EP2802682B1 - Use of a solution containing sulphate ions for reducing the blackening or tarnishing of a metal sheet during the storage thereof and metal sheet treated with such a solution - Google Patents

Description

The present invention relates to a sheet comprising a steel substrate coated on at least one of its faces with a coating comprising zinc and magnesium.

Such sheets are more particularly intended for the manufacture of parts for the automobile, without however being limited thereto.

Coatings comprising essentially zinc are traditionally used for their good protection against corrosion, whether in the automotive sector or in construction, for example. However, these coatings cause weldability problems and are now in competition with coatings comprising zinc and magnesium.

Indeed, the addition of magnesium clearly increases the resistance to perforating corrosion of these coatings, which can make it possible to reduce their thickness and therefore improve their weldability or even to maintain the coating thickness and increase the guarantee of protection against corrosion over time.

In addition, the improvement in corrosion resistance is such that it is now possible to reduce or even eliminate the use of secondary protective measures such as the use of waxes or sealants in places most susceptible to damage. 'be corroded.

However, the coils of sheets with such surface coatings can sometimes stay in storage sheds for several months and must not see this surface deteriorate by the appearance of surface corrosion, before being put into storage. form by the end user. In particular, no initiation of corrosion must appear whatever the storage environment, even in the event of exposure to the sun and / or to a humid or even saline environment.

Standard galvanized products, that is to say those whose coatings mainly comprise zinc, are also subject to these stresses and are coated with a protective oil which is generally sufficient.

However, the present inventors have observed that, in the event of dewetting of the protective oil, the coatings comprising zinc and magnesium exhibit, during storage, a slight surface oxidation which modifies the interaction of light with the surface and which therefore modifies its visual appearance.

In fact, the appearance of black dots is then observed for coatings comprising zinc and magnesium, this phenomenon being referred to by the term blackening. For coatings comprising zinc, magnesium and aluminum, the entire surface which is no longer covered with oil becomes tarnished. This is called tarnishing.

Moreover, the use of a temporary protective oil is quite restrictive because, on the one hand, the oil tends to dirty the working environment and the tools used to cut and shape the sheet coils and, on the other hand, degreasing is often necessary in a subsequent step in the manufacture of parts from these coils.

He is known to WO2005 / 071140 to use an aqueous treatment solution containing sulphate ions at a concentration greater than or equal to 0.01 mol / l on a galvanized steel sheet in order to reduce the degradation of the coating during the forming of the sheet.

It is also known from the article VOLOVITCH P ET AL: "Understanding corrosion via corrosion product characterization: II. Role of alloying elements in improving the corrosion resistance of ZnAIMg coatings on steel", CORROSION SCIENCE, OXFORD, GB, vol. 53, no. 8, 17 March 2011 (2011-03-17), pages 2437-2445 that zinc hydroxysulfate is a corrosion product observed on a Zn (Al) Mg surface exposed to an aggressive environment containing sulfate ions.

There is therefore a need to develop a temporary protection system for such coatings, in particular with respect to blackening and tarnishing phenomena, a system which is effective even in the absence of temporary protective oil.

To this end, the invention relates to a use according to claim 1.

The use according to the invention can also comprise the characteristics of claims 2 to 14, taken individually or in combination.

Other characteristics and advantages of the invention will become apparent on reading the description which follows, given solely by way of non-limiting example.

• la figure 1 est une vue schématique en coupe illustrant la structure d'une tôle selon l'invention,
• les figures 2A à 2D et 3A à 3F sont des clichés illustrant les résultats de tests de corrosion en humidotherme effectués sur différentes éprouvettes de tôles traitées selon l'invention ou non traitées,
• les figures 4 à 6 présentent des courbes illustrant les résultats de tests de vieillissement en exposition naturelle sous abri effectués sur différentes éprouvettes de tôle traitées selon l'invention ou non traitées.

The invention will now be illustrated by examples given by way of indication, and not by way of limitation, and with reference to the appended figures in which:

• the figure 1 is a schematic sectional view illustrating the structure of a sheet according to the invention,
• the figures 2A to 2D and 3A to 3F are clichés illustrating the results of humidity-temperature corrosion tests carried out on various specimens of sheets treated according to the invention or not treated,
• the figures 4 to 6 show curves illustrating the results of aging tests in natural exposure under shelter carried out on various test pieces of sheet metal treated according to the invention or not treated.

Sheet 1 of the figure 1 comprises a substrate 3 made of steel, preferably hot rolled and then cold rolled, and is for example coiled so as to be able to be used subsequently as a body part for the automobile.

The invention is not however limited to this field and can find a use for any steel part whatever its end use.

In this example, the sheet 1 will then be unwound from the coil then cut and shaped to constitute a part.

The substrate 3 is covered on one side 5 by a coating 7. In certain variants, such a coating 7 can be present on both sides of the substrate 3.

The coating 7 comprises at least one layer 9 based on zinc. Layer 9 preferably comprises 0.1 to 20% by weight of magnesium.

This layer 9 generally has a thickness less than or equal to 20 μm and aims to protect the substrate 3 against perforating corrosion, in a conventional manner. It will be observed that the relative thicknesses of the substrate 3 and of the various layers covering it have not been observed on the figure 1 in order to facilitate representation.

Layer 9 comprises at least 0.1% by weight of magnesium because no effect of protection against perforating corrosion is visible below.

In a preferred embodiment, layer 9 contains at least 0.5%, preferably at least 2% by weight of magnesium.

The magnesium content is limited to 20% by weight in the layer 9 because it has been observed that a greater proportion would lead to too rapid consumption of the coating 7 and therefore paradoxically degraded anticorrosion performance.

Layer 9 can in particular be obtained by a vacuum deposition process, such as magnetron sputtering, or vacuum evaporation by Joule effect, by induction or under an electron beam.

In this case, layer 9 generally only comprises zinc and magnesium, other elements, such as aluminum or silicon, however, being able to be added, if necessary, to improve other characteristics of layer 9 such as its ductility or its adhesion to the substrate 3.

When the layer 9 only comprises zinc and magnesium, it is particularly preferred that the layer 9 comprises between 14 and 18% by weight of magnesium, and better that it mainly corresponds to the intermetallic compound of formula Zn ₂ Mg, comprising approximately 16% by weight of magnesium, which exhibits particularly good puncture corrosion resistance properties.

In some variants, the coating 7 may comprise a layer 11 of zinc between the layer 9 and the face 5 of the substrate 3. This layer 11 may have been obtained for example by a vacuum deposition or electrodeposition process.

In the latter hypothesis, one can simply deposit magnesium under vacuum on zinc previously deposited by electrodeposition on the substrate 3 and then perform a heat treatment to combine the deposited magnesium and zinc and thus constitute the layer 9 comprising zinc and magnesium surmounting the layer 11 comprising zinc.

When the layer 9 comprises zinc, magnesium and aluminum, it is particularly preferred that the layer 9 comprises between 0.1 and 10% by weight of magnesium and between 0.1 and 20% by weight of aluminum. More preferably, layer 9 comprises between 2 and 4% by weight of magnesium and between 2 and 6% by weight of aluminum, and it is therefore close to the composition of the ternary zinc / aluminum / magnesium eutectic.

In this case, the layer 9 can be obtained by a hot dip coating process in a molten zinc bath containing magnesium up to a content of 10% by weight and aluminum up to a content of 20. % in weight. The bath can also contain up to 0.3% by weight of optional addition elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.

These various elements can make it possible, among other things, to improve the ductility or the adhesion of the layer 9 on the substrate 3. Those skilled in the art who know their effects on the characteristics of the layer 9 will know how to use them depending on the purpose. complementary sought. The bath may finally contain residual elements originating from the feed ingots or resulting from the passage of the substrate 3 through the bath.

The coating 7 is covered with a layer 13 of temporary protection.

Layer 13 was obtained for example by application to coating 7, optionally after degreasing, of an aqueous treatment solution containing SO ₄ ^2- sulfate ions at a concentration greater than or equal to 0.01 mol / l. The layer 13 thus formed is based on zinc hydroxysulphate and zinc sulphate. It is both sufficiently thick and adherent.

It is not possible to form such a layer 13 when the SO ₄ ^2- concentration is less than 0.01 mol / l, but it is also noted that too high a concentration does not appreciably improve the deposition rate and may even decrease it slightly.

The aqueous treatment solution is applied in a conventional manner, for example by dipping, by sprinkling or by coating.

In a preferred embodiment, the aqueous treatment solution also contains Zn ²⁺ ions at a concentration greater than or equal to 0.01 mol / l, which make it possible to obtain a more homogeneous deposit.

For example, the aqueous treatment solution is prepared by dissolving zinc sulfate in pure water. For example, zinc sulfate heptahydrate (ZnSO ₄ , 7 H ₂ O) is used. The concentration of Zn ²⁺ ions is then equal to that of SO ₄ ^2- anions.

The pH of the aqueous treatment solution preferably corresponds to the natural pH of the solution, without addition of base or acid. The value of this pH is generally between 5 and 7.

Preferably, the aqueous treatment solution is applied to the coating 7, under conditions of temperature, of contact time with the coating 7, of concentration of SO ₄ ^2- ions and of Zn ²⁺ ions adjusted so that the layer 13 contains an amount of sulfur greater than or equal to 0.5 mg / m ² .

Thus, the contact time of the aqueous treatment solution with the coating 7 is between 2 seconds and 2 minutes, and the temperature of the aqueous treatment solution is between 20 and 60 ° C.

Preferably, the aqueous treatment solution used contains between 20 and 160 g / l of zinc sulfate heptahydrate, corresponding to a concentration of Zn ²⁺ ions and a concentration of SO ₄ ^2- ions of between 0.07 and 0.55 mol / l. In fact, it has been observed that, in this range of concentrations, the rate of deposition was little influenced by the value of the concentration.

Advantageously, the aqueous treatment solution is applied under conditions of temperature, of contact time with the coating 7, and of concentrations of SO ₄ ^2- ions and of Zn ²⁺ ions adjusted to form a layer 13 having a quantity of sulfur. between 3.7 and 27 mg / m ² .

According to a variant of the invention, the aqueous treatment solution contains an agent which oxidizes zinc, such as hydrogen peroxide. This oxidizing agent can have a very marked accelerating effect of hydroxysulfation and sulfation at low concentration. It was found that the addition of only 0.03%, i.e. 8.10 ^-3 mol / liter of hydrogen peroxide, or 2.10 ^-4 mole / liter of potassium permanganate in the solution made it possible to (approximately) double the speed of deposit. It was observed, on the contrary, that concentrations 100 times higher no longer made it possible to obtain this improvement in the deposition rate.

After application of the aqueous treatment solution and before drying, the deposited layer 13 is adherent. The drying is adjusted to remove residual liquid water from the deposit.

Between the application step and the drying step, the sheet 1 is preferably rinsed so as to remove the soluble part of the deposit obtained. The absence of rinsing and the obtaining of a deposit which is partially soluble in water which results therefrom is not very detrimental to the reduction in the degradation of the coating 7 during the subsequent shaping of the sheet 1, as long as that the deposit obtained does indeed include a layer 13 insoluble in water.

According to another embodiment of the invention, the aqueous treatment solution comprises a concentration of SO ₄ ^2- ions greater than or equal to 0.01 mol / l and is applied under anodic polarization, and the pH of the aqueous solution of treatment is greater than or equal to 12, and less than 13.

If the pH of the solution is less than 12, no adherent hydroxysulphates are formed on the surface to be treated. If the pH of the solution is greater than or equal to 13, the hydroxysulfate re-dissolves and / or decomposes into zinc hydroxides.

When using sodium sulfate in the aqueous treatment solution, if the sodium sulfate concentration is less than 1.42 g / l in the solution, little hydroxysulfate formation is observed on the surface. This concentration is equivalent to 0.01 mol / l of SO ₄ ^2- and, more generally, it is therefore important that the concentration of SO ₄ ^2- ions is greater than or equal to 0.01 mol / l, and preferably greater or equal to 0.07 mol / l.

In addition, the concentration of sulfate ions is preferably less than or equal to 1 mole / liter. In the case of the use of sodium sulphate, at concentrations greater than 142 g / l, for example 180 g / l, a decrease in the yield of formation of the layer 13 is observed.

Preferably, the total amount of hydroxysulfates and sulfates deposited should be greater than or equal to 0.5 mg / m ² and less than or equal to 30 mg / m ² in sulfur equivalent, preferably between 3.5 and 27 mg / m ² in sulfur equivalent.

Preferably, the charge density applied is preferably between 10 and 100 C / dm ² of surface to be treated.

Preferably, it is appropriate to deposit the layer 13 based on zinc hydroxysulfate and zinc sulfate under a high polarization current density, in particular greater than 20 A / dm ² and, for example 200 A. / dm ² .

As a counter electrode, a titanium cathode can be used.

The temperature of the aqueous treatment solution is generally between 20 ° C and 60 ° C. Preferably, the procedure is carried out at a temperature greater than or equal to 40 ° C., so as to increase the conductivity of the solution and to reduce the ohmic losses.

After formation of the layer 13 according to this other embodiment, the treated surface is rinsed thoroughly with demineralized water. This rinsing step makes it possible to remove the alkaline reagents on the surface of the deposit, which would cause corrosion problems.

After production of the layer 13 according to one of the methods described above, it is optionally possible to lubricate the layer 13 of the sheet 1.

This lubrication can be ensured by applying a film of oil (not shown) with a weight of less than 2 g / m ² on layer 13.

As will be seen in the examples below, produced by way of indication and not by way of limitation, the inventors have shown that the presence of a layer 13 makes it possible to improve the resistance to blackening in the case of a coating 7 comprising zinc. and magnesium, and improve the tarnish resistance in cases where the coating 7 includes zinc, magnesium and aluminum. This increased resistance is particularly useful in the absence of an oil film, for example following a dewetting of an oil film applied to the coating 7.

This increase in resistance to blackening and tarnishing is mainly due to the formation of a conversion layer based on zinc hydroxysulfate Zn ₄ SO ₄ (OH) ₆ .

1. 1. Attaque acide du zinc métal (solution de SO₄ ^2- à pH compris entre 5 et 7), qui aboutit à la formation d'ions Zn²⁺ et à l'alcalinisation du milieu : Zn + 2H₂O -> Zn²⁺ + 2OH^- + H₂
2. 2. Précipitation d'hydroxysulfate de zinc sous l'effet de l'accumulation d'ions Zn²⁺ et OH^- dans la solution de sulfate : 4 Zn²⁺ + SO₄ ^2- + 6 OH^- -> Zn₄SO₄(OH)₆.

The reactions involved during the application of the aqueous treatment solution to the coating 7 are:

1. 1. Acid attack on zinc metal (solution of SO ₄ ^2- at pH between 5 and 7), which results in the formation of Zn ²⁺ ions and in the alkalinization of the medium: Zn + 2H ₂ O -> Zn ²⁺ + 2OH ^- + H ₂
2. 2. Precipitation of zinc hydroxysulphate under the effect of the accumulation of Zn ²⁺ and OH ^- ions in the sulphate solution: 4 Zn ²⁺ + SO ₄ ^2- + 6 OH ^- -> Zn ₄ SO ₄ (OH) ₆ .

Compared to a coating comprising only zinc, the presence of magnesium in coating 7 helps to stabilize the zinc hydroxysulphate layer over time and therefore to prevent its transformation into zinc carbonate under the effect of the CO ₂ contained. in the air. Zinc carbonates are indeed recognized to have a less protective power (less barrier) than zinc hydroxysulphates.

Blackening resistance:

• des tôles 1 dont le revêtement 7 comprend une couche 11 de zinc de 5,5 µm d'épaisseur et une couche 9 de 3,5 µm d'épaisseur, la couche 9 comprenant environ 84% en poids de zinc et 16% en poids de magnésium. Pour réaliser ces couches, on a tout d'abord déposé une couche de 7,5 µm de zinc par électrodéposition et une couche de 1,5 µm de magnésium par dépôt sous vide et on a soumis la tôle à un traitement thermique pour allier le magnésium et le zinc. Ces éprouvettes seront désignées ZEMg par la suite; et
• des tôles 1 dont le revêtement 7 comprend une couche 11 de zinc de 4 µm d'épaisseur déposée par dépôt sous vide et une couche 9 de 3,5 µm déposée par dépôt sous vide et comprenant environ 80% en poids de zinc et 20% en poids de magnésium. Ces éprouvettes seront désignées ZnMg Full PVD par la suite.

Le substrat 3 des éprouvettes ZEMg et ZnMgFullPVD est un acier laminé à froid pour emboutissage profond.
Certaines des éprouvettes ont été revêtues par une couche 13 ayant un poids compris entre 17 à 20 mg/m² de soufre.
Les conditions d'application pour former la couche 13 étaient les suivantes :

• Mode d'application = Spin Coater à une vitesse de 700 tr/min pendant 15 secondes,
• Concentration de la solution = 40 g/l de sulfate de zinc heptahydraté
• pH de la solution = 5
• Température de la solution = ambiante,
• Séchage = en portant les éprouvettes jusqu'à une température comprise entre 70 et 80°C.

Test specimens are cut from two types of sheet 1, namely:

• sheets 1 whose coating 7 comprises a layer 11 of zinc 5.5 μm thick and a layer 9 3.5 μm thick, the layer 9 comprising approximately 84% by weight of zinc and 16% by weight magnesium. To achieve these layers, we first deposited a 7.5 µm layer of zinc by electrodeposition and a 1.5 µm layer of magnesium by vacuum deposition and the sheet was subjected to a heat treatment to alloy the magnesium and zinc. These test pieces will be designated ZEMg hereafter; and
• sheets 1, the coating 7 of which comprises a layer 11 of zinc 4 μm thick deposited by vacuum deposition and a layer 9 of 3.5 μm deposited by vacuum deposition and comprising approximately 80% by weight of zinc and 20% by weight of magnesium. These test pieces will be referred to as ZnMg Full PVD hereafter.

The substrate 3 of the ZEMg and ZnMgFullPVD specimens is a cold rolled steel for deep drawing.
Some of the test pieces were coated with a layer 13 having a weight of between 17 to 20 mg / m ² of sulfur.
The conditions of application to form layer 13 were as follows:

• Application mode = Spin Coater at a speed of 700 rpm for 15 seconds,
• Concentration of the solution = 40 g / l of zinc sulphate heptahydrate
• pH of the solution = 5
• Temperature of the solution = ambient,
• Drying = by bringing the test pieces to a temperature between 70 and 80 ° C.

• Quaker (marque déposée) 6130 : poids du film d'huile environ 1 g/m², et
• Fuchs (marque déposée) 4107 S : poids du film d'huile environ 1.2 g/m².

The temporary protection of the specimens was assessed by a humidotherm test according to DIN EN ISO 6270-2 after application of the following protective oils on layers 13:

• Quaker (registered trademark) 6130: weight of the oil film approximately 1 g / m ² , and
• Fuchs (registered trademark) 4107 S: weight of the oil film approximately 1.2 g / m ² .

• une première phase de 8 heures à 40°C ± 3°C et à environ 98% d'humidité relative, suivie
• d'une seconde phase de 16 heures à 21°C ± 3°C et à moins de 98% d'humidité relative.

In a humidotherm test according to DIN EN ISO 6270-2, the test pieces are subjected to 24 hour aging cycles in a humidotherm, that is to say a chamber with controlled atmosphere and temperature. These cycles simulate the corrosion conditions of a coil of sheet metal or cut sheet metal during storage. Each cycle includes:

• a first phase of 8 hours at 40 ° C ± 3 ° C and at about 98% relative humidity, followed by
• a second phase of 16 hours at 21 ° C ± 3 ° C and less than 98% relative humidity.

• après 10 cycles avec 1 g/m² d'huile Quaker 6130 pour les constructeurs automobiles français,
• après 15 cycles avec 1.2 g/m² d'huile Fuchs 4107 S pour les constructeurs automobiles allemands.

At the end of these cycles, less than 10% of the surface of the test pieces must be visually altered:

• after 10 cycles with 1 g / m ² of Quaker 6130 oil for French car manufacturers,
• after 15 cycles with 1.2 g / m ² of Fuchs 4107 S oil for German car manufacturers.

The proportion of altered surface is determined by visual inspection of an operator.

The figure 2A shows the visual appearance of a ZEMg test piece without a layer 13 after Quaker 6130 oiling as described above. This test piece exhibits significant blackening making it unsuitable for use by French automobile manufacturers. On the contrary, the ZEMg test pieces with layer 13 ( figures 2B to 2D ) and after oiling Quaker 6130 as described above still meet the requirements of French car manufacturers at the end of the test.

The figures 3A to 3D allowing to show the visual aspects, at the end of the test, of ZnMg Full PVD test pieces, without layer 13 ( Fig. 3A ) and with layer 13 ( Fig. 3B to 3D ), after oiling Quaker 6130 as described above. As can be seen in these figures, the presence of layer 13 makes it possible to meet the requirements of French automobile manufacturers.

A ZnMg Full PVD test piece without layer 13 ( Fig. 3E ) after Fuchs oiling as described above does not meet the requirements of German car manufacturers, unlike a ZnMg Full PVD test tube with layer 13 ( Figure 3F ) after Fuchs oiling as described above.

The results of the humidothermal test were confirmed by polarization resistance measurements carried out from impedancemetry and polarization curve tests in a 5% sodium chloride solution at pH 7.

These measurements show that the ZnMg Full PVD test pieces without layer 13 have a polarization resistance of between 160Ω and 380Ω. With a layer 13, this resistance increases to values between 840Ω and 1200Ω, thus confirming the protective power of layer 13.

All the results obtained on the ZnMg Full PVD test pieces show that the layers 13 make it possible to reduce the blackening of sheets 1 with a coating 7 comprising zinc and magnesium. This effect is independent of the reduction in dewetting that the layers 13 make it possible to obtain, as described in the application. WO-2005/071 140 . Thus, one can use a coating 7 with a layer 13 without oiling, while maintaining good temporary protection.

Tarnish resistance:

Only one configuration of sheets 1 was studied in this case. The coating 7 only comprises a layer 9 of approximately 10 μm in thickness and comprising approximately 3% by weight of magnesium, approximately 3.7% by weight of aluminum, the remainder being zinc and the inevitable impurities. The substrate 3 is then a cold rolled steel for deep drawing. These test pieces will be referred to as ZnMgAI hereafter.

Two different supplies corresponding to this configuration were tested and they will be designated by the references AR 2596 and AR 2598 hereafter.

The temporary protection of the ZnMgAI test pieces was evaluated via an aging test in natural exposure under shelter according to the VDA 230-213 standard (test duration 12 weeks). It has in fact been observed that the humidotherm test did not make it possible to reproduce the tarnishing phenomenon observed under natural storage conditions.

The ZnMgAI specimens were tested in the oiled state (Quaker 6130 film with a weight of approximately 1 g / m ² ) and unoiled, with and without layer 13. For comparison, the same tests were carried out on specimens. from a standard galvanized sheet with a 10 µm thick zinc coating and a cold rolled steel substrate for deep drawing. These latter specimens will be referred to hereinafter as GI.

The evolution of the tarnish during the test was monitored using a colorimeter measuring the difference in luminance (measurement of ΔL *). The threshold value of | ΔL * | corresponding to the appearance of tarnish was set to 2.

The results obtained for the specimens GI, ZnMgAl AR 2596 and ZnMgAl AR 2598 are represented respectively on the figures 4 to 6 where the time, in weeks, is plotted on the abscissa and the evolution of ΔL * is plotted on the ordinate.

• ● : éprouvettes sans couche 13 ni huilage,
• × : éprouvettes sans couche 13 mais huilées avec un film d'huile Quaker 6130 d'un poids d'environ 1g/m²,
• ■ : éprouvettes avec couche 13 et huilées avec un film d'huile Quaker 6130 d'un poids d'environ 1g/m²,
• ◆ : éprouvettes avec couche 13 sans huilage.

The different curves are identified by the following symbols on each of the figures 4 to 6 :

• ●: test pieces without layer 13 or oiling,
• ×: test tubes without layer 13 but oiled with a film of Quaker 6130 oil weighing approximately 1 g / m ² ,
• ■: test specimens with layer 13 and oiled with a film of Quaker 6130 oil weighing approximately 1 g / m ² ,
• ◆: test specimens with layer 13 without oiling.

These results show the interest of the layers 13 for the temporary protection against tarnishing of coatings 7 comprising zinc, magnesium and aluminum, since all of the test pieces with a layer 13 exhibit a slowed tarnish kinetics compared to the test pieces without a layer 13, whether with or without oiling, and this even for the GI test pieces.

After 12 weeks, the level of tarnishing reached by the ZnMgAI test pieces with layer 13 is equivalent to that of the oiled GI test pieces.

Claims (14)

1. Use of an aqueous treatment solution containing sulphate ions SO₄ ^2- in a concentration greater than or equal to 0.01 mol/l, for treating a metal sheet (1) comprising a steel substrate (3) coated on at least one of its faces (5) with a coating (7) comprising at least zinc and magnesium, in order to reduce the blackening or the tarnishing of the metal sheet (1) when stored in the absence of a temporary protection oil.
2. Use according to claim 1, in which the coating (7) comprises at least zinc, magnesium and aluminium.
3. Use according to either claim 1 or claim 2, in which the pH of the aqueous treatment solution is between 5 and 7.
4. Use according to any of claims 1 to 3, in which the aqueous treatment solution furthermore contains Zn²⁺ ions in a concentration greater than or equal to 0.01 mol/l.
5. Use according to claim 4, in which the concentration of Zn²⁺ ions and the concentration of SO₄ ^2- ions are between 0.07 and 0.55 mol/l in the aqueous treatment solution.
6. Use according to claim 5, in which the aqueous treatment solution is applied to the coating (7) under conditions of temperature, contact time with the coating (7), and concentration of SO₄ ^2- ions and of Zn²⁺ ions which are adjusted to form a temporary protection layer (13) based on zinc hydroxysulphate and zinc sulphate, the amount of sulphur of which is greater than or equal to 0.5 mg/m² and less than or equal to 30 mg/m².
7. Use according to claim 6, in which the aqueous treatment solution is applied to the coating (7) under conditions of temperature, contact time with the coating (7), and concentration of SO₄ ^2- ions and of Zn²⁺ ions which are adjusted to form a temporary protection layer (13) based on zinc hydroxysulphate and zinc sulphate having an amount of sulphur of between 3.7 and 27 mg/m².
8. Use according to any of claims 1 to 7, in which, after the application of the aqueous treatment solution to the coating (7), the metal sheet (1) is dried after having possibly been rinsed.
9. Use according to either claim 1 or claim 2, in which the aqueous treatment solution is applied under anodic polarisation, and the pH of the aqueous treatment solution is greater than or equal to 12, and less than 13.
10. Use according to claim 9, in which the concentration of SO₄ ^2- ions is greater than 0.07 mol/l.
11. Use according to either claim 9 or claim 10, in which the density of electric charges flowing during the treatment through the coating (7) is adjusted to form a temporary protection layer (13) based on zinc hydroxysulphate and zinc sulphate, the amount of sulphur of which is greater than or equal to 0.5 mg/m² and less than or equal to 30 mg/m².
12. Use according to claim 11, in which the density of electric charges is adjusted to form a temporary protection layer (13) based on zinc hydroxysulphate and zinc sulphate, the amount of sulphur of which is between 3.7 and 27 mg/m².
13. Use according to any of claims 9 to 12, in which the polarisation current density applied during the treatment is greater than 20 A/dm².
14. Use according to any of claims 9 to 13, in which, after the application of the aqueous treatment solution to the coating (7), the metal sheet (1) is rinsed.

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