Method for the preparation 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

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

providing a steel substrate, at least one face of which is coated with a metal coating comprising at least 40% by weight of zinc,

applying on the outer surface of the metal coating an aqueous solution consisting of a mixture of:

water,

an amino acid selected from the group consisting of 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, and mixtures of said amino acids, each amino acid in the mixture independently in a neutral or salt form, and

optionally an acid, or a mixture of acids,

the aqueous solution being free of any compound comprising a metal from the group IIIB or from the group IVB and comprising less than 0.005 g/L of iron ions, 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%.

2. The method according to claim 1 , comprising a preliminary step for preparing the steel substrate, at least one face of which is coated with a metal coating, selected from among hot galvanization, a sonic vapor jet deposition and an electro-zinc-plating of the steel substrate.

3. The method according to claim 1 , wherein the metal coating 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.

4. The method according to claim 3 , wherein the metal coating 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.

5. The method according to claim 1 , wherein the amino acid is selected from the group consisting of alanine, aspartic acid, cysteine, glutamine, methionine, proline, serine, threonine, and a mixture thereof, each amino acid being in a neutral or salt form.

6. The method according to claim 1 , wherein the steel substrate, at least one face of which is coated with a metal coating was prepared by electro-zinc-plating and the amino acid is selected from the group consisting of aspartic acid, cysteine, methionine, proline and threonine, and a mixture thereof, each amino acid being in the neutral or salt form.

7. The method according to claim 1 , wherein the steel substrate at least one face of which is coated with a metal coating was prepared by hot galvanization and the amino acid is selected from the group consisting of alanine, arginine, glutamine, lysine, methionine, proline, serine, threonine and a mixture thereof, each amino acid being in a neutral or salt form.

8. The method according to claim 1 , wherein the amino acid is proline in the neutral or salt form, 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.

9. The method according to claim 1 , wherein the aqueous solution comprises from 1 to 200 g/L of the amino acid in the neutral or salt form or of a mixture of the amino acids in the neutral or salt forms or from 10 to 1,750 mmol/L of the amino acid in the neutral or salt form or of a mixture of the amino acids in the neutral or salt forms.

10. The method according to claim 1 , wherein 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 is greater than or equal to 90%.

11. The method according to claim 1 , 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].

12. The method according to claim 1 , wherein the aqueous solution is applied at a temperature comprised between 20 and 70° C. and/or for a period comprised between 0.5 s and 40 s on the outer surface of the metal coating.

13. The method according to claim 12 wherein the acid is phosphoric acid in a neutral form and in a salt form, or a mixture thereof.

14. The method according to claim 1 , wherein the solution is applied by coating with a roller.

15. The method according to claim 1 , further comprising, after the step of applying on the outer surface of the metal coating the aqueous solution, a drying step.

16. The method according to claim 1 , further comprising, after the step of applying on the outer surface of the metal coating the aqueous solution and an optional drying step, a step of applying a grease or oil film on the outer surface of the coating coated with a layer consisting of the amino acid or the mixture of said amino acids and optionally the acid or the mixture of acids.

17. The method according to claim 1 , further comprising, after the step of applying on the outer surface of the metal coating the aqueous solution, an optional drying step and an optional step of applying a grease or oil film, a step of shaping the metal sheet by drawing.

18. A method for improving the resistance to corrosion of an outer surface of a metal coating at least one face of a steel substrate, comprising at least the steps of:

providing a steel substrate having two faces, at least one of which is coated with a metal coating comprising at least 40% by weight of zinc,

application on the outer surface of the metal coating of an aqueous solution consisting of

water,

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, and

optionally an acid, or a mixture of acids,

the aqueous solution being free of any compound comprising a metal from the group IIIB or from the group IVB and comprising less than 0.005 g/L of iron ions, 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%.

19. A method for preparing a metal sheet comprising at least the steps of:

providing a steel substrate, at least one face of which is coated with a metal coating comprising at least 40% by weight of zinc,

applying on the outer surface of the metal coating an aqueous solution consisting of a mixture of:

water,

an amino acid selected from the group consisting of 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, and mixtures of said amino acids, each amino acid in the mixture independently in a neutral or salt form, and

optionally a base, a mixture of bases, an acid, or a mixture of acids,

the aqueous solution being free of any compound comprising a metal from the group IIIB or from the group IVB and comprising less than 0.005 g/L of iron ions, 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%,

upon drying, obtaining the metal sheet including a layer consisting of the amino or the mixture of said amino acids and optionally the base, the mixture of bases, the acid, or the mixture of acids, and

performing at least one of the following on the layer consisting of the amino acid or the mixture of said amino acids and optionally the base, the mixture of bases, the acid, or the mixture of acids:

degreasing the metal sheet including the layer;

shaping by drawing the metal sheet including the layer;

cutting the metal sheet including the layer;

applying a film of grease or oil onto the layer;

applying an adhesive based on epoxy onto the layer;

phosphating the metal sheet including the layer; or

performing a cataphoresis on the metal sheet including the layer.