Method and system for electrolytically coating a steel strip by means of pulse technology

Abstract

An electroplating method and a system for electrolytically coating a steel strip, in particular for the automotive sector, with a coating based on zinc and/or a zinc alloy utilizes pulse technology.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an electroplating method and a system for electrolytically coating a steel strip, in particular for the automotive sector, with a coating based on zinc and/or a zinc alloy.

BACKGROUND

[0002] Nowadays, electrolytically refined steel strip is used as a semi-finished product in many branches of industry, such as the automotive industry, aerospace technology, mechanical engineering, the packaging industry, and in the manufacture of household appliances and electrical devices. Traditionally, the production of such strips is carried out in continuously operating strip processing lines with a constant-speed passage of the steel strip through one or more electrolytic cells connected in series.

[0003] The coatings electrolytically deposited on one or both sides of the steel strip can perform various tasks and impart new product properties on the steel strip in question. 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.

[0004] For example, electrolytically galvanized steel strip is given active corrosion protection by the zinc coating and provides a good adhesion base for painting and/or laminating with plastic films. A chrome coating also imparts on a steel strip or a plastic strip increased corrosion and wear protection, along with decorative properties. Nickel and nickel alloys, on the other hand, can increase the surface hardness of the substrate in question.

[0005] The production of the respective coatings with the desired properties is, in particular under economic and business aspects, strongly dependent on various parameters, such as the type and composition of the electrolyte, its metal salt concentration and temperature, the geometrical arrangement of the electrolytic cells and their electrodes, the electrochemical current conduction along with its amount, time and polarity.

[0006] In the prior art, the electrolytic coating of steel strips is carried out by means of direct current, wherein thyristor technology is used thereby. Such so-called "DC electrolysis" can be designed to be unipolar and partially pole-reversible, but does not allow specific current sequences in magnitude, time and polarity. The high degree of hydrogen development is 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. Thus, the diffusing hydrogen is primarily responsible for so-called "spontaneous brittle fracture" and the reduction of the material yield strength or the required strength of a steel strip. Furthermore, the hydrogen trapped in a galvanized steel strip during the curing process of a painted component, preferably a component painted by means of a cathodic electrodeposition (CED) process, leads to the effusion of the trapped hydrogen, with the result that hydrogen bubbles form underneath the paint layer, resulting in so-called "paint bursts."

[0007] The hydrogen-induced reduction in material strength represents an additional significant process disadvantage in the prior art, because if the material is no longer strong enough, it is generally unusable for an application in the field of safety-relevant components, for example in the automotive sector.

SUMMARY

[0008] The present disclosure provides an improved process and an improved system for the electrolytic coating of steel strips with a coating based on zinc and/or a zinc alloy.

[0009] The method provides that the steel strip, after optionally 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 therein, wherein the steel strip is initially cathodically connected via at least one current roller and is guided within the at least one electrolytic cell at a defined distance parallel to at least one anode arranged in the electrolytic cell.

[0010] The at least one anode is supplied with a modulated current, wherein the coating process takes place within the coating section using a defined pulse pattern sequence, which is formed from at least one pulse pattern, wherein, in accordance with the pulse pattern sequence, zinc and/or a zinc alloy is deposited from an electrolyte on the steel strip and the coating is formed on the basis of zinc and/or a zinc alloy.

[0011] The present disclosure also provides for a system for the electrolytic coating of a steel strip. The system can comprise a cleaning and/or an activation unit, in which the steel strip can be cleaned and/or activated; a coating section with at least one, preferably at least two or more electrolytic cell(s), in which the steel strip can be successively electrolytically coated, and at least one current roller, via which the steel strip can be cathodically switched, wherein the at least one electrolytic cell comprises at least one anode, which is arranged in such a way that the steel strip that can be passed through the at least one electrolytic cell can be passed through at a defined and parallel distance from the at least one anode. The system comprises at least one pulse rectifier, which is designed in switching power supply technology, the negative pole of which is electrically connected to the at least one current roller and the positive pole of which is electrically connected to the at least one anode in such a way that the at least one anode can be supplied with a modulated current, in that the coating process can be carried out within the coating section using a defined pulse pattern sequence, wherein the pulse pattern sequence is formed from individual pulse patterns, wherein in accordance with a pulse pattern sequence a coating based on zinc and/or a zinc alloy can be deposited from an electrolyte on the steel strip.

[0012] Surprisingly, it has been shown that, using a defined pulse pattern sequence that is formed from individual pulse patterns, cathodic hydrogen evolution and its diffusion into the steel strip can be reduced to such an extent that hydrogen-induced brittle fracture, the decrease in tensile strength and the formation of surface defects in subsequent process steps can be effectively avoided. As such, a steel strip coated with a zinc and/or zinc alloy coating by the method in accordance with the disclosure can be produced directly in a strength-maintaining manner, such that a possibly necessary heat process step downstream of the coating process can be spared.

[0013] The coating process takes place within the coating section using a defined pulse pattern sequence, which is formed from individual pulse patterns. Thereby, the pulse pattern sequence can be formed from 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.

[0014] The features listed individually in the dependent claims can be combined with one another in a technologically useful manner and can define further embodiments. In addition, the features indicated in the claims are further specified and explained in the description, wherein further preferred embodiments are illustrated. In this connection, it is pointed out that all the present device features, which are explained in the course of the individual method steps or vice versa, can be combined in the same way with the system and/or the method in accordance with the disclosure, without explicitly referring to them.

[0015] Preferably, the steel strip is one that has a tensile strength of at least R.sub.e.gtoreq.500 MPa, more preferably of at least R.sub.e.gtoreq.600 MPa and most preferably of at least R.sub.e.gtoreq.800 MPa. In terms of maximum tensile strength, the steel strip is limited to a tensile strength of R.sub.e.ltoreq.2000 MPa, more preferably to a tensile strength of R.sub.e.ltoreq.1500 MPa, even more preferably to a tensile strength of R.sub.e.ltoreq.1200 MPa.

[0016] A preferred zinc alloy coating includes zinc-magnesium.

[0017] In principle, the coating section of the system can comprise an electrolytic cell with an anode, for example in the form of a plate anode. In a further development, the only one electrolytic cell can comprise two anodes, which are arranged one behind the other, for example in the direction of strip travel, in such a way that the steel strip can be coated on one side. In a preferred embodiment, the two anodes may be formed in an anode arrangement, in which the two anodes are then arranged parallel to each other within the one electrolytic cell.

[0018] In a preferred embodiment, the coating section comprises at least two electrolytic cells, more preferably at least three electrolytic cells, even more preferably at least four electrolytic cells, further preferably at least five electrolytic cells, and, for reasons of process economy, is limited to a maximum of twenty electrolytic cells, preferably to a maximum of 16, more preferably to a maximum of fifteen electrolytic cells. The plurality of electrolytic cells is preferably arranged one behind the other in the direction of strip travel, through which the steel strip is then fed within the coating section.

[0019] The individual electrolytic cells can be in the form of horizontal or preferably vertical electrolytic cells, through which the steel strip is guided by deflection rollers.

[0020] The deposition process within the individual electrolytic cells takes place in an electrolyte through which the steel strip is passed. The electrolyte medium is usually aqueous and usually has a pH value of less than 5.0. Alternatively, the electrolyte medium can be formed from a non-aqueous medium, such as an ionic liquid. A preferred ionic liquid comprises a mixture of choline chloride and urea.

[0021] The modulated current is provided by a pulse rectifier, which uses switching power supply technology. The use of such a pulse rectifier enables the defining of the magnitude, the time course along with the polarity of the respective desired pulse pattern and thus of the entire pulse pattern sequence, in such a way that the electrolytic process can be optimally adapted according to the given parameters.

[0022] A pulse rectifier formed in such a way is defined by the fact that the AC voltage on the line side is initially rectified and smoothed. The DC voltage then generated, which has much higher frequencies, as a rule in the range of 5 kHz to 300 kHz, is then divided, transformed at this high frequency and then rectified and screened. The superimposed voltage and current control usually works via pulse width modulation or pulse phase modulation.

[0023] Due to the high frequency at the power transformer, the transformer is much smaller, such that the energy losses are much lower. This results in a much higher power effectiveness of the DC power supply and thus of the overall production plant, due to the system.

[0024] Due to its design, the pulse rectifier can be provided in modular form. This leads to a much higher availability, since the power to be provided by a defective module can be taken over by another module, and upon the repair of a defective module, it can be replaced quickly.

[0025] An additional advantage is that the quality of the DC 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 to realize, and existing DC current/DC voltage supply systems can be expanded by additional modules at a later date by using appropriate control technology, by means of which the power of the DC current/DC voltage supply system can be increased.

[0026] The at least one pulse rectifier, which provides the modulated current, is advantageously electrically connected via its negative pole to the at least one current roller and the positive pole to the at least one anode. In this connection, 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. Via the control unit, the at least one pulse pattern of the pulse pattern sequence is transmitted to the at least one, preferably each, pulse rectifier, which then transmits it to the respective assigned electrolytic cell by means of signal technology.

[0027] 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 time-out, wherein the cathodic and anodic pulses are defined by a pulse duration and its respective shape, for example rectangular. The cathodic pulse is used to deposit the zinc and/or zinc alloy on the steel strip. In particular, an anodic pulse can be used to oxidize the nascent state hydrogen adsorbed on the steel strip surface back to the proton and thus remove it from the steel strip surface in a targeted manner.

[0028] The at least one anode is preferably formed as a plate anode. In principle, such plate anodes can be designed in the form of a soluble or an insoluble anode. In the case of soluble anodes, also known as active anode systems, the anode goes into solution during electrolysis. Insoluble anodes, also known as inert anode systems, on the other hand, do not pass 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 the active layer, on the other hand. Thereby, titanium, niobium or other reaction carrier metals are usually used as the carrier material, but in any case materials that passivate under the electrolysis conditions. Electron-conducting materials such as platinum, iridium or other precious metals, their mixed oxides or compounds of such elements are typically used as the material for the active layer. Thereby, the active layer can either be applied directly to the surface of the carrier material or be located on a substrate at a distance from the carrier material. Among other things, materials that can be considered as carrier materials, such as titanium, niobium or the like, can also serve as the substrate.

[0029] The at least one anode can preferably be formed in one piece and/or, in accordance with an advantageous embodiment, from at least two or more partial anodes formed in rod shape, wherein each of the partial anodes is then electrically connected to the current source. The at least two or more rod-shaped partial anodes are advantageously arranged in such a way that the distance of each partial anode from the strip can be adjusted over its width. Thereby, along the strip width of the steel strip, via the adjustment of the distance of each of the partial anodes to the strip and/or the current density, locally different layer thicknesses can be applied and/or corrected by means of desorption. For example, the partial anodes arranged at the strip edges, in comparison to those arranged in the middle segment, can be supplied with current 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 zinc alloy at the strip edges.

[0030] In a particularly advantageous embodiment, the at least one electrolytic cell comprises at least one anode arrangement consisting of two anodes arranged parallel to one another, through which the steel strip is passed. In such a configuration, 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, such that each of the anodes is electrically connected to a respective positive pole of each pulse rectifier and the negative pole of each pulse rectifier is electrically connected to the at least one current roller. In other words, the electrolytic cell in this configuration includes two anodes, two pulse rectifiers along with a current roller through which the strip substrate is cathodically switched.

[0031] In a further preferred embodiment, the at least one electrolytic cell comprises at least two anode arrangements, each with two anodes arranged parallel to one another, through which the steel strip is passed. If such an electrolytic cell is formed as an immersion tank, it is particularly preferred that the steel strip is deflected between the at least two anode arrangements by means of a deflection roller, which may be arranged inside the electrolytic cell. In a configuration formed in this way, each of the anodes of the at least two anode arrangement is also supplied with current via a separate pulse rectifier, such that a total of four pulse rectifiers are provided in this configuration. Thereby, each of the four anodes is electrically connected to one positive pole of each pulse rectifier and the negative pole of each two pulse rectifiers is electrically connected to one of the two current rollers. In other words, the electrolytic cell in this configuration comprises four anodes, four pulse rectifiers, two current rollers along with a deflection roller, which may be arranged inside the electrolytic cell.

[0032] In an additional preferred embodiment, the electrolytic cell can be formed substantially from the anode arrangement by closing the two open flanks thereof. Thereby, the steel strip passes through the partially enclosed chamber bounded by the anode arrangement and is washed around by the electrolyte in such chamber. The electrolyte can, for example, be fed to and flow through the entire cross-section of the chamber via corresponding pumps. Compared to an immersion tank, such a structure has a smaller installation space and thus requires smaller volumes of electrolyte.

[0033] In a particularly preferred embodiment, the coating section comprises a plurality of electrolytic cells arranged one behind the other in the direction of strip travel, through which the steel strip is passed. In this connection, it is advantageously provided that the steel strip is deflected between at least two, more preferably between each of the plurality of electrolytic cells, via at least one deflection roller formed as an intermediate current roller, and if necessary additionally cathodically switched. In an exemplary embodiment with two electrolytic 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, such that a total of eight pulse rectifiers are provided in this configuration. Thereby, each of the eight anodes is electrically connected to one positive pole of each pulse rectifier. With regard to the cathodic circuit, it is provided that this is distributed over the total of three current rollers in such a way that the negative pole of two pulse rectifiers in each case is electrically connected to one of the two outer current rollers (strip inlet current roller and strip outlet current roller) and the negative pole of the remaining four pulse rectifiers is electrically connected to the deflection roller formed as an intermediate current roller.

[0034] In a preferred embodiment, a hydrogen concentration is determined in the at least one electrolytic cell, more preferably in each of the electrolytic cells. The hydrogen concentration is preferably detected by hydrogen probes that directly measure the concentration in the exhaust air of the electrolytic cell(s). By detecting the hydrogen, it is possible to indirectly conclude the amount of hydrogen adsorbed on the steel strip and/or diffused into the steel strip, such that a correction can still be made in the coating process, by adjusting the pulse patterns within the pulse pattern sequence.

[0035] In a particularly preferred embodiment, the at least one pulse pattern of the pulse pattern sequence in the at least one, more preferably first, electrolytic cell of the plurality of electrolytic cells is selected with respect to its pulse type, i.e. cathodic and anodic pulse, its pulse shape, its pulse time-out, its pulse length along with its pulse number, in such a way that the steel strip is isolated from hydrogen adsorption.

[0036] For this purpose, a pulse pattern that enables the rapid formation of a fine-grained, closed zinc and/or zinc alloy coating is advantageously selected. A high number of uniformly distributed crystal nuclei can be formed on the steel strip surface via a sequence of short cathodic pulses, which nuclei can then be formed into a flat, closed zinc and/or zinc alloy layer with few defects as the crystals continue to grow on each nucleus. The reduction of imperfections, at which hydrogen preferentially deposits, reduces hydrogen adsorption and isolates the steel strip surface from protons present in the electrolyte. The increasing amount of adsorbed zinc and/or zinc alloy on the steel strip surface then reduces the hydrogen evolution in favor of the zinc and/or zinc alloy.

[0037] The pulse length of the at least one cathodic pulse and/or the at least one anodic pulse amount to 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.

[0038] Advantageous pulse times between any two of the plurality of pulses amount to 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.

[0039] With regard to the pulse number between the two types of pulses, it is advantageously provided that these amount to 1 to 5000, preferably 1 to 2500, more preferably 1 to 2000, even more preferably 1 to 1000, further preferably 1 to 200, more preferably 1 to 100 and most preferably 1 to 50.

[0040] In a particularly preferred embodiment, the ratio of pulse length to pulse time-out of the cathodic pulse amounts to 0.1 and/or 0.02, which advantageously leads to the reduction of the diffusion coefficient of hydrogen by up to 40% compared to DC electrolysis.

[0041] After the steel strip has been coated in the coating section of the system, it can be fed to a post-treatment unit, in which the coated steel strip is annealed. Preferably, the system for this purpose comprises an induction strip heating furnace and/or a gas-heated circulating air continuous furnace, in particular a floating strip continuous furnace, which enables contactless annealing and thus protects the zinc and/or zinc alloy coating.

[0042] Annealing of the coated steel strip is advantageously carried out at a maximum temperature of .ltoreq.300.degree. C. (PMT), more preferably in a range of 150 to 250.degree. C. (PMT).

[0043] The invention and the technical environment are explained in more detail below with reference to figures and examples. It should be noted that the invention is not intended to be limited by the embodiments shown. In particular, unless explicitly shown otherwise, it is also possible to extract partial aspects of the facts explained in the figures and combine them with other components and findings from the present description and/or figures. In particular, it should be noted that the figures and in particular the size relationships shown are only schematically. Identical reference signs designate identical objects, such that explanations from other figures can be used as a supplement if necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 shows a first embodiment of a part of a coating section of a system for the electrolytic coating of a steel strip with a coating in a schematic representation,

[0045] FIG. 2 shows a second embodiment of a part of the coating section of the system for the electrolytic coating of a steel strip with a coating in a schematic representation,

[0046] FIG. 3 shows an embodiment of a part of a coating section with n-cells,

[0047] FIG. 4 shows an embodiment of a partial anode arrangement,

[0048] FIG. 5 shows a third embodiment of a part of the coating section of the system for the electrolytic coating of a steel strip with a coating in a schematic representation,

[0049] FIG. 6 shows a first embodiment of a pulse pattern that can form part of the pulse pattern sequence,

[0050] FIG. 7 shows a second embodiment of a pulse pattern that can form part of the pulse pattern sequence,

[0051] FIG. 8 shows a third embodiment of a pulse pattern that can form part of the pulse pattern sequence, and

[0052] FIG. 9 shows a fourth embodiment of a pulse pattern that can form part of the pulse pattern sequence.

DETAILED DESCRIPTION

[0053] FIG. 1 shows a part of a coating section 1 of a system for the electrolytic coating (electroplating) of a steel strip with a coating based on zinc and/or a zinc alloy in a schematic representation. Such a system can comprise one or more coiling devices for uncoiling and recoiling the steel strip to be coated, an inlet accumulator, a stretcher leveler, a cleaning and activation unit, the coating section 1, a post-treatment unit, an outlet accumulator, an inspection section along an oiling device arranged upstream of the coiling station (coiling device).

[0054] In accordance with the coating section 1 shown herein, a steel strip 2 can be electrolytically coated with a coating based on zinc and/or a zinc alloy. For this purpose, the coating section 1 in the embodiment shown in FIG. 1 comprises an electrolytic cell 3, which is formed in the present case as an immersion tank and has a correspondingly electrochemically adjusted electrolyte 4 containing zinc and/or a zinc alloy in cationic form.

[0055] In the embodiment shown in the present case, 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 can be passed through the electrolytic cell 3 at a defined and parallel distance from them. Both anodes 5 are formed as one-piece plate anodes and are arranged one behind the other in direction of strip travel R in such a way that the steel strip 2 can be coated on one side with the coating based on zinc and/or zinc alloy.

[0056] In the present case, two current rollers 6, 7 are assigned to the electrolytic cell 3, wherein the first current roller 6 is 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 is arranged on the outlet side (strip outlet current roller) of the electrolytic cell 3. Via the strip inlet current roller 6, the steel strip 2, which may have been subjected to a previous cleaning and/or activation step, is deflected from a horizontal movement to a vertical movement, such that it enters the electrolytic cell 3, and is thereby simultaneously cathodically switched. After the coating process, the steel strip 2 is then deflected from the vertical back into the horizontal direction by the strip outlet current roller 7, wherein it can also be cathodically switched via the strip outlet current roller 7 if necessary. A deflection roller 8 is also arranged inside the electrolytic cell 3, via which the steel strip 2 is deflected.

[0057] To carry out the coating process, both anodes 5 are supplied with current by means of a modulated current, which is provided in each case by a separate pulse rectifier 9, which is designed in switching power supply technology. Thereby, each of the pulse rectifiers 9 is electrically connected via its negative pole to one of the two current rollers 6, 7 and the positive pole to one of the two anodes 5. The two anodes 5 can be supplied with current via the modulated current in such a way that the coating process can be carried out using a defined pulse pattern sequence 10 that is formed from individual pulse patterns 11.

[0058] Advantageously, both pulse rectifiers 9 are 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 controlled in an automated manner.

[0059] FIG. 2 shows a second embodiment of a part of the coating section 1. In contrast to the embodiment 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 passed. 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. Thereby, each of the four anodes 5 is electrically connected to a positive pole of each pulse rectifier 9, and the negative pole of each two pulse rectifiers 9 is electrically connected to one of the two current rollers 6 or 7, as the case may be.

[0060] FIG. 3 shows an embodiment of a part of a coating section 1 with n-electrolytic cells 3, of which four are shown as an example. All electrolytic cells 3 are arranged one behind the other in the direction of strip travel R. Thereby, a deflection roller in the form of an intermediate current roller 14 is arranged between each of the plurality of electrolytic cells 3, via which deflection roller the steel strip 2 is deflected from one preceding electrolytic cell 3 to the next and is also cathodically switched. 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. Thereby, 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 this is distributed over the different current rollers 6, 7, 14 in such a way that the negative pole of in each case two pulse rectifiers 9 is electrically connected to in each case one of the two outer current rollers 6, 7, i.e. the strip inlet current roller 6 and the strip outlet current roller 7, and the negative pole of the remaining pulse rectifiers 9 is electrically connected to the deflection roller formed as an intermediate current roller 14.

[0061] FIG. 4 shows an embodiment of a partial anode arrangement 15, which comprises a plurality of rod-shaped partial anodes 16, wherein each of the partial anodes 16 is electrically connected to the current source or to a negative pole of a pulse rectifier 9.

[0062] FIG. 5 shows a third embodiment of a part of a coating section 1. Thereby, the electrolytic cell 3 is substantially formed from the anode arrangement 13 by closing the two open flanks thereof. In this embodiment, the steel strip 2 is passed through the partially enclosed chamber bounded by the anode arrangement 13 and in this chamber the electrolyte 4 flows around it. The electrolyte 4 is conveyed from a reservoir 17 arranged below the anode arrangement 13 via a pump 18 into the chamber, where it flows through it over the entire cross-section.

[0063] FIGS. 6 to 9 show different embodiments of pulse patterns 11 that form part of the pulse pattern sequence 10.

[0064] FIG. 6 shows an initial current pulse of time length t, which is subsequently reduced to a constant current intensity. The initial current pulse can be used to increase the number of crystal nuclei on the cathode, resulting in the deposition of fine and small crystal forms. In contrast to this, the dashed line in FIGS. 6 to 8 shows a constant-time cathodic current as used in direct current (DC) electrolysis.

[0065] FIG. 7 shows an embodiment variant, which shows a repetitive pulse pattern 11 that is similar in current amount and time. The switching pauses in the current flow result in a relaxation of the Nernst double layer, which is associated with a reduction of the diffusion layer that impedes mass transfer and thus supports the formation of a homogeneous coating thickness over the surface of the strip.

[0066] FIG. 8 shows a pulse pattern 11 with a periodic current pulse formed in a rectangular shape, which can be used in combination with one of the preceding patterns to form a multilayer cathodic coating. Thereby, in the cathodic phase, the coating is deposited on the steel strip by electroplating, and the reverse pulse then applies it anodically, with currents that are lower in magnitude, and the deposition is prevented. The anodic switching time preferentially reduces crystal peaks and, again by cathodic switching, deposits another zinc and/or zinc alloy layer on top of the 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 corrosion resistance. This so-called reverse pulse current process is also called the bipolar pulse current process, since, thereby, the cathodic and anodic current conduction is alternated; i.e., the current flow is changed when the zero crossing is crossed. In other words, the cathode is temporarily switched to the anode, such that the electroplating deposition process can be temporarily reversed. The current amount, duration and polarity change can be designed according to the user's specification and can be optimized for the process.

EXAMPLES

[0067] To study hydrogen evolution and diffusion, a steel strip with a tensile strength of R.sub.e=1200 MPa was coated with a zinc coating in a system with ten electrolytic cells. For this purpose, each of the cells had a sulfuric acid aqueous electrolyte containing zinc sulfate at a concentration in the range of 280 and 320 g/l. The bath temperature was 50 and 70.degree. C.

Example 1

[0068] To isolate the steel strip from hydrogen adsorption, a pulse pattern sequence was selected with the following pulse pattern (FIG. 9), which allows the rapid deposition of a fine crystalline, dense, zinc coating.

[0069] Pulse pattern: [0070] Pulse: cathodic [0071] Pulse shape: rectangular [0072] Pulse time-out: 5 ms [0073] Pulse length: 5 ms [0074] Pulse number: 10 [0075] Pulse: anodic [0076] Pulse shape: rectangular [0077] Pulse length: 5 ms [0078] Pulse number: 2 [0079] Pulse time-out: 2 ms

[0080] The pulse current density was 100 A/dm.sup.2.

[0081] No significant reduction in yield strength (R.sub.e) was observed for the coated steel strip.

Example 2

[0082] To study the diffusion of hydrogen into the steel strip, a pulse pattern sequence with the following pulse pattern was selected.

[0083] Pulse pattern: [0084] Pulse: cathodic [0085] Pulse shape: rectangular [0086] Pulse time-out: 135 ms [0087] Pulse length: 3 ms

[0088] The pulse current density was 50 A/dm.sup.2.

[0089] Analysis of the steel strip coated in this way showed a significant reduction in the hydrogen measured compared to a pulse pattern with a pulse length to pulse time-out ratio of 3/1.

LIST OF REFERENCE SIGNS

[0090] 1 Coating section [0091] 2 Strip/fabric/cathode [0092] 3 Electrolytic cell [0093] 4 Electrolyte [0094] 5 Anode [0095] 6 First current roller/strip inlet current roller [0096] 7 Second current roller/strip outlet current roller [0097] 8 Deflection roller [0098] 9 Pulse rectifier [0099] 10 Pulse pattern sequence [0100] 11 Pulse pattern [0101] 12 Control unit [0102] 13 Anode arrangement [0103] 14 Intermediate current roller [0104] 15 Partial anode arrangement [0105] 16 Partial anodes [0106] 17 Reservoir [0107] 18 Pumps [0108] R Direction of strip travel

Claims

1.-21. (canceled)

22. A method for electrolytically coating a steel strip (2) with a coating based on zinc and/or a zinc alloy, comprising: feeding the steel strip (2) to a coating section (1) comprising at last one electrolytic cell (3) and successively electrolytically coating the steel strip (2) therein, wherein the steel strip (2) is initially cathodically connected via at least one current roller (6) and is guided within the at least one electrolytic cell (3) at a defined distance parallel to at least one anode (5) arranged in the electrolytic cell (3), wherein the at least one anode (5) is supplied with a modulated current and the coating takes place within the coating section (1) using a defined pulse pattern sequence (10), which is formed from at least one pulse pattern (11), wherein, in accordance with the pulse pattern sequence (10), the coating based on zinc and/or a zinc alloy is deposited and formed from an electrolyte (4) on the steel strip (2).

23. The method according to claim 22, wherein the modulated current is provided by at least one pulse rectifier (9), a negative pole of which is electrically connected to the at least one current roller (7) and a positive pole to the at least one anode (5).

24. The method according to claim 23, wherein the at least one pulse rectifier (9) is electrically connected to a central control unit (12) via which the coating is regulated.

25. The method according to claim 24, wherein the at least one pulse pattern (11) of the pulse pattern sequence (10) is transmitted from the central control unit (12) to the at least one pulse rectifier (9).

26. The method according to claim 22, 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 time-out, and wherein the cathodic pulse and the anodic pulse are defined by a pulse duration.

27. The method according to claim 22, wherein the at least one anode (5) is formed as a plate anode, which is formed in one piece or from two or more partial anodes (16) formed in rod shape.

28. The method according to claim 22, wherein the steel strip (2) is guided within the at least one electrolytic cell (3) through at least two anode arrangements (13), each comprising two anodes (5) arranged parallel to one another.

29. The method according to claim 28, wherein each of the anodes (5) of each anode arrangement (13) is supplied with current via a separate pulse rectifier (9), such that each of the anodes (5) is electrically connected to a respective positive pole of each pulse rectifier (9) and a negative pole of each pulse rectifier (9) is electrically connected to the at least one current roller (6, 7).

30. The method according to claim 28, wherein the steel strip (2) is deflected between the at least two anode arrangements (13) via a deflection roller (8) arranged within the electrolytic cell (3, 5).

31. The method according to claim 22, wherein the steel strip (2) is guided within the coating section (1) through a plurality of at least two electrolytic cells (3) arranged one behind the other in a direction of strip travel (R).

32. The method according to claim 31, wherein the steel strip (2) is deflected between the at least two electrolytic cells (3) via at least one deflection roller formed as an intermediate current roller (14).

33. The method according to claim 22, wherein a hydrogen concentration is determined in the at least one electrolytic cell (3).

34. The method according to claim 22, wherein the steel strip (2) has a tensile strength R.sub.e.gtoreq.1000 MPa.

35. The method according to claim 22, wherein the at least one pulse pattern (11) of the pulse pattern sequence (10) in the at least one electrolytic cell (3) is selected with respect to its pulse type, its pulse shape, its pulse off-time, its pulse length along with its pulse number in such a way that the steel strip (2) is isolated from hydrogen adsorption.

36. The method according to claim 35, wherein the pulse length of at least one cathodic pulse and/or at least one anodic pulse amounts to 3 to 5 ms.

37. The method according to claim 35, wherein the pulse off-time between each two of a plurality of pulses amounts to 1.0 to 5.0 ms.

38. The method according to claim 35, wherein the pulse number between each of two types of pulses, a cathodic pulse and an anodic pulse, amounts to 1 to 50.

39. The method according to claim 35, wherein a ratio of pulse length to pulse time-out of the cathodic pulse amounts to 0.1 or 0.02.

40. The method according to claim 22, wherein the steel strip (2), after coating in the coating section (1), is fed to a post-treatment unit, in which the coated steel strip (2) is annealed.

41. The method according to claim 39, wherein the annealing is performed at a temperature of .ltoreq.300.degree. C. (PMT).

42. A system for electrolytically coating a steel strip (2) with a coating based on zinc and/or a zinc alloy, comprising: optionally, a cleaning and/or an activation unit in which the steel strip (2) can be cleaned and/or activated; a coating section (1) with at least one electrolytic cell (3), in which the steel strip (2) can be successively electrolytically coated, and at least one current roller (6), via which the steel strip (2) can be cathodically switched, wherein the at least one 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), a negative pole of which is electrically connected to the at least one current roller (6) and a positive pole of which is electrically connected to the at least one anode (5), in such a way that the at least one anode (5) can be supplied with a modulated current, wherein a coating process can be carried out within the coating section (1) using a defined pulse pattern sequence (10), wherein the pulse pattern sequence (10) is formed from individual pulse patterns (11), wherein in accordance with a pulse pattern sequence (10) a coating based on zinc and/or a zinc alloy can be deposited from an electrolyte (4) on the steel strip (2).