DEVICE AND METHOD FOR COATING A COMPONENT OR A SEMI-FINISHED PRODUCT WITH A CHROME LAYER - Patent application

Abstract

The present invention relates to a device for coating components or semi-finished products with a chromium layer, the device comprising an undivided deposition cell in which an anode is present and which is suitable for receiving a cathodically connected component, in which an electrolyte solution containing chromium(II) is present, the device having an electrolysis cell divided by a membrane arranged in the electrolysis cell into a cathode chamber in which the cathode is present and an anode chamber in which the anode is present, the cathode chamber being connected to the deposition cell by means of a line and a pump arranged in the line, the pump being able to pump liquid from the cathode chamber into the deposition cell and / or pump liquid from the deposition cell into the cathode chamber.

Description

[Technical field]

[0001] The present invention relates to a device for coating a component or a semi-finished product with a chrome layer. Furthermore, the present invention relates to a method for coating a component with a chrome layer. [Background technology]

[0002] It is well known from practice that decorative and hard chromium layers are deposited on components from electrolyte solutions containing chromium (VI). The chromic acid used in this process is toxic and carcinogenic and is therefore included in the list of substances of very high concern (SVHC) of the EU Chemicals Regulation (REACH). For this reason, attempts have been made for many years to replace chromium (VI)-containing electrolyte solutions with chromium (III)-containing electrolyte solutions.

[0003] The fundamental problem here is that in aqueous electrolyte solutions, stable complexes formed between chromium(III) ions and six water molecules, the so-called hexaaquachromium(III) complexes, kinetically inhibit the reduction of chromium(III) ions and thus the deposition of chromium. Therefore, complexing agents such as formates, oxalates, or glycinates are usually added to the electrolyte solution. The formation of these chromium complexes allows for a faster deposition of chromium.

[0004] During the reduction of chromium (III) ions, first, chromium (III) ions are reduced to chromium (II) ions, and then chromium (II) ions are reduced to metallic chromium. With the reduction of chromium (III) ions to chromium (II) ions, the charge of the chromium positive ion is reduced, and therefore the stability of not only chromium aqua complexes, but also most other chromium complexes. The kinetic inhibition of chromium deposition is therefore caused in particular by the upstream reduction of chromium (III) ions to chromium (II) ions. Summary of the Invention [Means for solving the problem]

[0005] Against this background, the present invention is based on the object of creating devices and processes that make it possible to provide components with a chromium layer on a technological scale, without the need to use electrolyte solutions containing chromium(VI) compounds, or at least to reduce the use of solutions containing chromium(VI).

[0006] This object is solved by a device according to claim 1 and by a method according to claim 5. Advantageous embodiments are defined in the dependent claims and in the description that follows.

[0007] The basic idea of ​​the present invention is to carry out the reduction of chromium(III) ions to chromium(III) ions and the chromium(III) reduction to metallic chromium in two different electrochemical cells, whose electrolyte solutions are exchanged with each other via a circulation system.

[0008] The device according to the invention has a non-divided deposition cell in which the anode is located and which is suitable for receiving the cathodically connected component. Instead of a single anode, several anodes and additionally smaller auxiliary anodes can be arranged in the cell. This achieves in particular a good uniformity of the coating of the component. The deposition cell can be an immersion bowl. The use of an immersion bowl allows single or multiple parts, or a larger number of parts, to be immersed in a drum or on a rack. However, the deposition cell can also be a continuous cell in which the material is passed through the bowl, past one or more vertically, horizontally or radially arranged anodes. Furthermore, the deposition cell can also be combined with a reservoir and a pump for pumping the electrolyte solution in a circuit, in particular to ensure a good mixing of the electrolyte solution between the component and the anode. The deposition cell can also be a coating cell in which the electrolyte solution is pumped between the anode and the component surface or the semi-finished product surface, the component or the semi-finished product and / or the anode are not present in the bowl. In the present case, the cell is also combined with a pump and a reservoir for the electrolyte solution.

[0009] The present invention provides the advantage that conventional coating cells can be used not only for coating individual parts, but also for coating mass-produced and semi-finished products. For coating mass-produced products, the use of a rack and barrel process is preferred. For coating semi-finished products such as wires, strips, and tubes, a continuous line is preferably used. The present invention can also be carried out with an internal coating process, for example for containers, tubes, and bores. Here, the electrolyte solution is filled into the container or tube and the anode is introduced.

[0010] A deposition cell is an undivided cell. An undivided deposition cell is a space in which a liquid is located, by which it is understood that the space is not divided by a membrane into subcells that are interconnected only through the membrane.

[0011] The deposition cell contains a liquid in which the chromium(II)-containing material is dissolved. Depending on the chromium salt used, the liquid may also contain, in addition to the dissolved chromium(II) ions, negative ions such as chloride, sulfate, hydrogen sulfate, fluoride, formate, oxalate, methanesulfonate, glycinate, citrate or acetate. Furthermore, the liquid may contain at least one solvent such as water, ethylene glycol, acetic acid, dimethylsulfoxide, formamide, dimethylformamide or ethylene carbonate. To a certain extent, and particularly preferably, chromium(III) ions may also be present in the liquid during the deposition process, especially when chromium(II) ions are oxidized at the anode of the deposition cell. Additionally, the liquid may contain one or more acid buffers such as sodium sulfate, sodium chloride, sodium methanesulfonate, potassium sulfate, potassium chloride, potassium sulfate, aluminum sulfate, and boric acid, and / or one or more conductive salts, or non-charged complexing agents such as ammonia, glycine, thiosulfate, diethanolamine, thiourea, or urea, and additives such as polyethylene glycol.

[0012] According to the invention, the device additionally comprises an electrolytic cell divided by a membrane arranged within the electrolytic cell into a cathode chamber, within which the cathode is located, and an anode chamber, within which the anode is located.

[0013] The electrolysis cell can be designed as a bowl. In a preferred design, the cathode and the anode are geometrically designed and aligned so that their distance from each other is the same everywhere and the membrane is arranged approximately in the center. In a particularly preferred design, the cathode, the membrane and the anode are made plane parallel to each other and short distances in the range of 2 mm to 5 cm are selected between the cathode and the membrane and the anode and the membrane. If short distances are selected, it is advantageous to design both the cathode and the anode chambers as flow-through cells. In this case, the electrolyte solution in the anode chamber may also be circulated by means of a pump. An electrolyte reservoir may also be provided in the electrolyte circuit. Several cells may also be combined to form a cell stack with separate inlets and outlets.

[0014] The membrane separating the cathode chamber from the anode chamber is preferably a negative ion exchanger. However, operation with a positive ion exchanger or a diaphragm is also conceivable. For example, a negative ion exchange membrane with a polymer backbone based on polyetheretherketone, polysulfone, polyphenylene ether, polybenzimidazole, fluoropolymer, or polystyrene copolymer may be used. Trimethylammonium, pyridinium, sulfonium, phosphonium, guanidinium, imidazolium, or piperidinium may be bonded to the polymer backbone as a positive ion functional group.

[0015] The cathode located in the cathode chamber is preferably made of a conductive material characterized by a high overvoltage for cathodic water splitting. Particularly suitable are cathodes made of copper, lead, tin, titanium, lead-antimony alloy, or carbon. Solid but also porous electrodes such as metal foam or carbon tiles can be used. For example, a coating of copper or carbon fleece can be envisaged. For example, bismuth, indium, lead, bismuth-lead, silver-lead, gold-lead, or copper-lead are suitable.

[0016] The anode located in the anode chamber is preferably a titanium anode coated with iridium mixed oxide. However, other electrode materials such as platinized titanium, lead, lead-antimony, carbon, or stainless steel may also be used. The liquid introduced into the anode chamber preferably contains the same solvent as the liquid in the cathode chamber and an acid whose negative ions are the same as those present in the electrolyte solution in the cathode chamber.

[0017] According to the invention, the cathode chamber is connected to the deposition cell via a line and a pump arranged in the line, the pump being capable of pumping liquid from the cathode chamber into the deposition cell and / or from the deposition cell into the cathode chamber. According to the invention, methods can be envisaged which can be performed with the device according to the invention, in which the direction of flow through the lines is changed. The method according to the invention can be performed in the following manner: In a first operating condition, liquid is conveyed from the cathode chamber through a line into the deposition cell by means of a pump. In a second operating condition, liquid is conveyed from the deposition cell through a line into the cathode chamber by means of a pump.

[0018] The method may consist of a sequence of two operating states, however other operating states may also be provided in which no liquid is conveyed through the line.

[0019] In another embodiment, the line with the pump arranged therein is used only to transport the liquid in one direction. There are conceivable embodiments in which the liquid is transported from the cathode chamber into the deposition cell through the line by means of a pump. In such an embodiment, the cathode chamber is connected to the deposition cell through an additional return line, and the liquid can flow from the deposition cell into the cathode chamber through the return line. There are conceivable embodiments in which the liquid is transported from the deposition cell into the cathode chamber through a line by a pump. In such an embodiment, the deposition cell is connected to the cathode chamber through an additional return line, and the liquid can flow from the cathode chamber into the deposition cell through the return line.

[0020] For deposition cells connected to a reservoir via lines and pumps, the exchange of electrolyte solution described above can also take place between the reservoir and the cathode chamber of the membrane cell.

[0021] A line may also be a channel.

[0022] The present invention allows a high concentration of chromium (II) ions to be maintained in the non-split deposition cell. In this way, the kinetically significantly inhibited chromium deposition can be accelerated and the current requirement for chromium deposition in the deposition cell (amount of charge in ampere hours per mass of chromium deposited in kilograms) is reduced. Since the split deposition cell can be dispensed with, chain-shaped parts can be coated in electrolyte solutions containing chromium (II), in particular using additional auxiliary anodes. Furthermore, due to the lower oxidation potential of chromium (II) ions, only chromium (II) ions are oxidized to chromium (III) ions at the anode of the deposition cell, thus avoiding the oxidation of chloride to toxic chlorine, and chloride-containing electrolyte solutions may also be used in the non-split deposition cell. For the same reason, the oxidation of chromium (III) ions to chromium (VI) is also avoided. It is also a particular advantage that pulsed current deposition of chromium from chromium (II)-containing electrolyte solutions at high pulse current densities is enabled in the deposition cell. It is conceivable that the method may even be used to deposit microcracked chromium layers, which previously could only be produced from toxic chromium(VI)-containing electrolyte solutions.

[0023] In a preferred embodiment, a liquid containing chromium (III) is present in the cathode chamber. The chromium (III)-containing liquid contains chromium (III) ions. The liquid in the cathode chamber may contain the same constituents as the liquid in the deposition cell, especially if the liquid is constantly replaced. The concentration of chromium (II) ions may be slightly higher in the cathode electrolyte. Furthermore, the pH (acid concentration) in the cathode electrolyte and in the deposition cell may be different.

[0024] In a preferred embodiment, a reference electrode is provided in the cathode chamber. This reference electrode may also be located outside the cathode chamber. In this case, the electrolyte solution of the reference electrode can be connected to the electrolyte solution in the cathode chamber via a capillary, a so-called Herbalgin capillary. The opening of the capillary in the cathode chamber is preferably located in close proximity to the cathode surface. Suitable reference electrodes include a silver-silver chloride electrode, a mercurous chloride electrode, a lead sulfate electrode, or a mercuric sulfate electrode.

[0025] In a preferred embodiment, the device has a connector that can be electrically connected to the components to be coated, with which an electrical potential can be applied to the components. The connector can be, for example, a terminal. Depending on the deposition process, the electrical contact can also be made via a rack with receptacles for the components (rack process), via a discharge electrode in a drum (drum process), via a current roller in a continuous process, via sliding contact or other contact type discharge electrodes.

[0026] In a preferred embodiment, the device comprises a (first) current or voltage source having a first pole and a second pole. In a preferred embodiment, the anode of the deposition cell is electrically connected to the first pole of the first voltage source. In a preferred embodiment, a connector is provided that can be connected to the component to be coated and can be used to apply an electric potential to the component, the connector being electrically connected to the second pole of the first voltage source.

[0027] In a preferred embodiment, the device comprises a second current or voltage source having a first pole and a second pole. In a preferred embodiment, the anode of the anode chamber is electrically connected to the first pole of the second voltage source. In a preferred embodiment, the cathode of the cathode chamber is electrically connected to the second pole of the second voltage source.

[0028] The method according to the invention for coating a component with a chromium layer comprises: the component to be coated is immersed in a chromium(II)-containing liquid present in a deposition cell of the device according to the invention, the component to be coated is cathodically connected and the anode is anodically connected, Chromium deposition is carried out in a deposition cell from a liquid on a cathodically connected component; the liquid in the deposition cell is pumped through a line into the cathode chamber, or the liquid in the cathode chamber is pumped through a line into the deposition cell; to provide.

[0029] At the anode of the deposition cell, chromium (II) positive ions can be oxidized to chromium (III) positive ions. In a preferred embodiment, in the deposition cell, chromium (II) positive ions are depleted and an electrolyte solution rich in chromium (III) positive ions can be pumped into the cathode chamber. Both a constant liquid flow and an intermittent supply or delivery of the electrolyte solution are applicable. In a preferred embodiment, the liquid flow, i.e. the amount of exchanged liquid, can be dimensioned such that the concentration of chromium (II) ions in the deposition cell is only slightly lower than that in the cathode chamber of the electrolysis cell.

[0030] For the reduction of chromium (III) positive ions in the electrolytic cell, the cathode potential of the cathode may be measured and adjusted relative to a reference electrode. The cathode potential may be adjusted by controlling the cell voltage or current. Thus, the cathode potential may be set small enough to reduce chromium (III) ions to chromium (II) ions and large enough to avoid chromium deposition on the cathode of the electrolytic cell.

[0031] Depending on the chromium salt used as starting material, it may be appropriate to use a positive or negative ion exchange membrane. This makes it easier to keep the electrolyte concentration and pH constant in the electrolyte circulation system or to avoid oxidation to harmful substances at the anode. Chromium (III) and chromium (II) ions are usually present in solution as chained chromium positive ions. Depending on the number of negative ions and the number of charges attached to the chromium positive ions, the charge of the complex is positive, neutral, or negative. If the chromium (II) and chromium (III) complexes are completely or mainly positively charged, a negative ion exchange membrane can be used to prevent or minimize the transport of chromium ions into the anode chamber. If the chromium complexes are mainly present as negatively charged ions, a positive ion exchange membrane can also be used. If a chloride-containing starting salt is not used, a diaphragm electrolysis cell may also be used instead of a membrane electrolysis cell. It is also possible to use a membrane electrolysis cell divided into three chambers by negative and positive ion exchange membranes. A three-chamber cell with a cathode chamber, an anode chamber, and a central chamber without electrodes is suitable if the negative ions of the electrolyte solution in the cathode chamber are not allowed to reach the anode in the anode chamber, where they may be oxidized to toxic substances. For example, chlorides in the electrolyte solution in the cathode chamber are oxidized to toxic chlorine gas if they reach the anode in the anode chamber. This can be avoided by using a three-chamber cell whose central chamber is separated from the cathode chamber with a negative ion exchange membrane, and the central chamber is separated from the anode chamber with a positive ion exchange membrane. In this way, chlorides can pass through the negative ion exchange membrane into the central chamber, but their transport into the anode chamber is almost completely avoided by the use of a positive ion exchange membrane.

[0032] The invention can also be applied to the electrodeposition of chromium-containing alloys, such as zinc-chromium or chromium-iron electrodeposition. In addition, it may be used to electroplate iron or to plate alloys containing iron. The method may also be used for galvanic chromium and iron extraction or recovery of these metals from saline solutions. For this purpose, the electrolyte solution should contain the appropriate metal salts in a solvent. In the case of zinc-chromium deposition: at least one additional zinc-containing salt is required. For chromium-iron deposition: in addition, at least one ferrous salt. In the case of iron deposits: at least one ferrous salt must be used instead of a chromium-containing substance. Ferrous alloys: Instead of chromium-containing substances, at least one ferrous salt and an alloying partner salt must be used. [Brief description of the drawings]

[0033] The invention will now be described with reference to the drawings, which merely illustrate exemplary embodiments of the invention in more detail.

[0034] [Figure 1] FIG. 1 is a schematic depiction of a device according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The component 1 to be chromium plated is cathodically connected with a current source 4 and an anode 2 in a deposition cell 3 in an electrolyte solution containing chromium (II) ions, and thus chromium deposition takes place. The chromium (II) and chromium (III) ions, which are present almost exclusively as chained chromium cations, are represented in the exemplary embodiment as Cr 2+ and Cr 3+ ions (positive ions) are shown in simplified form only. The negative ions of the electrolyte solution in the deposition cell and in the cathode and anode chambers of the electrolysis cell are also not included.

[0036] On the component 1 to be chromium plated, the reduction of chromium(II) ions to metallic chromium (equation 1) and, as a secondary reaction (equation 2), the reduction of water with the release of hydrogen, take place preferentially. [ka]

[0037] At the anode 2 of the deposition cell 3, primarily the easily oxidizable chromium(II) ions are oxidized to chromium(III) ions (Equation 3). [ka]

[0038] Chromium salts are added to the electrolyte solution in the electrolytic cell 7 as chromium (II) or chromium (III) salts according to equation 4) below, and chromium (III) cations are reduced to chromium (II) cations at the cathode 8 of the divided electrolytic cell 7. [ka]

[0039] The electrolyte solution from the cathode chamber of the electrolytic cell 7 is transported via a line by means of a pump 5 to the deposition cell 3 and from there returned via a return line 6 to the cathode chamber of the electrolytic cell 7. The concentration of chromium(II) positive ions required for reactions 1) and 3) can be maintained in the deposition cell 3 by continuous or repeated recirculation of the electrolyte solution. To ensure that chromium is not deposited at the cathode 8, the voltage Uz of the voltage source 12 is adjusted to a voltage difference U between the cathode 8 and the reference electrode 11. B must be controlled to correspond to a target voltage suitable for reducing chromium(III) to chromium(II), but not to metallic chromium. The feasibility of such voltage control arises from the fact that the standard potential for the reaction in equation 1) is −0.913 V, and that for the reaction in equation 3) is only −0.41 V.

[0040] The membrane 10 in the electrolysis cell 7 can be a positive or negative ion exchange membrane, or a simple diaphragm is used. Depending on the choice of chromium salt, it may be appropriate to use a positive or negative ion exchange membrane. For example, if chromium(III) sulfate is used as the starting material, it makes sense to use a negative ion exchange membrane, since the sulfate anions supplied with chromium(III) sulfate are removed from the electrolyte circuit, i.e., transported to the anode chamber, using the negative ion exchange membrane. In this way, the electrolyte concentration can be kept constant despite the constant replenishment of chromium sulfate. At the same time, the chromium positive ions are almost completely retained by the negative ion exchange membrane, and oxidation of the chromium positive ions at the anode 9 to toxic chromium(VI) is avoided. At the anode 9 in the anode chamber of the electrolysis cell 7, water can be oxidized with the release of oxygen and an increase in the acid concentration according to the following equation 5): [ka]

[0041] When chromium sulfate is used as the chromium salt for the present method, it is advisable to introduce sulfuric acid into the anode chamber. The sulfuric acid generated in the anode chamber can then be used to adjust the pH of the chromium-containing electrolyte solution. In addition to the chromium salt, complexing agents such as formates, glycinates, or oxalates, and other additives can be added to the coating electrolyte.

[0042] Pulsed current deposition of chromium layers is also possible in the coating cell. For this purpose, a pulsed current source or a pulsed reverse current source must simply be used instead of the direct current source 4. Chromium deposition with intermittent or pulsed current reversal can also be performed with the present device.

Claims

1. A device for coating a component (1) or a semi-finished product with a chromium layer, said device having an undivided deposition cell (3) in which an anode (2) is located and which is suitable for receiving a cathodically connected component (1) or a semi-finished product, an electrolyte solution containing chromium (II) being located in said deposition cell, The device comprises an electrolytic cell (7) divided by a membrane (10) arranged in the electrolytic cell (7) into a cathode chamber in which a cathode (8) is located and an anode chamber in which an anode (9) is located, The membrane (10) is a negative ion exchange membrane, The device, wherein the cathode chamber is connected to the deposition cell (3) via a line and a pump (5) arranged in the line, the pump (5) being capable of pumping liquid from the cathode chamber into the deposition cell (3) and / or pumping liquid from the deposition cell (3) into the cathode chamber.

2. The device of claim 1 , wherein a liquid containing chromium (III) is present in the cathode chamber.

3. 2. The device of claim 1, wherein a reference electrode (11) is provided in the cathode chamber or is connected to the electrolyte solution in the cathode chamber via an electrolyte bridge.

4. 2. The device according to claim 1, wherein the cathode chamber is connected to the deposition cell (3) via an additional return line (6).

5. 2. The device according to claim 1, wherein the current source (4) is provided in the form of a rectifier or a pulsed current source or a pulsed inverse current source.

6. A method for coating a component (1) or a semi-finished product with a chromium layer using a device according to any one of claims 1 to 4, comprising the steps of: The component (1) or semi-finished product to be coated is immersed in the chromium(II)-containing electrolyte solution present in the deposition cell (3), the component (1) or semi-finished product to be coated is cathodically connected and the anode (2) is anodically connected, Chromium deposition is carried out in said deposition cell (3) from a liquid on said cathodically connected component (1) or semi-finished product, The liquid present in the deposition cell (3) is pumped through the line into the cathode chamber of the electrolysis cell (7) or the liquid present in the cathode chamber is pumped through the line into the deposition cell (3).

7. 7. The method according to claim 6, wherein the potential of a cathode located in the cathode chamber is set in the cathode chamber with the aid of a reference electrode (11) located in the cathode chamber.

8. 7. The method according to claim 6, wherein chromium (III) ions are reduced to chromium (II) ions at the cathode (8), thus replenishing consumed chromium (II) ions.

9. 7. The method according to claim 6, wherein the coating is performed using a direct current or the coating is performed using a pulsed current or an intermittent or pulsed reverse current.

10. Use of a device for coating a component (1) or a semi-finished product with a chrome layer, said device comprising: an undivided deposition cell (3) in which an anode (2) is located and which is suitable for receiving a cathodically connected component (1) or a semi-finished product, and an electrolyte solution containing chromium (II) is located in said deposition cell; an electrolytic cell (7) divided by a membrane (10) arranged within the electrolytic cell (7) into a cathode chamber in which a cathode (8) is located and an anode chamber in which an anode (9) is located; Equipped with The membrane (10) is a positive ion exchange membrane or diaphragm; The cathode chamber is connected to the deposition cell (3) via a line and a pump (5) arranged in the line, the pump (5) being capable of pumping liquid from the cathode chamber into the deposition cell (3) and / or pumping liquid from the deposition cell (3) into the cathode chamber.

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