Note: Descriptions are shown in the official language in which they were submitted.
CA 02855673 2014-07-03
Method for Passivation of Strip Black Plate
The invention concerns a method for passivation of strip black plate, where in
a first step, an
inert layer is formed on the black plate surface by an electrochemical
treatment of the black
plate, and in an additional step, an aqueous chromium-free treatment solution
is deposited on at
least one surface of the black plate to form a corrosion-protective conversion
layer, which
additionally forms a bonding layer for paints and organic coating materials.
Passivation here is
understood to mean the targeted production of a protective layer (here:
conversion layer) on the
black plate that prevents or at least greatly slows the corrosion of the black
plate. The invention
additionally concerns the use of black plates treated in accordance with the
invention as
packaging steel.
There are known processes for protection of metal surfaces from corrosion, in
which the metal
surface is provided with a coating of another, as a rule non-noble, metal
(such as zinc and
chromium). For example, coating steel sheet with zinc or chromium or even with
tin (which, to
be sure, is a more noble metal than steel) is known. For example, tin-plated
ultrafine plate
(tinplate) is very widely used for production of packaging, in particular in
the food area. Tinplate
is characterized by very good corrosion resistance and good forming behavior
as well as
weldability and is therefore very well suited for production of packagings
such as beverage cans.
To protect the metal coating, or the tin coating in the case of tinplate, from
corrosion and to
produce a good bonding base for paints and plastic coatings, conversion layers
are often applied
to the surface of the metal coating.
Conversion layers are understood to be very thin nonmetallic, mostly inorganic
layers on a metal
surface, which as a rule are produced by the chemical reaction of an aqueous
treatment solution
with the metal substrate. Conversion coatings guarantee, in particular in the
case of ultrafine
steels, a very effective corrosion protection, a very good bonding base for
paints and plastics, and
they reduce surface friction and wear.
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Depending on the substrate one distinguishes between iron, zinc, or manganese
phosphating,
electrolytic phosphating, or chromate, oxalate, and anodization processes.
Chromium-containing
conversion layers have proved to provide very effective corrosion protection.
In a chromating
process, the metal surface is treated with an acidic, chromium(VI) ion-
containing solution, in
which chromium(VI) is reduced to chromium(III). A chromium-containing
conversion layer that
protects against corrosion is formed on the metal surface by the treatment.
Chromium(VI) compounds are, however, acutely toxic and carcinogenic.
Passivation of metal
surfaces with chromium(VI)-containing substances has already been banned in
the EU for
applications in automobile manufacture and household appliances. For this
reason, chromium-
free conversion layers have been developed in the prior art. For example,
processes for
generation of chromium-free conversion layers on zinc and aluminum surfaces
are known from
WO 97/40208-A and EP 2532769 Al. Furthermore, treatment solutions for
generation of
chromium-free conversion layers that contain oxo cations and halogen complexes
and that lead
to colorless and slightly iridescent layers are described in WO 2008/119675.
Tinplate has outstanding properties as packaging material for foods and has
been manufactured
and processed for this purpose for many decades. However, tin, which is the
corrosion-inhibiting
coating in the case of tinplate, has become a relatively valuable material
because of the global
shortage of resources. As an alternative to tinplate, in particular for use as
packaging steel, steels
that have been electrolytically coated with chromium, which are called "tin-
free steel" (TFS) or
"electrolytic chromium coated steel" (ECCS), are known from the prior art.
These tin-free steels
are characterized on the one hand by good bonding capacity for paints or
organic protective
coatings (for example, of PP or PET), but on the other hand have considerable
disadvantages in
the conduct of the coating process because of the toxic and health-threatening
properties of the
chromium VI-containing materials that are used for the coating.
The task of this invention therefore consists of making available a chromium-
free packaging
steel, which is suitable as a substitute for tin-free steel (TFS or ECCS) and
as a substitute for
tinplate and which in particular should be comparable to tinplate or tin-free
steel both with
regard to corrosion resistance and bonding capacity for paints or organic
coatings.
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This task is solved by a process for passivation of strip black plate that has
the features as
described herein. According to this process, an uncoated black plate in strip
form is used and its
surface is inertized in a first process step by an electrochemical treatment,
then is rinsed with
water or another rinse liquid, and finally in an additional step is coated
with a corrosion-resistant
conversion coating by depositing an aqueous chromium-free treatment solution
onto at least one
surface of the black plate.
Other special embodiments of the process are described herein.
Cold-rolled, annealed, and rerolled or cold-finished steel strip made from an
unalloyed steel with
carbon content 20-1000 ppm is preferably used. The steel strip (black plate)
preferably has the
following properties:
= Strength: 300-1000 MPa
= Elongation at break: 1-40%
= Thickness: 0.05-0.49 mm
= Surface roughness: 0.1-1 pm.
The steel can be, for example, a ferritic steel or even a multiphase steel
that has a plurality of
structural components, in particular ferrite, martensite, bainite and/or
residual austenite. Such
multiphase steels are characterized by a high strength of more than 500 MPa,
while at the same
time having good elongation at break of more than 10%. In view of the intended
use of the black
plate treated in accordance with the invention as packaging steel, the steel
grades defined in DIN
EN 10202:2001: "Cold-rolled packaging steel products (electrolytically
tinplated and chrome
plated)" are used. Among other things, analysis and mechanical characteristics
of the steel are
defined in this standard. The qualities in particular lie between TS230 (soft
bell furnace grade,
tensile strength 230 MPa) to TH620 (DO, 620 MPa).
To conduct the process in accordance with the invention, the black plate,
which is in strip form,
is moved at a strip speed of preferably more than 200 m/min and up to 750
m/min. First there
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takes place a step to form an inert layer on the surface of the black plate
through an
electrochemical treatment. To prepare for the electrochemical treatment, the
moving black steel
plate is first cleaned and in particular degreased in an optionally necessary
pretreatment step,
after which it is rinsed, pickled, and again rinsed. This is optionally
necessary, since the cold-
rolled and recrystallization-annealed black plate as a rule is post rolled or
dressed after the
recrystallization annealing, where, for example, in wet rerolling with a
water/oil suspension or
even in dry rerolling, the black plate surface becomes contaminated by oil,
abraded iron, soap,
and other contaminants. This contamination is remedied by the (optional)
pretreatment step.
For this, the black plate is guided into a cleaning tank containing an
alkaline sodium or
potassium hydroxide solution. The concentration of the alkaline degreasing
agent is preferably
between 20 and 100 g/L at a bath temperature of 20-70 C. A degreasing of the
black plate
expediently takes place in two steps, where an immersion process is carried
out in a first step and
an electrolytic process with current densities of 2-30 A/dm2 is carried out in
a second step. After
the degreasing, both sides of the black plate strip are rinsed, for example,
by a triple cascade
rinse with 10-30 m3/h in each cascade. If necessary, oxide residues can be
removed by guiding
the black plate strip into additional cleaning tanks containing a salt or
sulfuric acid pickling
solution having a concentration of, for example, 10 to 120 g/L in two
successive immersion
operations, followed by an immersion rinse with an immersion dip. The
temperatures of the
pickling solution and the rinse water typically lie between 20 and 60 C.
After the pretreatment, a homogeneous, solid, inert steel surface is produced
by means of
electrochemical treatment by passing the black plate strip through an
electrolyte. The electrolyte
is preferably alkaline. The electrochemical, preferably alkaline, treatment of
the black plate
serves for inertization and for leveling out the surface properties of the
steel strip before
application of the conversion coating.
In the electrochemical treatment process step, the black plate strip is guided
at the strip speed
through an electrolyte bath while connected as the anode at a preferred
current density of 2-30
A/dm2. The electrolyte is, for example, a sodium hydroxide a sodium hydroxide
solution with a
preferred NaOH concentration of 20-100 g/L, and in particular a sodium
hydroxide bath
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containing a 3% NaOH solution. For example, a soda solution, in particular a
5% sodium
carbonate solution (Na2CO3), can also be used as electrolyte. The bath
temperatures of the
electrolysis bath are expediently kept between 20 and 80 C.
After the electrochemical treatment, the black plate is rinsed with water. The
rinse can take place
by immersing the strip in a water tank or by spraying with water.
Demineralized water (VE),
deionized water, osmosis water, or distilled water with temperatures of 20-60
C is preferably
used for this. However, untreated potable water or other rinse liquids can
also be used. The black
plate is dried after being rinsed.
Finally, in a concluding step, a conversion layer is deposited on at least one
surface of the black
plate by applying an aqueous chromium-free treatment solution to the surface
of the black plate
that was inertized beforehand in the electrochemical treatment.
The conversion layer is expediently deposited in a no-rinse process, i.e., a
rinsing is omitted after
generation of the conversion layer. The aqueous chromium-free treatment
solution that forms the
conversion coating is applied, for example, to the surface of the black plate,
for example, with an
application device that comprises a roll coater, a rotary sprayer, or spray
nozzles.
An application device with a rotary sprayer is preferably used for application
of the aqueous
treatment solution. Before application of the treatment solution, the surface
of the black plate to
which the conversion layer is to be applied should be as clean and dry as
possible. For this
reason, at least the surface of the black plate that is being coated with the
conversion layer is
dried with a drying device, for example an air knife. With this air knife, a
laminar hot air stream
is blown onto the surface of the moving strip, so that problematic foreign
particles are blown off
of the steel strip surface and the steel strip surface is dried.
The rotary sprayer has a plurality of spray rotors arranged side by side
across the strip direction
to which the aqueous treatment solution is supplied and which are set into
rotation by a drive in
order to spray the aqueous treatment solution by centrifugal force in the form
of a fine spray jet
onto the one or both surfaces of the strip and to form a wet film of the
aqueous solution there.
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After application of the wet film of aqueous treatment solution, it is leveled
on the surface of the
black plate by means of driven smoothing rollers. The smoothing rollers are
expediently
arranged in reference to the black plate surface(s) so that they exert only a
little pressure on the
wet film of aqueous treatment solution and do not squeeze out any, or at most
a minimal fraction,
of the applied treatment solution from the surface. The amount of treatment
solutions sprayed by
the rotary sprayer is appropriately adjusted so that an excess does not remain
on the black plate
surface. In this way, an otherwise necessary disposal or processing of the
excess amount of
treatment solution is no longer necessary. After leveling, the sprayed wet
film it is dried so that a
dry weight of the treatment substance remains on the treated surface or
surfaces. Expediently, the
dry weight of the treatment solution after drying is between 1 and 50 mg/m2
and preferably lies
in the range of 10 to 30 mg/m2. The amount of the aqueous treatment solution
delivered to the
spray rotors of the rotary sprayer per unit of time is expediently matched to
the strip speed.
Through this, it can be guaranteed that only the precisely required amount of
fresh treatment
solution in the appropriate concentration is applied as a wet film to the
black plate strip by the
rotary sprayer. In this way, for example, a constant application weight of wet
film in the range of
2 mL/m2 to 8 mL/m2 and preferably about 5 mL/m2 per strip side can be
established independent
of the strip speed.
After the application of the wet film of the treatment solution, the strip is
sent through a strip
dryer in order to dry the wet film. After drying, there remains on the surface
of the black plate,
per side, a dry application weight of the thus formed conversion layer of 2
mg/m2 to 30 mg/m2.
The desired dry weight of the conversion layer can be adjusted through the
amount of treatment
solution delivered to the rotary sprayer per unit of time.
An advantage of this kind of application lies in the fact that only fresh
treatment solution is
always used and it cannot be contaminated by stripped iron through contact and
recycling with
the steel strip. In addition, it turned out that the process is very
economical, since only the
precisely required amount is applied and an excess is not required, so that
excess treatment
solution no longer needs to be collected. This can prevent the formation of
wastewater that needs
to be treated later.
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Alternatively, the application of the treatment solution can also take place
via roll coaters
through a roller application, likewise on a predried black plate surface. Roll
coaters are
preferably used in the lower strip speed range, and in particular at strip
speeds of less than
200 m/min. Alternatively, the application can also take place by spraying the
treatment solution
or by immersing the strip in a bath containing the treatment solution. Since
in this case the
treatment solution is applied to the black plate in an excess amount, to
achieve a predetermined
desired application weight of the conversion layer, it is necessary to squeeze
off the excess
amount of the wet film, for example by means of squeeze rolls, where this can
take place "wet in
wet." In this process, however, the solution is not applied in a constant
uniform way independent
of strip speed, and the treatment solution can, moreover, become contaminated
by iron and then
must be refreshed and, after exceeding a contamination threshold, be disposed
of.
Finally, the wet film of treatment solution applied with the described
application process is dried
to form a dry conversion layer. This can take place, for example, by passing
the black plate
through a drying oven in which the wet film is dried by hot air or IR
radiation. The drying
preferably takes place at temperatures of 50-250 C. Then the surface of the
dried conversion
layer is lubricated or after treated with dioctyl sebacate (DOS), acetyl
tributyl citrate (ATBC),
butyl stearate (BSO), or polyalkylene glycol, in particular polyethylene
glycol (PEG, preferably
with a molecular weight of 6000 g/mol), or a combination thereof. An after
treatment by
lubrication with DOS, ATBC, BSO, or PEG expediently takes place
electrostatically, with
commercial lubricators as in the case of ECCS or tinplate, or also by means of
a rotary sprayer.
The treatment solution used for the conversion coating preferably contains at
least one of the
following substances:
- metal components: chosen from Ti, Zr, Mn, Zn, P, and combinations
thereof;
- organic components: chosen from polyacrylate, polycarboxylate and
combinations
thereof.
Metal and organic components can in turn be combined.
The treatment solution additionally contains at least one bonding agent for
paints or organic
coating materials, where the bonding agent in particular contains components
of maleic acid,
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isophthalic acid, and cyclohexanedimethanol (CHDM), or combinations thereof.
Compositions
that contain polyethylene terephthalate (PET) or polycyclohexylenedimethylene
terephthalate
(PCT), such as glycol-modified polyethylene terephthalate (PET-G, which
contains less than
about 30% CHDM) or PCTG (which contains more than about 30% CHDM), proved to
be
especially suitable bonding agents.
The application amounts are 1 to 50 mg/m2 for the relevant substances.
Some selected commercially available agents that are suitable for generation
of conversion
coatings using the process in accordance with the invention are listed below.
Trade name Important chemical Manufacturer/Distributor
component
Polycarboxylates BASF
EFKA 4560 Modified polyacrylates BASF
Ti, Zn, Mn phosphate Henkel
Granodine 1456 Ti, Zr Henkel
GTP 10861 Zn phosphate, Ti Chemetall
Gardo TP
GB X4744A Mn, Ti, Zr
Chemetall
Gardobond
(Chernetall )
GB X4591 Al Ti, Zr Chemetall
GB X4744 Ti, Zr Chemetall
Preferred treatment solutions for generation of the conversion layers can, for
example, be
composed as follows:
a) an aqueous solution that contains aluminum fluorozirconate having a mol
ratio of Al:Zr:F
of (0.15 to 0.67):1:(5 to 7), where the total concentration of Al + Zr + F is
0.1 to 2.0 g/L
and the pH is adjusted to below 5, preferably 3 to 5.
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b) an aqueous solution that essentially contains:
- 0.2 up to less than 10 g/L zinc ions,
- 0.5 to 25 g/L manganese ions, and
- 2 to 300 g/L phosphate ions, calculated as P205,
where the zinc:manganese weight ratio of the phosphating solution is
maintained in the
range of 0.05:1 to 1:1.
c) an aqueous solution that contains zinc and manganese, with zinc in the
range of 0.05 to
g/L, manganese in the range of 0.075 to 5.2 g/L, and copper in the range of
0.008 to
0.05 g/L, and/or a total of 0.002 to 0.5 g/L hexafluoride complexes of boron,
aluminum,
titanium, and/or zirconium, calculated as F.
d) an aqueous solution containing at least one film-forming agent, which
contains at least
one water-soluble or water-dispersed polymer having an acid number in the
range of 5 to
200, and at least one inorganic compound in particulate form with an average
particle
diameter, measured on a scanning electron microscope, in the range of 0.005 to
0.3 pm
diameter, where the polymer is selected from at least one plastic resin based
on acrylate,
ethylene, polyesters, polyurethane, silicone polyesters, epoxide, phenol,
styrene, urea
formaldehyde, their derivatives, copolymers, polymers, mixtures, and/or mixed
polymers,
and the inorganic compound in particulate form is selected from at least one
compound of
aluminum, silicon, titanium, zinc, and/or zirconium.
or
e) an aqueous solution containing:
i) at least one organic film forming agent, which contains at least one
water-soluble
or water-dispersed polymer, which is a plastic resin based on polyacrylic
acid,
polyacrylate, and/or polyethylene acrylic acid, or a plastic resin mixture,
and/or a
mixed polymer containing a plastic resin based on acrylate or polyacryl, and
ii) a content of cations and/or hexa- or tetrafluoro complexes of cations
selected from
the group consisting of titanium, zirconium, silicon, aluminum, and boron, in
the
range of 0.2 to 30 g/L with respect to the content of the elemental metal.
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The process in accordance with the invention can be integrated into an
existing coating plant, for
example in a strip coating plant for production of ECCS (or TFS), without
great installation
costs. The strip speed in such strip coating plants typically is 80-600 m/min.
The process in accordance with the invention has the advantages of a chromium-
free and thus
environmentally friendly and not health-hazardous, as well as low-cost coating
on traditional
black plate, in particular in the fine and ultrafine steel thickness range.
Moreover, an effective
savings of costs and energy is achieved through the selected application
process (no-rinse
process) for application of the conversion layer due to the omission of the
final rinse. Additional
advantages are achieved in particular through the combination of inorganic
passivation and
polymer-containing thin film coating. The black plates treated with the
process in accordance
with the invention are outstandingly suitable for production of packaging, in
particular cans, and
therefore can replace the tinplate and tin-free steel (TFS or ECCS) that are
traditionally used as
packaging steel. With regard to their corrosion resistance, these black plates
are comparable to
tinplate and have good adhesion properties for paints and plastic coatings,
for example of PP or
PET, that are comparable to tin-free steel (TFS or ECCS).
