Note: Descriptions are shown in the official language in which they were submitted.
CA 02018631 1999-01-1~
The present invention relates to a process for a
passivating postrinsing of phosphate layers on metals,
particularly steel, galvanized steel, zinc alloy-plated
steel and aluminum, with a chromium-free aqueous solution
before the application of a paint or adhesive.
Phosphating is industrially employed on a large
scale for the preparation of metal surfaces for a
subsequent application of paint. The phosphate layers thus
formed result, inter alia, in an improved adhesion of the
paint films on the metals, an increased resistance to
corrosion and an inhibition of subsurface corrosion which
might be initiated at damaged portions of the paint film.
The protective properties of the phosphate layers may
further be improved by a passivating postrinsing with an
aqueous fluid.
Desirable properties from the aspect of
application technology will result from a passivating
postrinsing with fluids which contain hexavalent and/or
trivalent chromium. But the toxicity of the trivalent and
particularly of the hexavalent chromium compounds is often
regarded as a disadvantage.
Whereas US-A-4376000 describes a chromium-free
postrinsing agent which contains polyvinylphenol, that
agent must be used in a comparatively high concentration so
that its use will result in an undesired pollution of the
sewage, particularly because a large amount of oxygen is
required for the decomposition.
CA 02018631 1999-01-1~
US-A-3695942 discloses the use of soluble
zirconium compounds for an aftertreatment of conversion
layers. In addition to zirconium, the postrinsing agents
contain cations consisting of alkali and ammonium. The
reference contains an explicit warning against the use of
alkaline earth metal cations. Said postrinsing agents,
which are used at a pH value from 3 to 8.5, do not result
in the same quality as the chromium-containing agents.
US-A-3895970 describes acid aqueous postrinsing
agents for treating phosphate layers. Said agents
containing simple or complex fluorides and chromium-
zirconium fluoride and zirconium fluoride are mentioned as
zirconium compounds. With the exception of chromium-
zirconium fluoride, the products mentioned in that patent
will meet only medium requirements and chromium-zirconium
fluoride has the above-mentioned disadvanta~e that it is
toxic.
It is an object of the invention to provide for
the passivating postrinsing of phosphate layers on metals
before an application of a paint or adhesive a process
which is free of the disadvantages of the known processes
and distinguishes by providing a high protection against
corrosion and a strong adhesion to paint and adhesive and
does not pollute or only very slightly pollutes the
environment.
That object is accomplished in that a process of
the kind described first hereinbefore is carried out in
accordance with the invention in such a manner that the
phosphated metal surfaces are rinsed with aqueous solutions
which have been adjusted to a pH value of 3 to 5 and which
contain an aluminum fluorozirconate in which the Al:Zr:F
CA 02018631 1999-01-1~
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mole ratio is (0.15 to 0.67):1:(5 to 7), and in which
solutions the total concentration of Al + Zr + F is 0.1 to
2.0 g/l.
According to a preferred feature of the invention
the phosphated metal surfaces are rinsed with aqueous
solutions in which the total concentration of Al + Zr + F
is 0.2 to 0.8 g/l.
The process in accordance with the invention is
suitable for treating phosphate layers of all types which
can be formed on metals, particularly on steel, galvanized
steel, steel plated with a zinc alloy, aluminum-plated
steel, zinc, zinc alloys, aluminum and aluminum alloys.
Such phosphates include, inter alia, zinc phosphate, iron
phosphate, manganese phosphate, calcium phosphate,
magnesium phosphate, nickel phosphate, cobalt phosphate,
zinc-iron phosphate, zinc-manganese phosphate, zinc-calcium
phosphate, and layers of other types, which contain two or
more divalent cations. The process is particularly suitable
for treating those phosphate layers which have been formed
by low-zinc phosphating processes with or without an
addition of further cations, such as Mn, Ni, Co, Mg.
When the metal surfaces have been phosphated, they
are suitably rinsed with water before they are
aftertreated, e.g., by dipping, spraying, flooding or
rolling by means of the process in accordance with the
nventlon.
The postrinsing agents used in the process in
accordance with the invention may be chemically classified
as
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aluminum fluorozirconates of low acidity. They may be produced,
e.g., in a process in which metallic zirconium or zirconium
carbonate is dissolved in aqueous hydrofluoric acid so that a
complex fluorozirconic acid is formed. Metallic aluminum or
aluminum hydroxide is then added and dissolved. Whereas that
production process is preferred, the postrinsing agents may
alternatively be produced by other processes.
In another preferred embodiment of the invention the
phosphated metal surfaces are rinsed with aqueous solutions
which additionally contain at least one of the anions benzoate,
caprylate, ethyl hexoate, salicylate in a total concentration
of 0.05 to 0.5 g/l. This will particularly result in a further
increase of the bare corrosion protection. The anions may be
added as the corresponding acids or salts.
The pH value of the postrinsing solutions is
preferably adjusted with cations of volatile bases, which
particularly include ammonium, ethanolammonium and di- and tri-
ethanolammonium.
The passivating postrinsing fluid may be applied to
the phosphated metal surfaces by dipping, flooding, spraying
and wetting, e.g., by means of rollers. The treating times are
between about 1 second and 2 minutes. The fluid may be applied
at a temperature from room temperature to about 80~C.
Temperatures between 20 and 50~C are usually preferred.
Deionized or low-salt water is usually employed to prepare the
postrinsing baths. Waters having a high salt content are less
suitable for preparing the baths.
In another preferred embodiment of the invention the
phosphated metal surfaces are finally rinsed with deionized
water.
,
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The process according to the invention is used to
prepare the phosphated metal surfaces for an application of
paint or adhesive. The process improves the adhesion of the
organic films to the metallic substrate, improves also the
resistance of the organic films to a formation of blisters
under corrosive conditions, and inhibits the progress of
subsurface corrosion from damaged portions of the film. The
process has proved to be particularly advantageous in
conjunction with paints applied by cathodic electrocoating
and powder coating or from low-solvent high-solids paints
and paints applied mainly with water as a solvent.
The process in accordance with the invention will
be explained more in detail and by way of example with
reference to the following Example.
Example
Degreased sheets made of steel, electrogalvanized
steel and AlMgSi are sprayed in a manganese-modified low-
zinc phosphating process at 55~C for 2 minutes. The
phosphating solution had the following composition:
0.7 g/l Zn0.04 g/l Fe(III)
1.0 g/l Mn13 g/l P2O5
1.0 g/l Ni2.1 g/l NO3
2.9 g/l Na 0.3 g/l F
0.15 g/l NH40.07 g/l NO2
Finely crystalline, uniformly covering phosphate
layers weighing 2.5 to 3 g/m2 were formed on the three
metal substrates. Thereafter the sheets were rinsed with
water and then subjected to a passivating postrinse. The
passivating postrinsing was effected by spraying at 30~C
for 1 minute. Thereafter the sheets were rinsed with
deionized water and were coated with a primer applied by
cathodic electro-
coating, with a filler and with a top coat. Each paint film wasseparately baked. The total thickness of the coating amounted
to 90 ~m.
The sheets were subsequently scribed by means of a
steel needle as far as to the metal substrate and were then
subjected to various tests. The results are compiled in Tables
1 to 3.
To prepare the postrinsing fluid used in the process
in accordance with the invention, 1.6 g of an aqueous
concentrate containing 0.855% Al + 8.62% Zr + 10.7% F by weight
was diluted with deionized water and was subsequently adjusted
with ammonia to a pH value of 3.5 to 4Ø This resulted in a
postrinsinq fluid containing 0.014 g/l Al + 0,14 g/l Zr
0.17 g/l F + 0.026 g/l NH3.
Control tests were conducted with: A postrinsing
solution containing Cr~VI) and Cr(III), specifically 0.2 g/l
CrO3 and 0.037 g/l Cr(III), and having a pH value of 3.5 to
4.0; a solution of chromium fluorozirconate containing
0.047 g/l Cr(III), 0.083 g/l Zr and 0.121 g/l F and having a pH
value of 3.5 to 4.0; and
a solution containing 0.6 g/l polyvinylphenol and having a pH
value of 3.5 to 4Ø
Each sheet specimen was tested by the salt spray test
in accordance with DIN 50021 SS(1008 h), the Filiform Test in
accordance with ASTM D 2803 (1008 h) and 20 cycles of the
General Motors Test Method TM 54-26 (GM Scab Test). The sub-
surface corrosion under the organic coating (rate of creep
back) was measured in mm.
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TABLE
Results on Steel
Afterrinsing pH- Subsurface corrosion under organic
agent value coating (mm) in
Salt spray Filiform GM Scab
test DIN Test ASTM Test
50021 SS D 2803
(1008 h) (1008 h) (20 cycles)
Cr(VI)- 3.5-
Cr(III) 4.0 0 - < 1 0 3.5
Chromium 3.5-
Fluorozir- 4.0 0 - c 1 0 - c 1 3.5
conate
Polyvinyl- 3.5-
phenol 4.0 0 - 1 < 1 4.0
Aluminum
fluorozir- 3.5-
conate 4.0 0 0 3.5
+ NH3 to pH
(invention)
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-
TABLE 2
Results on Galvanized Steel
Afterrinsing pH- Subsurface corrosion under organic
agent value coating (mm) in
Salt spray Filiform GM Scab
test DIN Test ASTM Test
50021 SS D 2803
(1008 h) (1008 h) (20 cycles)
Cr(VI)- 3,5-
Cr(III) 4.0 8.5 0 < 1 - 1
Chromium 3.5-
Fluorozir- 4.0 7.0 0 - < 1 < 1 - 1
conate
Polyvinyl- 3.5-
phenol 4.0 6.5 0 - < 1
Aluminum
fluorozir- 3.5-
conate 4.0 5.5 0 < 1 -1
+ NH3 to pH
(invention)
,
CA 02018631 1999-01-15
TABLE 3
Results on AlMgSi
Afterrinsing pH- Subsurface corrosion under organic
agent value coating (mm) in
Salt spray Filiform GM Scab
test DIN Test ASTM Test
50021 SS D 2803
(1008 h) (1008 h) (20 cycles)
Cr(VI)- 3.5-
Cr(III) 4.0 < 1 0 0.5 - 1
Chromium 3.5-
Fluorozir- 4.0 0 0
conate
Polyvinyl- 3.5-
phenol 4.0 0 - <1 0
Aluminum
fluorozir- 3.5-
conate 4.0 0 0 0.5 - 1
+ NH3 to pH
(invention)
A comparison of the data compiled in the Tables
will reveal that the results produced by the process in
accordance with the invention are at least as good in each
case as the best of the three controls which were also
tested.
