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.

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≥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 ≤300° 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).

Description

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° 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