Method for controlling chromium supply in an electrolytic process for producing a chromium layer and electrolytic cell therefor

Abstract

The present invention relates to a method for controlling chromium supply in an electrolytic process for producing a chromium layer using an anode (44, 144, 244) and a cathode (48, 148, 248) with a direct current, comprising the following steps during electrolytic deposition of chromium with formation of the chromium layer: (E) applying a cathodic voltage to a first auxiliary electrode (54, 154, 254), thereby dissolving a passive layer of chromium metal and causing chromium metal in the form of trivalent chromium ions to begin dissolving in an electrolyte (25, 125, 225); (F) following dissolution of the passive layer, terminating the current supply to the first auxiliary electrode (54, 154, 254) or turning off the voltage to the first auxiliary electrode (54, 154, 254); and (G) under the action of the electrolyte (25, 125, 225), chromium metal dissolves in the form of trivalent chromium ions from the first auxiliary electrode (54, 154, 254) without current. Steps (E), (F) and (G) can be repeated any number of times by replenishing the first auxiliary electrode with chromium metal during step (G).

Description

[Technical Field]

[0001] The present invention relates to a method for controlling the supply of chromium in an electrolytic process for producing a chromium layer and to an electrolytic bath therefor. [Background technology]

[0002] Galvanic treatments for coating the surfaces of objects have long been known in the art. The coated objects thus obtained have particularly advantageous surface properties, such as greater hardness, improved corrosion resistance, metallic appearance, gloss, etc. A galvanic bath containing the metal to be deposited as a salt in solution is used to deposit the metal on the object connected as a cathode by direct current. Thus, the objects to be coated are usually metallic materials or have a metallized surface to make them electrically conductive.

[0003] One metal used for this purpose is chromium. The application of a chromium layer using a galvanic bath produces a bright, highly reflective chrome layer that can be used for decorative purposes. It can also be used to plate objects for technical purposes, such as improving wear resistance, abrasion resistance, heat resistance, and corrosion resistance. For example, chrome plating is used on pistons, cylinders, cylinder liners, and axle bearings.

[0004] The use of hexavalent chromium salts, such as CrO3, and sulfuric acid in galvanic baths is well known. However, this has many drawbacks. For example, gases, especially hydrogen and, to a lesser extent, oxygen, are generated, resulting in the formation of acidic, corrosive, and partially toxic chromic acid mist. This necessitates the use of surfactants and wetting agents to intensively extract chromic acid from the galvanic bath surface and contain the resulting chromic acid mist. Furthermore, hexavalent chromium electrolytes are highly toxic and carcinogenic. Therefore, it is better to use non-toxic galvanic baths containing trivalent chromium salts.

[0005] In the case of galvanic chromium plating using trivalent chromium salts, care must be taken to ensure that the chromium layer is deposited with the appropriate thickness and that the system design is not too complex so that it can be used industrially.

[0006] The deposition of chromium from the trivalent chromium salts present in the electrolyte causes a decrease in the trivalent chromium ion concentration in the electrolyte. However, since trivalent chromium can only be added in the form of trivalent chromium salts, an undesirable gradual accumulation of anions present in the salts in the electrolyte occurs. This requires periodic dilution and subsequent re-dosing of the other components, necessitating constant control and monitoring of the system.

[0007] Furthermore, anodic voltage or anodic potential cannot be applied to a chromium metal anode. Chromium is a highly reactive metal, and an oxide film quickly forms on its surface, passivating it. Even if anodic voltage is applied to this passivated chromium, only a small amount of chromium dissolves. However, if the anodic voltage is increased, the chromium dissolves as hexavalent chromium ions. Hexavalent chromium is not only carcinogenic, but also interferes undesirably with trivalent chromium electrolytes, impairing their functionality. Therefore, it is extremely difficult to continuously supply chromium as trivalent chromium to the electrolyte.

[0008] The following solutions are already known from the prior art:

[0009] EP2640873A1 (WO2012 / 067725A1) describes a method for replenishing or increasing the chromium content in an electrolyte with trivalent chromium, which method comprises the following steps: a) Immersing the chromium-containing electrode and the second electrode in an electrolyte containing trivalent chromium ions. b) A pulsed AC current is applied to the chromium electrode and the second electrode. At this time, chromium dissolves from the chromium electrode in the form of trivalent chromium ions, replenishing or accumulating the trivalent chromium content in the electrolyte in which the chromium electrode is immersed. The duration of each forward and reverse pulse is typically between about 0.1 and 2 seconds. For example, a rectangular waveform can be used, with the AC pulse duration being about 400 ms for the cathodic forward pulse and about 400 ms for the anodic reverse pulse. The disadvantage of this proposal is the complex technology, and the constant polarity reversal requires the use of expensive pulse rectifiers.

[0010] Furthermore, GB414939 discloses a method for electroplating chromium, in which a direct current (DC) is passed from a chromium anode to a cathode to be plated, and an alternating current (AC) is superimposed on the plating current to activate and dissolve the chromium on the anode. For example, Figure 2 shows a circuit diagram illustrating an arrangement in which AC is superimposed only on the anode. H is an auxiliary electrode, and an AC generator WG is connected to anode A and auxiliary electrode H via a transformer T. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] EP2640873A1 [Patent Document 2] GB414939 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention aims to provide a method or electrolytic cell which avoids the drawbacks of the prior art and allows for a controlled supply of chromium metal during electrolysis without causing the accumulation of undesirable anions and without the formation of hexavalent chromium ions, and the chromium layer provided should meet the requirements imposed in particular on chromium coatings on gravure cylinders. [Means for solving the problem]

[0013] (Brief Description of the Invention) The stated object is achieved according to the invention by the teaching of the independent claims. The teaching of the dependent claims represents advantageous embodiments.

[0014] In particular, a method is provided for controlling the chromium supply in an electrolytic process for producing a chromium layer, the chromium layer being produced by electrolytic deposition of chromium from an electrolyte by direct current and using an anode and a cathode, comprising the steps of: (A) A first auxiliary electrode containing or made of chromium metal is prepared. (B) Providing a second auxiliary electrode in the form of an inert electrode. (C) Immerse both auxiliary electrodes in an electrolyte containing at least one trivalent chromium salt. (D) The first and second auxiliary electrodes are connected to each other in an electric circuit separate from the cathode and anode, and a chromium layer is formed during the electrolytic deposition of chromium. (E) A cathodic voltage is applied to the first auxiliary electrode, so that the passive layer of the chromium metal dissolves and the chromium metal in the form of trivalent chromium ions begins to dissolve in the electrolyte. (F) After the passive layer is dissolved, the current supply to the first auxiliary electrode is terminated or the voltage to the first auxiliary electrode is turned off. (G) Under the action of the electrolyte, chromium metal dissolves from the first auxiliary electrode in the form of trivalent chromium ions without current.

[0015] The process can also be terminated again in a targeted manner, for example when the object to be coated is completely coated. According to one embodiment of the present invention, the process can be terminated during step (G) as follows: · Apply an anodic voltage to the first auxiliary electrode. The first auxiliary electrode is withdrawn from the electrolyte or the electrolyte is drained from the electrolytic cell so that the first auxiliary electrode is in contact with the surrounding air. The chromium metal in the first auxiliary electrode is not replenished, and the dissolution of the chromium metal in the form of trivalent chromium ions from the first auxiliary electrode without current is completed by the action of the electrolyte when the chromium is completely dissolved.

[0016] The subject of the invention is also an electrolytic cell for controlling the supply of chromium to an electrolyte, comprising: ·anode ·cathode Electrolyte containing at least one trivalent chromium salt The anode and cathode must be immersed in the electrolyte. A first circuit that connects the anode and cathode and deposits a chromium layer by passing a direct current through the cathode to electrolytically deposit chromium from the electrolyte. A first auxiliary electrode containing or consisting of chromium metal A second auxiliary electrode in the form of an inert electrode Both auxiliary electrodes are immersed in the electrolyte. A second circuit that connects the first auxiliary electrode and the second auxiliary electrode to an electrical circuit separate from the cathode and anode, and that is one of the following: A cathodic voltage is applied to the first auxiliary electrode, dissolving the passive layer of chromium metal on the first auxiliary electrode. After dissolving the passive layer without applying a voltage to the first auxiliary electrode, chromium metal from the first auxiliary electrode dissolves in the electrolyte in the form of trivalent chromium ions without current. An anodic voltage is applied to the first auxiliary electrode and a passivation layer is again formed on the first auxiliary electrode.

[0017] The method and electrolytic cell according to the invention are therefore based on the chemical dissolution of chromium in the form of trivalent chromium ions in an electrolyte, which dissolution is activated by passing an electric current, continues without passing an electric current and can be terminated again, if desired, by passing an electric current or in another way. [Brief explanation of the drawings]

[0018] The above and other aspects, advantages, and features according to the present invention will also be described in more detail in the following paragraphs with the aid of the accompanying drawings.

[0019] [Figure 1] 1 is a flow chart illustrating an embodiment of a method according to the present invention. [Figure 2a]1 shows a schematic diagram of an embodiment of an electrolytic cell according to the present invention during a galvanizing stage, with a cathodic voltage applied to the first auxiliary electrode. [Figure 2b] 2b is a schematic view of the same embodiment as in FIG. 2a, but with no voltage applied to the first auxiliary electrode. [Figure 2c] 2b is a schematic diagram of the same embodiment as in FIG. 2a, where an anodic voltage is applied to the first auxiliary electrode. [Figure 3a] 1 is a schematic diagram illustrating one embodiment of a circuit according to the present invention, comprising first, second and third auxiliary electrodes, with a cathode voltage applied to the first auxiliary electrode. [Figure 3b] 3b is a schematic diagram of the same embodiment as in FIG. 3a, but with no voltage applied to the auxiliary electrode. [Figure 3c] 3b is a schematic diagram of the same embodiment as in FIG. 3a, with an anodic voltage applied to the first auxiliary electrode. [Figure 4a] 1 is a schematic diagram of one embodiment of a circuit according to the present invention, with auxiliary electrode units connected to each other to illustrate a series connection, and a cathode voltage applied to a first auxiliary electrode; [Figure 4b] 4b is a schematic diagram of the same embodiment as in FIG. 4a, but with no voltage applied to the auxiliary electrode. [Figure 4c] 4b is a schematic diagram of the same embodiment as in FIG. 4a, where an anodic voltage is applied to the first auxiliary electrode. [Figure 5a] FIG. 2 is a schematic diagram showing another embodiment of an electrolytic cell according to the present invention during a galvanizing stage, in which a cathodic voltage is applied to the first auxiliary electrode. [Figure 5b] 5b is a schematic diagram of the same embodiment as in FIG. 5a, but with no voltage applied to the auxiliary electrode. [Figure 5c] 5b is a schematic diagram of the same embodiment as in FIG. 5a, where an anodic voltage is applied to the first auxiliary electrode. [Figure 6a] FIG. 2 is a schematic diagram showing another embodiment of an electrolytic cell according to the present invention during a galvanizing stage, in which a cathode voltage is applied to the first auxiliary electrode. [Figure 6b] 6b is a schematic diagram of the same embodiment as in FIG. 6a, but with no voltage applied to the auxiliary electrode. [Figure 6c]6b is a schematic diagram of the same embodiment as in FIG. 6a, where an anodic voltage is applied to the first auxiliary electrode. DETAILED DESCRIPTION OF THE INVENTION

[0020] (Terms and definitions) "Electrolytic process" or "electrolysis" is understood to mean the deposition of a metal, in this case chromium, from a solution containing the corresponding metal ions using an electric current, thereby forming a metal layer (in this case, chromium plating).

[0021] The terms "cathode voltage" or "cathode potential" are used synonymously and interchangeably in the present invention and are taken to mean that a voltage is applied to an electrode or auxiliary electrode such that the electrode or auxiliary electrode functions as a cathode.

[0022] The terms "anode voltage" or "anode potential" are also used synonymously and interchangeably in the present invention and are taken to mean that a voltage is applied to an electrode or auxiliary electrode such that the electrode or auxiliary electrode functions as an anode.

[0023] (Detailed Description of the Invention) The method according to the invention for controlling the chromium supply is carried out during the electrolytic process for producing the chromium layer, which will therefore be described first.

[0024] The electrolytic deposition of the chromium layer is usually carried out in an electrolytic cell filled with an electrolyte, the container of which may be any suitable container for the skilled worker, in particular as used in galvanic techniques.

[0025] The cathode typically serves as the object onto which the chromium layer is deposited, for example the gravure cylinder.

[0026] The anode may be any anode known to those skilled in the art. In particular, an inert electrode may be used as the anode, and the inert electrode may be made of one or more conductive materials that are insoluble in the electrolyte. Examples of materials used for the insoluble anode or inert electrode include: Platinum-plated titanium Expanded titanium metal, optionally coated with mixed oxides or graphite Graphite and other carbon materials Indium and / or tantalum coated titanium Mixed metal oxides, in particular iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide, or iridium-tantalum mixed oxide Mixed metal oxides where titanium acts as the anode substrate coated with platinum, iridium, tantalum, and palladium oxides Mixed metal oxide coated titanium, niobium or tantalum sheets Titanium, tantalum or niobium coated with iridium transition metal mixed oxide and combinations of these materials

[0027] The shape of the anode can be adjusted by a person skilled in the art according to the purpose, for example, the anode can be a flat material, a plate material, a sintered material, or an expanded material.

[0028] The anode and cathode are immersed in an electrolyte, which can be any electrolyte known to those skilled in the art of galvanic technology. When a direct current voltage is applied to the two electrodes, the anode and cathode, trivalent chromium ions are deposited from the electrolyte onto the target, i.e., the cathode. If the target is not metallically conductive, it can be made conductive by pre-treatment.

[0029] As disclosed in WO 2008 / 014987 A2, the above setup can be modified to the extent that the electrolyte in the electrolytic cell is separated into catholyte (electrolyte in the cathode compartment) and anolyte (electrolyte in the anode compartment) by a semipermeable membrane. The cathode, as the object to be coated, is immersed in the catholyte containing the chromium ions to be deposited. When a voltage is applied, current flows from the membrane via the anolyte into the catholyte.

[0030] The anode arrangement can also be such that the anode is in direct contact with the membrane, i.e. the anode is covered with the membrane, a so-called direct contact membrane anode, as known from DE 10 2010 055 143 A1.

[0031] The chrome layer can be produced at a temperature of 20°C to 60°C, and the temperature of the electrolyte is set by suitable heating and cooling devices. The chrome layer can be produced at a current of, for example, 5 to 60 A / dm 2 can be produced at a current density of .

[0032] The electrolyte may be stirred or mixed while the electrodes are immersed in it. In particular, it may be circulated. Preferably, five bath volumes, i.e., five times the volume of the electrolyte, are circulated every hour.

[0033] The object to be coated can also be moved. In the case of a gravure cylinder, it can be moved at a rotation speed of, for example, 0.5 to 1.5 m / min.

[0034] The method according to the invention makes it possible to control the chromium supply in the electrolytic process for producing a chromium layer. For this purpose, a first auxiliary electrode (step (A)) and a second auxiliary electrode (step (B)) are provided, both of which are immersed in the electrolyte (step (C)).

[0035] The first auxiliary electrode includes or consists of chromium metal and is therefore also referred to herein as a "chrome electrode." The first auxiliary electrode, or chrome electrode, is made, for example, of a chrome molding that can be held in a framework or holder. This molding can have a regular or irregular shape and can be smooth or porous. These include, for example, nuggets, chunks, lumps, platelets, bars, wires, and / or grids, but not powders. The framework or holder is made of 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, for example, a plastic such as polypropylene or polyvinyl chloride. If the framework or holder does not conduct current, an additional conductive plate, for example, is attached to allow the chrome molding to be electrified.

[0036] According to one embodiment, the chrome metal piece, also referred to herein as a "chrome nugget," is housed within a plastic frame as a chrome molding.

[0037] The shape of the first auxiliary electrode can be appropriately selected by a person skilled in the art. Regarding the shape, particularly for a molded product made of chromium metal, it is important that the larger the surface area of ​​the chromium metal, the faster the dissolution rate in the electrolyte. Therefore, a person skilled in the art can select an appropriate shape.

[0038] 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 auxiliary electrode, thereby achieving particularly good dissolution of chromium and subsequent supply of trivalent chromium.

[0039] The second auxiliary electrode is an inert electrode and is made of one or more conductive materials insoluble in the electrolyte. The material of the inert electrode is not particularly limited as long as it has the described properties. For example, the same material as the anode in the above-mentioned electrolytic process can be used for chromium coating.

[0040] The shape of the second auxiliary electrode can be selected by those skilled in the art depending on the structural requirements, and the second auxiliary electrode may be, for example, a flat material, a plate-like material, a sintered material, or an expanded material.

[0041] According to a preferred embodiment, the surface area of ​​the first auxiliary electrode is selected to be the same as that of the second auxiliary electrode, in which case it is advantageous if the surface area of ​​the metallic chromium is 100% of the surface area of ​​the first auxiliary electrode.

[0042] An electrolytic cell has two independent electrical circuits. In the first circuit, the anode and cathode are connected to each other in order to apply trivalent chromium ions dissolved in the electrolyte to the object connected as a cathode in the form of a chromium layer. Direct current is used, and there is no reversal of polarity between the anode and the cathode. The anode always remains anode, and the cathode always remains cathode.

[0043] In one embodiment, the first auxiliary electrode and the second auxiliary electrode are connected to each other in a separate electric circuit from the cathode and the anode (step (D)). The first electric circuit of the anode and the cathode and the second electric circuit of the first auxiliary electrode and the second auxiliary electrode are not connected to each other but are completely separated from each other. Thus, the second electric circuit is controlled independently of the first electric circuit. The second electric circuit uses direct current, but the polarity is reversed after a specific time interval. However, this time interval is significantly longer than that of pulsed alternating current (e.g., 0.1 to 2 seconds in duration). That is, initially, the first auxiliary electrode functions as a cathode and the second auxiliary electrode as an anode, and at a later time, the first auxiliary electrode functions as an anode and the second auxiliary electrode as a cathode. This type of polarity reversal is known in the art, albeit in different contexts and for other purposes, and can be easily implemented by those skilled in the art. For example, polarity reversal can be achieved by a rectifier with a polarity inverter.

[0044] The second electric circuit functions independently of the first electric circuit, and can therefore be conveniently activated during the electrolytic deposition of chromium, which results in the formation of a chromium layer. For this purpose, a cathodic voltage is first applied to the first auxiliary electrode. Thus, the first auxiliary electrode is a cathode, and the second auxiliary electrode is an anode. The cathodic voltage, also referred to herein as a cathodic potential, has a reducing effect on chromium metal. As a result, the passive layer formed on the surface of the chromium metal of the first auxiliary electrode begins to decompose.

[0045] Although chromium is chemically more active than iron, it behaves almost like a noble metal when corroded by air or water. This is due to a very thin, virtually invisible chromium oxide layer, only a few nanometers thick (about 50 atomic layers in chromium-nickel steel and about 5 atomic layers in pure chromium), which protects the metal from the atmosphere and oxidation. The passive layer also prevents diffusion into the metal, preventing further corrosion of the metal. Therefore, the passive layer can be dissolved by applying a cathodic, i.e., reducing, voltage.

[0046] Therefore, a cathodic (i.e., reducing) voltage is first applied to the first auxiliary electrode (step (E)), while the second auxiliary electrode, which is the counter electrode, becomes the anodic electrode. The cathodic voltage reduces the passive layer, now in the form of a chromium oxide layer, formed on the surface of the chromium metal. In this process, the chromium oxide is converted to metallic chromium (Cr2O3 → Cr 金属 ) This cathode voltage or potential is selected to be high so that decomposition of the chromium oxide layer is achieved. The reducing direct current therefore dissolves the passive layer and trivalent chromium ions begin to migrate into solution.

[0047] For decomposition of the passive layer, the cathode voltage can be set in the range of, for example, 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 can be set in the range of, for example, 2.5 to 4 A / dm 2 , preferably 3.4A / dm 2 The current density is in the range of 4A / dm 2It has been found that it is preferable not to set the voltage higher than this value, as this will prevent the formation of undesirable hexavalent chromium in any case. Typically, the passive layer can be decomposed within about 5 to 60 seconds, preferably about 5 to 45 seconds, and more preferably about 5 to 30 seconds. However, this time may be shorter or longer in individual cases and depends on many process parameters, such as the selected pH value, current density, thickness of the deposited chromium metal layer, temperature, selected voltage, and type of auxiliary electrode used. The specified ranges are merely a guide for those skilled in the art, who can determine the optimal voltage level and duration of the cathodic current on the first auxiliary electrode for each application with a few experiments.

[0048] The method according to the invention makes it possible to decompose the passive layer on the first auxiliary electrode, which is formed on or above the chromium metal as a secondary reaction and has a passivating effect, so that trivalent chromium ions can be introduced into the solution without the formation of hexavalent chromium ions. It has been found that decomposition of the passive layer is a prerequisite for avoiding the formation of hexavalent chromium ions during the electrolytic dissolution of chromium.

[0049] Purely chemical dissolution of chromium with acid is only possible at very low pH values ​​below 0.5, which is unfavorable for the galvanic bath process conditions. In accordance with the present invention, the pH value of the electrolyte is typically in the range of 2.0 to 3.5, particularly 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. Therefore, the pH value of the electrolyte used alone is insufficient to remove the passive layer present on the chromium and initiate chemical dissolution. Therefore, dissolution of chromium metal as trivalent chromium ions is actually induced and initiated by the application of a cathodic voltage.

[0050] The dissolution of the passive layer on the first auxiliary electrode can be easily observed by the formation of bubbles on the surface of the auxiliary electrode. As soon as trivalent chromium ions dissolve in the electrolyte, hydrogen gas is evolved, which becomes visible in the form of bubbles. Simply put, the passive layer can be considered dissolved when bubbles form across the entire chromium metal surface of the first auxiliary electrode. This occurs, as already explained, after approximately 5 to 30 seconds.

[0051] Surprisingly, experiments have shown that after applying a cathodic voltage to the first auxiliary electrode as the cathode and dissolving the passive layer of metallic chromium, the dissolution reaction, which can be recognized by the generation of hydrogen bubbles, does not stop even when the voltage is turned off, but continues without current. Therefore, the dissolution of metallic chromium can be observed. Without being bound by this, it is thought that if trivalent chromium is present, divalent chromium is formed using the cathode during reduction. This should promote the dissolution of metallic chromium. At the same time, it prevents the formation of hexavalent chromium during oxidation. In other words, even if metallic chromium dissolves during electrolysis, hexavalent chromium is not produced.

[0052] Therefore, in the method according to the present invention, after the dissolution of the passive layer (step (F)), the current supply to the first auxiliary electrode is terminated, and the first and second auxiliary electrodes are no longer energized. If the cathodic voltage is maintained after the dissolution of the passive layer, hexavalent chromium ions will be formed. The formation of hexavalent chromium ions should be avoided for the reasons mentioned above. The generation of such ions can be prevented by switching off the voltage.

[0053] The chromium metal then dissolves from the first auxiliary electrode in the form of trivalent chromium ions without current flow under the action of the electrolyte (step (G)). As soon as the chromium oxide layer decomposes, the chromium is attacked by the acidic electrolyte and chemically dissolved. For this purpose, the electrolyte advantageously has an acidic pH value, for example, in the range of 2.0 to 3.5. In this pH range, the electrolyte is sufficiently acidic to dissolve the metal without current flow after the passive layer has dissolved. By applying a relatively short cathodic current, the passive layer decomposes to the extent that the acidic electrolyte can attack the chromium on the first auxiliary electrode. For electrolytes in a certain pH range, after the passive layer decomposes, the chromium metal does not repassivate, and dissolution of the chromium continues until the dissolution process is stopped or the chromium metal in the first auxiliary electrode is depleted and cannot be further replenished.

[0054] In one embodiment, the chromium metal in the first auxiliary electrode is replenished currentlessly during step (G), i.e., during dissolution of trivalent chromium ions into the electrolyte, after which steps (E), (F), and (G) are carried out consecutively in this order, preferably without intermediate steps, as disclosed. Thus, this embodiment includes the following steps: During step (G), the first auxiliary electrode is replenished with chromium metal. (E) A cathodic voltage is applied to the first auxiliary electrode, so that the passive layer of the chromium metal dissolves and the chromium metal in the form of trivalent chromium ions begins to dissolve in the electrolyte. (F) After the passive layer is dissolved, the current supply to the first auxiliary electrode is terminated or the voltage to the first auxiliary electrode is turned off. (G) Under the action of the electrolyte, chromium metal dissolves from the first auxiliary electrode in the form of trivalent chromium ions without current.

[0055] This process can be repeated as necessary, with chromium metal being replenished again and again, dissolving as trivalent chromium ions for deposition in the chromium layer.

[0056] There are several ways to terminate the process, i.e., stop the dissolution of trivalent chromium ions into the electrolyte.

[0057] According to one embodiment, an anodic voltage is applied to the first auxiliary electrode, which results in the re-formation of a passive chromium metal layer, thereby stopping the dissolution of trivalent chromium ions in the electrolyte. Therefore, the dissolution of chromium in the electrolyte can be stopped at any time by applying an anodic voltage (oxidizing potential) to the first auxiliary electrode. In response, the second auxiliary electrode (preferably in the form of an inert electrode as a counter electrode) becomes the cathode. As a result, metallic chromium initially dissolves as trivalent chromium for a short period of time. However, in parallel with the dissolution of the trivalent chromium ions, the chromium surface reacts with oxygen contained in the water, thereby re-forming the aforementioned Cr2O3 passive layer. In this process, the anodic voltage is selected so that the passive layer forms in the form of a chromium oxide layer. This phenomenon can be easily observed by the disappearance of bubbles formed on the surface of the auxiliary electrode. As soon as trivalent chromium ions cease to dissolve in the electrolyte, hydrogen gas, visible in the form of bubbles, ceases to be generated. Simply put, it can be considered that the passive layer re-forms after the bubbles disappear over the entire chromium metal surface on the first auxiliary electrode.

[0058] To rebuild the passivation layer, the anode voltage can be set in the range of, for example, 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 can be set in the range of, for example, 2.5 to 4 A / dm 2 , preferably 3.4A / dm 2 The passive layer is usually regenerated within about 5 to 60 seconds, preferably about 5 to 45 seconds, and more preferably about 5 to 30 seconds. However, this may be shorter or longer depending on the individual case, and is merely a guideline for those skilled in the art, who can determine the level of the anode voltage and the duration of the anode current flow on the first auxiliary electrode according to each application through a few experiments.

[0059] To terminate the process, the electrolyte can be drained from the electrolytic cell. Alternatively, the chromium electrode can be removed from the electrolyte and exposed to ambient air, allowing the passivation layer to reform and terminating the chemical reaction. Alternatively, the chromium electrode can be run idle by not further replenishing the chromium metal in the existing first auxiliary electrode.

[0060] That is, in this process, chromium is dissolved currentlessly and can be started in any way at a defined time using two auxiliary electrodes and a direct current, continued as many times as desired by replenishing chromium metal, and terminated again at a defined time. Thus, the method according to the invention according to this embodiment is based on the chemical dissolution of chromium in the form of trivalent chromium ions in an electrolyte, which can be activated by a defined current or a defined voltage, continued currentlessly, and terminated by any one of the methods described.

[0061] According to another embodiment, a third auxiliary electrode is provided in addition to the first and second auxiliary electrodes. The third auxiliary electrode is an inert electrode. According to one embodiment, the first auxiliary electrode in the form of a chromium electrode, the second auxiliary electrode of the second circuit, and the third auxiliary electrode are connected to each other to form one or more units. The units are configured as follows: Inert Electrode - Chrome Electrode - Inert Electrode (Second auxiliary electrode) (First auxiliary electrode) (Third auxiliary electrode) In other words, the first auxiliary electrode is surrounded by the second auxiliary electrode and the third auxiliary electrode.

[0062] The auxiliary electrodes may also connect two or more units together, in which case for two units: Inert Electrode - Chrome Electrode - Inert Electrode - Chrome Electrode - Inert Electrode

[0063] In this case, it is preferable to connect the units in series, similar to automobile batteries, which has the advantage that the units can be accommodated in a particularly space-saving manner and also has a high ability to dissolve trivalent chromium ions.

[0064] The process using three or more auxiliary electrodes is carried out in the same manner as the process using the first and second auxiliary electrodes detailed above.

[0065] To dissolve the passive layer on the chromium electrode, the surrounding inert electrode acts as the anode, and the middle chromium electrode acts as the cathode. After the passive layer dissolves, trivalent chromium ions from the chromium electrode dissolve into the electrolyte without current. To continue the process, the chromium metal from the chromium electrode can be replenished during the currentless dissolution. The passive layer can then be decomposed again, and the trivalent chromium ions can again dissolve into the electrolyte, continuing the process. In either case, the chromium metal can be replenished again just before it is used up. These steps can be repeated as many times as necessary.

[0066] To re-form a passivation layer on the chromium electrode, the inert electrode is connected as the cathode and the chromium electrode as the anode. Alternatively, the chromium electrode is withdrawn from the electrolyte or the electrolyte is removed during step (G). In either case, contact with the surrounding air re-forms a passivation layer on the chromium electrode. It is also possible not to replenish the chromium metal in the chromium electrode, in which case the supply of chromium metal is depleted.

[0067] FIG. 1 shows by means of a flow chart an embodiment of the method according to the invention.

[0068] During the electrolytic deposition of chromium at the cathode, the passive layer on the first auxiliary electrode or chromium electrodes is removed in step (E). After the passive layer dissolves, the current supply to the first auxiliary electrode is terminated or the voltage is switched off (step (F)), and trivalent chromium ions from the first auxiliary electrode dissolve into the electrolyte without current (step (G)). The process can then continue or terminate. This is depicted in the diamond-shaped section labeled "End the process?" in Figure 1. If the process is not terminated (branch "No" in Figure 1), chromium metal can be replenished to one or more chromium electrodes, if necessary, and process steps (E) to (G) can be performed again. This can be repeated as many times as necessary.

[0069] To terminate the process (branch "Yes" in Figure 1), the passive layer can be re-established by applying an anodic voltage to one or more chrome electrodes. Alternatively, the chrome electrodes can be run idle by not replenishing the chrome metal in the first auxiliary electrode. Another way to terminate the process is to drain the electrolyte from the cell or pull the chrome electrodes out of the electrolyte, exposing them to air, thereby re-establishing the passive layer and terminating the chemical reaction.

[0070] The electrolyte for the method according to the present invention is not particularly limited as long as it is suitable for electrolysis. Any electrolyte known to those skilled in the art can be used. The electrolyte contains water as a solvent. Preferably, the electrolyte has a pH value in the range of 2.0 to 3.5. In another embodiment, the electrolyte can have a pH value 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.

[0071] According to one embodiment, the electrolyte comprises: (a) One or more trivalent chromium salts (b) a compound of formula (I) JPEG2025527394000002.jpg25166 where R is NH2, OH, SO3H and / or salts thereof, in particular Na+ and / or K + or a salt with a divalent cation such as the following. n represents an integer of 1 to 3. (c) formic acid and / or its salts, in particular Na + and / or K + Salts with monovalent cations such as (d) optionally, one or more additives

[0072] Component (a) of the electrolyte according to this embodiment is one or more trivalent chromium salts. In the present invention, the term "trivalent chromium salt" is understood to mean any trivalent chromium salt capable of depositing chromium as a metal layer on an object. The trivalent chromium salt is selected from inorganic trivalent chromium salts, organic trivalent chromium salts, or mixtures thereof. Examples of inorganic trivalent chromium salts include, but are not limited to, potassium chromium alum, ammonium chromium alum, chromium sulfate, chromium (hydroxy)sulfate (alkaline chromium sulfate), chromium sulfoacetate, chromium nitrate, chromium sulfamate (chromium amidosulfonate), chromium chloride, chromium bromide, chromium iodide, chromium phosphate, chromium pyrophosphate (chromium diphosphate), chromium phosphonate, and mixtures of two or more thereof. Examples of organic trivalent chromium salts include, but are not limited to, chromium nitrate, chromium formate, chromium sulfoacetate, chromium oxalate, chromium methanesulfonate, chromium dimethanesulfonate, and mixtures of two or more thereof. Mixtures of inorganic and organic trivalent chromium salts can also be used.

[0073] For example, the amount of trivalent chromium salt is preferably selected in the range of 0.25 mol / L to 2.0 mol / L relative to the electrolyte, which range has proven to be particularly advantageous for the production of chromium layers on metal objects by electrolytic deposition.

[0074] Component (b) of the electrolyte according to this embodiment is a compound of formula (I) and / or a salt thereof. 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.

[0075] The amount of the compound of formula (I) in the electrolyte is preferably 0.5 mol / L to 1.5 mol / L relative to the electrolyte, which serves to set the pH value of the electrolyte.

[0076] Formic acid is present in the electrolyte as additional component (c) according to this embodiment and is used to remove oxygen released from the trivalent chromium salt by chemical decomposition into CO and HO. The amount of formic acid in the electrolyte is advantageously between 1.0 mol / L and 3.0 mol / L, relative to the electrolyte before chromium deposition. This range has proven particularly useful for setting the pH value of the electrolyte.

[0077] Instead of or in addition to formic acid, salts thereof can also be used, such as alkali and / or alkaline earth formates, especially sodium formate.

[0078] Component (d) of the electrolyte may optionally contain one or more additives, which may be compounds or mixtures of compounds capable of imparting advantageous properties to the galvanic bath. These compounds are known to those skilled in the art.

[0079] For example, the additive may be selected from a complexing agent, an alkali or alkaline earth salt, a wetting agent, a catalyst, or a mixture thereof.

[0080] The complexing agent is preferably a compound with a short alkyl chain (for example 1 to 5 C atoms) having one or two carboxyl groups or derivatives thereof, or one or two thio and / or sulfonic groups. The following compound is used as an example: JPEG2025527394000003.jpg25166 where R1 is C 1-5 It represents an alkyl radical, in particular CH3CH2-. X is one or more metal cations to balance the negative charge, e.g., Na + , K. + n is an integer of 1 to 5, particularly 3.

[0081] As a complexing agent, the compound N,N-dimethyl-dithiocarbamylpropylsulfonic acid sodium salt (DPS) is particularly preferred, as the use of DPS is advantageous since it gives a particularly good chromium layer.

[0082] Wetting agents reduce surface tension, allowing the formed H2 bubbles to detach from the cathode. This prevents the formation of pores in the chromium layer and results in a more uniform chromium layer. Preferred wetting agents are, for example, polyfluoromono- and / or dialkyl phosphates, PEG (polyethylene glycol) derivatives of phosphoric acid salts or esters, especially PEGylated phosphates.

[0083] To increase the conductivity, sulfates or acetosulfates, such as alkali or alkaline earth salts, especially sodium sulfate, sodium sulfoacetate, potassium sulfate or magnesium sulfate, can be used.

[0084] The amount of sulfate or acetosulfate can be 5 mM to 30 mM, for example, 10 mM to 20 mM.

[0085] The amount of additive present in the electrolyte can be between 0.01 g / L and 2.0 g / L of the electrolyte. For example, if PEG 6000 is used as a wetting agent, the concentration of the substance will be between 0.001 mMol / L and 0.3 mMol / L.

[0086] As already explained, the pH value in the electrolyte solution is preferably set in the range of 2.0 to 3.5. The pH value can be set, for example, by the compound of formula (I), formic acid and / or a salt thereof.

[0087] The electrolyte is essentially free of hexavalent chromium ions, i.e., only unavoidable impurities of hexavalent chromium ions are present in the electrolyte composition. In the method according to the invention, the content of hexavalent chromium ions is below the detection limit.

[0088] According to one embodiment, the electrolyte is preferably free of nitrogen-containing compounds, in which case the chromium layer formed is also free of nitrogen-containing compounds, resulting in a coating with particularly advantageous properties.

[0089] The subject of the invention is also an electrolytic cell for controlling the chromium supply in an electrolyte, comprising: ·anode ·cathode Electrolyte containing at least one trivalent chromium salt The anode and cathode must be immersed in the electrolyte. A first circuit that connects the anode and cathode and deposits a chromium layer by passing a direct current through the cathode to electrolytically deposit chromium from the electrolyte. A first auxiliary electrode containing or consisting of chromium metal A second auxiliary electrode in the form of an inert electrode Both auxiliary electrodes are immersed in the electrolyte. A second circuit that connects the first auxiliary electrode and the second auxiliary electrode to an electrical circuit separate from the cathode and anode.

[0090] The electrolytic cell assumes one of three states in each case: A cathodic voltage is applied to the first auxiliary electrode, dissolving the passive layer of chromium metal on the first auxiliary electrode. By not applying a voltage to the first auxiliary electrode, after the passive layer is dissolved, chromium metal from the first auxiliary electrode dissolves in the electrolyte in the form of trivalent chromium ions without current. An anodic voltage is applied to the first auxiliary electrode and a passivation layer is again formed on the first auxiliary electrode.

[0091] The vessel that can be used as the electrolytic cell can be any suitable container, vessel or tank known to those skilled in the art, particularly those commonly used in galvanic technology.

[0092] The above description of the method for controlling the chromium supply in the electrolytic process for producing the chromium layer applies equally to the electrolytic bath and will not be repeated.

[0093] The electrolytic cell may be one unit or may be divided into two. In one embodiment, the chromium plating bath can be placed in a first cell, in which a direct current flows between an anode and a cathode immersed in the electrolyte. In a second cell, which can be connected to the first cell, for example, a chromium electrode and an inert electrode, both immersed in the electrolyte, are connected to each other. Alternatively, one or more units consisting of an inert electrode, a chromium electrode, and an inert electrode can be connected to each other in the second cell. Chromium plating of the object is carried out in the first cell. In the second cell, trivalent chromium ions are accumulated in the electrolyte to the desired concentration, which can then be returned to the chromium plating bath. Other configurations are also possible.

[0094] The described process or electrolytic cell is used, inter alia, to replenish trivalent chromium used up in the electrolyte. The currentless dissolution of chromium is carried out for a time sufficient to bring the chromium content in the electrolyte to the desired level, for example, from a few minutes to a few hours.

[0095] Alternatively, the equilibrium is set up so that trivalent chromium ions are continuously supplied to the coating bath, which operates continuously. The time during which chromium dissolves as trivalent chromium ions is advantageously selected so that the concentration of trivalent chromium ions in the electrolyte is constant, in particular so that a stable equilibrium is maintained between the supply and consumption of trivalent chromium ions.

[0096] Preferably, chromium metal can be replenished during the process. The chromium molding of the first auxiliary electrode is preferably replenished during the currentless dissolution of trivalent chromium ions by the action of the electrolyte in step (G). In this state, replenishment can be carried out without any problems and therefore does not further interrupt the process. In the case of a newly replenished chromium molding, the passivation layer must first be removed again (step (E)), as already explained, before the trivalent chromium ions are again dissolved currentlessly in the electrolyte (steps (F) and (G)).

[0097] The present invention also relates to a method for maintaining a constant trivalent chromium content in the electrolyte in the process disclosed herein by comparing the weight of chromium metal used in the first auxiliary electrode with the weight of chromium metal used up for chromium layer formation, and replenishing the first auxiliary electrode with chromium metal before the trivalent chromium content in the electrolyte decreases. The trivalent chromium content in the electrolyte can be maintained constant by controlling the trivalent chromium content by measuring the weight of chromium metal present in the first auxiliary electrode compared to the chromium metal used up by coating. The weight can be measured, for example, by a pressure sensor.

[0098] By supplying metallic chromium to the electrolyte, it is possible to keep the trivalent chromium content almost constant during electrolysis, which is carried out over a long period of time, for example, from several hours to several months, and trivalent chromium ions are subsequently supplied to the electrolyte from which trivalent chromium has been depleted. Keeping the trivalent chromium content in the electrolyte constant is interpreted as meaning that the trivalent chromium content preferably varies by no more than ±10%.

[0099] Furthermore, the subject of the invention is the use of the method for producing a chromium layer on an object.

[0100] The invention also relates to the use of an electrolytic bath for producing a chromium layer on an object.

[0101] The benefits of the present invention are quite complex.

[0102] The method or electrolytic cell according to the present invention is based on a technical design that can be easily realized by a person skilled in the art. Continuous polarity reversal using expensive pulse rectifiers is not required. Rather, a simple rectifier with a polarity reversal can be used, since polarity reversal is only used at the beginning and end of the dissolution of trivalent chromium ions.

[0103] Another major advantage is that the dissolution of the trivalent chromium ions following the dissolution of the passive layer occurs without electrical current. This means that no additional energy needs to be used during the dissolution process. This is particularly important in large plants. As a result, the energy costs of the process and the electrolytic cell are significantly reduced.

[0104] Furthermore, the method according to the present invention can maintain a constant trivalent chromium content in the electrolyte. For example, the trivalent chromium content in the electrolyte can be maintained constant by correlating the weight of the chromium metal in the first auxiliary electrode with the weight of the chromium metal consumed by coating and controlling the weight accordingly. This makes it possible to maintain a nearly constant trivalent chromium content in electrolysis carried out over a long period of time, for example, several hours to several months, and trivalent chromium ions are subsequently supplied to the electrolyte from which trivalent chromium has been depleted. Maintaining a constant trivalent chromium content in the electrolyte is interpreted as meaning that the trivalent chromium content preferably varies by no more than ±10%.

[0105] In the method according to the invention, the electrolytic deposition can advantageously be carried out without the use of a semipermeable membrane. Conventionally, a semipermeable membrane has been used to separate the anode and the cathode to prevent the formation of hexavalent chromium. This is not necessary in the method according to the invention. In the method and electrolytic cell provided in the invention, the formation of hexavalent chromium during and after the electrolytic deposition of chromium is generally avoided. In the process according to the invention, hexavalent chromium is not detectable.

[0106] In this way, chrome coating can be applied simply, quickly and cost-effectively, even over long periods of time.

[0107] According to one embodiment, the electrolyte is preferably free of nitrogen-containing compounds, and the chromium layer formed thereby is also free of nitrogen-containing compounds, which results in a coating with particularly advantageous properties.

[0108] Chrome layers can be applied for decorative or technical reasons by electrolytic deposition of chromium. Examples of objects where chrome plating is used for technical reasons include rotationally symmetrical objects such as rods, pistons, and cylinders, especially gravure cylinders. A gravure cylinder or gravure roller is a printing cylinder used for gravure printing. The base cylinder is typically a tubular steel core that is first coated with copper in an electrolytic bath, and then coated with chrome after image data is applied. This process is carried out by galvanically coating the gravure cylinder with chromium.

[0109] The present invention provides chrome coatings of particularly high quality, which surprisingly meet the high demands placed on gravure cylinders. A smooth, uniform surface is obtained, essentially free of pores, depressions, and craters. The resulting chrome layers can be thicker than those typically achieved in the art. Thicknesses of 100 μm or more can be achieved. Furthermore, chrome layers with high hardness, particularly 900 HV or higher, can be produced. The resulting chrome layers are corrosion-resistant, wear-resistant, have good friction properties, heat resistance, and chemical resistance, and are also suitable for decorative purposes due to their gloss and high reflectivity.

[0110] In the following text, embodiments of the invention are described by way of example with reference to the accompanying drawings, which are drawn diagrammatically and not to scale, and therefore no assumptions can be made as to precise geometric values ​​with respect to original size. The figures of the present disclosure are part of and represent a part of the description and illustrate embodiments of the invention without being limited to the particular embodiments described. Together with the description, the drawings serve to explain the present disclosure.

[0111] 2a, 2b and 2c show an embodiment of the sequence or electrolytic cell states in the method according to the invention for the controlled supply of trivalent chromium ions by starting, continuing and terminating the dissolution of trivalent chromium ions in the context of an electrolytic process for the electrolytic deposition of a chromium layer. Figures 2a, 2b and 2c therefore show various states of the electrolytic cell illustrating the individual steps of the method according to the invention according to one embodiment.

[0112] 2a shows the electrolytic cell 10 in the form of a bath apparatus during which a chromium layer is produced by electrolytic deposition of chromium from the electrolyte 25 by means of a direct current using an anode 44 and a cathode 48. In the illustrated example, the cathode is a gravure cylinder 48, which is inserted into the bath apparatus, for example, by means of a crane (not shown). The gravure cylinder 48 is held by a bearing bridge 30 belonging to the bearing apparatus. The lateral surfaces of the gravure cylinder 48 are coated with chromium. Naturally, instead of the illustrated gravure cylinder 48, it is also possible to coat another object, in particular a rotationally symmetrical object.

[0113] The electrolytic cell 10 comprises a trough 15 containing a liquid electrolyte 25 containing water as a solvent and at least one trivalent chromium salt. In the illustrated example, 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: (a) One or more trivalent chromium salts as detailed above (b) a compound of formula (I) JPEG2025527394000004.jpg25166 where R is NH2, OH, SO3H and / or salts thereof, in particular Na + and / or K + or a salt with a divalent cation such as the following. n represents an integer of 1 to 3. (c) formic acid and / or its salts, especially salts with monovalent cations such as Na+ and / or K+, or divalent cations (d) optionally, one or more of the additives detailed above. Other electrolyte compositions are possible.

[0114] Additionally, a vertically movable anode device is provided within the trough 15, consisting essentially of an anode rail 42 and an anode basket 44, which is electrically and mechanically coupled to the anode rail 42 and functions as a metal holding device. The anode basket 44 may also be configured by combining multiple anode baskets or grids. The anode 44 represents an insoluble anode or inert electrode and may, for example, include 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 in which titanium serves as the anode substrate coated with platinum, iridium, tantalum and / or palladium oxide, titanium, niobium or tantalum sheet coated with mixed metal oxides, expanded metals consisting of titanium, tantalum or niobium, or titanium coated with iridium transition metal mixed oxides, or expanded metals consisting of titanium coated with mixed oxides, or expanded metals coated with graphite, and combinations of these materials.

[0115] For simplicity and ease of representation, only one of the bearing bridges 30 is shown in Figure 2a. For example, the two bearing bridges 30 can be moved on rails (not shown) in the axial direction of the gravure cylinder 48 by a spindle or other suitable adjustment mechanism, thereby sandwiching and holding the gravure cylinder 48 between them for rotation.

[0116] As can be seen in Figure 2a, part of the trough 15 remains freely accessible at the top by means of a bearing bridge 30 supporting one side, so that the anode rail 42 extending therein parallel to the axial direction of the gravure cylinder 48 can move freely vertically. The vertical movement of the anode rail 42 with the anode basket 48 is known to those skilled in the art and does not need to be described or illustrated in detail.

[0117] Figure 2a shows the electrolytic cell 10 at the galvanizing stage, with the gravure cylinder 48 almost completely immersed. In particular, depths of immersion of over 65% can be achieved for large cylinders (1500mm circumference) and up to about 80% for small cylinders (800mm circumference).

[0118] For galvanizing, i.e., depositing a chrome layer on the gravure cylinder 48, the anode basket 44 has already been raised to the side so that its large basket surface surrounds the immersed gravure cylinder 48.

[0119] Anode 44 and cathode 48 therefore form a first electrical circuit (not shown).

[0120] The first auxiliary electrode 54 and the second auxiliary electrode 56 are connected to each other by a second electrical circuit, which is connected independently and separately from the first electrical circuit so that there is no connection between the two electrical circuits.

[0121] The first auxiliary electrode 54 includes or consists of chromium metal and can therefore also be referred to as a "chrome electrode." This electrode is, for example, a chrome molding 54a, which is held in a holder, such as a framework or basket. The molding can have a regular or irregular shape and can be smooth or porous. For this purpose, for example, nuggets, chunks, lumps, platelets, bars, wires, and grids are suitable, rather than powders. The holder is made of a material that is resistant to the acidic electrolyte and may or may not carry a current. Conductive materials are metals, such as titanium. Non-conductive materials are plastics, such as polypropylene or polyvinyl chloride. In the illustrated exemplary embodiment, the chrome metal pieces 54a, also referred to as chrome nuggets, are housed in a plastic framework, such as a polypropylene basket.

[0122] The shape of the first auxiliary electrode 54 is not further limited, provided that it is suitable for the intended purpose, and suitable shapes are known to those skilled in the art.

[0123] The selected shape of the chromium metal part determines its surface area, the greater the surface area, the faster it will dissolve in the electrolyte, and therefore, one skilled in the art can select an appropriate shape.

[0124] The second auxiliary electrode 56 is an inert electrode, made of one or more conductive materials, and insoluble in the electrolyte. The material of the inert electrode is not particularly limited as long as it has the described properties. For example, the same material as the anode 44 can be used. The shape of the second auxiliary electrode 56 can be selected by those skilled in the art depending on the structural requirements. The second auxiliary electrode 56 can be, for example, a flat plate-like material, a plate-like material, a sintered material, or an expanded material.

[0125] In Figure 2a, the electrolytic cell 10 is depicted when a cathodic voltage is applied to the first auxiliary electrode 54 using direct current, so that the passive layer of chromium metal on the first auxiliary electrode 54 dissolves. This corresponds to step (E) of the method according to the invention. In this way, the first auxiliary electrode 54 becomes the cathode and the second auxiliary electrode 56 becomes the anode. A power supply 58 equipped with a rectifier and a polarity inverter (not shown) is used. The cathodic voltage has a reducing effect on the chromium metal nuggets 54a, and the passive layer formed on the surface of the chromium metal nuggets 54a of the first auxiliary electrode 54 begins to decompose and dissolve.

[0126] The applied cathode voltage is, for example, 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 2.5 to 4 A / dm 2 It is particularly preferably in the range of 3.4 A / dm 2The passivation layer decomposes after 5 to 60 seconds, preferably after 5 to 45 seconds, particularly preferably after 5 to 30 seconds. Depending on the electrolysis conditions selected, in particular the pH value, the temperature, the selected voltage, the current density, the thickness of the deposited chromium metal layer, and the type of auxiliary electrode used, the duration can be shorter or longer.

[0127] In FIG. 2b, the passive layer on the first auxiliary electrode 54 has already decomposed, and bubbles 55 (where trivalent chromium ions escape into the solution and generate hydrogen) have appeared all over the chromium metal surface on the first auxiliary electrode 54, indicating that the passive layer has dissolved.

[0128] In FIG. 2b, the passive layer has already dissolved, and the current supply to the first auxiliary electrode 54 and the second auxiliary electrode 56 is interrupted (step (F)). The voltage is then cut off. This is represented diagrammatically in FIG. 2b by the non-closed electric switch 59. Under the action of the electrolyte 25, chromium metal is dissolved from the first auxiliary electrode 54 in the form of trivalent chromium ions without current (step (G)). At this time, the pH value of the electrolyte 25 is in the range of 2.0 to 3.5. Therefore, the chromium metal nuggets 54a are attacked by the acidic electrolyte and chemically dissolved. This is done without current. The formed trivalent chromium ions migrate in the electrolyte 25 to the surface of the gravure cylinder 48, connected as the cathode, where they are deposited in the form of a chromium coating.

[0129] By dissolving trivalent chromium ions in the electrolyte, the chromium metal in the first auxiliary electrode (54) is depleted. The chromium metal in the first auxiliary electrode 54 can be replenished during step (G) depicted in FIG. 2b. Steps (E), (F), and (G) of the method according to the present invention are then repeated. This can continue as many times as necessary. Thus, a quasi-continuous process is established as follows: Figure 2a → Figure 2b → Refill → Figure 2a → Figure 2b → Refill → etc.

[0130] 2c, the dissolution of trivalent chromium ions into the electrolyte is stopped by applying an anodic voltage to the first auxiliary electrode 54, so that a passive layer of chromium metal is again formed on the first auxiliary electrode 54, thereby stopping the dissolution of trivalent chromium ions into the electrolyte 25. The illustrated first auxiliary electrode 54 then becomes the anode and the second auxiliary electrode 56, or inert electrode, becomes the cathode. A direct current is again applied, but with the polarity reversed.

[0131] For example, polarity reversal from FIG. 2 a (first auxiliary electrode 54 is the cathode) to FIG. 2 c (first auxiliary electrode 54 is the anode) can be achieved by a rectifier with a pole inverter connected to the power supply 58 .

[0132] In the example of FIG. 2c, the passivation layer has already been completely reformed and the bubbles 55 on the surface of the chromium metal, i.e., the chromium molding 54a, have completely disappeared since the trivalent chromium ions are no longer soluble in the electrolyte.

[0133] The applied anode voltage is, for example, 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, for example, 2.5 to 4 A / dm 2 preferably in the range of 3.4A / dm 2 The passive layer is formed after about 5 to 60 seconds, preferably after about 5 to 45 seconds, particularly preferably after about 5 to 30 seconds. The duration can be shorter or longer depending on the selected electrolysis conditions, in particular the pH value, the temperature, the selected voltage, the current density, the thickness of the deposited chromium metal layer, and the type of auxiliary electrode used. Alternatively, the process can be terminated during step (G) by withdrawing the first auxiliary electrode 54 from the electrolyte 25 or by draining the electrolyte 25 from the electrolytic cell 10 (not shown), so that the first auxiliary electrode 54, and in particular the chromium molding 54a, comes into contact with the ambient air, resulting in the establishment of a passivation layer. Alternatively, the chromium metal of the first auxiliary electrode 54 is not further replenished, so that the dissolution of chromium metal in the form of trivalent chromium ions from the first auxiliary electrode 54, currentlessly by the action of the electrolyte, is terminated as soon as the chromium metal present goes completely into solution.

[0134] No production of hexavalent chromium ions was detected during the process.

[0135] 3a-3c schematically illustrate another embodiment of the method according to the present invention, in which a third auxiliary electrode is present in addition to the first and second auxiliary electrodes. Like the second auxiliary electrode, the third auxiliary electrode is an inert electrode. An interconnected unit of auxiliary electrodes is available, which can, for example, replace the circuit consisting of the first auxiliary electrode 54 and the second auxiliary electrode 56 of FIG. 2a. FIG. 3a shows two inert electrodes 56.1 and 56.2, also referred to herein as the second and third auxiliary electrodes. These substantially surround the chromium electrode 54, also referred to herein as the first auxiliary electrode. In the illustrated embodiment, an anodic voltage is applied to the inert electrodes 56.1 and 56.2, and a cathodic voltage is applied to the chromium electrode 54. This is step (E), in which the passivation layer is dissolved.

[0136] FIG. 3b shows an embodiment according to FIG. 3a, but no voltage is applied to the auxiliary electrode (step (F)), and in FIG. 3c an anodic voltage is applied to the first auxiliary electrode (end of the process during step (G)).

[0137] According to another variant of the invention, the embodiments with a third auxiliary electrode (FIGS. 3a, 3b and 3c) can also be replaced by the second circuits of FIGS. 2a, 2b and 2c, respectively.

[0138] Figures 4a, 4b, and 4c show a schematic representation of another embodiment of the method according to the invention, in which a series connection of electrodes is shown. Inert electrodes 56.1, 56.2, 56.3, and 56.4 are shown, alternately connected with chrome electrodes 54.1, 54.2, and 54.3, respectively. Inert electrodes 56.1 and 56.4 are arranged around the edges of the electrodes and represent end electrodes. Auxiliary electrodes may be interconnected, e.g., to replace the second circuit consisting of first auxiliary electrode 54 and second auxiliary electrode 56 in Figures 2a, 2b, and 2c, respectively.

[0139] In the embodiment depicted in Figure 4a, an anodic voltage is applied to each of the inert electrodes 56.1, 56.2, 56.3, 56.4, and a cathodic voltage is applied to each of the chromium electrodes 54.1, 54.2, 54.3. This is step (E) and dissolves the passivation layer.

[0140] Figure 4b shows an embodiment according to Figure 4a, but no voltage is applied to the auxiliary electrode (step (F)), while in Figure 4c an anodic voltage is applied to the first auxiliary electrode in both cases (end of the process during step (G)).

[0141] Figures 5a, 5b and 5c show another embodiment of the present invention, which is a sequence of steps or electrolytic cell states of the method according to the present invention for the controlled supply of trivalent chromium ions by starting, continuing and terminating the dissolution of trivalent chromium ions in the context of an electrolytic process for the electrolytic deposition of a chromium layer.

[0142] In Figure 5a, the trough of the electrolytic cell 100 in the form of a bath apparatus is divided into an upper trough 110 and a lower trough 120 located below it. A liquid electrolyte 125 is placed in the upper trough 110 and the lower trough 120, is pumped from the lower trough 120 to the upper trough 110 by a pump 160, and flows back into the lower trough 120 via an overflow 127 that is vertically movable in at least two positions. Alternatively, two overflows that can be alternately opened and closed can be placed at different heights.

[0143] As previously described with respect to Figures 2a, 2b, and 2c, a vertically movable anode apparatus is disposed within the upper trough 110, consisting essentially of an anode rail 142 and an anode basket 144 that is electrically and mechanically coupled to the anode rail 142 and functions as a metal retention device. The anode basket 144 may also be comprised of a combination of multiple anode baskets or grids. The anode basket 144 is part of an insoluble anode.

[0144] The cathode 148, here a gravure cylinder, is held by two bearing bridges 130 (only one shown) between which the gravure cylinder 148 is sandwiched and rotatably held so that it can be moved on the rail in the axial direction of the gravure cylinder 148 by a suitable adjustment mechanism. The upper half of the upper trough 110 is therefore freely accessible, allowing the anode rail 142 to be moved vertically.

[0145] The degree of filling of the electrolyte 125 in the upper trough 110, i.e., the level of the electrolyte 125, can be appropriately adjusted using a vertically movable overflow 127.

[0146] 5a shows the electrolytic cell 100 at the galvanizing stage with the gravure cylinder 148 almost completely immersed. To galvanize, i.e., to deposit a chrome layer on the gravure cylinder 148, the anode basket 144 has already been raised to the side so that its large basket surface surrounds the immersed gravure cylinder 148.

[0147] The anode 144 and the cathode in the form of the gravure cylinder 148 thus form a first electrical circuit (not shown). In a second electrical circuit, which operates independently of the first electrical circuit, a first auxiliary electrode 154 and a second auxiliary electrode 156 are connected to each other within the lower trough 120.

[0148] The first auxiliary electrode 154 includes or consists of chromium metal and is also referred to herein as the "chrome electrode." The structures of the chrome electrode 154 and the inert electrode 156 are as previously described in Figure 2a (where they are labeled chrome electrode 54 and inert electrode 56).

[0149] In the electrolytic cell 100 depicted in FIG. 5a, a cathodic voltage is first applied to the first auxiliary electrode 154 using a direct current so as to dissolve the passive layer of chromium metal on the first auxiliary electrode 154 (step (E)). Thus, the first auxiliary electrode 154 serves as the cathode, and the second auxiliary electrode 156 serves as the anode. For example, a rectifier having a polarity inverter (not shown) connected to a power supply 158 is used. The cathodic voltage has a reducing effect on the chromium metal nuggets 154a, and the passive layer formed on the surface of the chromium metal nuggets 154a of the first auxiliary electrode 154 begins to decompose.

[0150] The applied cathode voltage is, for example, 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, for example, 2.5 to 4 A / dm 2 preferably in the range of 3.4A / dm 2 The passivation layer decomposes after about 5 to 60 seconds, preferably after about 5 to 45 seconds, particularly preferably after about 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 deposited chromium metal layer, and the type of auxiliary electrode used, shorter or longer durations may be possible.

[0151] In Figure 5b, the disappearance of the passive layer on the first auxiliary electrode 154 is indicated by the appearance of bubbles 155 due to the production of hydrogen by the trivalent chromium ions that have gone into solution. The bubbles 155 are present across the entire surface of the chromium metal on the first auxiliary electrode 154 when the passive layer has completely dissolved.

[0152] As soon as the passivation layer has dissolved, the current supply to the first auxiliary electrode 154 and the second auxiliary electrode 156 is interrupted (step (F)). This is represented diagrammatically in Figure 5b by an electric switch 159 which interrupts the current.

[0153] Hereinafter, dissolution of chromium metal from the first auxiliary electrode 154 in the form of trivalent chromium ions occurs without current flow due to the action of the electrolyte 125 in the lower trough 120. The electrolyte 125 has a pH value, for example, in the range of 2.0 to 3.5 (step (G)). The chromium metal nuggets 154a are attacked by the acidic electrolyte and chemically dissolved. This occurs without supplying current to the auxiliary electrodes 154, 156. The formed trivalent chromium ions are dispersed in the electrolyte 125, which is pumped from the lower trough 120 to the upper trough 110 by the pump 160 and then flows back into the lower trough 120 via the overflow 127, which is vertically movable in at least two positions. The trivalent chromium ions migrate through the electrolyte 125 to the surface of the gravure cylinder 144, connected as a cathode, and form a chromium coating thereon.

[0154] To reduce the amount of electrolyte in the upper trough 110, the upper trough 110 is tapered, for example, in its lower region. This tapering can be achieved by using an additional sheet plate 133 or by adapting the walls of the upper trough 110 accordingly. Blocks or boxes can also be used to change the volume. Restricting or reducing the volume of the upper trough 110 has the advantage that more electrolyte 125 than necessary does not need to be pumped upward from the lower trough 120. Therefore, there is no risk of the lower trough 120 becoming completely empty and causing the pump 160 to run dry.

[0155] If desired, the chromium metal of the first auxiliary electrode 154 can be replenished during step (G) according to Figure 5b, and the process can continue accordingly, repeating steps (E), (F) and (G), giving the following process sequence: Figure 5a → Figure 5b → Refill → Figure 5a → Figure 5b → Refill → ......

[0156] When the supply of trivalent chromium ions to the electrolyte is discontinued, an anodic voltage is applied to the first auxiliary electrode 154, which results in the re-formation of a passive layer of chromium metal on the first auxiliary electrode 154 and the cessation of dissolution of trivalent chromium ions into the electrolyte. This is shown in Figure 5c. The first auxiliary electrode 154 becomes the anode and the second auxiliary electrode 156, or inert electrode, becomes the cathode. A direct current is again applied, but with the polarity reversed.

[0157] The applied anode voltage is, for example, 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, for example, 2.5 to 4 A / dm 2 , preferably 3.4A / dm 2 The passivation layer is re-established after about 5 to 60 seconds, preferably about 5 to 45 seconds, particularly preferably about 5 to 30 seconds. Depending on the electrolysis conditions selected, in particular the pH value, the temperature, the selected voltage, the current density, the thickness of the deposited chromium metal layer, and the type of auxiliary electrode used, the duration can be shorter or longer.

[0158] For example, polarity reversal (first auxiliary electrode 154 becomes the cathode, then the anode) can be achieved by a rectifier with a polarity inverter connected to power supply 158 .

[0159] Once the passive layer has completely formed again, hydrogen gas is no longer formed and trivalent chromium ions are no longer dissolved in the electrolyte 125, so that no gas bubbles 155 are observed on the chromium metal surface of the first auxiliary electrode 154.

[0160] Alternatively, to terminate the process, the chromium metal in the first auxiliary electrode 154 may not be replenished, so that as soon as the chromium metal present goes completely into solution, the dissolution of chromium metal in the form of trivalent chromium ions from the first auxiliary electrode 154 ceases currentlessly under the action of the electrolyte 125. Alternatively, the process may be terminated by withdrawing the first auxiliary electrode 154 from the electrolyte 125 or draining the electrolyte 125 from the electrolytic cell 100 (not shown).

[0161] No production of hexavalent chromium ions was detected throughout the entire process.

[0162] After the galvanizing step is completed, the anode rail 142 with the anode basket 144 is moved downward into the upper trough 110. Simultaneously, or after a time delay, the overflow 127 is lowered so that the electrolyte 125 flows down to a suitable level in the lower trough 120. In this way, it is possible to achieve a state in which the anode basket 144 is still fully covered by the electrolyte 125, while the gravure cylinder 148 is positioned completely free above the level of the electrolyte 125 and can be easily lifted therefrom using a crane (not shown).

[0163] Figures 6a, 6b and 6c show a schematic representation of another embodiment of the present invention. In contrast to the embodiment of Figures 5a, 5b and 5c, there is no lower or upper trough, but rather a first trough 210 and a second trough 220 arranged adjacent to each other. Both troughs are connected to each other by a line with a pump 260, by which the electrolyte 225 can be pumped from the first trough 210 to the second trough 220 and back again to the first trough via an overflow (not shown). The rest of the operation corresponds to Figures 5a, 5b and 5c, so that their description need not be repeated here.

[0164] Alternatively, in Figures 5a to 5c and 6a to 6c, a third auxiliary electrode (not shown) can be additionally provided as an inert electrode, and as depicted in Figures 3a to 3c, the second auxiliary electrode (56, 156, 256) and the third auxiliary electrode are arranged so that the chromium electrode (54, 154, 254) is located between them.

[0165] According to another embodiment, several of these units consisting of inert electrode-chromium electrode-inert electrode can be connected in series as depicted in Figures 4a to 4c and replaced by the second circuits of Figures 5a to 5c and Figures 6a to 6c, respectively.

[0166] The method and electrolytic bath according to the invention can therefore be used in a particularly advantageous manner to obtain chromium layers with desired properties, in particular on gravure cylinders.

[0167] The following examples are intended to further illustrate the present invention and should not be construed as limiting the invention in any way. [Example]

[0168] Example 1 An exemplary embodiment of the method according to the present invention comprising steps (A) to (G) was carried out as follows.

[0169] An electrolyte solution having the following composition was prepared: ·Trivalent chromium sulfate (density=1.26g / mL; 3%Cr(III)->37.8g / L->0.727M) 18.6L ·Sodium sulfoacetate (8.3wt.%->104.6g / L->0.568M)6.27kg Sodium formate (8wt%->100.8g / L->1.482M) 6.05kg ·Sodium sulfate (1.7wt%->21.4g / L->17mM)1.29kg

[0170] One liter of electrolyte was placed in a beaker, heated to 40°C while stirring, and adjusted to a pH of 2.6. Two electrodes were then inserted into the beaker, parallel and facing each other with a 10 cm gap, and connected to a DC power source. One electrode was a mixed oxide-coated (MMO) titanium expanded metal, which was used as the anode. The other electrode was chromium, which was used as the cathode. The surfaces of the anode and cathode were connected to each other with a current of 3 A, resulting in an operating current density of approximately 4 A / dm. 2 The anode surface was chosen to be the same size as the cathode surface.

[0171] A cathodic voltage was applied to dissolve the passive layer on the chromium electrode (first auxiliary electrode according to the invention), the other electrode being an inert electrode and functioning as an anode.

[0172] In this example, after a short time of about 20 seconds, gas was generated on the chromium electrode, and gas bubbles were observed rising to the surface. This indicates that metallic chromium is dissolving in the form of trivalent chromium ions. After confirming the start of the dissolution process, the current flow was stopped, in this case after 60 seconds, by removing the DC current from the anode and cathode. In this case, the dissolution process did not stop and continued in the same manner.

[0173] During the experiment, 1.0 mL samples were taken at 0, 2, 4, and 6 hours, and the chromium content was measured. The results showed that the amount of trivalent chromium increased linearly during the observation period. This was also confirmed by gravimetric analysis of the chromium electrode. The only type of chromium dissolved was trivalent chromium. No hexavalent chromium was detected.

[0174] The currentless dissolution was then stopped by removing the chromium electrode from the electrolyte. Alternatively, the electrolyte could be removed from the beaker. This would allow the chromium electrode to undergo surface oxidation in air, again forming a passivation layer. After the passivation layer had formed, reinserting the chromium electrode into the electrolyte (e.g., after a 30-second dwell time in air) would not initiate the dissolution process.

[0175] Instead of removing the chromium electrode from the electrolyte, the currentless dissolution was stopped by changing the polarity: the chromium electrode was charged positively for a short period of time until hydrogen production on the electrode stopped. Hydrogen production stopped after 60 seconds. After visible gas evolution ceased, the current was turned off, permanently stopping the dissolution of chromium.

[0176] Example 2 pH fluctuations Example 1 was repeated, but with different pH values. Specifically, using the same setup as in Example 1, the pH value was gradually decreased from 3.1 to 2.8, 2.6, and 2.4. In another experiment, the pH value was increased from 3.1 to 3.3 and 3.5. In each case, the same results as in Example 1 were obtained. However, the dissolution rate of chromium was slower at higher pH values. [Explanation of symbols]

[0177] 10, 100, 200...electrolytic cell 15 Trough 25, 125, 225...electrolyte 30, 130, 230... Bearing bridge 42, 142, 242... Anode rail 44, 144, 244... Anode, anode basket 48, 148, 248···Cathode, gravure cylinder 54, 54.1, 54.2, 54.3, 154, 254... First auxiliary electrode, chromium electrode 54a, 154a, 254a Chrome moldings 55, 55.1, 55.2, 55.3, 155, 255... Hydrogen bubbles 56, 56.1, 156, 256...Second auxiliary electrode 56.2...Third auxiliary electrode 56.3, 56.4...Auxiliary electrode 58, 158, 258...Power supply 59, 159, 259... Switch 110 Upper Trough 120 Lower Trough 127...overflow 160, 260 pump 210 First Trough 220...Second trough

Claims

1. 1. A method for controlling the chromium supply in an electrolytic process for producing a chromium layer, said chromium layer being produced by electrolytic deposition of chromium from an electrolyte (25, 125, 225) by direct current and using an anode (44, 144, 244) and a cathode (48, 148, 248), comprising the steps of: (A) A first auxiliary electrode (54, 154, 254) containing or made of chromium metal is prepared. (B) Providing a second auxiliary electrode (56, 156, 256) in the form of an inert electrode. (C) Immersing both auxiliary electrodes (54, 56, 154, 156, 254, 256) in the electrolyte (25, 125, 225) containing at least one trivalent chromium salt. (D) connecting the first auxiliary electrode (54, 154, 254) and the second auxiliary electrode (56, 156, 256) to each other in an electrical circuit separate from the cathode (48, 148, 248) and the anode (44, 144, 244), and forming the chromium layer during the electrolytic deposition of chromium. (E) A cathodic voltage is applied to the first auxiliary electrode (54, 154, 254), so that the passive layer of the chromium metal dissolves and chromium metal in the form of trivalent chromium ions begins to dissolve in the electrolyte (25, 125, 225). (F) After the passive layer is dissolved, the supply of current to the first auxiliary electrode (54, 154, 254) is terminated. (G) Due to the action of the electrolyte (25, 125, 225), the chromium metal is dissolved from the first auxiliary electrode (54, 154, 254) without current in the form of trivalent chromium ions.

2. 2. The method of claim 1, wherein the chromium metal in the first auxiliary electrode (54, 154, 254) is replenished during step (G), and steps (E), (F), and (G) of claim 1 are thereafter performed consecutively in that order.

3. To terminate the process during step (G), an anodic voltage is applied to the first auxiliary electrode (54, 154, 254); or The first auxiliary electrode (54, 154, 254) is withdrawn from the electrolyte (25, 125, 225); or The electrolyte is drained from the electrolytic cell (10, 100, 200), or 3. The method of claim 1 or 2, characterized in that the chromium metal of the first auxiliary electrode (54, 154, 254) is not replenished.

4. 4. The method according to claim 1, wherein the pH value of the electrolyte (25, 125, 225) is set 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.

5. 5. The method according to claim 1, wherein one or more of the following conditions are met: The cathode voltage in step (E) is set 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 anode voltage for completing the process during step (G) is set 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 surface area of ​​the first auxiliary electrode (54, 154, 254) is selected to be the same as the surface area of ​​the second auxiliary electrode (56, 156, 256). - Current density is 2.5 to 4 A / dm ２ and preferably in the range of 3.4 A / dm ２ is. The passivation layer is decomposed within 5 to 60 seconds, preferably 5 to 45 seconds, and particularly preferably 5 to 30 seconds, by applying the cathodic voltage to the first auxiliary electrode (54, 154, 254) in step (E). To complete the process during step (G), the passivation layer is formed again by applying the anodic voltage to the first auxiliary electrode (54, 154, 254) within 5 to 60 seconds, preferably 5 to 45 seconds, particularly preferably 5 to 30 seconds. The chromium metal of the first auxiliary electrode (54, 154, 254) is provided as a chromium molding selected from nuggets, chunks, lumps, platelets, bars, wires, and grids held in a material resistant to the acidic electrolyte (25, 125, 225).

6. A third auxiliary electrode in the form of an inert electrode is provided, and said first auxiliary electrode (54, 154, 254), said second auxiliary electrode (56, 56.1, 156, 256) and said third auxiliary electrode (56b) in the form of a chromium electrode are connected to one another to form one or more units, one unit being Inert electrode (56.1) - chromium electrode (54) - inert electrode (56.2) 6. The method according to claim 1, wherein the number of inputs is selected from the group consisting of:

7. 7. The method according to any one of claims 1 to 6, characterized in that the components of the electrolyte (25, 125, 225) are selected from the following: (a) one or more trivalent chromium salts selected from inorganic and / or organic trivalent chromium salts; (b) a compound of formula (I) where R is NH ２ , OH, SO ３ H and / or its salts, especially Na ＋ and / or K ＋ or a salt with a divalent cation such as the following: where n represents an integer of 1 to 3. (c) formic acid and / or its salts, especially Na ＋ and / or K ＋ Salts with monovalent cations such as (d) optionally one or more additives selected in particular from complexing agents, alkali or alkaline earth salts, wetting agents, catalysts or mixtures thereof;

8. 8. The method according to claim 1, wherein the trivalent chromium content in the electrolyte (25, 125, 225) is kept constant by comparing the weight of the chromium metal used in the first auxiliary electrode (54, 154, 254) with the weight of the chromium metal used for the chromium layer, and the chromium metal in the first auxiliary electrode (54, 154, 254) is replenished before the trivalent chromium 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 trivalent chromium salt; The anode (44, 144, 244) and the cathode (48, 148, 248) are immersed in the electrolyte (25, 125, 225); a first circuit connecting the anode (44, 144, 244) and the cathode (48, 148, 248) and passing a direct current through the cathode (48, 148, 248) to electrolytically deposit chromium from the electrolyte (25, 125, 225) to deposit a chromium layer; a first auxiliary electrode (54, 154, 254) containing or consisting of chromium metal; a second auxiliary electrode (56, 156, 256) in the form of an inert electrode; Both of the auxiliary electrodes (54, 56, 154, 156, 254, 256) are 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) to an electrical circuit separate from the cathode (48, 148, 248) and the anode (44, 144, 244); A cathodic voltage is applied to the first auxiliary electrode (54, 154, 254) to dissolve the passive layer of chromium metal on the first auxiliary electrode (54, 154, 254); or no voltage is applied to the first auxiliary electrode (54, 154, 254), and after dissolution of the passivation layer, chromium metal from the first auxiliary electrode (54, 154, 254) dissolves in the electrolyte (25, 125, 225) in the form of trivalent chromium ions without current, or an anodic voltage is applied to said first auxiliary electrode (54, 154, 254) and said passivation layer is formed again on said first auxiliary electrode (54, 154, 254).

10. The electrolytic cell (10, 100, 200) Trough (15), or Two troughs arranged one above the other, in particular an upper trough (110) and a lower trough (120), or Electrolyzer (10, 100, 200) according to claim 9, characterized in that it comprises two troughs arranged adjacent to each other, in particular a first trough (210) and a second trough (220).

11. Electrolytic cell (10, 100, 200) according to claim 9 or 10, characterized in that it satisfies one or more of the following characteristics: The pH value of the electrolyte (25, 125, 225) is set in the range of 2.0 to 3.5, particularly 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 cathode voltage of the first auxiliary electrode (54, 154, 254) is set 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, or the anode voltage of the first auxiliary electrode (54, 154, 254) is set 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 cathodic voltage on the first auxiliary 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 on the first auxiliary 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 auxiliary electrode (54, 154, 254) is selected to be the same as the surface area of ​​the second auxiliary electrode (56, 156, 256). - Current density is 2.5 to 4 A / dm ２ and preferably in the range of 3.4 A / dm ２ is. The chromium metal of the first auxiliary electrode (54, 154, 254) is provided as a chromium molding selected from nuggets, chunks, lumps, platelets, bars, wires, and grids held in a material resistant to the acidic electrolyte (25, 125, 225). The components of the electrolyte (25, 125, 225) are selected from the following: (a) one or more trivalent chromium salts selected from inorganic and / or organic trivalent chromium salts; (b) a compound of formula (I) where R is NH ２ , OH or SO ３ H and / or salts thereof, in particular Na ＋ and / or K ＋ or a salt with a divalent cation such as the following: where n represents an integer of 1 to 3. (c) formic acid and / or its salts, especially Na ＋ and / or K ＋ Salts with monovalent cations such as (d) optionally one or more additives selected in particular from complexing agents, alkali or alkaline earth salts, wetting agents, catalysts or mixtures thereof;

12. A third auxiliary electrode in the form of an inert electrode is provided, and said first auxiliary electrode (54, 154, 254), said second auxiliary electrode (56, 56.1, 156, 256) and said third auxiliary electrode (56b) in the form of a chromium electrode are connected to one another to form one or more units, one unit being Inert electrode (56.1) - chromium electrode (54) - inert electrode (56.2) Electrolyzer (10, 100, 200) according to any one of claims 9 to 11, characterized in that the electrolytic cell (10, 100, 200) is selected from the group consisting of:

13. 13. Use of an electrolytic cell (10, 100, 200) according to any one of claims 9 to 12 for the production of chromium layers on rotationally symmetrical parts, in particular gravure cylinders (48, 148, 248).

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