EP3947777A1

EP3947777A1 - Method for reconditioning hot-dip galvanized surfaces

Info

Publication number: EP3947777A1
Application number: EP20714977.4A
Authority: EP
Inventors: Fabian JUNGE; Jennifer Schulz; Tobias LEWE; Burak William Cetinkaya
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2019-03-27
Filing date: 2020-03-24
Publication date: 2022-02-09
Also published as: DE102019204224A1; WO2020193548A1

Abstract

The invention relates to a method for at least partially removing a eutectic phase from a metallic zinc-magnesium-based protective layer, and to a flat steel product the surface of which has regions where eutectic phase and/or magnesium has/have been removed.

Description

Process for reconditioning hot-dip galvanized surfaces

The invention relates to a method for at least regionally removing a eutectic phase from a metallic protective layer based on zinc-magnesium and a flat steel product which has eutectic-free and / or magnesium-free areas on the surface.

The addition of various alloying elements when applying a metallic protective layer based on zinc has a different influence on the chemical composition of the immediate surface of the zinc coating, depending on the element. The composition of the areas of the protective layer close to the surface can change during aging due to oxidation processes. Such a change in the surface chemistry also influences the further processing steps such as pretreatment, gluing, phosphating and / or painting. In automotive engineering in particular, the processes are designed in such a way that the greatest possible number of material and surface concepts can be implemented in one process.

In the further processing of aged material or surface concepts, it can happen that these are not optimally covered by the existing process window, due to the newly created near-surface chemistry. In turn, this can have a negative effect on properties such as paint adhesion or the fracture behavior of bonded surfaces, so that these materials or surfaces can no longer be used or the existing process window would have to be laboriously adjusted.The further aging process after further processing steps can also lead to a deterioration in the surface properties to lead.

From EP 2824213A1 a method for improving the adhesion on a steel sheet provided with a protective coating based on Zn-Al-Mg is known, in which the natural oxide layer containing Al 2 O 3 and MgO is modified by applying an aqueous composition based on sodium fluoride, without picking them up.

US 2015125714A describes a method for producing a metal sheet, both sides of which have a metal coating containing zinc, 0.1-20 wt% aluminum and 0.1-10 wt% magnesium. For this purpose, the substrate is coated in an immersion bath and after cooling down, the layers of magnesium oxide or magnesium hydroxide are deposited have formed a metal coating on the outer surface by applying an acid solution to the outer surfaces and / or by applying mechanical forces using a roll straightener, a brushing device or a sandblasting device. A layer of oil is then applied to the outer surface of the metal coating.

US 2015/0352825 A1 also describes a method for producing metal sheets with a coating based on Zn-Al-Mg, which should have improved compatibility with adhesion promoters or adhesives by using a Zn-Al-Mg -coated first metal sheet is treated with an acidic solution, which has a pH of 1 to 4, and then an adhesion promoter or adhesive is applied to the coating treated with the acidic solution. A second metal sheet is then joined to the first metal sheet via the adhesion promoter or adhesive.

It is therefore known that metal coatings, in particular hot-dip galvanized surfaces, have alloying elements in the area close to the surface which have a disruptive effect on properties such as, for example, the breaking behavior of bonded surfaces or paint adhesion. In the case of ZM (zinc-magnesium) coatings, a process-related formation of a magnesium-rich oxide layer is responsible for this. Such an oxide layer interferes with further process steps.

There is therefore a need to modify surfaces of protective layers based on ZM (zinc-magnesia) and to improve them with regard to subsequent processes so that they have a corresponding profile of properties, which compared to a control or reference, an improved pretreatment, bonding , Cleaning, phosphating and / or painting are possible.

It is an object of the present invention to provide the above-described surfaces and a method for their production. It was essential that such a method should not only be used during or immediately after the production of a protective layer based on cement on a substrate. Rather, the same method should also be applicable to substrates that have already been produced and stored with such a protective layer, in other words, the method should eliminate negative effects of the aging process. The method should also meet the EHS requirements (Environment, Health and Safety) and be feasible by means of known and already available devices. These objects are achieved by the method according to the invention and the flat steel product according to the invention with the features according to the claims.

It was found that not only the oxide layer close to the surface can lead to a negative property profile up to a depth (determined from above) of up to approx. 100 nm, but also the components of the protective layer located underneath can, especially after aging, be an obstacle to subsequent processes and for achieving the corresponding process and surface-relevant properties.

For the purposes of the invention, the term “top” relates to the surface of the metallic protective layer and the term “bottom” relates to the substrate, which is accordingly arranged below the protective layer.

During the solidification of zinc-rich ZM melts, binary (zinc-magnesium) or ternary (zinc-aluminum-magnesium) eutectic phases develop locally to varying degrees between, above and below the primarily precipitated zinc grains. These eutectic phases are composed of the eutectic and pure metals, i.e. H. In addition to the eutectic, these eutectic phases also have (secondary) zinc grains and possibly Al grains. These secondary zinc grains and possibly Al grains are not to be confused with the zinc grains that are primarily separated (primary zinc grains), since they have a volume that is several orders of magnitude smaller than the volume of the primary zinc grains. While the primary zinc grains have a diameter of in some cases over 30 μm, the diameter of the secondary zinc grains located in the eutectic phases is up to 2 μm. Furthermore, these secondary grains are excreted before or after the eutectic. In the literature, the eutectic phases described above are referred to as hypo- or hyper-eutectic phases, depending on whether they are eliminated before or after the eutectic. The layer structure of a ZM coating shows an enrichment of eutectic phases that are not surface-covering but are distributed over the entire sample and are arranged over the zinc grains (and possibly also in the zinc grains). The eutectic and eutectic phases are magnesium-rich phases, for example in comparison to the primary tin grains.

The aging of the cementitious coatings when stored in air or in oxygen-containing atmospheres can result in a change in the chemical composition of the layers close to the surface, and thus also lead to a growth of the oxide layers in and on the metallic protective layer. The growth of these oxide layers is with the penetration of Oxygen is connected in the eutectic phases of the protective layer and leads to an additional worsening of further processing. Surprisingly, there is a preferred penetration of oxygen from the surface into and along the magnesium-rich phases, so that these are preferentially oxidized. Elementary metal atoms from the eutectic and the eutectic phases are oxidized to oxides and / or hydroxides or similar compounds; these are generally summarized below under the term metal oxides. In the corresponding or in the same period of time, zinc-rich and / or aluminum-rich phases are hardly oxidized. The metal oxides are arranged in layers close to the surface, i.e. in the upper layers of the protective layer, in particular at a depth of up to 75 nm.

In one embodiment, the present invention relates to a method for at least regionally removing a eutectic phase from a metallic protective layer based on zinc-magnesium comprising at least or consisting of the following steps, a) providing a substrate with a metallic protective layer; b) at least regionally removing at least one eutectic phase close to the surface by wetting the surface with an inorganic acid; c) exposing the upper surface of at least one zinc grain of the metallic protective layer; d) Ending the wetting as soon as the surface of the at least one zinc grain exposed in c) has essentially no eutectic phase.

The term “regionally” is to be interpreted three-dimensionally in the context of the present invention. On the one hand, of course, only certain, precisely defined areas of the surface can be wetted. It is true that these are two-dimensional areas; The wetting, however, acts in layers of the metallic protective layer close to the surface, i.e. in the depth and also in the third dimension. Due to the wetting with an inorganic acid, deeper layers of the metallic protective layer are exposed, until finally the upper surface of at least one (primary) zinc grain of the metallic layer is exposed. At least one eutectic phase close to the surface is removed in areas to a depth of at least 100 nm, preferably 120 nm, 125 nm, 150 nm, 175 nm, 200 nm, 250 nm, 300 nm, particularly preferably 400 nm, in particular 500 nm up to maximum 2 pm, before particularly 1.8 pm, 1.75 pm, 1.7 pm, particularly preferably 1.6 pm, 1.25 pm, in particular 1 mhh, in relation to the surface of an undressed protective layer. The eutectic phase arranged over the (primary) zinc grains is preferably removed. This can be retained between and below the (primary) zinc grains, provided that it is present before wetting. The same applies to the eutectic.

In the context of the invention, the depth of a layer is always determined from the top atom of the respective surface. Furthermore, the terms “metallic protective layer based on zinc-magnesium”, “metallic protective layer”, “protective layer” and “ZM coating” are equivalent according to the invention and are used synonymously.

In the context of the invention, the term “essentially no eutectic phase” means that the exposed surface of the primary zinc grains only accounts for 40%, preferably 30%, 25%, particularly preferably 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 1 1%, in particular 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1% or 0.5%, 0, 1% of eutectic and eutectic phase or is only covered with the aforementioned portion with eutectic and eutectic phase. The remaining and larger proportion of the exposed surface, i.e. well over 50%, is formed by zinc or zinc oxide or hydroxide (for the term zinc oxide, see the definition of metal oxide given above).

The exposed surface relates to a layer which is close to the surface in relation to the primary zinc grain and has at least one atomic layer to a depth of 99 nm, preferably 75 nm.

Correspondingly, the detection of a surface of the primary zinc grain essentially free of eutectic phase is determined by at least one of the following methods: SEM images (scanning electron microscope SEM) of a cross section, EDX mapping (energy dispersive X-ray spectroscopy, English energy dispersive X-ray spectroscopy), AES (Auger electron spectroscopy), XPS measurement, GD-OES measurement or ToF-SIMS measurement.

The same statements apply to the term “essentially no magnesium”. In an alternative, a relative magnesium concentration of a maximum of 20%, preferably 15%, particularly preferably 10%, 8%, is used in this regard on the entire exposed surface of the protective layer, i.e. not just on the exposed surface of the at least one primary zinc grain. 7%, in particular 5%, 4%, 3%, 2%, one percent or less, in an alternative with a lower limit of the magnesium concentration below the detection limit, or a lower limit of a maximum of 0.001%, 0.01% or 0, 1 %. The determination is made in an alternative by means of XPS measurement, GD-OES measurement or ToF-SIMS measurement. The relative concentration of zinc, magnesium and possibly aluminum is determined by determining the absolute concentration of these elements and then normalizing to 100%; the sum of the concentration of zinc, magnesium and possibly aluminum is set equal to 100 and the proportion of the respective element in this 100% is evaluated or weighted as a relative concentration, i.e. based on 100%. The relative concentration of an element (Al, Mg, Zn) therefore refers to the sum of the concentrations of the elements Mg, Zn and possibly Al, in that this sum represents 100%.

Since the absolute concentration of the elements Zn, Mg and possibly Al can vary from coating to coating, according to the invention the information for the method to be generally used is given as a relative concentration and in percentage points in order to precisely define changes.

The occurrence of the elements zinc, magnesium and aluminum within the meaning of the invention is recorded regardless of the form in which they are present, so it does not matter whether these elements are neutral atoms or ions, in a compound such as an alloy or inter-metallic phases or in a compound such as complexes, oxides, salts, hydroxides or the like. Thus, the terms “zinc”, “aluminum” and “magnesium” in the context of the invention can encompass not only the elements in pure form, but also oxidic and / or hydroxidic or any form of compounds that contain these elements.

In one embodiment, the substrate is a flat steel product. Flat steel products are understood here as rolled products, the length and width of which are each significantly greater than their thickness. Thus, when a flat steel product or a "sheet metal product" is mentioned below, this means rolled products such as steel strips or sheets which are cut off or blanks for the production of, for example, body parts. "Sheet metal parts" or "sheet metal parts" are made from such flat or sheet steel products, the terms "sheet metal part" and "sheet metal part" being used synonymously here.

Another embodiment relates to a method in which the metallic protective layer is a Zn-Al-Mg coating. Accordingly, the term ZM coating in an alternative also includes a Zn-Al-Mg coating.

Furthermore, in one embodiment, the metallic protective layer is produced by hot-dip coating. A coating based on zinc and magnesium is present on the substrate used according to the invention. In addition to Zn, the coating according to the invention preferably contains 0.1 to 3.0% by weight of Mg, preferably 0.6 to 2.0% by weight of Mg, and unavoidable impurities 0.1 to 3.0% by weight. Al, preferably 0.5 to 2.5% by weight of Al, particularly preferably 1.0 to 2.0% by weight of Al. According to the invention, the coating, in particular made of zinc or a zinc alloy, is preferably at a coating weight of at least 1 to a maximum of 600 g / m2, ie at least 0.5 to a maximum of 300 g / m2 per side, particularly preferably at least 20 to a maximum of 300 g / m2, i.e. at least 10 to a maximum of 150 g / m2 per side.

In a further embodiment, metal oxide layers of the metallic protective layer that are close to the surface are also removed in step b).

Furthermore, in one embodiment in step c) of the method according to the invention, an upper area of at least one zinc grain is also removed so that an exposed surface of the zinc grain is formed that is essentially free of eutectic and eutectic phase and possibly magnesium.

Accordingly, in one embodiment, the at least one eutectic phase is removed from the upper surface of at least one (primary) zinc grain or from at least one (primary) zinc grain.

In one embodiment, the method is characterized in that an inorganic acid selected from the group containing or consisting of: H2S04, HCl, HN03, H3P04, H2S03, HN02, H3P03, HF, or a mixture of 2 or more of these acids as aqueous Solution is used.

In a further embodiment, an aqueous solution of an inorganic acid with a pH between 0.01 and 2, preferably at least 0.01; particularly preferably at least 0.1 and at most 1.8, preferably at most 1.7, in particular a pH value less than 1.0 is used.

The invention also relates to an embodiment in which the metallic protective layer is wetted with the aqueous solution of an inorganic acid for a time of 0.5-600 seconds and / or at a temperature of 10 ° C. to 90 ° C.

In one embodiment, the protective layer is applied for a time of at least 0.5 seconds, preferably at least 1 second, particularly preferably at least 2, 3, 4 seconds, in particular at least 5 seconds, and a maximum of 600 seconds, preferably a maximum of 300 seconds, particularly preferably a maximum of 180 seconds, 120 seconds, 60 seconds, in particular a maximum of 50 seconds, 40 seconds, 30 seconds, 20 seconds, 10 seconds, 5 seconds with wetted by the inorganic acid.

In a further embodiment, the coated substrate is wetted with the inorganic acid at a temperature of 10 ° C to 90 ° C, 20 ° C to 70 ° C, preferably 20 ° C to 50 ° C, particularly preferably 20 ° C to 40 ° C, especially 10 ° C to 30 ° C, 20 ° C to 30 ° C. The coated substrate is wetted with the inorganic acid continuously in an alternative. For this purpose, the inorganic acid is applied to the coated substrate by a method selected from the group or consisting of spraying, spraying, dipping and coil coating methods. In a further alternative, the coated substrate is wetted with the inorganic acid in batches, for example by a method selected from the group or consisting of spraying, spraying and dipping.

Wetting by conventional spray application proves to be particularly effective. By using spray systems, a particularly high productivity can be achieved in a manner known per se. In diving systems, however, can be produced particularly inexpensively in an equally known manner.

In one embodiment, wetting is ended in step d) by rinsing with water or an aqueous solution. For this purpose, wetting with the inorganic acid is interrupted by rinsing with water and / or an alcohol, preferably selected from the group containing or consisting of methanol, ethanol, propanol, isopropanol, ethanol, preferably isopropanol or an aqueous solution. In an alternative, rinsing takes place in 2 sub-steps, in a first sub-step with water; in a second sub-step with an alcohol or an aqueous solution of an alcohol as indicated above. In another alternative, rinsing with water and an alcohol takes place in one step, preferably as a mixture of water with one of the alcohols specified above. In an alternative, step d) is also carried out continuously, a process selected from the group or consisting of spraying, spraying, dipping and coil coating processes being used. In a further alternative, step d) takes place batch-wise, a method selected from the group or consisting of spraying, spraying and being used.

In an alternative, the protective layer is used for a time of 0.5-600 seconds, preferably 1-300 seconds, 1-180 seconds, particularly preferably 1-120 seconds, 1-60 seconds, in particular 5-60 seconds, 10-50 seconds , 20 - 40 seconds, 5 - 30 seconds with Water or an aqueous solution is brought into contact. In a further alternative, the protective layer is dried by increasing the temperature (up to a maximum of 100 ° C) or by using a blower. In a further alternative, the protective layer is air-dried without any further aids. Another alternative is to dry the protective layer by reducing the pressure.

A further embodiment relates to the method according to the invention, characterized in that after wetting and / or termination of wetting, the protective layer is brought into contact with air or an oxygen-containing atmosphere in a further substep.

In one embodiment, the coated substrate is dried in this step, preferably by blowing in air, preferably with air at a temperature below room temperature.

In a further embodiment of the present invention, the coated substrates are degreased with alkaline cleaning agents before they are wetted with the inorganic acid.

In one embodiment, the substrate provided with the metallic protective layer is subjected to a dressing step prior to wetting with the aqueous solution of an inorganic acid.

Furthermore, in one embodiment, the exposed surface with at least one (primary) zinc grain has essentially no magnesium or magnesium oxide (for the definition of metal oxides, including magnesium oxide, see above).

Even if reference is made above to at least one (primary) zinc grain, in one embodiment the method according to the invention with its features and effects achieved, i.e. in particular the exposed surface of the (primary) zinc grains, relates to at least 40%, preferably 50%, 60%, particularly preferably 70%, 75%, 80%, in particular 85%, 90% and more of the zinc grains arranged in a previously defined area.

The present invention also relates to a method for activating or reactivating the surface of a cementitious protective layer, in which the above-mentioned steps are carried out. Using the method according to the invention, aged ZM protective layers are restored or activated or reactivated for further treatment. In comparison to a control or reference, less oxygen diffuses in the same time interval into the layers near the surface of the substrates treated according to the invention because it is rich in magnesium ok

Phases have been removed. This ensures long-term stable adhesive bonds between cementitious protective layer and adhesive or paint.

Another object of the present invention is a method for increasing the cohesive fracture surface portion and optionally the tensile shear strength of a disposition or connection of a substrate having a cementitious coating with a polymer layer by removing at least one near-surface eutectic phase by wetting the surface with a inorganic acid (as described above), optionally subsequent rinsing and / or drying (as described above), application, optionally curing of the polymer layer, the fracture surface fractions being determined in a tensile shear test.

The present invention also relates to a method for preventing or reducing the corrosion of a metallic protective layer in relation to a reference, by using an inorganic acid as described above, in particular after storage of the coated substrates, i.e. after aging of the protective layer.

The subject matter is also the use of an inorganic acid as described above to increase the cohesive fracture surface fraction and, if necessary, the tensile shear strength of a material or connection of a substrate having a protective coating with an organic polymer layer by removing at least one eutectic phase close to the surface by wetting the surface with an inorganic acid ( as described above), optionally subsequent rinsing and / or drying (as described above), application, optionally curing, of an organic polymer layer, the fractional area fractions being determined in a tensile shear test.

The invention also relates to a substrate with a metallic protective layer based on zinc-magnesium, which has at least one zinc grain on the surface with an upper, essentially eutectic-free and / or eutectic phase-free surface.

Another subject matter is a substrate with a metallic protective layer based on zinc-magnesium which has at least one zinc grain on the surface with an upper, essentially magnesium-free surface.

The present invention also provides a substrate as described above produced using the method according to the invention For the purposes of the invention, combinations of the embodiments and alternatives described above can also be used.

Examples

1. Preparation of the samples

A4 size printed circuit boards, which had a Zn-Al-Mg protective layer on both sides, were immersed in an aqueous solution of an inorganic acid for 5 seconds or sprayed with the corresponding solution of the acid.

The boards treated in this way were then immersed in a water bath or sprayed with water and blown off with cool air until they were dry.

The boards prepared in this way were exposed to the natural air atmosphere until further evaluation.

2. Measurement

2.1 XPS

The measurement is carried out with a device: Phi Quantera II SXM Scanning XPS Microprobe from Physical Electronics GmbH.

Parameter:

The element concentrations measured by means of the XPS are taken from overview spectra that are recorded at a transmission energy of 280 eV in the course of at least 7 cycles and relate to a measuring area of 100 × 100 pm ² .

2.2 GD-OES

The measurement is carried out with a glow discharge spectrometer "Spectruma GDA750" vacuum simultaneous spectrometer with a focal length of 750mm and a discharge source constructed according to the Grimm type. The measurement is carried out in RF mode.

Working conditions: The basic operation of the glow discharge spectrometer is carried out according to the operating instructions of the manufacturer (Spectruma). The device is operated with a 4mm anode and argon 5.0 (99.999%) gas. Typical parameters of the respective device for operation with a 4mm anode are a voltage of 800V, a current of 20mA, a power of 16W and a lamp pressure of 3-10 hPa. In addition, a pre-plasma lasting 25s is connected upstream as part of the measurements.

Using glow discharge spectroscopy (GD-OES), quantified sputter profiles are measured which, over a measuring spot with a diameter of 4 mm, show the relative proportion Xi of an element i (i.e. zinc, aluminum or magnesium) as a function of the mean sputtering depth d. An integral parameter Si can be calculated for each element using the trapezoidal rule, which is proximated to the area normalized to the length of the integration limits under the function Xi (d) ap. The integral parameter Si (d1, d2) depends on the limits d1 and d2, which specify the area to be examined under the function Xi (d). The characteristic values are determined by the integration limits of d1 = Onm and d2 = 75nm.

A large number of GD-OES measurements at different points on the same substrate Z (where Z is just a variable that can stand for any substrate) using the method described above, mean parameters Si (Z) and their standard deviation i (Z ) calculated for each element and assigned to the substrate Z. Then the element-specific characteristic values Si of the samples Z (1-n) described above are measured 1 to n times, which are drawn using the method according to the invention. These characteristic values lie for all elements i in a range of one standard deviation around Si (Z) ± 2 i (Z). Hence, who assigned the characteristic values to the respective element according to the control.

2.3. Tof-SIMS

The measurement was carried out with a device: TOF.SIMS 5, from ION-TOF GmbH, Münster.

Parameter:

Primary ion beam: 25 keV Bi3 +, ~ 0.3 pA, pulse duration: <1 ns, measuring chamber vacuum: ~ 2E-9mbar, measuring field:

(a) 500 x 500 pm ² with 512 x512 pixels, 30 scans, random raster, (b) 3.08 x 3.08 mm ² (308 x 308 pixels) with 50 shots per pixel, random raster (within each of the approx. 300 x 300 pm ² large "stitching" measuring fields)

The evaluation or determination of the relative concentration takes place as described above.

3. Results

3.1 SEM image

1 shows an SEM image of a sample prepared according to FIG.

The smooth areas (A) are the exposed zinc grains and the rough areas (B) are the continuous eutectic. The eutectic and the eutectic phases above the zinc grains were essentially completely removed.

3.2 Increase in the proportion of cohesive fracture surfaces

Substrates with a dressed Zn-Al-Mg coating were degreased with an alkaline cleaning agent and then subjected to the process according to the invention. The pre-treated substrates were immersed in the appropriate solutions of the inorganic acids for 5 seconds. This was followed by rinsing with water and isopropanol. All experiments were carried out under normal air atmosphere.

The fracture surface was examined after the adhesive bond had been separated from the acid-treated cementitious protective layer, bonded with an epoxy-based adhesive. AF gives the adhesive and CF the cohesive part of the fracture surface (special cohesive fracture SCF close to the substrate). The results are summarized in FIG. 2 and show a significant increase in the cohesive fraction of breakage due to the method according to the invention or the use according to the invention by means of various concentrations of inorganic acid before and after aging. The specified milliliter amount of concentrated sulfuric acid (approx. 96%) was used. The aging took place through 10 VDA, i.e. 10 cycles in the climate change test.

Claims

1. A method for at least regionally removing a eutectic phase and / or

Eutectic consisting of a metallic protective layer based on zinc-magnesium, comprising at least or consisting of the following steps:

a) providing a substrate with a metallic protective layer;

b) at least regionally removing at least one eutectic phase and / or eutectic near the surface by wetting the surface with an inorganic acid;

c) exposing the upper surface of at least one zinc grain of the metallic protective layer;

d) Termination of the wetting as soon as the surface of the at least one zinc grain exposed in c) has essentially no eutectic phase and / or eutectic.

2. The method according to claim 1, characterized in that the substrate is a steel flat product.

3. The method according to any one of the preceding claims, characterized in that the metallic protective layer is a Zn-Al-Mg coating.

4. The method according to any one of the preceding claims, characterized in that the metallic protective layer is produced by hot-dip coating.

5. The method according to any one of the preceding claims, characterized in that in step b) near-surface oxide layers of the metallic protective layer are also removed.

6. The method according to any one of the preceding claims, characterized in that in step c) an upper area of at least one zinc grain is removed.

7. The method according to any one of the preceding claims, characterized in that the at least one eutectic phase and / or eutectic is removed from the upper surface of at least one zinc grain or from at least one zinc grain.

8. The method according to any one of the preceding claims, characterized in that an inorganic acid selected from the group containing or consisting of: H2S04, HCl, HN03, H3P04, H2S03, HN02, H3P03, HF or a mixture of these acids is used as an aqueous solution.

9. The method according to any one of the preceding claims, characterized in that an aqueous solution of an inorganic acid with a pH value between 0.01 and 2, preferably at least 0.01 and at most 1.8, particularly preferably at least 0.1 and at most 1, 7, in particular with a pH value less than 1, is used.

10. The method according to any one of the preceding claims, characterized in that the metallic protective layer is wetted for a time of 0.5-600 seconds and / or at a temperature of 10 ° C to 90 ° C with the aqueous solution of an inorganic acid.

11. The method according to any one of the preceding claims, characterized in that in step d) the wetting is ended by rinsing with water or an aqueous solution.

12. The method according to any one of the preceding claims, characterized in that the substrate provided with the metallic protective layer is subjected to a dressing step prior to wetting with the inorganic acid.

13. The method according to any one of the preceding claims, characterized in that the exposed surface of at least one zinc grain has essentially no magnesium.

14. Substrate with a metallic protective layer based on zinc-magnesium, which has on the surface at least one zinc grain with an upper, essentially eutectic-free surface and / or surface free of eutectic phases.

15. Substrate with a metallic protective layer based on zinc-magnesium, which has at least one zinc grain on the surface with an upper, essentially magnesium-free surface.