Method for controlling the chromium feed in an electrolysis process for producing chromium layers, and an electrolysis cell for this purpose

Abstract

The invention relates to a method for controlling the chromium feed in an electrolysis process for producing a chromium layer by means of direct current and use of an anode (44, 144, 244) and a cathode (48, 148, 248), comprising, during the electrolytic deposition of chromium with formation of a chromium layer: (E) applying a cathode voltage to the first auxiliary electrode (54, 154, 254), whereby the passivation layer of the chromium metal dissolves and chromium metal in the form of chromium (III) ions starts to go into solution in the electrolyte (25, 125, 225); (F) following the dissolution of the passivation layer, ending the current supply to, or switching off, the voltage on the first auxiliary electrode (54, 154, 254); and (G) without current, leaving the chromium metal to go into solution from the first auxiliary electrode (54, 154, 254) in the form of chromium (III) ions by action of the electrolyte (25, 125, 225). By topping up the chromium metal in the first auxiliary electrode during step (G), steps (E), (F) and (G) can be repeated as often as desired.

Description

[0001] Method for controlling the chromium supply in an electrolysis process for producing chromium layers and an electrolysis cell therefor

[0002] TECHNICAL FIELD

[0003] The invention relates to a method for controlling the chromium supply in an electrolysis process for producing chromium layers and to an electrolysis cell therefor.

[0004] BACKGROUND OF THE INVENTION

[0005] Electroplating processes for the surface coating of objects have long been known in the art. The resulting coated objects have particularly advantageous surface properties, such as greater hardness, improved corrosion resistance, metallic appearance, luster, and the like. Using a galvanic bath containing the metal to be deposited as a salt in solution, the metal is deposited onto the object connected as a cathode using direct current. The object to be coated is therefore usually a metallic material or undergoes additional metallization of the surface to become electrically conductive.

[0006] One metal used for this purpose is chromium. The application of chromium layers using electroplating baths can serve decorative purposes, creating bright and highly reflective chrome layers. Chrome plating of objects can also be used for technical applications, for example, to increase wear resistance, improve abrasion stability, and enhance heat and corrosion resistance. This is used, for example, in chrome-plated pistons, cylinders, cylinder liners, axle bearings, and the like.

[0007] It is well known that chromium(VI) salts, such as CrOa, and sulfuric acid are used in electroplating baths. However, this has a number of disadvantages: The evolution of gas, particularly hydrogen, and to a lesser extent oxygen, leads to the formation of acidic, corrosive, and sometimes toxic chromic acid mists. This requires intensive extraction of the surface of the electroplating bath and the use of surfactants or wetting agents to contain the resulting chromic acid mists. In addition, the chromium(VI) electrolyte is highly toxic and carcinogenic. It would therefore be better to use non-toxic electroplating baths containing chromium(III) salts. When electroplating chrome using chromium(III) salts, it is important to ensure that chromium layers can be deposited in a suitable, but not too thin, thickness, and that the system design is not too complex to allow for industrial use.

[0008] The deposition of chromium from the chromium(III) salt present in the electrolyte causes a decrease in the concentration of chromium(III) ions in the electrolyte. However, the addition of Cr(III) can only occur in the form of chromium(III) salts, resulting in an undesirable gradual enrichment of the anion present in the salt in the electrolyte. This requires regular dilution with subsequent addition of the other components, thus requiring constant monitoring and control of the system.

[0009] Furthermore, it is not possible to apply an anodic voltage or an anodic potential to a chromium metal anode because chromium, being a highly active metal, immediately forms an oxide layer on its surface, which passivates the chromium. When an anodic voltage is applied to this passive chromium, only a small amount of chromium is dissolved. However, if a high anodic voltage is applied, the chromium dissolves as chromium(VI) ions, which are not only carcinogenic but also represent an undesirable interference with chromium(III) electrolytes and impair the function of the electrolyte. It is therefore very difficult to continuously supply chromium electrolytically to the electrolyte as trivalent chromium.

[0010] The following solutions have already been proposed in the state of the art:

[0011] According to EP 2 640 873 A1 (WO 2012 / 067725 A1), a method for supplementing or increasing the chromium content of an electrolyte with trivalent chromium is described, the method comprising the following steps: a) immersing a chromium-containing electrode and a second electrode in an electrolyte containing trivalent chromium ions; and b) applying a pulsed alternating current to the chromium electrode and the second electrode; whereby chromium is electrolytically dissolved from the chromium electrode in the form of trivalent chromium ions and the chromium(III) content of the electrolyte in which the chromium electrode is immersed is replenished or enriched. The duration of each forward pulse and each reverse pulse is typically between about 0.1 and about 2 seconds.For example, a square waveform can be used, with the duration of the alternating current pulse being approximately 400 ms for the cathodic forward pulse and 400 ms for the anodic reverse pulse. The disadvantage of this proposal is the complex technology, with permanent polarity reversal requiring the use of an expensive pulse rectifier.

[0012] Furthermore, GB 414 939 discloses a process for electroplating chromium, in which a direct current is passed from a chromium anode to a cathode to be plated, and an alternating current is superimposed on the plating current to activate and dissolve the chromium from the anode. For example, Fig. 2 shows a circuit diagram of an arrangement that can be used when the alternating current is superimposed only at the anode. H is an auxiliary electrode, with the alternating current generator WG connected to the anode A and the auxiliary electrode H via a transformer T.

[0013] The present invention is based on the object of avoiding the disadvantages of the prior art and providing a process and an electrolysis cell that enable a controlled supply of chromium metal during electrolysis without causing an accumulation of undesirable anions and without the formation of chromium(VI) ions. The chromium coatings provided are also intended to meet the requirements placed on chromium coatings, particularly on gravure cylinders.

[0014] BRIEF DESCRIPTION OF THE INVENTION

[0015] The described object is achieved according to the invention by the teachings of the independent claims. The teachings of the subclaims represent advantageous embodiments.

[0016] In particular, a method is provided for controlling the chromium supply in an electrolysis process for producing a chromium layer, wherein the chromium layer is produced by electrolytic deposition of chromium from an electrolyte by means of direct current and using an anode and a cathode, comprising the following steps:

[0017] (A) providing a first auxiliary electrode comprising or consisting of chromium metal;

[0018] (B) Providing a second additional electrode in the form of an inert electrode,

[0019] (C) immersing both additional electrodes in the electrolyte containing at least one chromium(III) salt;

[0020] (D) connecting the first additional electrode and the second additional electrode in a separate circuit from the cathode and anode; and during the electrolytic deposition of chromium to form a chromium layer: (E) applying a cathodic voltage to the first additional electrode, whereby the passivation layer of the chromium metal dissolves and chromium metal in the form of chromium(III) ions begins to dissolve in the electrolyte;

[0021] (F) after dissolving the passivation layer, stopping the current supply to or switching off the voltage at the first additional electrode; and

[0022] (G) electroless dissolution of the chromium metal from the first additional electrode in the form of chromium(III) ions by the action of the electrolyte.

[0023] The process can also be terminated again in a targeted manner, for example, when the object to be coated has been completely coated. According to one embodiment of the invention, the process can be terminated during step (G) by

[0024] Applying an anodic voltage to the first additional electrode or

[0025] Pulling out the first additional electrode from the electrolyte or pumping out the electrolyte from the electrolysis cell so that the first additional electrode comes into contact with the ambient air or not replenishing the chromium metal of the first additional electrode so that the currentless dissolution of the chromium metal from the first additional electrode in the form of chromium(III) ions by the action of the electrolyte ends as soon as the chromium has completely gone into solution.

[0026] The invention also relates to an electrolysis cell for controlling the supply of chromium to an electrolyte, comprising an anode; a cathode; an electrolyte containing at least one chromium(III) salt; the anode and cathode are immersed in the electrolyte; a first circuit connecting the anode and cathode and causing the deposition of a chromium layer by electrolytic deposition of chromium from an electrolyte by means of direct current on the cathode; a first additional electrode comprising or consisting of chromium metal; a second additional electrode in the form of an inert electrode, both additional electrodes being immersed in the electrolyte; a second circuit connecting the first additional electrode and the second additional electrode in a circuit separate from the cathode and anode; wherein either a cathodic voltage is applied to the first additional electrode so that the passivation layer of the chromium metal on the first additional electrode dissolves;or no voltage is applied to the first additional electrode so that the chromium metal, after the passivation layer has been dissolved, dissolves into the electrolyte in the form of chromium(III) ions without current from the first additional electrode; or an anodic voltage is applied to the first additional electrode so that the passivation layer on the first additional electrode is re-formed.

[0027] The process according to the invention and the electrolysis cell according to the invention are therefore based on the chemical dissolution of chromium in the form of chromium(III) ions in the electrolyte, which is activated by current flow, continues without current flow and, if desired, can be terminated again by current flow or in another way.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The described and other aspects, advantages, and features of the present invention are described in more detail in the following sections, also with reference to the accompanying drawings. In the drawings:

[0030] Fig. 1 is a flow chart illustrating an embodiment of the method according to the invention;

[0031] Fig. 2a is a schematic representation of an embodiment of the electrolysis cell according to the invention during a galvanization phase, wherein a cathodic voltage is applied to the first additional electrode;

[0032] Fig. 2b is a schematic representation of the same embodiment as in

[0033] Fig. 2a, wherein no voltage is applied to the first additional electrode;

[0034] Fig. 2c is a schematic representation of the same embodiment as in

[0035] Fig. 2a, wherein an anodic voltage is applied to the first additional electrode;

[0036] Fig. 3a is a schematic representation of an embodiment of a circuit according to the invention, comprising a first, second and third additional electrode, wherein a cathodic voltage is applied to the first additional electrode; Fig. 3b is a schematic representation of the same embodiment as in

[0037] Fig. 3a, where no voltage is applied to the additional electrodes;

[0038] Fig. 3c is a schematic representation of the same embodiment as in

[0039] Fig. 3a, wherein an anodic voltage is applied to the first additional electrode;

[0040] Fig. 4a is a schematic representation of an embodiment of a circuit according to the invention, comprising interconnected units of additional electrodes to illustrate a series circuit, wherein a cathodic voltage is applied to the first additional electrodes;

[0041] Fig. 4b is a schematic representation of the same embodiment as in

[0042] Fig. 4a, where no voltage is applied to the additional electrodes;

[0043] Fig. 4c is a schematic representation of the same embodiment as in

[0044] Fig. 4a, wherein an anodic voltage is applied to the first additional electrodes;

[0045] Fig. 5a is a schematic representation of another embodiment of the electrolysis cell according to the invention during a galvanization phase, wherein a cathodic voltage is applied to the first additional electrode;

[0046] Fig. 5b is a schematic representation of the same embodiment as in

[0047] Fig. 5a, where no voltage is applied to the additional electrodes;

[0048] Fig. 5c is a schematic representation of the same embodiment as in

[0049] Fig. 5a, wherein an anodic voltage is applied to the first additional electrode;

[0050] Fig. 6a is a schematic representation of another embodiment of the electrolysis cell according to the invention during a galvanization phase, wherein a cathodic voltage is applied to the first additional electrode; Fig. 6b is a schematic representation of the same embodiment as in Fig. 6a, wherein no voltage is applied to the additional electrodes; and

[0051] Fig. 6c is a schematic representation of the same embodiment as in Fig. 6a, wherein an anodic voltage is applied to the first additional electrode.

[0052] TERMINOLOGY AND DEFINITIONS

[0053] The term "electrolysis process" or "electrolysis" refers to the deposition of metal, in this case chromium, from a solution containing the corresponding metal ions using an electric current. This process is used to produce metal coatings, in this case, chrome plating.

[0054] The terms "cathodic voltage" or "cathodic potential" are used synonymously and interchangeably in the present invention and are to be understood as meaning that a voltage is applied to an electrode or additional electrode so that the electrode or additional electrode serves as a cathode.

[0055] The terms "anodic voltage" or "anodic potential" are also used synonymously and interchangeably in the present invention and are to be understood as meaning that a voltage is applied to an electrode or additional electrode so that the electrode or additional electrode serves as an anode.

[0056] DETAILED DESCRIPTION OF THE INVENTION

[0057] The inventive method for controlling the chromium supply takes place during an electrolysis process for producing a chromium layer. Therefore, the electrolysis process for producing a chromium layer will be described first, as the inventive method is closely related to it:

[0058] The electrolytic deposition of chromium layers is usually carried out in an electrolytic cell filled with electrolyte. Any vessel suitable for the expert, such as those used in electroplating, can be used as the container for the electrolytic cell. The object to be coated, on which the chromium layer is to be deposited, typically a gravure cylinder, serves as the cathode.

[0059] Anodes known to those skilled in the art can be used as the anode. In particular, an inert electrode is used as the anode, which is constructed from one or more electrically conductive materials that are insoluble in the electrolyte. Materials used for an insoluble anode or inert electrode include, for example: platinized titanium, an expanded metal made of titanium, optionally coated with a mixed oxide or coated with graphite,

[0060] Carbon materials such as graphite, titanium coated with indium and / or tantalum,

[0061] Mixed metal oxides: in particular iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide;

[0062] Mixed metal oxides, where titanium serves as the anode base material, which is coated with platinum, iridium, tantalum and / or palladium oxide; titanium, niobium or tantalum sheet coated with mixed metal oxides; titanium, tantalum or niobium coated with iridium-transition metal mixed oxide; and material combinations of these.

[0063] The shape of the anode can be adapted by a specialist to suit the specific purpose. The anode can be, for example, a flat material, a sheet material, a sintered material, or expanded material.

[0064] The anode and cathode are immersed in an electrolyte. Any electrolyte known to those skilled in electroplating can be used. When a direct current is applied to the two electrodes—anode and cathode—chromium(III) ions from the electrolyte are deposited onto the object, the cathode. If the object is not metallically conductive, it can be made electrically conductive through pretreatment.

[0065] The structure described above can also be varied, as disclosed in WO 2008 / 014987 A2, in such a way that in the electrolysis cell the electrolyte is separated by a semipermeable membrane into a catholyte (electrolyte in the cathode compartment) and an anolyte (electrolyte in the anode compartment). The cathode, as the object to be coated, is immersed in the catholyte, which contains the chromium ions to be deposited. When a voltage is applied, a current flows via the anolyte through the membrane into the catholyte. The anode system can, for example, also be one in which the anode is in direct contact with a membrane, i.e. the anode is coated with a membrane. This is a so-called direct-contact membrane anode, as known from DE 10 2010 055 143 A1.

[0066] The chromium layer can be produced at a temperature of 20°C to 60°C, with the temperature of the electrolyte being adjusted by appropriate heating and cooling devices. The chromium layer can be deposited at a current density of, for example, 5 to 60 A / dm 2 be manufactured.

[0067] The electrolyte can be stirred or mixed while the electrodes are immersed in it. Circulation is also possible. Preferably, five bath volumes (i.e., volumes of electrolyte) are circulated per hour.

[0068] The object to be coated can also be moved. If a gravure cylinder is to be coated, for example, it can be moved at a rotation speed of 0.5 to 1.5 m / min.

[0069] The method according to the invention now makes it possible to control the chromium supply in an electrolysis process for producing a chromium layer. For this purpose, a first additional electrode (step (A)) and a second additional electrode (step (B)) are provided, both of which are immersed in an electrolyte (step (C)).

[0070] The first additional electrode comprises or consists of chromium metal and is therefore also referred to here as a 'chromium electrode'. The first additional electrode or chromium electrode is constructed, for example, from shaped chromium bodies that can be held in a framework or holder. The shaped bodies can have a regular or irregular shape and can be smooth or porous. These are, for example, pieces, chunks, lumps, platelets, bars, wires and / or grids; a powder is not intended to be included here. The framework or holder is a material that is resistant to the acidic electrolyte and may or may not conduct current. Such a conductive material can be a metal, such as titanium. Such a non-conductive material is a plastic, for example polypropylene or polyvinyl chloride.If the frame or the holder does not conduct the current, an additional guide plate is attached, for example, in order to be able to energize the chrome molded bodies.

[0071] According to one embodiment, pieces of chromium metal, also referred to here as "chromium nuggets," are housed in a plastic frame as chromium molded bodies. The shape of the first additional electrode can be selected accordingly by a person skilled in the art. Regarding the shape, especially for molded bodies made of chromium metal, it is important that a larger surface area of ​​the chromium metal results in a higher dissolution rate in the electrolyte. The person skilled in the art can therefore select a suitable shape.

[0072] In one embodiment, the surface area of ​​the metallic chromium can be 1% to 50% or 1% to 100% of the surface area of ​​the first additional electrode. This allows for particularly good dissolution of the chromium and subsequent supply of chromium(III).

[0073] The second additional electrode is an inert electrode, which is made of one or more electrically conductive materials that are insoluble in the electrolyte. The material for the inert electrode is not further restricted, as long as it has the properties described. For example, the same materials as for the anode of the electrolysis process for chromium plating described above are suitable.

[0074] The shape of the second additional electrode can be selected by the expert according to the structural conditions. The second additional electrode can be, for example, a flat material, plate material, sintered material, or expanded material.

[0075] According to a preferred embodiment, the surface area of ​​the first additional electrode is chosen to be the same size as the surface area of ​​the second additional electrode. In this case, it is expedient if the surface area of ​​the metallic chromium is 100% of the surface area of ​​the first additional electrode.

[0076] The electrolysis cell contains two separate circuits: In the first circuit, the anode and cathode are connected together to deposit the chromium(III) ions dissolved in the electrolyte in the form of a chromium layer onto an object connected as the cathode. Direct current is used, and there is no polarity reversal between the anode and cathode. The anode always remains the anode, and the cathode always remains the cathode.

[0077] In one embodiment, a first additional electrode and a second additional electrode are connected together in a second circuit in a circuit separate from the cathode and anode (step (D)). The first circuit of anode and cathode and the second circuit of first and second additional electrode are not connected to one another, but are switched completely separately. The second circuit is therefore controlled independently of the first circuit. In the second circuit, direct current is used, but the polarity is reversed after certain time intervals, which are significantly longer than with pulsed alternating current (for example, with a duration of 0.1 to 2 s). This means that initially the first additional electrode functions as the cathode and the second additional electrode as the anode, and at a later time the first additional electrode functions as the anode and the second additional electrode as the cathode.This type of polarity reversal is known from the prior art, albeit in a different context and for different purposes, so that its technical implementation is readily possible for a person skilled in the art. For example, the polarity reversal can be achieved using a rectifier with a polarity inverter.

[0078] The second circuit can conveniently be activated during the electrolytic deposition of chromium to form a chromium layer, as it operates independently of the first circuit. For this purpose, a cathodic voltage is first applied to the first additional electrode. The first additional electrode is thus the cathode, and the second additional electrode is the anode. The cathodic voltage is also referred to as the cathodic potential and has a reducing effect on the chromium metal. As a result, the passivation layer that has formed on the surface of the chromium metal of the first additional electrode begins to degrade.

[0079] Although chromium is chemically less noble than iron, it behaves almost like a precious metal when exposed to corrosion in air and water. This is due to a very thin, virtually invisible chromium oxide layer a few nanometers thick (about 50 atomic layers in chromium-nickel steel, about 5 atomic layers in pure chromium), which protects the metal from the atmosphere and oxidation. The passivating layer also inhibits diffusion to the metal, preventing further corrosion of the metal. The passivating layer is therefore dissolved by applying a cathodic, and thus reducing, voltage.

[0080] Therefore, a cathodic (i.e., reducing) voltage is first applied to the first additional electrode (step (E)). Accordingly, the counter electrode, the second additional electrode, becomes the anode. The cathodic voltage reduces the passivation layer in the form of the existing chromium oxide layer that has formed on the surface of the chromium metal. In this process, chromium oxide is reduced to metallic chromium (Cr2Oa -> Cr meThis cathodic voltage or potential is chosen to be high enough to achieve degradation of the chromium oxide layer. The reducing direct current therefore dissolves the passivation layer, and chromium(III) ions begin to dissolve. The cathodic voltage can be set, for example, in a range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, and most preferably 3.0 to 7.0 volts to degrade the passivation layer. The current density is, for example, in the range from 2.5 to 4 A / dm 2 , preferably 3.4 A / dm 2 It has been found to be preferable if the current density is not higher than 4 A / dm 2is set, as this prevents the formation of undesirable chromium(VI) in any case. Typically, the passivation layer can then be broken down within about 5 to 60 seconds, preferably about 5 to 45 seconds, even more preferably about 5 to 30 seconds. However, this can also be exceeded or undershot in individual cases and depends on numerous process parameters, such as the set pH value, the current density, the thickness of the chromium metal layer to be deposited, the temperature, the selected voltage and the type of additional electrodes used. The specified range serves only as a guide for the person skilled in the art, who can determine the optimal level of voltage and duration for the cathodic current flow at the first additional electrode for the respective application based on a few tests.

[0081] The process according to the invention now succeeds in breaking down the passivation layer, which has formed on or on the chromium metal as a side reaction and has a passivating effect, at the first additional electrode, thereby dissolving chromium(III) ions without forming chromium(VI) ions. It has been shown that breaking down the passivation layer is a prerequisite for preventing the formation of chromium(VI) ions during the electrolytic dissolution of chromium.

[0082] A purely chemical dissolution of chromium by acids would only be possible at a very low pH of < 0.5, which would be disadvantageous for the process conditions in electroplating baths. According to the invention, the electrolyte typically has a pH in the range of 2.0 to 3.5, in particular 2.1 to 3.4, preferably 2.2 to 3.3, more preferably 2.3 to 3.2, even more preferably 2.4 to 3.1, and most preferably 2.5 to 3.0. The preferably used pH of the electrolyte alone is therefore not sufficient to remove the existing passivation layer on the chromium and initiate the chemical dissolution. For this reason, the dissolution of the chromium metal as chromium(III) ions is triggered and started by applying a cathodic voltage.

[0083] The dissolution of the passivation layer on the first additional electrode can be easily observed by the formation of bubbles on the surface of the additional electrode. As soon as chromium(III) ions dissolve in the electrolyte, hydrogen gas forms, which becomes visible in the form of bubbles. Therefore, it can be assumed that when bubbles appear across the entire surface of the chromium metal on the first additional electrode, the passivation layer has dissolved. This occurs, for example, after approximately 5 to 30 seconds, as already explained.

[0084] Surprisingly, experiments have shown that after applying the cathodic voltage to the first additional electrode as the cathode and dissolving the passivation layer of the chromium metal, the dissolution reaction, evident by the formation of hydrogen (bubbles), does not stop when the voltage is switched off, but continues without current. The dissolution of the metallic chromium can therefore be observed. Without being bound to this, it is assumed that in the presence of chromium(III) cathodically, chromium(II) is formed during reduction. This should promote the dissolution of the metallic chromium. At the same time, it prevents the formation of chromium(VI) during oxidation, i.e., when metallic chromium is dissolved during electrolysis, no chromium(VI) is formed.

[0085] In the process according to the invention, the current supply to the first additional electrode is therefore terminated after the passivation layer has dissolved (step (F)); the first and second additional electrodes are then no longer under voltage. If the cathodic voltage were maintained after the passivation layer has dissolved, chromium(VI) ions would form, which is to be avoided for the reasons already mentioned. This formation is prevented by switching off the voltage.

[0086] The chromium metal from the first additional electrode then dissolves in the form of chromium(III) ions without current through the action of the electrolyte (step (G)). As soon as the chromium oxide layer has been broken down, the chromium is attacked by the acidic electrolyte and chemically dissolved. For this purpose, it is advantageous for the electrolyte to have an acidic pH in the range of, for example, 2.0 to 3.5. In this pH range, the electrolyte is acidic enough to dissolve the metal without current, once the passivation layer has been dissolved. By applying a relatively short cathodic current, the passivation layer is broken down to such an extent that the acidic electrolyte can attack the elemental chromium in the first additional electrode.With an electrolyte in the specified pH range, after the passivation layer has been removed, the passivation of the chromium metal will not be rebuilt on its own, but the dissolution of the chromium will continue until the dissolution process is interrupted or the chromium metal in the first additional electrode is exhausted and is no longer replenished.

[0087] In one embodiment, the chromium metal in the first additional electrode is replenished during step (G), ie, during the electroless dissolution of the chromium(III) ions in the electrolyte, and then steps (E), (F), and (G), as disclosed, are carried out sequentially in this order, preferably without intermediate steps. This embodiment therefore comprises replenishing the chromium metal in the first additional electrode during step (G);

[0088] (E) applying a cathodic voltage to the first additional electrode, whereby the passivation layer of the chromium metal dissolves and chromium metal in the form of chromium(III) ions begins to dissolve in the electrolyte;

[0089] (F) after dissolving the passivation layer, stopping the current supply to or switching off the voltage at the first additional electrode; and

[0090] (G) electroless dissolution of the chromium metal from the first additional electrode in the form of chromium(III) ions by the action of the electrolyte.

[0091] This procedure can be repeated as often as required, with the chromium metal being replenished and dissolved as chromium(III) ions for the chromium layer to be deposited.

[0092] To stop the process, i.e. to stop the dissolution of chromium(III) ions into the electrolyte, there are several possibilities:

[0093] According to one embodiment, an anodic voltage is applied to the first additional electrode, whereby the passivation layer of the chromium metal is reformed and the dissolution of chromium(III) ions in the electrolyte is stopped. The dissolution of the chromium in the electrolyte can therefore be stopped at any desired time by applying an anodic voltage (oxidizing potential) to the first additional electrode. Accordingly, the second additional electrode, preferably in the form of an inert electrode as a counter electrode, then becomes the cathode. This initially leads to the metallic chromium dissolving as chromium(III) for a very short time, but parallel to the dissolution of the Cr(III) ions, the previously described passivation layer of Cr2Oa is re-formed, as the chromium surface reacts with the oxygen contained in the water. The anodic voltage is selected so that the passivation layer builds up in the form of a chromium oxide layer.This can be easily observed by the disappearance of bubble formation on the surface of the additional electrode. As soon as no more chromium(III) ions dissolve in the electrolyte, no more hydrogen gas is formed, which would be visible in the form of bubbles. Therefore, it can be assumed that once the bubble formation disappears, the passivation layer has been re-formed over the entire surface of the chromium metal at the first additional electrode.

[0094] The anodic voltage for rebuilding the passivation layer can be set, for example, in a range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, and most preferably 3.0 to 7.0 volts. The current density is, for example, in the range from 2.5 to 4 A / dm 2 , preferably 3.4 A / dm 2Typically, the passivation layer is then regenerated within approximately 5 to 60 seconds, preferably approximately 5 to 45 seconds, and even more preferably approximately 5 to 30 seconds. However, this time may be exceeded or undershot in individual cases and serves only as a guide for the person skilled in the art, who can determine the level of the anodic voltage and the duration of the anodic current flow at the first additional electrode for the respective application based on a few tests.

[0095] To terminate the process, it is also possible to pump the electrolyte out of the cell. Another option is to remove the chromium electrode from the electrolyte, exposing it to ambient air, which allows the passivation layer to regenerate and the chemical reaction to stop. Alternatively, the chromium metal in the existing first additional electrode can no longer be refilled, allowing the chromium electrode to run dry.

[0096] Thus, in this process, chromium can be dissolved without current and started at any time using direct current and two additional electrodes. The process can be continued as often as desired by replenishing the chromium metal, and terminated again at a specific time. The process according to the invention, according to this embodiment, is therefore based on the chemical dissolution of chromium in the form of chromium(III) ions in the electrolyte, which can be activated by a defined current flow or a defined voltage, continued without current, and terminated by one of the described options.

[0097] According to a further embodiment, a third additional electrode is provided in addition to the first and second additional electrodes. The third additional electrode is an inert electrode. According to one embodiment, the first additional electrode in the form of a chromium electrode, the second additional electrode, and the third additional electrode are then connected together in the second circuit to form one or more units. A unit is then constructed as follows:

[0098] Inert electrode - Chromium electrode - Inert electrode

[0099] (2nd additional electrode) (1st additional electrode) (3rd additional electrode)

[0100] In other words, the first additional electrode is surrounded by the second and third additional electrodes.

[0101] Two or more units of additional electrodes can also be connected together, whereby the following sequence applies for two units:

[0102] Inert electrode – chromium electrode – inert electrode – chromium electrode – inert electrode. The units are preferably connected in series, similar to a car battery. The series connection has the advantage of allowing for particularly space-saving placement of the units. It also provides a higher dissolving capacity for the chromium(III) ions.

[0103] The procedure with three or more additional electrodes is carried out analogously to the procedure with the first and second additional electrode already described in detail:

[0104] To dissolve the passivation layer on the chromium electrode(s), the surrounding inert electrodes serve as anodes, and the intermediate chromium electrode(s) serve as cathode(s). After the passivation layer has dissolved, the chromium(III) ions from the chromium electrode(s) dissolve electrolessly in the electrolyte. To continue the process, the chromium metal in the chromium electrodes can be replenished during the electroless dissolution. The process can then be continued by removing the passivation layer and dissolving the chromium(III) ions in the electrolyte again. Shortly before the chromium metal is consumed, it can be replenished. These process steps can be repeated as often as desired.

[0105] To rebuild the passivation layer on the chromium electrode(s), the inert electrodes are then connected as the cathode(s), and the chromium electrode(s) are the anode(s). Alternatively, the chromium electrode(s) are withdrawn from the electrolyte during step (G) or the electrolyte is removed; in both cases, a passivation layer builds up again on the chromium electrode(s) through contact with the ambient air. It is also possible to stop replenishing the chromium metal in the chromium electrode(s), so that no more chromium metal is available.

[0106] In Fig. 1, an embodiment of the method according to the invention is illustrated by means of a flow chart:

[0107] During the electrolytic deposition of chromium on the cathode, the passivation layer on the first additional electrode or chromium electrode is removed in step (E). After the passivation layer has dissolved, the current supply to the first additional electrode is stopped or the voltage is switched off (step (F)), and chromium(III) ions pass from the first additional electrode into the electrolyte without current (step (G)). After this, the process can either be continued or terminated. This is represented in Fig. 1 by the diamond that reads "Terminate the process?" If the process is not to be terminated (the "No" branch in Fig. 1), the one or more chromium electrodes can be refilled with chromium metal - if necessary - and process steps (E) to (G) can then be carried out again. This can be repeated as often as desired.

[0108] If the process is to be terminated (junction "Yes" in Fig. 1), the passivation layer can be rebuilt by applying an anodic voltage to the one or more chromium electrodes. Alternatively, the chromium metal in the first additional electrode(s) can no longer be replenished, so that the chromium electrode(s) are allowed to run dry. Another alternative for terminating the process is to pump the electrolyte out of the cell or to withdraw the chromium electrode(s) from the electrolyte so that the chromium electrode(s) are exposed to air, thereby re-forming the passivation layer and ending the chemical reaction.

[0109] The electrolyte for the process according to the invention is not particularly limited, provided it is suitable for electrolysis. Any electrolyte known to the person skilled in the art can be used. The electrolyte contains water as a solvent. The electrolyte preferably has a pH in the range of 2.0 to 3.5. In a further embodiment, the electrolyte can also have a pH in the range of 2.1 to 3.4, preferably 2.2 to 3.3, more preferably 2.3 to 3.2, even more preferably 2.4 to 3.1, and most preferably 2.5 to 3.0.

[0110] According to one embodiment, the electrolyte comprises:

[0111] (a) one or more chromium(III) salts,

[0112] (b) a compound of formula (I) where R is selected from NH2, OH or SO3H, and / or their salts, in particular salts with monovalent cations, such as Na + and / or K + - or divalent cations, where n is an integer from 1 to 3,

[0113] (c) Formic acid and / or its salts, in particular salts with monovalent cations such as Na + and / or K + , or divalent cations and

[0114] (d) optionally one or more additives. Component (a) of the electrolyte according to this embodiment is one or more chromium(III) salts. In the present invention, the term "chromium(III) salt" refers to any chromium(III) salt with which chromium can be deposited as a metal layer on objects. The chromium(III) salt is selected from an inorganic or organic chromium(III) salt or mixtures thereof. The inorganic chromium(III) salt is selected, for example, from the group consisting of, but not limited to, potassium chromium alum, ammonium chromium alum, chromium sulfate, chromium (hydroxy)sulfate (alkaline chromium sulfate), chromium sulfoacetate, chromium nitrate, chromium sulfamate (amidosulfonate), chromium chloride, chromium bromide, chromium iodide, chromium phosphate, chromium pyrophosphate (diphosphate), chromium phosphonate, and mixtures of two or more thereof.The organic chromium(III) salt is, for example, selected from the group consisting of, but not limited to, chromium citrate, chromium formate, chromium sulfoacetate, chromium oxalate, chromium methanesulfonate, chromium dimethanesulfonate, and mixtures of two or more of these. Inorganic and organic chromium(III) salts can also be used in a mixture.

[0115] For example, it is expedient to select the amount of chromium(III) salt in the range of 0.25 mol / L to 2.0 mol / L, based on the electrolyte. This range has proven particularly advantageous for the production of chromium coatings on metallic objects by electrolytic deposition.

[0116] Component (b) of the electrolyte according to this embodiment is the compound of formula (I) and / or its salt. Preferably, the compound of formula (I) is selected from glycine, glycolic acid, sulfoacetic acid, sodium sulfoacetate, potassium sulfoacetate, or a mixture of at least two of these compounds.

[0117] The amount of the compound of formula (I) in the electrolyte is preferably 0.5 mol / L to 1.5 mol / L, based on the electrolyte. This can be used to adjust the pH of the electrolyte.

[0118] According to the present embodiment, formic acid is present in the electrolyte as a further constituent (c), which is used to remove the oxygen released from the chromium(III) salt by conversion to CO2 and H2O. The amount of formic acid in the electrolyte is advantageously 1.0 mol / L to 3.0 mol / L, based on the electrolyte before chromium deposition. This range of amounts has proven particularly useful for adjusting the pH of the electrolyte. Instead of or in addition to formic acid, their salts can also be used. Examples include alkali and / or alkaline earth formates, in particular sodium formate.

[0119] One or more additives are optionally used as component (d) of the electrolyte. Any compound or mixture of compounds that can impart advantageous properties to the electroplating bath can be considered as an additive. These compounds are known to those skilled in the art.

[0120] For example, the additives are selected from complexing agents, alkali or alkaline earth salts, wetting agents, catalysts or mixtures thereof.

[0121] The complexing agents are preferably compounds with short alkyl chains (e.g., 1-5 C atoms) that contain 1 or 2 carboxyl groups or their derivatives or 1 or 2 thio and / or sulfone groups. For example, the following compound is used: where

[0122] Ri represents a Ci-s-alkyl radical, in particular CH3CH2-;

[0123] X one or more metal cations to balance the negative charge, such as Na + , K+ represents; where n is an integer from 1 to 5, in particular 3.

[0124] The compound N,N-dimethyldithiocarbamylpropylsulfonic acid sodium salt (DPS) is particularly preferred as a complexing agent. The use of DPS can be advantageous because particularly good chromium coatings can be obtained.

[0125] Wetting agents reduce the surface tension, allowing the formed chromium bubbles to detach from the cathode. This prevents pore formation in the chromium layer and allows for more uniform chromium layers to be obtained. Preferred wetting agents include polyfluorinated mono- and / or di-alkyl phosphates and PEG (polyethylene glycol) derivatives of salts or esters of phosphoric acid, especially PEGylated phosphates.

[0126] To increase conductivity, sulfates or acetosulfates can be used, such as alkali or alkaline earth salts, especially sodium sulfate, sodium sulfoacetate, potassium sulfate, or magnesium sulfate. The amount of sulfate or acetosulfate can be 5 mM to 30 mM, for example, 10 mM to 20 mM.

[0127] The amount of additive(s) present in the electrolyte can be 0.01 g / 1 to 2.0 g <l, bezogen auf den Elektrolyt, betragen. Bei Verwendung beispielsweise von PEG 6000 als Netzmittel ergibt sich eine Stoffmengenkonzentration von 0,001 mMol / L bis 0,3 mMol / L

[0128] As already explained, the pH value in the electrolyte is preferably set in the range of 2.0 to 3.5. The pH can be adjusted, for example, using the compound of formula (I), formic acid, and / or its salts.

[0129] The electrolyte is essentially free of chromium(VI) ions, meaning that only unavoidable chromium(VI) ion impurities are present in the electrolyte composition. In the process according to the invention, the content of chromium(VI) ions is below the detection limit.

[0130] According to one embodiment, it may be preferred if the electrolyte does not contain any nitrogen-containing compound. In this case, the resulting chromium layer also contains no nitrogen-containing compound, resulting in a coating with particularly advantageous properties.

[0131] The invention also relates to an electrolysis cell for controlling the supply of chromium to / in an electrolyte, comprising an anode; a cathode; an electrolyte containing at least one chromium(III) salt; the anode and cathode are immersed in the electrolyte; a first circuit connecting the anode and cathode and causing the deposition of a chromium layer by electrolytic deposition of chromium from an electrolyte by means of direct current on the cathode; a first additional electrode comprising or consisting of chromium metal; a second additional electrode in the form of an inert electrode, both additional electrodes being immersed in the electrolyte; a second circuit connecting the first additional electrode and the second additional electrode in a circuit separate from the cathode and anode.

[0132] The electrolysis cell assumes one of the following three states: a cathodic voltage is applied to the first additional electrode so that the passivation layer of the chromium metal on the first additional electrode dissolves; or no voltage is applied to the first additional electrode so that the chromium metal, after the passivation layer has dissolved, dissolves into the electrolyte in the form of chromium(III) ions without current from the first additional electrode; or an anodic voltage is applied to the first additional electrode so that the passivation layer on the first additional electrode forms again.

[0133] Any container, vessel or tank suitable for the person skilled in the art can be used as a container that can be used as an electrolysis cell, such as those commonly used in electroplating technology.

[0134] The above statements regarding the method for controlling the chromium supply in an electrolysis process for producing a chromium layer apply equally to the electrolysis cell and are therefore not repeated.

[0135] The electrolysis cell can be a one-piece, but also a two-piece design: In one embodiment, the chromium plating bath can be located in a first cell, in which a direct current flows between an anode and a cathode immersed in electrolyte. In a second cell, which can be connected to the first cell, a chromium electrode and an inert electrode, both immersed in electrolyte, are connected together. Alternatively, one or more units, each consisting of an inert electrode, a chromium electrode, and an inert electrode, can be connected together in the second cell. The chromium plating of an object takes place in the first cell. In the second cell, the electrolyte is enriched with chromium(III) ions to a desired concentration and can then be returned to the chromium plating bath. Other designs are also possible.

[0136] The described process or electrolysis cell is used in particular to recover Cr consumed in an electrolyte 3+ to replenish or top up. For example, the electroless dissolution of chromium is carried out over a period of time sufficient to bring the chromium content of the electrolyte to the desired level, which can take from a few minutes to several hours.

[0137] Alternatively, an equilibrium is established so that chromium(III) ions are continuously supplied to the plating bath, allowing it to operate continuously. The duration of the dissolution of chromium as chromium(III) ions is advantageously selected to ensure a constant concentration of chromium(III) ions in the electrolyte, particularly to maintain a stable equilibrium between chromium(III) ion supply and consumption.

[0138] The chromium metal can preferably be topped up or replenished during the process. The refilling of the chromium molded bodies of the first additional electrode is preferably carried out during the electroless dissolution of the chromium(III) ions by the action of the electrolyte in step (G). In this state, refilling can be carried out without difficulty and does not further interrupt the process. It is understood that for the newly refilled chromium molded bodies, the passivation layer must first be removed - as already described - (step (E)) before the chromium(III) ions can again dissolve electrolessly in the electrolyte (steps (F) and (G)).

[0139] The invention also relates to a method for maintaining the chromium(III) content in an electrolyte constant in the method disclosed here, by comparing the weight of the chromium metal used in the first additional electrode with the weight of the chromium metal consumed for the chromium layer and replenishing the chromium metal in the first additional electrode before the chromium(III) content in the electrolyte drops. Controlling the chromium(III) content by detecting the weight of the chromium metal present in the first additional electrode compared to the chromium metal consumed by the coating allows the chromium(III) content in the electrolyte to be kept constant. The weight detection can be carried out, for example, by pressure sensors.

[0140] The addition of metallic chromium to the electrolyte makes it possible to keep the chromium(III) content approximately constant during electrolysis over a long period of time, for example, from several hours to several months, with chromium(III) ions being replenished as the electrolyte becomes depleted of chromium(III). Keeping the chromium(III) content constant in the electrolyte means that the chromium(III) content preferably only changes by ± 10%.

[0141] The invention further relates to the use of the method for producing a chromium layer on an object.

[0142] The invention also relates to the use of the electrolysis cell for producing a chromium layer on an object.

[0143] The advantages of the invention are extremely multifaceted: The process according to the invention or the electrolysis cell according to the invention is based on a technical design that can be readily implemented by a person skilled in the art. Continuous polarity reversal using an expensive pulse rectifier is not required. Instead, a simple rectifier with a polarity inverter can be used, with the polarity reversal being used only at the beginning and end of the dissolution of the chromium(III) ions.

[0144] Another major advantage is that the chromium(III) ions are dissolved without electricity after the passivation layer has been dissolved. No additional energy input is required during the dissolution process. This is particularly important for large-scale industrial plants. This significantly reduces the energy costs for the process or the electrolysis cell.

[0145] The method according to the invention also makes it possible to keep the chromium(III) content in the electrolyte constant, for example by correlating the weight of the chromium metal in the first additional electrode with the weight of the chromium metal consumed by the coating and controlling it accordingly. This makes it possible to keep the chromium(III) content approximately constant during electrolysis over a long period of time, for example, several hours up to several months, with chromium(III) ions being replenished to the chromium(III)-depleted electrolyte. Keeping the chromium(III) content in the electrolyte constant means that the chromium(III) content preferably changes by only ± 10%.

[0146] In the process according to the invention, the electrolytic deposition can also advantageously be carried out without the use of a semipermeable membrane. Previously, semipermeable membranes were used to separate the anode from the cathode to prevent the formation of chromium(VI). This is not necessary with the process according to the invention. In the process according to the invention and the electrolysis cell provided, the formation of chromium(VI) during and after the electrolytic deposition of chromium is generally avoided. Chromium(VI) cannot be detected in the process according to the invention.

[0147] The chrome plating can thus be carried out in a simple, quick and cost-effective manner, even over longer periods of time.

[0148] According to one embodiment, it may be preferred if the electrolyte does not contain any nitrogen-containing compound, so that the chromium layer formed also does not contain any nitrogen-containing compound. This leads to a coating with particularly advantageous properties. The chromium layer can be applied for decoration or for technical reasons by electrolytic deposition of chromium. Examples of objects for which chromium plating is used for technical reasons are rotationally symmetrical objects such as rods, pistons and cylinders, in particular gravure cylinders. The term gravure cylinder or gravure roller refers to the printing form for gravure printing. The base cylinder is generally a steel tube core which is first coated with copper in an electrolytic bath and then, after the image data has been applied, with chromium. This process is carried out by electroplating the gravure cylinder with chromium.

[0149] According to the invention, chromium coatings of particularly excellent quality are obtained, which surprisingly also meet the high demands placed on gravure cylinders. Smooth, uniform surfaces are obtained that exhibit essentially no pores, pockmarks, or craters. The thickness of the resulting chromium coatings can be thicker than that of coatings commonly obtained in the prior art. Coating thicknesses of 100 μm and more can be obtained. Furthermore, chromium coatings of high hardness can be produced, in particular of over 900 HV. The resulting chromium coatings are corrosion-resistant, wear-resistant, have favorable friction properties, and are thermally and chemically stable; they are bright and highly reflective and are therefore also suitable for decorative purposes.

[0150] Embodiments of the present invention are described below by way of example with reference to the accompanying figures, which are schematic and not drawn to scale, so that no assumption can be made about exact geometric values ​​with respect to the original size. The figures of the present disclosure are part of and constitute a part of the description and illustrate embodiments of the invention without being limited to the specific embodiments described. The drawings, together with the description, serve to explain the present disclosure.

[0151] Figs. 2a, 2b and 2c show an embodiment of the process according to the invention and the states of an electrolysis cell for the controlled supply of chromium(III) ions by starting, continuing and ending the dissolution of the chromium(III) ions in connection with an electrolysis process for the electrolytic deposition of chromium layers. Figs. 2a, 2b and 2c therefore show the various states of an electrolysis cell, which illustrate the individual steps of the process according to the invention according to one embodiment: Fig. 2a shows an electrolysis cell 10 in the form of a bath device at a time during which a chromium layer is produced by electrolytic deposition of chromium from an electrolyte 25 by means of direct current using an anode 44 and a cathode 48. In the example shown, the cathode is a gravure cylinder 48, which was introduced into the bath device, for example, by means of a crane (not shown).The gravure cylinder 48 is held by bearing bridges 30 belonging to a bearing device. The outer surface of the gravure cylinder 48 is to be coated with chrome. Of course, another object, in particular a rotationally symmetrical object, could also be coated instead of the gravure cylinder 48 shown.

[0152] The electrolysis cell 10 has a tank 15 containing a liquid electrolyte 25, comprising the solvent water, which contains at least one Cr(III) salt. In the example shown, the electrolyte 25 has a pH value in the range of 2.0 to 3.5. According to one embodiment, the electrolyte has the following composition:

[0153] (a) one or more chromium(III) salts as already explained in detail;

[0154] (b) a compound of formula (I) where R is selected from NH2, OH or SO3H, and / or their salts, in particular salts with monovalent cations, such as Na + and / or K + or divalent cations, where n represents an integer from 1 to 3;

[0155] (c) Formic acid and / or its salts, in particular salts with monovalent cations such as Na + and / or K + , or divalent cations and

[0156] (d) optionally one or more additives as already described in detail.

[0157] Other electrolyte compositions are also possible.

[0158] Furthermore, a vertically movable anode device is provided in the tank 15, which essentially consists of an anode rail 42 and an anode basket 44 that is electrically and mechanically coupled to the anode rail 42 and serves as a metal holding device. The anode basket 44 can also consist of several assembled anode baskets or grids. The anode 44 represents an insoluble anode orInert electrode and can, for example, comprise or consist of the following materials: platinized titanium, carbon materials such as graphite, titanium coated with indium and / or tantalum and mixed metal oxides such as iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide; mixed metal oxides, where titanium serves as the anode base material which is coated with platinum, iridium, tantalum and / or palladium oxide; titanium, niobium or tantalum sheet coated with mixed metal oxides; titanium, tantalum or niobium coated with iridium-transition metal mixed oxide; or an expanded metal made of titanium, or an expanded metal made of titanium coated with a mixed oxide, or coated with graphite and material combinations of these.

[0159] For simplification and clarity, only one of the bearing bridges 30 is shown in Fig. 2a. For example, the two bearing bridges 30 are movable on rails (not shown) in the axial direction of the gravure cylinder 48 by means of spindles or other suitable adjustment mechanisms, so that they clamp the gravure cylinder 48 between them and hold it rotatably.

[0160] As can be seen in Fig. 2a, a portion of the tank 15 remains freely accessible at the top due to the single-sided supporting bearing bridges 30, so that the anode rail 42 extending there parallel to the axial direction of the gravure cylinder 48 can be freely moved vertically. The vertical movement of the anode rail 42 with the anode basket 48 is known to those skilled in the art from the prior art, so a detailed description and illustration is not necessary.

[0161] Fig. 2a shows the electrolysis cell 10 in the galvanization phase, with the gravure cylinder 48 almost completely submerged. In particular, immersion depths of more than 65% can be achieved for large cylinders (circumference 1500 mm) and up to approximately 80% for smaller cylinders (circumference 800 mm).

[0162] For electroplating, ie applying the chromium layer to the gravure cylinder 48, the anode basket 44 has already been pulled up laterally so that it surrounds the immersed gravure cylinder 48 with its large basket surface.

[0163] The anode 44 and cathode 48 therefore form a first circuit (not shown).

[0164] In a second circuit, which is independent and separately connected so that there is no connection between the two circuits, a first additional electrode 54 and a second additional electrode 56 are connected to one another. The first additional electrode 54 comprises or consists of chromium metal and can therefore also be referred to as a 'chromium electrode'. This is constructed, for example, from shaped chromium bodies 54a that are held in a holder, such as a frame or basket. The shaped bodies can have a regular or irregular shape and can be smooth or porous. For example, pieces, chunks, lumps, plates, bars, wires and grids, but not powders, are suitable for this purpose. The holder is a material that is resistant to the acidic electrolyte and can conduct current or not. Conductive materials include, for example, metals such as titanium.Non-conductive materials include, for example, plastics such as polypropylene and polyvinyl chloride. In the exemplary embodiment shown, chromium metal pieces 54a, also referred to as chromium nuggets, are housed in a plastic framework, for example, a polypropylene basket.

[0165] The shape of the first additional electrode 54 is not further limited, provided it is suitable for the intended purpose. Suitable shapes are known to those skilled in the art.

[0166] The chosen shape of the chromium metal molded bodies determines their surface area, with a larger surface area resulting in a higher dissolution rate in the electrolyte. The skilled person can therefore select a suitable shape.

[0167] The second additional electrode 56 is an inert electrode constructed from one or more electrically conductive materials and insoluble in the electrolyte. The material for the inert electrode is not further limited, provided it has the described properties. For example, the same material as for the anode 44 is suitable. The shape of the second additional electrode 56 can be selected by a person skilled in the art according to the structural conditions. The second additional electrode 56 can be, for example, a flat material, plate material, sintered material, or expanded material.

[0168] In Fig. 2a, the electrolysis cell 10 is shown at a time when a cathodic voltage using direct current is applied to the first additional electrode 54 so that the passivation layer of the chromium metal on the first additional electrode 54 dissolves. This represents step (E) of the method according to the invention. The first additional electrode 54 is thus the cathode and the second additional electrode 56 is the anode. A current source 58, which is provided with a rectifier and a polarity inverter (not shown), is used. The cathodic voltage has a reducing effect on the chromium metal pieces 54a, and the passivation layer that has formed on the surface of the chromium metal pieces 54a of the first additional electrode 54 begins to degrade and dissolve.The applied cathodic voltage can, for example, be in the range of 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, and most preferably 3.0 to 7.0 volts. The current density is preferably in the range of 2.5 to 4 A / dm. 2 , particularly preferably at 3.4 A / dm 2 The passivation layer is degraded after 5 to 60 seconds, preferably 5 to 45 seconds, and particularly preferably 5 to 30 seconds. Depending on the selected electrolysis conditions, in particular the pH value, the temperature, the selected voltage, the current density, the thickness of the chromium metal layer to be deposited, and the type of additional electrodes used, this time may be shorter or longer.

[0169] In Fig. 2b, the passivation layer on the first additional electrode 54 has already been degraded, whereby the appearance of bubbles 55 (hydrogen formation due to the dissolution of chromium(III) ions) over the entire surface of the chromium metal on the first additional electrode 54 indicates that the passivation layer has been dissolved.

[0170] In Fig. 2b, the passivation layer has therefore already dissolved and the current supply to the first additional electrode 54 and the second additional electrode 56 is interrupted (step (F)). The voltage is switched off. This is schematically illustrated in Fig. 2b using the non-closed electrical switch 59. The chromium metal from the first additional electrode 54 is dissolved in the form of chromium(III) ions without current through the action of the electrolyte 25 (step (G)), which here has a pH value in the range of 2.0 to 3.5. The chromium metal pieces 54a are therefore attacked by the acidic electrolyte and chemically dissolved. This occurs without current. The chromium(III) ions formed migrate in the electrolyte 25 to the surface of the gravure cylinder 48, which is connected as the cathode, where they are deposited in the form of a chromium coating.

[0171] As the chromium(III) ions dissolve in the electrolyte, the chromium metal in the first additional electrode (54) decreases. The chromium metal in the first additional electrode 54 can now be replenished during step (G), represented by Fig. 2b. Subsequently, steps (E), (F), and (G) of the inventive method are repeated. This can be continued as often as desired. A quasi-continuous process is therefore established, represented by: Fig. 2a -> Fig. 2b -> Replenish -> Fig. 2a -> Fig. 2b -> Replenish -> etc.

[0172] In Fig. 2c, the dissolution of the chromium(III) ions in the electrolyte is interrupted by applying an anodic voltage to the first auxiliary electrode 54, causing the passivation layer of the chromium metal to re-form on the first auxiliary electrode 54 and thus stopping the dissolution of chromium(III) ions in the electrolyte 25. The first auxiliary electrode 54 shown then becomes the anode, and the second auxiliary electrode 56, or inert electrode, becomes the cathode. Direct current flows again, but with reversed polarity.

[0173] For example, the polarity reversal from Fig. 2a (first additional electrode 54 is cathode) to Fig. 2c (first additional electrode 54 is anode) can be achieved by a rectifier with polarity reversal that is connected to a current source 58.

[0174] In the example case shown in Fig. 2c, the passivation layer has already completely formed again, the bubbles 55 on the chromium metal surface, ie the chromium moldings 54a, have completely disappeared, since no more chromium(III) ions dissolve in the electrolyte.

[0175] The applied anodic voltage can, for example, be in the range of 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts. The current density is, for example, in the range of 2.5 to 4 A / dm 2 , preferably at 3.4 A / dm 2The passivation layer is formed after approximately 5 to 60 seconds, preferably approximately 5 to 45 seconds, particularly preferably approximately 5 to 30 seconds. Depending on the selected electrolysis conditions, in particular the pH value, the temperature, the selected voltage, the current density, the thickness of the chromium metal layer to be deposited, and the type of additional electrodes used, this time may be shorter or longer.

[0176] The process can alternatively be terminated by withdrawing the first additional electrode 54 from the electrolyte 25 during step (G) or by pumping the electrolyte 25 out of the electrolysis cell 10 (not shown), so that the first additional electrode 54, in particular the chromium molded bodies 54a, comes into contact with the ambient air and thus builds up a passivation layer. Alternatively, the chromium metal of the first additional electrode 54 can no longer be refilled, so that the electroless dissolution of the chromium metal from the first additional electrode 54 in the form of chromium(III) ions by the action of the electrolyte ends as soon as the existing chromium metal has completely dissolved.

[0177] The formation of chromium(VI) ions could not be detected during the process.

[0178] Figures 3a to 3c schematically illustrate a further embodiment of the method according to the invention, wherein a third additional electrode is present in addition to the first and second additional electrodes. The third additional electrode, like the second additional electrode, is an inert electrode. There is an interconnected unit of additional electrodes, which can, for example, replace the circuit comprising the first additional electrode 54 and the second additional electrode 56 in Figure 2a. Fig. 3a shows two inert electrodes 56.1 and 56.2, which are also referred to here as the second and third additional electrodes. These virtually surround the chromium electrode 54, which is also referred to here as the first additional electrode. In the embodiment shown, an anodic voltage is applied to the inert electrodes 56.1 and 56.2 and a cathodic voltage to the chromium electrode 54. This is step (E), in which the passivation layer is dissolved.

[0179] Figure 3b shows the embodiment according to Figure 3a, but no voltage is applied to the additional electrodes (step (F)) and in Figure 3c an anodic voltage is applied to the first additional electrode (termination of the method during step (G)).

[0180] According to further variations of the present invention, the embodiment with a third additional electrode (Figs. 3a, 3b and 3c) could also replace the second circuit in Figs. 2a, 2b and 2c, respectively.

[0181] Figures 4a, 4b, and 4c schematically illustrate a further embodiment of the method according to the invention, illustrating a series connection of the electrodes. Inert electrodes 56.1, 56.2, 56.3, and 56.4 are shown, each alternating with chromium electrodes 54.1, 54.2, and 54.3. The inert electrodes 56.1 and 56.4 each represent edge electrodes, as they are located on the outside edge of the electrodes. Interconnected units of additional electrodes are present, which could, for example, replace the second circuit comprising the first additional electrode 54 and the second additional electrode 56 in Figures 2a, 2b, and 2c.

[0182] In the embodiment shown in Fig. 4a, an anodic voltage is applied to the inert electrodes 56.1, 56.2, 56.3, and 56.4, and a cathodic voltage is applied to the chromium electrodes 54.1, 54.2, and 54.3. This is step (E), in which the passivation layer is dissolved.

[0183] Figure 4b shows the embodiment according to Figure 4a, but with no voltage applied to the additional electrodes (step (F)), and in Figure 4c an anodic voltage is applied to the first additional electrode (ending the process during step (G)). Figures 5a, 5b, and 5c show a further embodiment of the invention. Figures 5a, 5b, and 5c show a further embodiment of the process according to the invention and the states of an electrolysis cell for the controlled supply of chromium(III) ions by starting, continuing, and ending the dissolution of the chromium(III) ions in connection with an electrolysis process for the electrolytic deposition of chromium layers.

[0184] In Fig. 5a, the tank of the electrolysis cell 100 is divided into an upper tank 110 and a lower tank 120 arranged below it in the form of a bath device. A liquid electrolyte 125 is located in the upper tank 110 and in the lower tank 120. This liquid electrolyte 125 is pumped from the lower tank 120 into the upper tank 110 by a pump 160 and flows back into the lower tank 120 via an overflow 127 that can be moved vertically into at least two positions. Alternatively, it is also possible to arrange two alternately openable overflows at different heights.

[0185] As already explained in Figures 2a, 2b, and 2c, a vertically movable anode device is arranged in the upper tank 110. This device essentially consists of an anode rail 142 and an anode basket 144, which is electrically and mechanically coupled to the anode rail 142 and serves as a metal holding device. The anode basket 144 can also consist of several assembled anode baskets or grids. The anode basket 144 is part of an insoluble anode.

[0186] The cathode 148, here a gravure cylinder, is held by two bearing bridges 130 (only one shown) on rails movable in the axial direction of the gravure cylinder 148 by means of suitable adjustment mechanisms, so that the gravure cylinder 148 is clamped between them and held rotatably. The upper half of the upper tank 110 is therefore freely accessible, allowing the anode rail 142 to be moved vertically.

[0187] The filling level of the electrolyte 125 in the upper tank 110, ie the height level of the electrolyte 125, can be adjusted in a suitable manner by means of the vertically movable overflow 127.

[0188] Fig. 5a shows the electrolysis cell 100 in the galvanization phase, in which the gravure cylinder 148 is almost completely submerged. For galvanization, i.e., applying the chromium layer to the gravure cylinder 148, the anode basket 144 has already been raised laterally so that its large basket surface surrounds the submerged gravure cylinder 148. The anode 144 and cathode in the form of a gravure cylinder 148 therefore form a first circuit (not shown). In a second circuit, which operates independently of the first circuit, a first auxiliary electrode 154 and a second auxiliary electrode 156 are connected to one another in the lower pan 120.

[0189] The first additional electrode 154 comprises or consists of chromium metal and is also referred to herein as the 'chromium electrode'. The structure of the chromium electrode 154 and inert electrode 156 are as already explained in Fig. 2a (there: chromium electrode 54 and inert electrode 56).

[0190] In the electrolysis cell 100 shown in Fig. 5a, a cathodic voltage is first applied to the first additional electrode 154 using direct current so that the passivation layer of the chromium metal on the first additional electrode 154 dissolves (step (E)). The first additional electrode 154 is thus the cathode, and the second additional electrode 156 is the anode. For example, a rectifier with a polarity reversal (not shown) connected to the power source 158 is used. The cathodic voltage has a reducing effect on the chromium metal pieces 154a, and the passivation layer that has formed on the surface of the chromium metal pieces 154a of the first additional electrode 154 begins to degrade.

[0191] The applied cathodic voltage can, for example, be in the range of 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts. The current density is, for example, in the range of 2.5 to 4 A / dm 2 , preferably at 3.4 A / dm 2 The passivation layer is degraded after approximately 5 to 60 seconds, preferably approximately 5 to 45 seconds, particularly preferably approximately 5 to 30 seconds. Depending on the selected electrolysis conditions, in particular the pH value, the temperature, the selected voltage, the current density, the thickness of the chromium metal layer to be deposited, and the type of additional electrodes used, this duration may be shorter or longer.

[0192] Fig. 5b shows the disappearance of the passivation layer on the first additional electrode 154 due to the appearance of bubbles 155, caused by hydrogen formation due to the dissolution of chromium(III) ions. The bubbles 155 are present over the entire surface of the chromium metal on the first additional electrode 154 when the passivation layer has completely dissolved.

[0193] As soon as the passivation layer has dissolved, the current supply to the first additional electrode 154 and the second additional electrode 156 is interrupted (step (F)). This is schematically illustrated in Fig. 5b using the electrical switch 159, which interrupts the current. Subsequently, the chromium metal from the first additional electrode 154, in the form of chromium(III) ions, is dissolved without current through the action of the electrolyte 125 in the lower pan 120 (step (G)), wherein the electrolyte 125 has, for example, a pH value in the range of 2.0 to 3.5. The chromium metal pieces 154a are attacked by the acidic electrolyte and chemically dissolved. This occurs without supplying current to the additional electrodes 154, 156.The formed chromium(III) ions are distributed in the electrolyte 125, which is pumped from the lower tank 120 into the upper tank 110 by means of the pump 160 and flows back into the lower tank 120 via an overflow 127 that can be moved vertically in at least two positions. The chromium(III) ions migrate via the electrolyte 125 to the surface of the gravure cylinder 144, which is connected as the cathode, and form a chromium coating there.

[0194] To reduce the amount of electrolyte in the upper pan 110, the upper pan 110 is tapered, for example, in the lower area. The tapering can be achieved with the help of additionally inserted sheets 133 or by appropriately adapting the walls of the upper pan 110. Blocks or boxes can also be used to displace volume. Limiting or reducing the volume of the upper pan 110 has the advantage that an excessive amount of electrolyte 125 does not have to be pumped upward from the lower pan 120. Accordingly, there is no risk of the lower pan 120 being completely emptied and the pump 160 running dry.

[0195] The chromium metal of the first additional electrode 154 can, if desired, be replenished during the execution of step (G) - according to Fig. 5b - and thus the process can be continued accordingly and steps (E), (F) and (G) can be repeated, so that a process sequence: Fig. 5a -> Fig. 5b -> Replenishing -> Fig. 5a -> Fig. 5b -> Replenishing -> is present.

[0196] If the supply of chromium(III) ions to the electrolyte is to be discontinued, an anodic voltage is applied to the first auxiliary electrode 154, causing the passivation layer of the chromium metal to re-form on the first auxiliary electrode 154 and stopping the dissolution of chromium(III) ions in the electrolyte. This is shown in Fig. 5c. The first auxiliary electrode 154 becomes the anode, and the second auxiliary electrode 156, or inert electrode, becomes the cathode. Direct current flows again, but with reversed polarity.

[0197] The applied anodic voltage can, for example, be in the range of 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts. The current density is, for example, again in the range of 2.5 to 4 A / dm 2 , preferably at 3.4 A / dm 2 The passivation layer is formed after approximately 5 to 60 seconds, preferably approximately 5 to 45 seconds, and particularly preferably approximately 5 to 30 seconds. Depending on the selected electrolysis conditions, in particular the pH value, the temperature, the selected voltage, the current density, the thickness of the chromium metal layer to be deposited, and the type of additional electrodes used, this duration may be shorter or longer.

[0198] For example, the polarity reversal (first additional electrode 154 as cathode, then as anode) can be achieved by a rectifier with polarity reversal, which is connected to a current source 158.

[0199] Once the passivation layer has fully formed again, bubbles 155 can no longer be observed on the chromium metal surface of the first additional electrode 154, since hydrogen gas is no longer formed and chromium(III) ions no longer dissolve in the electrolyte 125.

[0200] Alternatively, to terminate the process, the refilling of the chromium metal in the first additional electrode 154 could be omitted, so that the electroless dissolution of the chromium metal from the first additional electrode 154 in the form of chromium(III) ions by the action of the electrolyte 125 ends as soon as the existing chromium metal has completely dissolved. Alternatively, the first additional electrode 154 is withdrawn from the electrolyte 125 or the electrolyte 125 is pumped out of the electrolysis cell 100 (not shown).

[0201] The formation of chromium(VI) ions was not detectable during the entire process.

[0202] After the electroplating phase is complete, the anode rail 142 with the anode basket 144 is moved downward into the upper tank 110. Simultaneously or with a time delay, the overflow 127 is lowered so that the electrolyte 125 flows to a suitable level in the lower tank 120. This allows a state to be achieved in which the anode basket 144 is still completely covered by the electrolyte 125, while the gravure cylinder 148 stands completely free above the liquid level of the electrolyte 125 and can be easily lifted out using the crane (not shown).

[0203] A further embodiment of the invention is shown schematically in Figs. 6a, 6b and 6c. In contrast to the embodiment of Figs. 5a, 5b and 5c, no upper and lower troughs are provided, but rather a first trough 210 and a second trough 220 arranged side by side. Both troughs are connected to one another by a line with a pump 260, so that the electrolyte 225 can be pumped from the first trough 210 into the second trough 220 by means of the pump 260 and can flow back into the first trough via an overflow (not shown). Otherwise, the mode of operation corresponds to Figs. 5a, 5b and 5c, so that a repetition of the explanations there is not necessary here.

[0204] Alternatively, in Figures 5a-5c and 6a-6c, a third additional electrode (not shown) could be provided as an inert electrode, wherein the second additional electrode (56, 156, 256) and the third additional electrode, as shown in Figures 3a to 3c, are arranged such that the chromium electrode (54, 154, 254) is located between them.

[0205] According to a further embodiment, several of these units consisting of inert electrode-chromium electrode-inert electrode could be connected in series - as shown in Figures 4a to 4c - and could each replace the second circuit in Figures 5a-5c and 6a-6c.

[0206] With the method according to the invention and the electrolysis cell according to the invention, chromium layers with the desired properties can therefore be obtained in a particularly advantageous manner, in particular on gravure cylinders.

[0207] The following examples are intended to further illustrate the invention. They should in no way be construed as limiting the invention.

[0208] Examples

[0209] Example 1

[0210] An exemplary embodiment of the process according to the invention comprising steps (A) to (G) was carried out as follows:

[0211] An electrolyte with the following composition was provided:

[0212] - Chromium(III) sulfate (density = 1.26 g / mL; 3% Cr(III) -> 37.8 g / L -> 0.727 M) 18.6 L

[0213] - Na-sulfoacetate (8.3% by weight -> 104.6 g / L -> 0.568 M) 6.27 kg

[0214] - Na formate (8% by weight -> 100.8 g / L -> 1.482 M) 6.05 kg

[0215] - Na sulfate, 1.7% by weight -> 21.4 g / L -> 17 mM) 1.29 kg

[0216] One liter of electrolyte was heated to 40°C in a beaker with constant stirring, and a pH value of 2.6 was adjusted. Two electrodes were then placed parallel to and opposite each other at a distance of 10 cm from each other in the beaker and connected to a direct current source. One electrode was a mixed oxide-coated (MMO) titanium expanded metal, which served as the anode. The other electrode was a chromium plate, which served as the cathode. The anode and cathode surfaces were selected so that, at a current of 3 amperes, the working current density was approximately 4 A / dm². z The anode surface was chosen to be the same size as the cathode surface.

[0217] To dissolve the passivation layer on the chromium electrode (according to the invention: the first additional electrode), a cathodic voltage was applied. The other electrode was the inert electrode, which acted as the anode.

[0218] In the present example, after a short period of approximately 20 seconds, it was observed that gas evolution occurred at the chromium electrode, and gas bubbles rose to the surface. This indicated the dissolution of metallic chromium in the form of chromium(III) ions. After observing the start of the dissolution, the current flow was stopped, in this case after 60 seconds, so that the anode and cathode were no longer subjected to direct current. It was observed that the dissolution did not stop, but continued in the same manner.

[0219] At 0, 2, 4, and 6 hours during the test, a 1.0 mL sample was taken each day, and the chromium content was determined. It was found that the amount of Cr(III) had increased linearly during the observation period. This was further confirmed by a gravimetric analysis of the chromium electrode. The dissolved chromium species consisted exclusively of trivalent chromium. Hexavalent chromium (Cr(VI)) could not be detected at any time.

[0220] The electroless solution was then stopped by removing the chromium electrode from the electrolyte. Alternatively, the electrolyte could also be removed from the beaker. This exposes the chromium electrode to surface oxidation in the ambient air, causing the passivation layer to re-form. After the passivation layer had formed, reinserting the chromium electrode into the electrolyte—for example, after a residence time of 30 seconds in the ambient air—no longer initiated the dissolution process.

[0221] Instead of removing the chromium electrode from the electrolyte, the polarity was reversed to stop the electroless solution. This involved briefly applying a positive current to the chromium electrode until hydrogen formation at the chromium electrode stopped. Hydrogen formation stopped after 60 seconds. Once the visible gas evolution ceased, the current was turned off, thus permanently stopping the dissolution of chromium.

[0222] Example 2

[0223] Variation of pH value

[0224] Example 1 was repeated, but the pH was changed. Specifically, using the same setup as in Example 1, the pH was gradually lowered from 3.1 to 2.8, 2.6, and 2.4. In a further experiment, the pH was increased from 3.1 to 3.3 and 3.5. In both cases, the same results were obtained as described in Example 1. However, the chromium dissolution rate was lower at the higher pH.

[0225] List of reference symbols:

[0226] 10, 100, 200 electrolysis cells

[0227] 15 tub

[0228] 25, 125, 225 electrolyte

[0229] 30, 130, 230 bearing bridge

[0230] 42, 142, 242 anode rail

[0231] 44, 144, 244 anode, anode basket

[0232] 48, 148, 248 cathode, gravure cylinder

[0233] 54, 54.1, 54.2, 54.3, 154, 254 first additional electrode, chromium electrode

[0234] 54a, 154a, 254a chrome moldings

[0235] 55, 55.1, 55.2, 55.3, 155, 255 hydrogen bubbles

[0236] 56, 56.1,156, 256 second additional electrode

[0237] 56.2 third additional electrode

[0238] 56.3, 56.4 additional electrodes

[0239] 58, 158, 258 power source

[0240] 59, 159, 259 switches

[0241] 110 upper hull

[0242] 120 lower hull

[0243] 127 Overflow

[0244] 160, 260 pump

[0245] 210 first tub

[0246] 220 second tub

Claims

Patent claims 1. A method for controlling the chromium supply in an electrolysis process for producing a chromium layer, wherein the chromium layer is produced by electrolytic deposition of chromium from an electrolyte (25, 125, 225) by means of direct current and using an anode (44, 144, 244) and a cathode (48, 148, 248), comprising the following steps: (A) providing a first auxiliary electrode (54, 154, 254) comprising or consisting of chromium metal; (B) providing a second additional electrode (56, 156, 256) in the form of an inert electrode, (C) immersing both additional electrodes (54, 56, 154, 156, 254, 256) in the electrolyte (25, 125, 225) containing at least one chromium(III) salt; (D) connecting the first additional electrode (54, 154, 254) and the second additional electrode (56, 156, 256) in a separate circuit from the cathode (48, 148, 248) and anode (44, 144, 244); and during the electrolytic deposition of chromium to form a chromium layer: (E) applying a cathodic voltage to the first additional electrode (54, 154, 254), whereby the passivation layer of the chromium metal dissolves and chromium metal in the form of chromium(III) ions begins to dissolve in the electrolyte (25, 125, 225); (F) after dissolving the passivation layer, terminating the current supply to the first additional electrode (54, 154, 254); and (G) electroless dissolution of the chromium metal from the first additional electrode (54, 154, 254) in the form of chromium(III) ions by the action of the electrolyte (25, 125, 225).

2. The method according to claim 1, characterized in that the chromium metal in the first additional electrode (54, 154, 254) is replenished during step (G) and then steps (E), (F) and (G) of claim 1 are carried out successively in this order.

3. The method according to claim 1 or 2, characterized in that to terminate the method during step (G) an anodic voltage is applied to the first additional electrode (54, 154, 254) or the first additional electrode (54, 154, 254) is withdrawn from the electrolyte (25, 125, 225) or the electrolyte is pumped out of the electrolysis cell (10, 100, 200) or no refilling of the chromium metal of the first additional electrode (54, 154, 254) is carried out.

4. Method according to one of the preceding claims 1 to 3, characterized in that a pH value in the range from 2.0 to 3.5, in particular 2.1 to 3.4, preferably 2.2 to 3.3, more preferably 2.3 to 3.2, even more preferably 2.4 to 3.1, most preferably 2.5 to 3.0 is set in the electrolyte (25, 125, 225).

5. The method according to any one of the preceding claims 1 to 4, characterized in that one, two or more of the following conditions are met: the cathodic voltage in step (E) is set in the range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, very particularly preferably 3.0 to 7.0 volts; the anodic voltage for ending the method during step (G) is set in a range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, very particularly preferably 3.0 to 7.0 volts; the surface of the first additional electrode (54, 154, 254) is chosen to be the same size as the surface of the second additional electrode (56, 156, 256); the current density in the range of 2.5 to 4 A / dm 2 , preferably at 3.4 A / dm 2the passivation layer is broken down by applying the cathodic voltage to the first additional electrode (54, 154, 254) in step (E) within 5 to 60 seconds, preferably 5 to 45 seconds, particularly preferably 5 to 30 seconds; the passivation layer is reformed by applying the anodic voltage to the first additional electrode (54, 154, 254) within 5 to 60 seconds, preferably 5 to 45 seconds, particularly preferably 5 to 30 seconds to terminate the process during step (G); and the chromium metal of the first additional electrode (54, 154, 254) is present as a chromium shaped body selected from pieces, chunks, lumps, platelets, bars, wires and grids, which are held in a material resistant to the acidic electrolyte (25, 125, 225).

6. Method according to one of the preceding claims 1 to 5, characterized in that a third additional electrode in the form of an inert electrode is provided, wherein the first additional electrode (54, 154, 254) in the form of a chromium electrode, the second additional electrode (56, 56.1, 156, 256) and the third additional electrode (56b) are connected together to form one or more units, wherein a unit is selected from: inert electrode (56.1) - chromium electrode (54) - inert electrode (56.2), wherein a series connection is preferably used.

7. Method according to one of the preceding claims 1 to 6, characterized in that the components of the electrolyte (25, 125, 225) are selected from: (a) one or more chromium(III) salts selected from inorganic and / or organic chromium(III) salts; (b) a compound of formula (I) OH R-(CH2) n -C=O (I), where R is selected from NH2, OH or SO3H and n is an integer from 1 to 3, and / or their salts, in particular salts with monovalent cations, such as Na + and / or K + , or divalent cations; (c) Formic acid and / or its salts, in particular salts with monovalent cations such as Na + and / or K + , or divalent cations and (d) optionally one or more additives, in particular selected from complexing agents, alkali or alkaline earth salts, wetting agents, catalysts or mixtures thereof.

8. Method according to one of the preceding claims 1 to 7, characterized in that the chromium(III) content in the electrolyte (25, 125, 225) is kept constant by comparing the weight of the chromium metal used in the first additional electrode (54, 154, 254) with the weight of the chromium metal used for the chromium layer and replenishing the chromium metal in the first additional electrode (54, 154, 254) before the chromium(III) content in the electrolyte (25, 125, 225) decreases.

9. An electrolytic cell (10, 100, 200) for controlling the supply of chromium to an electrolyte (25, 125, 225), comprising an anode (44, 144, 244); a cathode (48, 148, 248); an electrolyte (25, 125, 225) containing at least one chromium(III) salt; the anode (44, 144, 244) and cathode (48, 148, 248) are immersed in the electrolyte (25, 125, 225); a first circuit connecting the anode (44, 144, 244) and cathode (48, 148, 248) and causing the deposition of a chromium layer by electrolytic deposition of chromium from an electrolyte (25, 125, 225) by means of direct current on the cathode (48, 148, 248); a first additional electrode (54, 154, 254) comprising or consisting of chromium metal; a second additional electrode (56, 156, 256) in the form of an inert electrode, both additional electrodes (54, 56, 154, 156, 254, 256) being immersed in the electrolyte (25, 125, 225);a second circuit connecting the first auxiliary electrode (54, 154, 254) and the second auxiliary electrode (56, 156, 256) in a separate circuit from the cathode (48, 148, 248) and anode (44, 144, 244); wherein either a cathodic voltage is applied to the first auxiliary electrode (54, 154, 254) to dissolve the passivation layer of the chromium metal on the first auxiliary electrode (54, 154, 254); or no voltage is applied to the first additional electrode (54, 154, 254) so ​​that the chromium metal, after the passivation layer has been dissolved, dissolves from the first additional electrode (54, 154, 254) in the form of chromium(III) ions into the electrolyte (25, 125, 225) without current; or an anodic voltage is applied to the first additional electrode (54, 154, 254) so ​​that the passivation layer on the first additional electrode (54, 154, 254) is re-formed.; 10. Electrolysis cell (10, 100, 200) according to claim 9, characterized in that the electrolysis cell (10, 100, 200) is constructed from a trough (15), two troughs arranged one above the other, in particular an upper trough (110) and a lower trough (120), or two troughs arranged next to one another, in particular a first trough (210) and a second trough (220).

11. Electrolysis cell (10, 100, 200) according to claim 9 or 10, characterized in that one, two or more of the following features are met: in the electrolyte (25, 125, 225) there is a pH value in the range from 2.0 to 3.5, in particular 2.1 to 3.4, preferably 2.2 to 3.3, more preferably 2.3 to 3.2, even more preferably 2.4 to 3.1, most preferably 2.5 to 3.0; the cathodic voltage at the first additional electrode (54, 154, 254) is in the range of 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, very particularly preferably 3.0 to 7.0 volts or the anodic voltage at the first additional electrode (54, 154, 254) is in a range of 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, very particularly preferably 3.0 to 7.0 volts;the cathodic voltage at the first additional electrode (54, 154, 254) is applied for 5 to 60 seconds, preferably 5 to 45 seconds, particularly preferably 5 to 30 seconds, or the anodic voltage at the first additional electrode (54, 154, 254) is applied for 5 to 60 seconds, preferably 5 to 45 seconds, particularly preferably 5 to 30 seconds; the surface area of ​​the first additional electrode (54, 154, 254) is selected to be the same size as the surface area of ​​the second additional electrode (56, 156, 256); the current density is in the range of 2.5 to 4 A / dm; 2 , preferably at 3.4 A / dm 2 set; the chromium metal of the first additional electrode (54, 154, 254) is present as a chromium shaped body, selected from pieces, chunks, lumps, platelets, bars, wires and grids, which are held in a material resistant to the acidic electrolyte (25, 125, 225); and / or the components of the electrolyte (25, 125, 225) are selected from: (a) one or more chromium(III) salts selected from inorganic and / or organic chromium(III) salts; (b) a compound of formula (I) OH R-(CH2)nC=O (I), wherein R is selected from NH2, OH or SO3H and n is an integer from 1 to 3; and / or their salts, in particular salts with monovalent cations, such as Na + and / or K + , or divalent cations; (c) Formic acid and / or its salts, in particular salts with monovalent cations, such as Na + and / or K + , or divalent cations and (d) optionally one or more additives, in particular selected from complexing agents, alkali or alkaline earth salts, wetting agents, catalysts or mixtures thereof.

12. Electrolysis cell according to one of the preceding claims 9 to 11, characterized in that a third additional electrode in the form of an inert electrode is provided, wherein the first additional electrode (54, 154, 254) in the form of a chromium electrode, the second additional electrode (56, 56.1, 156, 256) and the third additional electrode (56b) are connected together to form one or more units, wherein one unit is selected from: inert electrode (56.1) - chromium electrode (54) - inert electrode (56.2), wherein the units are preferably connected in series.

13. Use of the electrolysis cell (10, 100, 200) according to one of the preceding claims 9 to 12 for producing chromium layers on rotationally symmetrical components, in particular gravure cylinders (48, 148, 248).

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