EP3666931A1 - Process of fabricating a metal band having a chromium and chromium oxide coating using a trivalent chromium containing electrolyte - Google Patents

Abstract

The invention relates to a method for producing a metal strip (M) coated with a coating (B), the coating (B) containing chromium metal and chromium oxide and electrolytically from an electrolyte solution (E) containing a trivalent chromium compound onto the metal strip (M ) is applied by switching the metal strip (M) as the cathode into contact with the electrolyte solution (E) during an electrolysis period. An efficient deposition of a coating with a high chromium oxide content is achieved if the metal strip (M) is passed at a predetermined strip speed (v) in a strip running direction one after the other through several electrolysis tanks (1a, 1b, 1c; 1a to 1h) arranged one behind the other in the strip running direction is, at least in the last electrolysis tank (1c; 1h) seen in the direction of travel of the tape or in a rear group of electrolysis tanks (1g, 1h) the electrolyte solution (E) has an average temperature over the volume of the electrolysis tank of at most 40 ° C and the electrolysis time (t _E), in which the metal strip (M) is in electrolytically effective contact with the electrolytic solution (E), in the last electrolysis tank (1c) or in the rear group of electrolysis tanks (1g, 1h) is less than 2.0 seconds.

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

For coating the steel substrate with a coating containing metallic chromium and chromium oxide, electrolytic coating methods are known from the prior art, with which the coating is applied in a coil coating system to a strip-shaped steel sheet using an electrolyte containing chromium VI. However, these coating processes have considerable disadvantages due to the environmental and health-endangering properties of the chromium-VI-containing electrolytes used in the electrolysis process and must be replaced by alternative coating processes in the foreseeable future, since 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 do without the use of chromium-VI-containing electrolytes. For example, from WO 2015/177315-A1 a method for the electrolytic coating of an electrically conductive substrate, which is in particular a black plate (uncoated steel plate) or a tin plate (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 electrolytic solution which contains a trivalent chromium compound (Cr-III), an anode is provided which prevents or at least reduces the oxidation of chromium (III) ions to chromium (VI) ions and removes hydrogen bubbles which arise during the electrodeposition of the coating on the surface of the substrate. It was found that the deposition reaction and the surface properties of the electrodeposited coating depend on the temperature of the electrolytic solution and that temperatures of the electrolytic solution between 30 ° C and 70 ° C are suitable to produce coatings with a good surface appearance. A preferred temperature range between 40.degree. C. and 60.degree. C. has been recognized as being advantageous in relation to an efficient deposition reaction because the electrolyte solution has good conductivity at these temperatures.

From the WO 2015/177314-A1 A process for the electrolytic coating of a strip-shaped steel sheet with a chromium metal / chromium oxide (Cr-CrOx) layer in a strip coating installation is known, 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 contained in the electrolytic solution in addition to the trivalent chromium compound (Cr-III), can also contain chromium sulfates and chromium carbides in addition to the components chromium metal and chromium oxide, depends very much on the current densities of the electrolysis depends on which are set at the anodes during the electrolytic deposition process in the electrolytic tanks in which the electrolytic solution is contained. It was found that depending on the current density, three areas (Regime I, Regime II and Regime III) form, with no chromium-containing deposition on the steel substrate taking place in a first area with low current density up to a first current density threshold (Regime I) , in a second area with medium current density (Regime II) there is a linear relationship between the current density and the weight support of the deposited coating and at current densities above a second current density threshold (Regime III) the applied coating is partially decomposed so that the weight support of the chrome the applied coating in this area initially drops as the current density increases and then decreases to one at higher current densities sets constant value. In the area with medium current density (Regime II), essentially 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 Proportion of chromium oxide, which in the range of higher current densities accounts for between ¼ and 1/3 of the total weight of the coating. The values of the current density thresholds which delimit the areas (regimes I to III) depend on the belt speed at which the steel sheet is moved through the electrolyte solution.

In the WO 2014/079909 A1 It is mentioned that in order to achieve a corrosion resistance sufficient for packaging applications of a black plate (steel plate) coated with a chromium-chromium oxide coating, a minimum coating requirement 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 coating of chromium oxide of at least 5 mg / m ² is required in the coating in order to achieve a corrosion resistance sufficient for packaging applications.

The object of the present invention is to provide a process which is as efficient as possible and which can be carried out on a large scale in a coil coating installation for producing a metal strip coated with a coating of chromium and chromium oxide on the basis of an electrolyte solution with a trivalent chromium compound, the coating having the highest possible proportion of chromium oxide to ensure sufficient corrosion resistance of the coated metal strip and a good adhesive base for organic coatings, such as To achieve lacquers or polymer films made of PET or PP.

This object is achieved by a method having 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 which contains chromium metal and chromium oxide is applied electrolytically from an electrolyte solution which contains a trivalent chromium compound to a metal strip, in particular a steel strip, by bringing the metal strip into contact with the electrolyte solution as a cathode the metal strip is passed in succession at a predetermined strip speed in a strip running direction through a plurality of electrolysis tanks arranged one behind the other in the strip running direction, the electrolyte solution having a temperature averaged over the volume of the electrolysis tank, at least in the last electrolysis tank seen in the strip running direction or in a rear group of electrolysis tanks is less than 40 ° C and the electrolysis time in which the metal strip is in electrolytically effective contact with the electrolytic solution in the last electrolysis tank or in the rear group of electrolysis tanks is less than 2.0 seconds. The temperature of the electrolyte solution in the last electrolysis tank or in the rear group of electrolysis tanks, as seen in the direction of travel of the belt, is preferably between 25 and 38 ° C.

When one speaks of the temperature of the electrolyte solution or of the temperature in an electrolysis tank, the mean temperature in each case is the average temperature that results over the entire volume of an electrolysis tank. There is usually a temperature gradient in the electrolysis tanks with an increase in temperature from top to bottom. When we speak of chromium oxide, we mean all oxide forms of chromium (CrOx), including chromium hydroxides, in particular chromium (III) hydroxide and chromium (III) oxide hydrate, as well as mixtures thereof.

It has been shown that at temperatures of the electrolyte solution of 40 ° C or less, the formation of chromium oxide is promoted. Therefore, coatings with a higher proportion of chromium oxide can be produced at temperatures of the electrolyte solution of below 40 ° C. A higher proportion of chromium oxide in the coating proves to be dispersed with regard to the 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 with 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 coil coating system at high belt speeds, which are preferably more than 100 m / min.

The electrolysis time in which the metal strip is in electrolytically effective contact with the electrolyte solution is expediently less than 2 seconds in each of the electrolysis tanks, so that the metal strip can be guided at a constant strip speed through the plurality of electrolysis tanks which are arranged one behind the other in the strip running direction and which are expediently of identical design. 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 electrolytically effective 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 better deposition efficiency, it may be advantageous to choose the temperature of the electrolytic solution in the first electrolytic tank or in the front group of electrolytic tanks to be higher than in the last electrolytic 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.degree. C. and in particular between 53.degree. C. and 70.degree. C., since in this temperature range chromium is deposited more efficiently, in particular in the form of chromium metal , can be observed. Setting a higher temperature of the electrolytic solution in the first electrolytic tank or in the front group of electrolytic tanks of more than 50 ° C and simultaneously setting the temperature of the electrolytic solution in the last electrolytic tank or in the rear group of electrolytic tanks of less than 40 ° C a coating is deposited on the surface of the metal strip which comprises at least a lower and an upper layer, the lower layer 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 is deposited and the lower layer has a small proportion of chromium oxide and the upper layer has a higher 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%.

For apparatus reasons, however, 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 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 has to be cooled in order to maintain the preferred temperatures. This is made more difficult by the fact that the circulatory systems of the electrolysis tanks are usually coupled. It can therefore be expedient for apparatus reasons to maintain the same temperature in each case in the electrolysis tanks in order to avoid a different setting which is complex in terms of equipment. From a result-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.

In a preferred embodiment of the method 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 electrolytic solution in the second electrolysis tank or the rear group of electrolysis tanks.

In a further preferred embodiment, the metal strip is first passed 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 electrolytic solution but also on the current density of the electrolysis process. It has been shown that at higher current densities, which are in the region of Regime III, where a (partial) decomposition of the applied coating is already taking place, a higher proportion of the chromium oxide is produced in the coating compared to the lower current densities in Regime II, where a linear relationship can be observed between the deposited weight of the chromium and the current density. In order 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 coating layer, it is therefore advantageous in which Apply a low current density j ₁ or j ₂ when viewed in the direction of travel of the tape or in the front group of electrolysis tanks and possibly in the second electrolysis tank following in the direction of travel of the tape or in the middle group of electrolysis tanks, and in the last electrolysis tank as seen in the direction of travel of the tape or in the rear group of electrolysis tanks to provide a high current density j ₃ in regime III, j ₁ and j _{2 being} less than j ₃ and, for example at a belt speed of 100 m / min, the low current densities j ₁ and j ₂ each being greater than 20 A. / dm ² (and thus above the first current density threshold of approx. 20 A / dm2 and are therefore in the area of Regime II) and 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 at a 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 for a lower current density in the first electrolysis tank or in the front group of electrolysis tanks compared to the second electrolysis tank following in the direction of travel of the belt or in the middle group of electrolysis tanks, so that the relation 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 with respect to their proportion of chromium metal and chromium oxide, the lower layer facing the metal strip having an average proportion by weight of chromium oxide, which in particular in the Is in the range from 10% to 15%, the middle layer has a low proportion by weight of chromium oxide, which is in particular in the range from 2% to 10%, and the upper layer has a high proportion by weight of chromium oxide, which is in particular more than 30%, is preferably more than 50%. With regard to the adhesion of organic coatings, such as organic paints or polymer films made of PET or PP, it is advantageous if the layer with the high proportion of oxide is on the outside, since it has been shown that chromium oxide is a better basis for adhesion compared to chromium metal forms organic materials.

By dividing the electrolysis tanks arranged one behind the other in the strip running direction and setting different current densities increasing in the strip running direction in the individual electrolysis tanks, it is possible on the one hand to maintain high strip speeds of more than 100 m / min and on the other hand to have a sufficiently high weight coating on at least one side of the To deposit metal strip, the coating having the chromium oxide content of at least 5 mg / m ² , preferably more than 7 mg / m ² , required for adequate corrosion resistance. The total weight of the chromium oxide preferably does not exceed 15 mg / m ² , since with higher weights of the chromium oxide a reduced adhesion of organic coatings made of lacquers or thermoplastic polymer materials is observed. For this reason, a preferred range for the chromium oxide weight support 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, compared to the last electrolysis tank seen in the direction of travel of the belt or in the rear group of electrolysis tanks, j _{2 is} used, energy can also be saved since lower electric 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 support of chromium oxide is produced in the coating, since even at 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 certain proportion of chromium oxide is deposited on the metal substrate. The greater proportion of chromium oxide is seen in the last electrolysis tank or in the direction seen in the belt running direction rear group of electrolysis tanks, because it sets the high current density j ₃ , in which the proportion of chromium oxide in the total coating is higher.

As 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 proportion by weight of the total coating, which is approximately 9 to 15%, is due to the chromium oxide Chromium oxide crystals already form on the surface of the metal strip 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 germ cell for the growth of further oxide crystals, which is why the efficiency of the deposition of chromium oxide or the proportion of chromium oxide in the total coating layer in the last electrolysis tank or in the rear group of electrolysis tanks increases. Thus, with 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 coating of chromium oxide of preferably more than 5 mg / m ² on the surface of the Metal bands are generated.

The proportion of chromium oxide produced 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 with higher current densities (and consequently lower oxide proportion) due to the higher proportion of oxygen in the coating compared to electrolytic deposition ) 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, thereby increasing the efficiency of the method. It has been shown that a current density of at least 20 A / dm ^{2 is} required to maintain a preferred strip speed of at least 100 m / min, 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 is the first Current density threshold at a belt speed of approx. 100 m / min, which delimits regime I (no chrome deposition) from regime II (chrome deposition with a linear relationship between current density and the chrome weight support of the deposited coating).

The current densities (j ₁ , j ₂ , j ₃ ) in the electrolysis tanks are each adapted to the belt speed, with at least essentially a 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 lower 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, preferably at more than 8%, in particular between 8 and 15%, and directly on the surface of the metal strip is 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 pair of anodes with two opposite anodes is expediently arranged in each electrolysis tank, the metal strip passing between the opposite anodes of an anode pair. This enables a uniform current density distribution to be achieved around the metal strip. Expediently, 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 strip speed of the metal strip is expediently chosen so that the electrolysis time (t _E ) in which the metal strip is in electrolytically effective 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 support of at least 40 mg / m ² and in particular from 70 mg / m ² to 180 mg / m ² in order to achieve sufficient corrosion resistance of the coated metal strip. The one contained in the coating The proportion by weight of chromium oxide in 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 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 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 electrolytic 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 is both in this preferred composition and in other compositions at least 10 g / l and preferably more than 15 g / l and in particular is 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 electrolytic solution can comprise an alkali metal sulfate, preferably potassium or sodium sulfate. The electrolyte solution is preferably free of halides, in particular free of chloride and bromide ions and free of a buffering agent and in particular free of 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. An acid, for example sulfuric acid, can be added to adjust the pH of the electrolyte solution.

After the electrolytic application of the coating, an organic coating, in particular a lacquer or a thermoplastic material, for example a polymer film made of PET, PE, PP or a mixture thereof, can be applied to the surface of the chromium metal and chromium oxide coating for additional protection against corrosion and 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 illustrating the invention by way of example and not restricting it in relation 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 strip running direction v;

Figure 2:

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 strip running direction v;

Figure 3:

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

Figure 4:

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

Figure 5:

Graphical representation of the weight support of a coating, which contains chromium metal and chromium oxide, deposited on a metal strip, depending on the temperature of the electrolyte solution and the electrolysis time.

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 side by side 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 in succession. For this purpose, the metal strip M is drawn through a transport device, not shown here, into a strip running direction v at a predetermined strip speed through the electrolysis tanks 1a-1c. Current rolls S are arranged above the electrolysis tanks 1a-1c, via which the metal strip M as Cathode is switched. A deflection roller U is also arranged in each electrolysis tank, around which the metal strip M is guided and is thereby directed into and out of the electrolysis tank.

At least one anode pair AP is arranged in each electrolysis tank 1a-1c below the liquid level of the electrolyte solution E. In the exemplary embodiment shown, two anode pairs AP arranged one behind the other in the strip running direction are provided in each electrolysis tank 1a-1c. The metal strip M is passed between the opposite anodes of an anode pair AP. In the embodiment of Figure 1 Two anode pairs AP are thus arranged in each electrolysis tank 1a, 1b, 1c in such a way that the metal strip M is passed through these anode pairs AP in succession. The last anode pair APc in the downstream direction of the last electrolysis tank 1c seen in the strip running direction v has a shortened length in comparison to the other anode pairs AP. As a result, a higher current density can be generated with this last anode pair APc when an equally high electrical current 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 passed in this pretreated form through the electrolysis tanks 1 a - 1 c, the metal strip M being switched as a cathode by supplying electrical current 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 in each of the electrolysis tanks 1a-1c arranged one behind the other in the direction of tape travel v. The electrolytic 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 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 ° C. 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 electrolysis tanks 1a and 1b arranged upstream. In this embodiment of the method according to the invention, the temperature of the electrolytic solution in the last electrolysis tank 1c is preferably between 25 ° C. and 37 ° C. and in particular 35 ° C. The temperature of the electrolyte solution in the first two electrolysis tanks 1a, 1b in this exemplary embodiment is preferably between 50 ° C. and 75 ° C. and in particular 55 ° C. Due to the lower temperature of the electrolytic solution E, the deposition of a chromium / chromium oxide layer with a higher proportion of chromium oxide is promoted in the last electrolysis tank 1c.

This is from the diagram of the Figure 5 clearly shows the deposition of the chromium oxide content (CrOx in mg / m ² ) of a coating B 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). The diagram shows that with a given electrolysis time (e.g. 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 approximately 35 ° C., a maximum of the weight support of the chromium oxide can be observed. It follows from this that the deposition of coatings with a high chromium oxide content is promoted in the temperature range according to the invention of up to 40 ° C. and preferably in the range from 20 to 40 ° C.

Furthermore is off Figure 5 to recognize that the weight support of the chromium oxide increases with increasing electrolysis time t _E. In order to achieve a coating process which is as efficient as possible and 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 weight deposits of chromium oxide of more than 20 mg / m ² can be achieved if the temperature of the electrolyte solution 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 strip M, which is connected as the cathode and passed through the electrolysis tanks 1a-1c, is in electrolytic contact with the electrolyte solution _E during an electrolysis time t E. At belt speeds between 100 and 700 m / min, the electrolysis time is in each of the electrolysis tanks 1 a, 1b, 1c preferably between 0.5 and 2.0 seconds. According to the invention, in order to achieve a high coating efficiency and a 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 is in particular between 0.6 seconds and 1.8 seconds. The total electrolysis time in which the metal strip M is in electrolytically effective contact with the electrolyte solution E across all electrolysis tanks 1a-1c is accordingly between 1.8 and 5.4 seconds.

The anode pairs AP arranged in the electrolysis tanks 1a-1c can be supplied with direct electrical current in such a way that the same current density is present in the electrolysis tanks 1a, 1b, 1c. In order to deposit a coating B with several layers B1, B2, B3 of different composition on the metal strip M, 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 in the direction of tape travel v, a medium current density j ₂ in the second electrolysis tank 1b in the direction of tape travel and a high current density j ₃ in the last electrolysis tank 1c in the direction of tape travel , so that the relation j ₁ <j ₂ <j ₃ applies and the low current density j ₁ > 20 A / dm ² .

The current densities set in the electrolysis tanks 1a-1c electrolytically deposit a layer containing chromium metal and chromium oxide 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 by the proportion of chromium oxide.

In Figure 3 a sectional illustration of a metal strip M electrolytically coated on one side with the method according to the invention is shown schematically. A coating B is applied to one side of the metal strip M and is composed of the individual layers B1, B2, B3. 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 as essential components metallic chromium (chrome metal) and chromium oxides (CrOx), the composition of the individual layers B1, B2, B3 in relation to their respective weight fraction of Chromium metal and chromium oxide are different due to the different current densities j ₁ , j ₂ , j ₃ in the electrolysis tanks 1a, 1b, 1c. Furthermore, a possibly different temperature of the electrolyte solution in the electrolysis tanks 1a, 1b, 1c also contributes to the fact that the individual layers differ in terms of their composition since (as above with reference to FIG Figure 5 explained) at lower temperatures of 40 ° C or less, the formation of chromium oxide is promoted. In order to achieve the highest possible proportion of oxide in the 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. are preferably used in the last electrolysis tank 1c or less set.

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 compared to the layer B2 which is applied in the second (middle) electrolysis tank 1b, since lower current densities , which are within Regime II, form higher oxide fractions in the coating. In the last electrolysis tank 1c, a current density j3 is set which lies in regime III, in which an increased chromium oxide content in of the coating is produced, which is preferably more than 40% by weight and particularly preferably more than 50% by weight.

Suitable current densities j ₁ , j ₂ , j ₃ in the individual electrolysis tanks 1a, 1b, 1c at different belt speeds are shown in Table 1 by way of example. 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 support of chromium). The current density j _{1 of} the first electrolysis tank _{1 a} is expediently selected such that it is close to the first current density threshold, which delimits the regime I (in which no chromium deposition takes place yet) from the regime II. At these low current densities j ₁ , a chrome 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 the regime II. The layer deposited in the first electrolysis tank 1a therefore has B1 has a higher chromium oxide content in comparison to the layer B2 deposited in the second electrolysis tank 1b.

A current density j ₃ is preferably set in the last electrolysis tank 1a, which lies 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 in the current densities in regime II last electrolysis tank 1c deposited layer B3 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 coating 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 overlay can be applied either in a "coil coating" process or in a board process, the coated metal strip in the board process first being divided into boards which are then coated with an organic lacquer or coated with a polymer film .

In Figure 2 shows a second embodiment of a strip coating system with eight electrolysis tanks 1a-1h arranged one behind the other in the strip running direction v. 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 downstream electrolysis tanks 1c-1f and a rear group with the last two electrolysis tanks 1g and 1h. According to the invention, a temperature of the electrolytic solution of 40 ° C. or less is present in the rear group of the electrolysis tanks 1g and 1h. 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 a higher temperature can be present. To increase the separation efficiency, higher temperatures of more than 50 ° C. and in particular of approximately 55 ° C. in the electrolysis tanks 1a, 1b of the front group and the electrolysis tanks 1c-1f of the middle group are preferred. For apparatus reasons, 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.

Current densities j ₁ , j ₂ , j ₃ are preferably present in the groups of electrolysis tanks, with a low current density j ₁ in the front group of electrolysis tanks _{1 a} -1f and an average current density j ₂ in the middle group of electrolysis tanks ₁ c- ₁ f 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 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 of the front group in the electrolysis tanks 1c to 1f of the middle group each a current density j ₂ and in the electrolysis tanks 1g, 1h of the rear group a current density j _{3 is} set, where j ₁ <j ₂ <j ₃ .

In the front group of electrolysis tanks 1a, 1b electrolytically a first layer B1 containing chromium metal and chromium oxide and 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 on the metal strip M applied. As in the embodiment of Figure 1 Because of 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, layers B1, B2, B3 have different compositions, layer B1 containing a higher chromium oxide content than the second Layer B2 and the third layer B3 contains a higher chromium oxide content than the two layers B1 and B2.

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

With the coil coating line 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 the cathode is in electrolytically effective 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 chromium weight support 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 chromium oxide weight support contained in the chromium oxide, averaged over the entire support of layer B, is at least 5% and preferably between 10% and 15%. Coating B expediently has a total chromium oxide content with a chromium oxide bond weight of chromium bound of at least 3 mg chromium per m ² and in particular 3 to 15 mg / m ² . The weight of the chromium bound as chromium oxide, averaged over the entire coating B coating, is preferably at least 7 mg of chromium per m ² . Good adhesion of organic paints or thermoplastic polymer materials to the surface of coating B can be achieved with chromium oxide weights of up to approximately 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 the coating B is therefore between 5 and 15 mg / m ² .

Examples:

Laboratory tests for coating steel sheets with a chromium / chromium oxide coating are described in detail below to explain the embodiment of the invention:
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 experiments in a 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 part of the electrolyte solution should be as free as possible from organic residues. The Cr (III) salts can be prepared on an industrial scale by reduction from Cr (VI) salts. A less noble metal than chromium (variant 1), or alternatively an organic component (variant 2) is preferably used as the reducing agent. The pH of the electrolyte solution was adjusted by adding sulfuric acid and then filling up with deionized water.

A steel sheet already coated with a chromium / chromium oxide layer was used as the substrate for the coating experiments. This material was electrolytically coated with a chromium (III) electrolyte at 55 ° C. and Table 5 below describes the already 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 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 cathodically, 30 sec., 70 ° C in sodium hydroxide solution) and then rinsed with deionized water. The subsequent pickling process was dispensed with due to the already 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 strip with a belt speed of 100 m / min was simulated. At this speed, the selected current density of 60 A / dm ^2, which is kept constant during the test, is in Regime III (see Table 2) and thus (at least at the lower temperatures) mainly produces chromium oxide. 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 electrolysis time in the relevant regime III is given in Table 5 with "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 from approx. 40 ° C. a significantly lower proportion of the chromium oxide is present in the coating. 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. according to 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. As the duration of the electrolysis increased, a higher oxide occupancy was observed in the laboratory tests. However, short electrolysis times of less than 2 seconds are large in terms of efficiency Process control to be preferred 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 3rd 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 amount used / L Sodium formate 41.4 g basic chromium sulfate 120 g Sulfuric acid 96% ∼7.5 ml Sodium sulfate 100 g

Claims (15)

Method for producing a metal strip (M) coated with a coating (B), the coating (B) containing chromium metal and chromium oxide and being applied electrolytically to the metal strip (M) from an electrolyte solution (E) which contains a trivalent chromium compound, by bringing the metal strip (M) connected as the cathode into contact with the electrolyte solution (E) during an electrolysis period, characterized in that the metal strip (M) at a predetermined strip speed (v) in a strip running direction one after the other through a plurality of electrolysis tanks arranged one behind the other in the strip running direction (1a, 1b, 1c; 1a to 1h), wherein at least in the last electrolysis tank (1c; 1h) or in a rear group of electrolysis tanks (1g, 1h) as seen in the direction of travel of the tape, the electrolyte solution (E) is one over the volume of the Electrolysis tanks averaged temperature of less than 40 ° C and preferably between 25 ° C and 38 ° C and the electrolysis time er (t _E ), in which the metal strip (M) is in electrolytically effective contact with the electrolytic solution (E), in the last electrolysis tank (1c) or in the rear group of electrolysis tanks (1g, 1h) for less than 2.0 seconds is. A method according to claim 1, characterized in that the electrolysis time (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 is preferably between 0.3 and 2.0 seconds, in particular between 0.6 seconds and 1.8 seconds, the total electrolysis time (t _E ) in which the metal strip (M) is electrolytically active in contact with the electrolyte solution (E) is preferably between 2 seconds and 16 seconds and particularly preferably between 4 seconds and 14 seconds. Method according to one of the preceding claims, characterized in that the average temperature of the electrolytic solution in the first electrolysis tank (1a) or in the front group of electrolysis tanks (1a, 1b) is greater than 40 ° C and in particular between 50 ° C and 75 ° C lies. Method according to one of claims 1 to 4, characterized in that the temperature of the electrolyte solution averaged over the volume of the respective electrolysis tank in all electrolysis tanks (1a - 1c; 1a - 1h) and in particular in the last electrolysis tank or the rear group of electrolysis tanks between 20 ° C and less than 40 ° C and preferably between 25 ° C and 38 ° C. Method according to one of the preceding claims, characterized in that the metal strip 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, 1h ), the mean temperature of the electrolytic solution in the first electrolysis tank (1a) or the front group of electrolysis tanks (1a, 1b) being greater than the mean temperature of the electrolytic solution in the last electrolysis tank (1c) or the rear group of electrolysis tanks ( 1g, 1h). A method according to claim 7, characterized in that a low current density (j ₁ ) in the first electrolysis tank (1a) or in the front group of electrolysis tanks (1a, 1b), in the second electrolysis tank (1b) or there is a medium current density (j ₂ ) in the middle group of electrolysis tanks (1c - 1f) and a high current density (j ₃ ) in a last electrolysis tank (1c) as seen in the direction of travel of the strip or in a rear group of electrolysis tanks (1g, 1h), where j ₁ ≤ j ₂ ≤ j ₃ and the low current density (j ₁ ) is greater than 20 A / dm ² . Method according to one of the preceding claims, characterized in that the electrolyte solution in addition to the trivalent chromium compound, which preferably comprises basic Cr (III) sulfate (Cr ₂ (SO ₄ ) ₃ ), at least one complexing agent, 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, between 1: 1.1 and 1: 1.4 and preferably between 1: 1.2 and 1: 1 , 3 and particularly preferably 1: 1.25. Method according to one of the preceding claims, characterized in that the electrolyte solution to increase the conductivity comprises an alkali metal sulfate, preferably potassium or sodium sulfate, and / or free of halides, in particular free of chloride and bromide ions and free of a buffering agent and in particular free of a boric acid buffer. Method according to one of the preceding claims, characterized in that the concentration of the trivalent chromium compound in the electrolytic solution is at least 10 g / l and preferably more than 15 g / l and particularly preferably 20 g / l or more. Method according to one of the preceding claims, characterized in that the pH of the electrolyte solution (measured at a temperature of 20 ° C) between 2.0 and 3.0 and preferably between 2.5 and 2.9 and particularly preferably at 2 , 7 lies. Method according to one of the preceding claims, characterized in that the metal strip is moved through the electrolyte solution at a strip speed of at least 100 m / min. Method according to one of the preceding claims, characterized in that the coating applied from the electrolyte solution has a total weight support of chromium of at least 40 mg / m ² , preferably from 70 mg / m ² to 180 mg / m ² , the one contained in the chromium oxide The proportion of the total weight of the chromium is at least 5%, preferably 10 to 15%. Method according to one of the preceding claims, characterized in that the coating applied from the electrolyte solution has a chromium oxide content with a chromium oxide bond weight of chromium bound of at least 5 mg Cr per m ² , preferably at least 7 mg Cr per m ² and particularly preferably between 5 and 15 mg / m ² . Method according to one of the preceding claims, characterized in that the coating (B) deposited on the surface of the metal strip (M) is composed of at least two layers (B1, B3) with a different composition with respect to their proportion of chromium metal and chromium oxide, wherein the lower layer (B1) facing the metal strip has an average proportion by weight of chromium oxide, which is in particular in the range from 10% to 15%, and the upper layer (B3) has a high proportion by weight of chromium oxide, which in particular is more than 30%, is preferably more than 50%. Method according to one of the preceding claims, characterized in that the coating deposited on the surface of the metal strip (M) is composed of three layers (B1, B2, B3) with different compositions in relation to their proportion of chromium metal and chromium oxide, the The lower layer (B1) facing the metal strip has an average proportion by weight of chromium oxide, which is in particular in the range from 10% to 15%, a middle layer (B2) has a low proportion by weight in chromium oxide, which is in particular in the range from 2% to 10% , and the upper layer (B3) one has a high proportion by weight of chromium oxide, which is in particular more than 30%, preferably more than 50%.

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