DE102021002197A1 - Device and method for coating a component or semi-finished product with a chromium layer - Google Patents

Abstract

The invention relates to a device for coating a component or semi-finished product with a chromium layer with an undivided deposition cell in which there is an anode and which is suitable for accommodating a cathodically connected component, the deposition cell containing a chromium(II)-containing electrolyte solution is located, and an electrolytic cell which is separated by a membrane arranged in the electrolytic cell into a cathode compartment in which a cathode is located and an anode compartment in which an anode is located, the cathode compartment being connected to the deposition cell via a line and a connected to a pump arranged in the line, which pump can pump liquid from the cathode compartment into the deposition cell and/or liquid from the deposition cell into the cathode compartment.

Description

The invention relates to a device for coating a component or a semi-finished product with a chromium layer. Furthermore, the invention relates to a method for coating a component with a chromium layer.

It is known from practice that decorative chromium layers and hard chromium layers are deposited on a component from electrolytic solutions containing chromium(VI). The chromic acid used is toxic and carcinogenic and has therefore been included in the lists of substances of very high concern (SVHC) of the EU Chemicals Regulation (REACH). Attempts have therefore been made for many years to replace the chromium(VI)-containing electrolyte solutions with chromium(III)-containing electrolyte solutions.

A fundamental problem is that stable complexes are formed between chromium(III) ions and six water molecules in aqueous electrolyte solutions, the so-called hexaaquachrome(III) complexes, which prevent the reduction of the chromium(III) ion and thus the separation of the Chroms kinetically inhibit. For this reason, complexing agents such as formates, oxalates or glycinates are usually added to the electrolyte solutions. Faster separation of the chromium from the chromium complexes that form is possible.

During the reduction of the chromium(III) ion, the chromium(III) ion is first reduced to the chromium(II) ion and then the chromium(II) ion to the metallic chromium. With the reduction of the chromium(III) ion to the chromium(II) ion, the charge of the chromium cation and thus the stability of the chromaqua complex, but also of most other chromium complexes, decreases. The kinetic inhibition of the chromium deposition is therefore caused in particular by the preceding reduction of the chromium(III) ion to the chromium(II) ion.

Against this background, the invention was based on the object of creating a device and a method that allow a component to be provided with a chromium layer on an industrial scale without having to use electrolyte solutions with chromium(VI) compounds, at least to be able to reduce the use of chromium(VI)-containing solutions.

This object is achieved by the device according to claim 1 and by the method according to claim 5. Advantageous embodiments are specified in the subclaims and the description below.

The basic idea of the invention is to carry out the reduction of the chromium(III) ion to the chromium(II) ion and the chromium(II) reduction to the metallic chromium in two different electrochemical cells whose electrolyte solutions are mutually exchanged via a circulatory system .

The device according to the invention has an undivided deposition cell in which there is an anode and which is suitable for accommodating a cathodically connected component. Instead of a single anode, a number of anodes and, in addition, smaller auxiliary anodes can also be arranged in the cell. In this way, good uniformity of the coating of the component is achieved in particular. The deposition cell can be a dip tank. The use of a dip tank allows individual or multiple parts or even a larger number of parts to be immersed in drums or on racks. However, the separation cell can also be a continuous cell in which the material is guided past one or more vertically, horizontally or radially arranged anodes through a basin. Furthermore, the deposition cell can also be combined with a reservoir and a pump in order to circulate the electrolyte solution in order to ensure thorough mixing of the electrolyte solution, in particular between the component and the anode. The deposition cell can also be a coating cell in which the electrolyte solution is guided between the anode and the surface of the component or semi-finished product without the component or semi-finished product and/or the anode being located in a basin. In this case, the cell is also combined with a pump and a reservoir for the electrolyte solution.

The invention offers the advantage that conventional coating cells for the coating of individual parts, but also mass-produced goods and semi-finished products can be used for its implementation. For the coating of mass-produced goods, preference is given to using rack and barrel processes. Continuous systems are preferably used for semi-finished products such as wire, strip and pipes. The invention can also be practiced with internal coating methods, e.g. B. for containers, pipes and bores. Here the electrolytic solution is poured into the container or tube and an anode is inserted.

The deposition cell is an undivided cell. An undivided separation cell is understood as meaning a space in which a liquid is located, which space is not divided by a membrane into sub-cells which are only connected to one another via the membrane.

In the separation cell there is a liquid in which a substance containing chromium(II) is dissolved. Depending on the chromium salts used, the liquid can also contain anions such as chloride, sulphate, hydrogen sulphate, fluoride, formate, oxalate, methanesulphonate, glycinate, citrate or acetate in addition to the dissolved chromium(II) ions. Furthermore, the liquid can contain at least one solvent such as water, ethylene glycol, acetic acid, dimethyl sulfoxide, formamide, dimethylformamide or ethylene carbonate. To a certain extent, particularly preferably during the deposition process, there can also be chromium(III) ions in the liquid, in particular if the chromium(II) ions are oxidized at the anode of the deposition cell. Furthermore, the liquid can contain one or more acid buffers and/or one or more conductive salts such as sodium sulfate, sodium chloride, sodium methanesulfonate, potassium sulfate, potassium chloride, potassium sulfate, aluminum sulfate and boric acid or uncharged complexing agents such as ammonia, glycine, thiosulfate, diethanolamine, thiourea or urea and additives such as polyethylene glycol contain.

According to the invention, the device also has an electrolytic cell. The electrolytic cell is separated by a membrane arranged in the electrolytic cell into a cathode compartment, in which a cathode is located, and an anode compartment, in which an anode is located.

The electrolytic cell can be designed as a basin. In a preferred design, the cathode and anode are designed and aligned geometrically in such a way that their distances from one another are the same everywhere and the membrane is arranged approximately in the middle. In a particularly preferred design, the cathode, membrane and anode are designed to be plane-parallel to one another and small distances in the range from 2 mm to 5 cm are selected between the cathode and membrane and the anode and membrane. If small distances are selected, it is advantageous to design both the cathode and the anode space as flow cells. In this case, the electrolyte solution can also be circulated in the anode chamber with the aid of a pump. An electrolyte reservoir can also be present in the electrolyte circuit. Several cells can also be combined to form a cell stack with separate inlets and outlets.

The membrane that separates the cathode space from the anode space is preferably an anion exchanger. However, operation with a cation exchanger or diaphragm is also possible.

The cathode located in the cathode space is preferably made of an electrically conductive material which is characterized by a high overvoltage for the cathodic decomposition of water. Cathodes made of copper, lead, tin, titanium, lead-antimony alloys or carbon are particularly suitable. Solid but also porous electrodes such as metal foams or carbon tiles can be used. Coatings of e.g. copper or carbon fleece are conceivable.

For example, bismuth, indium, lead, bismuth-lead, silver-lead, gold-lead or copper-lead are suitable for this.

The anode located in the anode space 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 can also be used. The liquid that is placed in the anode compartment preferably contains the same solvent as the liquid in the cathode compartment and an acid whose anion is identical to an anion that is in the electrolyte solution in the cathode compartment.

• • in einem ersten Betriebszustand Flüssigkeit mittels der Pumpe durch die Leitung vom Kathodenraum in die Abscheidungszelle gefördert wird und
• • in einem zweiten Betriebszustand Flüssigkeit mittels der Pumpe durch die Leitung von der Abscheidungszelle in den Kathodenraum gefördert wird.

According to the invention, the cathode compartment is connected to the deposition cell via a line and a pump arranged in the line, the pump being able to pump liquid from the cathode compartment into the deposition cell and/or liquid from the deposition cell into the cathode compartment. According to the invention, a method that can be carried out with the device according to the invention is conceivable, in which the direction of flow through the line is changed. The inventive method can be carried out in such a way that

• • in a first operating state, liquid is conveyed by means of the pump through the line from the cathode space into the deposition cell and
• • In a second operating state, liquid is conveyed by means of the pump through the line from the deposition cell into the cathode space.

The method can consist of a series of the two operating states. However, other operating states can also be provided in which no liquid is conveyed through the line.

In another embodiment, the line with the pump arranged in it is only used to convey the liquid in one direction. Embodiments are conceivable in which liquid is conveyed by means of the pump through the line from the cathode space into the deposition cell. In such an embodiment, the cathode compartment with the deposition cell be connected via an additional return line, whereby liquid flows via the return line from the deposition cell into the cathode space. Embodiments are conceivable in which liquid is conveyed by means of the pump through the line from the deposition cell into the cathode space. In such an embodiment, the deposition cell may be connected to the cathode compartment via an additional return line, liquid flowing from the cathode compartment into the deposition cell via the return line.

For deposition cells that are connected to a reservoir via lines and a pump, the previously described exchange of the electrolyte solution can also take place between the reservoir and the cathode space of the membrane cell.

The line can also be a gutter.

The invention enables a high concentration of chromium(II) ions to be maintained in an undivided deposition cell. In this way, the kinetically strongly inhibited chromium deposition can be accelerated and the power requirement (charge quantity in ampere-hours per mass of deposited chromium in kilograms) for the chromium deposition in the deposition cell can be reduced. Since there is no need for a divided deposition cell, parts with complex shapes can also be coated in a chromium(II)-containing electrolyte solution, in particular using additional auxiliary anodes. Furthermore, electrolyte solutions containing chloride can also be used in an undivided coating cell, since only the chromium(II) ion is oxidized to the chromium(III) ion at the anode of the deposition cell due to the lower oxidation potential of the chromium(II) ion and thus the oxidation of the bleach to the toxic chlorine is avoided. For the same reason, the oxidation of the chromium(III) ion to chromium(VI) is avoided. A particular advantage is also that in the deposition cell, pulsed current deposition of the chromium from the chromium(II)-containing electrolyte solution is made possible at high pulsed current densities. It is conceivable that even finely cracked chromium layers can be deposited with this process, which previously could only be produced from the toxic chromium(VI)-containing electrolyte solutions.

In a preferred embodiment, a chromium(III)-containing liquid is located in the cathode compartment. The liquid containing chromium(III) contains chromium(III) ions. The liquid in the cathode compartment can contain the same components as the liquid in the deposition cell, especially if the liquids are constantly changed. The concentration of chromium(II) ions can be slightly higher in the catholyte. Furthermore, the pH (acid concentration) in the catholyte and in the separation cell can be different.

In a preferred embodiment, a reference electrode is provided in the cathode compartment. This reference electrode can also be located outside the cathode compartment. In this case, the electrolyte solution of the reference electrode can be connected to the electrolyte solution in the cathode compartment via a capillary, the so-called Haber-Luggin capillary. The opening of the capillary in the cathode space is preferably positioned in the immediate vicinity of the cathode surface. Suitable reference electrodes include silver-silver chloride electrodes, calomel electrodes, lead sulphate electrodes or mercury sulphate electrodes.

In a preferred embodiment, the device has a connecting piece which can be electrically connected to the component to be coated and with which a potential can be applied to the component. The connector can be a clamp, for example. Depending on the deposition process, the electrical contact can also be made via frames with holders for the components (frame process), via conductive electrodes in drums (drum process), via current rollers in a continuous process, via sliding contacts or other contact-making conductive electrodes.

In a preferred embodiment, the device has a (first) current or voltage source with 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 connecting piece is provided which can be connected to the component to be coated and with which a potential can be applied to the component, the connecting piece being electrically connected to the second pole of the first voltage source.

In a preferred embodiment, the device has a second current or voltage source with a first pole and a second pole. In a preferred embodiment, the anode of the anode compartment is electrically connected to the first pole of the second voltage source. In a preferred embodiment, the cathode of the cathode compartment is electrically connected to the second pole of the second voltage source.

• • das zu beschichtende Bauteil in die in der Abscheidungszelle der erfindungsgemäßen Vorrichtung befindliche Chrom(II)-haltige Flüssigkeit getaucht wird,
• • das zu beschichtende Bauteil kathodisch geschaltet wird und die Anode anodisch geschaltet wird,
• • in der Abscheidungszelle aus der Flüssigkeit heraus an dem kathodisch geschalteten Bauteil eine Chromabscheidung erfolgt,
• • die in der Abscheidungszelle befindliche Flüssigkeit über die Leitung in den Kathodenraum gepumpt oder die in dem Kathodenraum befindliche Flüssigkeit über die Leitung in die Abscheidungszelle gepumpt wird.

The inventive method for coating a component with a chromium layer provides that

• • the component to be coated in the deposition cell of the invention device containing chromium(II)-containing liquid is immersed,
• • the component to be coated is connected cathodically and the anode is connected anodically,
• • in the deposition cell, chromium is deposited from the liquid on the cathodic connected component,
• • the liquid in the deposition cell is pumped into the cathode space via the line or the liquid in the cathode space is pumped into the deposition cell via the line.

At the anode of the deposition cell, the chromium(II) cations can oxidize to chromium(III) cations. In a preferred embodiment, the electrolyte solution which is depleted in chromium(II) cations and enriched in chromium(III) cations in the coating cell can be pumped into the cathode space. A constant flow of liquid can be used as well as an intermittent supply and removal of the electrolyte solution. In a preferred embodiment, the flow of liquid or the amounts of liquid exchanged can be measured in such a way that the concentration of chromium(II) ions in the deposition cell is only slightly lower than in the cathode space of the electrolytic cell.

For the reduction of the chromium(III) cations in the electrolytic cell, the cathode potential of the cathode can be measured against a reference electrode and adjusted. The cathode potential can be set by controlling the cell voltage or current. In this way, a cathode potential can be set which is small enough to reduce the chromium(III) ions to chromium(II) ions and large enough to avoid chromium deposition on the cathode of the electrolytic cell.

Depending on which chromium salt is used as the starting material, it may be appropriate to use a cation exchange or an anion exchange membrane. This makes it easier to keep the electrolyte concentration and the pH value constant in the electrolyte circulation system or to avoid oxidation to toxic substances at the anode. The chromium(III) ions and chromium(II) ions are usually present in solutions as complex-bound chromium cations. Depending on the number of anions bound to the chromium cation and the number of charges, the charge on the complexes is positive, neutral or negative. If the chromium(II) and chromium(III) complexes are completely or predominantly positively charged, an anion exchange membrane can be used to prevent or minimize the transfer of chromium ions into the anode compartment. If the chromium complexes are predominantly present as negatively charged ions, the use of a cation exchange membrane can also be considered. If no starting salt containing chloride is used, a diaphragm electrolysis cell can also be used instead of a membrane electrolysis cell. The use of membrane electrolysis cells, which are divided into three chambers by an anion and cation exchange membrane, is also possible. A three-chamber cell with a cathode compartment, anode compartment and a middle compartment without an electrode is ideal if anions from the electrolyte solution in the cathode compartment must not reach the anode in the anode compartment, as they can be oxidized there to form toxic substances. For example, chlorides in the electrolyte solution in the cathode compartment are oxidized to form poisonous chlorine gas if they reach the anode in the anode compartment. This can be avoided with a three-chamber cell, the center space of which is separated from the cathode space by an anion exchange membrane and the center space is separated from the anode space by a cation exchange membrane. In this way, the chlorides can get through the anion exchange membrane into the middle space, but their passage into the anode space is almost completely avoided by using the cation exchange membrane.

• • für Zink-Chrom-Abscheidung: Zusätzlich mindestens ein zinkhaltiges Salz erforderlich
• • für Chrom-Eisen-Abscheidung: Zusätzlich mindestens ein eisenhaltiges Salz
• • für Eisenabscheidung: Anstelle von chromhaltigen Substanzen muss mindestens ein eisenhaltiges Salz verwendet werden
• • Eisenlegierungen: Anstelle von chromhaltigen Substanzen müssen mindestens ein eisenhaltiges Salz sowie Salze der Legierungspartner verwendet werden

The invention can also be applied to the electroplating of alloys containing chromium, e.g. B. in the galvanic zinc-chromium or chromium-iron deposition. In addition, it is applicable to galvanic iron plating or plating of an alloy containing iron. The process can also be used for the galvanic chromium and iron recovery or recovery of these metals from salt solutions. For this, the electrolyte solutions should contain appropriate metal salts in the solvent:

• • for zinc-chromium separation: At least one salt containing zinc is also required
• • for chromium-iron separation: additionally at least one ferrous salt
• • for iron separation: Instead of substances containing chromium, at least one salt containing iron must be used
• • Iron alloys: Instead of substances containing chromium, at least one salt containing iron and salts from the alloy partners must be used

• 1 eine schematische Darstellung der erfindungsgemäßen Vorrichtung.

The invention is explained below with reference to a drawing that merely shows an exemplary embodiment of the invention in more detail. In it shows

• 1 a schematic representation of the device according to the invention.

The component 1 to be chromium-plated is connected cathodically with the aid of a power source 4 and an anode 2 in the deposition cell 3 in an electrolytic solution containing chromium(II) ions, so that chromium is deposited. The chromium(II) and chromium(III) ions, which are present almost exclusively as complex-bound chromium cations, are simply shown as Cr ²⁺ or Cr ³⁺ ions (cations) in the exemplary embodiment. The anions of the electrolyte solution in the deposition cell and in the cathode and anode compartment of the electrolytic cell were also not included.

The reduction of the chromium(II) ion to metallic chromium (equation 1) and, as a side reaction (equation 2), the water reduction with the release of hydrogen, preferably take place on the component 1 to be chrome-plated: Cr ²⁺ +2 e ^- ⇌ Cr(0) 1) H2O + 2e ^- _⇌½H2 + OH ^- ₂ )

At the anode 2 of the deposition cell 3, mainly chromium(II) ions, which are easily oxidizable, are oxidized to chromium(III) ions (equation 3): 2cr ²⁺ ⇌ 2e ^- + 2cr ³⁺ 3)

The chromium salt is supplied to the electrolytic solution in the electrolytic cell 7 as chromium (II) or chromium (III) salt and the chromium (III) cation at the cathode 8 of the divided electrolytic cell 7 to chromium (II) cation according to the following equation 4) reduced:- - 2 Cr ³⁺ +2 e ^- ⇌ 2 Cr ²⁺ 4)

The electrolyte solution from the cathode compartment of the electrolytic cell 7 is transferred via a line by means of a pump 5 into the deposition cell 3 and from there returned to the cathode compartment of the electrolytic cell 7 via a return line 6 . The concentration of chromium(II) cations required for reactions 1) and 3) can be maintained in the deposition cell 3 by continuous or repeated pumping around, ie by circulating the electrolyte solution. So that no chromium is deposited on the cathode 8, the voltage Uz of the voltage source 12 must be regulated in such a way that the voltage difference U _B between the cathode 8 and a reference electrode 11 corresponds to a target voltage which is suitable for converting the chromium(III) into the chromium( II) but not to reduce to metallic chromium. The feasibility of such voltage regulation is evident from the fact that the standard potential of the response in Equation 1) is -0.913V and that of the response in Equation 3) is only -0.41V.

The membrane 10 in the electrolytic cell 7 may be a cation or anion exchange membrane, or a simple diaphragm may be used. Depending on the selection of the chromium salt, it may be appropriate to use a cation or anion exchange membrane. If, for example, chromium(III) sulfate is used as the starting material, it makes sense to use an anion exchange membrane, since the sulfate anions supplied with chromium(III) sulfate are removed from the electrolyte circuit by the anion exchange membrane, ie transferred to the anode compartment. In this way, the concentration of the electrolyte can be kept constant despite constant replenishment of the chromium sulphate. At the same time, the chromium cations are almost completely retained by the anion exchange membrane and oxidation of the chromium cations to form toxic chromium(VI) at the anode 9 is avoided. At the anode 9 in the anode compartment of the electrolytic cell 7, water can be oxidized with the release of oxygen as the acid concentration increases according to the following equation 5): H ₂ O ⇌ ½ O ₂ + 2 H ⁺ + 2 e ^- 5)

If chromium sulfate is used as the chromium salt for the process, sulfuric acid is expediently initially introduced into the anode compartment. The sulfuric acid produced in the anode compartment can then be used to adjust the pH of the chromium-containing electrolyte solution. In addition to the chromium salt, complexing agents such as formate, glycinate or oxalate and other bath additives can be added to the coating electrolyte.

Pulsed current deposition of the chromium layer is also possible in the coating cell. For this purpose, instead of the direct current current source 4, a pulsed current source only has to be used. Chromium deposits with temporary or pulsed current reversal can also be carried out with this device.

Claims (6)

Device for coating a component (1) or a semi-finished product with a chromium layer with an undivided deposition cell (3), in which there is an anode (2) and which is suitable for receiving a cathodically connected component (1) or semi-finished product, with a chromium(II)-containing electrolyte solution is located in the deposition cell, characterized by an electrolytic cell (7) which is arranged in a cathode by a membrane (10) in the electrolytic cell (7). space, in which a cathode (8) is located, and an anode space, in which an anode (9) is located, is separated, the cathode space being connected to the deposition cell (3) via a line and a pump (5 ) is connected, wherein the pump (5) can pump liquid from the cathode compartment into the deposition cell (3) and/or liquid from the deposition cell (3) into the cathode compartment. device after claim 1 , characterized in that there is a liquid containing chromium (III) in the cathode space. device after claim 1 or 2 , characterized in that a reference electrode (11) is provided in the cathode compartment or this is connected to the electrolyte solution in the cathode compartment via an electrolyte bridge. Device according to one of Claims 1 until 3 , characterized in that the cathode compartment is connected to the deposition cell (3) via an additional return line (6). Method for coating a component (1) or a semi-finished product with a chromium layer using a device according to one of Claims 1 until 4 , characterized in that • the component (1) or semi-finished product to be coated is immersed in the chromium(II)-containing electrolyte solution located in the deposition cell (3), • the component (1) or semi-finished product to be coated is connected cathodically and the anode (2) is connected anodically, • in the deposition cell (3), chromium is deposited from the liquid on the cathodically connected component (1) or semi-finished product, • the liquid in the deposition cell (3) is pumped into the cathode chamber via the line or the liquid in the cathode compartment is pumped via the line into the deposition cell (3). procedure after claim 5 , characterized in that the potential of a cathode located in the cathode space is set in the cathode space with the aid of a reference electrode (11) located in the cathode space.

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