. This is
also referred to as "in situ" electrochemical etching. In this
process, ions are detached from the surface of the workpiece by
applying a suitable voltage (usually over a short period of time),
i.e. the workpiece functions as an anode. On the one hand,
microscopic bumps are thus removed, and on the other hand
microscopic contamination is removed from or out of the surface.
This method is also suitable, for example, to remove oxide layers
from steel, which would interfere with the adhesion of any coating
to be deposited. This cleaning step can be carried out in the ionic
liquid that is also used for coating, wherein the voltage is
reversed with respect to the coating process. It is also
conceivable, however, to provide a separate bath for this purpose.
While the first variant needs simpler apparatuses and saves time,
the latter variant helps to avoid that the coating liquid is
contaminated by substances removed from the workpiece.
[0038] While it is possible to remove most of the adhering liquid
from the workpiece by spinning the workpiece in method step d),
residues will usually remain. For this reason, in a further
development of the method, the workpiece is rinsed by means of a
rinsing liquid after removing the excess liquid. The term rinsing
liquid also includes liquids in which the ionic liquid can be
emulsified, in addition to those in which it is dissolvable.
Rinsing on the one hand is for cleaning the workpiece. The
workpiece can also be prepared for any other coating processes by
means of the rinsing process. Finally there is the possibility, in
a further improvement of the method, to recover the residues of the
coating substance removed from the workpiece from the rinsing
liquid and to recycle them into the bath. In this case, care must
be taken that the rinsing liquid used for rinsing does not react
with the ionic liquid.
[0039] After the rinsing process, rinsing liquid residue often
still adheres to the workpiece. In order to remove this, it is
preferred to dry the workpiece after rinsing. This can be done by
renewed spinning, drying in a cold or hot airflow, or by other
means known from the state of the art.
[0040] While it is possible, with the method according to the
present invention, to create closed layers, ensuring excellent
corrosion protection, for example, it may be desirable to adjust
certain surface properties by means of an additional coating. It
may be desirable, for example, to create a colored graphic design
or to adjust a coefficient of friction. Moreover, the
electrochemically applied coating can be protected against
mechanical damage by an additional coating. For this reason,
according to a further development of the method, a top coat is
applied after coating. Suitable top coats are known from the state
of the art.
[0041] The usability of an ionic liquid in the context of an
industrial coating process is ensured over long periods of time by
the method according to the present invention. Degredation of the
conductivity of the coating liquid due to temperature changes is
prevented. The method thus enables the deposition of high quality
metal layers, in particular of aluminum, from a coating bath, which
remains useful over a long period. The coating bath thus only
rarely needs to be refilled or exchanged, which results in
substantial cost savings. Furthermore, the disposal of ionic liquid
that has become useless is far less frequent, which is advantageous
both from an economic and an ecological point of view.
[0042] The method according to the present invention can be carried
out by means of an apparatus for coating a workpiece. Since coating
is achieved by means of deposition from a coating liquid, which
comprises an ionic liquid containing ions of at least one element,
the apparatus must comprise at least two electrodes according to
the state of the art (one electrode for contacting the workpiece,
and a counter electrode). Usually a coating container for receiving
the coating liquid during the coating process is also necessary. It
may be advantageous to carry out the coating process with a
so-called "three-electrode arrangement" to apply a precise
potential to the workpiece.
[0043] According to the present invention, such an apparatus
comprises means for temperature measurement, by means of which a
deviation of the temperature of the workpiece by 10.degree. C.,
preferably by 5.degree. C., from a predetermined set temperature
can be determined, and means for heating and/or cooling the
workpiece.
[0044] The means for temperature measurement can either work in a
contact-free manner (by measuring the infrared radiation) or by
contacting the workpiece or the coating liquid respectively (for
example as a bimetal thermometer or a resistance thermometer). Such
temperature sensors are known from the state of the art and usually
work with a sufficiently high (usually substantially better)
measuring accuracy to be able to determine temperature differences
of 5.degree. C. or 10.degree. C.
[0045] As already explained, such an apparatus can optionally
comprise further components, for example, for spinning off coating
liquid from the workpiece or for mixing the coating liquid.
[0046] Furthermore, in view of the chemical offensiveness of many
ionic liquids, it is also suitable to manufacture those parts of
the apparatus that come into contact with the coating liquid of a
chemically insensitive material, such as ceramics.
[0047] The functioning of the invention will now be explained with
reference to exemplary embodiments.
EXAMPLE 1
State of the Art
[0048] 20 kg of steel bolts are to be coated with aluminum.
80.degree. C. has been determined as the set temperature for the
coating process. The bolts are prepared for coating first by sand
blasting and then by degreasing in a basket in a cleaning solution
consisting of water in which 9 g of potassium phosphate and 27 g of
potassium hydroxide have been dissolved per one liter of water, at
85.degree. C.
[0049] A thermostat connected to the bath is used to ensure that
its temperature is within a range of between 80.degree. C. and
90.degree. C. After a soaking time of 5 minutes, the basket is
lifted out of the bath. The basket with the bolts is rinsed with
tap water at a temperature of about 80.degree. C. and subsequently
spun dry. Thereafter, the bolts are further dried by means of an
airflow preheated to about 90.degree. C.
[0050] After completion of drying, the basket is introduced through
a first lock door into a lock chamber, the first lock door is
closed, and the lock chamber is partially evacuated to 0.05 bar. By
these means, the last moisture residue evaporates. Subsequently,
the lock chamber is flooded with nitrogen. Induction coils are
integrated in the walls of the chamber, by means of which the bolts
can be inductively heated, if necessary. Herein, it is checked by
means of an infrared camera, whether the temperature of the bolts
is within the predetermined range of between 70.degree. C. and
90.degree. C.
[0051] The lock chamber communicates with a coating chamber filled
with a nitrogen atmosphere via a second lock door. The bottom of
the coating chamber is configured as a basin filled with a coating
bath. The coating bath consists of a melt of
1-ethyl-3-methyl-1H-imidazolium chloride (EMIC), in which aluminum
chloride is dissolved. The mass ratio of EMIC:AlCl.sub.3 is 1.7:1.
Temperature sensors are used to continuously check whether the
temperature of the walls of the coating chamber deviates from the
set temperature. In the case of a deviation, additional heating is
carried out by means of heating elements integrated into the walls,
wherein the heating power is adjusted in dependence on the
magnitude of the deviation. In this way it is ensured that the
temperature never deviates from the set temperature by more than
10.degree. C. A plurality of temperature sensors is also spatially
distributed in the basin, which check the temperature of the
coating liquid itself. Furthermore, a vertically traversable
coating drum with perforated walls is arranged in the coating
chamber. The drum can be rotated about its longitudinal axis by
means of a motor. The drum itself is also heatable and is
maintained within the predetermined temperature range by means of a
thermostat.
[0052] The basket is introduced into the coating chamber through
the second lock door, then the second lock door is closed. At this
point, the coating drum is located outside of the coating bath. The
bolts are introduced into the coating drum through an opening,
subsequently the drum is traversed downwards into the coating bath.
In the interior of the drum, the coating liquid flows around an
aluminum electrode, which is connected to the outer wall of the
drum via a voltage supply. For coating, the drum is slowly rotated
at 20 rpm while a voltage of 20V is applied between the aluminum
electrode and the outer wall of the drum so that the aluminum
electrode functions as an anode. As a result, the bolts in contact
with the walls of the drum are coated by the deposition of aluminum
from the coating liquid, while aluminum ions are continuously
detached from the anode by oxidation, so that the aluminum
concentration remains constant in the coating liquid.
[0053] After a coating duration of 5 minutes, an Al layer having a
thickness of about 10 .mu.m has been deposited on the bolts, the
voltage is switched off, the drum is stopped and lifted up out of
the bath, wherein most of the liquid drains off. Subsequently, the
drum is caused to rotate in a fast rotation at 300 rpm, to spin off
liquid residue. After spinning, the basket is traversed to a
position below the drum to receive the bolts, the drum is opened
and emptied into the basket through the opening rotated into a
bottom position.
[0054] Hereafter, the basket is introduced into a rinsing chamber
filled with nitrogen through a third lock door, and the third lock
door is closed. There are a number of spraying nozzles in the
rinsing chamber, by means of which an aprotic rinsing substance is
sprayed onto the bolts. In this way final residues of the coating
liquid are removed, which are carried off together with the rinsing
substance for separation and recycling into the coating bath.
Subsequently, the basket is spun again and traversed out of the
rinsing chamber through a fourth lock door.
EXAMPLE 2
[0055] As in example 1, 20 kg of steel bolts are to be coated with
aluminum. This time, 20.degree. C. is set as the set temperature
for the coating process. Again, the bolts are sand blasted and
degreased in a fashion analogous to example 1. Since the degreasing
process is substantially more efficient at higher temperatures, it
is similarly carried out at 85.degree. C. for 5 min.
[0056] Rinsing, in this case, is carried out with tap water at a
temperature of about 20.degree. C. This is followed, again, by a
spin drying process.
[0057] Treatment in diluted hydrochloric acid at about 20.degree.
C. for 5 min. is next, whereby most of the oxide layer on the bolts
is removed. Subsequently, they are rinsed in distilled water at
20.degree. C. and spun dry.
[0058] Then the bolts are further dried by means of an airflow
heated to about 40.degree. C. The temperature of the airflow which
is increased with respect to the set temperature, is for
compensating heat losses due to the evaporation of liquid.
[0059] After drying, the basket is, again, introduced through a
first lock door into a lock chamber, where last liquid residues are
evaporated by means of partial evacuation. Subsequently, the lock
chamber is flooded with nitrogen having a temperature of 20.degree.
C. Induction coils are integrated in the chamber walls, by means of
which the bolts can be inductively heated as needed. There is an
additional possibility of directing a flow of nitrogen at a
temperature of 0.degree. C. onto the bolts by means of a nozzle
integrated in the chamber wall to cool them if necessary. An
infrared camera is used to check whether the temperature of the
bolts is within the predetermined range of between 10.degree. C.
and 30.degree. C.
[0060] The lock chamber communicates via a second lock door with a
coating chamber filled with a nitrogen atmosphere. The structure of
the coating chamber is similar to the one in example 1. Again, the
bottom of the coating chamber is configured as a basin filled with
a coating bath. This basin is made of a ceramic material that is
chemically particularly insensitive. The coating bath consists of a
melt of 1-ethyl-3-methylimidazolium chloride, in which aluminum
chloride is dissolved. The molar ratio of
1-ethyl-3-methylimidazolium chloride to ALCl.sub.3 is 2:3.
Temperature sensors are used to continuously check whether the
temperature of the walls of the coating chamber deviates from the
set temperature. If there is any deviation, heating or cooling can
be carried out by passing water at a suitable temperature through
the heating/cooling pipes integrated into the walls. In the normal
state, the above mentioned pipes have water flowing in them at a
temperature of 20.degree. C. A plurality of temperature sensors is
also spatially distributed in the basin for checking the
temperature of the coating liquid itself.
[0061] In order to be able to carry out cooling of the coating
liquid if needed, the coating chamber comprises a snake-like heat
exchanger system arranged within the basin and extending through
the coating liquid. To avoid damage, the arrangement is chosen in
such a manner that any contact between the drum and the heat
exchanger system is prevented. Cool water can be caused to flow
through this heat exchanger system while, in the normal state,
water at a temperature of 20.degree. C. is used.
[0062] The above mentioned cooling systems are necessary since,
even if the workpieces are carefully temperature-controlled, the
workpiece and the coating liquid can be heated by the deposition
process, since it involves the transformation of electrical energy
into heat.
[0063] Also, at the bottom of the basin, there is a continuously
operated magnetic stirrer, which, on the one hand, homogenizes the
coating liquid and, on the other hand, ensures uniform temperature
control and, by moving the coating liquid, also promotes the heat
exchange between the liquid and the heat exchanger system.
[0064] A coating drum with an aluminum electrode is used as in
example 1. The drum is coated with a ceramic material, a series of
electrodes for contacting the bolts are, however, provided on the
inner wall.
[0065] The introduction of the workpieces and the traversal of the
coating drum is carried out as in example 1. To prepare the bolts
for the coating process, they are treated with in situ
electrochemical etching. For this purpose, a voltage of 0.8V is
applied between the electrodes in the wall of the drum and the
aluminum electrode. The drum is caused to rotate at a slow 20 rpm,
wherein the bolts function as an anode due to their contact with
the electrodes in the drum wall. By these means, the last residue
of oxide is removed. After 2 min., the etching process is finished,
and an opposite voltage of -0.2V is applied, whereby the aluminum
electrode now functions as an anode, while the bolts are coated by
the deposition of aluminum from the coating liquid. The rotation of
the drum is continued during the coating process lasting 10
min.
[0066] Lifting the drum out of the bath, spinning off of liquid
residues and the further treatment of the bolts is analogous to
example 1.
* * * * *