Note: Descriptions are shown in the official language in which they were submitted.
CA 02465273 2004-04-27
Specification for entering the national phase
of PCT/BE02/00162
METHOD FOR COATING METAL SURFACES
Subject of the invention
[0001] The present invention relates to a method
for continuously coating a substrate in motion, more
particularly a metal sheet of coated or galvanised steel,
by means of an ultra-thin protective layer of oxide
nanoparticles, preferably silicon, titanium or zirconium.
Technological background
[0002] The benefits of different materials such as
zinc or aluminium are well known for the protection of
steel sheets against corrosion. Numerous methods allow the
continuous deposition of a layer of zinc or of aluminium
onto a sheet passing through a bath containing one or the
other of these metals or one of their alloys in a molten
state. These deposits obtained by immersion generally have
a thickness of 5 to 25 microns.
[0003] Thinner deposits, at the most of a few
microns, can be achieved by electro-deposition or sometimes
from the vapour phase of the protective metal.
First approach: coating without chromate
[0004] After the provision of anticorrosion
protection by the layer of zinc or another expendable metal
described above, it is necessary to achieve a further layer
which at the same time facilitates the attachment of a
possible subsequent coating and in addition protects the
surface of the coated metal against changes of aspect in
the course of storage. Several types of coating are known,
among which phosphate treatments with zinc, alkaline
treatment, silanation, treatment with chromate, etc. The
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choice of a specific treatment depends on the type of use
for which the product is intended.
[0005] At the present time, the best resistance
against corrosion is obtained with methods including at
least one chromic rinse. Unfortunately, the hexavalent
chrome that is used in these treatments is a very toxic
substance whose use is increasingly subjected to
regulation. In view of this situation, a growing demand for
steels coated without hexavalent chrome is observed.
Second approach: coil-coating at the end of the
galvanisation line
[0006] One of the techniques allowing to apply
undercoat and paint in a particularly effective manner is
the "coil-coating" technique, that is to say the continuous
deposition of an organic coating on a sheet in motion which
is re-coiled at the end of the line. However, these days,
the painting operation is most often dissociated from the
galvanisation operation. This situation is explained by the
great difficulty of applying paint on lines as fast as
galvanisation lines. Thus, the deposition of paint is
performed either by the buyer of the metal plates or on
lines exclusively dedicated to pre-treatment and painting.
[0007] Enormous joint efforts of metallurgists and
paint manufacturers aim to simplify the methods for
depositing paints so as to make them practical at high
speed. The aim is to be able to apply a metal coating, such
as zinc or another metal, and paint on the same line. The
benefits of such a combination are multiple. Firstly, the
use of oil for anticorrosion protection of the sheet during
storage or transport is eliminated. In addition, the number
of installations and in particular the major investment and
maintenance costs associated with uncoiling and re-coiling
are reduced.
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Promising solutions: oxide of silicon, titanium, zirconium,
aluminium, cerium or antimony
[0008] If the methods for producing the protective
layer of metal surfaces, whether coated or not, are
examined, oxides of silicon, titanium, zirconium,
aluminium, cerium or antimony are very promising compounds.
They are first of all resistant to oxidants. They are also
electrical insulators and moreover, they are relatively
inert from a chemical point of view. Thus, an ultra-thin,
dense layer of this type of oxides could suffice to ensure
good protection against corrosion.
[0009] Paradoxically, it is known that metal
surfaces always bear an oxide film but that, in its natural
form, it cannot really be used to fulfil the functions of
protection and attachment. Indeed, procedures such as hot
galvanisation or thermal oxidation lead to the formation of
oxides, for instance of zinc or of aluminium, on the
surface of steel sheets. However, this protection is weak
and these oxides are moreover inert and do not allow, or do
not allow so well, the subsequent attachment of an organic
coating such as paint (see for example "Le livre de
1'acier" (The book of steel), G. BERANGER et al., Ed.
Lavoisier Tec & Doc (1996) pp. 700-701). In order to
overcome this difficulty, one may take the trouble to
chemically transform the galvanised surface, achieved by
the dissolution and/or removal of these inert oxides, into
a reactive surface available for subsequent coating.
[0010] The above-mentioned beneficial oxides (of
silicon, etc.) have the advantage of being both compatible
with metals and with the organic materials used in the
composition of paints. Indeed, the Applicant noted that
these oxides and the metal surfaces to be protected may, in
certain conditions, chemically bond via an oxygen bridge.
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Likewise, with the formation of organic silanes, it is
possible to obtain covalent bonds of great energy between
organic molecules and oxides of this type. The latter
therefore make ideal candidates for allowing an excellent
bond between the substrate and paint.
[0011] Moreover, the need to find new methods for
coating galvanised steels has increased the importance of
the chemical compounds that had been neglected up to now.
[0012] Thus, nanoparticles which have however been
commercially available for over 60 years have only been
actually incorporated as a principal component of a
treatment layer before paint for a dozen years or so.
[0013] The advantages of the use of nanoparticles
are multiple. First of all, they are less reactive than
molecular precursors such as silanols, mineral salts or the
organometallic precursors and therefore allow to achieve
far more stable solutions. Furthermore, they are
sufficiently small to allow the creation of ultra-thin
coatings (a few hundred nanometres). In addition, their use
allows the production of more malleable coatings than dense
coatings (for instance in molten glass). Lastly, although
solutions of these nanoparticles are classified as
corrosive or harmful, they are neither toxic nor dangerous
to the environment as solutions based on hexavalent chrome
can be.
State of the art
[0014] Attempts at wet deposition of ultra-thin
coatings of, amongst other things, silica nanoparticles,
have been carried out but these tests have proved to be
unsatisfactory in terms of reaction speed. The deposition
speed must in fact be very rapid because the coating must
be achieved on the galvanising lines themselves, the speed
of which is typically 2 or 3 metres a second.
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[0015] Amongst other attempts, one may cite:
- the deposition of a ethysilicate/silica mixture (sol-gel
technology) on aluminium; this method requires slow
drying, hence takes a long time, in order to limit the
5 formation of cracks when the solvent evaporates (American
patents US-A-5 514 211 and US-A-5 879 437 in the name of
Alcan Inc.);
- the rinse, which also takes a long time, in a solution of
silicate/metallic salt followed by a treatment based on
silane, which is necessary given the lack of energy
supplied to the system. This method has been developed by
Armco Steel and is the subject of European application
EP-0 492 306-A2;
- the immersion of coated parts in a mixture based on a
mineral or organic silicate, generally of potassium
silicate. This operation is carried out at a slightly
higher temperature (125°C). Good properties of resistance
against corrosion are claimed but the attachment of
subsequent coatings is not taken into account and the
treatment times are still long since times of up to 60
seconds are reported. This method was patented by Zaclon
Corporation (US-A-5 068 134);
- the treatment of galvanised steel in a solution mainly
containing nanometric silica, followed by a drying
operation. Patent applications were filed by NKK Corp.
(JP-A-92 96276 or JP-A-92 96277); here again the drying
time is prohibitive;
- the treatment of galvanised steel by a solution of oxide
particles or a mixture of oxide (Si02, Sb203, A1203, Zr02
or Ti02) the surface of which adsorbs Ni or Co ions. A
drying operation is also required here (patent
application JP-A-21 04675 filed by Sumitomo Metal Ind.
Ltd.)
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[0016] In each of the solutions proposed above, the
fact of working at low temperatures implies a low speed of
conglomeration of the nanoparticles. The lack of energy
does not favour good bonding of the particles to each other
either. This will eventually have consequences on the
cohesion of the layer which will have a tendency to turn
out brittle.
[0017] Other solutions have been proposed, in
particular electrolysis treatments. This type of method is
effective since thermal energy is replaced by electricity:
- the deposition of a zinc-silica compound on a steel plate
by electrolysis by means of a solution containing
colloidal silica, surfactants and zinc salts. The
morphology of the coating is not the same as that of a
coating of pure silica. There are in particular far less
attachment points for an organic coating layer such as
paint (American patent US-A-4 655 882 by Okayama - Ken);
- the production of a layer of Cr oxide/nanoparticles of
silica on a galvanised plate by cathode electrolysis.
Here, the silica plays the part of a matrix for the
coating. Unfortunately, this method requires the use of
Cr(VI) (European patent application EP-0 247 290 by
Kawasaki Steel ) .
Aims of the invention
[0018] The present invention aims to provide a
method for coating a metal with an ultra-thin protective
oxide deposit, preferably of silicon, titanium, zirconium,
cerium, yttrium or antimony.
[0019] A complementary aim of the invention is to
provide an alternative method to the current procedures
relying on the use of materials listed as toxic, in
particular a method which does not use chrome(VI).
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[0020] Another aim of the invention is to provide a
method which is very quick and easy to implement, in
particular capable of being implemented in the context of
the "coil coating" technique.
Main characteristics of the invention
[0021] A first aim of the present invention relates
to a method for continuously coating a substrate in motion,
such as for instance a metal sheet of coated steel, said
coating being deposited on the substrate comprising a layer
of ultra-thin thickness between 20 and 2000nm, and
preferably between 40 and 500nm, characterised in that the
deposition is achieved:
- from an aqueous solution containing oxide nanoparticles,
- in conditions of controlled pH,
- at high temperature, preferably higher than 200°C,
- the deposition time being less than 10 seconds, and
preferably less than 2 seconds.
[0022] According to the invention, the deposition
is achieved on a substrate of bare metal, preferably steel,
aluminium, zinc or copper, or of a first metal coated by a
second metal, preferably a steel sheet covered with a layer
of zinc, aluminium, tin or an alloy of at least two of
these metals.
[0023] As an advantage, the nanoparticles comprise
oxides, preferably Si02, Ti02, Zr02, A1203, Ce02, Sb205, Y203,
ZnO, Sn02 or mixtures of these oxides, are hydrophilic
and/or hydrophobic, have a size between 1 and 100nm and are
included in the solution at a density between 0.1 and 10%,
and preferably between 0.1 and 1%.
[0024] According to an important characteristic of
the invention, the pH of the solution is adjusted so as to
allow the dissolution and/or removal of surface oxides on
the metallic substrate during its contact with the solution
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and to give the particles present in the solution a
sufficient electrical charge. Thus, any conglomeration in
the solution is hopefully avoided and the particles are
made as reactive as possible without destabilising the
solution.
[0025] In addition, the pH of the solutions based
on nanoparticles of Si02, Sn02, Ti02, ZnO, Sb205 or of
mixtures thereof is basic, and preferably between 9 and 13,
whereas the pH of solutions based on nanoparticles of Zr02,
Ce02, Si02, Sbz05 or mixtures thereof is acid, and
preferably between 1 and 5.
[0026] Preferably, the pH of solutions based on a
mixture of nanoparticles is adjusted so that the solution
is stable during its period of use.
[0027] Still more preferably, in cases where the
substrate has a surface layer comprising a compound of
zinc, aluminium, iron, tin, chrome, nickel or copper, the
pH of the solution may be basic. Similarly, in cases where
the substrate has a surface layer comprising a compound of
zinc, aluminium, iron, tin, chrome, nickel or copper, the
pH of the solution may be acid.
[0028] According to a first preferred embodiment of
the invention, the deposition is achieved by immersion of
the substrate for a controlled period in an immersion tank
containing the solution.
[0029] According to a second preferred embodiment
of the invention, the deposition is achieved by spraying
the solution over the substrate by means of one or more
jets. By jets) is meant any system, assisted by compressed
gas or not, which sprays droplets of the solution.
[0030] According to a third preferred embodiment of
the invention, the deposition is achieved by depositing the
solution on the substrate by means of a roller.
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[0031] Preferably, the solution which comes into
contact with the sheet is maintained at a temperature below
50°C, and preferably below 35°C and the temperature of the
substrate at the start of the deposition is higher than
200°C.
[0032] Still preferably, when the substrate already
has a metallic coating before treatment, the temperature of
the substrate at the start of the deposition is higher than
200°C and lower by 30 to 100°C than the melting point of
said coating metal.
[0033] According to a particular characteristic of
the invention, when the substrate has a metallic coating
obtained by immersion, preferably by galvanised hot
dipping, the deposition is achieved just after the metal
deposition.
[00034] According to another characteristic of the
invention, in cases where substrates already have a metal
coating produced by immersion, said substrate is protected
from significant contact with the air.
[0035] As an advantage, the deposition is limited
in time by varying the depth of immersion in the case of a
deposition in a solution or the length sprayed in the
direction of the flow in the case of spraying the solution
with (a) jet (s) .
[0036] According to a general aspect of the
invention, the solvent used comprises water with possibly a
co-solvent capable of dispersing the nanoparticles in an
effective manner.
[0037] As an advantage, agents are added to the
solution of nanoparticles in order to improve the
resistance against corrosion and/or the adhesion to the
substrate or paint and/or to improve the flow during
formation.
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[0038] After further investigation, the inventors
noted that the coated substrate may be rinsed after coating
with water or with a solution based on organic silanes or
carboxylic acid containing an function capable of
5 subsequently forming a strong organic bond.
[0039] It is moreover advantageous for the method
of the invention to comprise means for:
- measuring and controlling the pH continuously,
- ensuring that the solution is renewed and that excess
10 products of the reaction are eliminated,
- ensuring the homogeneous mixture of the bath so as to
avoid turbulence at its surface.
[0040] More specifically, as in the first preferred
embodiment, the temperature of the sheet and of the bath is
controlled, as are the time that the sheet stays in the
bath, the concentration of nanoparticles in the bath and
the pH of the bath.
[0041] More specifically, according to the second
preferred embodiment, the temperature of the sheet, the
spraying time, the concentration of the nanoparticles in
the sprayed solution, the flow rate of the spray and the pH
are controlled.
[0042] Another significant advantage of the coating
method of the present invention is that it does not require
any additional drying operation after the production of the
coating per se.
[0043] A second aim of the present invention
relates to an installation for the coating of a steel sheet
by hot dipping, comprising a device for producing a second
coating. layer by the implementation of the method in the
invention, characterised in that said device is located
after the units ensuring the spinning and solidification
operations of the first coating layer, said method being
carried out in this device at a temperature about 100°C
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below the solidification temperature, preferably between
200 and 350°C.
[0044] Finally, a third aim of the present
invention relates to a flat or long metal product,
preferably a sheet, thread, panel or tube, coated with an
ultra-thin protective deposit by means of the method in the
invention, characterised in that said protective deposit
comprises nanoparticles of oxides or a mixture of these
oxides, preferably Si02, Ti02, Zr02, A1203, Ce02, Sb205, Yz~3,
Zn0 or Sn02, without hexavalent chrome and has a thickness
between 20 and 2000nm, preferably between 40 and 500nm.
Detailed description of the invention
[0045] The present invention aims to provide a
Z5 method for coating metal surfaces so as to create a surface
layer which ensures the protection and at the same time the
ability to attach a subsequent layer. The substrate to be
coated by an ultra-thin protective layer is either a bare
metal such as steel, aluminium, zinc, copper, etc. or a
metal coated with a layer of another metal, such as a layer
of zinc, aluminium, tin or an alloy of these metals.
[0046] According to the invention, this treatment
is characterised by the use of oxide nanoparticles.
[0047] The particles used are preferably the
following oxides: SiOz, Ti02, Zr02, A1203, Ce02, Sb205, Yz03,
ZnO, Sn02. They are either pure or in the form of a
hydrophilic and/or hydrophobic mixture. The size of the
particles is between 1 and 100nm. The solvent used is
either water or alcohol, or even a mixture of water and
alcohol. Another solvent which can disperse the
nanoparticles in an effective manner may also be used.
[0048] Various deposition techniques may be used:
- immersion for a controlled period in an immersion tank;
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- spraying a solution (vaporiser, jets), that is to say
propulsion under the effect of the pressure of the
solution or by a carrier gas under pressure;
- deposition with a roller ("roll coater")
[0049] The production of an ultra-thin layer is
associated with deposition times of less than 5-10 seconds,
and preferably 2 seconds. In addition, a short time is
required since the temperature of the substrate falls
during the deposition; it is vital to use a short time
period in order to dry the sheet by the heat present there
because of its own temperature at the end of the treatment.
This type of "natural" drying in fact avoids the risk of
damaging the coating by forced external drying.
[0050] The temperature of the substrate plays an
important role in the method in the invention. Preferably,
if the temperature is higher than 200°C, one may benefit
from the temperature of the sheet in the case of coating by
immersion. In this case, indeed, the sheet is continuously
tempered in an oven, immersed in the bath of liquid metal,
then, after the spinning and solidification of the coating,
it is still at a high temperature. However, it still cannot
be cooled too quickly because that would affect its
evenness. According to an important characteristic of the
invention, the deposition will be achieved with a substrate
temperature of the order of the temperature at which the
coating metal solidifies less about 30 to 100°C.
[0051] According to the present invention, the
conglomeration of particles on the metal sheet is mainly
achieved in the first second of contact between the above-
mentioned solution and the hot sheet, for example in an
immersion bath. With the time in the bath preferably less
than 2 seconds, the residual heat of the plate at its exit
from the tank allows rapid "self-drying" of the coating
layer formed.
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[0052] In the case of an uncoated metal, one may in
certain cases benefit from the stored heat of the metal,
for instance, in continuous tempering of the steel sheets,
high temperature cleaning, etc. The metal to be treated can
also be heated with a flame, by induction, etc.
[0053] As for the temperature of the solution, it
will affect its reactivity and stability, as well as the
cooling speed. It will be maintained before the deposition
at a temperature below 50°C, and preferably below 35°C.
[0054] The pH of the solution at the moment of the
deposition represents a crucial aspect since it affects the
attachment of the present treatment to the metal surface,
whether coated or not. The presence of a non-protective
oxide such as A1203 or Zn0 on the surface of zinc is,
according to the Applicant, not a good thing. Therefore,
its removal is a priority. To this end, the colloidal
solution of nanoparticles is modified by the addition of a
basic compound such as soda, potassium hydroxide or
ammonium carbonate. The resulting increase in pH is
intended to make the surface oxide present soluble by
forming a hydroxide. The boiling of water at the surface of
the zinc then easily wipes away this compound and cleans
the surface of all pre-existing or unwanted oxides. It is
intended to electrically charge to the maximum the
nanoparticles of the surface to be coated in order to avoid
any conglomeration in the colloidal solution and to make
the particles as reactive as possible without destabilising
the solution. To this end, it is recommended that the pH of
the solutions used is adjusted.
[0055] In the case of substrates already having a
metal coating produced by immersion, the substrate is
protected from significant contact with the air so as to
prevent the formation of a too thick layer of inert oxides.
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This could not in fact be removed within acceptable
treatment times on line.
[0056] This increase in pH also has other
advantages. It makes the silica more reactive thanks to a
thicker surface concentration of silanolate. Moreover,
according to certain sources, this would allow the
production of dense coatings. The best results in terms of
the coating adhesion and powdering are obtained with pH's
between 9 and 13. With pH's of less than 9, the silica is
powdery and adheres poorly. With a pH higher than 13, the
colloidal solution becomes unstable: the silica polymerises
and falls off by itself.
[0057] The use of alkaline baths is recommended
with solutions of nanoparticles of oxides such as Si02,
Sn02, Ti02, Zn0 or Sb205. On the contrary, with solutions
based on nanoparticles of oxides such as Zr02, CeOz or
again Si02 and Sbz05, their use is recommended with an acid
pH, and preferably between 1 and 5.
[0058] However, solutions of either basic or acid
pH may be used if the substrate comprises a compound of
zinc, aluminium, iron, tin, chrome, nickel or copper.
[0059] Table 1 shows the influence of the pH of a
colloidal solution of silica nanoparticles on the
subsequent adhesion of paint, in the case of galvanised
plates pre-treated from said colloidal solution with
different values of pH.
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Table I
Adhesion pH 7 pH pH 11 pH 12
9
Grid pattern + tape (*) - - 0 +
Bending OT + tape (**) - 0 0 +
Key: - large visible surface of bare metal
0 small visible surface of bare metal
+ no surface of bare metal exposed, no trace of paint
5 on the tape
( * ) scratch test with a comb so as to make a grid pattern on
the paint followed by an adhesion test with scotch tape;
(**) test: bending of the plate by 180°, the radius of the
curve of the bend not allowing the insertion of a same
10 plate into the bend. Followed by an adhesion test with
scotch tape.
[0060] With regard to the chemistry of the
deposition bath, on the one hand the concentration on
particles in the bath is between 0.1 and 10%, and
15 preferably between 0.1 and 1%. On the other hand, from the
point of view of the chemical management of the treatment
bath, means are provided for the continuous measurement and
control of the pH, renewal of the solution, removal of the
reaction products and an adjusted mixing system in order to
prevent turbulence at the surface of the bath, it being
understood that the surface of the bath must be as even as
possible.
[0061] The thickness of the deposit is typically 20
to 5000nm, preferably between 50 and 1000nm. The thickness
is for instance controlled by ellipsometric measurements in
line or by nuclear measurements. In the case of immersion
in a bath, the control parameters are specifically the
temperature of the sheet and of the bath, the time that the
sheet stays in the bath, the concentration of nanoparticles
and the pH of the bath. In the case of spraying with jets,
the control parameters are specifically the temperature of
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the sheet, the spraying period, the concentration of the
nanoparticles in the vaporised solution, the flow rate of
the sprays and the pH.
[0062] Additives may be added to the basic
particles:
- either to improve the resistance against corrosion (based
on organic or mineral compounds such as CrX, MoX, etc.)
- or to improve flow during formation (MoS2, PTFE, etc).
[0063] After deposition, it may be worth rinsing
with an aqueous solution at a few parts per thousand of
organic silane. There are two reasons for doing this:
firstly, to obtain a good rinse of the excess silica and
then to benefit from the opportunity to give certain
functions of organic substances of the type amine, alcohol,
epoxy or even double carbon-carbon bond (for example
acrylate) to the surface of the oxide layer. This allows
the subsequent strengthening of the silica/organic
substance bond.
Description of a preferred embodiment of the invention
[0064] By way of an application example of the
invention, a continuous coating line for a steel sheet will
be described below.
[0065] A continuous line for coating by immersion
generally comprises the following successive steps:
- the sheet continuously passes through a tempering oven;
- it is then plunged into a bath of liquid metal which is
intended to coat it;
- upon its exit from the bath, the sheet follows a vertical
route: first, the excess coating metal is removed from it
by gas spinners and then this coating solidifies while
the sheet moves up to the upper roller;
- lastly, the sheet passes through a section where the
following operations are performed: cooling by air, mist
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Z7
and/or immersion in water, cold-rolling, conversion of
the surface (chroming).
[0066] The speed of the line is typically of the
order of 120 m/min (i.e. 2 m/sec). The sheet temperature is
of the order of 460°C in the bath. In the case of
galvanised coating, the temperature gradually falls so as
to reach 340-390°C at the upper roller; it then falls
progressively. In the case of "galvanneal" coating (ZnFe
alloy), the sheet is immediately reheated after spinning to
490-560°C, then it is cooled again before it reaches the
upper roller.
[0067] In such a line, the coating may be applied
for example:
- by spraying during the vertical movement of the sheet
towards the upper roller or just after this roller, the
temperature of the sheet at this point being typically
between 200 and 350°C;
- by immersion in a bath of solution during a descending
vertical movement.
[0068] The treated surfaces are metals or alloys
which may be made of iron (steels), aluminium, zinc or
copper as well as stainless steels. It is also very
interesting to use this coating method to protect coated
surfaces such as galvanised steels (that is to say steels
coated with an alloy based on zinc or aluminium).
(0069] The method of the invention can be applied
to any metal piece of a particular shape (for example
tubes, panels, threads, etc.) but also to metal sheets
which will be cut into plates.
[0070] The production of this layer gives the
advantage of protecting the substrate against premature
deterioration caused by aggressive external agents
(chemical, thermal, mechanical, etc.). This coating also
has the advantages of:
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1$
- limiting the formation of corrosion;
- creating an electrically insulating layer, especially
intended for application to plates used in electrical and
electronic construction;
- ensuring protection against fingerprints during
production or service;
- improving the resistance of the product against scratches
and abrasion.
[0071] The coating in the invention also has the
advantage of helping the metal to withstand the various
treatments which it will subsequently undergo, and in
particular:
- of improving the adhesion of organic coatings
subsequently added in order to protect or give a
different aspect (colour, shine, etc.);
- of improving the strength of glued assemblies;
- of improving suitability for shaping.
[0072] The method of the invention also has the
advantage that it can be used in a very short time
interval. This short time is required on the one hand
because of the rapid processing on the line (and thus with
a limit to the length of the bath or the spraying) and on
the other hand because of the product itself, which
requires that the reaction of the coating formation is
rapid. This choice made by the Applicant is deliberate and
linked to the constitution and structure of the layer
formed.
[0073] According to the invention, the drying after
coating being very rapid, it turns out that the sheet can
be directly sent onto a "skinpass" line (a series of damp
rollers) intended to modify the mechanical properties of
the sheet without any additional drying operation. In this
respect, the invention has the advantage over the present
state of the art that, in the case of a long time in the
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bath, the plate loses its heat in the bath and an
additional drying unit must be provided (see for example
JP-A-92 96275).
[0074] Apart from these requirements, the method of
the invention allows to meet the current requirements in
terms of environmental protection ("chrome-free" methods)
and simplification of processes.
