Note: Descriptions are shown in the official language in which they were submitted.
, ,, CA 02303877 2000-03-17
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"Process for phosphating steel strip" _
This invention relates to a process for the phosphating of steel strip, or of
steel strip coated
on one or both sides with zinc or zinc alloy, by a spraying or dipping
treatment which,
depending on the speed of the conveyor belt, takes place over a period of
about 2 to about
20 seconds.
Processes for phosphating surfaces of iron, steel, zinc and alloys thereof as
well as of
aluminum and alloys thereof are long-standing prior art. The phosphating of
the above-
mentioned surfaces serves to increase the adhesive strength of paint layers
and to improve
the protection from corrosion. The phosphating is carried out by dipping the
metal surfaces
into the phosphating solutions and by spraying the metal surfaces with the
phosphating
solutions. Combined processes are also known. Formed metal parts such as car
bodies, may
be phosphated, as may also metal strips on high-speed production lines. The
present
invention is concerned with such a phosphating of strip. Phosphating of strip
differs from
phosphating of parts in that, because of the high conveyor belt speeds, the
phosphating, i.e.
the development of a closed metal phosphate layer, has to take place within a
short period
of time, for example, within about 2 to about 20 seconds.
Processes for phosphating metal strips, in particular steel strips which have
been zinc-coated
electrolytically or by hot dipping, are known in the art. For example, WO
91/02829
describes a process for phosphating steel strip which has been zinc-coated
electrolytically
and/or by hot dipping; the process involves short-term treatment using acid
phosphating
solutions which, besides zinc ions and phosphate ions, contain manganese
cations and nickel
canons, as well as anions of oxygen-containing acids having an accelerating
action. The latter
term refers, in particular, to nitrate ions. DE-A-35 37 108 likewise describes
a process for
phosphating electrolytically zinc-coated steel strips by treatment using acid
phosphanng
solutions which, besides zinc ions, manganese ions and phosphate ions, contain
other metal
cations, such as nickel ions and/or anions of oxygen-containing acids having
an accelerating
action, in particular nitrate ions. Here, the zinc cation content is within
the relatively low
range of 0.1 to 0.8 gll.
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DE-A-39 20 296 describes a process for the production of manganese- and
magne~sium-
containing zinc phosphate coatings on steel, zinc, aluminum and/or alloys
thereof by
spraying, spray-dipping and/or dipping using an aqueous solution which
contains 0.2 to 1.0
g/1 zinc ions, 0.2 to 2.0 g/1 manganese ions, 0.5 to 2 gll magnesium ions, 10
to 20 g/1
phosphate ions, 0.2 to 10 g/1 nitrate ions and up to 1.0 g/1 fluoride ions.
This phosphating
solution necessarily also contains 0.02 to 2 g/1 nitrite ions and/or 0.4 to 1
g/1 chlorate ions
and/or 0.2 to 1 g/1 of an organic oxidising agent as an accelerator. The
nitrate ions essential
in this process may, however, in the case of zinc-coated steel surfaces, lead
to loss of quality
in the phosphate layer as a result of pinholing. In this connection,
"pinholes" mean whitish
corrosion points on the metal surface which have a crater-like appearance in
rnicrographs.
From the phosphating times of more than 2 minutes mentioned in the Examples,
it is obvious
that this is not a process for the phosphating of strip.
The German Patent Application 196 39 596 attempts to provide a phosphating
process which
on the one hand solves the problem of pinholing and, on the other hand, also
makes it
possible, within the short phosphating times usual on production lines, to
produce a closed
crystalline phosphate layer on steel strips which are not zinc-coated and on
the uncoated side
of steel strips zinc-coated on one side. According to that document, the
object is fulfilled by
a process for the phosphating of steel strip, or of steel strip coated on one
or both sides with
zinc or zinc alloy, by spraying treatment or dipping treatment for a period of
2 to 15
seconds, using an acid phosphating solution containing zinc and manganese, at
a temperature
of 40 to 70°C, characterised in that the phosphating solution contains:
1 to 4 g/1 zinc ions;
0.8 to 3.5 g/1 manganese ions;
to 30 g/1 phosphate ions;
0.1 to 3 gll hydroxylamine in free, ionic or bound form;
and not more than 1 g/1 nitrate ions; a free acid content of 0.4 to 4 points;
and a total acid
content of 12 to 50 points.
The crystalline zinc phosphate layers produced by this process already very
adequately meet
the standards being set nowadays for a zinc phosphating solution. This is
particularly the case
when this process is used in an embodiment in which either 0.8 to 3.5 g/1
nickel ions or
CA 02303877 2000-03-17
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0.002 to about 0.2 g/1 copper ions are simultaneously present. If the process
is carried out
without these rations, disadvantages with regard to paint adhesion and
protection from
corrosion have to be accepted.
For environmental and toxicological reasons, the use of nickel ions in
phosphating baths is
becoming increasingly less desirable nowadays. For the phosphating of parts,
such as the
phosphating of car bodies using phosphating times of more than about 2
minutes, phosphating
processes have been employed recently in which, instead of the nickel ions,
copper ions are
used within the low concentration range given above. Processes which function
without the
use of the toxicologically- and environmentally-disadvantageous nickel ions
are also being
intensively sought for high-speed production lines with short phosphating
times of about 2
to about 20 seconds. For production lines, however, the replacement of the
nickel ions by
low concentrations of copper ions is not a solution, because for the large
area to be
phosphated per unit time and with the small bath volume compared with that in
the
phosphating of parts, the copper content would have to be determined and
controlled in time
intervals of shorter than 1 minute. There are no suitable measuring and
controlling techniques
available.
Accordingly, an object of the present invention is to provide a phosphating
process for high-
speed production lines using phosphating times of about 2 to about 20 seconds,
which, on
the one hand, is operated free from nickel and which, on the other hand,
results in phosphate
layers whose quality as regards paint adhesion and protection against
corrosion is equal to
that of nickel-containing phosphate layers.
This object is fulfilled by a process for the phosphating of steel strip, or
of steel strip coated
on one or both sides with zinc or zinc alloy, by a spraying or dipping
treatment for a period
of 2 to 20 seconds, using an acid phosphating solution containing zinc,
magnesium and
manganese, at a temperature of 50 to 70°C, characterised in that the
phosphating solution is
free from nitrate ions and in that it contains:
1 to 4 g/1 zinc ions;
1.2 to 4 g/1 manganese ions;
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1 to 4 g/1 magnesium ions;
to 30 g/1 phosphate ions;
0.1 to 3 g/1 hydroxylamine in free, ionic or bound form;
a free acid content of 0.4 to 4 points; and a total acid content of 15 to 45
points.
In this process, the steel strips may be zinc-coated electrolytically or zinc-
coated by hot
dipping or coated with zinc alloy. "Coated with zinc alloy" means that the
steel surface is
coated with an alloy which in addition to zinc contains other metals, such as
iron, nickel or
aluminum. A zinc alloy coating using a zinc-iron alloy may be carried out, for
example, by
tempering a zinc-coated steel strip, as a result of which a diffusion of iron
atoms into the
zinc layer and vice versa takes place. The layer thicknesses of the zinc
coating layers are
generally within the range of about 5 to about 20 ~cm.
The terms "free acid" and "total acid" are generally known in the field of
phosphating. They
are determined by titrating a sample from the acid bath using 0.1 N sodium
hydroxide
solution and measuring the consumption of the latter. The consumption in ml is
expressed
as a number of points. In the present context, the number of points of the
free acid means
the consumption in ml of 0.1 N sodium hydroxide solution required to titrate
10 ml of bath
solution, which has been diluted to 50 ml using deionised water, to a pH of
4Ø Similarly,
the number of points of the total acid gives the consumption in ml to attain a
pH of 8.2.
Preferably, the free acid is adjusted to within the range of 1.5 to 2.8 points
and the total acid
to within the range of 25 to 35 points.
Hydroxylamine may be used as the free base, as a hydroxylamine-abstracting
compound,
such as hydroxylamine complexes, and as ketoximes or aldoximes, or in the form
of
hydroxylammonium salts. If free hydroxylamine is added to the phosphating bath
or to a
phosphating bath concentrate, it will be present largely in the form of
hydroxylammonium
cations owing to the acid character of these solutions. If it is used in the
form of a hydroxyl-
ammonium salt, the sulfates and the phosphates are particularly suitable. In
the case of the
phosphates, the acid salts are preferred owing to the better solubility
thereof. In order, on
the one hand, to take account of economic aspects and, on the other hand, not
to load the
phosphating baths with excessive sulfate ions, a combination of free
hydroxylamine and
CA 02303877 2000-03-17
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hydroxylammonium sulfate may be used with advantage. Hydroxylamine or
derivatives
thereof are added to the phosphating bath in quantities such that the
calculated concentration
of the free hydroxylamine is between about 0.1 and about 3 g/1, preferably
between about
0.15 and about 1 g/1.
For the purpose of stating the phosphate concentration, the total phosphorus
content of the
phosphating bath is regarded as being present in the form of phosphate ions
PO43-. In the
calculation or determination of the concentration, no consideration is
therefore given to the
known fact that, at the acidic pH of the phosphanng baths, which is within the
range of about
2.0 to about 3.6, only a very small proportion of the phosphate is actually
present in the
form of the triply negatively charged anions. At these pH values, it is rather
to be expected
that the phosphate exists chiefly as a singly negatively charged dihydrogen
phosphate anion,
together with undissociated phosphoric acid and with smaller quantities of
doubly negatively
charged hydrogen phosphate anions.
No nickel ions are added to the phosphating solution. In the ideal case, this
solution is
therefore completely free from nickel ions. However, if the phosphating unit
is made of
specialised steel containing nickel, or if the objects being phosphated are
zinc-coated steel
strips in which the zinc layer additionally contains nickel, the possibility
that low
concentrations of nickel ions may enter the solution as a result of attack by
the phosphating
solution cannot be ruled out. The phosphating solution should nevertheless
contain not more
than about 0.05 g/1 nickel ions and in particular not more than about 0.015
g/1 nickel ions.
In addition to the above-mentioned layer-forming cations, the phosphating
solutions contain
alkali metal canons and/or ammonium canons in order to adjust the value of the
free acid to
within the required range.
The presence of fluoride ions in the phosphating solution is not as a rule
necessary for the
phosphating of steel which is not zinc-coated. The phosphating of steel strips
zinc-coated by
hot dipping is, however, facilitated by fluoride ions, and even for the
phosphating of
electrolytically zinc-coated steel strip, the presence of fluoride ions may be
advantageous in
forming a uniform layer. Accordingly, another preferred. embodiment of the
present invention
CA 02303877 2000-03-17
involves using phosphating solutions which contain up to about 0.8 gll
fluoride in free or in
complexed form. For example, for phosphating electrolytically zinc-coated
steel strip, the
preferred fluoride contents are within the range of 0.0 to about 0.5 g/1 and
in particular
within the range of about 0.1 to 0.2 g/1.
The phosphating solutions are generally prepared in the manner known to those
skilled in the
art. Phosphate, for example, is introduced into the phosphating solutions in
the form of
phosphoric acid. The canons are added to the phosphoric acid in the form of
acid-soluble
compounds, such as the carbonates, the oxides or the hydroxides, so that the
acid is partly
neutralised. The further neutralisation to within the required pH range is
carried out
preferably by addition of sodium hydroxide or sodium carbonate. A suitable
source of free
fluoride anions is, for example, sodium fluoride or potassium fluoride.
Tetrafluoroborate or
hexafluorosilicate, for example, may be used as complex fluorides.
A further aspect of the present invention relates to the use of the above
phosphating process
for the production of phosphate layers having a mass per unit area within the
range of about
0.4 to about 2.0 g/m2 on both sides of steel strip or on both sides of steel
strip coated on one
or both sides with zinc or zinc alloy. Preferably, phosphate layers having a
mass per unit
area within the range of about 0.9 to about 1.8 g/m2 are produced. The mass
per unit area
("layer weight"), as those skilled in the art know, may be determined by
weighing a
phosphated sample sheet, dissolving the phosphate layer in 5 % chromic acid
solution and
reweighing the sample sheet. This method is described, for example, in DIN
50942. For the
production of phosphate layers having the required mass per unit area, it is
preferable to use
phosphating solutions wherein the free acid content is within the range of
about 1.5 to about
2.8 points and the total acid content is within the range of about 25 to about
35 points. The
temperature of the treatment solution is preferably within the range of about
50 to about 70°C
and in particular within the range of about 55 to about 65°C. Preferred
treatment times are
within the range of about 5 to about 10 seconds. The phosphating solution is
preferably
sprayed onto the metal surface and rinsed off with water after the required
treatment period.
Before the application of the phosphating solution, the metal surface must be
completely
wettable with water. This is generally catered for in continuously operating
production lines.
CA 02303877 2000-03-17
If the surface of the strip is however to be oiled, the oil should be removed
with a suitable
cleaner prior to phosphating. The processes for this are common in the
industry. Before the
phosphating, an activation is generally carried out using activating agents
known in the art.
Generally, solutions or suspensions containing titanium phosphates and sodium
phosphates
are used. The activation is followed by the application of the present
phosphating process,
which is advantageously followed by a passivating secondary washing. Here an
intermediate
washing with water generally takes place between phosphating and passivating
secondary
washing. Treatment baths containing chromic acid are widely used for
passivating secondary
washing. On grounds of industrial safety and environmental protection and for
reasons
associated with toxic waste disposal, there is, however, a tendency to replace
these
chromium-containing passivating baths with chromium-free treatment baths. For
this purpose,
purely inorganic bath solutions, in particular those based on
hexafluorozirconates, or even
organic reactive bath solutions, for example, based on substituted
poly(vinylphenols) are
known. One may also use secondary washing solutions containing 0.001 to 10 g/1
of one or
more of the following cations: lithium ions, copper ions, silver ions and/or
bismuth ions.
The metal strips phosphated according to the present invention may be coated
directly with
an organic coating. Even in the initially uncoated state they may, however,
after cutting,
forming and joining, be assembled to form structural parts, such as car bodies
or domestic
appliances. The forming processes associated with this are facilitated by the
phosphate layer.
If the corrosive stress on the finished structural parts is low as, for
example, in the case of
domestic appliances, the appliances assembled from the previously phosphated
metal may be
directly coated. For higher corrosion-prevention standards, such as are set,
for example, in
automobile construction, it is advantageous for another phosphating treatment
to be carried
out following the assembly of the car bodies.
The present invention is illustrated in more detail by the Examples below
(Table 1).
Phosphating processes according to DE-A-39 20 296 were used as Comparison
Examples.
Cleaned sample sheets were activated using an activating agent containing
titanium phosphate
and were phosphated by spraying. After an intermediate washing, they were post-
treated
using a post-passivating solution based on chromate and, after having been
rinsed with
deionised water, were given a grey coating using BASF KTL FT 85-7042. Table 2
shows
CA 02303877 2000-03-17
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results of investigations into corrosion protection by the VDA alternating
climate test 621-415
over 10 cycles, together with the stone impact test in accordance with VW
Standard P 1210
(K = 10: worst value, K = l: best value). The creepage of paint was measured
along a
scribe above the half scribe-width.
' CA 02303877 2000-03-17
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