EP3250730B1 - Method for the production of a coated metal sheet, comprising the application of an aqueous solution containing an amino acid, and associated use in order to improve corrosion resistance - Google Patents

Description

The present invention relates to a sheet comprising a steel substrate having two faces of which at least one is coated with a metal coating comprising at least 40% by weight of zinc, its preparation process and the use of an amino acid for improving the corrosion resistance of zinc-coated coatings.

The invention relates to a coated steel sheet. Before being used, the coated steel sheets are generally subjected to various surface treatments.

The demand US 2010/0261024 discloses the application of an aqueous solution of glycine or neutral form glutamic acid or salt to a steel sheet coated with a zinc-based coating to improve the corrosion resistance of the sheet.

The demand WO 2008/076684 discloses the application to zinc-coated steel sheet, electrogalvanised steel plate or galvanized steel sheet of a pretreatment composition consisting of an aqueous solution comprising a compound comprising a Group IIIB (Sc, Y) metal , La, Ac) or group IVB (Ti, Zr, Hf, Rf) and a copper-based compound, for example aspartate or copper glutamate, followed by the application of a composition comprising a resin film-forming material and a yttrium-based compound. The addition of copper to a solution comprising a Group IIIB metal or IVB group is described as improving the corrosion resistance of the sheet.

The demand EP 2 458 031 describes the application on a galvanized steel sheet GI, or galvanized alloy GA, a conversion treatment solution comprising a compound (A) selected from water-soluble titanium or zirconium compounds and an organic compound (B) which can in particular, be glycine, alanine, asparagine, glutamic acid or aspartic acid in neutral form or salt. According to this application, the compound (A) forms on the sheet a conversion film which improves the compatibility of the sheet with subsequently applied coatings, such as cataphoretic paints, and its resistance to corrosion. Compound (B) is described as stabilizing compound (A).

The demand US2014 / 0360630 discloses a zinc-based pretreatment process performed prior to the application of an anti-corrosion coating, which can be implemented for the manufacture of automotive parts. This pretreatment process is a "ferrization" process by forming a homogeneous layer of iron oxide and / or iron on the surface to improve the resistance to corrosion.

These coated steel sheets are for example intended for the automotive field. Metal coatings essentially comprising zinc are traditionally used for their good protection against corrosion.

An object of the invention is to provide a method for preparing a steel sheet coated with a metal coating comprising zinc which has an even greater corrosion resistance.

For this purpose, the invention relates to a method according to claim 1.

The method may also include the features of claims 2 to 14, taken alone or in combination.

The invention also relates to a sheet according to claim 15 to 17, and uses according to claims 18 and 19.

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 figure, which is a diagrammatic sectional view illustrating the structure of a sheet 1 obtained by a method according to the invention. invention.

Sheet 1 of the figure comprises a substrate 3 made of steel coated on each of its two faces 5 by a metal coating 7. It will be observed that the relative thicknesses of substrate 3 and coatings 7 covering it have not been respected in FIG. to facilitate representation.

The coatings 7 present on the two faces 5 are similar and only one will be described in detail later. Alternatively (not shown), only one of the faces 5 has a metal coating 7.

The metal coating 7 comprises more than 40% by weight of zinc, in particular more than 50% by weight of zinc, preferably more than 70% by weight of zinc, more preferably more than 90%, preferably more than 95%, of preferably more than 99%. The complement may consist of metal elements Al, Mg, Si, Fe, Sb, Pb, Ti, Ca, Sr, Mn, Sn, La, Ce, Cr, Ni or Bi, taken alone or in combination. The measurement of the composition of a coating is generally carried out by chemical dissolution of the coating. The result given corresponds to an average content throughout the layer.

The metal coating 7 may comprise several successive layers of different compositions, each of these layers comprising more than 40% by weight of zinc (or more, as defined above). The metal coating 7, or one of its constituent layers, may also have a concentration gradient in a given metal element. When the metal coating 7, or one of its constituent layers, has a zinc concentration gradient, the average proportion of zinc in the metal coating 7, or in this constituent layer, is more than 40% by weight of zinc (or more, as defined above).

To produce the sheet 1, one can for example proceed as follows.

The method may comprise a prior step of preparing the substrate 2 made of steel having two faces 5, at least one of which is coated with a metal coating 7 comprising at least 40% by weight of zinc. A steel substrate 3 is used, for example obtained by hot rolling and then cold rolling. The metal coating 7 comprising more than 40% by weight of zinc may be deposited on the substrate 3 by any known deposition method, in particular by electrogalvanization, vapor deposition ("physical vapor deposition" PVD in English), deposition by jet of Sonic steam ("Jet Vapor Deposition" JVD in English) or hot dip galvanizing.

According to a first alternative, the steel substrate 3 having two faces 5, at least one of which is coated with a metal coating 7 comprising at least 40% by weight of zinc is obtained by electrogalvanizing the substrate 3 made of steel. The application of the coating may take place on one side (the sheet 1 then comprises only a metal coating 7), or on both sides (the sheet 1 then comprises two metal coatings 7).

According to a second alternative, the steel substrate 3 having two faces 5, at least one of which is coated with a metal coating 7 comprising at least 40% by weight of zinc is obtained by hot-dip galvanizing the steel substrate 3.

Generally, the substrate 3 is then in the form of a strip which is scrolled in a bath to deposit the metal coating 7 by hot dipping. The composition of the bath varies according to whether the desired sheet 1 is a galvanized steel sheet GI, a galvanized steel sheet or a galvanized steel sheet coated with a zinc alloy. and magnesium, an alloy of zinc and aluminum or an alloy of zinc, magnesium and aluminum. The bath may also contain up to 0.3% by weight of additional optional elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi. These various additional elements can in particular make it possible to improve the ductility or adhesion of the metal coating 7 to the substrate 3. The skilled person, who knows their effects on the characteristics of the metal coating 7, will know how to use them according to the purpose complementary research. The bath may finally contain residual elements from the ingots, or resulting from the passage of the substrate 3 in the bath, a source of unavoidable impurities in the metal coating 7.

In one embodiment, the steel substrate 3 having two faces 5, at least one of which is coated with a metal coating 7 comprising at least 40% by weight of zinc, is a galvanized steel sheet GI. The metal coating 7 is then a GI zinc coating. Such a coating comprises more than 99% by weight of zinc.

In another embodiment, the steel substrate 3 having two faces 5, at least one of which is coated with a metal coating 7 comprising at least 40% by weight of zinc is a galvanized steel sheet GA. The metal coating 7 is then a coating of zinc GA. A galvanized steel sheet GA is obtained by annealing ("annealing" in English) of a sheet galvanized steel GI. In this case, the method therefore comprises a step of hot-dip galvanizing the steel substrate 3, and then an annealing step. The annealing causes the iron of the steel substrate 3 to diffuse into the metal coating 7. The metal coating 7 of a GA sheet typically comprises from 10% to 15% by weight of iron.

In another embodiment, the metal coating 7 is an alloy of zinc and aluminum. The metal coating 7 may for example comprise 55% by weight of aluminum, 43.5% by weight of zinc and 1.5% by weight of silicon, such as Aluzinc® sold by ArcelorMittal.

In another embodiment, the metal coating 7 is an alloy of zinc and magnesium, preferably comprising more than 70% by weight of zinc. Metal coatings comprising zinc and magnesium will be generally referred to herein as zinc-magnesium or ZnMg coatings. The addition of magnesium to the metal coating 7 significantly increases the corrosion resistance of these coatings, which can reduce their thickness or increase the guarantee of protection against corrosion over time.

The metal coating 7 may especially be an alloy of zinc, magnesium and aluminum, preferably comprising more than 70% by weight of zinc. Metal coatings comprising zinc, magnesium, and aluminum will generally be referred to herein as zinc-aluminum-magnesium or ZnAIMg coatings. The addition of aluminum (typically of the order of 0.1% by weight) to a coating based on zinc and magnesium also improves the corrosion resistance, and makes the coated sheet easier to be formatting. Thus, metal coatings essentially comprising zinc are now competing with coatings comprising zinc, magnesium and possibly aluminum.

Typically, the metal coating 7 of ZnMg or ZnAIMg type comprises between 0.1 and 10% by weight, typically between 0.3 and 10% by weight, especially between 0.3 and 4% by weight of magnesium. Below 0.1% by weight of Mg, the coated sheet is less resistant to corrosion and above 10% by weight of Mg, the ZnMg or ZnAIMg coating oxidizes too much and can not be used.

For the purposes of the present application, when a range of numbers is described as being between a low terminal and a high terminal, it is understood that these terminals are included. For example a coating comprising 0.1% or 10% by weight of magnesium is included when the expression "The metal coating 7 comprises between 0.1 and 10% by weight of magnesium" is used.

The metal coating 7 of ZnAIMg type comprises aluminum, typically between 0.5 and 11% by weight, especially between 0.7 and 6% by weight, preferably between 1 and 6% by weight of aluminum. Typically, the mass ratio between the magnesium and the aluminum in the metal coating 7 of the ZnAIMg type is strictly less than or equal to 1, preferably strictly less than 1, and more preferably strictly less than 0.9.

The most common unavoidable impurity present in the metal coating 7 and resulting from the passage of the substrate in the bath is iron which may be present at a content of up to 3% by weight, generally less than or equal to 0.4% by weight, typically between 0.1 and 0.4% by weight relative to the metal coating 7.

The unavoidable impurities from the ingots, for the ZnAIMg baths, are generally lead (Pb), present at a content of less than 0.01% by weight with respect to the metal coating 7, Cadmium (Cd), present at a content of less than 0.005% by weight with respect to the metal coating 7 and tin (Sn) present at a content of less than 0.001% by weight relative to the metal coating 7.

Additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi may be present in the metal coating 7. The content by weight of each additional element is generally less than 0 , 3%.

The metal coating 7 generally has a thickness less than or equal to 25 microns and conventionally aims to protect the steel substrate 3 against corrosion.

After depositing the metal coating 7, the substrate 3 is for example spun by means of nozzles throwing a gas on either side of the substrate 3.

The metal coating 7 is then allowed to cool in a controlled manner to solidify. Controlled cooling of the metal coating 7 is provided at a rate of preferably greater than or equal to 15 ° C / s or more than 20 ° C / s between the onset of solidification (i.e. when the metal coating 7 falls just below the temperature of the liquidus) and the end of solidification (that is to say when the metal coating 7 reaches the temperature of the solidus).

In a variant, the spin may be adapted to remove the metal coating 7 deposited on one face 5 so that only one of the faces 5 of the sheet 1 is finally coated with a metal coating 7.

The band thus treated can then be subjected to a so-called skin-pass step which allows the harden and give it a roughness facilitating its subsequent shaping.

The outer surface 15 of the metal coating 7 is subjected to a surface treatment step which comprises applying to them an aqueous solution comprising an amino acid selected from alanine, arginine, aspartic acid, cysteine, glutamine, lysine, methionine, proline, serine, threonine, and a mixture thereof. Each amino acid can be in neutral form or salt. As used herein, an amino acid is one of the 22 proteinaceous amino acids (L isomer) or an isomer thereof, especially their D isomers. The amino acid is preferably an L-amino acid for cost reasons.

The invention is based on the unexpected discovery that the application on the outer surface of the metal coating 7 of an aqueous solution comprising an amino acid from the above mentioned list makes it possible to improve the corrosion resistance of the sheet obtained. This improvement is not observed regardless of the amino acid used. For example, the corrosion resistance has not been improved by applying valine to a metal-coated sheet comprising at least 40% by weight of zinc. No theory has yet been put forward to explain why some amino acids can improve corrosion resistance and not others.

The aqueous solution applied may comprise an amino acid selected from alanine, arginine, aspartic acid, cysteine, glutamine, lysine, methionine, proline, serine, threonine, and a mixture of these. ci, each amino acid being in neutral form or salt.

The aqueous solution applied may comprise an amino acid selected from alanine, arginine, aspartic acid, glutamine, lysine, methionine, proline, serine, threonine, and a mixture thereof, each amino acid being in neutral form or salt. The aqueous solution applied can in particular comprise an amino acid chosen from alanine, aspartic acid, cysteine, glutamine, methionine, proline, serine, threonine, and a mixture thereof, each amino acid being under neutral form or salt.

The aqueous solution applied may, for example, comprise an amino acid chosen from alanine, aspartic acid, glutamine, methionine, proline and serine. threonine, and a mixture thereof, each amino acid being in neutral form or salt.

Preferably, in the first alternative in which the sheet 1 is an electrogalvanized steel sheet, the amino acid of the aqueous solution applied is selected from aspartic acid, cysteine, methionine, proline and threonine, and a mixture of of these, each amino acid being in neutral or salt form, in particular among aspartic acid, methionine, proline and threonine, and a mixture thereof, each amino acid being in neutral or salt form.

Preferably, in the second alternative in which the sheet 1 is a sheet obtained by hot-dip galvanizing the steel substrate 3, the amino acid of the aqueous solution applied is chosen from alanine, arginine, glutamine and lysine. , methionine, proline, serine, threonine and a mixture thereof, each amino acid being in neutral form or salt. For example, the amino acid of the applied aqueous solution is selected from alanine, glutamine, methionine, proline, serine, threonine and a mixture thereof, each amino acid being in neutral form or salt.

Preferably, in the third alternative in which the sheet 1 is indifferently an electrogalvanized steel sheet or a sheet obtained by hot-dip galvanizing the steel substrate 3, the amino acid of the aqueous solution applied is chosen from methionine, proline and threonine and a mixture thereof, each amino acid being in neutral form or salt.

The amino acid is especially selected from proline in neutral form or salt, cysteine in neutral form or salt, and a mixture thereof. Proline is particularly effective in improving corrosion resistance. The cysteine advantageously makes it possible to assay the amount of amino acid deposited on the surface by virtue of its thiol function, for example by X-ray fluorescence spectrometry (SFX).

Preferably, the amino acid is selected from proline in neutral form or salt, threonine in neutral form or salt, and a mixture thereof. Proline and threonine not only make it possible to improve the corrosion resistance of the sheet, but also to improve the compatibility of the surface with an adhesive and to improve the tribological properties of the surface of the sheet (this which makes it well adapted to its subsequent shaping, especially by stamping).

The improvement of the compatibility of the surface of the sheet with an adhesive can for example be demonstrated by carrying out tensile tests on samples of sheets assembled by means of an adhesive and possibly aged, up to rupture of the assembly and by measuring the maximum tensile stress and the nature of the fracture. The improvement of the tribological properties can for example be demonstrated by measuring the coefficient of friction (μ) as a function of the contact pressure (MPa), for example from 0 to 80 MPa.

It is particularly surprising that threonine and / or proline make it possible to improve these three properties at the same time. In the conditions tested, the other amino acids did not allow an improvement of these three properties on any type of metal coating comprising at least 40% by weight of zinc (at best, the other amino acids have allowed to observe an improvement of two of these properties, but not of the three).

The aqueous solution applied generally comprises from 1 to 200 g / l, in particular from 5 g / l to 150 g / l, typically from 5 g / l to 100 g / l, for example from 10 to 50 g / l of amino acid. in neutral form or salt or mixture of amino acid in neutral form or salts. The most significant improvement in the corrosion resistance of the metal coating 7 of the sheet 1 was observed using an aqueous solution comprising from 5 g / l to 100 g / l, in particular from 10 to 50 g / l d amino acid or amino acid mixture.

The aqueous solution applied generally comprises from 10 to 1750 mmol / l, especially from 40 mmol / l to 1300 mmol / l, typically from 40 mmol / l to 870 mmol / l, for example from 90 to 430 mmol / l of amino acid. in neutral form or salt or mixture of amino acid in neutral form or salts. The most significant improvement in the corrosion resistance of the metal coating 7 of the sheet 1 was observed using an aqueous solution comprising from 40 mmol / L to 870 mmol / L, in particular from 90 to 430 mmol / L amino acid or amino acid mixture.

Of course, the mass and molar proportions of the amino acid (or of each of the amino acids when an amino acid mixture is used) in the aqueous solution can not be greater than the proportions corresponding to the limit of solubility of the amino acid at the temperature. to which the aqueous solution is applied. The mass percentage of dry extract of the amino acid in neutral form or of salt or of the mixture of neutral amino acids or of salts in the aqueous solution is greater than or equal to 75%, in particular greater than or equal to 90%, preferably greater than or equal to 95%. Likewise, generally, the molar percentage of dry extract of the amino acid in neutral form or of salt in the aqueous solution is greater than or equal to 75%, especially greater than or equal to 90%, preferably greater than or equal to 95%.

The aqueous solution may comprise zinc sulfate and / or iron sulfate. The proportion of zinc sulphate in the aqueous solution is generally less than 80 g / l, preferably less than 40 g / l. Preferably, the aqueous solution is free of zinc sulfate and iron sulfate.

Generally, the aqueous solution comprising an amino acid comprises less than 10 g / l, typically less than 1 g / l, generally less than 0.1 g / l, especially less than 0.05 g / l, for example less than 0.01 g. / L of zinc ions. Preferably, the aqueous solution is free of zinc ion (in addition to unavoidable traces, which could for example arise from the pollution, by the substrate, of the bath of aqueous solution).

The aqueous solution comprising an amino acid generally comprises less than 0.005 g / L of iron ions. The aqueous solution comprising an amino acid generally comprises few metal ions other than potassium, sodium, calcium and zinc, typically less than 0.1 g / l, especially less than 0.05 g / l, for example less 0.01 g / L, preferably less than 0.005 g / L of metal ions other than potassium, sodium, calcium and zinc. Typically, the aqueous solution is free of metal ions other than zinc, calcium, sodium and potassium. The aqueous solution comprising an amino acid generally comprises few metal ions other than zinc, typically less than 0.1 g / l, especially less than 0.05 g / l, for example less than 0.01 g / l, preferably less than 0.005 g / L of metal ions other than zinc. Typically, the aqueous solution is free of metal ions other than zinc. In particular, the aqueous solution comprising an amino acid generally comprises few cobalt and / or nickel ions, typically less than 0.1 g / l, especially less than 0.05 g / l, for example less than 0.01 g / l of cobalt and / or nickel ions. Preferably, the aqueous solution is free of cobalt ions and / or free of nickel ions and / or free of copper ions and / or free of chromium ions. The aqueous solution is free of a compound comprising a Group IIIB metal (Sc, Y, La, Ac) or Group IVB (Ti, Zr, Hf, Rf). Preferably, it is free of metal ions (in addition to unavoidable metallic impurities, which could for example come from the pollution, by the substrate, of the aqueous solution bath).

In general, the absence of metal ions in the aqueous solution makes it possible to avoid disturbing the action of the active ingredient that is the amino acid or the mixture of amino acids.

In addition, the aqueous solution comprising an amino acid generally comprises less than 0.1 g / l, in particular less than 0.05 g / l, for example less than 0.01 g / l of compounds comprising chromium VI, or more generally chromium. Generally, it is free of compounds comprising chromium VI, or more generally chromium.

Moreover, the aqueous solution is generally free of oxidizing agent.

On the other hand, the aqueous solution is generally free of resin, in particular of organic resin. A resin is a polymeric product (natural, artificial or synthetic) which is a raw material for manufacturing, for example, plastics, textiles, paints (liquid or powder), adhesives, varnishes, polymer foams. It can be thermoplastic or thermosetting. More generally, the aqueous solution is generally free of polymer.

The absence of resin makes it possible to obtain a thin treatment layer and thus to facilitate its removal during degreasing before phosphating and painting. Under these conditions, a resin tends to leave residues which disturb phosphatation.

The pH of the aqueous solution applied is generally comprised of a pH equal to the [isoelectric point of the 3-amino acid] at a pH equal to the [isoelectric point of the amino acid + 3], in particular of a pH equal to isoelectric point of the amino acid - 2] at a pH equal to the [isoelectric point of the amino acid + 2], preferably of a pH equal to the [isoelectric point of the amino acid - 1] at a pH equal to isoelectric of the amino acid + 1]. For example, when the amino acid is proline whose isoelectric point is 6.3, the pH of the aqueous solution is generally from 3.3 to 9.3, especially from 4.3 to 8.3, preferably from 5.3 to 7.3.

The pH of the aqueous solution applied is generally comprised of a pH equal to the [isoelectric point of the amino acid -3] at a pH equal to the [isoelectric point of the amino acid + 1], preferably of a pH equal to [isoelectric point of the amino acid - 3] at a pH equal to the [isoelectric point of the amino acid - 1], in particular of a pH equal to the [isoelectric point of the amino acid - 2.5] at a pH equal to [isoelectric point of the amino acid - 1.5], typically a pH equal to the [isoelectric point of the amino acid - 2]. For example, when the amino acid is proline whose isoelectric point is 6.3, the pH of the aqueous solution is preferably from 3.3 to 5.3, especially from 3.8 to 4.8, typically from the order of 4.0, such as 4.3. Such a pH makes it possible to promote the bond between the amino acid and the metal coating 7. In particular, a process carried out with a solution having such a pH makes it possible to obtain a sheet which retains its properties. improved corrosion resistance, even when subjected to a washing / re-oiling treatment. Generally, once the sheet according to the invention has been prepared, it can be blanked before forming, typically by stamping. In order to eliminate the impurities deposited on the sheet resulting from this cutting, a washing / re-oiling treatment can be implemented. This consists in applying a low-viscosity oil to the surfaces of the sheet, then in brushing and then applying an oil of greater viscosity. Without wishing to be bound by any particular theory, it is assumed that a solution having such a pH makes it possible to obtain the amino acid in protonated form (NH ₃ ⁺ ), which would promote the bond between the amino acid and the metal coating 7 and therefore the maintenance of the amino acid on the surface despite the washing / re-oiling treatment. At different pH and in particular greater than the [isoelectric point of the amino acid-1], the amine of the amino acid is little or not protonated: the bonds between the amino acid and the metal coating 7 would be weaker and the amino acid would be more likely to dissolve in the oil used during the washing / re-oiling treatment, leading to its at least partial removal, and therefore to less good corrosion resistance properties.

Those skilled in the art know how to adapt the pH of the aqueous solution, by adding a base if it wishes to increase the pH, or an acid, such as phosphoric acid, if it wishes to reduce it. In the sense of the application, a base or an acid is indifferently in neutral form and / or salt. Generally, the proportion of acid is less than 10 g / l, in particular 1 g / l in the solution. Preferably, the phosphoric acid is added together in neutral form and in salt form (for example sodium, calcium or potassium), for example in a H ₃ PO ₄ / NaH ₂ PO ₄ mixture. Phosphoric acid advantageously makes it possible to measure the amount of aqueous solution (and therefore of amino acid) deposited on the surface by means of phosphorus and / or sodium, for example by X-ray fluorescence spectrometry (SFX).

In one embodiment, the aqueous solution consists of a mixture of water, amino acid in neutral form or salt or a mixture of amino acids independently in neutral forms or salts and optionally a base or a mixture of bases, or an acid or a mixture of acids. The base or acid serves to adjust the pH of the aqueous solution. The amino acid imparts the properties of improving the corrosion resistance. The base or the acid makes it possible to reinforce this effect. Addition of other compounds is not necessary.

In the process according to the invention, the aqueous solution comprising an amino acid can be applied at a temperature of between 20 and 70 ° C. The duration of application of the aqueous solution can be between 0.5s and 40s, preferably between 2s and 20s.

The aqueous solution comprising an amino acid can be applied by immersion, spraying or any other system.

The application of the aqueous solution on the outer surface 15 of the metal coating 7 can be carried out by any means, for example by immersion, by spraying ("spray" in English) or by roll coating ("roll coat" in English). . The latter technique is preferred because it makes it easier to control the amount of aqueous solution applied while ensuring a homogeneous distribution of the aqueous solution on the surface. Generally, the wet film thickness consisting of the aqueous solution applied to the outer surface of the metal coating 7 is 0.2 to 5 μm, typically between 1 and 3 μm.

By "application on the outer surface 15 of the metal coating 7 of an aqueous solution comprising an amino acid" is meant that the aqueous solution comprising an amino acid is brought into contact with the outer surface 15 of the metal coating 7. It is therefore sub- It is to be understood that the outer surface 15 of the metal coating 7 is not covered with an intermediate layer (a film, a coating or a solution) which would prevent the contact of the aqueous solution comprising an amino acid with the outer surface of the coating. metallic 7.

Typically, the method comprises, after the step of applying to the outer surface of the metal coating 7 an aqueous solution comprising an amino acid, a drying step, which provides on the outer surface 15 of the metal coating 7 a layer comprising (or consisting of) an amino acid (in neutral or salt form) or a mixture of amino acids (independently in neutral forms or salts). This can be done by subjecting the sheet 1 to a temperature between 70 and 120 ° C, for example between 80 and 100 ° C, generally for 1 to 30 seconds, especially 1 to 10 seconds, for example 2 s. In particular, a process carried out with such a drying step makes it possible to obtain a sheet which retains its improved properties of corrosion resistance, even when it has undergone a washing / re-oiling treatment.

The metal coating 7 of the sheet 1 obtained is then typically coated with a layer comprising from 0.1 to 200 mg / m ² , in particular from 25 to 150 mg / m ² , in particular from 50 to 100 mg / m ² , example of 60 to 70 mg / m ² of amino acid (in neutral or salt form) or a mixture of amino acids (independently in neutral forms or salts). The amount of amino acid deposited on the outer surface of the metal coating 7 can be determined by assaying the amount of amino acid deposited (eg by infrared), or by assaying the amount of amino acid remaining in the aqueous solution (for example by acidobasic and / or conductimetric assay) since the initial amino acid concentration of the aqueous solution is known. In addition, when the amino acid or one of the amino acids is cysteine, the amount of cysteine deposited on the surface can be determined by X-ray fluorescence spectrometry (SFX).

Generally, the layer comprising an amino acid (in neutral or salt form) or a mixture of amino acids (independently in neutral forms or salts) which coat the metal coating 7 of the sheet 1 obtained comprises from 75 to 100% by weight, typically 90 to 100% by weight of amino acid (in neutral or salt form) or mixture of amino acids (independently in neutral forms or salts).

The method may comprise (or be free from) other surface treatment step (s) than that of applying an aqueous solution comprising an amino acid (for example alkaline oxidation surface treatment and / or chemical conversion). When this (s) surface treatment step (s) leads (s) to the formation of a layer on the metal coating 7, this (these) other (s) surface treatment step (s) is (are) carried out simultaneously or after the step of applying an aqueous solution comprising an amino acid on the outer surface 15 of the metal coating 7, so that there is no intermediate layer between the outer surface 15 of the coating metal 7 and the aqueous solution comprising an amino acid. These possible surface treatment steps mentioned above may include other sub-stages of rinsing, drying ....

After applying the aqueous solution comprising an amino acid, a film of fat or oil is generally applied to the outer surface of the metal coating 7 coated with a layer comprising an amino acid or a mixture of amino acids to protect it against corrosion.

The band may possibly be wound before being stored. Typically, before putting the piece into shape, the strip is cut. A grease or oil film may then be reapplied to the outer surface of the metal coating 7 coated with a layer comprising an amino acid or a mixture of amino acids prior to shaping.

Preferably, the process is free of a degreasing step (typically performed by applying a basic aqueous solution of pH generally greater than 9 to the outer surface 15 of the metal coating 7) prior to shaping. Indeed, the treatment with a basic aqueous solution on the outer surface 15 of the coating metal coated with a layer comprising an amino acid or a mixture of amino acids could lead to the partial or total elimination of the amino acid (s) which has (have) been deposited on the surface outer 15 of the metal coating 7, which is sought to avoid.

The sheet may then be shaped by any method adapted to the structure and shape of the parts to be manufactured, preferably by stamping, such as for example cold stamping. The shaped sheet 1 then corresponds to a part, for example a car part.

• une étape de dégraissage, typiquement réalisée en appliquant une solution aqueuse basique sur la surface extérieure 15 du revêtement métallique 7, et/ou
• d'autre(s) étape(s) de traitement de surface, par exemple une étape de phosphatation, et/ou
• une étape de cataphorèse.

Once the sheet 1 has been shaped, the process can then include (or be free from):

• a degreasing step, typically performed by applying a basic aqueous solution to the outer surface 15 of the metal coating 7, and / or
• other step (s) of surface treatment, for example a phosphating step, and / or
• a stage of cataphoresis.

The disclosure also relates to the sheet 1 that can be obtained by the method. Such a sheet comprises at least a portion of at least one outer surface 15 of the metal coating 7 coated with a layer comprising from 0.1 to 200 mg / m ² , in particular from 25 to 150 mg / m ² , in particular from 50 to at 100 mg / m ² , for example 60 to 70 mg / m ² of amino acid in neutral form or salt.

The disclosure also relates to the use of an aqueous solution comprising an amino acid selected from alanine, arginine, aspartic acid, cysteine, glutamine, lysine, methionine, proline, serine, threonine and a mixture thereof, each amino acid being in neutral form or salt, the aqueous solution being free of a compound comprising a Group IIIB or Group IVB metal, to improve the corrosion resistance of an outer surface. a metal coating 7 coating at least one face 5 of a substrate 3 of steel, wherein the metal coating 7 comprises at least 40% by weight of zinc.

The preferred embodiments described above for the aqueous solution, the conditions of application of the aqueous solution and the metal coating 7 are of course applicable.

• fourniture d'un substrat 3 en acier présentant deux faces 5, dont au moins l'une est revêtue par un revêtement métallique 7 comprenant au moins 40% en poids de zinc,
• application sur la surface extérieure 15 du revêtement métallique 7 d'une solution aqueuse comprenant un aminoacide choisi parmi l'alanine, l'arginine, l'acide aspartique, la cystéine, la glutamine, la lysine, la méthionine, la proline, la sérine, la thréonine, et un mélange de ceux-ci, chaque aminoacide étant sous forme neutre ou de sel, la solution aqueuse étant exempte de composé comprenant un métal du groupe IIIB ou du groupe IVB.

The disclosure also relates to a method for improving the corrosion resistance of an outer surface of a metal coating 7 coating at least one face of a steel substrate 3, comprising at least the steps of:

• supplying a steel substrate 2 having two faces 5, at least one of which is coated with a metal coating 7 comprising at least 40% by weight of zinc,
• application on the outer surface 15 of the metal coating 7 of an aqueous solution comprising an amino acid selected from alanine, arginine, aspartic acid, cysteine, glutamine, lysine, methionine, proline, serine , threonine, and a mixture thereof, each amino acid being in neutral form or salt, the aqueous solution being free of a compound comprising a Group IIIB metal or Group IVB.

The preferred embodiments described above for the aqueous solution, the conditions of application of the aqueous solution, the metal coating 7 and any additional steps in the process are of course applicable.

• améliorer la compatibilité, avec un adhésif 13, d'au moins une partie d'une surface extérieure 15 d'un revêtement métallique 7 revêtant au moins une face 5 d'un substrat 3 en acier,
• améliorer la résistance à la corrosion de la surface extérieure 15 du revêtement métallique 7 revêtant au moins une face 5 du substrat 3 en acier, et
• améliorer les propriétés tribologiques de la surface extérieure 15 du revêtement métallique 7 revêtant au moins une face 5 du substrat 3 en acier,

où le revêtement métallique 7 comprend au moins 40% en poids de zinc.The disclosure also relates to the use of an aqueous solution comprising an amino acid selected from proline, threonine and a mixture thereof, wherein proline and threonine are independently in neutral form or salt, the aqueous solution being free from compound comprising a Group IIIB metal or Group IVB for:

• improving the compatibility, with an adhesive 13, of at least part of an outer surface 15 of a metal coating 7 coating at least one face 5 of a substrate 3 made of steel,
• improving the corrosion resistance of the outer surface 15 of the metal coating 7 coating at least one face 5 of the steel substrate 3, and
• to improve the tribological properties of the outer surface 15 of the metal coating 7 coating at least one face 5 of the steel substrate 3,

wherein the metal coating 7 comprises at least 40% by weight of zinc.

• améliorer la compatibilité, avec un adhésif 13, d'au moins une partie d'une surface extérieure 15 d'un revêtement métallique 7 revêtant au moins une face 5 d'un substrat 3 en acier,
• améliorer la résistance à la corrosion de la surface extérieure 15 du revêtement métallique 7 revêtant au moins une face 5 du substrat 3 en acier, et
• améliorer les propriétés tribologiques de la surface extérieure 15 du revêtement métallique 7 revêtant au moins une face 5 du substrat 3 en acier,

ledit procédé comprenant au moins les étapes de :

• fourniture d'un substrat 3 en acier présentant deux faces 5, dont au moins l'une est revêtue par un revêtement métallique 7 comprenant au moins 40% en poids de zinc,
• application sur la surface extérieure 15 du revêtement métallique 7 d'une solution aqueuse comprenant un aminoacide choisi parmi la proline, la thréonine et un mélange de celles-ci, la proline et la thréonine étant indépendamment sous forme neutre ou de sel, la solution aqueuse étant exempte de composé comprenant un métal du groupe IIIB ou du groupe IVB.

The preferred embodiments described above for the aqueous solution, the conditions of application of the aqueous solution and the metal coating 7 are of course applicable. The disclosure also relates to a method for:

• improving the compatibility, with an adhesive 13, of at least part of an outer surface 15 of a metal coating 7 coating at least one face 5 of a substrate 3 made of steel,
• improving the corrosion resistance of the outer surface 15 of the metal coating 7 coating at least one face 5 of the steel substrate 3, and
• to improve the tribological properties of the outer surface 15 of the metal coating 7 coating at least one face 5 of the steel substrate 3,

said method comprising at least the steps of:

• supplying a steel substrate 2 having two faces 5, at least one of which is coated with a metal coating 7 comprising at least 40% by weight of zinc,
• applying to the outer surface 15 of the metal coating 7 an aqueous solution comprising an amino acid selected from proline, threonine and a mixture thereof, the proline and the threonine being independently in neutral form or salt, the aqueous solution being free of a compound comprising a Group IIIB metal or Group IVB.

The preferred embodiments described above for the aqueous solution, the conditions of application of the aqueous solution, the metal coating 7 and any additional steps in the process are of course applicable.

Example 1: Corrosion Resistance Tests

• une solution aqueuse d'aminoacide tel que défini ci-dessus dont le pH avait été éventuellement ajusté par ajout de H₃PO₄, puis
• de l'huile Fuchs® 3802-39S en une quantité de 3 g/m2,
• et ayant alors été embouties.

In order to illustrate the invention, corrosion resistance tests were carried out according to the ISO 6270-2 standards of 2005 and / or VDA 230-213 of 2008 on metal sheets 1 covered with a metal coating 7 comprising about 99% zinc (sheet steel GI), or samples of electrogalvanized sheet steel 1 comprising 100% zinc (steel sheet EG), on which was applied:

• an aqueous solution of amino acid as defined above whose pH was possibly adjusted by adding H ₃ PO ₄ , then
• Fuchs® 3802-39S oil in an amount of 3 g / m2,
• and having then been stamped.

It appears that the sheets 1 obtained by a process according to the invention have a better resistance to corrosion. The other properties of the sheets 1 obtained by the process according to the invention (mechanical properties, compatibility with a subsequent stage (s) of cataphoresis and / or phosphatation and / or painting) have not been degraded.

Example 2: Tests for Measuring the Friction Coefficient (μ) as a Function of the Contact Pressure (MPa) and Tensile Tests for the Proline and Threonine Amino Acids 2.1. Traction tests

Tensile tests have been carried out and are described by way of non-limiting examples.

Samples of steel sheet 1 covered with a metal coating 7 comprising approximately 99% zinc (GI steel sheet), or samples of electrogalvanised steel sheet 1 comprising 100% zinc (EG sheet steel) were used.

Each specimen 27 was prepared as follows. Tabs 29 were cut from sheet 1 to be evaluated. These tongues 29 had dimensions of 25 mm × 12.5 mm × 0.2 mm.

Tabs 29 were immersed for an immersion time of 20 seconds at a temperature of 50 ° C. in an aqueous solution of proline or threonine whose pH had been adjusted by addition of H ₃ PO ₄ , with the exception of reference plates (Ref) having not been subjected to any treatment with an amino acid.

Fuchs® 3802-39S oil was applied to the tabs 29 in an amount of 3 g / m ² .

Two tabs 29 were glued by a seal 31 of BM1496V, BM1440G or BM1044 adhesive, which are epoxy-based "crash" adhesives marketed by Dow® Automotive. These adhesives have been selected because they are adhesives conventionally leading to adhesive fractures before aging and / or after aging of the adhesive.

The test piece 27 thus formed was then raised to 180 ° C. and held at this temperature for 30 minutes, which makes it possible to bake the adhesive.

Aging tests were carried out with the test pieces 27 whose tabs 29 were glued with BM1044 glue. The natural aging of the adhesive is simulated by aging in a wet poultice at 70 ° C for 7 or 14 days.

The tensile test was then carried out at an ambient temperature of 23 ° C. by imposing a tensile speed of 10 mm / min on one tongue 29, parallel to the latter, while the other tongue 29 of the test specimen It was fixed. The test was continued until the test piece 27 was broken.

At the end of the test, the maximum tensile stress was noted and the nature of the failure (cohesive failure, when the rupture occurred in the thickness of the adhesive - adhesive failure) was visually assessed rupture takes place at one of the interfaces between the sheet and the adhesive - superficial cohesive rupture, when the rupture takes place in the adhesive in the vicinity of an interface between the tongues and the sheet) (knowing that in the automotive industry, we try to avoid adhesive breaks that translate poor compatibility of the adhesive with the sheet).

In Table 1 are grouped the results on GI sheet.

In Table 2 are grouped the results on galvanized sheet metal (EG). SCR means superficial cohesive rupture.

As illustrated by Tables 1 and 2 below, the sheets 1 which have undergone treatment with an aqueous solution comprising proline or threonine promote the appearance of superficial cohesive fractures, in contrast to the reference sheets for which more than Adhesive fractures have been observed.

• avec l'adhésif BM1496V, les faciès de rupture observés sur les essais avec la proline ou la thréonine sont uniquement constitués de rupture cohésive superficielle, contrairement à la référence n'ayant pas subi de traitement (Ref 1) où il est constaté 30% de rupture adhésive.
• Avec l'adhésif BM1440G, les faciès de rupture observés sur les essais avec la proline ou la thréonine sont également uniquement constitués de rupture cohésive superficielle, contrairement à la référence n'ayant pas subi de traitement (Ref 2) où il est constaté 20% de rupture adhésive,
• Avec l'adhésif BM1044, il est observé que l'adhérence de l'adhésif sur les tôles avec la proline ou la thréonine (essais 7A à 7C) vieillit mieux que sur la référence, après 7 et 14 jours de cataplasme humide.

In particular, on GI sheets (Table 1):

• with the BM1496V adhesive, the fracture facies observed on the proline or threonine tests only consist of superficial cohesive rupture, unlike the non-treated reference (Ref 1) where 30% of adhesive break.
• With the BM1440G adhesive, the fracture facies observed on the proline or threonine tests also consist only of superficial cohesive rupture, unlike the non-treated reference (Ref 2) where it is found to be 20% adhesive break,
• With the BM1044 adhesive, it is observed that the adhesion of the adhesive on the plates with proline or threonine (tests 7A to 7C) is better than the reference, after 7 and 14 days of wet poultice.

In particular, on electrogalvanized sheets (Table 2), with the BM1496V adhesive, the fracture facies observed on tests 8A to 9B with proline or threonine mainly consist of superficial cohesive rupture, contrary to the reference not undergone treatment (Ref 6) where it is found 40% of adhesive rupture. Table 1: Maximum tensile stresses and nature of the fracture for the specimens based on tested GI sheets. No. Test Adhesive Amino acid Concentration g / L pH Aging (days) Maxi strain MPa Facies of rupture (RCS) 2A BM1496V L-Proline 20 4 N / A 17.8 100% 2B 50 16.8 100% 2C 100 15.1 100% 2D 150 14.4 100% 4A L-threonine 20 4 N / A 16.8 100% 4B 50 15.9 100% 4C 80 15 100% 4D 100 14.8 100% Ref 1 N / A N / A N / A N / A 17.9 70% 6 BM1440G L-Proline 50 natural N / A 14.5 100% Ref 2 N / A N / A N / A N / A 14.9 80% 7A BM1044 L-Proline 50 natural N / A 10.6 100% 7B 7 11.5 100% 7C 14 11.3 90% Ref 3 N / A N / A N / A N / A 11.8 100% Ref 4 7 12 80% Ref 5 14 11.5 60% No. Test Adhesive Amino acid Concentration (g / L) pH Aging (days) Max. Stress (Mpa) Facies of rupture (RCS) 9A BM1496V L-Proline 20 natural N / A 12.2 95% 9B 50 10 100% Ref 6 N / A N / A N / A N / A 14.6 60%

2.2. Measurements of the coefficient of friction (μ) as a function of the contact pressure (MPa)

Tests for measuring the coefficient of friction (μ) as a function of the contact pressure (MPa) have been carried out and are described by way of non-limiting examples.

Samples of steel sheet 1 covered with a metal coating 7 comprising approximately 99% zinc (DX56D grade steel sheet, thickness 0.7 mm), samples of electrogalvanised steel sheet 1 whose coating included 100% of Zinc (EG steel sheet grade DC06, thickness 0.8 mm), electro-galvanized Fortiform® steel sheet 1 samples with 100% zinc coating (7.5 μm on both sides) or sheet metal samples 1 steel sonic vapor deposition (Zn JVD) coated with 100% zinc (7.5 μm on both sides) were used.

Samples having dimensions of 450 mm × 35 mm × thickness (0.7 mm for GI and 0.8 mm for EG) were cut from these steel sheets. The samples were immersed for an immersion time of 20 seconds at a temperature of 50 ° C. in an aqueous solution of proline or threonine whose pH had possibly been adjusted by addition of H ₃ PO ₄ . Fuchs® 3802-39S (in an amount of 3 g / m ² ), Fuchs® 4107S (reject) or QUAKER 6130 (reject) were applied to one side of the samples.

• sur l'échantillon de la tôle traité par la solution aqueuse de proline ou de thréonine ainsi préparé, et
• sur un échantillon de tôle revêtue non traitée par un aminoacide (témoin).

The coefficient of friction (μ) was then measured as a function of the contact pressure (MPa) by varying the contact pressure from 0 to 80 MPa:

• on the sample of the sheet treated with the aqueous solution of proline or threonine thus prepared, and
• on a sample of coated sheet untreated with an amino acid (control).

Several test phases were performed (phases A, B, and C in Table 3 below).

• de réduire le coefficient de frottement par rapport à une tôle revêtue non traitée par une telle solution (témoin), et/ou
• d'éviter un frottement pas à-coups ou broutage (« stick slip » en anglais), alors qu'à certaines pressions, un broutage est observé pour une tôle revêtue non traitée par une telle solution (témoin),
• de conserver les propriétés tribologiques améliorées, même lorsque la tôle revêtue traitée a subi un traitement de lavage/rehuilage.

Tableau 3 : Propriétés tribologiques (Observation d'un broutage et coefficient de friction (µ) en fonction de la pression exercée) pour les échantillons de tôles testés. Tôle Huile Solution aqueuse appliquée Pression (MPa) à laquelle un broutage est observé Coefficient de friction (µ) Aminoacide (nature) Concentration en aminoacide (g/L) pH de la solution aqueuse à 40 MPa à 60 MPa à 80 MPa GI Fuchs® 3802-39S A Aucun(témoin) NA NA 21 0,180 0,190 0,200 Proline 50 6,3 NA 0,145 0,160 0,150 100 6,3 NA 0,120 0,120 0,105 150 6,3 NA 0,110 0,105 0,105 Thréonine 20 5,6 NA 0,130 0,155 0,140 50 5,6 NA 0,110 0,110 0,100 80 5,6 NA 0,110 0,100 0,090 100 5,6 NA 0,115 0,110 0,100 GI C Aucun(témoin) NA NA 18 0,18 0,19 0,17 Proline 80 4,0* NA 0,13 0,13 0,12 Proline** 80 4,0* NA 0,14 0,14 0,13 EG DC06 Aucun(témoin) NA NA 43 0,170 0,200 0,190 Proline 50 Naturel NA 0,120 0,120 0,120 Thréonine 20 naturel NA 0,125 0,125 0,110 EG DC06 Quaker Aucun(témoin) NA NA 18 0,19 0,16 0,14 Proline 70 naturel NA 0,15 0,12 0,11 Fortiform Aucun(témoin) NA NA NA 0,18 0,15 0,13 Proline 70 naturel NA 0,13 0,12 0,11 Zn JVD Fuchs® 4107S A Aucun(témoin) NA NA NA 0,25 0,22 0,18 Proline 10 naturel NA 0,24 0,20 0,17 Proline 20 naturel NA 0,20 0,17 0,14 B Aucun(témoin) NA NA NA 0,27 0,23 0,20 Proline 10 naturel NA 0,24 0,20 0,17 Proline 20 naturel NA 0,20 0,17 0,14 Proline 70 naturel NA 0,14 0,12 0,10 Quaker A Aucun(témoin) NA NA NA 0,26 0,23 0,20 Proline 10 naturel NA 0,25 0,20 0,18 Proline 20 naturel NA 0,20 0,17 0,15 B Aucun(témoin) NA NA NA 0,26 0,23 0,20 Proline 10 naturel NA 0,25 0,20 0,18 Proline 20 naturel NA 0,20 0,17 0,15 Proline 70 naturel NA 0,14 0,12 0,10 EG : substrat électrozingué
* : pH ajusté par ajout de H₃PO₄
** : test après avoir subi un traitement de lavage/rehuilage As illustrated by Table 3 below, it has been observed that the application of an aqueous solution of proline or threonine enables:

• to reduce the coefficient of friction with respect to a coated sheet untreated by such a solution (control), and / or
• to avoid a friction not jerking or grazing ("stick slip" in English), while at certain pressures, grazing is observed for a coated sheet untreated by such a solution (control),
• to preserve the improved tribological properties, even when the treated coated sheet has undergone a washing / re-oiling treatment.

Table 3: Tribological properties (Observation of a chatter and coefficient of friction (μ) as a function of the pressure exerted) for the tested sheet samples. sheet metal Oil Applied aqueous solution Pressure (MPa) at which grazing is observed Coefficient of friction (μ) Amino acid (nature) Amino acid concentration (g / L) pH of the aqueous solution at 40 MPa at 60 MPa at 80 MPa GI Fuchs® 3802-39S AT None (Control) N / A N / A 21 0,180 0,190 0,200 proline 50 6.3 N / A 0,145 0,160 0,150 100 6.3 N / A 0,120 0,120 0.105 150 6.3 N / A 0.110 0.105 0.105 threonine 20 5.6 N / A 0,130 0.155 0,140 50 5.6 N / A 0.110 0.110 0,100 80 5.6 N / A 0.110 0,100 0.090 100 5.6 N / A 0.115 0.110 0,100 GI C None (Control) N / A N / A 18 0.18 0.19 0.17 proline 80 4.0 * N / A 0.13 0.13 0.12 proline ** 80 4.0 * N / A 0.14 0.14 0.13 EG DC06 None (Control) N / A N / A 43 0,170 0,200 0,190 proline 50 Natural N / A 0,120 0,120 0,120 threonine 20 natural N / A 0,125 0,125 0.110 EG DC06 Quaker None (Control) N / A N / A 18 0.19 0.16 0.14 proline 70 natural N / A 0.15 0.12 0.11 Fortiform None (Control) N / A N / A N / A 0.18 0.15 0.13 proline 70 natural N / A 0.13 0.12 0.11 Zn JVD Fuchs® 4107S AT None (Control) N / A N / A N / A 0.25 0.22 0.18 proline 10 natural N / A 0.24 0.20 0.17 proline 20 natural N / A 0.20 0.17 0.14 B None (Control) N / A N / A N / A 0.27 0.23 0.20 proline 10 natural N / A 0.24 0.20 0.17 proline 20 natural N / A 0.20 0.17 0.14 proline 70 natural N / A 0.14 0.12 0.10 Quaker AT None (Control) N / A N / A N / A 0.26 0.23 0.20 proline 10 natural N / A 0.25 0.20 0.18 proline 20 natural N / A 0.20 0.17 0.15 B None (Control) N / A N / A N / A 0.26 0.23 0.20 proline 10 natural N / A 0.25 0.20 0.18 proline 20 natural N / A 0.20 0.17 0.15 proline 70 natural N / A 0.14 0.12 0.10 EG: electrozinced substrate
*: pH adjusted by addition of H ₃ PO ₄
**: test after having undergone a washing / re-oiling treatment

Claims (19)

1. A method for preparing a metal sheet (1) comprising at least the steps of:

   - providing a steel substrate (3), at least one face (5) of which is coated with a metal coating (7) comprising at least 40% by weight of zinc,

   - applying on the outer surface (15) of the metal coating (7) an aqueous solution comprising an amino acid selected from among alanine, arginine, aspartic acid, cysteine, glutamine, lysine, methionine, proline, serine, threonine, and a mixture thereof, each amino acid being in a neutral or salt form,

   the aqueous solution being free of any compound comprising a metal from the group IIIB or from the group IVB, and
   the mass percentage as a dry extract of the amino acid in neutral or salt form or of the mixture of amino acids in the neutral or salt forms in the aqueous solution being greater than or equal to 75%, preferably greater than or equal to 90%..
2. The method according to claim 1, comprising a preliminary step for preparing the steel substrate (3), at least one face (5) of which is coated with a metal coating (7), selected from among hot galvanization, a sonic vapor jet deposition and an electro-zinc-plating of the steel substrate (3).
3. The method according to any of claims 1 to 2, wherein the metal coating (7) is selected from among a zinc coating GI, a zinc coating GA, a zinc and aluminum alloy, a zinc and magnesium alloy and a zinc, magnesium and aluminum alloy, preferably the metal coating (7) is a zinc and magnesium alloy comprising between 0.1 and 10% by weight of Mg and optionally between 0.1 and 20% by weight of Al, the remainder of the metal coating being Zn, inevitable impurities and optionally one or several added elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi.
4. The method according to one of the preceding claims, wherein the amino acid is selected from among alanine, aspartic acid, cysteine, glutamine, methionine, proline, serine, threonine, and a mixture thereof, each amino acid being in a neutral or salt form, preferably the amino acid is selected from among proline in the neutral or salt form, cysteine in the neutral or salt form, and a mixture thereof, preferably proline in a neutral or salt form.
5. The method according to any of claims 1, 2 or 4, wherein the steel substrate (3), at least one face (5) of which is coated with a metal coating (7) was prepared by electro-zinc-plating and the amino acid is selected from among aspartic acid, cysteine, methionine, proline and threonine, and a mixture thereof, each amino acid being in the neutral or salt form.
6. The method according to any of claims 1 to 4, wherein the steel substrate (3) at least one face (5) of which is coated with a metal coating (7) was prepared by hot galvanization and the amino acid is selected from among alanine, arginine, glutamine, lysine, methionine, proline, serine, threonine and a mixture thereof, each amino acid being in a neutral or salt form.
7. The method according to any of claims 1 to 6, wherein the amino acid is threonine in the neutral or salt form or a mixture of proline and of threonine, the proline and the threonine being in the neutral or salt form.
8. The method according to one of the preceding claims, wherein the aqueous solution comprises from 1 to 200 g/L of amino acid in the neutral or salt form or of a mixture of amino acids in the neutral or salt forms, or from 10 to 1,750 mmol/L of amino acid in the neutral or salt form or of a mixture of amino acids in the neutral or salt forms.
9. The method according to one of the preceding claims, wherein the aqueous solution has a pH comprised between a pH equal to [isoelectric point of the amino acid -3] and a pH equal to the [isoelectric point of the amino acid +1], preferably comprised between a pH equal to the [isoelectric point of the amino acid -3] and a pH equal to the [isoelectric point of the amino acid -1].
10. The method according to one of the preceding claims, wherein the aqueous solution is applied at a temperature comprised between 20 and 70°C and/or wherein the solution is applied for a period comprised between 0.5s and 40s on the outer surface (15) of the metal coating (7).
11. The method according to one of the preceding claims, wherein the solution is applied by coating with a roller.
12. The method according to one of the preceding claims, comprising, after the step for applying on the outer surface (15) of the metal coating (7) an aqueous solution comprising an amino acid, a drying step, preferably carried out by subjecting the metal sheet (1) to a temperature comprised between 70 and 120°C for 1 to 30 seconds.
13. The method according to one of the preceding claims, comprising, after the step for applying on the outer surface (15) of the metal coating (7) an aqueous solution comprising an amino acid and the optional drying step, a step for applying a grease or oil film on the outer surface (15) of the coating (7) coated with a layer comprising an amino acid or a mixture of amino acids.
14. The method according to one of the preceding claims, comprising, after the step for applying on the outer surface (15) of the metal coating (7) an aqueous solution comprising an amino acid, the optional drying step and the optional step for applying a grease or oil film, a step for shaping the metal sheet (1), preferably achieved by drawing.
15. A metal sheet (1) obtainable by a method according to any of claims 1 to 14.
16. The metal sheet (1) according to the preceding claim, for which at least one portion of at least one outer surface (15) of the metal coating (7) is coated with a layer comprising from 0.1 to 200 mg/m² of amino acid in the neutral or salt form or of a mixture of amino acids in the neutral or salt forms.
17. The metal sheet (1) according to claim 15 or 16, for which at least one portion of at least one outer surface (15) of the metal coating (7) is coated with a layer comprising from 75 to 100% by weight of amino acid in the neutral or salt form, or of a mixture of amino acids in the neutral or salt forms.
18. Use of an aqueous solution comprising an amino acid selected from among alanine, arginine, aspartic acid, cysteine, glutamine, lysine, methionine, proline, serine, threonine, and a mixture thereof, each amino acid being in the neutral or salt form,
    the aqueous solution being free of any compound comprising a metal from the group IIIB or from the group IVB, and
    the mass percentage in dry extract of the amino acid in the neutral or salt form or of the mixture of amino acids in the neutral or salt forms in the aqueous solution being greater than or equal to 75%,
    for improving the resistance to corrosion of an outer surface (15) of a metal coating (7) coating at least one face (5) of a steel substrate (3), wherein the metal coating (7) comprises at least 40% by weight of zinc.
19. Use of an aqueous solution comprising an amino acid selected from among proline, threonine and a mixture thereof, the proline and the threonine being independently in a neutral or salt form, the aqueous solution being free of any compound comprising a metal from the group IIIB or from the group IVB, for:

    - improving the compatibility, with an adhesive 13, of at least one portion of an outer surface (15) of a metal coating (7) coating at least one face (5) of a steel substrate (3),

    - improving the resistance to corrosion of the outer surface (15) of the metal coating (7) coating at least one face (5) of the steel substrate (3), and

    - improving the tribological properties of the outer surface (15) of the metal coating (7) coating at least one face (5) of the steel substrate (3),

    wherein the metal coating (7) comprises at least 40% by weight of zinc.

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