WO2021023778A1

WO2021023778A1 - Method and system for electroytically coating a steel strip by means of pulse technology

Info

Publication number: WO2021023778A1
Application number: PCT/EP2020/072020
Authority: WO
Inventors: Henry GÖRTZ; Thomas Daube; Frank PLATE; Walter Timmerbeul
Original assignee: Sms Group Gmbh
Priority date: 2019-08-05
Filing date: 2020-08-05
Publication date: 2021-02-11
Also published as: EP4010517A1; WO2021023789A1; CN114174560A; EP4010515A1; US20240229276A1; EP4010516A1; CN114207191A; WO2021023783A1; US20220275530A1; EP4010518A1; WO2021023779A1; CN114207190A; CN114174559A

Abstract

The present invention relates to a galvanic method and to a system for electrolytically coating a steel strip, in particular for the motor vehicle industry, comprising a coating based on zinc and/or a zinc alloy, by means of pulse technology.

Description

Process and system for the electrolytic coating of a steel strip using pulse technology

The present invention relates to a galvanic method and a system for the electrolytic coating of a steel strip, in particular for the automotive sector, with a coating based on zinc and / or a zinc alloy

Nowadays, electrolytically refined steel strips are used as semi-finished products in many branches of industry, such as the automotive industry, aerospace technology, mechanical engineering, the packaging industry, as well as household and electrical appliance manufacture. The production of such strips is traditionally carried out in continuously operating strip treatment plants with a constant-speed passage of the steel strip through one or more electrolysis cells connected in series.

The coatings deposited electrolytically on one or both sides of the steel strip can perform various tasks and give the respective steel strip new product properties. These are, for example, protection against corrosion or oxidation, wear protection, the production of decorative product properties, and / or the production of magnetic and / or electrical surface properties.

For example, the zinc coating gives an electrolytically galvanized steel strip an active protection against corrosion and offers a good primer for painting and / or laminating with plastic films. A chrome coating also gives a steel band or a plastic band increased protection against corrosion and wear, as well as decorative Characteristics. Nickel and nickel alloys, on the other hand, can increase the surface hardness of the respective substrate.

The production of the respective coatings with the desired properties depends, especially under economic and economic aspects, on various parameters such as the type and composition of the electrolyte, its metal salt concentration and temperature, the geometric arrangement of the electrolysis cells and their electrodes, the electrochemical current flow and their Amount, time and polarity, strongly dependent

The electrolytic coating of steel strips is carried out in the prior art by means of direct current, the thyristor technology being used here. This so-called DC electrolysis can be designed to be unipolar and partially reversible, but does not allow any specific current sequences in terms of amount, time and polarity. In particular, the high level of hydrogen evolution proves to be particularly problematic here, since the hydrogen diffusing into the steel strip has a massive negative impact on the product properties of the steel strip in the subsequent production steps. The diffusing hydrogen is primarily responsible for the so-called spontaneous brittle fracture and the lowering of the material yield point or the required strength of a steel strip. Furthermore, the hydrogen trapped in a galvanized steel strip leads to the effusion of the trapped hydrogen during the curing process of a painted component, preferably a component painted by means of the KTL process, with the result that hydrogen bubbles form below the paint layer, which lead to so-called "paint bursts".

The hydrogen-related decrease in material strength represents a further significant process disadvantage in the prior art, because if the strength of the material is no longer available, it is usually unusable for an application in the area of safety-relevant components, for example in the automotive sector. The object of the present invention is therefore to provide a method which is improved over the prior art and an improved system for the electrolytic coating of steel strips with a coating based on zinc and / or a zinc alloy.

According to the invention, the object is achieved by a method with the features of patent claim 1 and a system with the features of patent claim 21

With regard to the method according to the invention, it is provided that the steel strip, after possibly prior cleaning and / or activation, is fed to a coating section comprising at least one, preferably at least two or more, electrolytic cell (s) and is successively electrolytically coated in this, the steel strip initially Connected cathodically via at least one current roller and is guided within the at least one electrolytic cell at a defined distance parallel to the at least one anode arranged in the electrolytic cell. According to the invention, the at least one anode is energized by means of a modulated current, the coating process taking place within the coating section using a defined pulse pattern sequence which is formed from at least one pulse pattern, with zinc and / or a zinc alloy being deposited from an electrolyte on the steel strip according to the pulse pattern sequence and the coating is formed on the basis of zinc and / or a zinc alloy.

In the same way, the present invention provides a system for the electrolytic coating of a steel strip. The system may include a cleaning and / or an activation unit in which the steel strip can be cleaned and / or activated; a coating line with at least one, preferably at least two or more electrolytic cell (s) in which the steel strip can be electrolytically coated successively, and at least one current roller via which the steel strip can be connected cathodically, the at least one electrolytic cell comprising at least one anode which is arranged in such a way that the through the at least one electrolytic cell can be fed through the steel strip at a defined and parallel distance to the at least one anode. According to the invention, the system comprises at least one pulse rectifier which is implemented using switched-mode power supply technology, the negative pole of which is electrically connected to the at least one current roller and the positive pole is electrically connected to the at least one anode, in such a way that the at least one anode can be energized by means of a modulated current, that the coating process can be carried out within the coating section using a defined pulse pattern sequence, the pulse pattern sequence being formed from individual pulse patterns, with a coating based on zinc and / or a zinc alloy from an electrolyte being able to be deposited on the steel strip according to the pulse pattern sequence.

Surprisingly, it has been shown that using a defined pulse pattern sequence, which is formed from individual pulse patterns, the cathodic hydrogen deposition and its diffusion into the steel strip can be reduced to such an extent that the hydrogen-induced brittle fracture, the decrease in tensile strength and the formation of surface defects in subsequent process steps are effective can be avoided. A steel strip coated with a zinc and / or a zinc alloy coating by means of the method according to the invention can therefore be produced directly in a manner that maintains its strength, so that a possibly required heating process step downstream of the coating process can be saved.

The coating process according to the invention takes place within the coating section using a defined pulse pattern sequence that is formed from individual pulse patterns. The pulse pattern sequence can be off a single pulse pattern and / or from a combination of at least two or a plurality of identical and / or different pulse patterns of a pulse pattern collection. Further advantageous refinements of the invention are specified in the dependent claims. The features listed individually in the dependently formulated claims can be combined with one another in a technologically meaningful manner and can define further embodiments of the invention. In addition, the features specified in the claims are specified and explained in more detail in the description, with further preferred embodiments of the invention being presented. In this context, it is pointed out that all of the device features in question, which are explained in the course of the individual process steps or vice versa, can be combined in the same way with the system according to the invention and / or the process without explicit reference to them.

Preferably, the steel strip is one which has a tensile strength of at least R _e ^ 500 MPa, more preferably at least R _e ^ 600 MPa, and most preferably at least R _e ^ 800 MPa. With regard to the maximum tensile strength, the steel strip is limited to a tensile strength of R _e ^ 2000 MPa, more preferably to a tensile strength of R _e ^ 1500 MPa, even more preferably to a tensile strength of R _e ^ 1200 MPa.

A preferred zinc alloy coating comprises zinc-magnesium.

The coating section of the system can in principle comprise an electrolysis cell with an anode, which is designed, for example, in the form of a plate anode. In a further development, the only one electrolysis cell can comprise two anodes, which are arranged one behind the other in the direction of belt travel, for example, in such a way that the steel strip can be coated on one side. In a preferred embodiment, the two anodes in one An anode arrangement can be formed in which the two anodes are then arranged parallel to one another within the one electrolytic cell.

In a preferred embodiment variant, the coating section comprises at least two electrolysis cells, more preferably at least three electrolysis cells, even more preferably at least four electrolysis cells, further preferably at least five electrolysis cells, and for reasons of process economy it is limited to a maximum of twenty electrolysis cells, preferably a maximum of 16, more preferably a maximum of fifteen Electrolysis cells limited. The plurality of electrolysis cells are preferably arranged one behind the other in the direction of travel of the strip, through which the steel strip is then guided within the coating section.

The individual electrolysis cells can be designed in the form of horizontally or preferably in the form of vertically designed electrolysis cells, through which the steel strip is guided over deflection rollers.

The deposition process within the individual electrolysis cells takes place in an electrolyte through which the steel strip is passed. The electrolyte medium is usually aqueous and usually has a pFI value of less than 5.0. Alternatively, the electrolyte medium can also be formed from a non-aqueous medium such as an ionic liquid. A preferred ionic liquid comprises a mixture of choline chloride and flarnea.

The modulated current is provided by a pulse rectifier that uses switched-mode power supply technology. The use of such a pulse rectifier allows the amount, the temporal course and the polarity of the respective desired pulse pattern and thus the entire pulse pattern sequence to be defined in such a way that the electrolytic process can be optimally adapted according to the specified parameters. A pulse rectifier designed in this way is defined in that the mains-side alternating voltage is first rectified and smoothed. The DC voltage then generated, which has significantly higher frequencies, usually in the range from 5 kHz to 300 kHz, is then divided, transformed with this high frequency and then rectified and screened. The superimposed voltage and current regulation usually works via pulse width modulation or pulse phase modulation.

Due to the high frequency at the power transmitter, the transformer is made much smaller so that the energy losses are much lower. Depending on the system, this results in a significantly higher power efficiency of the direct current supply and thus of the entire production plant. Depending on the design, the pulse rectifier can be made available in a modular design.

This leads to a significantly higher availability, since the performance to be provided by a defective module can be taken over by another module and when a defective module is repaired, it can be quickly replaced.

Another advantage is that the quality of the direct current, in particular its lower residual ripple, is much better with lower losses than with conventional thyristor-based DC electrolysis, the repair of defective devices is much faster and easier, and existing direct current / DC voltage supply systems through additional modules by using appropriate control technology, by means of which the output of the direct current

/ DC voltage supply system can be increased, are expandable. The at least one pulse rectifier that provides the modulated current is advantageously connected to the at least one current roller and via its negative pole the positive pole is electrically connected to the at least one anode. In this context it is preferably provided that the at least one pulse rectifier is electrically connected to a central control unit via which the entire coating process is regulated. The at least one pulse pattern of the pulse pattern sequence is transmitted via the control unit to the at least one, preferably each, pulse rectifier, which then transmits this signal to the respective associated electrolysis cell.

Usually, a pulse pattern of the pulse pattern sequence comprises at least one cathodic pulse, at least one anodic pulse, and / or at least one pulse off-time, the cathodic and anodic pulse being defined by a pulse duration and its respective shape, for example rectangular. The zinc and / or the zinc alloy is deposited on the steel strip via the cathodic pulse. In particular, the nascent hydrogen adsorbed on the steel strip surface can be oxidized to a proton again via an anodic pulse and can thus be specifically removed from the steel strip surface.

The at least one anode is preferably designed as a plate anode. Such plate anodes can in principle be designed in the form of a soluble or an insoluble anode. In the case of soluble anodes, which are also known as active anode systems, the anode dissolves during the electrolysis. In contrast, insoluble anodes, also known as inert anode systems, do not go into solution during electrolysis. Insoluble anodes consist of a carrier material on the one hand and a coating applied to it, which can be referred to as an active layer, on the other hand. Titanium, niobium or other reaction carrier metals are usually used as the carrier material, but in any case those materials which passivate under the electrolysis conditions. The material used for the active layer is usually electron-conducting materials, such as platinum, iridium or other noble metals, their mixed oxides or compounds of these elements. The active layer can either be applied directly to the Be applied to the surface of the carrier material or be located on a substrate arranged at a distance from the carrier material. Materials which can be used as carrier material, for example titanium, niobium or the like, can also serve as the substrate.

The at least one anode can preferably be formed in one piece and / or, according to an advantageous embodiment variant, from at least two or more rod-shaped partial anodes, each of the partial anodes then being electrically connected to the power source. The at least two or more rod-shaped partial anodes are advantageously arranged in such a way that the distance between each partial anode and the strip can be adjusted across its width. As a result, locally different layer thicknesses can be applied and / or corrected by desorption along the strip width of the steel strip by adjusting the distance between each of the partial anodes and the strip and / or the current density. For example, the partial anodes arranged on the strip edges, compared to those arranged in the middle segment, can be energized with a lower current density and / or positioned a greater distance from the strip in order to control the deposition of the zinc and / or the zinc alloy on the strip edges .

In a particularly advantageous embodiment variant, the at least one electrolytic cell comprises at least one anode arrangement made up of two anodes arranged parallel to one another, through which the steel strip is guided. In a configuration designed in this way, it is preferably provided that each of the anodes of the at least one anode arrangement is supplied with current via a separate pulse rectifier, in such a way that each of the anodes is electrically connected to a positive pole of each pulse rectifier and the negative pole of each pulse rectifier is electrically connected to the at least one current roller is. In other words, the electrolytic cell in this configuration comprises two anodes, two pulse rectifiers and a current roller, via which the strip substrate is connected cathodically. In a further preferred embodiment variant, the at least one electrolysis cell comprises at least two anode arrangements, each with two anodes arranged parallel to one another, through which the steel strip is guided. If such an electrolytic cell is designed as an immersion tank, it is particularly preferably provided that the steel strip is deflected between the at least two anode arrangements via a deflection roller, possibly arranged within the electrolytic cell. In a configuration designed in this way, each of the anodes of the at least two anode arrangement is also supplied with current via a separate pulse rectifier, so that a total of four pulse rectifiers are provided in this configuration. Here, each of the four anodes is electrically connected to a positive pole of each pulse rectifier and the negative pole of two pulse rectifiers is electrically connected to one of the two current rollers. In other words, the electrolysis cell in this configuration comprises four anodes, four pulse rectifiers, two current rollers and a deflection roller, possibly arranged within the electrolysis cell.

In a further preferred embodiment variant, the electrolysis cell can essentially be formed from the anode arrangement in that the two open flanks of this are closed. The steel strip is guided through the partially closed space delimited by the anode arrangement and the electrolyte flows around it. The electrolyte can, for example, be supplied to the space over the entire cross section via appropriate pumps and flow through it. Such a structure has a smaller installation space than an immersion tank and therefore requires smaller volumes of the electrolyte.

In a particularly preferred embodiment variant, the

Coating section a plurality of electrolysis cells arranged one behind the other in the direction of travel of the strip, through which the steel strip is guided. In this context, it is advantageously provided that the steel strip between at least two, more preferably between each of the plurality of Electrolysis cells, deflected via at least one deflection roller designed as an intermediate current roller, and optionally additionally connected cathodically. In an exemplary embodiment variant with two electrolysis cells each comprising two anode arrangements, each of the anodes of the four anode arrangements is also supplied with current via a separate pulse rectifier, so that a total of eight pulse rectifiers are provided in this configuration. Each of the eight anodes is electrically connected to a positive pole of each pulse rectifier. With regard to the cathodic circuit, it is provided that it is distributed over a total of three current rollers, in such a way that the negative pole of two pulse rectifiers each with one of the two outer current rollers (strip inlet current roller and strip outlet current roller) and the negative pole of the other four pulse rectifiers is electrically connected to the deflecting roller designed as an intermediate current roller. In a preferred embodiment variant, a hydrogen concentration is determined in the at least one electrolysis cell, more preferably in each of the electrolysis cells. The hydrogen concentration is preferably detected using hydrogen probes that measure the concentration in the exhaust air from the electrolysis cell (s) directly. By detecting the hydrogen, conclusions can be drawn indirectly about the amount of hydrogen adsorbed on the steel strip and / or diffused into the steel strip, so that a correction can be made during the coating process by adapting the pulse pattern within the pulse pattern sequence. In a particularly preferred embodiment, the at least one pulse pattern of the pulse pattern sequence in the at least one, more preferably first, electrolysis cell of the plurality of electrolysis cells is selected in this way with regard to its pulse type, i.e. cathodic and anodic pulse, its pulse shape, its pulse timeout, its pulse length and its number of pulses that the steel strip is insulated against hydrogen adsorption. For this purpose, a pulse pattern is advantageously selected that enables a fast formation of a fine-grain, closed zinc and / or zinc alloy coating. A series of short cathodic pulses can be used to form a large number of uniformly distributed seed cells on the steel strip surface, which can then be formed into a flat, closed zinc and / or zinc alloy layer with few defects with further crystal growth on each seed cell. The reduction of imperfections, where hydrogen is preferentially deposited, reduces hydrogen adsorption and isolates the steel strip surface from the protons present in the electrolyte. The hydrogen separation is then reduced in favor of the zinc and / or the zinc alloy via the increasing amount of the adsorbed zinc and / or the zinc alloy on the steel strip surface. The pulse length of the at least one cathodic pulse and / or the at least one anodic pulse is advantageously 3.0 to 100 ms, more preferably 3.0 to 50 ms, even more preferably 3.0 to 20 ms, further preferably 3.0 to 10 ms and most preferably 3.0 to 5 ms. Advantageous pulse pauses between two of the plurality of pulses are 1.0 to 200 ms, preferably 1.0 to 100 ms, more preferably 1.0 to 50 ms, even more preferably 1.0 to 25 ms and most preferably 1.0 to 5.0 ms.

With regard to the number of pulses between the two types of pulses, it is advantageously provided that this is 1 to 5000, preferably 1 to 2500, more preferably 1 to 2000, even more preferably 1 to 1000, more preferably 1 to 200, more preferably 1 to 100, and most preferably 1 to 50.

In a particularly preferred embodiment, the ratio of the pulse length to the pulse off-time of the cathodic pulse is 0.1 and / or 0.02, which is advantageously leads to a reduction in the diffusion coefficient of hydrogen by up to 40% compared to DC electrolysis.

After the steel strip has been coated in the coating section of the system, it can be fed to an aftertreatment unit in which the coated steel strip is tempered. For this purpose, the system preferably comprises an inductive belt heating furnace and / or a gas-heated continuous-air furnace, in particular a continuous-flow furnace, which enables contactless tempering and thus protects the zinc and / or zinc alloy coating.

The tempering of the coated steel strip is advantageously carried out at a maximum temperature of <300 ° C. (PMT), more preferably in a range from 150 to 250 ° C. (PMT).

The invention and the technical environment are explained in more detail below with the aid of figures and examples. It should be pointed out that the invention is not intended to be restricted by the exemplary embodiments shown. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and to combine them with other components and findings from the present description and / or figures. In particular, it should be pointed out that the figures and in particular the size relationships shown are only schematic. The same reference symbols denote the same objects, so that explanations from other figures can be used in addition, if necessary. Show it:

1 shows a first variant embodiment of part of a

Coating line of a system for the electrolytic coating of a steel strip with a coating in a schematic

Presentation, 2 shows a second embodiment variant of a part of the coating section of the system for electrolytically coating a steel strip with a coating in a schematic representation,

3 shows a variant of a part of a coating line with n-cells, FIG. 4 shows a variant of a partial anode arrangement,

5 shows a third embodiment variant of a part of the coating section of the installation for electrolytically coating a steel strip with a coating in a schematic representation,

6 shows a first variant embodiment of a pulse pattern which can form part of the pulse pattern sequence,

7 shows a second embodiment variant of a pulse pattern which can form part of the pulse pattern sequence,

8 shows a third variant embodiment of a pulse pattern which can form part of the pulse pattern sequence, and FIG. 9 shows a fourth variant embodiment of a pulse pattern which can form part of the pulse pattern sequence.

In FIG. 1, a part of a coating line 1 of a system for electrolytically coating a steel strip with a coating based on zinc and / or a zinc alloy is shown in a schematic representation. Such a system can have one or more flasher devices for dismantling and Winding up of the steel strip to be coated, an infeed store, a straightener, a cleaning and activation unit, the coating line 1, a post-treatment unit, an outfeed store, an inspection line and an oiling device arranged in front of the winding station (reel device).

According to the coating line 1 shown here, a steel strip 2 can be electrolytically coated with a coating based on zinc and / or a zinc alloy. For this purpose, the coating line 1 in the variant shown in FIG. 1 comprises an electrolysis cell 3, which in the present case is designed as an immersion tank and has a correspondingly electrochemically adjusted electrolyte 4 containing zinc and / or a zinc alloy in cationic form. In the embodiment variant shown here, the electrolytic cell 3 comprises two anodes 5, which are positioned in the electrolytic cell 3 in such a way that the steel strip 2 to be coated, which can be passed through the electrolytic cell 3, can be passed through at a defined and parallel distance therefrom. Both anodes 5 are designed as one-piece plate anodes and are arranged one behind the other in the strip running direction R in such a way that the steel strip 2 can be coated on one side with the coating based on zinc and / or the zinc alloy.

The electrolytic cell 3 is assigned two current rollers 6, 7, with the first current roller 6 being arranged within the coating section 1 on the inlet side (strip inlet current roller) of the electrolytic cell 3 and the second current roller 7 on the outlet side (strip outlet current roller) of the electrolytic cell 3. The steel strip 2, which may have been subjected to a previous cleaning and / or activation step, is diverted from a horizontal movement to a vertical movement via the strip infeed current roller 6, so that it enters the electrolytic cell 3, and at the same time connected cathodically . About the belt outlet current roller 7, the steel belt 2 is then after The coating process is diverted from the vertical back to the horizontal movement, whereby it can optionally also be connected cathodically via the strip outlet current roller 7. In addition, a deflection roller 8 is arranged inside the electrolysis cell 3, via which the steel strip 2 is deflected.

To carry out the coating process, both anodes 5 are energized by means of a modulated current which is provided by a separate pulse rectifier 9, which is implemented using switched-mode power supply technology. Each of the pulse rectifiers 9 is electrically connected to one of the two current rollers 6, 7 via its negative pole and the positive pole is electrically connected to one of the two anodes 5. The two anodes 5 can be energized via the modulated current in such a way that the coating process can be carried out using a defined pulse pattern sequence 10, which is formed from individual pulse patterns 11.

Both pulse rectifiers 9 are advantageously electrically connected to a central control unit 12, via which the respective desired pulse pattern 11 of the pulse pattern sequence 10 is transmitted to each of the pulse rectifiers 9. This allows the entire coating process to be regulated in an automated manner.

In FIG. 2, a second variant of a part of the coating line 1 is shown. In contrast to the embodiment variant shown in FIG. 1, the electrolytic cell 3 comprises two anode arrangements 13, each with two anodes 5 arranged parallel to one another, through which the steel strip 2 is guided. As can be seen from FIG. 2, each of the anodes 5 of the two anode arrangements 13 is also supplied with current via a separate pulse rectifier 9. Here, each of the four anodes 5 is electrically connected to a positive pole of each pulse rectifier 9 and the negative pole of two pulse rectifiers 9 is electrically connected to one of the two current rollers 6 and 7, respectively. FIG. 3 shows a variant of a part of a coating line 1 with n-type electrolysis cells 3, four of which are shown by way of example. All the electrolysis cells 3 are arranged one behind the other in the direction R of the strip. Here, between each of the plurality of electrolytic cells 3, a deflecting roller designed as an intermediate current roller 14 is arranged, via which the steel strip 2 is deflected from a previous to the next electrolytic cell 3 and is additionally connected cathodically. As can be seen from FIG. 3, each of the anodes 5 of the plurality of anode arrangements 13 is supplied with current via a separate pulse rectifier 9. Here, each of the anodes 5 is electrically connected to a positive pole of each pulse rectifier 9. With regard to the cathodic circuit, it is provided that it is distributed over the different current rollers 6, 7, 14 in such a way that the negative pole of two pulse rectifiers 9 each with one of the two outer current rollers 6, 7, i.e. the strip inlet current roller 6 and the Tape exit current roller 7, and the negative pole of the remaining pulse rectifiers 9 is electrically connected to the deflection roller designed as an intermediate current roller 14.

FIG. 4 shows a variant of a partial anode arrangement 15 which comprises a plurality of rod-shaped partial anodes 16, each of the partial anodes 16 being electrically connected to the power source or to a negative pole of a pulse rectifier 9.

In Figure 5, a third variant of a part of a coating line 1 is shown. In this case, the electrolysis cell 3 is essentially formed from the anode arrangement 13 in that the two open flanks of this are closed. In this embodiment variant, the steel strip 2 is guided through the partially closed space delimited by the anode arrangement 13 and the electrolyte 4 flows around it in this space. The electrolyte 4 is conveyed from a reservoir 17 arranged below the anode arrangement 13 via a pump 18 into the space, where it flows through it over the entire cross section. In FIGS. 6 to 9, different embodiment variants of pulse patterns 11 are shown, which form part of the pulse pattern sequence 10. In Figure 6, an initial current pulse of the time length t is shown, which is then reduced to a constant current strength. The initial current pulse can be used to increase the number of nuclei on the cathode, with the result that fine and small crystal forms are deposited. In contrast to this, the dashed line in FIGS. 6 to 8 shows a cathodic current that is constant over time, as is used in direct current electrolysis (DC electrolysis).

In FIG. 7, an embodiment variant is shown, which shows a repetitive pulse pattern 11 of identical design in terms of current amount and time. The pauses in the current flow result in a relaxation of the Nernst double layer, which is associated with a breakdown of the diffusion layer that hinders the transport of substances and thus supports the formation of a homogeneous coating thickness over the surface of the strip. FIG. 8 shows a pulse pattern 11 with a periodic, square-wave current pulse which can be used in combination with one of the preceding patterns to form a multilayer, cathodic coating. In this case, the coating is galvanically deposited on the steel strip in the cathodic phase, then applied anodically by the reverse pulse, with currents that are lower in magnitude, and the deposition is prevented. Due to the anodic switching time, crystal peaks are preferably broken down and, again due to cathodic switching, another zinc and / or

Zinc alloy layer deposited on the already existing layer. By means of the pulse pattern shown in FIG. 8, the metallic coatings can be built up periodically and in layers, which is associated with an improvement in the corrosion resistance. This so-called Reverse pulse current method is also called the bipolar pulse current method, since the cathodic and anodic current flow is changed, i.e. the current flow is changed when the zero crossing is cut. In other words, the cathode is temporarily switched to the anode, so that the galvanic deposition process can temporarily be carried out reversibly. The amount of current, the duration and the polarity change can be designed according to the specifications by the user and optimized for the process.

Examples:

To investigate hydrogen separation and hydrogen diffusion, a steel strip with a tensile strength of R _e = 1200 MPa was coated with a zinc coating in a system with ten electrolysis cells. For this purpose, each of the cells had a sulfuric acid, aqueous electrolyte with zinc sulfate in a concentration in the range of 280 and 320 g / l. The bath temperature was 50 and 70 ° C.

Example 1: To isolate the steel strip from hydrogen adsorption, a pulse pattern sequence with the following pulse pattern (FIG. 9) was selected, which allows a fast deposition of a finely crystalline, dense, zinc coating. Pulse pattern:

Pulse: cathodic

Pulse shape: rectangular

Pulse off time: 5 ms Pulse length: 5 ms Number of pulses: 10 Pulse: anodic

Pulse shape: rectangular

Pulse length: 5 ms Number of pulses: 2

Pulse off time: 2 ms

The pulse current density was 100 A / dm ^2. No significant reduction in the yield point (R _e ) could be detected with the coated steel strip.

Example 2: To investigate the diffusion of hydrogen into the steel strip, a pulse pattern sequence with the following pulse pattern was selected.

Pulse pattern: Pulse: cathodic

Pulse shape: rectangular

Pulse off time: 135 ms

Pulse length: 3 ms The pulse current density was 50 A / dm ² .

The analysis of the steel strip coated in this way showed a significant reduction in the measured hydrogen compared to a pulse pattern with a ratio of pulse length to pulse off-time of 3/1. List of reference symbols

1 coating line

2 tape / fabric / cathode

3 electrolytic cell

4 electrolyte

5 anode

6 first power roller / tape infeed power roller

7 Second power roller / belt exit power roller

8 pulley

9 pulse rectifiers

10 pulse pattern sequence 11 pulse pattern 12 control unit

13 Anode arrangement

14 Intermediate current role

15 Partial anode arrangement

16 partial anodes

17 reservoir

18 pumps

R direction of tape travel

Claims

Patent claims:

1 . Method for the electrolytic coating of a steel strip (2) with a coating based on zinc and / or a zinc alloy, the steel strip (2), after possibly prior cleaning and / or activation, preferably comprising a coating section (1) comprising at least one at least two or more electrolytic cell (s) (3) are supplied and are successively electrolytically coated in this, the steel strip (2) initially connected cathodically via at least one current roller (6) and within the at least one electrolytic cell (3) at a defined distance parallel to at least one anode (5) arranged in the electrolysis cell (3), the at least one anode (5) being energized by means of a modulated current and the coating process within the coating section (1) taking place using a defined pulse pattern sequence (10) , which is formed from at least one pulse pattern (11), wherein according to the pulse pattern sequence (10) the coating on basis s of zinc and / or a zinc alloy is deposited and formed from an electrolyte (4) on the steel strip (2).

2. The method according to claim 1, wherein the modulated current is provided by at least one pulse rectifier (9), the negative pole of which is electrically connected to the at least one current roller (7) and the positive pole to the at least one anode (5).

3. The method according to claim 2, wherein the at least one pulse rectifier (9) is electrically connected to a central control unit (12) via which the coating process is regulated.

4. The method of claim 3, wherein the at least one pulse pattern (11) of the pulse pattern sequence (10) from the central control unit (12) to the at least one pulse rectifier (9), preferably to each of the pulse rectifiers (9), is transmitted.

5. The method according to any one of the preceding claims, wherein the at least one pulse pattern (11) of the pulse pattern sequence (10) comprises at least one cathodic pulse, at least one anodic pulse, and / or at least one pulse timeout, and wherein the cathodic and the anodic pulse via a Pulse duration can be defined.

6. The method according to any one of the preceding claims, wherein the at least one anode (5) is designed as a plate anode, which is preferably formed in one piece and / or from at least two or more rod-shaped partial anodes (16).

7. The method according to any one of the preceding claims, wherein the steel strip (2) within the at least one electrolytic cell (3) by at least one anode arrangement (13) of two anodes (5) arranged parallel to one another, preferably by at least two anode arrangements (13) each with two anodes (5) arranged parallel to one another.

8. The method according to claim 7, wherein each of the anodes (5) of an anode arrangement (13) is energized via a separate pulse rectifier (9), such that each of the anodes (5) each with a positive pole of each pulse rectifier (9) and the The negative pole of each pulse rectifier (9) is electrically connected to the at least one current roller (6, 7).

9. The method according to claim 7 or 8, wherein the steel strip (2) is deflected between the at least two anode assemblies (13) via a deflection roller (8), optionally arranged within the electrolysis cell (3, 5).

10. The method according to any one of the preceding claims, wherein the

Steel strip (2) is guided within the coating section (1) through a plurality of at least two electrolysis cells (3) arranged one behind the other in the strip running direction (R).

11. The method according to claim 10, wherein the steel strip (2) is deflected between the at least two electrolytic cells (3) via at least one deflecting roller designed as an intermediate current roller (14) and, if necessary, is additionally connected cathodically.

12. The method according to any one of the preceding claims, wherein in the at least one electrolysis cell (3), preferably in each of the electrolysis cells (3), a hydrogen concentration is determined.

13. The method according to any one of the preceding claims, wherein the steel strip (2) has a tensile strength of at least R _e ^ 500 MPa, preferably of at least R _e ^ 800 MPa, more preferably of at least R _e ^ 1000 MPa.

14. The method according to any one of the preceding claims, wherein the at least one pulse pattern (11) of the pulse pattern sequence (10) in the at least one, preferably first electrolysis cell (3) with regard to its pulse type, its pulse shape, its pulse timeout, its pulse length and its number of pulses is selected such that the steel strip (2) is insulated against hydrogen adsorption.

15. The method according to claim 14, wherein the pulse length of the at least one cathodic pulse and / or the at least one anodic pulse 3 to 100 ms, preferably 3 to 50 ms, more preferably 3 to 20 ms, even more preferably 3 to 10 ms and most preferably 3 to 5 ms.

16. The method according to claim 14 or 15, wherein the pulse off time between each two of the plurality of pulses is 1.0 to 200 ms, preferably 1.0 to 100 ms, more preferably 1.0 to 50 ms, even more preferably 1.0 to 25 ms and most preferably 1.0 to 5.0 ms.

17. The method according to any one of the preceding claims 14 to 16, wherein the number of pulses between the two types of pulses is 1 to 5000, preferably 1 to 2500, more preferably 1 to 2000, even more preferably 1 to 1000, more preferably 1 to 200, more preferably 1 to 100, and most preferably 1 to 50.

18. The method according to any one of the preceding claims 14 to 17, wherein the ratio of pulse length to pulse off-time of the cathodic pulse is 0.1 and / or 0.02.

19. The method according to any one of the preceding claims, wherein the steel strip (2) after the coating in the coating section (1) is fed to an aftertreatment unit in which the coated steel strip (2) is tempered.

20. The method according to claim 19, wherein the annealing is carried out at a maximum temperature of <300 ° C (PMT).

21. Plant for the electrolytic coating of a steel strip (2) with a coating based on zinc and / or a zinc alloy, comprising: possibly a cleaning and / or an activation unit in which the steel strip (2) is cleaned and / or can be activated; a coating line (1) with at least one, preferably at least two or more electrolytic cell (s) (3), in which the steel strip (2) can be electrolytically coated successively, and at least one power roller (6) over which the steel strip (2) is cathodic can be switched, the at least an electrolytic cell (3) comprises at least one anode (5) which is arranged in such a way that the steel strip (2) that can be passed through the at least one electrolytic cell (3) can be passed through at a defined and parallel distance from the at least one anode (5), wherein the system comprises at least one pulse rectifier (9), the negative pole of which is electrically connected to the at least one current roller (6) and the positive pole is electrically connected to the at least one anode (5), such that the at least one anode (5) by means of a modulated current can be energized in such a way that the coating process within the coating section (1) using a defined

Pulse pattern sequence (10) can be carried out, the pulse pattern sequence (10) being formed from individual pulse patterns (11), wherein according to the pulse pattern sequence (10) the coating based on zinc and / or the zinc alloy from an electrolyte (4) on the steel strip ( 2) is separable.