Note: Descriptions are shown in the official language in which they were submitted.
CA 02632720 2014-05-20
WET ON WET METHOD AND CHROME-FREE ACIDIC SOLUTION FOR THE
CORROSION CONTROL TREATMENT OF STEEL SURFACES
won The present invention relates to a novel product for the anticorrosion
treatment of metal surfaces. In addition, a specific procedure is described
for the
anticorrosion treatment, specifically of steel surfaces. The anticorrosion
treatment is
primarily intended as a pre-treatment for a subsequent cathodic electro-dip
coating.
[0002] Anticorrosion agents that involve an acidic aqueous solution of fluoro
complexes have been known for some considerable time. They are increasingly
employed as a replacement for chromating processes that due to the
toxicological
properties of chromium compounds are less and less used. Generally, solutions
of
fluoro complexes of this type contains additional anticorrosion agents that
further
improve the corrosion protection and paint adhesion.
[0003] DE-A-19 33 013, for example, describes in one embodiment a treatment
solution that is an aqueous solution of ammonium hexafluorozirconate, sodium
nitrate, cobalt nitrate and sodium m-nitrobenzene sulfonate and has a pH of
5.2. The
solution can be used to treat zinc-, steel- or aluminum surfaces. EP-A-1 571
237
describes a treatment solution and treatment methods for iron-, zinc-,
aluminum- and
magnesium-containing surfaces. This solution has a pH in the range of 2 to 6
and
contains 5 to 5000 ppm zirconium and/or titanium as well as 0.1 to 100 ppm
free
fluoride. In addition, the solution can further contain components selected
from
chlorate, bromate, nitrite, nitrate, permanganate, vanadate, hydrogen
peroxide,
tungstate, molybdate or each of the corresponding acids. Organic polymers can
also
be present. After the treatment with a solution of this type, the metal
surfaces can be
rinsed with a further passivating solution. EP-A-1 405 933 discloses a
composition
that contains at least one metal from the group Ti, Zr, Hf and Si as well as a
fluoride
ion source for the treatment of iron and/or zinc surfaces, wherein defined
conditions
concerning the concentration ratios of both components are set. In addition,
this
solution can contain the same additional active substances as in EP 1 571 237.
CA 02632720 2014-05-20
,
Metal ions selected from the group Ag, Al, Cu, Fe, Mn, Mg, Ni, Co and Zn can
be
present as further components. DE-A-100 10 758 discloses a treatment solution
containing complex fluorides of Ti, Zr, Hf, Si and/or B as well as organic
polymers
particularly for zinc, aluminum and/or magnesium surfaces. The solution can
also
contain one or more of the metals Mn, Ce, Li, V, W, Mo, Mg, Zn, Co and Ni.
Further
potential additives are compounds that are known phosphatization accelerators
for
the phosphatization layer formation. WO 95/14539 describes treatment solutions
for
metal surfaces, particularly for aluminum, which contain complex fluorides of
Ti, Zr,
Hf, Si, Ge, Sn or B as well as organic hydroxycarboxylic acids containing at
least 2
hydroxyl groups (wherein the hydroxyl groups of the carboxylic acid groups are
not
counted) per carboxylic acid group. A specific example of such an acid is
gluconic
acid.
[0004] In all these documents, mention is made - either in general terms in
the
description or specifically in the embodiments - that the metal surfaces
treated with
the stated solution are dried prior to their being coated with an additional
organic
coating, such as for example a varnish. However, especially for the
manufacture,
pre-treatment and painting of automobile body work, because of the short cycle
times and the shortest possible length of the pre-treatment line, it is
desired that the
pre-treated body work be fed into the dip coating bath without being dried,
i.e. while
still wet. Accordingly, a drying step, in which the pre-treatment layer can
be, for
example, chemically modified and/or hardened by dewatering, is not wanted.
Therefore, during the pre-treatment, an anticorrosion layer has to be produced
-
without a drying step- and which possesses the required corrosion protection
and
adhesion to paint, prior to the application of a first organic paint layer.
[0005] A coating process that meets these specifications is described in EP-A-
1 433
876. Here, the treatment of steel, zinc and aluminum surfaces for example, is
carried
out with a solution that contains Zr, Ti and/or Hf as well as fluoride ions
and
additionally a water-soluble resin that can be regarded as a polyvinylamine or
polyallylamine. In the sole embodiment that describes the painting of a still
wet, pre-
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CA 02632720 2014-05-20
treated metal surface, an aqueous solution of fluorozirconic acid,
polyallylamine, zinc
nitrate, silica and ascorbic acid is employed. It is predicted here, that the
organic
polymer is a significant component in regard to the corrosion protection and
the
adhesion to paint. However, the presence of organic polymers in pre-treatment
solutions is frequently undesirable as they can lead to difficulties in the
waste water
treatment.
[0006] Surfaces of zinc, aluminum and galvanized steel can be pre-treated with
very
differently formulated aqueous solutions of fluoro complexes, and the
previously
stated requirements can be satisfied. With steel surfaces that have not been
pre-
treated, the previously stated requirements can only be satisfied up to now
with a
layer-forming zinc phosphatization. Compared with the standard of the layer-
forming
zinc phosphatization, a pre-treatment with aqueous solutions of fluoro
complexes
shows significant disadvantages in regard to corrosion protection and adhesion
to
paint when the pre-treated surfaces are not dried before painting.
[0007] However, this problem can be solved if the aqueous solution of a fluoro
complex contains certain additional components. Consequently, in a first
aspect, the
present invention relates to a process for the anticorrosion treatment of
bright metal
surfaces, which are at least in part steel surfaces, wherein the metal
surfaces are
brought into contact with an acidic aqueous solution of a fluoro complex of at
least
one element M selected from the group B, Si, Ti, Zr and Hf, are rinsed with
water
and thereafter coated with a cathodically depositable electro-dipcoating,
characterised in that
a) the aqueous solution contains no more than 1 mg/I of an organic polymer
with
allylamine or vinylamine monomers,
b) the aqueous solution additionally contains at least one further component
which is
selected from among: nitrate ions, copper ions, silver ions, vanadium or
vanadate
ions, bismuth ions, magnesium ions, zinc ions, manganese ions, cobalt ions,
nickel
ions, tin ions, buffer systems for the pH range from 2.5 to 5.5, silica
particles with an
average particle size of below 1 pm,
3
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a
c) after having been brought into contact with the aqueous solution of the
fluor
complex and before being coated with the cathodically depositable electro-
dipcoating, the metal surface is not dried.
[mos] In this context, "steel surfaces" are understood to mean surfaces of
steel,
which are not top coated with an additional metal, such as for example zinc or
its
alloys with nickel or aluminum. "Bright" metal surfaces are understood to mean
metal
surfaces, which are not yet coated with an anticorrosion coating. Accordingly,
the
process according to the invention concerns the first or sole treatment step,
which
produces an anticorrosion layer that in turn can serve as the basis for a
subsequent
coat of paint. Therefore, it is not a post-treatment of a previously produced
anticorrosion layer, such as for example a phosphate layer.
[0009] Derivatives of the stated aromatic carboxylic acids can also be
employed.
Among these are meant those molecules, in which one or more hydrogen atoms of
the basic structure (e.g. hydrogen atoms on the aromatic core, hydrogen atoms
of
the hydroxyl or amino groups or hydrogen atoms of the carboxyl groups) are
replaced by other atoms or groups of atoms.
[0olo] The stated silica particles with an average particle size of below 1 pm
are
known under various generic names to the person skilled in the art. They are
called,
for example, colloidal silica, precipitated silica or pyrogenic silica. The
average
particle size, which is preferably in the range of about 0.01 pm to about 1
pm, can be
determined by light scattering methods or by electron microscopy.
[0011] In the process according to the invention, an aqueous solution is added
that
on toxicological grounds is essentially free of chromium (VI) compounds and
preferably contains no chromium compounds of any kind. Traces of chromium
compounds, which can arrive in the treatment solution by being leached out of
stainless steel containers, are not considered. In this context, treatment
solutions
containing no more than 1 ppm, particularly no more than 0.1 ppm chromium, are
4
CA 02632720 2014-05-20
. .
understood as "chromium free". The treatment solutions to be employed
according
to the invention do not represent phosphatization solutions, i.e. they do not
lead to
the formation of an amorphous or crystalline phosphate layer. This is achieved
in
that the treatment solutions preferably contain no more than 1 g/I inorganic
phosphate or phosphoric acid, calculated as P043-. However, phosphate contents
in
the range of 10 to 500 mg/I, for example, can be tolerated and can even
improve the
action of the treatment solution
[0012] The stability of concentrates, from which the aqueous treatment
solutions to
be used in the process according to the invention can be manufactured by
dilution
with water, can be improved by adding thickeners and/or dispersants. These
thickeners and/or dispersants are then also present in a suitably diluted
state in the
ready for use aqueous treatment solution. Their presence in the treatment
solution
can be tolerated, such that a treatment solution of this type can be used in
the
process according to the invention. Accordingly, the aqueous treatment
solution can
be wherein it contains no more than 1 mg/I of another organic polymer than
such a
polymer that does not contain allylamine or vinylamine monomers, and that, at
a
concentration of no more than 50 g/I, has thickening or dispersing properties.
Examples of such polymers are polymers or copolymers of unsaturated carboxylic
acids, carbohydrates or proteins.
[0013] The aqueous treatment solution should contain no more than 1 mg/I of an
organic polymer containing allylamine or vinylamine monomers. However, other
polymers can be present. Among these are the previously stated polymers with
thickening and/or dispersing properties. Apart from these, the treatment
solution can
contain additional polymers with known positive activity in anticorrosion
treatment.
Exemplary polymers of this type (including those with thickening and/or
dispersing
properties) are:
a) polymers or copolymers of unsaturated alcohols or the esters or ethers
thereof,
b) polymers or copolymers of unsaturated carboxylic acids, organophosphonic
CA 02632720 2014-05-20
acids, organophosphinic acids or in each case the salts, esters or amides
thereof,
c) polyamino acids or proteins or in each case the salts, esters or amides
thereof,
d) carbohydrates or the esters or ethers thereof,
e) polyamines, in which the nitrogen atoms are incorporated into the
polymer
chain,
f) polyethers,
g) polyvinylphenols and the substitution products thereof,
h) epoxy resins, amino resins, tannins, phenol-formaldehyde resins,
i) polymers and copolymers of vinyl pyrrolidone.
[0014] In so far as these types of polymer are present, their concentration in
the
aqueous treatment solution is preferably less than 2000 mg/I. On secondary
technical grounds, such as for example the simplification of the waste water
treatment, it can be advantageous to largely or completely dispense with the
presence of organic polymers in the aqueous treatment solution. Consequently,
a
preferred embodiment of the present invention is wherein the aqueous solution
contains no more than 1 mg/I of organic polymer.
[0015] The pH of the acidic treatment solution is preferably in the range 2 to
5.5,
particularly 3.5 to 5. The pH is preferably adjusted to the stated acidic
range by
adding the fluoro complex at least partially in the form of an acid. However,
it can
also be adjusted by means of another acid, for example nitric acid. According
to the
invention and according to this first aspect of the invention, no measures are
required, and preferably should be even avoided, which would dry the metal
surface
after it has been contacted with the aqueous solution of the fluoro complex
and
before being coated with the cathodically depositable electro-dipcoating. An
unintentional drying can however occur during a unit shutdown when the treated
metal surface, for example an automobile bodywork or a part thereof, is
exposed to
air between the bath containing the aqueous solution of the fluoro complex and
the
electro-dipcoating bath. This unintentional drying is harmless, however.
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CA 02632720 2014-05-20
[0016] The additional components stated in feature b) are preferably present
in the
following concentration ranges:
nitrate ions: 0.1 to 5000 mg/I, preferably 1 to 3000 mg/I, in particular 10 to
1000 mg/I,
copper-, silver-, cobalt- or nickel ions: each 0.1 to 300 mg/I, preferably 1
to 30 mg/I,
vanadium- or vanadate ions: 1 to 2000 mg/I, preferably 5 to 500 mg/I
(calculated as
vanadium),
bismuth-, magnesium-, zinc-, manganese- or tin ions: each 1 to 2000 mg/I,
preferably 5 to 500 mg/I,
buffer system for the ph range 2.5 to 5.5: in sufficient quantity that the ph
of the
solution does not change by more than 0.2 units when a 1 n acid or base per
liter
solution is added,
aromatic carboxylic acids containing at least two groups containing donor
atoms, or
derivatives of such acids: 0.01 to 1000 mg/I, preferably Ito 500 mg/I,
silicon in the form of silica particles with an average particle size of less
than 1 pm:
to 1000 mg/I, preferably 50 to 500 mg/I
[0017] In the context of the present invention, when an "acid", specifically a
"carboxylic acid" is mentioned, then the free acid and/or its anions are to be
understood. The person skilled in the art is aware that independently of
whether the
acid is added in the form of a free acid or in the form of soluble salts in
the given
concentration range, an equilibrium is reached between the free acid and the
salt
form, which depends on the pKs of the acid in question and the pH of the
aqueous
solution. Concentrations are calculated as the free acid. The same is true,
for
example, for H2ZrF6 or other acids, which are present in the aqueous solution.
[0018] An acetic acid/acetate buffer is particularly suitable as the buffer
system for
the stated pH range. A further suitable buffer system is based on potassium
hydrogen phthalate.
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CA 02632720 2014-05-20
[0019] Preferably the aqueous solution contains a quantity of fluoro complex
such
that the concentration of the metal M is in the range from 1 to 5000 mg/I,
preferably
in the range from 5 to 1000 mg/I and in particular in the range from 10 to 500
mg/I.
Zirconium and/or titanium is particularly preferred as the metal M.
[0020] It is further preferred that in the fluoro complex the element M is
selected from
the group Si, Ti, Zr and Hf, and in that the aqueous solution contains on
average at
least 1, preferably at least 3, in particular at least 5 fluorine ions per ion
of the
element M. Here, the statement "on average" means the calculated atom ratio of
fluoride ions to M ions in the aqueous solution. The stability of fluoro
complexes of
the stated metals M leads to the expectation that if the aqueous solution
contains
less than 6 fluoride ions per M ion, then the fluoride ions are almost
completely
attached to the M ions in the form of fluoride complexes. Therefore, in this
case the
fluoride is essentially totally present as "complex fluoride".
[0021] However, the aqueous solution can also contain more fluoride ions than
is
required for the complete formation of hexafluoro complexes. In this case, one
can
assume that 6 fluoride ions are present in the fluoro complex and that the
excess
fluoride ions are present as so-called "free fluoride". They can be in the
form of, for
example HF and/or water-soluble salts thereof. This embodiment of the present
invention is wherein 6 fluoride ions per ion of the metal M (M selected from
Si, Ti, Zr,
Hf) are present in the fluoro complex, and that moreover the aqueous solution
still
contains 1 to 1000 mg/I of fluoride ions which are not attached to zirconium.
[0022] In a particularly preferred embodiment, the aqueous solution contains
at least
0.1 mg/I, preferably at least 1 mg/I and particularly at least 10 mg/I of
nitrate ions.
The upper limit of the nitrate concentration is chosen more from economic than
technical grounds, the economic grounds also including the costs of the waste
water
disposal. The upper limit of the nitrate ion concentration can be chosen, for
example
as 5000 mg/I, preferably 3000 mg/I and particularly 1000 mg/I. The aqueous
solution
can contain copper ions and/or silver ions as additional components according
to
8
CA 02632720 2014-05-20
, .
feature b). They can be present instead of the nitrate ions or be together
with them.
In this embodiment, the aqueous solution preferably contains 0.1 to 300 mg/I,
particularly 1 to 30 mg/I of copper ions and/or silver ions.
[0023] In a further preferred embodiment, the aqueous treatment solution
contains a
buffer system for the pH range 2.5 to 5.5, as already described.
[0024] Accordingly, preferably employable aqueous treatment solutions contain
at
least one of the components described above in more detail: nitrate ions,
copper
ions and/or silver ions, silica particles with an average particle size of
below 1 pm
and/or a buffer system for the pH range 2.5 to 5.5. Two or more of these
components can also be present together. The presence of further components in
addition to the stated components can have a favorable effect on anticorrosion
and
paint adhesion. For example, in addition to one or more of the previously
stated
components (nitrate ions, copper ions and/or silver ions, silica particles,
buffer
system), one or more of the following components can be present: vanadium- or
vanadate ions, cobalt ions, nickel ions, manganese ions, tin ions, bismuth
ions,
magnesium ions and zinc ions. Their preferred concentration ranges have
already
been given above. In this regard, a treatment solution, which in addition to
one of the
stated preferred components (nitrate ions, copper ions and/or silver ions,
aromatic
carboxylic acids, silica particles, buffer system), contains both zinc ions as
well as
magnesium ions, is particularly preferred.
[0025] Moreover, the aqueous treatment solution can additionally contain
aluminum
ions. They can be introduced in the form of soluble salts, for example in the
form of
the nitrates. In this case, the aqueous treatment solution preferably contains
1 to
1000 mg/I, especially 10 to 500 mg/I of aluminum ions. Aluminum ions can serve
as
"getters" for excess free fluoride ions, as with these they form stable fluoro
complexes. Free fluoride ions are produced in the aqueous treatment solution
because the metal M, for example zirconium, precipitates out, probably in the
form of
oxides, onto the treated metal surface. In this way, the fluoride ions that
were
9
CA 02632720 2014-05-20
originally attached to the metal M are released. The increased pickling effect
of the
aqueous solution caused by free fluoride ions can be reduced by the presence
of the
aluminum ions owing to the complex formation.
[0026] In addition to the already stated components, the aqueous treatment
solution
can contain compounds that are employed in the layer forming phosphatization
as
the so-called "accelerators". These accelerators have the property of
capturing
hydrogen atoms that are produced by the pickling attack of the acids on the
metal
surface. This reaction, also known as "depolarization", facilitates the attack
of the
acidic treatment solution on the metal surface and thereby accelerates the
formation
of the anticorrosion layer. Accelerators, which are listed in the previously
stated
document DE-A-199 33 189, can be employed, for example:
0.05 to 2 g/I m-nitrobenzene sulfonate ions,
0.1 to 10 g/I hydroxylamine in free or bound form,
0.05 to 2 g/I m-nitrobenzoate ions,
0.05 to 2 g/I p-nitrophenol,
1 to 70 mg/I of hydrogen peroxide in free or bound form,
0.05 to 10 g/I organic N-oxides,
0.01 to 3 g/I, preferably up to 0.5 g/I nitroguanidine,
1 to 500 mg/I of nitrite ions
0.5 to 5 g/I chlorate ions.
[0027] It is known from the previously stated document EP-A-1 571 237 that
after the
treatment with the aqueous solution of a fluoro complex, the treated metal
surface is
then rinsed with an aqueous solution that contains one or more components
selected from compounds or salts of the elements cobalt, nickel, tin, copper,
titanium
and zirconium and/or from water-soluble or water-dispersible organic polymers.
Corrosion protection and paint adhesion are further improved by this final
rinse. A
CA 02632720 2014-05-20
final rinse of this type also has a positive effect in the course of the
process
according to the invention. Accordingly, the present invention also includes a
process variant, in which after having been brought into contact with the
aqueous
solution of a fluoro complex and before being coated with the cathodically
depositable electro-dipcoating, the metal surface is rinsed with an aqueous
solution
that contains one or more components selected from among compounds or salts of
the elements cobalt, nickel, tin, copper, titanium and zirconium and/or from
among
water-soluble or water-dispersible organic polymers.
[0028] In the context of the experiments which led to the previously described
inventive process steps, it was clear that an addition of one or more
components
selected from among tin ions, bismuth ions, buffer systems for the pH range
2.5 to
5.5, aromatic carboxylic acids or derivatives thereof, generally improved the
anticorrosion effect of aqueous solutions of fluoro complexes for surfaces of
steel,
aluminum, zinc and galvanized steel. This is true independently of whether the
surfaces are dried or not between the contact with this solution and a
subsequent
painting. These types of treatment solution are therefore not only
advantageously
employable in the context of the previously described inventive process cycle,
but
also show a positive effect for anticorrosion and paint adhesion of metal
surfaces in
general.
[0029] Accordingly, a second aspect of the present invention rests on the
provision
of an acidic, chromium-free aqueous solution of a fluoro complex of at least
one
element M selected from among the group B, Si, Ti, Zr and Hf with a pH in the
range
from 2 to 5.5 for the treatment of metal surfaces, wherein it additionally
contains one
or more components selected from among: tin ions, bismuth ions, buffer system
for
the pH range from 2.5 to 5.5, aromatic carboxylic acids with at least two
groups
containing donor atoms, or derivatives of such carboxylic acids.
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CA 02632720 2014-05-20
[0030] For this second aspect of the present invention, the previously made
explanations to the essential or additional facultative components are
correspondingly valid:
[0031] An aqueous solution is added that on toxicological grounds is
essentially free
of chromium (VI) compounds and preferably contains no chromium compounds of
any kind. Traces of chromium compounds, which can arrive in the treatment
solution
by being leached out of stainless steel containers, are not considered. In
this
context, treatment solutions containing no more than 1 ppm, particularly no
more
than 0.1 ppm chromium, are understood as "chromium free". The treatment
solutions
according to the invention do not represent phosphatization solutions, i.e.
they do
not lead to the formation of an amorphous or crystalline phosphate layer. This
is
achieved in that the treatment solutions preferably contain no more than 1 g/I
inorganic phosphate or phosphoric acid, calculated as P043-. However,
phosphate
contents in the range of 10 to 500 mg/I, for example, can be tolerated and can
even
improve the action of the treatment solution
[0032] The pH of the acidic treatment solution is preferably in the range 2 to
5.5,
particularly 3.5 to 5. The pH is preferably adjusted to the stated acidic
range by
adding the fluor complex at least partially in the form of an acid. However,
it can
also be adjusted by means of another acid, for example nitric acid.
[0033] The one or more essential components are preferably present in the
following
concentrations:
tin ions: 1 to 2000 mg/I, preferably 5 to 500 mg/I,
bismuth ions: 1 to 2000, preferably 5 to 500 mg/I,
buffer system for the pH range 2.5 to 5.5: in sufficient quantity that the pH
of the
solution does not change by more than 0.2 units when a 1 N acid or base per
liter
solution is added,
aromatic carboxylic acids: 0.1 to 1000, preferably 1 to 500 mg/I.
12
CA 02632720 2014-05-20
I ,
,
[0034] In addition, this aqueous solution can contain one or more of the
following
components:
nitrate ions: 0.1 to 5000 mg/I, preferably 1 to 1000 mg/I,
copper-, cobalt-, nickel- and/or silver ions: each 0.1 to 300 mg/I, preferably
1 to 30
mg/I,
vanadium- or vanadate ions: 1 to 2000, preferably 5 to 500 mg/I (calculated as
vanadium),
magnesium ions: 1 to 2000, preferably 5 to 500 mg/I,
manganese ions: 1 to 2000 mg/I, preferably 5 to 500 mg/I,
zinc ions: 1 to 2000, preferably 5 to 500 mg/I,
[0035] In this regard, it can be preferred that the aqueous solution contains
both zinc
ions as well as magnesium ions in addition to at least one of the stated
essential
components (tin ions, bismuth ions, buffer system for the pH range 2.5 to 5.5,
aromatic carboxylic acids or derivatives thereof). Moreover, the presence of
copper
ions and/or silver ions is preferred.
[0036] An acetic acid/acetate buffer is particularly suitable as the buffer
system for
the stated pH range. A further suitable buffer system is based on potassium
hydrogen phthalate.
[0037] Preferably the aqueous solution contains a quantity of fluoro complex
such
that the concentration of the metal M is in the range from 1 to 5000 mg/I,
preferably
in the range from 5 to 1000 mg/I and in particular in the range from 10 to 500
mg/I.
Zirconium and/or titanium is particularly preferred as the metal M.
[0038] It is further preferred that in the fluoro complex the element M is
selected from
the group Si, Ti, Zr and Hf, and in that the aqueous solution contains on
average at
least 1, preferably at least 3, in particular at least 5 fluorine ions per ion
of the
13
CA 02632720 2014-05-20
. .
element M. Here, the statement "on average" means the calculated atom ratio of
fluoride ions to M ions in the aqueous solution. The stability of fluoro
complexes of
the stated metals M leads to the expectation that if the aqueous solution
contains
less than 6 fluoride ions per M ion then the fluoride ions are almost
completely
attached to the M ions in the form of fluoro complexes. Therefore, in this
case the
fluoride is essentially totally present as "complex fluoride".
[0039] However, the aqueous solution can also contain more fluoride ions than
is
required for the complete formation of hexafluoro complexes. In this case, one
can
assume that 6 fluoride ions per M ion are present in the fluoro complex and
that the
excess fluoride ions are present as so-called "free fluoride". They can be in
the form
of, for example HF and/or water-soluble salts thereof. This embodiment of the
present invention is wherein 6 fluoride ions per ion of the metal M (M
selected from
Si, Ti, Zr, Hf) are present in the fluoro complex, and that moreover the
aqueous
solution still contains Ito 1000 mg/I of fluoride ions which are not attached
to
zirconium.
[0040] In the context of the second aspect of the present invention, when an
"acid",
specifically a "carboxylic acid" is mentioned, then the free acid and/or its
anions are
to be understood. The person skilled in the art is aware that independently of
whether the acid is added in the form of a free acid or in the form of soluble
salts in
the given concentration range, an equilibrium is reached between the free acid
and
the salt form, which depends on the pKs of the acid in question and the pH of
the
aqueous solution. Concentrations are calculated as the free acid. The same is
true,
for example, for H2ZrF6 or other acids, which are present in the aqueous
solution.
[0041] In a further preferred embodiment in the context of the second aspect
of the
invention, the aqueous treatment solution contains a buffer system for the pH
range
2.5 to 5.5, as already described.
14
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,
[0042] Moreover, the aqueous treatment solution can additionally contain
aluminum
ions. They can be introduced in the form of soluble salts, for example in the
form of
the nitrates. In this case, the aqueous treatment solution preferably contains
1 to
1000 mg/I, especially 10 to 500 mg/I of aluminum ions. Aluminum ions can serve
as
"getters" for excess free fluoride ions, as with these they form stable fluoro
complexes. Free fluoride ions are produced in the aqueous treatment solution
because the metal M, for example zirconium, precipitates out, probably in the
form of
oxides, onto the treated metal surface. In this way, the fluoride ions that
were
originally attached to the metal M are released. The increased pickling effect
of the
aqueous solution caused by free fluoride ions can be reduced by the presence
of the
aluminum ions owing to the complex formation.
[0043] In addition to the already stated components according to the second
aspect
of the invention, the aqueous treatment solution can contain compounds that
are
employed in the layer forming phosphatization as the so-called "accelerators".
These
accelerators have the property of capturing hydrogen atoms that are produced
by
the pickling attack of the acids on the metal surface. This reaction, also
known as
"depolarization", facilitates the attack of the acidic treatment solution on
the metal
surface and thereby accelerates the formation of the anticorrosion layer.
Accelerators can be employed, for example, which are listed in the previously
stated
document DE-A-199 33 189:
0.05 to 2 g/I m-nitrobenzene sulfonate ions,
0.1 to 10 g/I hydroxylamine in free or bound form,
0.05 to 2 g/I m-nitrobenzoate ions,
0.05 to 2 g/I p-nitrophenol,
1 to 70 mg/I of hydrogen peroxide in free or bound form,
0.05 to 10 g/I organic N-oxides
0.01 to 3 g/I, preferably up to 0.5 g/I nitroguanidine
CA 02632720 2014-05-20
to 500 mg/I of nitrite ions
0.5 to 5 g/I chlorate ions.
[0044] Moreover, the treatment solution can contain polymers with known
positive
activity in anticorrosion treatment. Examples of this type of polymers are:
a) polymers or copolymers of unsaturated alcohols or the esters or ethers
thereof,
b) polymers or copolymers of unsaturated carboxylic acids, organophosphonic
acids, organophosphinic acids or in each case the salts, esters or amides
thereof,
c) polyamino acids or proteins or in each case the salts, esters or amides
thereof,
d) carbohydrates or the esters (including esters of xanthic acid) or ethers
thereof,
e) polyamines, in which the nitrogen atoms are incorporated into the
polymer
chain,
f) polyethers,
g) polyvinylphenols and the substitution products thereof,
h) epoxy resins, amino resins, tannins, phenol-formaldehyde resins,
i) polymers and copolymers of vinyl pyrrolidone.
[0045] In so far as these types of polymer are present, their concentration in
the
aqueous treatment solution is preferably less than 2000 mg/I. On secondary
technical grounds, such as for example the simplification of the waste water
treatment, it can be advantageous to largely or completely dispense with the
presence of organic polymers in the aqueous treatment solution. Consequently,
a
preferred embodiment of the present invention is wherein the aqueous solution
contains no more than 1 mg/I of organic polymer. Under this condition, it is
further
preferred that the aqueous solution additionally contains 10 to 1000 mg/I,
preferably
50 to 500 mg/I of silicon in the form of silica particles with an average
particle size of
less than 1 pm. The stated silica particles with an average particle size of
less than 1
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pm are known under various generic names to the person skilled in the art.
They are
called, for example, colloidal silica, precipitated silica or pyrogenic
silica. The
average particle size, which is preferably in the range of about 0.01 pm to
about 1
pm, can be determined by light scattering methods or by electron microscopy.
[0046] According to the second aspect of the present invention, the treatment
solution can be manufactured at the place of use by dissolving the stated
components in water and adjusting the pH. However, this procedure is unusual
in
practice. In practice, instead of this, aqueous concentrates are usually
provided to
the place of use, diluted with water and the pH optionally adjusted to produce
the
ready-for-use treatment solution. Accordingly, an aqueous concentrate belongs
to
the second aspect of the invention, which on dilution with water by a factor
of about
to about 100, particularly by a factor of about 20 to about 50 and optionally
adjusted in pH, results in an acidic, chromium-free, aqueous solution of
fluoro
complexes according to the previous description.
[0047] For stabilization, the concentrates can contain polymers with
thickening
and/or dispersing properties. Examples of such polymers are polymers or
copolymers of unsaturated carboxylic acids, carbohydrates or proteins. They
can be
present in a concentration of up to 50 g/I.
[0048] On stability grounds, concentrates of this type are often adjusted such
that on
dilution with water, the pH is not directly in the required range. In this
case, after
dilution with water, the pH has to be corrected either downwards or upwards. A
downwards adjustment is made by adding an acid, wherein either the acid form
of
the fluoro complex of the metal M or nitric acid lends itself in the present
case. An
upwards adjustment of pH can be effected with any basic substance, for example
with a solution of alkali metal hydroxides or -carbonates, ammonia or organic
amines. However, basic compounds or salts, for example metal oxides, -
hydroxides
or -carbonates, which represent the possible active components in the
treatment
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=
solution, can also be added to increase the pH. For example, magnesium or zinc
oxides, -hydroxides or -carbonates can be used here.
[0049] A process for the anticorrosion treatment of bright metal surfaces
further
belongs to the second aspect of the present invention, wherein the metal
surface is
contacted with a previously described aqueous solution according to the second
aspect of the invention.
[0050] It can be preferred that after contact with the aqueous solution of a
fluoro
complex, the metal surface is then rinsed with an aqueous solution that
contains one
or more components selected from compounds or salts of the elements cobalt,
nickel, tin, copper, titanium and zirconium and/or from water-soluble or water-
dispersible organic polymers.
[0051] The term "bright" metal surface was explained further above in
connection
with the first aspect of the present invention. This explanation is also
correspondingly valid for the second aspect of the present invention.
[0052] Independently of whether the treatment of the metal surface with the
aqueous
solution of a fluoro complex corresponding to the process cycle according to
the first
aspect of the invention or with an aqueous solution according to the second
aspect
of the invention, the following is generally valid for this process step:
[0053] The bright metal surface is contacted with the acidic, aqueous solution
of a
fluoro complex for a period of 0.5 to 10 minutes, preferably for 1 to 5
minutes. This
can be carried out by dipping into the treatment solution or by spraying the
treatment
solution. In this regard, the temperature of the aqueous solution of a fluoro
complex
is preferably in the range 15 to 60 C, especially in the range of 25 to 50
C. After
this contact, rinsing is preferably carried out with water, especially fully
deionized
water. After this, the previously described final rinse can optionally follow.
In this
case, another rinse is subsequently made with water.
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[0054] In the process cycle according to the first aspect of the present
invention, the
treated metal surfaces are transferred without drying into a bath for the
cathodic
electro-dipcoating. One can proceed in exactly the same way for the treatment
with
a solution of fluoro complexes according to the second aspect of the present
invention. However, one can also dry the treated metal surface before coating
it with
a cathodic electro-dipcoat or with another coating, such as for example a
powder
coating.
[0055] The subsequent embodiments demonstrate the technical advantages of the
process according to the invention or the novel aqueous treatment solutions
according to the invention.
[0056] Sample sheets of cold rolled steel, as is used in the automotive
construction
industry, were used as the substrate in the following experiments. All process
steps
were carried out as dip processes. Abbreviations: FD water = fully deionized
water,
RT = room temperature, min. = minutes, CED = cathodic electro-dipcoating.
[0057] Table 1: General procedure
Process step Bath composition Bath pH Treatment
Temp time
Cleaning Alkaline cleanser of the 60 C Alkaline 5 min.
Applicant: 3% Ridoline 1562 +
0.3% Ridosol 1561
Rinse Plant water RT 1 min.
Rinse FD water RT 1 min.
Pre-treatment See Tables (zirconium is added 30 C See 3 min.
as H2ZrF6) Tables
Rinse FD water RT 0.5 min.
Optional drying Drying cabinet 50 C 60 min.
(see Tables)
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. .
CED Electro-dipcoat "CathoguardC:)"
310 from BASF
[0058] Table 2:
Bath composition for the pre-treatment in the process cycle according to Table
1
with drying after pre-treatment, and corrosion results: Climate change test
according
to VDA 621-415: average corrosion after 70 days in mm, and stone impact damage
after 70 days, scale from 0.5 to 5 according to DIN 55996-1 (the smaller the
better).
Bath composition Climate change Climate change
test:
test: corrosion stone impact
damage
Comparative 1: 150 mg/I Zr, pH 4 3.4 4.5
Example 1:150 mg/I Zr + 50 mg/I 1.6 4.0
salicylic acid, pH 4
Example 2: 150 mg/I Zr + 200 1.9 3.5
mg/I salicylic acid, pH 4
Comparative 2: 150 mg/I Zr + 200 9.3 5.0
mg/I citric acid, pH 4
[0059] Results:
The inventive examples 1 and 2 prove the favorable effect of an addition of
salicylic
acid, when the pre-treatment layer is dried (second aspect of the invention).
In
contrast, an addition of citric acid (comparison 2) has a rather negative
result.
[0060] Table 3:
Bath composition for the pre-treatment in the process cycle according to Table
1
without drying after pre-treatment ("wet on wet"), and corrosion results:
Climate
change test according to VDA 621-415: average corrosion after 35 days in mm.
Bath composition Climate
change
test: corrosion
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Comparative 3: 150 mg/I Zr, pH 4 1.5
Example 3: 150 mg/I Zr, 400 mg/I nitrate (added as nitric 1.3
acid), 200 mg/I Si (added as colloidal silica), pH 4
[0061] Results:
The inventive example 3 proves the favorable effect of an addition of nitrate
and
silica, when the pre-treatment layer is not dried (first aspect of the
invention).
[0062] Table 4:
In the procedure according to Table 1, the following treatment solution was
employed for the pre-treatment (treatment time: 5 min.), wherein firstly
(comparison
4) drying was made after the pre-treatment and secondly (example 4) no drying
was
made after the pre-treatment and prior to the dipcoating: 150 mg/I Zr, 400
mg/I
nitrate (added as nitric acid), 200 mg/I Si (added as colloidal silica), pH
3.8.
Climate change test according to VDA 621-415: average corrosion after 70 days
in
mm:
Bath composition Climate change test: corrosion
Comparative 4: (with drying) 3.9 mm
Example 4: (without drying) 3.6 mm
[01363] Results:
The inventive example 4 shows that in the presence of nitrate and silica,
better
results are obtained without drying than with drying (first aspect of the
invention).
[0064] Table 5:
Bath composition for the pre-treatment in the process cycle according to Table
5
without drying ("wet on wet"), after pre-treatment and corrosion results:
Climate change test according to VDA 621-415:
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average corrosion after 70 days in mm, and stone impact damage after 70 days,
scale from 0.5 to 5 according to DIN 55996-1 (the smaller the better).
Bath composition Climate change test: Climate change test:
corrosion stone impact
damage
Comparative 5: 150 mg/I Zr, pH 4 3.1 4.5
Example 5: 150 mg/I Zr + 20 mg/I Cu, 1.3 3.7
pH 4
Example 6: 150 mg/1Zr + 20 mg/I Cu 1.1 3.2
+ 200 mg/I Si, pH 4
[0065] Results:
The inventive example 5 proves that the addition of 20 mg/I of copper (as
Cu(NO)3)
to the conversion bath in the "wet on wet" process yields significantly better
infiltration values in the climate change test. The further addition of 20
mg/I of silicon
in the form of colloidal silica (Example 6) yields a significant improvement
in the K-
value in the stone impact test.
[0066] Table 6:
Bath composition for the pre-treatment in the process cycle according to Table
1
without drying ("wet on wet"), after pre-treatment and corrosion results:
Climate change test according to VDA 621-415: average corrosion after 70 days
in
mm, and stone impact damage after 70 days, scale from 0.5 to 5 according to
DIN
55996-1 (the smaller the better).
Bath composition Climate change Climate change test:
test: corrosion stone impact damage
Example 7: 150 mg/I Zr + 20 mg/I Cu + 0.9 3.7
50 mg/I Si, pH 4
Example 8: 150 mg/I Zr + 5 mg/I Cu + 50 0.7 2.3
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. :
mg/I Si + 50 mg/I nitroguanidine, pH 4
,
[0067] Results:
The inventive examples 7 and 8 in comparison show that the addition of the
accelerator nitroguanidine (50 mg/I) to the conversion bath result in a
further
improvement in the climate change test in regard to the corrosive paint
infiltration
and also significantly smaller K-values in the stone impact damage test.
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