EP3733932A1 - Method for producing a metal strip coated with a coating of chromium and chromium oxide based on an electrolytic solution containing a trivalent chromium compound and electrolytic system for performing the method - 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 containing a trivalent chromium compound and at least one salt to increase conductivity and at least contains an acid or a base for setting a desired pH value, is applied to the metal strip (M) by bringing the metal strip (M) into contact with the electrolyte solution (E) in an electrolytically effective manner during an electrolysis period, the metal strip (M ) is passed through several electrolysis tanks (1a, 1b, 1c; 1a to 1h) arranged one behind the other in the direction of tape travel at a predetermined tape speed (v), at least the first electrolysis tank (1c; 1h) or a front group as seen in the tape travel direction of electrolysis tanks (1a, 1b) filled with a first electrolyte solution (E1) i st and the last electrolysis tank (1c; 1h) or a rear group of electrolysis tanks (1g, 1h) is filled with a second electrolyte solution (E2), the second electrolyte solution (E2) not containing any other components apart from the trivalent chromium compound and the at least one salt and the at least one acid or base and in particular is free from organic complexing agents and free from buffering agents.

Description

The invention relates to a method for producing a metal strip coated with a coating according to the preamble of claim 1, as well as an electrolysis system for the electrolytic deposition of a coating containing chromium and chromium oxide on the surface of a metal strip.

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 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 harmful 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 2015/177314-A1 a method for electrolytic coating of a strip-shaped steel sheet with a chromium-metal-chromium oxide (Cr-CrOx) layer in a coil-coating system, in which the steel sheet, connected as a cathode, is passed through a single electrolyte solution, which is a trivalent chromium compound, at high belt speeds of more than 100 m / min (Cr-III) as well as a complexing agent and a conductivity-increasing salt and is free of chlorides and buffering agents such as boric acid. Organic substances, in particular formates and preferably sodium or potassium formate, are used as complexing agents. To set a preferred pH value in the range from 2.5 to 3.5, the electrolyte solution can contain sulfuric acid. The coating of chromium metal and chromium oxide can be deposited in layers in successive electrolysis tanks or strip coating systems arranged one after the other, the electrolysis tanks each being filled with the same electrolyte solution.

It was observed that the electrodeposited coating can also contain chromium sulfates and chromium carbides in addition to the components chromium metal and chromium oxide and that the proportion of these components in the total weight of the coating depends to a large extent on the current densities set in the electrolysis tanks. 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 of the applied coating in this area initially drops with increasing current density and then adjusts to a constant value with higher current densities. 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 Proportion of chromium oxide, 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, which the areas (Regime I to III) They are dependent on the belt speed with which the steel sheet is moved through the electrolyte solution.

In the WO 2014/079909 A1 it is mentioned that in order to achieve sufficient corrosion resistance for a black sheet (uncoated steel sheet) coated with a chromium-chromium oxide coating, a minimum of 20 mg / m ^{2 of} the coating is required in order to achieve a corrosion resistance comparable to that of conventional ECCS. It has also been shown that in order to achieve sufficient corrosion resistance for packaging applications, a minimum amount of chromium oxide of at least 5 mg / m ² in the coating is necessary. In order to ensure such a minimum amount of chromium oxide in the coating, it appears expedient to apply high current densities in the electrolysis process so that work can be carried out in the area (regime III) in which a coating with a relatively high proportion of chromium oxide is on the steel substrate separates. In order to obtain a coating with a high chromium oxide content, high current densities would have to be used. Achieving high current densities in the electrolysis tanks, however, requires a considerable amount of energy to apply high currents to the anodes.

The object of the present invention is to provide a method that is as efficient, cost-effective and energy-saving as possible for producing a metal strip with a coating containing chromium oxide and based on an electrolyte solution with a trivalent chromium compound. In any case, the use of chromium VI-containing substances, including as intermediate products of the electrolysis process, should be avoided in order to be able to fully comply with the legal requirements regarding the ban on chromium VI-containing substances. Furthermore, the metal strip coated according to the method should have the highest possible corrosion resistance and a good adhesive base for organic coatings, e.g. for organic paints and for polymer coatings, in particular for polymer films, e.g. made of PET, PE or PP.

This object is achieved by a method with the features of claim 1 and by an electrolysis system with the features of claim 14 and by a metal strip according to claim 15. Preferred embodiments of the method and the electrolysis system can be found in the dependent claims.

In the method according to the invention, a coating which contains chromium metal and chromium oxide / chromium hydroxide is electrolytically produced from an electrolyte solution which contains a trivalent chromium compound and at least one salt to increase conductivity and at least one acid or base to set a desired pH , applied to a metal strip, in particular a steel strip, in that the metal strip is brought into contact with the electrolyte solution in an electrolytically effective manner, the metal strip being guided successively at a predetermined strip speed in a strip running direction through several electrolysis tanks arranged one behind the other in the strip running direction, at least the one in the strip running direction seen first electrolysis tank or a front group of electrolysis tanks is filled with a first electrolyte solution and the last electrolysis tank or a rear group of electrolysis tanks with a second electrolyte solution when viewed in the direction of belt travel is filled, wherein the second electrolyte solution, apart from a trivalent chromium compound and at least one salt and at least one acid or base, contains no further components and in particular is free from organic complexing agents and free from buffering agents. No chromium VI-containing substances are used, not even as intermediate products, so that the process is completely free of chromium VI-containing substances and is therefore environmentally and health-friendly when carrying out the process.

It has surprisingly been shown that it is possible to electrolytically deposit a chromium-containing layer on the surface of the metal strip even without the use of organic complexing agents, such as formates, as a component of the electrolyte solution, the deposited using an electrolyte solution without organic complexing agents Layer consists at least essentially only of chromium oxide and / or chromium hydroxide. It has also been shown that a layer of pure chromium oxide / chromium hydroxide forming the surface of the coating is advantageous with regard to the corrosion resistance and the adhesive effect of organic coatings such as paints or polymer layers. It is therefore provided in the method according to the invention that a second electrolyte solution, which does not contain any organic complexing agents, is contained in the last electrolysis tank seen in the strip running direction or in the rear group of electrolysis tanks. As a result, a layer consisting at least essentially of pure chromium oxide and / or chromium hydroxide can be produced on the surface of the coating. The underlying layers of the coating contained in the or The electrolysis tank (s) upstream in the direction of travel of the strip, on the other hand, contain not only a chromium oxide / chromium hydroxide component but also a proportion of metallic chromium, because the upstream electrolysis tank contains a first electrolyte solution, the (organic) complexing agents, in particular formates such as sodium or potassium formate , contains. The coating applied electrolytically to the surface of the metal strip is thus composed of several superimposed layers, the lower layer (s) containing or containing a mixture of chromium metal and chromium oxide and / or chromium hydroxide, and possibly other chromium compounds such as chromium carbides. and the top layer, which forms the surface of the coating, consists at least essentially of pure chromium oxide and / or chromium hydroxide. The weight of the individual layers can be controlled in particular by the electrolysis times in the individual electrolysis tanks and set to desired values.

When chromium oxide is spoken of, 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. The compounds of chromium and oxygen in which the chromium is present in trivalent form, in particular as dichromium trioxide (Cr ₂ O ₃ ), are preferred. The coating thus contains (in addition to metallic chromium) preferably exclusively trivalent chromium compounds and in particular only trivalent chromium oxides and / or chromium hydroxides.

The top layer of the coating, which is applied in the last electrolysis tank or in the rear group of electrolysis tanks, preferably has a proportion by weight of chromium oxides, including chromium hydroxides, of more than 90%, particularly preferably more than 95%. This ensures a good adhesive base with good adhesion for organic coatings such as paints or polymer layers made of thermoplastics such as PET or PP.

For the electrolytic deposition of the layers, the metal strip, which can be, for example, a (initially uncoated) steel strip (black plate strip) or a tin-plated steel strip (tinplate strip), is placed in the first electrolysis tank or in the front group of electrolysis tanks during a first Electrolysis time t1 in contact with the first electrolyte solution and then in the second electrolysis tank or in the rear group of electrolysis tanks during a second electrolysis time t2 in contact with the second electrolyte solution brought. The total electrolysis time tG = t1 + t2 is preferably in the range from 0.5 to 5.0 seconds and particularly preferably between 1.0 and 1.5 seconds. Here, the metal strip is guided successively through the electrolysis tanks arranged one behind the other in the strip running direction, the strip speed being at least 100 m / min and preferably between 200 m / min and 750 m / min. Due to the high belt speeds, a high efficiency of the process can be guaranteed.

At the preferred belt speeds, the first electrolysis period in which the metal belt is in effective electrolytic contact with the first electrolyte solution is less than 2.0 seconds and the second electrolysis period in which the metal belt is in effective electrolytic contact with the second electrolyte solution is also preferably less than 2.0 seconds.

The electrolysis times in the last electrolysis tank or the rear group of electrolysis tanks are expediently set via the belt speed so that the layer of chromium oxide applied from the second electrolyte solution has a total weight of the chromium oxide of at least 3 mg / m ² and preferably 7 mg / m ² to 10 mg / m ² . These chromium oxide weights ensure adequate corrosion resistance and provide a good adhesive base for organic coatings such as paints or thermoplastic films. To achieve sufficient corrosion resistance for packaging applications, a weight of the chromium oxide in the top layer of at least 5 mg / m ² , preferably more than 7 mg / m ² , is preferred.

To improve the corrosion resistance and to create a barrier against sulfur-containing materials, in particular against contents of packaging containing sulphide or sulphite, after the electrolytic application of the coating, a layer made of an organic material, in particular a lacquer or a thermoplastic plastic, in particular a polymer film made of PET, PE, PP or a mixture thereof, can be applied to the surface of the coating (i.e. to the upper layer made of chromium oxide / chromium hydroxide).

In order to ensure a process that is completely free of substances containing chromium VI, a suitable anode is expediently selected for the electrolytic deposition of the coating and placed in the electrolysis tanks, which oxidizes chromium (III) from the trivalent chromium compound of the electrolyte solution to chromium ( VI) prevents. In particular, anodes with an outer surface or a coating made of a metal oxide, in particular iridium oxide, or of a mixed metal oxide, in particular of iridium-tantalum oxide, have proven to be suitable for this. The anode preferably contains neither stainless steel nor platinum. By using such anodes, coatings can be deposited on the black or tin plate which contain exclusively trivalent chromium oxides and / or chromium hydroxides, in particular Cr ₂ O ₃ and / or Cr (OH) ₃ .

1. (1) Chrom (Cr) Cr⁶⁺ + 3 e^- ⇄ Cr³⁺ +1,33 V
2. (2) Sauerstoff (O) O₂ + 4 H⁺ + 4 e^- ⇄ 2 H₂O +1,23 V

As in every electroplating process, electroplating from a chromium (III) electrolyte also involves at least one anodic oxidation in addition to the cathodic reduction. In the galvanic chrome plating from a chromium (III) electrolyte, the anodic oxidation consists on the one hand of the oxidation of Cr (III) to Cr (VI) and on the other hand of the oxidation of water to oxygen. Both potentials are closely related in the electrochemical series:

1. (1) Chromium (Cr) Cr ⁶⁺ + 3 e ^- ⇄ Cr ³⁺ +1.33 V
2. (2) Oxygen (O) O ₂ + 4 H ⁺ + 4 e ^- ⇄ 2 H ₂ O +1.23 V

The measurement in the associated Daniell element serves as the basis for the potentials. The potential of the redox equation depends on the anode material used. The choice of the anode material therefore largely determines whether reaction (1) is suppressed and only reaction (2) takes place. In order to prevent the formation of Cr ⁶⁺ in the method ^according to the invention, anodes based on metal oxides, especially iridium oxide, or mixed metal oxides, such as mixed metal oxides, which mainly consist of multilayered layers of tantalum oxide and iridium oxide , can be used to suppress reaction 1 consist. The anodes can have an outer surface or an outer coating made of a mixed metal oxide. In particular, anodes with a core made of titanium and an outer coating made of a tantalum oxide-iridium oxide have proven to be suitable. When using such anodes, polarographic measurements (mercury drip electrode) showed that no Cr (VI) was produced.

When using anodes made of stainless steel, the oxidation of Cr (III) to Cr (VI) (reaction 1) is not (sufficiently) suppressed. Comparative measurements with stainless steel anodes show a clearly detectable Cr (VI) concentration after a total coating time of just a few seconds. Stainless steel as an anode material therefore does not at least completely suppress the oxidation of Cr (III) to Cr (VI). This leads to an accumulation of Cr (VI) in the Cr (III) electrolyte and thus to a different deposition mechanism. Therefore, in the method according to the invention, preference is given to using anodes which are free of stainless steel. This ensures that no Cr (VI) is produced during the electrolysis and that the deposited coating only contains Cr (III) compounds or metallic chromium. Furthermore, a post-treatment, e.g. with thiosulfate, which would otherwise be necessary when using stainless steel anodes to reduce deposited Cr (VI) to Cr (III).

Since already in the first electrolysis tank or in the front group of electrolysis tanks and possibly in a middle electrolysis tank or in a middle group of electrolysis tanks a certain weight proportion of the total application of the deposited coating, which is typically around 9 to 25%, is on the chromium oxide , including chromium hydroxide is omitted, chromium oxide crystals are already formed on the surface of the metal strip in the first electrolysis tank or in the front group of electrolysis tanks and in the middle 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 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 is improved. In this way, a sufficiently high weight of chromium oxide, including chromium hydroxide, of preferably more than 5 mg / m ^{2 can be produced} on the surface of the coating in an efficient manner.

The belt speed of the metal belt is preferably 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 2.0 seconds in each of the electrolysis tanks and in particular between 0.5 and 1, 9 seconds and is preferably less than 1.0 seconds and in particular between 0.6 seconds and 0.9 seconds. This ensures on the one hand a higher efficiency of the process and on the other hand the deposition of a coating with a sufficient total weight of the chromium in the coating of preferably at least 40 mg / m ² and in particular from 70 mg / m ² to 180 mg / m ² . The proportion by weight of chromium oxide contained in the coating in relation to the total weight of the coating is preferably more than 10%, particularly preferably more than 20% and in particular between 25 and 50%.

The total electrolysis time (t _E ) in which the metal strip is in electrolytically effective contact with the electrolyte solution (E) is - added up over all electrolysis tanks - preferably less than 16 seconds and is in particular between 3 and 16 seconds. The total electrolysis time is particularly preferably less than 8 seconds and is in particular between 4 seconds and 7 seconds.

Due to the arrangement of the electrolysis tanks through which the metal strip is passed in the direction of travel of the strip, the coating is deposited in layers, whereby in each of the electrolysis tanks, depending on the selected current density in the respective electrolysis tank and depending on the composition of the electrolyte solution, a layer with a different composition, in particular with a different chromium oxide content in the respective layer is generated. For example, in the first electrolysis tank or in the front group of electrolysis tanks, a layer containing chromium metal and chromium oxide / chromium hydroxide with a proportion by weight of chromium oxide, including chromium hydroxide, of less than 15%, in particular from 6 to 10%, on the surface of the metal strip are deposited and in the last electrolysis tank or in the rear group of electrolysis tanks a layer which consists at least essentially of pure chromium oxide and / or chromium hydroxide.

The first and second electrolyte solutions each preferably have a temperature in the range from 20 ° C to 65 ° C and preferably in the range from 30 ° C to 55 ° C and particularly preferably between 35 ° C and 45 ° C. At these temperatures the electrolytic deposition of the layers is very efficient. When the temperature of the electrolyte solution or the temperature in an electrolysis tank is spoken of, 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.

Both the first electrolyte solution and the second electrolyte solution preferably contain, in addition to the trivalent chromium compound, at least one salt which increases the conductivity and at least one acid or base for setting a suitable pH value. Both the first electrolyte solution and the second electrolyte solution are preferably free from chloride ions and free from buffering agents, in particular free from a boric acid buffer.

The trivalent chromium compound of the first electrolyte solution and / or the second electrolyte solution is preferably selected from the group consisting of basic Cr (III) sulfate (Cr ₂ (SO ₄ ) ₃ ), Cr (III) nitrate (Cr (NO ₃ ) ₃ ), Cr (III) oxalate (CrC ₂ O ₄ ), Cr (III) acetate (C ₁₂ H ₃₆ ClCr ₃ O ₂₂ ), Cr (III) formate (Cr (OOCH) ₃ ) or a mixture thereof . The concentration of the trivalent chromium compound in the first electrolyte solution and / or in the second electrolyte solution is preferably at least 10 g / l and particularly preferably more than 15 g / l and in particular 20 g / l or more.

To increase the conductivity, both the first electrolyte solution and the second electrolyte solution contain at least one salt, which is preferably an alkali metal sulfate, in particular potassium or sodium sulfate.

A very efficient deposition of a chromium and / or chromium oxide-containing layer is achieved if the pH (measured at a temperature of 20 ° C.) of the first electrolyte solution and / or the second electrolyte solution is in a range from 2.3 to 5.0 and is preferably between 2.5 and 2.9. The desired pH can be adjusted by adding an acid or base to the first or second electrolyte solution. If basic Cr (III) sulfate is used as the trivalent chromium compound, sulfuric acid or an acid mixture containing sulfuric acid is particularly suitable for setting the desired pH.

The composition of the first electrolyte solution and the second electrolyte solution differs in that the first electrolyte solution contains organic complexing agents, in particular in the form of formates, preferably in the form of potassium or sodium formate, whereas the second electrolyte solution is preferably free of complexing agents and in particular no organic Contains complexing agents such as formates.

Particularly preferred compositions of the first and the second electrolyte solution each comprise basic Cr (III) sulfate (Cr ₂ (SO ₄ ) ₃ ) as a trivalent chromium compound. The concentration of the trivalent chromium compound in the first and the second electrolyte solution is at least 10 g / l and preferably more than 15 g / l and is in particular 20 g / l or more.

Further components of the first electrolyte solution are a salt to increase conductivity, as well as organic complexing agents, in particular the salts of formic acid, such as 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.

In addition to the trivalent, chromium-containing substance, the at least one salt to increase conductivity and at least one acid or base to adjust the pH, the second electrolyte solution preferably contains no further components. This ensures a simple and inexpensive production of the second electrolyte solution.

To produce the second electrolyte solution, the trivalent chromium compound, which has initially been largely freed from organic residues, as well as the at least one salt and the at least one acid or base can be dissolved in water to set a desired pH. Since the second electrolyte solution does not contain any complexing agents, the solution thus obtained should expediently be left to stand for complex formation for at least 5 days, preferably 7 days (in atmospheric oxygen). Thereafter, no fine adjustment of the desired pH value can be made by adding an acid or base.

With the method according to the invention, metal strips, in particular steel strips, can be produced with an electrolytically deposited coating which contains chromium and chromium oxide / chromium hydroxide, the coating being composed of a first layer facing the surface of the metal strip and a second layer lying thereon and the first layer being metallic Contains chromium and the second layer consists at least essentially only of chromium oxide and / or chromium hydroxide, preferably only of trivalent chromium oxides and / or chromium hydroxides and preferably has a proportion by weight of chromium oxide and / or chromium hydroxide of more than 90%, particularly preferably of more than 95%.

Such metal strips according to the invention are distinguished by a high level of corrosion resistance and good adhesion for organic coatings such as paints or polymer layers. The coating preferably contains at least essentially only compounds of chromium and oxygen in which the chromium is in trivalent form, in particular as Cr ₂ O ₃ and / or Cr (OH) _3, apart from unavoidable impurities.

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 unter Verwendung einer Elektrolytlösung mit einem einer dreiwertigen Chromsubstanz (basisches Cr-(III)-Sulfat) und einem organischen Komplexierungsmittel (Natriumformiat) auf einem Stahlband abgeschiedenen Schicht, welche Chrommetall, Chromoxid und Chrom-Carbide enthält, wobei das Chromoxid hauptsächlich an der Oberfläche der Schicht liegt;

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;

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 direction of travel v of the coil;

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 deposited electrolytically using an electrolyte solution with a trivalent chromium substance (basic Cr (III) sulfate) and an organic complexing agent (sodium formate) on a steel strip, which layer contains chromium metal, chromium oxide and chromium carbides, the chromium oxide is mainly on the surface of the layer;

In Figure 1 a coil coating system for performing 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. Through the Electrolysis tanks 1a-1c, an initially uncoated metal strip M, for example a black plate or tin plate strip, is directed one after the other. For this purpose, the metal strip M is pulled through the electrolysis tanks 1a-1c in a strip running direction v at a predetermined strip speed by a transport device (not shown here). Above the electrolysis tanks 1a-1c, power rollers S are arranged, via which the metal strip M is connected as a cathode. In each electrolysis tank, a deflection roller U is also arranged, 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 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 anode pairs AP one after the other. 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 compared to the other anode pairs 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 an initially uncoated steel strip (black plate strip) or also 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 1a-1c, the metal strip M being connected 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 front electrolysis tanks 1a and 1b seen in the strip running direction v are each filled with the same electrolyte solution E1. This first electrolyte solution E1 contains a trivalent chromium compound, preferably basic Cr (III) sulfate, Cr ₂ (SO ₄ ) ₃ . Besides the trivalent chromium compound, the first electrolyte solution E1 contains at least one organic 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 formates, is preferably between 1: 1.1 and 1: 1.4 and particularly preferably 1: 1.25. To increase the conductivity, the first electrolyte solution E1 contains a salt, in particular. an alkali metal sulfate, for example potassium or sodium sulfate. The concentration of the trivalent chromium compound in the first electrolyte solution E1 is at least 10 g / l and particularly preferably 20 g / l or more. The pH of the first electrolyte solution E1 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 first electrolyte solution E1 is expediently the same in the two front electrolysis tanks 1a, 1b and is preferably between 25.degree. C. and 70.degree. However, different temperatures of the electrolyte solution can also be set in the two front electrolysis tanks 1a-1b. For example, the temperature of the electrolyte solution in the middle electrolysis tank 1b can be lower than in the front electrolysis tank 1a arranged upstream. The temperature of the electrolyte solution in the middle electrolysis tank 1b is preferably between 25 ° C and 37 ° C and in particular 35 ° C and the temperature of the first electrolyte solution E1 in the front electrolysis tank 1a is preferably between 40 ° C and 75 ° C and in particular at 55 ° C. Due to the lower temperature of the first electrolyte solution E1, the deposition of a chromium / chromium oxide / chromium hydroxide layer with a higher proportion of chromium oxide / chromium hydroxide is promoted in the central electrolysis tank 1b.

The rear or last electrolysis tank 1c seen in the strip running direction v is filled with a second electrolyte solution E2, the composition of which differs from the first electrolyte solution E1 at least in that the second electrolyte solution E2 contains no organic constituents and in particular no complexing agents. Otherwise, the constituent parts of the second electrolyte solution E2 can correspond to the first electrolyte solution E1. In particular, the second electrolyte solution E2 also contains a trivalent chromium compound, preferably basic Cr (III) sulfate, Cr ₂ (SO ₄ ) _3, and at least one salt and one acid or base for setting a suitable pH value. The salt, which, like the first electrolyte solution E1, is an alkali metal sulfate, for example Potassium or sodium sulfate, serves to increase the conductivity. The concentration of the trivalent chromium compound in the second electrolyte solution E2 is at least 10 g / l and particularly preferably 20 g / l or more. The pH of the second electrolyte solution E2 is adjusted to a preferred value between 2.0 and 5.0 and in particular to pH = 4.0 by adding the acid or base, for example sulfuric acid.

The temperature of the second electrolyte solution E2 in the rear electrolysis tank 1c is expediently between 25 ° C and 70 ° C and is preferably between 25 ° C and 40 ° C, and is in particular 35 ° C.

The anode pairs AP arranged in the electrolysis tanks 1a-1c are supplied with electrical direct current in such a way that the electrolysis tanks 1a, 1b, 1c each have a sufficiently high current density to allow electrolytic deposition of a chromium-containing (in particular a Cr-III-containing) layer produce. The minimum current density required for this depends on the belt speed and is, for example, at a (minimum) belt speed of 100 m / min approx. 15 to 20 A / dm ² . The minimum current density required for electrolytic deposition of a chromium-containing layer increases with higher belt speed.

Depending on the belt speed, the metal belt M connected as a cathode and passed through the electrolysis tanks 1a-1c is in electrolytically effective contact with the first electrolyte solution E1 during an electrolysis period t1 in the two front electrolysis tanks 1a, 1b and subsequently during an electrolysis period t2 in the rear electrolysis tank 1c in electrolytically effective contact with the second electrolyte solution E2. At belt speeds between 100 and 700 m / min, the electrolysis time in each of the electrolysis tanks 1a, 1b, 1c is between 0.5 and 2.0 seconds. Belt speeds are preferably set so high that the electrolysis time in each electrolysis tank 1a, 1b, 1c is less than 2 seconds and in particular is between 0.6 seconds and 1.8 seconds. The total electrolysis time tG = t1 + t2, in which the metal strip M is in electrolytically effective contact with the first and second electrolyte solutions E1, E2 across all electrolysis tanks 1a-1c, is accordingly between 1.8 and 5.4 seconds. The duration of the electrolysis in the individual electrolysis tanks 1a, 1b, 1c can be adjusted on the one hand by the belt speed and on the other hand by the dimensioning of the electrolysis tanks 1a-1c.

If a current density is set in the respective electrolysis tanks 1a-1c that is greater than the minimum current density, a layer is deposited on at least one side of the metal strip M in the front electrolysis tank 1a and in the middle electrolysis tank 1b, which layer contains chromium and chromium oxide / Chromium hydroxide and chromium carbides and, if a sulfate-containing first electrolyte solution E1 is used, may contain chromium sulfates. A layer B1 or B2 is produced in each of the two electrolysis tanks 1a, 1b, the composition of the layers B1, B2, in particular with regard to the proportion of chromium oxide / chromium hydroxide, being able to differ if in the front electrolysis tank 1a and the middle electrolysis tank 1b different electrolysis parameters, especially. Different current densities and temperatures, are used.

In the rear electrolysis tank 1c, an upper layer B3 is deposited on at least one side of the metal strip M, which layer consists at least essentially of pure chromium oxide and / or chromium hydroxide. The proportion by weight of the chromium oxide / chromium hydroxide in the total weight of the upper layer B3 is expediently at least 90%, preferably more than 95%.

In Figure 3 a schematic sectional view of a metal strip M electrolytically coated using the method according to the invention is shown. A coating B is applied to one side of the metal strip M, which 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 two lower layers B1, B2 facing the surface of the metal strip M contain metallic chromium (chromium metal) and chromium oxides (CrOx) / chromium hydroxides as well as chromium carbides and possibly chromium sulfates as essential components, the composition of the individual layers B1, B2 in particular with regard to their respective weight percentages of chromium metal and chromium oxide / chromium hydroxide can be the same or different, depending on whether the same or different electrolysis parameters have been used in the two front electrolysis tanks 1a, 1b. The upper layer B2 facing away from the surface of the metal strip M essentially contains only chromium oxides (CrOx) and / or chromium hydroxides and in particular no chromium carbides and hardly any metallic chromium and hardly any chromium sulfates.

The layer structure of the layers B1, B2, B3 deposited on the metal substrate can be demonstrated by GDOES spectra ( Glow Discharge Optical Emission Spectroscopy ). A metallic chrome layer with a thickness of 10-15 nm is first deposited on the metal strip M in the two front electrolysis tanks 1a, 1b. The surface of this layer oxidizes and is mainly in the form of chromium oxide in the form of Cr ₂ O ₃ or as a mixed oxide hydroxide in the form of Cr ₂ O ₂ (OH) ₂ . This oxide layer is a few nanometers thick. In addition, chromium-carbon and chromium sulfate compounds, which are formed from the reduction of the organic complexing agent or the sulfate of the electrolyte solution, are formed evenly throughout the entire layer. Typical GDOES spectra of the layers B1, B2 deposited in the individual electrolysis tanks show a strong increase in the oxygen signal in the first nanometers of the layer, from which it can be concluded that the oxide layer is concentrated on the surface of the respective layer ( Figure 4 ).

The composition of the layers can be determined according to the EURO standard DIN 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).

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. The organic layer is applied in a known manner, for example by painting or laminating a plastic film onto the surface of the coating B, that is to say onto the upper layer B3 made of chromium oxide / chromium hydroxide. The upper layer B3 made of chromium oxide / chromium hydroxide offers a good adhesive base for the organic material of the overlay. The organic coating can be, for example, an organic lacquer or polymer films made from thermoplastic polymers such as PET, PE, PP or mixtures thereof. The organic coating can be applied, for example, 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 with a polymer film be coated.

In Figure 2 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 is shown. 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.

The first electrolyte solution E1, which contains organic complexing agents, in particular formates, is located in the front group of electrolysis tanks 1a, 1b and in the middle group of electrolysis tanks 1c, 1d, 1e, 1f. The rear group of electrolysis tanks 1g, 1h is filled with the second electrolyte solution E2, which is free from organic substances and in particular free from complexing agents.

With the in Figure 2 The arrangement of a coil coating system shown in the figure can produce the same layer structure with layers B1, B2 and B3 as with the one in FIG Figure 1 shown system. By arranging the electrolysis tanks 1a to 1h in groups, however, the total electrolysis time can be increased. As a result, it is possible to work with a higher belt speed or higher weight loadings of the layers B1, B2, B3 can be generated with the same belt speed.

To achieve sufficient corrosion resistance for packaging applications, 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 by weight of the chromium contained in the chromium oxide / chromium hydroxide, averaged over the entire amount of coating B, is at least 15% and preferably between 20% and 40%. The coating B expediently has a total of chromium oxide with a weight of the chromium bound as chromium oxide or chromium hydroxide 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 or chromium hydroxide, averaged over the entire amount of coating B, is preferably at least 5 and preferably more than 7 mg of chromium per m ² . Good adhesion of organic paints or thermoplastic polymer materials on the surface of the coating B can be achieved with weight limits of the chromium oxide / chromium hydroxide of up to approx. 15 mg / m ² . One the preferred range for the weight of the chromium oxide / chromium hydroxide in coating B is therefore between 5 and 15 mg / m ² .

The thickness or the weight of the individual layers B1, B2, B3 can be adjusted in the method according to the invention by means of the electrolysis times t1, t2. As soon as a sufficiently high current density is selected in the electrolysis tanks, the thickness or the weight of the deposited layers B1, B2, B3 does not depend on the current density but only on the electrolysis time t1, t2 (if the temperature of the electrolyte solution remains constant) the metal strip M is in electrolytically effective contact with the first or the second electrolyte solution E1, E2.

The chromium oxide / chromium hydroxide proportion of the total weight of the coating B can therefore be set by the electrolysis time t2, in which the metal strip M in the rear electrolysis tank 1c or the rear group of electrolysis tanks 1g, 1h is in electrolytically effective contact with the second electrolyte solution E2 is located. This electrolysis duration t2 is in turn dependent on the dimensioning of the rear electrolysis tank 1c or 1g, 1h and the belt speed.

Claims (15)

A process for the production of a metal strip (M) coated with a coating (B), the coating (B) containing chromium metal and chromium oxide / chromium hydroxide and electrolytically from an electrolyte solution containing a trivalent chromium compound and at least one salt to increase conductivity and at least one acid or contains a base for setting a desired pH value, to which the metal strip (M) is applied by bringing the metal strip (M) into contact with the electrolyte solution (E) in an electrolytically effective manner during an electrolysis period, characterized in that the metal strip ( M) is passed through several electrolysis tanks (1a, 1b, 1c; 1a to 1h) arranged one behind the other in the direction of belt travel at a predetermined belt speed (v), at least the first electrolysis tank (1c; 1h) or a front one as seen in the direction of belt travel Group of electrolysis tanks (1a, 1b) with a first electrolyte solution (E1) is filled and the last electrolysis tank (1c; 1h) or a rear group of electrolysis tanks (1g, 1h) is filled with a second electrolyte solution (E2), the second electrolyte solution (E2) not containing any other components apart from the trivalent chromium compound and the at least one salt and the at least one acid or base and in particular is free from organic complexing agents and free from buffering agents. Method according to claim 1, characterized in that a layer consisting at least essentially of chromium oxide and / or chromium hydroxide is deposited in the last electrolysis tank (1c; 1h) or in the rear group of electrolysis tanks (1g, 1h), this layer preferably being a Has a weight fraction of chromium oxide and / or chromium hydroxide of more than 90%, particularly preferably of more than 95%. Method according to claim 1 or 2, characterized in that the composition of the first electrolyte solution (E1) and the second electrolyte solution (E2) differs, the first electrolyte solution (E1) and the second electrolyte solution (E2) preferably at least one besides the trivalent chromium compound Contains salt and at least one acid or base and is free from chloride ions and free from buffering agents and the first electrolyte solution (E1) preferably contains organic complexing agents, in particular in the form of formates, preferably in the form of potassium or sodium formate. Method according to one of the preceding claims, characterized in that the metal strip (M) is guided successively through the electrolysis tanks (1a, 1b, 1c; 1a - 1h) at a predetermined strip speed (v), the strip speed (v) being at least 100 m / min and is preferably between 200 m / min and 750 m / min and that the metal strip (M) for the electrolytic deposition of the coating (B) during a first electrolysis period (t1) in contact with the first electrolyte solution (E1) and during a second electrolysis time (t2) is brought into contact with the second electrolyte solution (E2), the total electrolysis time (tG = t1 + t2) preferably in the range from 0.5 to 6.0 seconds and particularly preferably between 1.5 and 3, 5 seconds and the first electrolysis time (t1), in which the metal strip (M) is in electrolytically effective contact with the first electrolyte solution (E1), is preferably less than 2.0 seconds and the second electrolysis time (t2), in which the metal strip (M) is in electrolytically effective contact with the second electrolyte solution (E2), is preferably less than 2.0 seconds. Method according to one of the preceding claims, characterized in that the first and the second electrolyte solution (E1, E2) have a temperature in the range from 20 ° C to 65 °, averaged over the volume of the respective electrolysis tank (1a, 1b, 1c; 1a to 1h) ° C and preferably in the range from 30 ° C to 55 ° C and particularly preferably between 35 ° C and 45 ° C. Method according to one of the preceding claims, characterized in that the first electrolyte solution (E1) and the second electrolyte solution (E2) contain a trivalent chromium compound which is selected from the group consisting of basic Cr (III) sulfate (Cr ₂ (SO ₄ ) ₃ ), Cr (III) nitrate (Cr (NO ₃ ) ₃ ), Cr (III) oxalate (CrC ₂ O ₄ ), Cr (III) acetate (C ₁₂ H ₃₆ ClCr ₃ O ₂₂ ), Cr (III) formate (Cr (OOCH) ₃ ) or a mixture thereof and that the first electrolyte solution (E1) and the second electrolyte solution (E2) contain at least one conductivity-increasing salt, the salt preferably at least one alkali metal sulfate, in particular potassium - Or sodium sulfate, and the first electrolyte solution (E1) and the second electrolyte solution (E2) are free from halides and in particular free from chloride and bromide ions. Method according to one of the preceding claims, characterized in that the first electrolyte solution (E1) and / or the second electrolyte solution (E2) has a pH value (measured at a temperature of 20 ° C) in the range from 2.3 to 5.0 , preferably between 2.5 and 2.9, the pH being adjusted by adding at least one acid to the first or second electrolyte solution, the at least one acid preferably comprising sulfuric acid or preferably consisting of sulfuric acid. Method according to one of the preceding claims, characterized in that the concentration of the trivalent chromium compound in the first electrolyte solution (E1) and / or in the second electrolyte solution (E2) is at least 10 g / l and preferably more than 15 g / l and particularly preferably at 20 g / l or more. Method according to one of the preceding claims, characterized in that the layer of chromium oxide / chromium hydroxide applied from the second electrolyte solution (E2) has a total weight of chromium oxide and / or chromium hydroxide of at least 3 mg / m ² , preferably 7 mg / m ² to 10 mg / m ² . Method according to one of the preceding claims, characterized in that, after the electrolytic application of the coating (B), an overlay made of an organic material, in particular a lacquer or a thermoplastic, in particular a polymer film made of PET, PE, PP or a mixture thereof the coating (B) is applied, in particular to the upper layer (B3) of the coating (B). Method according to one of the preceding claims, characterized in that by using a suitable anode during the electrolytic deposition of the coating (B) an oxidation of chromium (III) from the trivalent chromium compound of the electrolyte solution (E) to chromium (VI) is prevented, whereby the anode is in particular free of stainless steel and platinum. Method according to Claim 11, characterized in that the anode contains or consists of an outer surface or coating made of a metal oxide, in particular iridium oxide, or of a mixed metal oxide, in particular of iridium-tantalum oxide. Method according to one of the preceding claims, wherein for the production of the second electrolyte solution (E2) the trivalent chromium compound, which has been freed from organic residues except for unavoidable impurities or residual constituents, the at least one salt and the at least one acid or base to set a desired one pH value dissolved in water and the solution thus obtained is left to complex for at least 5 days, preferably 7 days, and then fine adjustment of the pH value is carried out by adding an acid or base. Electrolysis system for the electrolytic deposition of a chromium and chromium oxide and / or chromium hydroxide-containing coating (B) on the surface of a metal strip (B), the electrolysis system comprising: - At least one first electrolysis tank (1a) filled with a first electrolyte solution (E1), or a group of front electrolysis tanks (1a, 1b, 1c) which are each filled with a first electrolyte solution (E1), - A last electrolysis tank (1c) filled with a second electrolyte solution (E1), or a group of rear electrolysis tanks (1g, 1h), which are each filled with a second electrolyte solution (E2), the composition of the first electrolyte solution (E1) and the second electrolyte solution (E2) differentiates from one another in that the first electrolyte solution (E1) contains a trivalent chromium compound and at least one salt to increase the conductivity and at least one acid or a base to set a desired pH value and organic complexing agents, and the second Apart from a trivalent chromium compound and at least one salt to increase the conductivity and at least one acid or a base to set a desired pH value, the electrolyte solution (E2) does not contain any other components and is in particular free of organic components and free of buffering agents, - wherein the metal strip (M) for the electrolytic deposition of the passivation layer is passed at a predetermined belt speed (v) in a belt running direction through the electrolysis tanks (1a, 1b, 1c; 1a to 1h), whereby the surface is electrolytically effective in contact with the first and the second electrolyte solution (E1, E2) comes, - whereby a layer of chromium metal and chromium oxide and possibly other chromium compounds is deposited in the first electrolysis tank (1a) or in the front group of electrolysis tanks (1a, 1b, 1c), - and in the last electrolysis tank (1c) or in the rear group of electrolysis tanks (1g, 1h) a layer is deposited which at least essentially consists only of chromium oxide. Metal strip, in particular steel strip, with a coating (B) produced by electrolytic deposition on a surface of the metal strip and containing chromium and chromium oxide / chromium hydroxide, characterized in that the coating (B) consists of a first layer facing the surface of the metal strip and a composed of the second layer lying thereon, the first layer containing metallic chromium and the second layer at least essentially consisting only of chromium oxide and / or chromium hydroxide and preferably a proportion by weight of chromium oxide and / or chromium hydroxide of more than 90%, particularly preferably more than 95% % having.

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