Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02551492 2006-06-22
Hydroxysulfata surfsc~ treatment
The present invention relates to the use of a treatment
solution for treating the surface of a steel sheet
coated with a metal coating based on zinc or its
alloys. It also relates to a method of lubricating such
a coated sheet.
Steel sheet coated with zinc or its alloys is widely
used in the automotive field and in industry in
general, as'it exhibits excellent, corrosion resistance.
However, such galvanized steel sheet has a number of
difficulties when it is being formed, for example by
drawing, in order to manufacture parts.
Usually, to give galvanized steel sheet better
tribological properties, a film of lubricating oil is
applied to its surface so as to facilitate the forming
operation.
However, despite applying a suitable lubricating oil
film, very substantial friction exerted by forming
tools on the surface of the sheet causes, at the
surface of the sheet, powder or particles based on zinc
or its alloys that are generated by degradation of the
coating. The accumulation and/or agglomeration o~ these
particles or this powder in the forming tools may
damage the formed parts, by the formation of barbs
and/or constrictions.
Furthermore; because of the high friction coefficient
that characterizes the sliding of a galvanized surface
in contact with the surface of a forming tool, the
sheet runs the risk of fracturing shouJ.d there be
insufficient sliding of the sheet i,n the forming tool
gap. Such fractures may appear even when an oil film of
sufficient weight, i.e. greater than 1 g/mz, is applied
to the surface of the sheet as it is not possible to
maintain a uniform distribution of the oil film over
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the surface of the sheet. This is due to the phenomenon
of dewetting, which corresponds to the presence of
areas deficient in oil.
However, the fact of depositing a relatively thick oil
film on the surface of the sheet poses problems of
contamination of the workshops and drawing tools, and
requires the use of large quantities o~ degreasing
agents in order to clean the sheet and considerable
means for treating the effluents coming from the
cleaning operation.
Moreover, the oil deficiency in certain areas of the
oil film due to the dewetting effect is also
responsible for lesser temporary corrosion protection
of the steel sheet while it is being stored.
The object of the present invention is therefore to
propose a treatment solution which, when applied to the
surface of a steel sheet coated with a metal layer
based on zinc ox its alloys, makes it possible to
reduce the degradation of the galvanized surface of
this sheet while it is being formed; to reduce the
quantity of lubricating oil to be deposited on the
sheet before it is formed, and to improve the temporary
corrosion protection of the sheet.
For this purpose, the subject of the invention is the
use of an aqueous treatment solution containing sulfate
ions SO42- with a concentration of not less than
0.01 mol/1 in order to treat the surface of a steel
sheet treated on at least one of its sides with a metal
coating based on zinc or its alloys, for the purpose of
reducing the formation of metal powder or particles
based on zinc or its alloys generated by the
degradation of the coating while said sheet is being
formed.
The expression "metal coating based on a zinc alloy" is
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understood to mean a zinc coating comprising one or
more alloying elements, such as for example but not
being restricted thereto, iron, aluminum, silicon,
magnesium and nickel.
According to the invention, the use of a steel sheet
coated with an almost pure zinc coating is prefexxed.
when the surface of a steel sheet coated with a metal
coating based on zinc or its alloys is treated by means
of an aqueous treatment solution according to the
invention, a layer forms on the surface of the sheet
that is based on zinc hydroxysulfate and zinc sulfate
and is both sufficiently thick and sufficiently
adherent. However, such a layer cannot be formed when
the So42- concentration is less than 0.01 mol/1, but it
has also been found that too high a concentration does
not substantially impxove the rate of deposition and
may even slightly reduce it.
In a first embodiment of the invention, the treatment
solution is applied in a conventional manner, for
example by dipping, by spraying or by coating, both on
electrogalvanized sheet and on hot-dip galvanized
sheet.
In a preferred embodiment, the aqueous treatment
solution furthermore contains Zn2+ ions with a
concentration of not less than 0.01 mol/1 which makes
it possible to obtain a more uniform deposition.
For example, the treatment solution is prepared by
dissolving zinc sulfate in pure water; for example,
zinc sulfate heptahydrate (ZnS09~7H20) is used, the Zn2+
ion concentration then being equal to that of the 5092-
ani.ons .
Preferably, the pH of the treatment solution
corresponds to the natural pH of the solution without
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the addition of either base or acid - the value of this
pH is generally between 5 and 7.
To minimize the formation of powder or particles of
zinc or its alloys resulting from degradation of the
coating on the sheet while it is being formed, the
conditions under which the treatment solution is
applied to the surface of the sheet, namely the
temperature, the time during which the solution is in
contact with the galvanized surface, the 5042- ion
concentration and the Zn2+ ion concentration, are
adjusted so as to form a layer based on zinc
hydroxysulfate and zinc sulfate, the sulfur content of
which, is not less than 0.5 mg/mz. This is because when
the sulfur content is less than 0.5 mg/mz, the reduction
in degradation of the coating is less substantial.
Thus, the time during which the treatment solution is
in contact with the galvanized suxface is between two
seconds and two minutes and the temperature of the
treatment solution is between 20 and 60°C.
Preferably, the treatment solution used contains
between 20 and 160 g/1 of zinc sulfate heptahydrate,
which corresponds to a Zn2+ ion concentration and an
SO92~ ion concentration that lie between 0.07 and
0.55 mol/1. In fact, it has been found that in this
concentration range the rate of deposition is barely
influenced by the value of the concentration.
Advantageously, the conditions under which the
treatment solution is applied, namely the temperature,
the time during which the solution is in contact with
the galvanized surface and the sOQZ- ion az~d Zn2+ ion
concentrations, are adjusted so as to form a hydroxy-
sulfate/sulfate-based layex having a sulfur content of
between 3.7 and 27 mg/mz.
According to a variant of the invention, the treatment
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solution contains an oxidizing agent for zinc, such as
hydrogen peroxide. This oxidizing agent may have a very
pronounced sulfation/hydroxysulfation accelerator
effect at low concentration. It has been found that the
addition of only 0.03, i.e. 8 x 10-3 mol/1 of hydrogen
peroxide, ox of 2 x 10-4 mol/1 of potassium
permanganate, to the solution makes it possible for the
rate of deposition to be doubled (approximately).
However, it has been found that concentrations 100
times higher no longer allaw this improvement in
deposition rate to be obtained.
After the treatment solution has been applied, but
before drying, the layer deposited on the sheet is
adherent. The drying is adjusted in order to remove the
residual liquid water from the deposit.
Between the application step and the drying step, the
sheet is preferably xinsed so as to remove the soluble
portion of the deposit obtained. The absence of rinsing
and the formation of a resulting deposit that is
partially soluble in water do not greatly prejudice the
reduction in degradation of the galvanized coating
during the operation of forming the sheet, since the
deposit obtained does indeed comprise a water-insoluble
zinc.hydroxysulfate/zinc sulfate-based layer in contact
with the sheet.
According to a second embodiment of the invention, the
aqueous treatment solution having an SO42' ion
concentration of not less than 0.07. mol/1 is applied
under anodic polarization and the pH of the treatment
solution is equal to 12 or higher, but less than 1,3.
Tf the pH of the solution is less than 12, adherent
hydroxysulfates are not formed on the surface to be
treated. If the pH of the solution is equal to 13 or
higher, the hydroxysulfate redissolves and/or
decomposes into zinc hydroxides.
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when sodium sulfate is used in the treatment solution,
if the sodium sulfate concentration is less than
1.4 g!1 in the solution, little formation of
hydroxysulfates on the surface is observed.. More
generally, it is therefore important for the SO4z~ ion
concentration to be not less than 0.01 moll, and
preferably not less than 0.07 mol/1.
Furthermore, the sulfate ion concentration is
preferably not more than 1 mol/1. If sodium sulfate is
used, at concentrations greater than 142 g/1
(equivalent to 2 mol/1 of SOQZ-), for example 180 g/1, a
reduction in the efficiency of formation of the zinc
hydroxysulfate/zinc sulfate-based layer is observed.
It has been found that the reduction in degradation of
the galvanized coating of the sheet while it is being
formed is obtained only if the thickness of the zinc
hydroxysulfate/zinc sulfate-based layer deposited
corresponds to more than 0.5 mg/m2 of~equivalent, sulfur,
preferably at least 3.5 mg/m2 of equivalent sulfur.
However, it has been found that the reduction in
degradation of the galvanized coating decreases if the
amount of sulfur in the zinc hydroxysulfate/zinc
sulfate-based layer deposited greatly exceeds 30 mg/mz,
owing, it would seem, to the deterioration in adhesion
of this layer.
Thus, to reduce the deterioration o.~ the galvanized
coating, significantly, it is necessary for the total
deposited amount of hydroxysulfates and sulfates to be
not less than 0.5 mg/m2 but not to exceed 30 mg/m2 of
equivalent sulfur, preferably to be between 3.5 and
27 mg/m2 of equivalent sulfur.
The zinc needed to form the zinc hydroxysulfate/zinc
sulfate-based deposit comes from the anodic dissolution
CA 02551492 2006-06-22
7
of zinc under the effect of the polarization of the
galvanized surface.
~t is therefore necessary for the density of electrical
charges flowing during the treatment through the
surface of the sheet to be adjusted so as to form a
zinc hydroxysulfate/zinc sulfate-based layer having a
sulfur content of not less than 0.5 mg/mz.
Thus, the applied charge density is preferably between
10 and 100 C/dm~ of surface to be treated.
~f the charge density exceeds 100 C/dm2, it has been
found that the amount of suJ,fur deposited on the
surface no longer increases, and even decreases.
Thanks to the anodic polarization of the galvanized
surface to be treated, there is rapid dissolution of
the zinc in the immediate vicinity of the galvanized
surface, which promotes the precipitation of zinc salts
on this surface.
Thus, to carry out this treatment in as productive a
manner as possible with. a satisfactory coulombic
efficiency, it is necessary for the zinc hydroxy-
sulfate/zinc sulfate-based layer to be deposited under
a high polarization current density, especiaJ.J.y greater
than 20 A/dm2, for example 200 A/dm2.
At a current density of less than or equal to 20 A/dm2,
the deposition efficiency is very low and the amount of
sulfur in the deposited layer does not make it possible
to significantly reduce the degradation of the zinc
coating on the sheet whi7.e it is being formed.
As counter-electrode, it is possible to use a titanium
cathode.
The temperature of the treatment solution is generally
CA 02551492 2006-06-22
between 20°C and 60°C. preferably, the treatment is
carried out at a temperature of 40°C or higher, so as
to incxease the conductivity of the solution and to
reduce the ohmic losses.
The flow rate of the solution at the surface of the
sheet does not, here, have any majox impact on the
treatment according to the invention.
After the hydroxysulfateJsulfate-based layer has been
formed on the surface, the coated surface is thoroughly
rinsed wi~,h demineralized water. This rinsing step is
important in order to remove the alkaline reactants at
the surface of the deposit, reactants that might cause
corrosion problems,
The subject of the invention is also a method of
lubricating a steel sheet coated with a layer
consisting of a metal coating based on zinc or its
alloys, in which method:
- said~sheet is coated with an upper layer based
on zinc hydroxysulfate and zinc sulFate, said upper
layer having been obtained by using a treatment
solution according to the invention; and then
- a lubricating oil film is app3.ied to the upper
layer with a weight of less than 1 g/mz.
The weight of lubricating oil film applied is
preferably less than 0.9 g/m2 and is more particularly
between 0.2 and 0.5 g/m2, since such weights are
sufficient to obtain excellent temporary corrosion
protection and to avoid any risk of contamination of
workshops and forming tools.
Finally, the subject of the invention is the use of an
aqueous treatment solution comprising sulfate ions with
a concentration of not less than 0.01 mol/1, in order
to improve temporary corrosion protection of a steel
sheet coated with a metal layer based on zinc or its
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alloys.
This aqueous treatment solution is applied to the steel.
sheet according to the embodiments described in the
paragraphs relating to the use of an aqueous treatment
solution containing sulfate ions for treating a
galvanized steel sheet for the purpose of reducing
degradation of the zinc coating while the sheet is
being forri~ed. For this purpose, the reader may refer to
the paragraphs relating thereto.
As will be seen in the examples illustrating the
invention, the inventors have shown that the temporary
corrosion protection of a galvanized steel sheet
firstly treated with a treatment solution according to
the invention and then coated with an oil film is very
much better than that of a galvanized steel sheet that
has not been treated beforehand.
The invention will now be described by examples given
by way of nonlimiting indication and with reference to
the appended figures in which:
- Figure 1, with reference to Example 2,
illustrates the results of the friction tests carried
out on various sheet test pieces treated according to
the invention or untreated: and
- Figure 2, with reference to Example 3,
illustrates the results of the hot/wet corrosion tests
carried out on various sheet test pieces according to
the invention or untreated.
1. Reduction in the formation o~ aoatirig ~owdsr or
particies when drawing a galvanized sheet
Test pieces were cut from a steel sheet, of "aluminum-
killed steel" grade and of ES quality, with a thickness
of 0.7 mm and coated on each of its sides with a zinc
coating produced by hot dipping in a zinc bath.
CA 02551492 2006-06-22
An aqueous treatment solut~.on according to the
invention, obtained from 125 g/1 of zinc sulfate
heptahydrate ZnS0q~7H20 was prepared.
Next, this treatment solution was applied to some of
the test pieces by spraying the treatment solution at a
temperature of 40°C. After the sheet was in contact
with the solution for a time of 3 to 4 s, the treated
sheet was drained and then dried.
A lubricating oil film was then applied to the zinc
hydroxysulfate/zinc sulfate-based layer formed on the
surface of the galvanized steel sheet test pieces, said
oil being either QUAKER 6130 oil (from Quaker) or
FUCHS 41075 oil (from Fuchs), with a film weight of
1.5 g/mz.
The other test pieces that had not been pretreated with
the treatment solution according to the invention were
oiled, either with the QUAKER 6130 oil or with the
FUCHS 4107s oil, again with a film weight of 1.5 g/mz.
The two series of test pieces were then subj ected to a
controlled deformation test by means of a pxess
comprising a punch, a die and a blank holder,
recreating in the laboratory the stresses undergone by
the sheet during a drawing operation, especially in the
die radii and/or in the retaining rings with which the
drawing tools are equipped. Various blank-holder
clamping forces were applied to the test pieces
undergoing the test.
Each of the test pieces of the two series was weighed
before the oiling operation and then at the end of the
test, after de-oiling, by means of a balance accurate
to 0.0001 grams. The measured difference in weight was
normalized to a weight loss per square meter, taking
into account the area affected by the friction during
the simulation of drawing the test piece, which was
CA 02551492 2006-06-22
identical for each of the test pieces.
Moreover, after having formed one test pzece and before
the next test piece was formed, the press was wiped so
as to identify the zinc coating powder or particles
lost by the test piece in the press.
The weight loss results of the test pieces after
drawing and the identification of zinc powder and/or
particles coming from the coating are given in Table 1.
The particles and/or powder are identified by a rating
in the following manner, according to a scale ranging
from 1 to 9, where:
rating 1: very few particles oz very little
powder;
rating 2: few particles or little powdery
rating 3: many particles or a lot of powder; and
rating 4: very high level of particles or powder.
Table 1: Test results
Type of Identification
oil on
ClampingWeight
(quantity: the
tools
force loss
1.5 g/m2
(daN) (g/m2~ Powdex Particles
per side)
Steel sheet 0.63
QUAKER oil 900 3 3
coated with 0.09
lubricating 0.55 t
FUCHS oil 400 3 3
oil film 0.04
Steel sheet 0.12 -~-
A00 2 1
coated with 0.1
QUAKER oil
a hydroxy- 0.22 f
X50 3 1 to 2
sulfate 0.1
layer and
with a 0.20 t
FUCHS oil 750 3 1
lubricating 0.1
oil film
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The measured weight losses and the amounts of powder
and particles observed when wiping the tools show that
the loss of material from the zzne coating, due to the
galvanized steel sheet passing over the punch, is
significantly reduced when the sheet, before, being
oiled, is treated with the treatment solution according
to the invention.
2. Reduction in the effect of dewetting - e~~~eat on
the txiboloqical behayior - ~ziction test
Test pieces 1 cm2 in area were cut from a steel sheet,
of "aluminum-killed steel" grade and of ES quality,
with a thickness of 0.7 mm and coated on each of its
sides with a zinc coating pxoduced by hot dipping in a
zinc bath.
Some of these test pieces were treated with a treatment
solution according to the invention, under the same
conditions as those indicated in Example 1, so as to
form a zinc hydroxysulfate/zinc sulfate-based layer. A
lubricating oil film (QUAI<ER 6130 oxl) is then applied
to this layer in amounts ranging between 0.25 and
2.5 g/m2.
The other test pieces were oiled in the same manner as
previously, but without having been pretreated with the
treatment solution according to the invention.
The friction behavior of each of the test pieces was
then characterized using a tribology tester in the
following manner.
The tester was a flat-on--flat tribometer known per se.
The test pieces to be tested were clamped with a
clamping force F~ between two high-speed steel plates
offering a beaxing (or sliding) surface to the test
pieces. The friction coefficient N was measured while
moving the test piece relative to the plates over a
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total travel D of 180 mm and at a speed of 10 mm/s,
while progressively increasing the clamping force F~.
A curve showa.ng the variation in friction coefficient
as a function of the clamping force F~ for different
lubricating oil film weights can then be plotted (see
Figure 1~ .
The various curves are identified by the following
symbols:
+: sheet treated according to the invention and
then coated with 0.25 g/mz per side of a QUAKER 6130 oil
f i. lm;
x: sheet treated according to the invention and
then coated with 1.0 g/m2 per side of a QUAKER 6130 oil;
~: sheet treated according to the invention and
then coated with 2.5 g/mZ per side of a QUAKER 6130 oil
film;
~: untreated sheet coated with 0.25 g/mZ per side
of a QUAKER 6130 oil film;
~: untreated sheet coated with 1.0 g/mZ per side of
a QUAKER 6130 oil film; and
~: untreated sheet coated with 2.5 g/m2 per side of
a QUAKER 6130 oil film.
Given in Table 2, for each of the test pieces tested,
is the mean value of the friction coefficient for a
given clamping force F~.
CA 02551492 2006-06-22
_ ~~ _
m~hl 0 7
Friction
coefficient
Weight
of oil
on the
Clamping sheet Weight of oil on the
treated
force according untreated
to the sheet
(MPa) invention
0.2S I.4 2.5 0.25 1.0 2.5
(9/m2) (g/mzl ( /m2) (g/m2) (g/m2) (g/m2)
30 0.13 0.12 0.12 0.20 0.15 0.15
50 0.11 0.11 0.21 0.20 0.17 0.17
The results obtained show that a reduction in the
weight of oil results in a substantial increase in the
friction coefficient when no treatment solution
according to the invention is applied before applying
the oil film.
However, when the treatment solution according to the
invention has been applied to the galvan~.zed sheet
prior to application of the lubricating oil film, the
friction coefficients obtained are very low, even with
oil weights of less than 0.5 g/mz.
3. Reduction in the effect of the dewetting - the
effect on temporary aorrosian~roteation
Test pieces were cut from a steel sheet, of ~~aluminum.-
killed steel" gxade and of ES quality, with a thickness
of 0.7 mm and coated on each of its sides with a zinc
coating produced by hot dipping in a zinc bath.
Some of these test pieces were treated with a treatment
solution according to the invention, under the same
conditions as those indicated iIt Example 1, so ds to
form a zinc hydroxysulfate/zinc sulfate-based layer. A
lubricating oil film (QUAKER 6130 oil) was then applied
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to this layer in weights ranging between 0.25 and
1.0 g/m2.
The other test pieces were oiled in the same manner as
previously, but without having been pretreated with the
treatment solution according to the invention.
The lubricating oils applied to the steel sheets.. coated
with the zinc-based metal coating guaranteed corrosion
protection ~luxzng the period of time that elapsed
between manufacture of the sheets and their processing,
for example by drawing.
The conformity of the product delivered up to this
point was checked via the results of an accelerated
hotlwet corrosion test.
Specifically, the test pieces to be tested were placed
in an environmental chamber corresponding to the
DIN 50017 standard, which simulates the conditions for
corrosion of the.external turn of a coil of sheet or of
an individual cut sheet during storage.
The details of the hot/wet cycle (one cycle = 24 hours)
are given below:
- 8 h at 40°C and 95-100% RH (relative humidity);
16 h at 20°C and 75~ RH.
The individual test pieces were suspended vertically.
The results of the test, given in Table 3, were
obtained by measuring the number of successive cycles
before any traces of corrosion appeared on the test
piece.
A curve showing the variation in the percentage content
of white rust as a function of the number of cycles for
each of the test pieces tested can then be plotted (cf.
Figure 2).
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The various curves are identified by 'the following
symbols:
sheet treated according to the invention and
then coated with 0.25 g/m2 per side of a QUAKER 6130 oil
film;
sheet treated according to the invention and
then coated with 0.5 g/m2 per side of a QUAKER 6.30 oil
film;
~: sheet treated according to the invention and
then coated with 1.0 g/mz per side of a QUAKER 6130 oil
film;
~: untreated sheet coated with 0.25 g/m' per side
of a QUAKER 6130 oil film;
~: untreated sheet coated with 0.5 gtm2 per side of
a QUAKER 6130 oil film; and
~: untreated sheet coated with 1.0 g/m2 per side of
a QUAKER 6130 oil film.
Table 3
rust
Weight
Number of oil Weight
of on the of oil
cycles sheet on the
treated untreated
according sheet
to the
invention
0.25 0.5 1.0 0.25 0.5 1.0
(g/m2) (g/m2) (g/m2) (grm2) (g/mz) (glmZ)
0.5 5% 0~ 05 60% 20% 0%
1.5 0% $5~ 2%
2.5 2% 0% 2~
3.5 20% 2$ 0~ 2%
4.5 30% 0% 40~ 2%
5.5 55$ 0% 50% 20
8.5 12% 0% 65% I 2%
Tt has been found that it is possible to very
significantly improve the temporary coxxosion
CA 02551492 2006-06-22
- z~ -
protection of galvanized steel sheets to which a
treatment solution according to the invention has been
applied before application of the lubricating oil film,
this being so even when the weight of oil is less than
1 glm2.
