METHODE ET PRODUIT DE PHOSPHATAGE DE SURFACES EN METAL

PROCESS AND COMPOSITION FOR PHOSPHATING METAL SURFACES

Abrégé anglais

ABSTRACT OF THE DISCLOSURE
In a process for applying phosphate coatings to
metal surfaces by means of nitrite-accelerated phosphating
solutions based on zinc phosphate, the layer weight on iron
or steel is reduced and controlled by bringing the metal
surfaces in contact with a phosphating solution which addi-
tionally contains a nitrobenzenesulfonate and in which the
nitrobenzenesulfonate content is such that the weight of the
resulting layer is less than 2 g/m2, preferably less than
1.5 g/m .
The content of nitrobenzenesulfonate is greater than
100 mg/liter and that of nitrite should preferably be 10 to
300 mg/liter. The process according to the invention is
applicable both to low zinc and high zinc phosphating solutions.

Revendications

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows.
1. A process for applying phosphate coatings to
metal surfaces by means of nitrite-accelerated phosphating
solutions based on zinc phosphate, which comprises bringing the
metal surfaces in contact with a phosphating solution which
additionally contains a nitrobenzenesulfonate compound in an
amount so that the weight of the resulting layer on iron or steel
is less than 2 g/m2.
2. The process as in Claim 1, wherein the nitro-
benzenesulfonate content is an amount such that the weight of
the resulting layer is less than 1.5 g/m2.
3. The process of Claim 1, wherein the content of
nitrobenzenesulfonate is greater than 100 mg/liter.
4. The process of Claim 1, wherein the nitrite
content is 10 to 300 mg/liter.
5. The process of Claim 1, wherein the weight ratio
of Zn to P2O5 is 1 : (8 to 85).
6. The process of Claim 1, wherein the weight ratio
of Zn to P2O5 is 1 : (0.3 to 7).
7. An aqueous acidic composition comprising in dis-
solved form zinc, phosphate, nitrite and a nitrobenzenesulfonate
compound in an amount sufficient to generate a coating weight
less than 2 g/m when contacted with an iron or steel surface.
8. The composition of Claim 7, wherein the nitro-
benzenesulfonate compound is present in a content greater than
100 mg/l.
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Description

Case: P30,029 ~Z~85Z
PROCESS AND COMPOSITION FOR Foisting METAL SURFACES
Background of the In notion
The invention relates Jo a composition and process for
applying phosphate coatings to metal surfaces by means ox nitrite-
accelerated phosphating solutions based on zinc phosphate.
It is known to add nitrites, in particular alkali metal nitrites to aqueous acid solutions of zinc phosphate in order to
accelerate layer formation and to oxidize the dissolved iron into
the trivalent form (German Patent Specifications 845,135 and
907,966~ German Auslegeschriften 1,031,083 and 1~277,645 and
German Offenlegungsschrift 1,771,769)~ The processes are used
in industry on a large scale. They are distinguished by a high
speed of phosphating, but give phosphate layers having relatively
high weights per unit area, unless special layer-improving
substances, for example polyphosphates (German Auslegeschrift
1,062,083), are added.
Admittedly the weight of the phosphate layer can be
somewhat reduced by the concomitant use of chlorates in nitrite-
accelerated phosphating solutions German Auslegeschrift
1,521~678). However, this procedure has the disadvantage that
I; major reductions in the weight of the layer are only possible
using high concentrations of chlorate, and thereby the side
reaction between chlorate and nitrite, with the formation of
chloride and nitrate, assumes undesirably high proportions.
In a recent phosphating~process a mixture of chlorate
and an organic vitro compound, for example a nitrobenzenesulfonate,
is used for acceleration (European Laid-Open Specification 36,689).
,. . _ .
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I issue
Thin, finely crystalline phosphate layers are produced by this
means. Here too, however, a disadvantage is the accumulation
of chloride in the treatment baths; in major amounts this can
have an adverse effect on the quality ox the layer.
The object of the invention is to provide a process
which does not have the disadvantages ox the known processes
and which has a high accelerating effect and gives extremely
finely gained, flexible phosphate layers having a low weight
per unit area.
I
Summary of the Invention
The object is achieved by modifying the process of
the type initially mentioned, in accordance with the invention,
in such a way that the metal surfaces are brought in contact
lo with a phosphating solution which additionally contains a
I` nitrobenzenesulfonate and in which the nitrobenzenesulfonate
content is such that the weight of the resulting layer on iron
or steel is less than 2 g/m2.
,
Admittedly it is known from German Offenlegungsschrift
~3~004~927/ for a special, so-called low-zinc process, to employ
a phosphating solution which also embraces the accelerator
combination of a nitrite and an aromatic vitro compound, such as
a nitrobenzenesulfonate. It is evident/ however, that neither
the significance of precisely this special combination nor the
possibility of being able to reduce the weight of the layer
for iron or steel below 2 g/m2 by controlling the dosage of
nitrobenzenesulfonate has been recognized (cf., in particular,
also the illustrative embodiments in this text).
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Detailed Description of the I've 8 52
The process according to the invention is intended
particularly for the treatment of iron and steel. However, it
is also suitable or phosphating zinc and aluminum materials
and also steel using coatings produced by a very wide variety
of processes and composed of zinc, zinc alloys, aluminum and
aluminum alloys.
The treatment of the metal surfaces can be carried
out by the spray-coating process, the flow-coating process and
also by the dip-coating process. However, it is also applicable
in combined procedures, for example spraying-dipping, dipping-
spraying, spraying-dipping-spraying and the like.
The contact times for the phosphating solution with
the metal surface are within the usual limits and can be, for
example, 45 seconds to 3 minutes for spraying, 2 to 10 minutes
for dipping and 20 seconds of spraying and 3 minutes of dipping
for spraying-dipping.
The temperatures of the phosphating solutions used in
the process according to the invention are usually between 30
and 60C. The lower the temperature, the longer the contact
time must be, in general.
When carrying out phosphating of iron or steel with
the accelerators nitrite and nitrobenzenesulfonate, it is possible
to obtain the layer weights below 2 g/m which are desired in a
particular case by suitable metering. Layer weights down to
about 1 g/m2 can be obtained.
.
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With regard to particularly hug lily of the
phosphate layer produced, as well as high gloss, but also a low
consumption of chemicals, an advantageous embodiment of the
invention envisages bringing the iron or steel surface in contact
with a phosphating solution in which the nitrobenzenesulfonate
content is such that the weight of the resulting layer on iron
or steel is less than 1.5 g/m . It is advantageous for this
purpose to bring the metal surfaces into contact with a phosphating
solution in which the content of nitrobenzenesulfonate is at
least 300 mg/liter.
The contents of the individual active ingredients of
the bath vary within the customary limits. The nitrite content
is preferably within the range from 10 to 300 mg/liter. The
upper limit for the nitrobenzenesulfonate content is about
2 g/liter. Higher contents are associated in practice with no
additional effect.
The nitrite and nitrobenzenesulonate accelerators
are usually added to the phosphating bath in the form of their
alkali metal salts. The nitrobenzenesulfonate can however, also
be added in the form of the corresponding sulfonic acid. In
special cases it is possible to generate the nitrite within the
phosphating bath by auto catalytic formation from nitrate and,
if necessary, to add nitrite separately only when starting the
phosphating baths after prolonged interruptions in working.
The aqueous acid solutions, to be used within the
scope of the invention, based on zinc phosphate contain primary
zinc phosphate and a certain amount of free acid, this amount
being adjusted to the particular bath concentration, mode of use
.
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issue
and bath -temperature. Depending on the process conditions, the
pi of the-baths it, for instance, between 2.0 and 3.9.
The baths can also contain further cations which are
known in the technology of phosphating, for example Nix Co, Cut
My, Cay My, Fe, Nay K, H, NH4 and others. In order to adjust
the acidity and to achieve special technical effects it can be
necessary or desirable to use concomitantly further anions other
than phosphate, for example NO, C103, Of, Suave F, BF4, Sift,
citrate, tart rate and others.
lo It has been found, surprisingly, that the accelerator
combination nitrite/nitrobenzenesulfonate is effective in the
same manner either in conjunction with the suckled low-zinc
technology or in conjunction with the standard zinc technology.
Accordingly, advantageous embodiments of the invention envisage
bringing the metal surfaces in contact with a phosphating solution
in which the ratio by weight of Zen to P205 has been adjusted to
l : I to 85), or bringing the metal surfaces in contact with a
phosphating solution in which the ratio by weight of Zen to P205
has been adjusted to l : (OWE to 7).
In the first case, phosphate layers containing a high
proportion of phosphophyllite to hopeite are produced on steel.
Because of their excellent resistance to corrosion, these coatings
are particularly suitable as a preparatory measure for painting,
the best protective properties being achieved in conjunction with
cathodic application of paint by electrode position.
In the second case the result is phosphating processes
which are distinguished by a higher speed of phosphating compared
with the low zinc processes. The phosphate layers produced by
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sluice
these processes possess good properties for a broad spectrum of
use in the fields of corrosion resistance and cold deformation
without machining.
The process according to the invention is exemplified
and illustrated in detail on the basis of the hollowing examples.
Examples:
Steel sheets were degrade in an aqueous alkaline
cleansing agent, rinsed with water and then treated by spraying
with various phosphating solutions for 2 minutes at 55C.
Bath solution 1:
1.4 g/liter of Zen
0.4 g/liter of No
17.8 g/liter of P205
6.0 g/liter of NO
100 mgjliter of NO as Nina
+ Noah to adjust the phosphating equilibrium.
Added quantity of No nutria
benzenesulfonate (mg/liter) 0 200 400 600 800
Weight of phosphate layer
(g/m ) 4.1 3.9 2.9 1.7 1.3
Appearance of the
phosphate layer: increase in fineness of grain
I with increasing amount of
nitrobenzenesulfonate
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Bath solution I 685~
. . .
0.8 g/liter of Zen
0.5 g/liter of No
1.5 g/liter of Clue
14 g/liter of P205
2 g/liter of N03
100 mg/liter of N02 as Nina
+ Noah to adjust the phosphating equilibrium.
Added quantity of
No m-nitrobenzenesulfonate
(mg/liter) o 100 200 30V 400 500
Weight of phosphate layer
(g/m ) 2.5 2.1 1.8 1.6 1.4 1.2
Appearance of the
15 phosphate layer: increase in fineness of grain with
increasing amounts of nitrobenzenesulfonate
Bath solution 3:
3 g/liter of Zen
20 0.012 g/liter of No
1.5 g/liter of Clue
7.2 g/liter f~P205
:: 1.3 g/liter of N03
133 mg/liter of NO as Nina
+ Noah to adjust the phosphating equilibrium.
Added quantity of
No m-nitrobenzenesulfonate
(mg/liter) 0 100 200 300 400 500 700
Weight of phosphate layer
~g/m2) 3.5 3.1 2.6 2.2 1.9 1.6 1.2
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I
Appearance of the
phosphate layer: increase in fineness of grain with
increasing amounts of nitrobenzenesul~onate
Bath solution 4:
3.2 g/liter of Zen
6.8 g/liter of P205
2~6 g/liter of N03
150 mg/liter of N02 as Nina
Noah to adjust the phosphating equilibrium.
Added quantity of
No m-nitrobenzenesulfonate
(mg/liter) o 100 200 400 600 800
Weight of phosphate layer
(g/m2) 3.8 3.5 3.2 2.5 2.0 1.4
: Appearance of the
phosphate layer: increase in fineness of grain with
increasing amounts of nitrobenzenesulfonate
"Bath solution 1" and "bath solution 2" correspond to
low-zinc technology, while "bath solution 3" and "bath solution 4"
: correspond to standard zinc technology. The data shows that,
; within both procedures, the controlled addition of nitrobenzene-
~;~ 25 sulfonate is effective in the same manner and is associated with
I: a considerable reduction in the layer weight.
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