EP3666931B1 - Process of fabricating a metal band having a chromium and chromium oxide coating using a trivalent chromium containing electrolyte - Google Patents

Description

The invention relates to a method for producing a metal strip coated with a coating of chromium and chromium oxide according to the preamble of claim 1.

For the production of packaging, steel sheets which are electrolytically coated with a coating of chromium and chromium oxide are known from the prior art, which are referred to as tin-free steel sheet ("Tin Free Steel", TFS) or as "Electrolytic Chromium Coated Steel (ECCS)" and a Represent an alternative to tinplate. These tin-free steel sheets are particularly characterized by good adhesion for paints or organic protective coatings (such as, for example, polymer coatings made of PP or PET). Despite the low thickness of the coating made of chromium and chromium oxide, which is usually less than 20 nm, these chromium-coated steel sheets have good corrosion resistance and good processability in forming processes for the production of packaging, e.g. in deep-drawing and ironing processes.

For coating the steel substrate with a coating containing metallic chromium and chromium oxide, electrolytic coating processes are known from the prior art, with which the coating is applied in a strip coating system to a strip-shaped steel sheet using an electrolyte containing chromium VI. However, due to the properties of the chromium VI-containing electrolytes used in the electrolysis process, which are hazardous to the environment and health, these coating processes have considerable disadvantages and will have to be replaced by alternative coating processes in the foreseeable future, as the use of chromium VI-containing materials will be prohibited in the future.

For this reason, electrolytic coating processes have already been developed in the prior art, which can dispense with the use of electrolytes containing chromium VI. For example, from the WO 2015/177315-A1 a method for the electrolytic coating of an electrically conductive substrate, which is in particular a black sheet (uncoated steel sheet) or a tinplate (tinned steel sheet) can act, known with a chrome metal-chromium oxide (Cr-CrOx) layer, in which the substrate connected as a cathode is brought into contact with an electrolyte solution which contains a trivalent chromium compound (Cr-III), with an anode is provided that prevents or at least reduces the oxidation of chromium (III) ions to chromium (VI) ions and hydrogen bubbles that arise during the electrolytic deposition of the coating on the surface of the substrate are removed. It was found that the deposition reaction and the surface properties of the electrolytically deposited coating depend on the temperature of the electrolyte solution and that temperatures of the electrolyte solution between 30 ° C and 70 ° C are suitable for producing coatings with a good surface appearance. A preferred temperature range between 40 ° C. and 60 ° C. has been recognized as advantageous with regard to an efficient deposition reaction because the electrolyte solution has good conductivity at these temperatures.

From the WO 2015/177314-A1 a method is known for the electrolytic coating of a strip-shaped steel sheet with a chromium metal / chromium oxide (Cr-CrOx) layer in a strip coating system, in which the steel sheet, connected as a cathode, is passed through an electrolyte solution at high strip speeds of more than 100 m / min, which contains a trivalent chromium compound (Cr-III). It was observed that the composition of the coating, which, depending on the components still contained in the electrolyte solution in addition to the trivalent chromium compound (Cr-III), can also contain chromium sulfate and chromium carbide in addition to the constituents chromium metal and chromium oxide, depends to a large extent on the current densities of the electrolysis depends, which are set at the anodes during the electrolytic deposition process in the electrolysis tanks in which the electrolyte solution is contained. It was found that, depending on the current density, three areas (regime I, regime II and regime III) develop, whereby in a first area with a low current density up to a first current density threshold (regime I) there is no chromium-containing deposition on the steel substrate , in a second area with medium current density (regime II) there is a linear relationship between the current density and the weight of the deposited coating and, with current densities above a second current density threshold (regime III), the applied coating is partially decomposed, so that the weight of the chromium the applied coating in this area initially drops with increasing current density and then increases to one with higher current densities sets a constant value. In the area with medium current density (regime II), metallic chromium with a weight fraction of up to 80% (based on the total weight of the coating) is deposited on the steel substrate and above the second current density threshold (regime III) the coating contains a higher one Chromium oxide content, which makes up between ¼ and 1/3 of the total weight of the coating in the area of higher current densities. The values of the current density thresholds that delimit the areas (regime I to III) depend on the belt speed with which the steel sheet is moved through the electrolyte solution.

Another method for the electrolytic coating of a black sheet with a chromium metal / chromium oxide (Cr-CrOx) layer made of an electrolyte with a trivalent chromium compound is from EP 3 378 973-A1 known, in which the black plate connected as cathode is passed at a speed of at least 50 m / min in a coating line through the electrolyte, which has a temperature between 30 and 70 ° C and preferably at least 40 ° C.

In the WO 2014/079909 A1 It is mentioned that in order to achieve sufficient corrosion resistance for a black sheet (steel sheet) coated with a chrome-chrome oxide coating for packaging applications, a minimum coating of at least 20 mg / m ^{2 is} required in order to achieve a corrosion resistance comparable to that of conventional ECCS. ^{It has also been shown that a minimum amount of chromium oxide of at least 5 mg / m 2} in the coating is necessary in order to achieve a corrosion resistance which is sufficient for packaging applications.

The object of the present invention is to provide a method that is as efficient as possible and can be carried out on an industrial scale in a coil coating system for producing a metal strip coated with a coating of chromium and chromium oxide based on an electrolyte solution with a trivalent chromium compound, the coating having the highest possible proportion of chromium oxide is intended to achieve adequate corrosion resistance of the coated metal strip and a good adhesive base for organic coatings, such as paints or polymer films made of PET or PP.

This object is achieved by a method with the features of claim 1. Preferred embodiments of this method can be found in the subclaims.

In the method according to the invention, a coating containing chromium metal and chromium oxide is applied electrolytically from an electrolyte solution containing a trivalent chromium compound to a metal strip, in particular a steel strip, by bringing the metal strip into contact with the electrolyte solution connected as a cathode , the metal strip successively at a predetermined strip speed in a strip running direction at least through a first electrolysis tank (1a) or a front group of electrolysis tanks (1a, 1b) and through a last electrolysis tank (1c) or a rear group of electrolysis tanks (1g) as seen in the strip running direction , 1h), the temperature of the electrolyte solution in the first electrolysis tank (1a) or the front group of electrolysis tanks (1a, 1b) averaged over the volume of the electrolysis tank being greater than the average temperature of the electrolyte solution in the last electrolysis tank (1c) or the rear group of E electrolysis tanks (1g, 1h) and in the last electrolysis tank seen in the direction of belt travel or in a rear group of electrolysis tanks, the electrolyte solution has a temperature averaged over the volume of the electrolysis tank of less than 40 ° C and the electrolysis time in which the metal strip is electrolytically effective Contact with the electrolyte solution is less than 2.0 seconds in the last electrolysis tank or in the rear group of electrolysis tanks. The temperature of the electrolyte solution in the last electrolysis tank or in the rear group of electrolysis tanks, viewed in the direction of belt travel, is preferably between 25 and 38 ° C.

When speaking of the temperature of the electrolyte solution or the temperature in an electrolysis tank, what is meant is the mean temperature which is averaged over the entire volume of an electrolysis tank. As a rule, there is a temperature gradient in the electrolysis tanks with a temperature increase from top to bottom. When chromium oxide is mentioned, all oxide forms of chromium (CrOx), including chromium hydroxides, in particular chromium (III) hydroxide and chromium (III) oxide hydrate, as well as mixtures thereof are meant.

It has been shown that at temperatures of the electrolyte solution of 40 ° C. or less, the formation of chromium oxide is promoted. Therefore, at temperatures of the electrolyte solution below 40 ° C, coatings with a higher proportion of chromium oxide can be used be generated. A higher proportion of chromium oxide in the coating proves to be beneficial in terms of improved corrosion resistance of the coated metal strip. Due to the short electrolysis time at least in the last electrolysis tank or in the rear group of electrolysis tanks of 2.0 seconds or less, the proportion of chromium oxide in the coating can also be increased. In addition, the short electrolysis time in the last electrolysis tank or in the rear group of electrolysis tanks enables the electrolytic coating to be carried out in a continuous process in a strip coating system at high strip speeds, which are preferably more than 100 m / min.

The electrolysis time in which the metal strip is in effective electrolytic contact with the electrolyte solution is expediently less than 2 seconds in each of the electrolysis tanks, so that the metal strip moves at a constant belt speed through the several electrolysis tanks arranged one behind the other in the direction of belt travel, which are expediently designed in the same way are, can be guided. At preferred belt speeds of more than 100 m / min, the electrolysis time in each of the electrolysis tanks is preferably between 0.5 and 2.0 seconds, in particular between 0.6 seconds and 1.8 seconds. Depending on the belt speed selected, the electrolysis time in each of the electrolysis tanks can also be between 0.3 and 2.0 seconds and preferably between 0.5 seconds and 1.4 seconds.

Depending on the number of electrolysis tanks arranged one behind the other, the total electrolysis time (t _E ) in which the metal strip is in effective electrolytic contact with the electrolyte solution is preferably between 2 and 16 seconds and in particular between 4 seconds and 14 seconds across all electrolysis tanks.

For reasons of better separation efficiency, it can be advantageous to select the temperature of the electrolyte solution in the first electrolysis tank or in the front group of electrolysis tanks to be higher than in the last electrolysis tank. The temperature of the electrolyte solution in the first electrolysis tank or in the front group of electrolysis tanks is expediently more than 50 ° C and in particular between 53 ° C and 70 ° C, since in this temperature range a more efficient deposition of chromium, in particular in the form of chromium metal , can be observed. When setting a higher temperature of the electrolyte solution in the first electrolysis tank or in the front group of Electrolysis tanks of more than 50 ° C and at the same time setting the temperature of the electrolyte solution in the last electrolysis tank or in the rear group of electrolysis tanks of less than 40 ° C, a coating can be deposited on the surface of the metal strip, at least one lower and one upper Layer comprises, wherein the lower layer is deposited in the first electrolysis tank or in the front group of electrolysis tanks and the upper layer in the last electrolysis tank or in the rear group of electrolysis tanks and the lower layer has a low proportion of chromium oxide and the upper layer a higher one Has proportion of chromium oxide. The proportion by weight of chromium oxide in the lower layer facing the surface of the metal strip is preferably less than 15% and in the upper layer preferably more than 40%.

In the context of an embodiment not according to the invention, however, for reasons of apparatus it can also be expedient to set a uniform temperature of the electrolyte solution in the electrolysis tanks, which (averaged over the volume of the respective electrolysis tank) in all electrolysis tanks is preferably between 20 ° C and less than 40 ° C and particularly preferably between 25 ° C and 38 ° C.
Due to the exothermic deposition process, the electrolyte solution in the electrolysis tanks must be cooled in order to maintain the preferred temperatures. This is made more difficult by the fact that the circulation systems of the electrolysis tanks are usually coupled. Therefore, for reasons of apparatus, it can be expedient to maintain the same temperature in each case in the electrolysis tanks in order to avoid different settings that are expensive in terms of apparatus. From a results-oriented point of view, in particular with regard to improved corrosion resistance of the coated metal strip, it is advantageous, however, to set a higher temperature in the first electrolysis tank or in the front group of electrolysis tanks than in the last electrolysis tank or in the rear group of electrolysis tanks.

According to the invention it is therefore provided that the metal strip is guided at least through a first electrolysis tank or a front group of electrolysis tanks and then through a second electrolysis tank or a rear group of electrolysis tanks, the average temperature of the electrolyte solution in the first electrolysis tank or the front group of Electrolysis tanks is greater than the average temperature of the electrolyte solution in the second electrolysis tank or the rear group of electrolysis tanks.

In a further preferred embodiment, the metal strip is first guided through a first electrolysis tank or a front group of electrolysis tanks, then through a second electrolysis tank or a middle group of electrolysis tanks and finally through a last electrolysis tank or a rear group of electrolysis tanks, the average temperature being the Electrolyte solution in the first electrolysis tank or the front group of electrolysis tanks and / or in the second electrolysis tank or the middle group of electrolysis tanks is greater than the average temperature of the electrolyte solution in the last electrolysis tank or the rear group of electrolysis tanks.

The composition of the coating deposited electrolytically on the metal strip depends not only on the temperature of the electrolyte solution but also on the current density of the electrolysis process. It has been shown that at higher current densities, which are in the range of regime III, where (partial) decomposition of the applied coating already takes place, a higher proportion of chromium oxide is generated in the coating, compared to the lower current densities in regime II, where a linear relationship between the deposited weight of the chromium and the current density can be observed. To produce a coating with a lower layer, which has a high proportion of chromium metal, and an upper layer with a high chromium oxide proportion, which is preferably more than 40% by weight of the total application of the layer, it is therefore advantageous in the _{A low current density j 1} or j _{2 should} be applied in the first electrolysis tank or in the front group of electrolysis tanks and, if necessary, in the second electrolysis tank following in the direction of strip travel or in the middle group of electrolysis tanks, and in the last electrolysis tank or in In the rear group of electrolysis tanks, a high current density j _{3 must be provided} in regime III, where j ₁ and j _{2 are} less than j ₃ and, for example, at a belt speed of 100 m / min, the low current densities j ₁ and j _{2 are} each greater than 20 A / dm ² (and thus are above the first current density threshold of approx. 20 A / dm2 and are therefore in the range of regime II) and the high current density j ₃ greater than 50 A / dm ² (and therefore above the second current density threshold and therefore in the range of regime III). Depending on the belt speed, the current densities j ₁ , j ₂ and j _{3 are} increased, so that for example Belt speed of 300 m / min, the current densities j ₁ and j _{2 are} greater than 70 A / dm ² and the high current density j _{3 is} greater than 130 A / dm ² .

A particularly preferred embodiment provides that a lower current density is present in the first electrolysis tank or in the front group of electrolysis tanks compared to the second electrolysis tank following in the direction of belt travel or in the middle group of electrolysis tanks, so that the ratio 20 A / dm ² <j ₁ ≤ j ₂ <j ₃ applies.

As a result, a coating can be deposited on the surface of the metal strip, which is composed of three layers with different compositions in terms of their proportion of chromium metal and chromium oxide, the lower layer facing the metal strip having an average weight proportion of chromium oxide, which is in particular in the range of 10% to 15%, the middle layer has a low percentage by weight of chromium oxide, which is in particular in the range from 2% to 10%, and the upper layer has a high percentage by weight of chromium oxide, which is in particular more than 30%, preferably at is more than 50%. With regard to the adhesion of organic coatings, such as organic lacquers or polymer films made of PET or PP, it is advantageous if the layer with the high oxide content is on the outside, since it has been shown that chromium oxide is a better adhesive base for chromium metal than chromium metal forms organic materials.

By dividing the electrolysis tanks arranged one behind the other in the direction of belt travel and setting different current densities in the individual electrolysis tanks, which increase in the direction of belt travel, it is possible on the one hand to maintain high belt speeds of more than 100 m / min, and on the other hand to ensure that the coating is sufficiently heavy on at least one side of the To deposit metal strip, the coating having the proportion of chromium oxide required for adequate corrosion resistance of at least 5 mg / m ² , preferably more than 7 mg / m ² . Preferably, the total weight of the chromium oxide ^{does not exceed 15 mg / m 2} , since with higher weight of the chromium oxide, reduced adhesion of organic coatings made of lacquers or thermoplastic polymer materials is observed. For this reason, a preferred range for the weight of the chromium oxide is between 5 and 15 mg / m ² . The fact that in the first electrolysis tank or in the front group of electrolysis tanks and in the second electrolysis tank or in the middle group of electrolysis tanks a lower current density j ₁ or j _{2 is} used, energy can also be saved, since lower electrical currents are required to act on the anodes in the first electrolysis tank or in the front group of electrolysis tanks and in the second electrolysis tank or in the middle group of electrolysis tanks. Nevertheless, a sufficiently high weight of chromium oxide is produced in the coating, since even with the lower current densities j ₁ and j ₂ , which are set in the first and the second electrolysis tank or the front and the middle group of electrolysis tanks, already to one a certain amount of chromium oxide is deposited on the metal substrate. The greater proportion of chromium oxide is deposited in the last electrolysis tank or in the rear group of electrolysis tanks, as seen in the direction of strip travel, because the high current density j _{3 is} set therein, at which the proportion of chromium oxide in the total application of the coating turns out to be higher.

Since already in the first electrolysis tank or in the front group of electrolysis tanks and in the second electrolysis tank or in the middle group of electrolysis tanks a certain weight proportion of the total amount of the deposited coating, which is approx. 9 to 15%, is made up of chromium oxide Chromium oxide crystals develop on the surface of the metal strip already in the first electrolysis tank or in the front group of electrolysis tanks and in the second electrolysis tank or in the middle group of electrolysis tanks. These chromium oxide crystals act in the last electrolysis tank and / or in the rear group of electrolysis tanks as a nucleus for the growth of further oxide crystals, which is why this increases the efficiency of the deposition of chromium oxide or the proportion of chromium oxide in the total application of the coating in the last electrolysis tank or in the rear group of electrolysis tanks increases. Thus, with the energy-saving use of lower current densities j ₁ and j ₂ in the first and second electrolysis tanks or the front and middle group of electrolysis tanks, a sufficiently high level of chromium oxide of preferably more than 5 mg / m ² on the surface of the Metal bands are produced.

The proportion of chromium oxide generated in the first electrolysis tank or in the front group of electrolysis tanks and in the second electrolysis tank or in the middle group of electrolysis tanks forms due to the higher oxygen content in the coating compared to electrolytic deposition with higher current densities (and consequently a lower oxide content ) a denser coating, which leads to improved corrosion resistance.

The use of at least two, preferably three, electrolysis tanks or groups of electrolysis tanks arranged one behind the other makes it possible to maintain a high belt speed with the lowest possible current densities, which increases the efficiency of the process. It has been shown that to maintain a preferred belt speed of at least 100 m / min, a current density of at least 20 A / dm ^{2 is} required so that a chromium-chromium oxide layer can be deposited on at least one surface of the metal strip. This current density of 20 A / dm ² represents the first current density threshold value at a belt speed of approx. 100 m / min, which corresponds to regime I (no chromium deposition) of regime II (chromium deposition with a linear relationship between current density and the chromium weight of the deposited coating ) delimits.

The current densities (j ₁ , j ₂ , j ₃ ) in the electrolysis tanks are each adapted to the belt speed, with at least an essentially linear relationship between the belt speed and the respective current density (j ₁ , j ₂ , j ₃ ). It is advantageous if the current density in the first electrolysis tank or in the front group of electrolysis tanks is smaller than in the second electrolysis tank or in the middle group of electrolysis tanks. A lower current density in the first electrolysis tank or in the front group of electrolysis tanks produces a dense and therefore corrosion-resistant chromium-chromium oxide coating with a relatively high chromium oxide content, which is preferably more than 8%, in particular between 8 and 15%, directly on the surface of the metal strip particularly preferably more than 10% by weight.

To generate the current densities (j ₁ , j ₂ , j ₃ ) in the electrolysis tanks, at least one anode pair with two opposing anodes is expediently arranged in each electrolysis tank, the metal strip running through between the opposing anodes of an anode pair. This allows an even current density distribution around the Metal band can be achieved. The anode pairs of each electrolysis tank can be supplied with electrical current independently of one another, so that different current densities (j ₁ , j ₂ , j ₃ ) can be set in the electrolysis tanks.

The belt speed of the metal belt is expediently chosen so that the electrolysis time (t _E ) in which the metal belt is in effective electrolytic contact with the electrolyte solution is less than 1.0 seconds in each of the electrolysis tanks and in particular between 0.5 and 1, 0 seconds and is preferably between 0.6 seconds and 0.9 seconds.

The coating deposited on the metal strip with the method according to the invention preferably has a chromium weight of at least 40 mg / m ² and in particular 70 mg / m ² to 180 mg / m ² to achieve adequate corrosion resistance of the coated metal strip. The proportion by weight of chromium oxide contained in the coating in relation to the total weight of the coating is preferably at least 5%, in particular more than 10% and, for example, between 11 and 16%. The chromium oxide portion of the coating has a weight of the chromium bound as chromium oxide of at least 3 mg Cr per m ² , in particular from 3 to 15 mg / m ² and preferably of at least 7 mg Cr per m ² .

A single electrolyte solution is expediently used in the method according to the invention, i.e. the electrolysis tanks are all filled with the same electrolyte solution.

A preferred composition of the electrolyte solution comprises basic Cr (III) sulfate (Cr ₂ (SO ₄ ) ₃ ) as a trivalent chromium compound. The concentration of the trivalent chromium compound in the electrolyte solution, both in this preferred composition and in other compositions, is at least 10 g / l and preferably more than 15 g / l and is in particular 20 g / l or more. Further useful constituents of the electrolyte solution can be complexing agents, in particular an alkali metal carboxylate, preferably a salt of formic acid, in particular potassium format or sodium format. The ratio of the proportion by weight of the trivalent chromium compound to the proportion by weight of the complexing agents, in particular the formates, is preferably between 1: 1.1 and 1: 1.4 and preferably between 1: 1.2 and 1: 1.3 and in particular 1: 1 , 25. To increase the conductivity, the electrolyte solution can be an alkali metal sulfate, preferably potassium or sodium sulfate. The electrolyte solution is preferably free from halides, in particular free from chloride and bromide ions and free from a buffering agent and in particular free from a boric acid buffer.

The pH of the electrolyte solution (measured at a temperature of 20 ° C.) is preferably between 2.0 and 3.0 and particularly preferably between 2.5 and 2.9 and in particular 2.7. To adjust the pH of the electrolyte solution, an acid, for example sulfuric acid, can be added.

After the electrolytic application of the coating, an organic coating, in particular a lacquer or a thermoplastic, for example a polymer film made of PET, PE, PP or a mixture thereof, can be applied to the surface of the coating made of chrome metal and chrome oxide in order to provide additional protection against corrosion and to form a barrier against acidic contents of packaging.

The metal strip can be a (initially uncoated) steel strip (black plate strip) or a tinned steel strip (tin plate strip).

Figur 1:

schematische Darstellung einer Bandbeschichtungsanlage zur Durchführung des erfindungsgemäßen Verfahrens in einer ersten Ausführungsform mit drei in Bandlaufrichtung v hintereinander angeordneten Elektrolysetanks;

Figur 2:

schematische Darstellung einer Bandbeschichtungsanlage zur Durchführung des erfindungsgemäßen Verfahrens in einer zweiten Ausführungsform mit acht in Bandlaufrichtung v hintereinander angeordneten Elektrolysetanks;

Figur 3:

Schnittdarstellung eines mit dem erfindungsgemäßen Verfahren in der ersten Ausführungsform beschichteten Metallbands;

Figur 4:

GDOES-Spektrum einer elektrolytisch auf einem Stahlband abgeschiedenen Schicht, welche Chrommetall, Chromoxid und Chrom-Carbide enthält, wobei das Chromoxid an der Oberfläche der Schicht liegt;

Figur 5:

Graphische Darstellung der auf einem Metallband abgeschieden Gewichtsauflage einer Beschichtung, welche Chrommetall und Chromoxid enthält, in Abhängigkeit der Temperatur der Elektrolytlösung und der Elektrolysedauer.

The invention is explained in more detail below on the basis of exemplary embodiments with reference to the accompanying drawings, these exemplary embodiments merely explaining the invention by way of example and not restricting it with reference to the scope of protection defined by the following claims. The drawings show:

Figure 1:

schematic representation of a coil coating system for carrying out the method according to the invention in a first embodiment with three electrolysis tanks arranged one behind the other in the direction of travel v of the coil;

Figure 2:

a schematic representation of a coil coating system for carrying out the method according to the invention in a second embodiment with eight electrolysis tanks arranged one behind the other in the direction of travel v of the coil;

Figure 3:

Sectional representation of a metal strip coated with the method according to the invention in the first embodiment;

Figure 4:

GDOES spectrum of a layer deposited electrolytically on a steel strip, which layer contains chromium metal, chromium oxide and chromium carbides, the chromium oxide being on the surface of the layer;

Figure 5:

Graphic representation of the weight of a coating deposited on a metal strip, which contains chromium metal and chromium oxide, as a function of the temperature of the electrolyte solution and the duration of the electrolysis.

In Figure 1 a coil coating system for carrying out the method according to the invention is shown schematically in a first embodiment. The coil coating system comprises three electrolysis tanks 1a, 1b, 1c arranged next to one another or one behind the other, each of which is filled with an electrolyte solution E. An initially uncoated metal strip M, in particular a steel strip, is passed through the electrolysis tanks 1a-1c one after the other. For this purpose, the metal strip M is pulled through the electrolysis tanks 1a-1c by a transport device (not shown here) in a strip running direction v at a predetermined strip speed. Current rollers S are arranged above the electrolysis tanks 1a-1c, via which the metal strip M is connected as a cathode. In each electrolysis tank there is also a deflection roller U, around which the metal strip M is guided and is thereby directed into and out of the electrolysis tank.

Within each electrolysis tank 1a-1c, at least one pair of anodes AP is arranged below the liquid level of the electrolyte solution E. In the exemplary embodiment shown, two pairs of anodes AP arranged one behind the other in the direction of travel of the strip are provided in each electrolysis tank 1a-1c. The metal strip M is passed between the opposing anodes of an anode pair AP. In the embodiment of Figure 1 Thus, two pairs of anodes AP are arranged in each electrolysis tank 1a, 1b, 1c in such a way that the metal strip M is passed through these pairs of anodes AP one after the other. The last pair of anodes APc in the downstream direction of the last electrolysis tank 1c seen in the direction of travel v of the strip has a shorter length than the other pairs of anodes AP. As a result, a higher current density can be generated with this last pair of anodes APc when an electric current of the same level is applied.

The metal strip M can be a cold-rolled, initially uncoated steel strip (black plate strip) or a tinned steel strip (tin plate strip). To prepare for the electrolysis process, the metal strip M is first degreased, rinsed, pickled and rinsed again and in this pretreated form passed one after the other through the electrolysis tanks la -1c, the metal strip M being switched as the cathode in that electrical current is supplied via the current rollers S. The belt speed at which the metal belt M is passed through the electrolysis tanks 1a-1c is at least 100 m / min and can be up to 900 m / min.

The same electrolyte solution E is filled into each of the electrolysis tanks 1a-1c arranged one behind the other in the strip running direction v. The electrolyte solution E contains a trivalent chromium compound, preferably basic Cr (III) sulfate [Cr ₂ (SO ₄ ) ₃ ]. In addition to the trivalent chromium compound, the electrolyte solution preferably contains at least one complexing agent, for example a salt of formic acid, in particular potassium or sodium format. The ratio of the proportion by weight of the trivalent chromium compound to the proportion by weight of the complexing agents, in particular the formats, is preferably between 1: 1.1 and 1: 1.4 and particularly preferably 1: 1.25. To increase the conductivity, the electrolyte solution E can contain an alkali metal sulfate, for example potassium or sodium sulfate. The concentration of the trivalent chromium compound in the electrolyte solution E is at least 10 g / l and particularly preferably 20 g / l or more. The pH of the electrolyte solution is adjusted to a preferred value between 2.0 and 3.0 and in particular to pH = 2.7 by adding an acid, for example sulfuric acid.

The temperature of the electrolyte solution E can be the same in all electrolysis tanks 1a-1c and, according to the invention, is at most 40.degree. In preferred exemplary embodiments of the method according to the invention, however, different temperatures of the electrolyte solution can also be set in the electrolysis tanks 1a-1c. For example, the temperature of the electrolyte solution in the last electrolysis tank 1c can be at most 40 ° C., and a higher temperature can be present in the upstream electrolysis tanks 1a and 1b. In this embodiment of the method according to the invention, the temperature of the electrolyte solution in the last electrolysis tank 1c is preferably between 25 ° C and 37 ° C and in particular 35 ° C. In this exemplary embodiment, the temperature of the electrolyte solution in the first two electrolysis tanks 1a, 1b is preferably between 50.degree. C. and 75.degree. C. and in particular 55.degree. The lower temperature of the electrolyte solution E promotes the deposition of a chromium / chromium oxide layer with a higher proportion of chromium oxide in the last electrolysis tank 1c.

This is evident from the diagram of the Figure 5 This is clearly shown by the weight of the chromium oxide component (CrOx in mg / m ² ) of a coating B deposited on the metal strip as a function of the temperature (T in ° C.) of the electrolyte solution and the electrolysis time (t _E in seconds). It can be seen from the diagram that for a given electrolysis duration (of, for example, t _E = 0.5 seconds) at temperatures T below 40 ° C., a higher weight of chromium oxide (CrOx) is deposited than at higher temperatures. At a temperature T of the electrolyte solution of approx. 35 ° C, a maximum of the weight of the chromium oxide can be observed. It follows from this that in the temperature range according to the invention of up to 40 ° C. and preferably in the range from 20 to 40 ° C., the deposition of coatings with a high chromium oxide content is promoted.

It is still off Figure 5 It can be seen that with increasing electrolysis time t _E, the weight of the chromium oxide increases. To achieve the most efficient coating process possible, which can be carried out in a coil coating process with the highest possible belt speeds of preferably more than 100 m / min, low electrolysis times of less than 2 seconds in each of the electrolysis tanks 1a-1c are preferred. The diagram of the Figure 5 shows that even with short electrolysis times of less than 1 second, sufficiently high chromium oxide weights of more than 20 mg / m ² can be achieved if the temperature of the electrolyte solution is in the range according to the invention of 40 ° C or less and in particular between 20 ° C and 38 ° C.

Depending on the belt speed, the metal belt M connected as a cathode and guided through the electrolysis tanks 1a-1c is in _{electrolytically effective contact with the electrolyte solution E} during an electrolysis period t E. At belt speeds between 100 and 700 m / min, the electrolysis period is in each of the electrolysis tanks 1a, 1b, 1c preferably between 0.5 and 2.0 seconds. According to the invention, to achieve high coating efficiency and high throughput, belt speeds are set so high that the electrolysis time t _E in each electrolysis tank 1a, 1b, 1c is less than 2 seconds and in particular between 0.6 seconds and 1.8 seconds. The total electrolysis time in which the metal strip M over all electrolysis tanks 1a-1c is in electrolytically effective contact with the electrolyte solution E away, is accordingly between 1.8 and 5.4 seconds.

The anode pairs AP arranged in the electrolysis tanks 1a-1c can be supplied with electrical direct current in such a way that the same current density is present in each of the electrolysis tanks 1a, 1b, 1c. In order to deposit a coating B with a plurality of layers B1, B2, B3 of different composition on the metal strip M, however, it is also possible to set different current densities in the electrolysis tanks 1a, 1b, 1c. _{For example, a low current density j 1} can be set in the first electrolysis tank 1a upstream as seen in the strip running direction v, an average current density j _{2 in the second electrolysis tank 1b following in the strip running direction and a high current density j 3} in the last electrolysis tank 1c as seen in the strip running direction so that the relation j ₁ <j ₂ <j ₃ applies and the low current density is j ₁ > 20 A / dm ² .

Due to the current densities set in the electrolysis tanks 1a-1c, a layer containing chromium metal and chromium oxide is electrolytically deposited on at least one side of the metal strip M, a layer B1, B2, B3 being produced in each of the electrolysis tanks 1a, 1b, 1c. Due to the different current densities j ₁ , j ₂ , j ₃ in the individual electrolysis tanks 1a, 1b, 1c, each electrolytically applied layer B1, B2, B3 has a different composition, which differs in particular through the proportion of chromium oxide.

In Figure 3 a schematic sectional view of a metal strip M which is electrolytically coated on one side using the method according to the invention is shown. A coating B, which is composed of the individual layers B1, B2, B3, is applied to one side of the metal strip M. Each individual layer B1, B2, B3 is applied to the surface in one of the electrolysis tanks 1a, 1b, 1c.

The coating B, which is composed of the individual layers B1, B2, B3, contains metallic chromium (chromium metal) and chromium oxides (CrOx) as essential components, the composition of the individual layers B1, B2, B3 in relation to their respective weight fraction of Chromium metal and chromium oxide due to the different current densities j ₁ , j ₂ , j ₃ in the electrolysis tanks 1a, 1b, 1c is different. Furthermore, a possibly different temperature of the electrolyte solution in the electrolysis tanks 1a, 1b, 1c contribute to the fact that the individual layers differ in terms of their composition, since (as above based on Figure 5 explained) at lower temperatures of 40 ° C or less, the formation of chromium oxide is promoted. To achieve the highest possible oxide content in layer B3, a high current density j ₃ (which is higher than the current density j ₁ , j ₂ in the preceding electrolysis tanks) and, at the same time, a low temperature of the electrolyte solution of 40 ° C. is preferred in the last electrolysis tank 1c or less.

Due to the low current density j ₁ in the first electrolysis tank 1a, the layer B1 applied in the first electrolysis tank 1a has a higher oxide content than the layer B2 applied in the second (middle) electrolysis tank 1b, since lower current densities , which are within the regime II, form higher oxide proportions in the coating. In the last electrolysis tank 1c, a current density j3 is set which is in regime III, in which an increased chromium oxide content is generated in the coating, which is preferably more than 40% by weight and particularly preferably more than 50% by weight.

Table 1 shows examples of suitable current densities j ₁ , j ₂ , j ₃ in the individual electrolysis tanks 1a, 1b, 1c at different belt speeds. It can be seen from Table 1 that the current densities j ₁ in the first electrolysis tank 1a are slightly smaller than the current densities j2 in the second electrolysis tank 1b and are above a lower limit value of j ₀ = 20 A / dm ² . _{The current densities j 1} , j ₂ present in the first two electrolysis tanks 1a, 1b are therefore each in regime II, in which there is a linear relationship between the current density and the electrolytically deposited amount of chromium (or the deposited weight of chromium). The current density j _{1 of} the first electrolysis tank 1a is expediently selected so that it is close to the first current density threshold which delimits regime I (in which no chromium is yet deposited) from regime II. At these low current densities j ₁ , a chromium metal-chromium oxide coating (layer B1) is deposited on the surface of the metal strip M with a higher chromium oxide content than at higher current densities within regime II B1 has a higher chromium oxide content than the layer B2 deposited in the second electrolysis tank 1b.

In the last electrolysis tank 1a, a current density j ₃ is preferably set which is above the second current density threshold which delimits regime II from regime III. The current density j _{3 of} the last electrolysis tank 1c is therefore in regime III, in which the chromium metal-chromium oxide coating is partially decomposed and a significantly higher proportion of chromium oxide is deposited than with the current densities in regime II. For this reason, the im Layer B3 deposited in the last electrolysis tank 1c has a high chromium oxide content, which is higher than the chromium oxide content in layers B1 and B2.

After the electrolytic coating, the metal strip M provided with the coating B is rinsed, dried and oiled (for example with DOS). The metal strip M electrolytically coated with the coating B can then be provided with an organic layer on the surface of the coating B. The organic coating can be, for example, an organic lacquer or polymer films made from thermoplastic polymers such as PET, PP, PE or mixtures thereof. The organic coating can be applied either in a "coil coating" process or in a panel process, the coated metal strip being first divided into panels in the panel process, which are then coated with an organic lacquer or coated with a polymer film will.

In Figure 2 shows a second embodiment of a coil coating system with eight electrolysis tanks 1a-1h arranged one behind the other in the direction of travel v of the coil. The electrolysis tanks 1a-1h are grouped into three groups, namely a front group with the first two electrolysis tanks 1a, 1b, a middle group with the subsequent electrolysis tanks 1c-1f in the direction of belt travel and a rear group with the last two electrolysis tanks 1g and 1h. According to the invention, in the rear group of the electrolysis tanks 1g and 1h, the temperature of the electrolyte solution is 40 ° C. or less. In the front group with the two first electrolysis tanks 1a, 1b and the middle group with the electrolysis tanks 1c-1f, either the same or at least approximately the same temperature or also a higher temperature can be present. To increase the separation efficiency, higher temperatures of more than 50 ° C. and in particular of approx. 55 ° C. are preferred in the electrolysis tanks 1a, 1b of the front group and the electrolysis tanks 1c-1f of the middle group. For reasons of apparatus, however, it can also be expedient to set the same temperature in all electrolysis tanks 1a to 1h and to maintain it by cooling the electrolyte solution during the electrolysis process.

In the groups of electrolysis tanks there are preferably different current densities j ₁ , j ₂ , j _{3 , with a low current density j 1} in the front group of electrolysis tanks 1a, 1b _{and an average current density j 2} in the middle group of electrolysis tanks 1c-1f and in the rear group of electrolysis tanks 1g, 1h there is a high current density j ₃ , where j ₁ <j ₂ <j ₃ and the low current density is j ₁ > 20 A / dm ² .

Analogous to Table 1, table 2 shows examples of suitable current densities j ₁ , j ₂ , j ₃ in the individual electrolysis tanks 1a to 1h at different belt speeds, with a current density j ₁ in each of the electrolysis tanks 1a, 1b in the front group and a current density j 1 in the electrolysis tanks 1c to 1f of the middle group each have a current density j _{2 and a current density j 3 is} set in each of the electrolysis tanks 1g, 1h of the rear group, where j ₁ <j ₂ <j ₃ .

In the front group of electrolysis tanks 1a, 1b a first layer B1 containing chromium metal and chromium oxide is electrolytically applied to the metal strip, in the second group of electrolysis tanks 1c-1f a second layer B2 and in the rear group of electrolysis tanks 1g, 1h a third layer B3 M applied. As with the embodiment of Figure 1 Layers B1, B2, B3 have _{different compositions due to the different current densities j 1} , j ₂ , j ₃ and possibly different temperatures in the groups of electrolysis tanks arranged one behind the other, with layer B1 containing a higher chromium oxide content than the second Layer B2 and the third layer B3 contain a higher proportion of chromium oxide than the two layers B1 and B2.

With the method according to the invention in the coil coating system from Figure 2 Coating B applied to the surface of the metal strip M thus has essentially the same composition and structure as in FIG Figure 3 shown.

The entire electrolysis time in which the metal strip M is in electrolytically effective contact with the electrolyte solution E is in the embodiment of FIG Figure 2 across all electrolysis tanks 1a-1h, preferably for less than 16 seconds and in particular between 4 and 16 seconds.

With the coil coating system from Figure 2 Because of the higher number of electrolysis tanks and the associated higher total electrolysis time in which the metal strip connected as a cathode is in effective electrolytic contact with the electrolyte solution E, coatings B with higher weight requirements can be produced.

To achieve adequate corrosion resistance, the coatings B preferably have a total weight of the chromium of at least 40 mg / m ² and particularly preferably from 70 mg / m ² to 180 mg / m ² . The proportion of the total amount of chromium by weight contained in the chromium oxide, averaged over the entire amount of layer B, is at least 5% and preferably between 10% and 15%. The coating B expediently has a total of chromium oxide with a weight add-on of the chromium bound as chromium oxide of at least 3 mg chromium per m ² and in particular from 3 to 15 mg / m ² . The amount by weight of the chromium bound as chromium oxide, averaged over the entire amount of coating B, is preferably at least 7 mg of chromium per m ² . Good adhesion of organic lacquers or thermoplastic polymer materials on the surface of the coating B can be achieved with weight of the chromium oxide of up to approx. 15 mg / m ² . With higher chromium oxide coatings, a deterioration in the adhesion of organic coatings such as paints or polymer films can be observed. A preferred range for the weight of the chromium oxide in coating B is therefore between 5 and 15 mg / m ² .

Examples:

To explain the implementation of the invention, laboratory tests for coating steel sheets with a chromium / chromium oxide coating are shown in detail below:
Table 3 shows an example of the composition of an electrolyte solution which contains a Cr (III) salt (Cr ₂ (SO ₄ ) ₃ ) and has been used for coating tests in laboratory apparatus for the electrolytic coating of a metal strip. The parameters of the electrolyte solution used can be found in Table 4. The Cr (III) salt used as a component of the electrolyte solution should be as free of organic residues as possible. The preparation of the Cr (III) salts can be carried out on an industrial scale by means of reduction from Cr (VI) salts be performed. The reducing agent used is preferably a less noble metal than chromium (variant 1), or alternatively an organic component (variant 2). The pH of the electrolyte solution was adjusted by adding sulfuric acid followed by topping up with deionized water.

A steel sheet already coated with a chromium / chromium oxide layer was used as the substrate for the coating tests. This material was electrolytically coated at 55 ° C. with a chromium (III) electrolyte and Table 5 below describes the existing coating of the steel sheet with chromium metal and chromium oxide. It can be seen that mainly chromium metal and only a little chromium oxide was formed.

The chromium metal determination was carried out according to the EURO norm EN 10202 (Cr-metal photometric (euro-norm) step 2: 120 ml NaCO ₃ and 15 mA / plane; successfull dissolution visible by potential step, oxidation with 10 ml 6% H ₂ O ₂ , photometric @ 370 nm). The chromium oxide determination was also carried out according to the EURO standard EN 10202 (Cr-oxides photometric: (euro-norm) step 1: 40 ml NaOH (330g / L), reaction at 90 ° C for 10 minutes, oxidation with 10 ml 6% H ₂ O ₂ , photometric @ 370 nm).

In preparation for the laboratory coating, the substrate was degreased (2.5 A / dm ² connected to the cathode, 30 seconds, 70 ° C. in sodium hydroxide solution) and then rinsed with deionized water. The subsequent pickling process was dispensed with due to the existing metallic coating.

Coating parameters and results:

Tables 6 and 7 summarize the parameters and the results of the coating tests. A large-scale coating of a steel belt with a belt speed of 100 m / min was simulated. At this speed, the selected current density of 60 A / dm ^2, which was kept constant during the test, is in regime III (see Table 2) and thus mainly generates chromium oxide (at least at the lower temperatures). In the laboratory tests, both the temperatures of the electrolyte solutions and the holding times (electrolysis time) were varied in regime III. The underside of the substrate was coated in each case. The duration of the electrolysis in the relevant regime III is given in Table 5 as “time (s) segment 1”.

It can be observed that at temperatures of the electrolyte solution in the range from 22 ° C to approx. 37 ° C there is an increase in the chromium oxide content of the coating and at temperatures above approx. 40 ° C a significantly lower proportion of the chromium oxide is present in the coating. In order to achieve chromium-containing coatings with a high proportion of chromium oxide, electrolyte temperatures of at most 40 ° C. are therefore used according to the invention. In order to produce a coating that has the highest possible chromium oxide content on the surface, the coating is carried out at electrolyte temperatures below 40 ° C. in accordance with the invention in the last electrolysis tank or in a rear group of electrolysis tanks.

In the laboratory tests, the electrolysis times in the respective regime (segment) were less than 2 seconds. With increasing electrolysis time, a higher oxide coating was observed in the laboratory tests. However, short electrolysis times of less than 2 seconds are to be preferred in terms of efficiency in a large-scale process, since high belt speeds of preferably more than 100 m / min are used here. <b> Table 1: </b> Current densities j ₁, j ₂, j ₃ in the individual electrolysis tanks of the first exemplary embodiment (with 3 Electrolysis tanks 1a - 1c) at different belt speeds v: tank 1a 1b 1c v [m / min] J ₁ / [A / dm ² ] J ₂ / [A / dm ² ] J ₃ / [A / dm ² ] 100 25th 29 75 150 41 45 91 200 57 61 107 300 73 77 133 400 89 93 149 500 105 109 165 tank 1a 1b 1c 1d 1e 1f 1g 1h v [m / min] J ₁ / [A / dm ² ] J ₁ / [A / dm ² ] J ₂ / [A / dm ² ] J ₂ / [A / dm ² ] J ₂ / [A / dm ² ] J ₂ / [A / dm ² ] J ₃ / [A / dm ² ] J ₃ / [A / dm ² ] 100 25th 25th 29 29 29 29 75 75 150 41 41 45 45 45 45 91 91 200 57 57 61 61 61 61 107 107 300 73 73 77 77 77 77 133 133 400 89 89 93 93 93 93 149 149 500 105 105 109 109 109 109 165 165 substance used amount / L Sodium formate 41.4 g basic chromium sulfate 120 g Sulfuric acid 96% ∼7.5 ml Sodium sulfate 100 g Serial no. Production date Temperature TCCT tank [° C] pH value VA08 Conductance VA08 [mS / cm] pH value (55 ° C) Conductivity 55 ° C [mS / cm] Chromium concentration electrolyte (g / L) Iron concentration electrolyte (mg / L) Chloride concentration electrolyte (mg / l) Electrolyte surface explosion 1st cycle at ∼55 ° C (µC / cm ² ) Electrolyte surface explosion 2nd cycle at ∼55 ° C (µC / cm ² ) 13th 02/28/2016 55 2.4 88.5 2.3 158.3 22.6 270 182 348.8 327.8 Serial no. Ø chromium metal OS (mg / m ² ) Ø chromium metal US (mg / m ² ) Ø chromium oxide OS (mg / m ² ) Ø chromium oxide US (mg / m ² ) 11 63 111 3 1

Claims (14)

1. Method for producing a metal strip (M) coated with a coating (B), wherein the coating (B) contains chromium metal and chromium oxide and is applied to the metal strip (M) electrolytically from an electrolyte solution (E) containing a trivalent chromium compound by bringing the metal strip (M) into contact with the electrolyte solution (E) connected as the cathode during a period of electrolysis, characterised in that the metal strip (M) is guided at a predetermined strip speed (v) in a travel direction of the strip successively at least through a first electrolysis tank (1a) or a front group of electrolysis tanks (1a, 1b) and through a last electrolysis tank (1c) seen in the travel direction of the strip or a rear group of electrolysis tanks (1g, 1h), wherein the temperature of the electrolyte solution averaged over the volume of the electrolysis tank in the first electrolysis tank (1a) or the front group of electrolysis tanks (1a, 1b) is greater than the average temperature of the electrolyte solution in the last electrolysis tank (1c) or the rear group of electrolysis tanks (1g, 1h) and in the last electrolysis tank (1c, 1h) or in the rear group of electrolysis tanks (1g, 1h), the electrolyte solution (E) has an average temperature of less than 40°C, and the period of electrolysis (t_E), in which the metal strip (M) is in electrolytically effective contact with the electrolyte solution (E), in the last electrolysis tank (1c) or in the rear group of electrolysis tanks (1g, 1h) is less than 2.0 seconds.
2. Method according to claim 1, characterised in that the period of electrolysis (t_E), in which the metal strip (M) is in electrolytically effective contact with the electrolyte solution (E) in each of the electrolysis tanks (1a - 1h), is less than 2.0 seconds, and lies preferably between 0.3 and 2.0 seconds, in particular between 0.6 seconds and 1.8 seconds, wherein the total period of electrolysis (t_E), in which the metal strip (M) is in electrolytically effective contact with the electrolyte solution (E), lies preferably between 2 seconds and 16 seconds and particularly preferably between 4 seconds and 14 seconds.
3. Method according to one of the preceding claims, characterised in that the average temperature of the electrolyte solution in the first electrolysis tank (1a) or in the front group of electrolysis tanks (1a, 1b) lies between 50°C and 75°C.
4. Method according to one of claims 1 to 3, characterised in that the temperature of the electrolyte solution averaged over the volume of the respective electrolysis tank in the last electrolysis tank or the rear group of electrolysis tanks lies between 20°C and less than 40°C and preferably between 25°C and 38°C.
5. Method according to claim 1, characterised in that the first electrolysis tank (1a) seen in the travel direction of the strip or the front group of electrolysis tanks (1a, 1b) has a low current density (j₁), a second electrolysis tank (1b) following in the travel direction of the strip or a middle group of electrolysis tanks (1c-1f) has a moderate current density (j₂) and the last electrolysis tank (1c) seen in the travel direction of the strip or the rear group of electrolysis tanks (1g, 1h) has a high current density (j₃), wherein j₁ ≤ j₂ < j₃ and the low current density (j₁) is greater than 20 A/dm².
6. Method according to one of the preceding claims, characterised in that the electrolyte solution comprises, in addition to the trivalent chromium compound which preferably comprises basic Cr(III) sulphate (Cr₂(SO₄)₃), at least one complexing agent, in particular an alkali metal carboxylate, preferably a salt of formic acid, in particular potassium formate or sodium formate, wherein the ratio of the weight proportion of the trivalent chromium compound to the weight proportion of the complexing agent, in particular of the formates, lies between 1:1.1 and 1:1.4 and preferably between 1:1.2 and 1:1.3 and particularly preferably at 1:1.25.
7. Method according to one of the preceding claims, characterised in that the electrolyte solution comprises an alkali metal sulphate, preferably potassium sulphate or sodium sulphate, to increase the conductivity, and/or is free of halides, in particular free of chloride ions and bromide ions, and free of a buffering agent and in particular free of a boric acid buffer.
8. Method according to one of the preceding claims, characterised in that the concentration of the trivalent chromium compound in the electrolyte solution is at least 10 g/l and preferably more than 15 g/l and lies particularly preferably at 20 g/l or more.
9. Method according to one of the preceding claims, characterised in that the pH value of the electrolyte solution, measured at a temperature of 20°C, lies between 2.0 and 3.0 and preferably between 2.5 and 2.9 and particularly preferably at 2.7.
10. Method according to one of the preceding claims, characterised in that the metal strip is moved through the electrolyte solution at a strip speed of at least 100 m/minute.
11. Method according to one of the preceding claims, characterised in that the coating applied from the electrolyte solution has a total weight overlay of chromium of at least 40 mg/m², preferably of 70 mg/m² to 180 mg/m², wherein the proportion of the total weight overlay of chromium present in the chromium oxide lies at least at 5%, preferably at 10 to 15%.
12. Method according to one of the preceding claims, characterised in that the coating applied from the electrolyte solution has a chromium oxide proportion with a weight overlay of chromium bound as chromium oxide of at least 5 mg Cr per m², preferably of at least 7 mg Cr per m² and lies particularly preferably between 5 and 15 mg/m².
13. Method according to one of the preceding claims, characterised in that the coating (B) deposited on the surface of the metal strip (M) is composed of at least two layers (B1, B3) with different composition with regard to their proportion of chromium metal and chromium oxide, wherein the lower layer (B1) facing the metal strip has an average weight proportion of chromium oxide which lies in particular in the range from 10% to 15%, and the upper layer (B3) has a high weight proportion of chromium oxide which lies in particular at more than 30%, preferably at more than 50%.
14. Method according to one of the preceding claims, characterised in that the coating deposited on the surface of the metal strip (M) is composed of three layers (B1, B2, B3) with different composition with regard to their proportion of chromium metal and chromium oxide, wherein the lower layer (B1) facing the metal strip has an average weight proportion of chromium oxide which lies in particular in the range from 10% to 15%, a middle layer (B2) has a low weight proportion of chromium oxide which lies in particular in the range from 2% to 10%, and the upper layer (B3) has a high weight proportion of chromium oxide which lies in particular at more than 30%, preferably at more than 50%.

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