EP0565070A1 - Electroplating process - Google Patents

Abstract

The invention relates to a process for electrochemical (electro-)deposition of a surface coating on an object, in particular a roller of a machine, employing an electrical variable, which effects the application of the coating, of the electroplating process. It is provided that, in order to achieve a desired, structured surface topography by means of at least one initial pulse of the electrical variable, nuclei of the deposition material are formed on the face to be coated, and that then, by means of at least one subsequent pulse, growth of the deposition material nuclei is brought about by accretion of further deposition material. <IMAGE>

Description

The invention relates to a method for the electrochemical (galvanic) application of a surface coating to an object, preferably a machine component, in particular a machine roller, using an electrical variable of the galvanic process which causes the layer application.

In many areas of technology, for example, machine components with special surface properties are required. It is known to apply surface coatings to machine components by means of galvanic processes. If one considers, for example, machine rollers or cylinders for the graphic industry, for example for textile printing or cylinders for printing machines, these are required, among other things, with regard to dampening cylinders with a special, "rough" surface. To produce such surface qualities, the dampening cylinder is hard chrome-plated and then subjected to a customized grinding process. This is followed by a structure etching in order to bring about the desired roughness of the surface.

A hard chrome layer is then applied to the surface structure created in this way. These various work steps required for the creation are quite complex and require complicated process engineering. The costs are essentially determined by the complex processing steps such as mechanical grinding to size and chemical structure etching; these machining processes are relatively expensive.

The invention is therefore based on the object of specifying a method for the electrochemical application of a surface coating to an object, which enables the desired, structured surface topographies to be created in a simple and inexpensive manner.

This object is achieved in that, in order to achieve the desired structured surface topography, nucleation of the deposition material takes place on the surface to be coated by means of at least one initial impulse of the electrical quantity and that the deposition material nuclei are subsequently grown by the addition of further deposition material by means of at least one subsequent impulse. This procedure according to the invention leads to a uniform, optimal structuring of the surface without the need for complex intermediate grinding as well as chemical etching processes. Rather, the desired surface structure is set during the galvanic coating process. It is essential here that the nucleation with separating material is first carried out by means of the initial impulse of the electrical quantity and that the germs formed are subsequently grown by the subsequent impulse.

According to a development of the invention, it is provided that an electrical voltage and / or an electrical current is used as the electrical variable such that the initial pulse and / or the subsequent pulse have a defined shape by means of a corresponding voltage and / or current function as a function of time Has.

On the basis of the procedure according to the invention, it is possible to produce the structured surface coatings, preferably by means of galvanic chrome or chromium alloy electrolytes, by means of galvanic nickel or nickel alloy electrolytes, by means of galvanic cobalt or cobalt alloy electrolytes, by means of galvanic copper or copper alloy electrolytes or by means of galvanic noble metal or noble metal alloy electrolytes. With the structure of the surface created according to the invention, the requirements of the most varied fields of application can be met. For example, the structure forms defined lubricant deposits or has a storage capacity for substances that come into contact with the surface. In addition, the structuring leads to low-glare devices, for example in medical or optical technology. In this way, precisely defined degrees of reflection can be achieved, such as for functional but also needed for decorative applications. In particular, it is possible with the method according to the invention to coat rollers for printing machines, in particular dampening cylinders from dampening units of such printing machines, which have optimal properties for the printing process.

According to a preferred embodiment of the invention, a multilayer structure is provided, at least one of the layers being provided with the structured surface topography. In the course of this multilayer structure, a nickel strike layer is preferably first applied to the object. This nickel strike layer is applied with a thickness of 0.2 to 2 μm, preferably <1 μm. As with all the layers mentioned below, the application is preferably carried out by means of a galvanic process. The object is, for example, a roller or a cylinder of a printing press. The cylinder is preferably made of steel (St.52 / Nirosta).

A sulfamate-nickel layer is applied to the nickel strike layer. This layer is preferably produced with a thickness of 25 to 40 μm, in particular 30 μm.

It is particularly advantageous if a chromium layer, in particular a low-crack chromium layer, is applied to the sulfamate-nickel layer. This chrome layer preferably has a thickness of 5 up to 15 µm, in particular from 10 µm. The structured surface coating generated by means of the initial and subsequent pulse is now applied to the chrome layer. This surface coating is preferably designed as a structured chromium layer, a chromium or a chromium alloy electrolyte being used in the galvanic process. According to the invention, the application is carried out by first applying germs of the deposition material to the surface to be coated (for example the mentioned chrome layer) by means of at least one initial pulse of the electrical quantity of the galvanic process. Subsequently, growth of these germs is then brought about by means of at least one subsequent pulse until the desired structuring is achieved. The structured surface coating is preferably produced with a maximum thickness of 5 to 20 μm, preferably 7 to 10 μm. The "maximum thickness" is understood to mean the measure up to the highest elevations, because due to the structuring, that is to say higher and lower lying areas, a thickness measure specification is not otherwise clearly defined. The so-called "load share", which is also defined as the "material share" according to DIN 4762, can also be used as the design. This load share is the percentage ratio of the length of the profile cut in a certain cutting line to a reference distance. The profile results from the surface structure, the cutting line being below the highest elevations of the structure, so that it cuts the corresponding elevations leads in some areas between the surveys. The method according to the invention preferably achieves a load share of 25%, the cutting line being 2 μm below the highest point of the structure.

According to a preferred embodiment of the invention it is provided that a finishing layer of micro-cracked chromium is applied to the structured surface coating. This final layer is preferably produced with a thickness of 5 to 20 μm, in particular 8 to 10 μm.

While the structured surface coating produced by the method according to the invention has the correspondingly desired roughness or the correspondingly desired load-bearing component, the other layers mentioned here (nickel strike layer, sulfamate-nickel layer, low-crack chrome layer (base layer) and finishing layer made of micro-cracked chromium ) on the other hand, to be regarded as having the same thickness and unstructured.

According to a preferred embodiment of the invention, it is provided that a chromium electrolyte is used for the electrochemical process for applying the structured surface coating. This chrome electrolyte preferably has a temperature of about 45 ° C.

It is advantageous if the object is set in rotation while the structured surface coating is being applied. Preferably this takes place in the cylinders of the printing presses mentioned in that they are rotated about their longitudinal central axis.

Anodes made of PbSn7 or platinized titanium are particularly preferably used when applying the structured surface coating. In contrast, the object to be coated forms the cathode when the structured surface coating is applied.

It has proven to be particularly advantageous if, when the structured surface coating is applied, there is an electrode distance between the anode and cathode of 10 to 40 cm, in particular 25 cm.

A trapezoidal initial voltage pulse and also an approximately trapezoidal subsequent voltage pulse are preferably used to generate the structure of the surface coating. First, in the course of the process, the machine component is immersed in the electrolytes, in particular chrome electrolytes, and the initial impulse is only started after a voltage-free or current-free waiting time. This waiting time serves, among other things, to adjust the temperature, i.e. the base material (machine component) assumes approximately the temperature of the electrolyte. This waiting time is preferably 60 s.

Furthermore, it is advantageous if between the end of the initial pulse and the beginning of the subsequent pulse a voltage or current-free intermediate period elapses. This intermediate period is thus between the period of the nucleation mentioned above and the growth phase of the deposition process.

. Diese Anfangsflanke wird solange beibehalten, bis eine Amplitude von etwa 4 V vorliegt. Diese wird mit konstantem Wert über einen Zeitraum von etwa 600 s beibehalten. Der Grundimpuls endet mit einem Abfall von $\text{δU/δt = ca. 0,4 V/5 s}$

, wobei sich dieser Abfall an die konstante Amplitude anschließt und im spannungs- bzw stromfreien Zustand endet. Damit ist der Grundimpuls abgeschlossen und es tritt nunmehr eine spannungs- bzw. stromfreie Ruhephase ein, die sich an die Endflanke des Grundimpulses anschließt und mit dem Anfangsimpuls zur Herbeiführung der Keimbildung endet.According to a preferred embodiment, in order to form the structured surface coating, it is provided that a basic pulse (voltage or current pulse) is connected upstream of the initial pulse. This serves to build up a base layer already mentioned above. The basic pulse preferably has an initial edge with a slope of $\text{δU / δt = approx.0.25 V / 5 s}$

. This initial edge is maintained until there is an amplitude of approximately 4 V. This is maintained with a constant value over a period of approximately 600 s. The basic pulse ends with a drop of $\text{δU / δt = approx.0.4 V / 5 s}$

, with this drop following the constant amplitude and ending in the voltage or current-free state. This completes the basic pulse and a voltage or current-free resting phase now occurs, which follows the end flank of the basic pulse and ends with the initial pulse for inducing nucleation.

hat, wobei diese Steigung bis zu einer Amplitude von etwa 5 V beibehalten wird. Ist diese Amplitude erreicht so ist der Anfangsimpuls abgeschlossen. Es schließt sich an den Anfangsimpuls eine Startflanke eines Folgeimpulses an, wobei die Startflanke des Folgeimpulses eine Steigung von $\text{δU/δt = ca. 0,1 V/6 s}$

aufweist. Mittels dieser Startflanke wird der Strom im galvanischen Prozeß bis auf eine maximale Stromstärke von ca. 950 A bezogen auf eine Normfläche gesteigert. Diese maximale Stromstärke wird nun über eine Zeitspanne von etwa 60s beibehalten. Anschließend wird der Folgeimpuls heruntergefahren, das heißt, er weist eine Rückflanke auf, welche mit einem Abfall von $\text{δU/δt = ca. 0,5 V/4 s}$

versehen und bis zur Strom- beziehungsweise Spannungsfreiheit heruntergefahren wird. Damit ist die gewünschte, strukturierte Oberflächentopographie auf dem Gegenstand (Maschinenbauteil) hergestellt.This initial pulse receives a starting edge that has a slope of $\text{δU / δt = approx. 0.3 V / 5 s}$

has, this slope is maintained up to an amplitude of about 5 V. When this amplitude is reached, the initial pulse is complete. A starting edge of a follows the initial pulse Follow pulse on, the starting edge of the follow pulse an increase of $\text{δU / δt = approx.0.1 V / 6 s}$

having. This starting edge increases the current in the galvanic process up to a maximum current of approx. 950 A based on a standard area. This maximum current is now maintained over a period of approximately 60s. The following pulse is then shut down, that is to say it has a trailing edge, which has a drop of $\text{δU / δt = approx.0.5 V / 4 s}$

provided and shut down until there is no current or voltage. This creates the desired, structured surface topography on the object (machine component).

To vary the surface topography, it is possible to vary the aforementioned voltage and / or current values and / or voltage difference values and / or time and / or time difference values. This variation is possible, based on the exemplary embodiment mentioned, with deviations of ± 10%, preferably ± 5%.

Figur 1

einen Längsschnitt durch einen Feuchtreibzylinder eines Feuchtwerks einer Druckmaschine,

Figur 2

eine vergrößerte Schnittansicht durch einen Oberflächenschichtaufbau des Feuchtreibzylinders der Figur 1,

Figur 3

ein Spannungs-Zeit-Diagramm eines galvanischen Beschichtungsprozesses zum Aufbringen einer strukturierten Oberflächenbeschichtung,

Figur 4

eine Darstellung der strukturierten Oberflächenbeschichtung in 200-facher Vergrößerung,

Figur 5

die strukturierte Oberflächenbeschichtung der Figur 4, jedoch mit 500-facher vergrößerung,

Figur 6

eine nach dem Stand der Technik hergestellte Oberflächenbeschichtung und

Figur 7

eine nach dem erfindungsgemäßen Verfahren hergestellte Oberflächenbeschichtung.

The drawings illustrate the invention, in which:

Figure 1

2 shows a longitudinal section through a dampening cylinder of a dampening unit of a printing press,

Figure 2

2 shows an enlarged sectional view through a surface layer structure of the dampening friction cylinder of FIG. 1,

Figure 3

a voltage-time diagram of a galvanic coating process for applying a structured surface coating,

Figure 4

a representation of the structured surface coating in 200x magnification,

Figure 5

the structured surface coating of Figure 4, but with 500x magnification,

Figure 6

a surface coating produced according to the prior art and

Figure 7

a surface coating produced by the method according to the invention.

FIG. 1 shows a longitudinal section through a dampening cylinder 1 of a printing press. The dampening cylinder 1 has a cylindrical base body 2, the outer surface 3 of which is provided with a layer structure 4. The layer structure 4 is identified by the dash-dot line in FIG. 1. It is guided around the edge regions 5 of the dampening cylinder 1 with the length of a portion of the radius.

The layer structure 4 is composed of individual layers, each of which is applied electrochemically, that is to say by means of galvanic processes.

FIG. 2 shows a section through the layer structure 4. A nickel strike layer 6 is electrodeposited on the base body 2 of the dampening cylinder 1. The basic body is made of steel St.52 / Nirosta. The nickel strike layer 6 is a pre-nickel plating. The electrolyte used for this is very acidic with a high chloride concentration. The nickel strike layer 6 has a uniform thickness of preferably 1 to 2 μm.

A sulfamate-nickel layer 7 is applied electrolytically to the nickel strike layer 6. This sulfamate-nickel layer 7 is sulfur-free; it has a thickness of 30 to 40 µm and a Vickers hardness of 200 to 250 HV.

A low-crack chrome layer 8 is electroplated onto the sulfamate-nickel layer 7; it has a uniform thickness of 10 to 15 µm and forms a so-called base layer.

A structured surface coating 9 is applied to the chrome layer 8 by means of a galvanic process. This surface coating 9 represents a structured chrome layer 10. Due to the structuring, there are corresponding elevations and depressions, the maximum thickness, measured from the sole to the apex of the maximum elevation, of this structured chrome layer 10 being 7 to 10 μm.

A uniformly thick final layer 11, which consists of micro-cracked chromium, is applied galvanically to the structured surface coating 10. Its thickness is preferably 8 to 10 microns. The hardness is about 900 HV or greater than 900 HV.

Overall, a surface of the dampening cylinder 1 with a roughness Rz = 6 to 10 μm is thus provided.

FIG. 3 shows a voltage-time diagram which illustrates the control of an electrical variable (voltage U) of the galvanic process for applying the base layer and for subsequently applying the structured surface coating 9. For the electrochemical process, the dampening cylinder 1 is switched as a cathode and anodes made of PbSn7 or platinized titanium are used. The electrode distance between the anode and cathode is set to approx. 25 cm. The dampening cylinder 1 is continuously rotated about its longitudinal axis 12 (FIG. 1) while the structured surface coating 9 is being applied.

According to FIG. 3, the electrochemical process for applying the base layer (chrome layer 8) and the structured surface coating 9 is carried out as follows:

First, the base body 2 of the dampening cylinder 1 is introduced into a chrome electrolyte with a temperature of approx. 45 ° C., after which Initially, a waiting time t _w passes, which is approximately 60 s long. During this time (without applying current or voltage), the temperature of the base material (base body 2) is adjusted to the electrolyte temperature.

For the application of the base layer (chrome layer 8), an electrical base pulse 13 is first applied between the anode and cathode after the waiting time t _w . Then, by means of an initial and a subsequent pulse 14, nucleation of the separating material (initial pulse 14 ') and then growth of the separating material nuclei by addition with further separating material is then brought about (subsequent pulse 14''), as a result of which the structured surface coating 9 is formed.

The basic pulse 13 and the initial and subsequent pulse 14 are designed as follows: The basic pulse 13 is a voltage pulse with a trapezoidal shape. The start and follow pulse 14 is also a voltage pulse, which is composed of the start pulse 14 'and the directly following pulse 14''and also has a trapezoidal shape; the pure trapezoidal shape is disturbed insofar as the starting edge (leading edge) of the initial pulse 14 'has a different slope than the starting edge of the subsequent pulse 14''. This will be discussed in more detail below.

aufweist. An die Anfangsflanke 15 schließt sich eine konstante Amplitude 16 mit 4 V über einen Zeitraum von 600 s an. Daran schließt sich eine Endflanke 17 an, die einen Abfall von $\text{δU/δt = 0,4 V/5 s}$

aufweist. Es folgt dann eine Zwischenzeitspanne t_z, die strombeziehungsweise spannungsfrei ist und eine Länge von 60 s aufweist. Es schließt sich dann der Anfangsimpuls 14' mit einer Startflanke 18 an, wobei diese eine Steigung von $\text{δU/δt = 0,3 V/5 s}$

aufweist. Diese Steigung wird bis zu einer Amplitude A von 5 V durchgeführt. Hier endet der Anfangsimpuls 14'. Dies ist durch die gestrichelte Linie 22 angedeutet. Es schließt sich an den Anfangsimpuls 14' unmittelbar die Startflanke 20 des Folgeimpulses 14'' an, die eine Steigung $\text{δU/δt = 0,1 V/6 s}$

aufweist. Mittels dieser Startflanke 20 wird der Strom, der dem galvanischen Prozeß zugrunde liegt, bis auf eine maximale Stromstärke I_max von 950 A gesteigert. Diese maximale Stromstärke I_max wird über eine Zeitspanne von 60 s beibehalten. Anschließend folgt eine Rückflanke 21 des Folgeimpulses 14'', die einen Abfall von $\text{δU/δt = 0,5 V/4 s}$

aufweist. Am Ende der Rückflanke 21 besteht Strom- beziehungsweise Spannungsfreiheit.The basic pulse 13 has an initial edge 15 which starts after the waiting time t _w and an increase of $\text{δU / δt = 0.25 V / 5 s}$

having. A constant amplitude 16 with 4 V over a period of 600 s connects to the starting edge 15. This is followed by an end flank 17, which has a drop of $\text{δU / δt = 0.4 V / 5 s}$

having. An intermediate time period t _z then follows, which is current-free or voltage-free and has a length of 60 s. The starting pulse 14 'then follows with a starting edge 18, this having an increase of $\text{δU / δt = 0.3 V / 5 s}$

having. This slope is carried out up to an amplitude A of 5 V. The initial pulse 14 'ends here. This is indicated by the dashed line 22. The starting edge 20 of the following pulse 14 ″ immediately follows the starting pulse 14 ′, which has an incline $\text{δU / δt = 0.1 V / 6 s}$

having. By means of this starting edge 20, the current on which the galvanic process is based is increased to a maximum current I _max of 950 A. This maximum current I _max is maintained over a period of 60 s. This is followed by a trailing edge 21 of the subsequent pulse 14 ″, which drops from $\text{δU / δt = 0.5 V / 4 s}$

having. At the end of the trailing edge 21 there is no current or voltage.

In FIG. 3, the total flank formed by the initial pulse 14 'and the subsequent pulse 14''is designated by 19. It is composed of the two starting edges 18 and 20.

By means of the described electrochemical process for applying the structured surface coating 9, a roughness Rz = 9 μm is achieved with a load share of 25%.

Subsequently, the finishing layer 11 is applied to the structured surface coating 9 produced according to the invention using a conventional electrochemical process.

FIG. 4 shows the structure chrome of the structured surface coating 9 in a 200-fold magnification. FIG. 5 shows a 500-fold magnification. It can be clearly seen that there is a very uniform structured distribution. FIGS. 6 and 7 show a comparison of a known surface coating with the surface coating according to the invention: namely, FIG. 6 shows a conventional surface coating subjected to a grinding and etching process in 200 times magnification and FIG. 7 shows the structured surface coating 9 according to the invention, also in FIG 200x magnification. It can be clearly seen that the structure according to the invention is constructed in a substantially more uniform and orderly manner than in the case of the prior art.

When a dampening cylinder 1 is created, a degreasing process and a de-papering are of course carried out first, as is customary, before the layer structure 4 is applied. These processes may also be repeated several times.

Only then is the nickel strike layer 6, then the sulfamate nickel layer 7 and then the chrome layer 8 applied. The structured surface coating 9 is then deposited and then the final layer 10 is applied, which consists of micro-cracked chromium and with which the dimensional accuracy can be controlled.

As already mentioned at the beginning, the invention is not limited to chromium or chromium alloy layers, but can also be carried out with other deposition materials.

Furthermore, according to a further exemplary embodiment, not shown, it is possible to insert a current-free or voltage-free pause, namely an intermediate period, between the initial pulse and the subsequent pulse.

Claims (33)

Process for the electrochemical (galvanic) application of a surface coating to an object, preferably a machine component, in particular a machine roller, using an electrical quantity of the galvanic process which effects the layer application, characterized in that in order to achieve a desired structured surface topography by means of at least one initial pulse (14 ') of the electrical size on the surface to be coated nucleation of the separating material take place and that subsequently the growth of the separating material nuclei is brought about by the addition of further separating material by means of at least one subsequent pulse (14''). Method according to Claim 1, characterized in that an electrical voltage (U) and / or an electrical current is used as the electrical variable such that the initial pulse (14 ') and / or the subsequent pulse (14'') has a defined shape by a corresponding one Has voltage and / or current function depending on the time (t). Method according to one of the preceding claims, characterized in that a nickel strike layer (6) is applied to the object. Method according to one of the preceding claims, characterized in that the nickel strike layer (6) has a thickness of 0.2 µm to 2 µm, preferably <1 µm. Method according to one of the preceding claims, characterized in that a sulfamate-nickel layer (7) is applied to the nickel strike layer (6). Method according to one of the preceding claims, characterized in that the sulfamate-nickel layer (7) is produced with a thickness of 25 µm to 40 µm, in particular of 30 µm. Method according to one of the preceding claims, characterized in that a chromium layer (8), in particular a low-crack chromium layer, is applied to the sulfamate-nickel layer (7). Method according to one of the preceding claims, characterized in that the chrome layer (8) is produced with a thickness of 5 µm to 15 µm, in particular of 10 µm. Method according to one of the preceding claims, characterized in that the chrome layer (8) is a base layer which is applied galvanically by means of an electrical base pulse (13). Method according to one of the preceding claims, characterized in that on the chrome layer (8) the structured surface coating (9) produced by means of the initial and subsequent pulse (14, or 14 'and 14'') is applied. Method according to one of the preceding claims, characterized in that the structured surface coating (9) is designed as a structured chrome layer (10). Method according to one of the preceding claims, characterized in that the structured surface coating (9) is produced with a maximum thickness of 5 µm to 20 µm, preferably 7 µm to 10 µm. Method according to one of the preceding claims, characterized in that a finishing layer (11) made of micro-cracked chromium is applied to the structured surface coating (9). Method according to one of the preceding claims, characterized in that the closing layer (11) is produced with a thickness of 5 µm to 20 µm, in particular 8 µm to 10 µm. Method according to one of the preceding claims, characterized in that a chromium electrolyte is used for the electrochemical process for applying the structured surface coating (9). Method according to one of the preceding claims, characterized in that the chromium electrolyte has a temperature of approximately 45 ° C. Method according to one of the preceding claims, characterized in that the object is set in rotation during the application of the structured surface coating (9). Method according to one of the preceding claims, characterized in that anodes made of PbSn7 or platinized titanium are used when applying the structured surface coating (9). Method according to one of the preceding claims, characterized in that the object, in particular the machine component to be coated, forms the cathode when the structured surface coating (9) is applied. Method according to one of the preceding claims, characterized in that when the structured surface coating (9) is applied, an electrode distance between the anode and cathode of 10 cm to 40 cm, in particular 25 cm, is maintained. Method according to one of the preceding claims, characterized in that an approximately trapezoidal voltage pulse is used to generate the structure of the surface coating (9). Method according to one of the preceding claims, characterized in that to form the the chrome layer (8) forming the base layer, the object, in particular the machine component, is immersed in the electrolyte, in particular chrome electrolyte, and the initial pulse (14 ') is only started after a voltage-free or current-free waiting time (t _w ). Method according to one of the preceding claims, characterized in that a voltage-free or current-free intermediate time period (t _z ) elapses between the end of the basic pulse (13) and the beginning of the initial pulse (14 '). Method according to one of the preceding claims, characterized in that the basic pulse (13) has an initial flank (15) with a slope of $\text{δU / δt = approx.0.25 V / 5 s}$

receives. Method according to one of the preceding claims, characterized in that the initial flank (15) is followed by a constant amplitude of approximately 4 V over a period of approximately 600 s. Method according to one of the preceding claims, characterized in that the basic pulse (13) has an end flank (17) following the constant amplitude with a drop of $\text{δU / δt = approx.0.4 V / 5 s}$

receives. Method according to one of the preceding claims, characterized in that the intermediate period (t _z ) adjoins the end flank (17). Method according to one of the preceding claims, characterized in that the initial pulse (14 ') has a starting flank (18) with a gradient of $\text{δU / δt = approx. 0.3 V / 5 s}$

receives, this slope is maintained up to an amplitude (A) of about 5 V. Method according to one of the preceding claims, characterized in that the start pulse (14 ') is followed by the follow-up pulse (14'') immediately (without a pause) and that the end of the start edge (18) of the start pulse (14') is continuous (without jumps) ) passes into the beginning of the start edge (20) of the following pulse (14 ''). Method according to one of the preceding claims, characterized in that the starting flank (20) of the following pulse (14 '') has an incline of $\text{δU / δt = approx.0.1 V / 6 s}$

The current is increased up to a maximum current (I _max ) of approx. 950 A based on a standard area. Method according to one of the preceding claims, characterized in that the maximum current strength (I _max ) is maintained over a period of approximately 60 s. Method according to one of the preceding claims, characterized in that the subsequent pulse (14 '') has a back flank (21) which adjoins the maximum current (I _max ) and which has a drop of $\text{δU / δt = approx.0.5 V / 4 s}$

Mistake and is shut down until there is no current or voltage. Method according to one of the preceding claims, characterized in that the aforementioned voltage and / or current values and / or voltage difference values and / or time and / or time difference values with deviations of ± 10%, preferably ± 5%, are used.

Images