A method for controlling the supply of chromium in an electrolytic process for producing a chromium layer, and an 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 cell for the same purpose. [Background technology]

[0002] Galvanic treatment for coating the surface of an object has long been known in the art. The coated object obtained in this way possesses particularly advantageous surface properties, such as greater hardness, improved corrosion resistance, metallic appearance, and luster. A galvanic bath containing the metal to be deposited as a salt in a solution is used, and the metal is deposited onto the object connected as a cathode by a direct current. Therefore, the object to be coated is usually a metallic material, or its surface is metallized to become conductive.

[0003] One metal used for this purpose is chromium. A chromium coating using a galvanic bath forms a bright, highly reflective chromium layer that can be used for decorative purposes. Furthermore, chromium plating can be applied to objects for technical applications such as improved wear resistance, heat resistance, and corrosion resistance. For example, pistons, cylinders, cylinder liners, and axle bearings are often chromium-plated.

[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, it generates chromic acid mist, which is acidic, corrosive, and somewhat toxic, due to the production of gases, particularly hydrogen and, to a lesser extent, oxygen. Therefore, it is necessary to use surfactants or wetting agents to intensively extract chromic acid from the surface of the galvanic bath and contain the generated chromic acid mist. Furthermore, hexavalent chromium electrolytes are highly toxic and carcinogenic. Therefore, it is preferable to use a non-toxic galvanic bath containing trivalent chromium salts.

[0005] In the case of galvanic chromium plating using trivalent chromium salts, the system design must not be overly complex so that the chromium layer is deposited to an appropriate thickness and is suitable for industrial use.

[0006] The precipitation of chromium from trivalent chromium salts present in the electrolyte leads to a decrease in the concentration of trivalent chromium ions 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 occurs in the electrolyte. Therefore, periodic dilution and subsequent re-dosing of other components are necessary, requiring constant control and monitoring of the system.

[0007] Furthermore, it is not possible to apply an anode voltage or anode potential to a chromium metal anode. This is because chromium is a highly reactive metal and quickly forms an oxide film on its surface, passing it away. Even if an anode voltage is applied to this passedivated chromium, only a small amount of chromium dissolves. However, if the anode voltage is increased, chromium dissolves as hexavalent chromium ions. Hexavalent chromium is not only carcinogenic, but it also interferes undesirably with the trivalent chromium electrolyte, impairing its function. Therefore, it is extremely difficult to continuously supply chromium as trivalent chromium to the electrolyte.

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

[0009] EP2640873A1 (WO2012 / 067725A1) describes a method for supplementing or increasing the chromium content in an electrolyte with trivalent chromium, and this method includes the following steps: a) Immerse the chromium-containing electrode and the second electrode in an electrolyte solution containing trivalent chromium ions. b) A pulsed alternating 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, and the trivalent chromium content in the electrolyte in which the chromium electrode is immersed is replenished or accumulated. The duration of each forward pulse and each reverse pulse is usually between approximately 0.1 seconds and approximately 2 seconds. For example, a rectangular waveform can be used as the waveform, with AC pulse durations of approximately 400 ms for the cathode forward pulse and approximately 400 ms for the anode reverse pulse. The drawback of this proposal is that it is a complex technique and requires the use of an expensive pulse rectifier for continuous polarity reversal.

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

[0011] [Patent Document 1] EP2640873A1 [Patent Document 2] GB414939 [Overview of the Initiative] [Problems that the invention aims to solve]

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

[0013] (Brief Description of the Invention) The object described is achieved according to the present invention by the teachings of the independent claims. The teachings of the dependent claims represent advantageous embodiments.

[0014] In particular, a method is provided for controlling the chromium supply in an electrolysis process for manufacturing a chromium layer, the chromium layer being manufactured by electrolytic deposition of chromium from an electrolytic solution by direct current using an anode and a cathode, and including the following steps. (A) Prepare a first auxiliary electrode containing or consisting of chromium metal. (B) Prepare a second auxiliary electrode in the form of an inert electrode. (C) Immerse both auxiliary electrodes in an electrolytic solution containing at least one trivalent chromium salt. (D) In an electrical circuit separate from the cathode and the anode, connect the first auxiliary electrode and the second auxiliary electrode to each other, and a chromium layer is formed during the electrolytic deposition of chromium. (E) Apply a cathode voltage to the first auxiliary electrode, as a result, the passive layer of chromium metal dissolves, and chromium metal in the form of trivalent chromium ions begins to dissolve in the electrolytic solution. (F) After the dissolution of the passive layer, terminate the current supply to the first auxiliary electrode or switch off the voltage of the first auxiliary electrode. (G) By the action of the electrolytic solution, chromium metal dissolves from the first auxiliary electrode without current in the form of trivalent chromium ions.

[0015] The process can also be terminated again in a targeted manner, for example, when the coating object is completely covered. According to an embodiment of the present invention, the process can be terminated as follows during step (G). · Apply an anode voltage to the first auxiliary electrode. · Withdraw the first auxiliary electrode from the electrolytic solution or drain the electrolytic solution from the electrolytic cell so that the first auxiliary electrode comes into contact with the surrounding air. · Without replenishing the chromium metal of the first auxiliary electrode, the dissolution of chromium metal from the first auxiliary electrode without current in the form of trivalent chromium ions ends by the action of the electrolytic solution when the chromium is completely dissolved.

[0016] The subject of the present invention is also an electrolytic cell for controlling the supply of chromium to an electrolyte, which includes the following: ·anode ·cathode • Electrolyte containing at least one type of trivalent chromium salt • The anode and cathode must be immersed in the electrolyte. The first circuit connects the anode and cathode, and by passing a direct current through the cathode, chromium is electrolytically extracted from the electrolyte, thereby depositing a chromium layer. • First auxiliary electrode containing or made of chromium metal • Second auxiliary electrode in the form of an inert electrode Both auxiliary electrodes are immersed in the electrolyte. • The second circuit connecting the first auxiliary electrode and the second auxiliary electrode to an electrical circuit separate from the cathode and anode is one of the following: A cathode voltage is applied to the first auxiliary electrode, causing the passive layer of chromium metal on the first auxiliary electrode to dissolve. Without applying voltage to the first auxiliary electrode, after the passivation layer dissolves, the chromium metal dissolves from the first auxiliary electrode into the electrolyte in the form of trivalent chromium ions without any current. A positive voltage is applied to the first auxiliary electrode, and a passivation layer is formed again on the first auxiliary electrode.

[0017] Therefore, the method and electrolytic cell according to the present invention are based on the chemical dissolution of chromium in the form of trivalent chromium ions in an electrolyte, which is activated by the application of electricity, continues without electricity, and can be terminated again by applying electricity or by other means as desired. [Brief explanation of the drawing]

[0018] The matters described, as well as other embodiments, advantages, and features of the present invention, are further described in the following paragraphs with reference to the accompanying drawings.

[0019] [Figure 1] This is a flowchart showing one embodiment of the method according to the present invention. [Figure 2a]This diagram shows a schematic representation of one embodiment of the electrolytic cell according to the present invention during the zinc plating stage, with a cathode voltage applied to the first auxiliary electrode. [Figure 2b] This is a schematic diagram of the same embodiment as Figure 2a, where no voltage is applied to the first auxiliary electrode. [Figure 2c] This is a schematic diagram of the same embodiment as Figure 2a, where the anode voltage is applied to the first auxiliary electrode. [Figure 3a] This is a schematic diagram showing one embodiment of the 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] This is a schematic diagram of the same embodiment as Figure 3a, but with no voltage applied to the auxiliary electrode. [Figure 3c] This is a schematic diagram of the same embodiment as Figure 3a, where the anode voltage is applied to the first auxiliary electrode. [Figure 4a] This is a schematic diagram of one embodiment of a circuit according to the present invention, which includes auxiliary electrode units connected to each other to illustrate a series connection, with a cathode voltage applied to the first auxiliary electrode. [Figure 4b] This is a schematic diagram of the same embodiment as Figure 4a, but with no voltage applied to the auxiliary electrode. [Figure 4c] This is a schematic diagram of the same embodiment as Figure 4a, where the anode voltage is applied to the first auxiliary electrode. [Figure 5a] This is a schematic diagram showing another embodiment of the electrolytic cell according to the present invention during the zinc plating stage, in which a cathode voltage is applied to the first auxiliary electrode. [Figure 5b] This is a schematic diagram of the same embodiment as Figure 5a, but with no voltage applied to the auxiliary electrode. [Figure 5c] This is a schematic diagram of the same embodiment as Figure 5a, where the anode voltage is applied to the first auxiliary electrode. [Figure 6a] This is a schematic diagram showing another embodiment of the electrolytic cell according to the present invention during the zinc plating stage, in which a cathode voltage is applied to the first auxiliary electrode. [Figure 6b] This is a schematic diagram of the same embodiment as Figure 6a, but with no voltage applied to the auxiliary electrode. [Figure 6c]This is a schematic diagram of the same embodiment as Figure 6a, where the anode voltage is applied to the first auxiliary electrode. [Modes for carrying out the invention]

[0020] (Terms and Definitions) The term "electrolytic process" or "electrolysis" is interpreted as using an electric current to deposit a metal, in this case chromium, from a solution containing the corresponding metal ions. This forms a metal layer (in this case, chromium plating).

[0021] The terms “cathode voltage” or “cathode potential” are used synonymously and interchangeably in this invention and are interpreted to mean that a voltage is applied to the 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 this invention, and are interpreted to mean that a voltage is applied to the electrode or auxiliary electrode so that the electrode or auxiliary electrode functions as an anode.

[0023] (Detailed description of the invention) The method according to the present invention for controlling chromium supply is carried out during an electrolytic process for producing a chromium layer. Therefore, the electrolytic process for producing a chromium layer will be described first.

[0024] Electrolytic extraction of chromium layers is typically carried out in an electrolytic cell filled with electrolyte. The electrolytic cell container can be any container suitable for those skilled in the art, particularly those used in galvanic technology.

[0025] The cathode typically serves as an object from which a chromium layer is deposited, such as a gravure cylinder.

[0026] As the anode, any material known to those skilled in the art can be used. In particular, an inert electrode is used as the anode, and this inert electrode is composed of one or more conductive materials that are insoluble in the electrolyte. Examples of materials used for the insoluble anode or inert electrode include the following: Platinum-plated titanium • Expanded titanium metal, coated with mixed oxide or graphite as needed. • Carbon materials such as graphite Titanium coated with indium and / or tantalum • Mixed metal oxides, particularly iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide, or iridium-tantalum mixed oxide • A mixed metal oxide in which titanium acts as an anode substrate coated with platinum, iridium, tantalum, and palladium oxides. • Titanium, niobium, or tantalum sheets coated with mixed metal oxides • Titanium, tantalum, or niobium coated with iridium transition metal mixed oxides and combinations of these materials

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

[0028] The anode and cathode are immersed in an electrolyte. Any electrolyte known to those skilled in galvanic technology can be used. When a DC voltage is applied to the two electrodes, the anode and cathode, trivalent chromium ions are deposited from the electrolyte onto the object, i.e., the cathode. If the object is not metallically conductive, it can be made conductive by pretreatment.

[0029] As disclosed in WO2008 / 014987A2, the above setup can also be modified to the extent that the electrolyte in the electrolytic cell is separated into a cathode solution (electrolyte in the cathode compartment) and an anode solution (electrolyte in the anode compartment) by a semipermeable membrane. The cathode is immersed in the cathode solution containing chromium ions to be deposited as the object to be coated. When a voltage is applied, current flows from the membrane through the anode solution into the cathode solution.

[0030] The anode mechanism can also be such that the anode is in direct contact with a film, i.e., the anode is covered with a film. This is a so-called direct-contact film anode, as known from DE102010055143A1.

[0031] The chromium layer can be manufactured at temperatures of 20°C to 60°C, and the electrolyte temperature is set by appropriate heating and cooling equipment. The chromium layer can be manufactured at, for example, 5 to 60 A / dm³. 2 It can be manufactured at the current density.

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

[0033] Furthermore, the object being coated can also be moved. For example, a gravure cylinder can be moved at a rotational speed of 0.5 to 1.5 m / min.

[0034] The method according to the present invention makes it possible to control the supply of chromium in an 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 an electrolyte (step (C)).

[0035] The first auxiliary electrode contains or consists of chromium metal and is therefore also referred to herein as the “chromium electrode.” The first auxiliary electrode, or chromium electrode, is made of a chromium molded article that can be held in a framework or holder, for example. This molded article has a regular or irregular shape and can be smooth or porous. These are, for example, nuggets, chunks, ramps, platelets, bars, wires and / or grids, and do not contain powder. The framework or holder is made of a material resistant to acidic electrolytes and may or may not conduct electricity. Such conductive materials may be metals such as titanium. Such non-conductive materials are plastics such as polypropylene or polyvinyl chloride. If the framework or holder does not conduct electricity, a conductive plate may be added to allow current to flow through the chromium molded article.

[0036] According to one embodiment, a chromium metal piece, also referred to herein as a "chromium nugget," is housed within a plastic frame as a chromium molded product.

[0037] The shape of the first auxiliary electrode can be appropriately selected by a person skilled in the art. Regarding the shape, particularly for molded products made of chromium metal, it is important that a larger surface area of ​​chromium metal results in a higher 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 ​​metallic chromium can be 1% to 50% or 1% to 100% of the surface area of ​​the first auxiliary electrode. In this way, particularly good dissolution of chromium and subsequent supply of trivalent chromium can be achieved.

[0039] The second auxiliary electrode is an inert electrode and is composed 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 properties described. For example, the same material as the anode in the electrolytic process described above can be used for the chromium coating.

[0040] The shape of the second auxiliary electrode can be selected by those skilled in the art according to the structural requirements. The second auxiliary electrode can be, for example, a flat material, a plate-shaped 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 size as the surface area of ​​the second auxiliary electrode. In this case, it is advantageous that the surface area of ​​metallic chromium is 100% of the surface area of ​​the first auxiliary electrode.

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

[0043] In the second electrical circuit, the first and second auxiliary electrodes are connected to each other in a separate electrical circuit from the cathode and anode in one embodiment (step (D)). The first electrical circuit of the anode and cathode and the second electrical circuit of the first and second auxiliary electrodes are not connected to each other, but are connected in complete isolation. Thus, the second electrical circuit is controlled independently of the first electrical circuit. A direct current is used in the second electrical circuit, but the polarity is reversed after a certain time interval. However, this time interval is considerably longer than in the case of pulsed alternating current (e.g., duration 0.1 to 2 seconds). That is, initially the first auxiliary electrode functions as the cathode and the second auxiliary electrode as the anode, and at a later point, the first auxiliary electrode functions as the anode and the second auxiliary electrode as the cathode. This type of polarity reversal is known from the art, albeit in different contexts and for other purposes, and is readily implementable by those skilled in the art. For example, polarity reversal can be achieved by a rectifier equipped with a polarity inverter.

[0044] The second electrical circuit functions independently of the first electrical circuit and can therefore be conveniently operated during the electrolysis of chromium, which involves the formation of a chromium layer. For this purpose, a cathode voltage is initially applied to the first auxiliary electrode. Thus, the first auxiliary electrode is the cathode and the second auxiliary electrode is the anode. The cathode voltage, also referred to herein as the cathode potential, has a reducing effect on the chromium metal. As a result, the passivation layer formed on the surface of the chromium metal of the first auxiliary electrode begins to decompose.

[0045] Although chromium is chemically more reactive than iron, when corroded by air and water, it behaves almost like a precious metal. This is due to an extremely thin, virtually invisible layer of chromium oxide, 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 passivation layer also prevents diffusion into the metal, thus preventing further corrosion. Therefore, the passivation layer dissolves when a cathode, or a reducing voltage, is applied.

[0046] Therefore, initially, a cathode (i.e., reduction) voltage is applied to the first auxiliary electrode (step (E)). In contrast, the second auxiliary electrode, which is the counter electrode, becomes the anode. The cathode voltage reduces the passivation layer in the current form of chromium oxide layer formed on the surface of the chromium metal. In this process, chromium oxide is converted to metallic chromium (Cr2O3 → Cr 金属 The cathode voltage or cathode potential is selected to be high enough so that the decomposition of the chromium oxide layer is achieved. Thus, the reducing DC current dissolves the passivation layer, and the trivalent chromium ions begin to move into the solution.

[0047] For the decomposition of the passivation layer, the cathode voltage can be set to a 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 is, for example, 2.5 to 4 A / dm². 2 Preferably 3.4 A / dm 2 It is within the range. Current density is 4A / dm 2It has been found that setting the pH lower than the specified value is preferable because it prevents the formation of undesirable hexavalent chromium in any case. Typically, the passivation 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 can be shorter or longer in individual cases and depends on many process parameters, such as the set pH value, current density, thickness of the chromium metal layer to be deposited, temperature, selected voltage, and type of auxiliary electrode used. The specified range is merely a guideline for those skilled in the art, and they can determine the optimal voltage level and energizing time of the cathode current on the first auxiliary electrode for each application through a few experiments.

[0048] The method according to the present invention makes it possible to decompose the chromium metal or the passivation layer on the first auxiliary electrode, which is formed on the chromium metal and has a passivation effect, as a secondary reaction. As a result, trivalent chromium ions can be introduced into the solution without forming hexavalent chromium ions. It has been found that decomposition of the passivation layer is an essential condition for avoiding the generation of hexavalent chromium ions during the electrolytic dissolution of chromium.

[0049] The pure chemical dissolution of chromium by acid is only possible at very low pH values ​​below 0.5, which is unfavorable for the process conditions of a galvanic bath. According to the present invention, the pH value of the electrolyte is usually given 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 alone is insufficient to remove the passivation layer present on the chromium and initiate chemical dissolution. For this reason, the dissolution of chromium metal as trivalent chromium ions is effectively induced and initiated by the application of a cathode voltage.

[0050] The dissolution of the passivation 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 generated and becomes visible in the form of bubbles. Simply put, the passivation layer can be considered dissolved when bubbles form across the entire surface of the chromium metal on the first auxiliary electrode. This occurs, as already explained, for example, after about 5 to 30 seconds.

[0051] Surprisingly, experiments revealed that when the first auxiliary electrode is used as the cathode and a cathode voltage is applied, the passive layer of metallic chromium is dissolved. The dissolution reaction, which can be recognized by the generation of hydrogen (bubbles), does not stop even when the voltage is removed, but continues without current. Therefore, the dissolution of metallic chromium can be observed. Not limited to this, if trivalent chromium is present, it is thought that divalent chromium will be 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 will not be formed.

[0052] Therefore, in the method according to the present invention, after the passivation layer is dissolved (step (F)), the supply of current to the first auxiliary electrode is terminated, and the first and second auxiliary electrodes are no longer energized. If the cathode voltage is maintained after the passivation layer is dissolved, 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] Subsequently, due to the action of the electrolyte, the chromium metal dissolves from the first auxiliary electrode in the form of trivalent chromium ions without current (step (G)). As soon as the chromium oxide layer decomposes, the chromium is attacked by the acidic electrolyte and chemically dissolved. For this purpose, it is advantageous for the electrolyte to have an acidic pH value in the range of, for example, 2.0 to 3.5. In this pH range, the electrolyte is acidic enough to dissolve the metal without current after the passivation layer has dissolved. By applying a relatively short cathode current, the passivation layer decomposes to the extent that the acidic electrolyte can attack the chromium in the first auxiliary electrode. In the case of an electrolyte in a specific pH range, after the passivation layer has decomposed, the chromium metal does not become passivated again, and the dissolution of chromium continues until the dissolution process is stopped or the chromium metal in the first auxiliary electrode is depleted and can no longer be replenished.

[0054] In one embodiment, the chromium metal in the first auxiliary electrode is replenished without current during step (G), i.e., during the dissolution of trivalent chromium ions into the electrolyte, and then steps (E), (F), and (G) are carried out sequentially in this order, preferably without intermediate steps, as disclosed. Thus, this embodiment includes the following steps. During process (G), chromium metal is replenished in the first auxiliary electrode. (E) A cathode voltage is applied to the first auxiliary electrode, causing the passive layer of chromium metal to dissolve, and chromium metal in the form of trivalent chromium ions to begin dissolving in the electrolyte. (F) After the passivation layer has dissolved, terminate the supply of current to the first auxiliary electrode or turn off the voltage to the first auxiliary electrode. (G) Due to the action of the electrolyte, the chromium metal dissolves from the first auxiliary electrode in the form of trivalent chromium ions without the use of current.

[0055] This process can be repeated as needed, and chromium metal is replenished multiple times, dissolving as trivalent chromium ions to precipitate in the chromium layer.

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

[0057] In one embodiment, an anode voltage is applied to the first auxiliary electrode, resulting in the re-formation of a passive layer of chromium metal, thereby stopping the dissolution of trivalent chromium ions in the electrolyte. Therefore, by applying an anode voltage (oxidation potential) to the first auxiliary electrode, the dissolution of chromium in the electrolyte can be stopped at any desired timing. Accordingly, the second auxiliary electrode (preferably in the form of an inert electrode as a counter electrode) becomes the cathode. As a result, the metallic chromium initially dissolves as trivalent chromium for a short time, but in parallel with the dissolution of trivalent chromium ions, the surface of the chromium reacts with oxygen contained in the water, and the aforementioned passive layer of Cr2O3 is re-formed. In this process, the anode voltage is selected so that the passive layer is formed 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 trivalent chromium ions stop dissolving in the electrolyte, the generation of hydrogen gas, which is visible in the form of bubbles, also stops. In simplified terms, it can be considered that the passive layer is re-formed after the bubbles disappear from the entire surface of the chromium metal on the first auxiliary electrode.

[0058] The anode voltage can be set to a 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, in order to rebuild the passivation layer. The current density is, for example, 2.5 to 4 A / dm². 2 Preferably 3.4 A / dm 2 The passivation layer is typically 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 in individual cases and is merely a guideline for those skilled in the art, who can determine the appropriate anode voltage level and anode current duration on the first auxiliary electrode through several 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, exposed to ambient air, and allowed to re-form a passivation layer, thus ending the chemical reaction. Or, the chromium electrode can be run dry by not replenishing the existing first auxiliary electrode with chromium metal.

[0060] In other words, in this process, chromium dissolves without current, and can be started in any way at a specified time by using a direct current with two auxiliary electrodes, continued for a desired number of times by replenishing the chromium metal, and terminated again at a specified time. Thus, the method according to the present invention, as described in 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 specified current or a specified voltage, continued without current, 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 - Chromium electrode - Inert electrode (Second auxiliary electrode) (First auxiliary electrode) (Third auxiliary electrode) In other words, the first auxiliary electrode is surrounded by the second and third auxiliary electrodes.

[0062] The auxiliary electrodes can also consist of two or more units connected to each other, in which case the following applies to the two units: Inert electrode - Chromium electrode - Inert electrode - Chromium electrode - Inert electrode

[0063] In this case, it is preferable to connect the units in series, similar to automotive batteries. Series connection has the advantage of allowing the units to be housed in a particularly space-saving manner. Furthermore, it enhances the ability to dissolve trivalent chromium ions.

[0064] Processes using three or more auxiliary electrodes are carried out in the same manner as the processes using the first and second auxiliary electrodes, which have already been described in detail.

[0065] To dissolve the passivation layer on the chromium electrode, the surrounding inert electrode acts as the anode, and the intermediate chromium electrode acts as the cathode. After the passivation layer has dissolved, trivalent chromium ions from the chromium electrode dissolve into the electrolyte without current. To continue the process, the chromium metal in the chromium electrode can be replenished during the current-free dissolution. The process can then be continued by decomposing the passivation layer again and dissolving the trivalent chromium ions into the electrolyte once more. 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 needed.

[0066] To re-form a passivation layer on the chromium electrode, an inert electrode is connected as the cathode, and the chromium electrode is used as the anode. Alternatively, the chromium electrode is withdrawn from the electrolyte, or the electrolyte is removed during process (G). In either case, contact with the surrounding air causes a re-formation of a passivation layer on the chromium electrode. It is also possible not to replenish the chromium metal in the chromium electrode, in which case there is no supply of chromium metal.

[0067] Figure 1 shows a flowchart illustrating one embodiment of the method according to the present invention.

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

[0069] To terminate the process (branch "yes" in Figure 1), the passivation layer can be rebuilt by applying an anode voltage to one or more chromium electrodes. Alternatively, the chromium electrodes can be run dry by not replenishing the chromium metal in the first auxiliary electrode. Another way to terminate the process is to drain the electrolyte from the cell or withdraw the chromium electrodes from the electrolyte so that the chromium electrodes are exposed to air, thereby allowing the passivation layer to be rebuilt and the chemical reaction to end.

[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 may 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 includes the following: (a) One or more trivalent chromium salts (b) Compounds of formula (I) JPEG0007900518000001.jpg25166 Here, R is NH2, OH, SO3H and / or their salts, especially Na+ and / or K + is a salt with a monovalent cation such as or a salt with a divalent cation. n represents an integer of 1 to 3. (c) Formic acid and / or its salts, particularly Na + and / or K + a monovalent cation such as or a salt with a divalent cation (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 construed 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 chrome alum, ammonium chrome alum, chromium sulfate, chromium (hydroxy) sulfate (alkaline chromium sulfate), chromium sulfacetate, chromium nitrate, chromium sulfamate (amide sulfonic acid chromium), chromium chloride, chromium bromide, chromium iodide, chromium phosphate, chromium pyrophosphate (diphosphate chromium), 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 sulfacetate, chromium oxalate, chromium methanesulfonate, chromium dimethanesulfonate, and mixtures of two or more thereof. It is also possible to use a mixture of inorganic and organic trivalent chromium salts.

[0073] For example, it is preferable that the amount of the trivalent chromium salt is selected in the range of 0.25 mol / L to 2.0 mol / L with respect to the electrolytic solution. This range has been proven to be particularly advantageous for the formation of a chromium layer on a metal object 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 compound (I) in the electrolyte is preferably 0.5 mol / L to 1.5 mol / L relative to the electrolyte. This plays a role in setting 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 a chemical reaction that decomposes it into CO2 and H2O. The amount of formic acid in the electrolyte is favorably 1.0 mol / L to 3.0 mol / L with respect to the electrolyte before chromium deposition. This range has proven particularly useful for setting the pH value of the electrolyte.

[0077] Formic acid can also be used in place of, or in addition to, its salts. Examples include alkaline and / or alkaline earth formate salts, particularly sodium formate.

[0078] Optionally, one or more additives are used as component (d) of the electrolyte. As additives, compounds or mixtures of compounds that can impart properties favorable to the galvanic bath can be used. These compounds are known to those skilled in the art.

[0079] For example, additives are selected from complexing agents, alkali salts or alkaline earth salts, wetting agents, catalysts, or mixtures thereof.

[0080] The complexing agent is preferably a compound comprising one or two carboxyl groups or derivatives thereof, or a short alkyl chain (e.g., 1 to 5 carbon atoms) having one or two thio groups and / or sulfone groups. The following compounds are used as examples. JPEG0007900518000002.jpg25166 Here, R1 is C 1-5 This represents an alkyl radical, specifically CH3CH2-. X is one or more metal cations to balance the negative charge, for example, Na + , K + This represents n as an integer between 1 and 5, especially 3.

[0081] As a complexing agent, the compound N,N-dimethyl-dithiocarbamylpropylsulfonate sodium salt (DPS) is particularly preferred. Using DPS is advantageous because it yields a particularly good chromium layer.

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

[0083] To enhance conductivity, sulfates or acetosulfates, such as alkali salts or alkaline earth salts, particularly 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 relative to the electrolyte. For example, when PEG6000 is used as a wetting agent, the substance concentration will be between 0.001 mMol / L and 0.3 mMol / L.

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

[0087] The electrolyte is essentially free of hexavalent chromium ions; that is, only the unavoidable impurity of hexavalent chromium ions is present in the electrolyte composition. In the method according to the present invention, the hexavalent chromium ion content is below the detection limit.

[0088] According to one embodiment, it is preferable that the electrolyte does not contain nitrogen-containing compounds. In this case, the chromium layer formed also does not contain nitrogen-containing compounds, so a coating with particularly advantageous properties can be obtained.

[0089] The subject of the present invention is also an electrolytic cell for controlling the supply of chromium to an electrolyte, which includes the following: ·anode ·cathode • Electrolyte containing at least one type of trivalent chromium salt • The anode and cathode must be immersed in the electrolyte. The first circuit connects the anode and cathode, and by passing a direct current through the cathode, chromium is electrolytically extracted from the electrolyte, thereby depositing a chromium layer. • First auxiliary electrode containing or made of chromium metal • Second auxiliary electrode in the form of an inert electrode Both auxiliary electrodes are immersed in the electrolyte. • The second circuit connects the first auxiliary electrode and the second auxiliary electrode to an electrical circuit separate from the cathode and anode.

[0090] Each electrolytic cell assumes one of the following three conditions: A cathode voltage is applied to the first auxiliary electrode, causing the passive layer of chromium metal on the first auxiliary electrode to dissolve. By not applying voltage to the first auxiliary electrode, after the passivation layer dissolves, the chromium metal dissolves from the first auxiliary electrode into the electrolyte in the form of trivalent chromium ions without any current. A positive voltage is applied to the first auxiliary electrode, and a passivation layer is formed again on the first auxiliary electrode.

[0091] Any container suitable to those skilled in the art can be used as an electrolytic cell, such as a suitable container, vessel, or tank, and those commonly used in galvanic technology are particularly preferred.

[0092] The above description regarding the chromium supply control method in the electrolytic process for manufacturing the chromium layer applies equally to the electrolytic cell and therefore will not be repeated.

[0093] The electrolytic cell may be a single unit or it may be divided into two parts. In one embodiment, the chromium plating bath can be placed in a first cell through which a direct current flows between an anode and a cathode immersed in the electrolyte. In a second cell connectable 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 inert electrode-chromium electrode-inert electrode can be connected to each other within the second cell. Chromium plating of the object is performed in the first cell. In the second cell, trivalent chromium ions are accumulated in the electrolyte to a desired concentration and then returned to the chromium plating bath. Other configurations are also possible.

[0094] The described process or electrolytic cell is used, in particular, to replenish the trivalent chromium that has been used up in the electrolyte. The current-free dissolution of chromium is carried out for a sufficient amount of time, for example, to bring the chromium content in the electrolyte to the desired level, which can range from a few minutes to several hours.

[0095] Alternatively, an equilibrium is set so that trivalent chromium ions are continuously supplied to the coating bath, and this operates continuously. The time for chromium to dissolve as trivalent chromium ions is favorably selected so that the concentration of trivalent chromium ions in the electrolyte remains constant, and 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 molded product of the first auxiliary electrode is preferably replenished in step (G) while trivalent chromium ions are dissolved without current by the action of the electrolyte. 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 molded product, the passivation layer must first be removed again (step (E)) as previously described, before the trivalent chromium ions are dissolved again in the electrolyte without current (steps (F) and (G)).

[0097] The present invention also relates to a method for maintaining a constant trivalent chromium content in an electrolyte by comparing the weight of chromium metal used in a first auxiliary electrode with the weight of chromium metal used 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 by the coating. The weight can be measured, for example, by a pressure sensor.

[0098] By supplying metallic chromium to the electrolyte, it becomes possible to maintain a nearly constant trivalent chromium content during electrolysis, which can take place over long periods ranging from several hours to several months. Trivalent chromium ions are then supplied to the electrolyte as the trivalent chromium is depleted. Maintaining a constant trivalent chromium content in the electrolyte is interpreted as meaning that the trivalent chromium content preferably changes by only ±10%.

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

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

[0101] The advantages of this invention are extremely complex.

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

[0103] Another major advantage is that the dissolution of trivalent chromium ions following the dissolution of the passivation layer occurs without electric current. Therefore, no additional energy is required during the dissolution process. This is particularly important in large-scale plants. As a result, the energy costs of the process and electrolytic cells are significantly reduced.

[0104] Furthermore, the method according to the present invention makes it possible to maintain a constant trivalent chromium content in the electrolyte. For example, by correlating the weight of chromium metal in the first auxiliary electrode with the weight of chromium metal used up by coating and controlling accordingly, the trivalent chromium content in the electrolyte can be kept constant. This makes it possible to maintain a nearly constant trivalent chromium content in electrolysis carried out over a long period of time, such as several hours to several months, and trivalent chromium ions are supplied later to the electrolyte where the trivalent chromium has been depleted. Maintaining a constant trivalent chromium content in the electrolyte is interpreted as meaning that the trivalent chromium content preferably changes by only ±10%.

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

[0106] Thus, chrome coating can be applied easily, quickly, and cost-effectively, even over long periods.

[0107] According to one embodiment, it is preferable that the electrolyte does not contain nitrogen-containing compounds, and that the chromium layer formed therefrom also does not contain nitrogen-containing compounds. This results in a coating with particularly advantageous properties.

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

[0109] According to the present invention, a chromium coating of particularly excellent quality can be obtained, which remarkably meets the high requirements imposed on gravure cylinders. A smooth and uniform surface is obtained, essentially free of pores, depressions, and craters. The thickness of the resulting chromium layer can be greater than that of layers normally obtained in the art. A layer thickness of 100 μm or more can be obtained. Furthermore, a chromium layer with high hardness, especially 900 HV or more, can be produced. The resulting chromium layer has corrosion resistance, wear resistance, good friction properties, heat resistance, and chemical resistance, and is also suitable for decorative purposes because it is glossy and has high reflectivity.

[0110] In the following text, embodiments of the present invention are described illustratively with reference to the accompanying drawings, which are schematic and not scaled, so no precise geometric values ​​regarding the original sizes can be assumed. The drawings in this disclosure are part of the description and illustrate embodiments of the present invention without limiting them to the specific embodiments described. The drawings, together with the description, are helpful in illustrating this disclosure.

[0111] Figures 2a, 2b, and 2c illustrate embodiments of sequences or electrolytic cell states in the method according to the present invention for controlling the supply of trivalent chromium ions by initiating, continuing, and terminating the dissolution of trivalent chromium ions in relation to the electrolytic process for electrolytic extraction of the chromium layer. Accordingly, Figures 2a, 2b, and 2c show various states of the electrolytic cell illustrating individual steps of the method according to the present invention according to one embodiment.

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

[0113] The electrolytic cell 10 is equipped with a trough 15 containing a liquid electrolyte 25 with water as the solvent, and this electrolyte contains 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 already described in detail (b) Compounds of formula (I) JPEG0007900518000003.jpg25166 Here, R is NH2, OH, SO3H and / or their salts, especially Na + and / or K + It is a salt with a monovalent cation, or a salt with a divalent cation, such as n. n represents an integer from 1 to 3. (c) Formic acid and / or salts thereof, in particular salts with monovalent cations such as Na+ and / or K+, or with divalent cations. (d) Optionally, one or more additives as described above. Other electrolyte compositions are also possible.

[0114] Furthermore, within the trough 15 is provided a vertically movable anode device, which essentially consists of an anode rail 42 and an anode basket 44 electrically and mechanically coupled to the anode rail 42 and functioning as a metal retaining device. The anode basket 44 can also be configured by combining multiple anode baskets or grids. The anode 44 represents an insoluble anode or inert electrode and may include or consist of the following materials, for example: Platinumized 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 functions as an anode substrate coated with platinum, iridium, tantalum and / or palladium oxide, titanium, niobium or tantalum sheets coated with mixed metal oxides, expanded metals made of titanium, tantalum or niobium or titanium coated with iridium transition metal mixed oxides, or expanded metals made 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, two bearing bridges 30 can be moved along rails (not shown) in the axial direction of the gravure cylinder 48 by a spindle or other suitable adjustment mechanism, thereby sandwiching the gravure cylinder 48 between them and holding it in a rotatable position.

[0116] As shown in Figure 2a, a portion of the trough 15 remains freely accessible at the top by a bearing bridge 30 supporting one side, thereby allowing the anode rail 42, which extends therein parallel to the axial direction of the gravure cylinder 48, to move freely in the vertical direction. The vertical movement of the anode rail 42 with the anode basket 48 is known to those skilled in the art and therefore does not require detailed description or illustration.

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

[0118] For zinc plating, i.e., coating the gravure cylinder 48 with a chromium layer, the anode basket 44 has already been pulled laterally so that the surface of its large basket surrounds the immersed gravure cylinder 48.

[0119] Therefore, the anode 44 and cathode 48 form a first electrical circuit (not shown).

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

[0121] The first auxiliary electrode 54 contains or consists of chromium metal and can therefore also be called a "chromium electrode." This electrode is composed of, for example, chromium molded articles 54a, which are held in a holder such as a framework or basket. These molded articles can have regular or irregular shapes and can be smooth or porous. For this purpose, nuggets, chunks, lamps, platelets, bars, wires, and grids are suitable, for example, rather than powder. The holder is made of a material resistant to acidic electrolytes and may or may not conduct current. Conductive materials are metals such as titanium, for example. Non-conductive materials are plastics such as polypropylene or polyvinyl chloride, for example. In the illustrated exemplary embodiment, chromium metal pieces 54a, also called chromium nuggets, are housed in a plastic framework such as a polypropylene basket, for example.

[0122] The shape of the first auxiliary electrode 54 is not limited as long as it is suitable for the intended purpose. Suitable shapes are known to those skilled in the art.

[0123] The selected shape of a chromium metal molded product determines its surface area, and a larger surface area results in a faster dissolution rate in the electrolyte. Therefore, a person skilled in the art can select an appropriate shape.

[0124] The second auxiliary electrode 56 is an inert electrode, composed 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 properties described. 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 according to structural requirements. The second auxiliary electrode 56 can be, for example, a flat plate, a plate, a sintered material, or an expanded material.

[0125] In Figure 2a, the electrolytic cell 10 is depicted when a cathode voltage is applied to the first auxiliary electrode 54 using DC 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 present 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 an electrode inverter (not shown) is used. The cathode voltage has a reducing effect on the chromium metal nugget 54a, and the passive layer formed on the surface of the chromium metal nugget 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 / dm². 2 It is within the range of 3.4 A / dm², and is particularly preferably 3.4 A / dm². 2The passivation layer decomposes after 5 to 60 seconds, preferably after 5 to 45 seconds, and especially preferably after 5 to 30 seconds. Depending on the selected electrolytic conditions, particularly the pH value, temperature, selected voltage, current density, thickness of the deposited chromium metal layer, and type of auxiliary electrode used, the duration can be shorter or longer.

[0127] In Figure 2b, the passivation layer on the first auxiliary electrode 54 has already decomposed, and bubbles 55 (indicating the release of trivalent chromium ions into the solution and the generation of hydrogen) have appeared across the entire chromium metal surface on the first auxiliary electrode 54, indicating that the passivation layer has dissolved.

[0128] In Figure 2b, the passivation layer has already dissolved, and the supply of current to the first auxiliary electrode 54 and the second auxiliary electrode 56 is interrupted (step (F)). The voltage is then cut off. This is schematically represented in Figure 2b by an open electrical switch 59. Due to the action of the electrolyte 25, chromium metal dissolves 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 nugget 54a is attacked by the acidic electrolyte and dissolves chemically. This occurs without current. The formed trivalent chromium ions move to the surface of the gravure cylinder 48 connected as the cathode in the electrolyte 25, where they precipitate in the form of a chromium film.

[0129] By dissolving trivalent chromium ions in the electrolyte, the amount of chromium metal in the first auxiliary electrode (54) decreases. The chromium metal in the first auxiliary electrode 54 can be replenished during step (G), shown in Figure 2b. Thereafter, steps (E), (F), and (G) of the method according to the present invention are repeated. This can be continued any number of times as needed. Thus, a quasi-continuous process is established as follows. Figure 2a → Figure 2b → Supplement → Figure 2a → Figure 2b → Supplement → etc.

[0130] In Figure 2c, by applying an anode voltage to the first auxiliary electrode 54, the dissolution of trivalent chromium ions into the electrolyte is stopped. As a result, a passive layer of chromium metal is formed again on the first auxiliary electrode 54, thereby stopping the dissolution of trivalent chromium ions into the electrolyte 25. Subsequently, the first auxiliary electrode 54 becomes the anode, and the second auxiliary electrode 56 or the inert electrode becomes the cathode. A DC current flows again, but with the polarity reversed.

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

[0132] In the example in Figure 2c, the passivation layer has already been completely reformed, and the bubbles 55 on the surface of the chromium metal, i.e., the chromium molded product 54a, have completely disappeared because the trivalent chromium ions no longer dissolve 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 / dm². 2 It is within the range of 3.4 A / dm 2 The passivation layer is formed after approximately 5 to 60 seconds, preferably after approximately 5 to 45 seconds, and particularly preferably after approximately 5 to 30 seconds. Depending on the selected electrolytic conditions, particularly the pH value, temperature, selected voltage, current density, thickness of the chromium metal layer to be deposited, and the type of auxiliary electrode used, the duration can be shorter or longer. 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), thereby bringing the first auxiliary electrode 54, in particular the chromium molded product 54a, into contact with the ambient air, resulting in the formation of a passivation layer. Alternatively, the chromium metal of the first auxiliary electrode 54 is not further replenished, thereby terminating the dissolution of the chromium metal from the first auxiliary electrode 54 in the form of trivalent chromium ions without current due to the action of the electrolyte, as soon as the present chromium metal is completely dissolved.

[0134] No hexavalent chromium ions were detected during the process.

[0135] Figures 3a and 3c schematically represent 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. Similar to the second auxiliary electrode, the third auxiliary electrode is an inert electrode. There is an auxiliary electrode interconnection unit that can replace, for example, the circuit consisting of the first auxiliary electrode 54 and the second auxiliary electrode 56 in Figure 2a. Figure 3a shows two inert electrodes 56.1 and 56.2, which are also referred to herein as the second and third auxiliary electrodes. They substantially surround the chromium electrode 54, which is also referred to herein as the first auxiliary electrode. In the illustrated embodiment, an anode voltage is applied to the inert electrodes 56.1 and 56.2 and a cathode voltage is applied to the chromium electrode 54. This is step (E), in which the passivation layer is dissolved.

[0136] Figure 3b shows the embodiment according to Figure 3a, but no voltage is applied to the auxiliary electrode (step (F)), while in Figure 3c, the anode voltage is applied to the first auxiliary electrode (end of process in step (G)).

[0137] According to another modification of the present invention, embodiments comprising a third auxiliary electrode (Figures 3a, 3b, and 3c) can also be replaced with the second circuits in Figures 2a, 2b, and 2c, respectively.

[0138] Figures 4a, 4b, and 4c schematically represent another embodiment of the method according to the present invention, where a series connection of electrodes is illustrated. Inert electrodes 56.1, 56.2, 56.3, and 56.4 are depicted, alternately connected to chromium electrodes 54.1, 54.2, and 54.3, respectively. Inert electrodes 56.1 and 56.4 are arranged around the ends of the electrodes, respectively, and represent end electrodes. There are units in which auxiliary electrodes are interconnected, which can replace, for example, the second circuit consisting of the first auxiliary electrode 54 and the second auxiliary electrode 56 in Figures 2a, 2b, and 2c, respectively.

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

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

[0141] Figures 5a, 5b, and 5c illustrate another embodiment of the present invention. Figures 5a, 5b, and 5c illustrate another embodiment relating to a sequence of the method according to the present invention for controlled supply of trivalent chromium ions or the state of an electrolytic cell, relating to an electrolytic process for electrolytic extraction of a chromium layer, by initiating, continuing, and terminating the dissolution of trivalent chromium ions.

[0142] In Figure 5a, the trough of the electrolytic cell 100, which is shaped like a bath apparatus, is divided into an upper trough 110 and a lower trough 120 located below it. Liquid electrolyte 125 is placed in the upper trough 110 and the lower trough 120, and 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 at at least two positions. Alternatively, two alternately openable and closable overflows can be placed at different heights.

[0143] As already described with respect to Figures 2a, 2b, and 2c, a vertically movable anode device is located within the upper trough 110, which essentially consists 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 retaining device. The anode basket 144 can also be configured by combining multiple anode baskets or grids. The anode basket 144 is part of the insoluble anode.

[0144] The cathode 148, in this case a gravure cylinder, is held by two bearing bridges 130 (only one shown) so that it can move along the rail in the axial direction of the gravure cylinder 148 by a suitable adjustment mechanism, and the gravure cylinder 148 is held between these so that it can rotate. Thus, the upper half of the upper trough 110 is freely accessible and the anode rail 142 can be moved vertically.

[0145] The degree to which the electrolyte 125 is filled in the upper trough 110, i.e., the water level of the electrolyte 125, can be appropriately adjusted using the vertically movable overflow 127.

[0146] Figure 5a shows the electrolytic cell 100 during the zinc plating stage, with the gravure cylinder 148 almost completely immersed. For zinc plating, i.e., coating the gravure cylinder 148 with a chromium layer, the anode basket 144 has already been pulled up laterally so that the surface of its large basket surrounds the immersed gravure cylinder 148.

[0147] Therefore, the anode 144 and cathode in the form of the gravure cylinder 148 form a first electrical circuit (not shown). In a second electrical circuit operating 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 contains or consists of chromium metal and is also referred to herein as the "chromium electrode." The structures of the chromium electrode 154 and the inert electrode 156 have already been described in Figure 2a (where they are labeled as chromium electrode 54 and inert electrode 56).

[0149] In the electrolytic cell 100 shown in Figure 5a, a cathode voltage is initially applied to the first auxiliary electrode 154 using a DC current so that the passive layer of chromium metal on the first auxiliary electrode 154 dissolves (step (E)). Thus, the first auxiliary electrode 154 becomes the cathode and the second auxiliary electrode 156 becomes the anode. For example, a rectifier with an electrode inverter (not shown) connected to a power supply 158 is used. The cathode voltage has a reducing effect on the chromium metal nugget 154a, and the passive layer formed on the surface of the chromium metal nugget 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 / dm². 2 It is within the range of 3.4 A / dm 2 The passivation layer decomposes after approximately 5 to 60 seconds, preferably after approximately 5 to 45 seconds, and particularly preferably after approximately 5 to 30 seconds. Depending on the selected electrolytic conditions, particularly the pH value, temperature, selected voltage, current density, thickness of the chromium metal layer to be deposited, and the type of auxiliary electrode used, the duration can be shortened or lengthened.

[0151] In Figure 5b, the disappearance of the passivation layer on the first auxiliary electrode 154 is indicated by the appearance of bubbles 155, which are caused by the generation of hydrogen by trivalent chromium ions in the solution. The bubbles 155 are present across the entire surface of the chromium metal on the first auxiliary electrode 154 when the passivation layer has completely dissolved.

[0152] As soon as the passivation layer dissolves, the supply of current to the first auxiliary electrode 154 and the second auxiliary electrode 156 is interrupted (step (F)). This is schematically represented in Figure 5b by an electrical switch 159 that cuts off the current.

[0153] In the following, the dissolution of chromium metal from the first auxiliary electrode 154 in the form of trivalent chromium ions occurs without current by the action of the electrolyte 125 in the lower trough 120 (step (G)), where the electrolyte 125 has a pH value in the range of, for example, 2.0 to 3.5. The chromium metal nugget 154a is attacked and chemically dissolved by the acidic electrolyte. This is done without supplying current to the auxiliary electrodes 154 and 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 flows back into the lower trough 120 via an overflow 127 that is vertically movable at at least two positions. The trivalent chromium ions move through the electrolyte 125 to the surface of the gravure cylinder 144 connected as the cathode, where they form a chromium film.

[0154] To reduce the amount of electrolyte in the upper trough 110, the upper trough 110 is tapered, for example, in the lower region. This tapering can be achieved using additionally used sheet plates 133 or by appropriately adapting the walls of the upper trough 110. Blocks or boxes can also be used to change the volume. Limiting or reducing the volume of the upper trough 110 has the advantage that there is no need to pump more electrolyte 125 upward from the lower trough 120 than necessary. Therefore, there is no risk of the lower trough 120 becoming completely empty and the pump 160 running dry.

[0155] If desired, the chromium metal in the first auxiliary electrode 154 can be replenished during the execution of step (G) according to Figure 5b, and the process can be continued accordingly, and steps (E), (F), and (G) can be repeated. This gives the following sequence of processes. Figure 5a → Figure 5b → Supplement → Figure 5a → Figure 5b → Supplement → ...

[0156] When the supply of trivalent chromium ions to the electrolyte is stopped, an anode voltage is applied to the first auxiliary electrode 154, resulting in the formation of a chromium metal passivation layer on the first auxiliary electrode 154 again, and the dissolution of trivalent chromium ions into the electrolyte ends. This is shown in Figure 5c. The first auxiliary electrode 154 becomes the anode, and the second auxiliary electrode 156 or the inert electrode becomes the cathode. A DC current flows again, 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 / dm². 2 Preferably 3.4 A / dm 2 The passivation layer is rebuilt after approximately 5 to 60 seconds, preferably after approximately 5 to 45 seconds, and particularly preferably after approximately 5 to 30 seconds. Depending on the selected electrolytic conditions, particularly the pH value, temperature, selected voltage, current density, thickness of the deposited chromium metal layer, and type of auxiliary electrode used, the duration can be shorter or longer.

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

[0159] Once the passivation layer is completely formed again, hydrogen gas will no longer be formed, and trivalent chromium ions will no longer dissolve in the electrolyte 125. As a result, bubbles 155 will no longer be 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, and as a result, as soon as the present chromium metal is completely dissolved, the dissolution of chromium metal in the form of trivalent chromium ions from the first auxiliary electrode 154 is terminated without current by the action of the electrolyte 125. Alternatively, the process may be terminated by withdrawing the first auxiliary electrode 154 from the electrolyte 125 or by draining the electrolyte 125 from the electrolytic cell 100 (not shown).

[0161] The formation of hexavalent chromium ions was not detected throughout the entire process.

[0162] After the zinc plating stage is complete, the anode rail 142, which has the anode basket 144, is moved downward into the upper trough 110. Simultaneously, or with a time delay, the overflow 127 is lowered so that the electrolyte 125 flows down to a suitable height level in the lower trough 120. In this way, the anode basket 144 is still completely covered by the electrolyte 125, while the gravure cylinder 148 is positioned completely free above the electrolyte level 125 and can be easily lifted from there using a crane (not shown).

[0163] Figures 6a, 6b, and 6c schematically represent another embodiment of the present invention. In contrast to the embodiments of Figures 5a, 5b, and 5c, there are no lower and upper troughs, and instead, there are 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, and the electrolyte 225 can be pumped from the first trough 210 to the second trough 220 by the pump 260 and returned to the first trough via overflow (not shown). The other modes of operation correspond to Figures 5a, 5b, and 5c, so there is no need to repeat their description here.

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

[0165] In another embodiment, multiple of these units, each consisting of an inert electrode, a chromium electrode, and an inert electrode, can be connected in series as shown in Figures 4a-4c, replacing the second circuits in Figures 5a-5c and 6a-6c, respectively.

[0166] Therefore, the method and electrolytic cell according to the present invention can be used in a particularly advantageous manner to obtain a chromium layer having desired properties, especially on a gravure cylinder.

[0167] The following examples are for further illustration of the present invention. They should not be construed as limiting the present invention in any way. [Examples]

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

[0169] An electrolyte solution with 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 (8 wt% -> 100.8 g / L -> 1.482 M) 6.05 kg ·Sodium sulfate (1.7wt%->21.4g / L->17mM)1.29kg

[0170] One liter of electrolyte was placed in a beaker and heated to 40°C while stirring, adjusting the pH to 2.6. Then, two electrodes were inserted into the beaker parallel to each other, 10 cm apart, and connected to a DC power supply. One electrode was a mixed oxide (MMO) coated titanium expanded metal, used as the anode. The other electrode was chromium, used as the cathode. The surfaces of the anode and cathode had an operating current density of approximately 4 A / dm² at a current of 3 A. 2 It was chosen so that the anode surface would be the same size as the cathode surface.

[0171] A cathode voltage was applied to dissolve the passivation layer on the chromium electrode (first auxiliary electrode according to the present invention). The other electrode was an inert electrode and functioned as the anode.

[0172] In this embodiment, after a short period of approximately 20 seconds, gas was observed to be generated on the chromium electrode, and gas bubbles were seen 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 not applying direct current to the anode and cathode. In this process, the dissolution 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 their chromium content was measured. The results showed a linear increase in trivalent chromium during the observation period. This was also confirmed by gravimetric analysis of the chromium electrode. Only trivalent chromium was dissolved; hexavalent chromium was not detected.

[0174] Subsequently, the dissolution without current was stopped by removing the chromium electrode from the electrolyte. Alternatively, the electrolyte can be removed from the beaker. This allows the chromium electrode to undergo surface oxidation in the air, forming a passivation layer again. After the passivation layer has formed, reinserting the chromium electrode into the electrolyte (for example, after a residence time of 30 seconds in the air) will not initiate the dissolution process.

[0175] Instead of removing the chromium electrode from the electrolyte, the polarity was reversed to stop the current-free dissolution. That is, the chromium electrode was positively charged for a short time until hydrogen production on the chromium electrode stopped. Hydrogen production stopped after 60 seconds. After the visible gas generation stopped, the current was switched off, permanently stopping the dissolution of chromium.

[0176] (Example 2) • pH value fluctuations The experiment in Example 1 was repeated, but with a different pH value. Specifically, using the same setup as in Example 1, the pH value was gradually lowered 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 all cases, the same results as in Example 1 were obtained. However, the dissolution rate of chromium was lower 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 rails 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 molded products 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... switches 110... Upper trough 120... Lower trough 127... Overflow 160, 260... pumps 210...First Trough 220...Second Trough

Claims

1. A method for controlling the supply of chromium in an electrolytic process for manufacturing a chromium layer, wherein the chromium layer is manufactured by electrolytically discharging chromium from an electrolyte (25, 125, 225) by direct current, using an anode (44, 144, 244) and a cathode (48, 148, 248), and the method includes the following steps. (A) Prepare first auxiliary electrodes (54, 154, 254) that contain or are made of chromium metal. (B) Prepare second auxiliary electrodes (56, 156, 256) in the form of inert electrodes. (C) The two auxiliary electrodes (54, 56, 154, 156, 254, 256) are immersed in the electrolyte (25, 125, 225) which contains at least one type of trivalent chromium salt. (D) In ​​an electrical circuit separate from the cathodes (48, 148, 248) and anodes (44, 144, 244), the first auxiliary electrodes (54, 154, 254) and the second auxiliary electrodes (56, 156, 256) are connected to each other, and the chromium layer is formed on the cathodes (48, 148, 248) during electrolysis of the chromium in the electrolyte (25, 125, 225). (E) A cathode voltage is applied to the first auxiliary electrodes (54, 154, 254) in the electrolyte (25, 125, 225), and as a result, the passivation layer of the chromium metal present on the first auxiliary electrodes (54, 154, 254) dissolves, and the chromium metal in the form of trivalent chromium ions begins to dissolve in the electrolyte (25, 125, 225). (F) After the passivation layer has dissolved, the supply of current to the first auxiliary electrodes (54, 154, 254) is terminated. (G) Due to the action of the electrolyte (25, 125, 225), the chromium metal dissolves from the first auxiliary electrodes (54, 154, 254) in the form of trivalent chromium ions without any current. Here, the chromium metal in the first auxiliary electrodes (54, 154, 254) is replenished during step (G), and steps (E), (F), and (G) are then performed sequentially in this order, the first auxiliary electrodes (54, 154, 254) being selected from a holder or framework in which the chromium metal is held in the form of a chromium molded product.

2. In order to complete process (G), The anode voltage is applied to the first auxiliary electrodes (54, 154, 254), or The first auxiliary electrodes (54, 154, 254) are withdrawn from the electrolyte (25, 125, 225), or The electrolyte is discharged from the electrolytic cell (10, 100, 200), or The method according to claim 1, characterized in that the chromium metal in the first auxiliary electrodes (54, 154, 254) is not replenished.

3. The method according to claim 1, characterized in that the pH value of the electrolyte (25, 125, 225) is set in the range of 2.0 to 3.

5.

4. The method according to claim 1, characterized in that it satisfies one or more of the following conditions. - The cathode voltage in process (E) is set to a range of 1.0 to 10.0 volts. The anode voltage for terminating process (G) is set to a range of 1.0 to 10.0 volts. - The surface area of ​​the first auxiliary electrode (54, 154, 254) is selected to be the same size as the surface area of ​​the second auxiliary electrode (56, 156, 256). • Current density is 2.5–4 A / dm² ２ It is within the range. - The passivation layer is decomposed within 5 to 60 seconds by applying the cathode voltage to the first auxiliary electrodes (54, 154, 254) in step (E). - In order to terminate the process during step (G), the passivation layer is reformed by applying the anode voltage to the first auxiliary electrodes (54, 154, 254) within 5 to 60 seconds. The chromium metal of the first auxiliary electrodes (54, 154, 254) is provided as a chromium molded article having a regular or irregular shape, held in a material resistant to acidic electrolytes (25, 125, 225).

5. A third auxiliary electrode in the form of an inert electrode is provided, and the first auxiliary electrodes (54, 154, 254), the second auxiliary electrodes (56, 56.1, 156, 256), and the third auxiliary electrode (56.2), all in the form of chromium electrodes, are connected to each other to form one or more units, one of which is Inert electrode (56.1) - Chromium electrode (54) - Inert electrode (56.2) The method according to claim 1, characterized in that it is selected from.

6. (a) The electrolyte (25, 125, 225) contains one or more trivalent chromium salts selected from inorganic and / or organic trivalent chromium salts. The method according to claim 1, wherein the electrolyte (25, 125, 225) further comprises one or more components selected from (b) to (d) below. (b) Compound of formula (I) Here, R is NH ２ , OH, SO ３ It is H and / or a salt thereof. n represents an integer from 1 to 3. (c) Formic acid and / or its salts (d) One or more additives selected from complexing agents, alkali salts or alkaline earth salts, wetting agents, catalysts or mixtures thereof.

7. The method according to claim 1, characterized in that 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 electrodes (54, 154, 254) with the weight of the chromium metal used up for the chromium layer, and the chromium metal in the first auxiliary electrodes (54, 154, 254) is replenished before the trivalent chromium content in the electrolyte (25, 125, 225) decreases.

8. An electrolytic cell (10, 100, 200) for carrying out the method according to any one of claims 1 to 7, Anodes (44, 144, 244) and Cathode (48, 148, 248) and, The system comprises an electrolyte (25, 125, 225) containing at least one type of trivalent chromium salt, The anodes (44, 144, 244) and cathodes (48, 148, 248) are immersed in the electrolyte (25, 125, 225). A first circuit connects the anode (44, 144, 244) and the cathode (48, 148, 248), and deposits a chromium layer by passing a direct current through the cathode (48, 148, 248) to electrolyze chromium from the electrolyte (25, 125, 225), First auxiliary electrodes (54, 154, 254) containing or made of chromium metal, It comprises a second auxiliary electrode (56, 156, 256) in the form of an inert electrode, The two auxiliary electrodes (54, 56, 154, 156, 254, 256) are immersed in the electrolyte (25, 125, 225). The first auxiliary electrodes (54, 154, 254) and the second auxiliary electrodes (56, 156, 256) are connected to a second circuit separate from the cathode (48, 148, 248) and the anode (44, 144, 244), and the circuit is further provided. A cathode voltage is applied to the first auxiliary electrodes (54, 154, 254), and the passivation layer of the chromium metal on the first auxiliary electrodes (54, 154, 254) dissolves. Without applying voltage to the first auxiliary electrodes (54, 154, 254), after the passivation layer has dissolved, the chromium metal from the first auxiliary electrodes (54, 154, 254) dissolves in the electrolyte (25, 125, 225) in the form of trivalent chromium ions without any current, or The anode voltage is applied to the first auxiliary electrodes (54, 154, 254), and the passivation layer is formed again on the first auxiliary electrodes (54, 154, 254). The electrolytic cell wherein the first auxiliary electrodes (54, 154, 254) are selected from a holder or framework in which the chromium metal is provided in the form of a chromium molded product, and the chromium metal in the first auxiliary electrodes (54, 154, 254) is replenished while no voltage is applied to the first auxiliary electrodes (54, 154, 254).

9. The electrolytic cells (10, 100, 200) are Container (15), or The upper container (110) and the lower container (120) are two containers arranged vertically, or The electrolytic cell (10, 100, 200) according to claim 8, characterized in that it consists of two containers, a first container (210) and a second container (220), which are arranged adjacent to each other.

10. The electrolytic cell (10, 100, 200) according to claim 8, characterized in that it satisfies one or more of the following features. The pH values ​​of the electrolytes (25, 125, 225) are set to a range of 2.0 to 3.

5. - The cathode voltage of the first auxiliary electrodes (54, 154, 254) is set to a range of 1.0 to 10.0 volts, or the anode voltage of the first auxiliary electrodes (54, 154, 254) is set to a range of 1.0 to 10.0 volts. - The cathode voltage on the first auxiliary electrodes (54, 154, 254) is applied for 5 to 60 seconds, or the anode voltage on the first auxiliary electrodes (54, 154, 254) is applied for 5 to 60 seconds. - The surface area of ​​the first auxiliary electrode (54, 154, 254) is selected to be the same size as the surface area of ​​the second auxiliary electrode (56, 156, 256). • Current density is 2.5–4 A / dm² ２ It is within the range. The chromium metal of the first auxiliary electrodes (54, 154, 254) is provided as a chromium molded article having a regular or irregular shape, held in a material resistant to acidic electrolytes (25, 125, 225). (a) The electrolyte (25, 125, 225) contains one or more trivalent chromium salts selected from inorganic and / or organic trivalent chromium salts. Furthermore, the components of the electrolyte (25, 125, 225) include one or more selected from the following (b) to (d). (b) Compound of formula (I) Here, R is NH ２ , OH or SO ３ It is selected from H and / or a salt thereof. n represents an integer from 1 to 3. (c) Formic acid and / or its salts (d) One or more additives selected from complexing agents, alkali salts or alkaline earth salts, wetting agents, catalysts or mixtures thereof.

11. A third auxiliary electrode in the form of an inert electrode is provided, and the first auxiliary electrodes (54, 154, 254), the second auxiliary electrodes (56, 56.1, 156, 256), and the third auxiliary electrode (56.2), all in the form of chromium electrodes, are connected to each other to form one or more units, one of which is Inert electrode (56.1) - Chromium electrode (54) - Inert electrode (56.2) The electrolytic cell (10, 100, 200) according to claim 8, characterized in that it is selected from among.

12. Use of the electrolytic cell (10, 100, 200) according to claim 8 for the production of a chromium layer on a rotationally symmetric component.

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