EP2450470A1 - Surface treatment by hydroxysulphate - Google Patents

Abstract

Aqueous treatment solution contains sulphate ions (SO 42->) at a concentration of greater than or equal to 0.02 mol/l, for the treating the surface of a steel sheet with a zinc or zinc alloy based coating on at least one of its surfaces. The treatment is aimed at reducing the formation of powder or metal particles generated by the degradation of the coating during the shaping of the sheet. Independent claims are also included for: (a) lubrication of a coated steel sheet using the aqueous treatment solution; (b) utilisation of the aqueous treatment solution for treating coated steel sheet to improve temporary protection against corrosion.

Description

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 sheets coated with zinc or its alloys are commonly used in the automotive and industrial sectors in general, as they have excellent corrosion resistance. However, these galvanized steel sheets have a number of difficulties when they are shaped, for example by stamping, to manufacture parts.

Usually, in order to give the galvanized steel sheets better tribological properties, a film of lubricating oil is applied to their surface which facilitates shaping.

However, despite the application of a suitable lubricating oil film, the very high friction exerted by the forming tools on the surface of the sheet causes, on the surface of the sheet, powder or particles based on zinc or its alloys generated by the degradation of the coating. The accumulation and / or agglomeration of these particles or powder in the forming tools can cause damage to the formed parts, by formation of pins and / or strictions.

In addition, because of the high coefficient of friction which characterizes the sliding of a galvanized surface in contact with the surface of a shaping tool, the sheet may break in the event of insufficient sliding of the sheet into the surface. grip of the formatting tool. These breaks can occur, even by applying a film of oil with a sufficient grammage to the surface of the sheet, ie greater than 1 g / m ² , because it is not possible to keep a distribution on the surface of the sheet. homogeneous oil film. This is due to the dewetting phenomenon which corresponds to the presence of areas with an oil deficit.

However, the fact of depositing on the surface of the sheet a relatively thick film of oil poses problems of pollution of the workshops and stamping tools, and imposes the use of large quantities of products. degreasers for cleaning the sheet as well as important means for treating the effluents resulting from the cleaning.

Furthermore, the oil deficits in certain areas of the oil film due to the dewetting phenomenon, are also responsible for a less temporary protection against corrosion of the steel sheet during storage.

The present invention therefore aims to provide a treatment solution which, applied to the surface of a steel sheet coated with a metal layer based on zinc or its alloys, reduces the degradation of the galvanized surface of this sheet during its shaping, reduce the amount of lubricating oil to be deposited on the sheet before shaping, and improve the temporary protection against corrosion of the sheet.

For this purpose, the subject of the invention is the use of an aqueous treatment solution containing sulphate ions SO ₄ ^2- at a concentration greater than or equal to 0.01 mol / l, for treating the surface of a sheet coated steel on at least one of its faces, a metal coating based on zinc or its alloys, to reduce the formation of powder or metal particles based on zinc or its alloys generated by degradation coating during the shaping of said sheet.

The term metal coating based on zinc alloy, a zinc coating comprising one or more alloying elements, such as for example and without limitation, iron, aluminum, silicon, magnesium, and nickel.

According to the invention, the use of a steel sheet coated with a virtually pure zinc coating is preferred.

When treating the surface of a steel sheet coated with a metal coating based on zinc or its alloys, by means of an aqueous treatment solution according to the invention, it is formed on the surface of the sheet a layer based on zinc hydroxysulfate and zinc sulfate both sufficiently thick and adherent. On the other hand, it is not possible to form such a layer when the SO ₄ ^2- concentration is less than 0.01 mol / l, but it is also found that a too high concentration does not appreciably improve the deposition rate and can even decrease it slightly.

In a first embodiment of the invention, the treatment solution is conventionally applied, for example by dipping, by spraying or by coating, as well on electrogalvanized sheets as on dip galvanized sheets.

In a preferred embodiment, the aqueous treatment solution additionally 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 deposition.

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

The pH of the 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.

In order to reduce to a maximum the formation of powder or particles of zinc or its alloys resulting from the degradation of the coating of the sheet during its shaping, the treatment solution is applied to the surface of the sheet under conditions of temperature, time of contact with the galvanized surface, concentration of SO ₄ ^2- ions and Zn ²⁺ ions adjusted to form a layer based on zinc hydroxysulfate and zinc sulphate whose sulfur content is greater or equal to 0.5 mg / m ² . Indeed, when the amount of sulfur is less than 0.5 mg / m ² , the improvement in the degradation of the coating is less sensitive.

Thus, the contact time of the treatment solution with the galvanized surface is between 2 seconds and 2 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 / l of zinc sulphate 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. Indeed, it was found that in this range of concentrations, the deposition rate was little influenced by the value of the concentration.

Advantageously, the treatment solution is applied under conditions of temperature, time of contact with the galvanized surface, and of concentrations of SO ₄ ^2- ions and Zn ²⁺ ions adjusted to form a layer based on hydroxysulfate and sulfate having a sulfur content of between 3.7 and 27 mg / m ² .

According to a variant of the invention, the treatment solution contains an oxidizing agent for zinc, such as hydrogen peroxide. This oxidizing agent can have a very marked hydroxysulfation and sulphation accelerator effect at low concentration. It was found that the addition of only 0.03%, ie 8.10 ^-3 mol / liter of hydrogen peroxide, or 2.10 ^-4 mol / liter of potassium permanganate in the solution allowed to double (approximately) the speed of deposit. It has been found, 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 treatment solution and before drying, the layer deposited on the sheet is adherent; Drying is adjusted to remove residual liquid water from the deposit.

Between the application step and the drying step, the sheet is preferably rinsed so as to eliminate the soluble part of the deposit obtained; the absence of rinsing and the obtaining of a partially water-soluble deposit resulting therefrom are not very detrimental to the reduction of the degradation of the galvanized coating during the shaping of the sheet, as long as the obtained deposit includes a layer based on zinc hydroxysulfate and zinc sulfate insoluble in water in contact with the sheet.

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

If the pH of the solution is less than 12, no adhering hydroxysulfates are formed on the surface to be treated. If the pH of the solution is greater than or equal to 13, the hydroxysulfate is redissolved and / or decomposed into zinc hydroxides.

When sodium sulphate is used in the treatment solution, if the sodium sulphate concentration is less than 1.4 g / l in the solution, little formation of hydroxysulphates is observed on the surface; more generally, it is therefore important that the concentration of SO ₄ ^2- ions is greater than or equal to 0.01 mol per liter, and preferably greater than or equal to 0.07 mol / l.

In addition, the concentration of sulphate ions is preferably less than or equal to 1 mol / liter; in the case of the use of sodium sulphate, at concentrations greater than 142 g / l (equivalent to 1 mol SO ₄ ^2- / liter), for example 180 g / l, there is a decrease in the formation yield of the layer based on zinc hydroxysulfate and zinc sulfate.

It has been found that the reduction of the degradation of the galvanized coating of the sheet during its shaping was obtained only if the thickness of the layer based on zinc hydroxysulfate and zinc sulphate deposited corresponded to more than 0.5 mg / m ² sulfur equivalent, preferably at least 3.5 mg / m ² sulfur equivalent.

Conversely, it has been found that the reduction in the degradation of the zinc-coated coating decreases if the amount of sulfur in the layer based on zinc hydroxysulfate and zinc sulphate deposited largely exceeded 30 mg / m ² , because, it seems it, the degradation of the adhesion of this layer.

Thus, in order to obtain a reduction in the deterioration of the significant galvanized coating, the total quantity of hydroxysulfates and sulphates deposited should be greater than or equal to 0.5 mg / m ² and less than or equal to 30 mg / m ² in equivalent sulfur, preferably between 3.5 and 27 mg / m ² sulfur equivalent.

The zinc required for the formation of the zinc hydroxysulfate and zinc sulphate deposit results from the anodic dissolution of the zinc under the effect of the polarization of the galvanized surface.

It is therefore appropriate that the density of electrical charges, circulating during the treatment through the surface of the sheet, be adjusted so as to form a layer based on zinc hydroxysulfate and zinc sulphate, the sulfur content is greater or equal to 0.5 mg / m ² .

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

If the charge density exceeds 100 C / dm ² , it is found that the amount of sulfur deposited on the surface no longer increases and even decreases.

Thanks to the anodic polarization of the galvanized surface to be treated, there is a rapid dissolution of zinc in the immediate vicinity of the galvanized surface, which promotes the precipitation of zinc salts on this surface.

Thus, in order to carry out this treatment as productively as possible with a satisfactory faradic yield, it is necessary to deposit the layer based on zinc hydroxysulfate and zinc sulphate at a high polarization current density, in particular greater than 20 A / dm ² and, for example 200 A / dm ² .

For a current density less than or equal to 20 A / dm ² , the deposition efficiency is very low and the amount of sulfur in the deposited layer does not make it possible to obtain a significant reduction in the degradation of the zinc coating of the sheet metal. when formatting.

As a counter-electrode, it is possible to use a titanium cathode.

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

The speed of circulation of the solution at the surface of the sheet does not have a decisive impact on the treatment according to the invention.

After formation of the hydroxysulfate and sulfate layer on the surface, the treated surface is thoroughly rinsed with deionized water. This rinsing step is important for removing alkaline reagents from the deposition surface, which would cause corrosion problems.

• on revêt ladite tôle d'une couche supérieure à base d'hydroxysulfate de zinc et de sulfate de zinc, ladite couche supérieure ayant été obtenue par l'utilisation d'une solution de traitement selon l'invention, puis
• on applique un film d'huile de lubrification sur la couche supérieure avec un grammage inférieur à 1 g/m².

The invention also relates to a method of lubricating a steel sheet coated with a layer consisting of a metal coating based on zinc or its alloys, and wherein:

• said sheet is coated with a top layer based on zinc hydroxysulfate and zinc sulphate, said upper layer having been obtained by the use of a treatment solution according to the invention, then
• a film of lubricating oil is applied to the top layer with a basis weight of less than 1 g / m ² .

The lubricating oil film is applied with a grammage of preferably less than 0.9 g / m ² , and more preferably between 0.2 and 0.5 g / m ² , because these weights are sufficient to obtain excellent protection. temporary protection against corrosion and avoid any risk of pollution of the workshops and the tools of formatting.

Finally, the subject of the invention is the use of an aqueous treatment solution comprising sulphate ions at a concentration greater than or equal to 0.01 mol / l, to improve the temporary protection against corrosion of a metal sheet. steel coated with a metal layer based on zinc or its alloys.

The application of this aqueous treatment solution to the steel sheet is carried out according to the embodiments described in the paragraphs concerning the use of an aqueous treatment solution containing sulphate ions to treat a zinc-coated steel sheet. to reduce the degradation of the zinc coating during its shaping. For this purpose, see the relevant paragraphs.

As will be seen in the examples illustrating the invention, the inventors have shown that the temporary protection against corrosion of a zinc-coated steel sheet first treated with a treatment solution according to the invention, then coated with an oil film was much superior to that of a previously untreated galvanized steel sheet.

• la figure 1, en référence à l'exemple 2, illustre les résultats des tests de frottement effectués sur différentes éprouvettes de tôle traitées selon l'invention ou non traitées,
• la figure 2, en référence à l'exemple 3, illustre les résultats des tests de corrosion en humidotherme effectués sur différentes éprouvettes de tôle traitées selon l'invention ou non traitées.

The invention will now be illustrated by examples given for information only, and not limiting, and with reference to the appended figures in which:

• the figure 1 , with reference to example 2, illustrates the results of the friction tests carried out on different specimens of sheet treated according to the invention or untreated,
• the figure 2 , with reference to example 3, illustrates the results of the corrosion tests in humidotherm on various specimens of treated sheet according to the invention or untreated.

1. Reduction of the formation of powder or coating particles during the stamping of a galvanized sheet

Test specimens are cut from 0.7 mm thick ES-grade "ES-coated steel" steel sheet coated on both sides with a hot dipped zinc coating. in a bath of zinc.

An aqueous treatment solution according to the invention prepared with 125 g / l zinc sulfate heptahydrate ZnSO ₄ , 7 H ₂ O was prepared.

This treatment solution was then applied to a portion of the specimens by spraying the treatment solution at a temperature of 40 ° C. After a contact time of the solution with the sheet of 3 to 4 s, the treated sheet is dewatered and then dried.

Then, on the zinc hydroxysulfate and zinc sulphate layer formed on the surface of the zinc-coated steel sheet specimens, a film of lubricating oil which can be either QUAKER 6130 oil (from Quaker Company), which is FUCHS 4107S (from Fuchs), with a basis weight of 1.5 g / m ² .

Another part of the test specimens not previously treated with the treatment solution according to the invention was oiled either with QUAKER 6130 oil or with FUCHS 4107S oil also with a weight of 1.5 g / m ² .

The two sets of test pieces were then subjected to a controlled strain test by means of a press comprising a punch, a die and a blank holder, recreating in the laboratory the stresses undergone by the sheet during the stamping operation. especially in the matrix rays and / or in the retaining rings that equip the stamping tools. Different clamping forces of the blank holder were applied to the test pieces.

Each of the specimens of both series was weighed before the oiling operation, then at the end of the test after de-oiling, using a balance specifies at 0.0001 gram. The difference in mass measured was reduced to a loss of mass per square meter, taking into account the area affected by the friction during the simulation of the stamping of the specimen, identical for each of the specimens.

Furthermore, after shaping a specimen and before shaping the next specimen, the press was wiped so as to identify the powder or zinc coating particles lost by the specimen in the press.

• cotation 1 : très peu de particules ou de poudre,
• cotation 2 : peu de particules ou de poudre,
• cotation 3 : beaucoup de particules ou de poudre, et
• cotation 4 : niveau très élevé de particules ou de poudre.

Tableau 1 : résultats des essais Quantité de l'huile Forces de serrage Perte de masse Identification sur outils (1,5 g/m²/face) (daN) (g/m²) poudre particules Tôle d'acier revêtue d'un film d'huile lubrifiante huile QUAKER 400 0,63 ± 0,04 3 3 huile FUCHS 400 0,55 ± 0,04 3 3 Tôle d'acier revêtue d'une couche d'hydroxy-sulfate, et d'un film d'huile lubrifiante huile QUAKER 400 0,12 ± 0,1 2 1 750 0,22 ± 0,1 3 1 à 2 huile FUCHS 750 0,20 ± 0,1 3 1 The results of mass loss of the specimens after stamping, as well as the identification of the powder and / or zinc particles from the coating are summarized in Table 1. The identification of the particles and / or the powder is quoted from the following way on a scale from 1 to 4, with:

• rating 1: very few particles or powder,
• rating 2: few particles or powder,
• rating 3: a lot of particles or powder, and
• rating 4: very high level of particles or powder.

Table 1: Test results Quantity of the oil Clamping forces A loss of mass Identification on tools (1.5 g / m ² / side) (DaN) (g / m ² ) powder particles Sheet steel coated with a film of lubricating oil QUAKER oil 400 0.63 ± 0.04 3 3 FUCHS oil 400 0.55 ± 0.04 3 3 Steel sheet coated with a hydroxy-sulfate layer, and a film of lubricating oil QUAKER oil 400 0.12 ± 0.1 2 1 750 0.22 ± 0.1 3 1 to 2 FUCHS oil 750 0.20 ± 0.1 3 1

The measured mass losses as well as the quantities of powder and particles observed during the wiping of the tools show that the loss of material of the zinc coating, due to the passage of the galvanized steel sheet on the punch, is significantly decreased when the sheet was treated with the treatment solution according to the invention prior to its oiling.

2. Decrease in the effect of dewetting, effect on tribological behavior - Friction test

1 cm ² specimens are cut from a 0.7 mm thick steel sheet of the so-called "aluminum-killed steel" grade ES, coated on each of its faces with a zinc coating by dipping hot in a zinc bath.

A portion of these test pieces is treated with a treatment solution according to the invention, under the same conditions as those indicated in Example 1, so as to form a layer based on zinc hydroxysulfate and zinc sulfate. Then a film of lubricating oil (QUAKER 6130 oil) is applied to this layer with amounts ranging from 0.25 to 2.5 g / m ² .

Another part of the test pieces was oiled in the same way as before, but was not previously treated with the treatment solution according to the invention.

• L'appareil de test est un tribomètre plan-plan connu en lui-même. Les éprouvettes à tester sont serrées selon une force de serrage Fs entre deux plaquettes en acier rapide offrant une surface d'appui (ou de glissement) sur les éprouvettes. On mesure le coefficient de frottement N, tout en déplaçant l'éprouvette par rapport aux plaquettes sur une course D totale de 180 mm et à la vitesse de 10 mm/Fs en augmentant progressivement la force de serrage Fs.

Then the friction characterization of each of the test pieces is carried out using a tribology test apparatus, as follows:

• The test apparatus is a plane-plane tribometer known in itself. The test pieces to be tested are tightened according to a clamping force Fs between two fast steel plates providing a bearing surface (or sliding) on the test pieces. The coefficient of friction N is measured while moving the test piece relative to the plates over a total travel D of 180 mm and at a speed of 10 mm / Fs by progressively increasing the clamping force Fs.

An evolution curve of the coefficient of friction can then be plotted as a function of the clamping force Fs, with a given lubricating oil basis weight (see figure 1 ).

• + : tôle traitée selon l'invention, puis revêtue d'un film d'huile QUAKER 6130 de 0,25 g/m²/face
• × : tôle traitée selon l'invention, puis revêtue d'un film d'huile QUAKER 6130 de 1,0 g/m²/face
• ● : tôle traitée selon l'invention, puis revêtue d'un film d'huile QUAKER 6130 de 2,5 g/m²/face
• ■ : tôle non traitée revêtue d'un film d'huile QUAKER 6130 de 0,25 g/m²/face
• ▲ : tôle non traitée revêtue d'un film d'huile QUAKER 6130 de 1,0 g/m²/face
• ◆ : tôle non traitée revêtue d'un film d'huile QUAKER 6130 de 2,5 g/m²/face

The different curves are identified by the following symbols:

• +: treated sheet according to the invention, then coated with a film of oil QUAKER 6130 0.25 g / m ² / face
• ×: treated sheet according to the invention, then coated with a film of oil QUAKER 6130 of 1.0 g / m ² / face
• ●: treated sheet according to the invention, then coated with a film of oil QUAKER 6130 of 2.5 g / m ² / face
• ■: untreated sheet coated with a QUAKER 6130 oil film 0.25 g / m ² / face
• ▲: untreated sheet coated with QUAKER 6130 oil film of 1.0 g / m ² / face
• ◆: Untreated sheet coated with a 2.5 g / m ² QUAKER 6130 oil film

Table 2 shows, for each of the test pieces tested, an average value of the coefficient of friction for a given clamping force Fs. <u> Table 2 </ u> Clamping force (MPa) Coefficient of friction Oil weight of the treated sheet according to the invention Oil weight of untreated sheet 0.25 (g / m ² ) 1.0 (g / m ² ) 2.5 (g / m ² ) 0.25 (g / m ² ) 1.0 (g / m ² ) 2.5 (g / m ² ) 30 0.13 0.12 0.12 0.20 0.15 0.15 50 0.11 0.11 0.11 0.20 0.17 0.17

The results obtained show that a reduction in the weight of oil causes a significant increase in the coefficient of friction in the absence of application of the treatment solution according to the invention prior to the application of the oil film.

On the other hand, when the treatment solution according to the invention has been applied to the galvanized sheet prior to the application of the lubricating oil film, the friction coefficients obtained are very low, even with oil weights below 0.5 g / m ² .

3. Decrease in the effect of dewetting, effect on temporary protection against corrosion

Test specimens are cut from 0.7 mm thick ES-grade "ES-coated steel" steel sheet coated on both sides with a hot dipped zinc coating. in a bath of zinc.

A portion of these test pieces is treated with a treatment solution according to the invention, under the same conditions as those indicated in Example 1, so as to form a layer based on zinc hydroxysulfate and zinc sulfate. Then a film of lubricating oil (QUAKER oil 6130) with weights of between 0.25 and 1.0 g / m ² is applied to this layer.

Another part of the test pieces was oiled in the same way as before, but was not previously treated with the treatment solution according to the invention.

The lubricating oils applied to steel sheets coated with a zinc-based metal layer provide protection against corrosion during the time period between the manufacture of the sheets and their implementation, for example by stamping.

The compliance of the product delivered on this point is verified through the results of an accelerated moisture-thermal corrosion test.

For this purpose the test pieces are placed in a climatic chamber corresponding to the DIN 50017 standard, which simulates the corrosion conditions of an outer coil of sheet metal coil or sheet metal sheet during storage.

• 8 h à 40°C et 95-100% de RH (humidité relative)
• 16 h à 20°C et 75% de RH.

The detail of the cycle (one cycle = 24 hours) in the humidotherm is described below:

• 8 h at 40 ° C and 95-100% RH (relative humidity)
• 16 h at 20 ° C and 75% RH.

The specimens are suspended vertically individually.

The result of the test, shown in Table 3, is obtained by recording the number of successive cycles before the appearance of corrosion on the test specimen.

An evolution curve of the percentage of white rust can then be plotted as a function of the number of cycles for each of the test pieces tested (cf. figure 2 ).

The different curves are identified by the following symbols:
+: treated sheet according to the invention, then coated with a film of oil QUAKER 6130 0.25 g / m ² / face
*: treated sheet according to the invention, then coated with a film of oil QUAKER 6130 of 0.5 g / m ² / face
▲: treated sheet according to the invention, then coated with a film of oil QUAKER 6130 of 1.0 g / m ² / face
◆: untreated sheet coated with a QUAKER 6130 oil film of 0.25 g / m ² / face
■: untreated sheet coated with QUAKER 6130 oil film 0.5 g / m ² / face
●: untreated sheet coated with a 1.0 g / m ² QUAKER 6130 oil film <u> Table 3 </ u> Number of cycles % rust Oil weight of the treated sheet according to the invention Oil weight of untreated sheet 0.25 (g / m ² ) 0.5 (g / m ² ) 1.0 (g / m ² ) 0.25 (g / m ² ) 0.5 (g / m ² ) 1.0 (g / m ² ) 0.5 5% 0% 0% 60% 20% 0% 1.5 0% 85% 2% 2.5 2% 0% 2% 3.5 20% 2% 0% 2% 4.5 30 % 0% 40% 2% 5.5 55% 0% 50% 2% 8.5 12% 0% 65% 2%

It has been found that it is possible to very significantly improve the temporary protection against corrosion of galvanized steel sheets on which a treatment solution according to the invention has been applied before the application of the film. lubrication oil, even when the weight of oil is less than 1 g / m ² .

Claims (17)

Use of an aqueous treatment solution containing sulphate ions SO ₄ ^2- at a concentration greater than or equal to 0.01 mol / l, for treating the surface of a steel sheet coated on at least one of its faces, a metal coating based on zinc or its alloys, in order to reduce the formation of powder or metal particles based on zinc or its alloys generated by the degradation of the coating during the shaping of said sheet. Use according to claim 1, wherein the aqueous treatment solution additionally contains Zn ²⁺ zinc ions at a concentration greater than or equal to 0.01 mol / l, Use according to one of claims 1 or 2, wherein the treatment solution is applied to the surface of the sheet under conditions of temperature, time of contact with the galvanized surface, concentration of SO ₄ ^2- ions and Zn ²⁺ ions adjusted to form a layer based on zinc hydroxysulfate and zinc sulphate with a sulfur content greater than or equal to 0.5 mg / m ² . Use according to any one of claims 1 to 3, wherein the concentration of Zn ²⁺ ions and the concentration of SO ₄ ^2- ions are between 0.07 and 0.55 mol / l. Use according to any one of claims 1 to 4, wherein the pH of the treatment solution is between 5 and 7. Use according to any one of claims 1 to 5, wherein the treatment solution is applied under conditions of temperature, time of contact with the galvanized surface, concentrations of SO ₄ ^2- ions and adjusted Zn ²⁺ ions. to form a layer of hydroxysulfate and sulfate having a sulfur content of between 3.7 and 27 mg / m ² . Use according to any one of claims 1 to 6, wherein after the application of the treatment solution to the sheet, said sheet is dried, after being optionally rinsed to remove the soluble portion of the hydroxysulfate and sulfate layer. Use according to claim 1, wherein the treatment solution is applied under anodic polarization, and the pH of the treatment solution is greater than or equal to 12, and less than 13. Use according to claim 8, wherein the density of electrical charges circulating during the treatment through the surface of the sheet is adjusted to form a layer based on zinc hydroxysulfate and zinc sulphate whose sulfur content is greater than or equal to 0.5 mg / m ² . Use according to one of claims 8 or 9, wherein the concentration of SO ₄ ^2- ion is greater than 0.07 mol / l. Use according to any one of claims 8 to 10, wherein the density of electrical charges is adjusted to form a layer based on zinc hydroxysulfate and zinc sulphate whose sulfur content is between 3, 7 and 27 mg / m ² . Use according to any one of claims 8 to 11, wherein the bias current density applied during the treatment is greater than 20 A / dm ² . Use according to any one of claims 8 to 12, wherein after the application of the treatment solution to the sheet, said sheet is rinsed. A method of lubricating a steel sheet coated with a layer consisting of a metal coating based on zinc or its alloys, and wherein: said sheet is coated with a top layer based on zinc hydroxysulfate and zinc sulfate, said top layer having been obtained by the use of a treatment solution as defined in any one of claims 1 at 13 and then a film of lubricating oil is applied to the top layer with a grammage of less than 1 g / m ² . Process according to Claim 14, characterized in that the basis weight of the oil film is less than 0.9 g / m ² . Process according to claim 15, characterized in that the basis weight of the oil film is between 0.2 and 0.5 g / m ² . Use of an aqueous treatment solution containing sulphate ions SO ₄ ^2- at a concentration greater than or equal to 0.01 mol / l, for treating the surface of a steel sheet coated on at least one of its faces, a metal coating based on zinc or its alloys, to improve the temporary protection against corrosion of said sheet.

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