EP3666928B1 - Process of fabricating a metal band having a chromium and chromium oxide coating using a trivalent chromium containing electrolyte - Google Patents

Description

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

For the production of packaging, steel sheets 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 their good adhesion to paints or organic protective coatings (such as polymer coatings made of PP or PET). Despite the low thickness of the coating made of chromium and chromium oxide, which is usually less than 20 nm, these chromium-coated steel sheets have good corrosion resistance and good processability in forming processes for the production of packaging, for example 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 to a strip-shaped steel sheet in a strip coating system using an electrolyte containing chromium VI. Out of US 3,316,160-A a process for the electrolytic chromium coating of a steel sheet with a chromic anhydride electrolyte is known, in which the steel sheet is passed through several coating baths, the steel sheet being coated with a chromium layer in the first coating bath at a higher current density of at least 30 A/dm ² and in In the second coating bath or the further subsequent coating baths, a cathodic treatment of the steel sheet is first carried out at a low current density of 0.1 to 10 A/dm ² and then an electrolytic deposition of a chromium layer takes place at the higher current density.

However, these coating processes with a hexavalent chromium electrolyte, such as chromic anhydride, have significant disadvantages due to the environmentally 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 banned in the future.

For this reason, electrolytic coating processes have already been developed in the prior art that do not require the use of electrolytes containing chromium VI. For example, from the WO 2015/177314-A1 and the EP 3378973 A1 a method for electrolytically coating a strip-shaped steel sheet with a chromium metal-chromium oxide (Cr-CrOx) layer in a strip coating system is known, in which the steel sheet, switched as a cathode, is passed through an electrolyte solution at high belt speeds of more than 100 m / min, which is a contains trivalent chromium compound (Cr-III). It was observed that the composition of the coating, which, depending on the components contained in the electrolyte 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 significantly on the current densities of the electrolysis depends on which are set on the anodes during the electrolytic deposition process in the electrolysis tanks in which the electrolyte solution is contained. It was found that three regions (Regime I, Regime II and Regime III) are formed depending on the current density, with no chromium-containing deposition occurring on the steel substrate in a first region 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) a partial decomposition of the applied coating occurs, so that the weight support of the chromium of the applied coating in this area initially drops as the current density increases and then settles to a constant value at higher current densities. In the area with medium current density (Regime II), essentially metallic chromium is deposited on the steel substrate with a weight proportion of up to 80% (based on the total weight of the coating), and above the second current density threshold (Regime III), the coating contains a higher current density Proportion of chromium oxide, which is in the range of higher Current densities are between ¼ and 1/3 of the total weight of the coating. The values of the current density thresholds that 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 corrosion resistance of a black sheet (uncoated steel sheet) coated with a chromium-chromium oxide coating that is sufficient for packaging applications, a minimum coating layer of at least 20 mg/m ² is required in order to achieve corrosion resistance comparable to conventional ECCS. It has also been shown that in order to achieve corrosion resistance sufficient for packaging applications, a minimum chromium oxide layer of at least 5 mg/m ² is required in the coating. In order to ensure such a minimum amount of chromium oxide in the coating, it appears advisable to apply high current densities in the electrolysis process so that work can be carried out in the area (Regime III) in which there is a coating with a relatively high proportion of chromium oxide on the steel substrate separates. In order to obtain a coating with a high proportion of chromium oxide, high current densities would therefore have to be used. However, achieving high current densities in the electrolysis tanks requires considerable energy expenditure for applying high currents to the anodes.

The under WO 2019/121582 A1 A subsequently published European patent application discloses a method for producing a black sheet metal strip coated with a coating containing chromium metal and chromium oxide from an electrolyte solution which contains a trivalent chromium compound, in which the black sheet metal strip, connected as a cathode, is brought into contact with the electrolyte solution and is passed one after the other through several electrolysis tanks arranged one behind the other , where the current density in the front electrolysis tanks is in a regime II and the current density in the rear electrolysis tanks is in a regime III and the current density in the regime III is higher than the current density in the regime II and the current densities in the regime II a chromium oxide-containing layer is deposited on the black plate, which has a lower proportion of chromium oxide compared to the layer which is deposited in regime III.

The object of the present invention is to provide the most efficient and energy-saving method for producing a tinplate strip coated with a coating of chromium and chromium oxide based on an electrolyte solution with a trivalent chromium compound.

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

In the method according to the invention, a coating containing chromium metal and chromium oxide is electrolytically applied to a tinplate strip from an electrolyte solution containing a trivalent chromium compound by bringing the tinplate strip into contact with the electrolyte solution connected as a cathode, the tinplate strip being sequentially is passed at a predetermined belt speed in a belt running direction through several electrolysis tanks arranged one behind the other in the belt running direction, with a low current density j ₁ in the first electrolysis tank seen in the belt running direction or in a front group of electrolysis tanks, in a second electrolysis tank following in the belt running direction or in a middle group of electrolysis tanks there is a medium current density j ₂ and in a last electrolysis tank seen in the direction of strip travel or in a rear group of electrolysis tanks there is a high current density j ₃ , where j ₁ ≤ j ₂ < j ₃ and the low current density j ₁ is greater than 20 A/ dm is ² .

The low current density j ₁ > 20 A/dm ² is selected so that a coating which contains chromium and/or chromium oxide is already deposited on the tinplate strip in the first electrolysis tank or in the front group of electrolysis tanks. With the selected lower limit value for the current density of 20 A/dm ² , coatings containing chromium and/or chromium oxide can be deposited even at low belt speeds (e.g. v = 100 m/min). To achieve high throughput, belt speeds of v ≥ 100 m/min are preferred.

By dividing the electrolysis tanks arranged one behind the other in the direction of strip travel and setting different current densities in the individual electrolysis tanks that increase in the direction of strip travel, it is possible, on the one hand, to maintain high belt speeds of 100 m/min or more, and on the other hand to maintain a sufficiently high weight support Coating to be deposited on at least one side of the tinplate strip, the coating having the proportion of chromium oxide required for sufficient corrosion resistance of at least 5 mg/m ² , preferably more than 7 mg/m ² .

When we talk about 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.

Because 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 there is a lower current density j ₁ or j ₂ is used, energy can be saved because lower electrical currents are required to act on the anodes in the first electrolysis tank or in the front group of electrolysis tanks and in the second electrolysis tank or in the middle group of electrolysis tanks. Nevertheless, a sufficiently high weight of chromium oxide is created in the coating, since even at the lower current densities j ₁ and j ₂ , which are set in the first and second electrolysis tanks or the front and middle groups of electrolysis tanks, there is already one A certain amount of chromium oxide is deposited on the metal substrate. The larger proportion of chromium oxide is deposited in the last electrolysis tank seen in the direction of strip travel or in the rear group of electrolysis tanks, because the high current density j ₃ is set there, at which the proportion of chromium oxide in the total layer of the coating is higher.

Since 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 deposit of the deposited coating, which is approximately 9 to 25%, is accounted for by chromium oxide Chromium oxide crystals already form on the surface of the tinplate 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 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 coverage of the coating in the last one 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 concentration of chromium oxide of preferably more than 5 mg/m ² on the surface of the Tinplate strips are produced.

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 due to the higher oxygen content in the coating compared to an electrodeposition with higher current densities (and consequently lower oxide content ) a denser coating that results in improved corrosion resistance.

The use of at least three 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 process. It has been shown that in order to maintain a preferred strip speed of at least 100 m/min, a current density of at least 25 A/dm ² is required so that a chromium-chromium oxide layer can be deposited on at least one surface of the tinplate strip. This current density of 25 A/dm ² represents the first current density threshold at a belt speed of approx. 100 m/min, which separates regime I (no chromium deposition) from regime II (chromium deposition with a linear relationship between current density and the chromium weight 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 smaller than in the second electrolysis tank or in the middle group of electrolysis tanks. A lower current density in the first electrolysis tank or in the front group of electrolysis tanks produces a dense and therefore corrosion-resistant chromium-chromium oxide coating directly on the surface of the tinplate strip with a relatively high chromium oxide content, preferably at more than 8%, in particular between 8 and 15% and 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, with the tinplate strip passing between the opposite anodes of a pair of anodes. This allows a uniform current density distribution around the tinplate strip to be achieved. The anode pairs of each electrolysis tank can expediently 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.

In order to be able to set a high current density j ₃ in the last electrolysis tank viewed in the direction of strip travel, at least one pair of anodes can be provided therein, which has a smaller expansion in the direction of strip travel compared to the anode pairs in the previous electrolysis tanks. This means that all anode pairs can be operated with the same amount of electrical current and yet a high current density j ³ can be set in the last electrolysis tank, which is higher than the current density in the previous electrolysis tanks. Furthermore, by using a shortened pair of anodes in the final electrolysis tank, the anodes can be coupled to a rectifier that has a lower rectifier capacity.

The belt speed of the tinplate strip is preferably selected so that the electrolysis time (t _E ), during which the tinplate strip is in electrolytically effective 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 weight of chromium 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 the chromium oxide contained in the coating of the total weight of the coating is at least 5%, preferably more than 10% and in particular 11 to 16%. A short electrolysis period of less than 1 second in each of the electrolysis tanks promotes (at constant current density) the formation of chromium oxide and inhibits the formation of metallic chromium, which is why short electrolysis periods (t _E ) are also important With regard to the formation of a coating with the highest possible chromium oxide content, it is preferable.

The total electrolysis time (t _E ), during which the tinplate strip is in electrolytically effective contact with the electrolyte solution (E), is - added up across all electrolysis tanks (1c - 1h) - 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 tinplate strip is passed in the strip running direction, the coating is deposited in layers, with a layer with a different composition of the coating, in particular with a different chromium oxide, in each of the electrolysis tanks, depending on the current density selected in the respective electrolysis tank. Proportion 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 with a weight proportion of chromium oxide of more than 5%, in particular 6 to 15%, can be deposited on the surface of the tinplate strip and in the second electrolysis tank or in the middle group of electrolysis tanks a layer containing chromium metal and chromium oxide with a weight proportion of the chromium oxide of less than 5%, in particular from 1 to 3%. In the third electrolysis tank or in the rear group of electrolysis tanks, a layer with a higher proportion by weight of chromium oxide is deposited in any case at the high current density j ₃ , the higher proportion by weight of chromium oxide preferably being more than 40%, in particular between 50 and 80%. lies.

The coating applied from the electrolyte solution, which contains at least the components chromium metal and chromium oxide and possibly also chromium sulfates and chromium carbides, preferably has a total weight of chromium of at least 40 mg/m ² and in particular 70 mg/m to achieve a sufficiently high corrosion resistance ² to 180 mg/m ² , the proportion of the total weight of chromium contained in the chromium oxide being at least 5%, preferably 10 to 15%. The chromium oxide portion has a weight of the chromium bound as chromium oxide of at least 3 mg Cr per m ² , in particular from 3 to 15 mg/m ² and preferably 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, with both the composition and the temperature of the electrolyte solution in all electrolysis tanks preferably being at least essentially the same. With regard to the temperature of the electrolyte solution, an (average) temperature in all electrolysis tanks of less than 40 ° C has proven to be suitable in terms of deposition of the highest possible proportion of chromium oxide in the coating. It has been shown that at electrolyte solution temperatures of up to 40°C, the formation of chromium oxide is promoted and the formation of metallic chromium is suppressed. It is also possible to set different temperatures of the electrolyte solution in the electrolysis tanks. For example, in order to achieve the highest possible proportion of chromium oxide in the last electrolysis tank or in the rear group of electrolysis tanks, a lower temperature can be set than in the first and second electrolysis tanks or the front and middle group of electrolysis tanks. For example, the (average) temperature of the electrolyte solution in the last electrolysis tank or the rear group of electrolysis tanks can be between 20 ° C and less than 40 ° C and preferably between 25 ° C and 38 ° C and in particular at 35 ° C and the The temperature of the electrolyte solution in the electrolysis tanks preceding the last electrolysis tank can be at higher temperatures, in particular between 40 ° C and 70 ° C and preferably at 55 ° C.

When we talk about the temperature of the electrolyte solution or the temperature in an electrolysis tank, what is meant is the average temperature that 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.

A preferred composition of the electrolyte solution includes basic Cr(III) sulfate (Cr ₂ (SO ₄ ) ₃ ) as a trivalent chromium compound. The concentration of the trivalent chromium compound in the electrolyte solution, both in this preferred composition and in other compositions, is at least 10 g/l and preferably more than 15 g/l and is in particular 20 g/l or more. Further useful components of the electrolyte solution can be complexing agents, in particular an alkali metal carboxylate, preferably a salt of formic acid, in particular potassium formate or sodium formate. The ratio of the weight proportion is preferred trivalent chromium compound to the weight proportion 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 in particular at 1:1.25. To increase conductivity, the electrolyte 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 as well as free of a buffering agent and in particular free of a boric acid buffer.

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

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

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 Weißblechbands;

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.

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

Figure 1:

Schematic representation of a strip 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 strip 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 tinplate 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, with the chromium oxide lying on the surface of the layer.

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

Within each electrolysis tank 1a-1c, at least one pair of anodes AP is arranged below the liquid level of the electrolyte solution E. In the exemplary embodiment shown, two pairs of anodes AP arranged one behind the other in the direction of strip travel are provided in each electrolysis tank 1a-1c. The tinplate strip M is passed between the opposite anodes of an anode pair AP. In the embodiment of Figure 1 Two pairs of anodes AP are thus arranged in each electrolysis tank 1a, 1b, 1c in such a way that the tinplate strip M is passed through these pairs of anodes 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 remaining anode pairs AP. As a result, a higher current density can be generated with this last pair of anodes APc when an equally high electrical current is applied.

The tinplate strip M is a tin-plated steel strip. To prepare for the electrolysis process, the tinplate strip M is first degreased, rinsed, pickled and rinsed again and, in this pretreated form, is successively passed through the electrolysis tanks 1a - 1c, with the tinplate strip M being switched as a cathode by using the Power rollers S electrical current is supplied. The belt speed at which the tinplate strip M is passed through the electrolysis tanks 1a-1c is at least 100 m/min and can be up to 900 m/min.

The same electrolyte solution E is filled into the electrolysis tanks 1a-1c arranged one behind the other in the strip running direction v. The electrolyte solution E contains a trivalent chromium compound, preferably basic Cr(III) sulfate, Cr ₂ (SO ₄ ) ₃ . In addition to the trivalent chromium compound, the electrolyte solution preferably contains at least one complexing agent, for example a salt of formic acid, in particular potassium or sodium formate. The ratio of the proportion by weight of the trivalent chromium compound to the proportion by weight of the complexing agents, in particular the formats, is preferably between 1:1.1 and 1:1.4 and particularly preferably 1:1.25. To increase the conductivity, the electrolyte solution E can contain an alkali metal sulfate, for example potassium or sodium sulfate. The concentration of the trivalent chromium compound in the electrolyte solution E is at least 10g/l and particularly preferably 20g/l or more. The pH value 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 is expediently the same in all electrolysis tanks 1a-1c and is preferably between 25 ° C and 70 ° C. However, in particularly preferred embodiments of the method according to the invention, 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 lower than in the electrolysis tanks 1a and 1b arranged upstream. In this embodiment of the method, the temperature of the electrolyte solution in the last electrolysis tank 1c is preferably between 25 ° C and 38 ° C and in particular at 35 ° C. In this exemplary embodiment, the temperature of the electrolyte solution in the first two electrolysis tanks 1a, 1b is preferably between 40 ° C and 75 ° C and in particular at 55 ° C. The lower temperature of the electrolyte solution E promotes the deposition of a chromium/chromium oxide layer with a higher proportion of chromium oxide in the last electrolysis tank 1c.

The anode pairs AP arranged in the electrolysis tanks 1a-1c are supplied with direct electrical current in such a way that one in each of the electrolysis tanks 1a, 1b, 1c different current density exists. In the first electrolysis tank 1a, which is upstream in the direction of strip travel, there is a low current density j ₁ , in the second electrolysis tank 1b following in the direction of strip travel, there is an average current density j ₂ , and in the last electrolysis tank 1c, as seen in the direction of strip travel, there is a high current density j ₃ , so that the relation j ₁ < j ₂ < j ₃ holds and the low current density j ₁ > 20 A/dm ² .

Due to the set current density in the respective electrolysis tank, a layer containing chromium and chromium oxide is electrolytically deposited on at least one side of the tinplate strip M, with a layer B1, B2, B3 being produced in each of the electrolysis tanks. 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 in the proportion of chromium oxide.

In Figure 3 A sectional view of a tinplate strip M electrolytically coated using the method according to the invention is shown schematically. A coating B is applied to one side of the tinplate 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 coating B, which is composed of the individual layers B1, B2, B3, contains metallic chromium (chromium metal) and chromium oxides (CrOx) as essential components, with the composition of the individual layers B1, B2, B3 in relation to their respective weight proportion 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.

The layer structure of the layers deposited on the metal substrate can be demonstrated using GDOES spectra ( Glow Discharge Optical Emission Spectroscopy ). A metallic chromium layer with a thickness of 10-15 nm is first deposited on the tinplate strip substrate. The surface of this layer oxidizes and is present primarily as chromium oxide in the form Cr ₂ O ₃ or as a mixed oxide-hydroxide in the form Cr ₂ O ₂ (OH) ₂ . This oxide layer is a few nanometers thick. In addition, through the complete Layer evenly installed, chromium-carbon and chromium sulfate compounds, which are formed from the reduction of the organic complexing agent or the sulfate of the electrolyte solution. Typical GDOES spectra of the layers B1, B2, B3 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 ).

Depending on the belt speed, the tinplate strip M, which is connected as a cathode and passed through the electrolysis tanks 1a-1c, is in electrolytically effective contact with the electrolyte solution E during an electrolysis period t _E. At belt speeds between 100 and 700 m/min, the electrolysis period is in each of the electrolysis tanks 1a, 1b, 1c between 0.5 and 2.0 seconds. Belt speeds are preferably set so high that the electrolysis time t _E in each electrolysis tank 1a, 1b, 1c is less than 2 seconds and in particular between 0.6 seconds and 1.8 seconds. The total electrolysis time in which the tinplate strip M is in electrolytically effective contact with the electrolyte solution E across all electrolysis tanks 1a-1c is between 1.8 and 5.4 seconds.

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 at lower current densities , which are within regime II, form higher oxide contents in the coating. In the last electrolysis tank 1c, a current density j3 is set which is in regime III, in which an increased proportion of chromium oxide is produced in the coating, which is preferably more than 40% by weight and particularly preferably more than 50% by weight.

Table 1 shows examples of suitable current densities j ₁ , j ₂ , j ₃ in the individual electrolysis tanks 1a, 1b, 1c at different belt speeds. From Table 1 it can be seen that the current densities j ₁ in the first electrolysis tank 1a are slightly smaller compared to the current densities j2 in the second electrolysis tank 1b and are above a lower limit of j ₀ = 20 A/dm ² . The current densities j 1 , j 2 present in the first two electrolysis tanks _1a , _1b are in regime II, in which there is a linear relationship between the current density and the electrolytically deposited Amount of chromium (or the deposited weight of chromium) is present. The current density j ₁ of the first electrolysis tank 1a is expediently selected so that it is close to the first current density threshold, which delimits regime I (in which no chromium deposition yet takes place) from regime II. At these low current densities j _1, a chromium metal-chromium oxide coating (layer B1) is deposited on the surface of the tinplate strip M with a higher chromium oxide content than at higher current densities within regime II. Therefore, the layer deposited in the first electrolysis tank 1a B1 has a higher chromium oxide content compared to the coating B2 deposited in the second electrolysis tank 1b.

In the last electrolysis tank 1a, a current density j ₃ is set which is above the second current density threshold, which delimits regime II from regime III. The current density j ₃ of the last electrolysis tank 1c is therefore in regime III, in which a partial decomposition of the chromium metal-chromium oxide coating occurs and a significantly higher proportion of chromium oxide is deposited than at current densities in regime II. For this reason, the current density in the last Electrolysis tank 1c deposited coating B3 has a high chromium oxide content, which is higher than the chromium oxide contents in the coatings B1 and B2.

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

In Figure 2 a second embodiment of a strip coating system is shown 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, and a middle group with those in the direction of strip travel subsequent electrolysis tanks 1c-1f and a rear group with the last two electrolysis tanks 1g and 1h. The groups of electrolysis tanks each have different current densities j ₁ , j ₂ , j ₃ , with a low current density j ₁ in the front group of electrolysis tanks 1a, 1b and a medium current density j ₂ in the middle group of electrolysis tanks 1c-1f and in the rear group of electrolysis tanks 1g, 1h there is a high current density j ₃ , where j ₁ < j ₂ < j ₃ and the low current density j ₁ > 20A/dm ² .

In the front group of electrolysis tanks 1a, 1b a layer B1 containing chromium and chromium oxide is electrolytically applied 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 is applied to the tinplate strip M applied. As in the embodiment of Figure 1 the layers B1, B2, B3 have different compositions due to the different current densities j ₁ , j ₂ , j ₃ in the groups of electrolysis tanks arranged one behind the other, with the layer B1 containing a higher proportion of chromium oxide than the second layer B2 and the third Layer B3 contains a higher proportion of chromium oxide than the two layers B1 and B2.

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 v, with a low current density j ₁ , in in each of the electrolysis tanks 1a, 1b of the front group an average current density j ₂ is set in the electrolysis tanks 1c to 1f of the middle group and a high current density j ₃ is set in the electrolysis tanks 1g, 1h of the rear group, where j ₁ <j ₂ <j ₃ .

The process according to the invention in the coil coating system of Figure 2 The coating B produced on the surface of the tinplate strip M therefore has essentially the same composition and structure as in Figure 3 shown.

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

In order to achieve sufficient corrosion resistance, the coatings B preferably have a total weight of 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 weight of chromium contained in the chromium oxide, averaged over the entire coating of coating B, is at least 5% and preferably between 10% and 15%. The coating B expediently has a total chromium oxide content with a weight of the chromium bound as chromium oxide of at least 3 mg chromium per m ² and in particular from 3 to 15 mg/m ² . The weight of the chromium bound as chromium oxide, averaged over the entire coating of coating B, 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 . A preferred range for the weight of the chromium oxide in the coating B is therefore between 5 and 15 mg/m ² .

The entire electrolysis period during which the tinplate 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 less than 16 seconds and in particular between 4 and 16 seconds. <b>Table 1:</b> Current densities j ₁ , j ₂ , j ₃ in the individual electrolysis tanks of the first exemplary embodiment (with 3 electrolysis tanks 1a - 1c) at different belt speeds v: tank 1a 1b 1c v [m/min] J ₁ / [A/dm ² ] J ₂ / [A/dm ² ] J ₃ / [A/dm ² ] 100 25 29 75 150 41 45 91 200 57 61 107 300 73 77 133 400 89 93 149 500 105 109 165 Current densities j ₁ , j ₂ , j ₃ in the individual electrolysis tanks of the second exemplary embodiment (with 8 electrolysis tanks 1a - 1h, which are grouped into three groups) at different belt speeds v: 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 25 25 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

Claims (15)

1. Method for producing a tinplate strip (M) coated with a coating (B), wherein the coating (B) contains chromium metal and chromium oxide and is electrolytically applied to the tinplate strip (M) from an electrolyte solution (E), which contains a trivalent chromium compound in a concentration of at least 10 g/l, by bringing the tinplate strip (M) as a cathode in contact with the electrolyte solution (E), characterised in that the tinplate strip (M) is passed successively at a predetermined strip speed (v) in a strip running direction through a plurality of electrolysis tanks (1a to 1h) arranged one behind the other in the strip running direction, wherein a low current density (j₁) is present in the first electrolysis tank (1a) seen in the strip running direction or in a front group of electrolysis tanks (1a, 1b), a medium current density (j₂) is present in a second electrolysis tank (1c) following in the strip running direction or in a middle group of electrolysis tanks (1c - 1f) and a high current density (j₃) is present in a last electrolysis tank (1h) seen in the strip running direction or in a rear group of electrolysis tanks (1g, 1h), wherein j₁ ≤ j₂ < j₃ and the low current density (j₁) is greater than 20 A/dm².
2. Method according to claim 1, characterised in that the current densities (j₁ , j₂ , j₃ ) in the electrolysis tanks (1a - 1h) are each adapted to the strip speed (v), wherein in particular and at least essentially a linear relationship between the strip speed (v) and the respective current density (j₁ , j₂ , j₃) is given.
3. Method according to claim 1 or 2, characterised in that at least one anode pair (AP) with two opposing anodes is arranged in each electrolysis tank (1a - 1h), wherein the tinplate strip is passing between the opposing anodes of an anode pair (AP).
4. Method according to claim 3, characterised in that at least one anode pair (APc) is provided in the last electrolysis tank (1c; 1h) as seen in the strip running direction, said anode pair having a smaller extension in the strip running direction compared to the anode pairs (AP) in the preceding electrolysis tanks (1a, 1b or 1a to 1g).
5. Method according to one of the preceding claims, characterised in that the electrolysis time (t_E), in which the tinplate strip (M) is in electrolytically effective contact with the electrolyte solution (E), is less than 2.0 seconds in each of the electrolysis tanks (1a-1c; 1a - 1h) and in particular is between 0.5 and 1.9 seconds and preferably is less than 1.0 seconds and in particular is between 0.6 seconds and 0.9 seconds.
6. Method according to one of the preceding claims, characterised in that the total electrolysis time (t_E), in which the tinplate strip (M) is electrolytically effective in contact with the electrolyte solution (E), is less than 16 seconds over all electrolysis tanks (1a-1c; 1a - 1h) and in particular is between 4 and 16 seconds and preferably is less than 8 seconds and in particular is between 5 seconds and 7 seconds.
7. Method according to one of the preceding claims, characterised in that the electrolysis tanks (1a-1c; 1a to 1h) are filled with the electrolyte solution (E), wherein the composition and/or the temperature of the electrolyte solution (E) is at least substantially the same in all electrolysis tanks (1a to 1h).
8. Method according to one of the preceding claims, characterised in that the mean temperature of the electrolyte solution (E) in all electrolysis tanks (1a-1c; 1a to 1h) is less than 40°C.
9. Method according to one of the preceding claims, characterised in that the mean temperature of the electrolyte solution in the last electrolysis tank (1c; 1h) or the rear group of electrolysis tanks (1g, 1h) is between 20°C and 40°C and preferably between 25°C and 38°C and in particular at 35°C.
10. Method according to one of claims 1 to 6, characterised in that the temperature of the electrolyte solution in the last electrolysis tank (1c; 1h) is below 40°C and in particular is between 25°C and 38°C and that the temperature of the electrolyte solution in the electrolysis tanks (1a, 1b; 1a to 1g) preceding the last electrolysis tank (1h) is higher than 40°C and in particular is between 40°C and 70°C.
11. Method according to one of the preceding claims, characterised in that the electrolyte solution (E) comprises, in addition to the trivalent chromium compound, which preferably comprises basic Cr(III) sulphate (Cr₂ (SO )₄₃ ), at least one complexing agent, in particular an alkali metal carboxylate, preferably a salt of formic acid, in particular potassium formate or sodium formate, wherein the ratio of the proportion by weight of the trivalent chromium compound to the proportion by weight of the complexing agents, in particular the formates, is between 1:1.1 and 1:1.4 and preferably between 1:1.2 and 1:1.3 and particularly preferably at 1:1.25, and that the electrolyte solution for increasing the conductivity preferably comprises an alkali metal sulphate, in particular potassium or sodium sulphate, and/or is free from halides, in particular free from chloride and bromide ions and free from a buffering agent and in particular free from a boric acid buffer.
12. Method according to one of the preceding claims, characterised in that the concentration of the trivalent chromium compound in the electrolyte solution is more than 15 g/l and is particularly preferably 20 g/l or more and/or in that the pH value of the electrolyte solution, measured at a temperature of 20°C, is between 2.0 and 3.0 and preferably between 2.5 and 2.9 and particularly preferably 2.7.
13. Method according to one of the preceding claims, characterised in that the tinplate strip is moved through the electrolysis tanks (1a-1c; 1a to 1h) at a strip speed of at least 100 m/min.
14. Methods according to one of the preceding claims, characterised in that the coating applied from the electrolyte solution has a total weight of chromium of at least 40 mg/m², preferably from 70 mg/m² to 180 mg/m² , the proportion of the total weight of chromium contained in the chromium oxide being at least 5%, preferably from 10 to 15% , and/or that the coating applied from the electrolyte solution has a chromium oxide content with a weight of the chromium bound as chromium oxide of at least 3 mg Cr per m², in particular from 3 to 15 mg/m² and preferably of at least 7 mg Cr per m².
15. Method according to one of the preceding claims, characterized in that in the first electrolysis tank (1a) or in the front group of electrolysis tanks (1a, 1b) a coating (B) containing chromium metal and chromium oxide with a weight proportion of the chromium oxide of more than 5%, in particular from 6 to 15%, is deposited on the surface of the tinplate strip, and/or in that in the second electrolysis tank (1b) or in the middle group of electrolysis tanks (1c - 1f) a coating (B) containing chromium metal and chromium oxide with a chromium oxide content by weight of less than 5%, in particular from 1 to 3%, is deposited on the surface of the tinplate strip, and/or in that in the third electrolysis tank (1c) or in the rear group of electrolysis tanks (1g, 1h) a coating (B) containing chromium metal and chromium oxide with a chromium oxide content by weight of more than 40%, in particular from 50 to 80%, is deposited on the surface of the tinplate strip.

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