KR20230173685A - Coating device and coating method for coating components or semi-finished products with a chrome layer - Google Patents

Abstract

The present invention relates to a device for coating a component or semi-finished product with a chromium layer, the device comprising an anode-positioned, non-split deposition cell suitable for receiving a cathodically connected component, the deposition cell comprising a chromium layer. (II) A containing electrolyte solution is provided, and the device has an electrolytic cell divided into a cathode chamber in which the cathode is located and an anode chamber in which the anode is located by a membrane disposed therein, the cathode chamber having a line, and this line It is connected to the deposition cell by a pump disposed in, and the pump is capable of pumping liquid from the cathode chamber to the deposition cell and/or pumping liquid from the deposition cell to the cathode chamber.

Description

Coating device and coating method for coating components or semi-finished products with a chrome layer

The present invention relates to a coating device for coating components or semi-finished products with a chromium layer. Additionally, the present invention relates to a coating method for coating a component with a chromium layer.

It is well known in the field that decorative chrome layers and hard chrome layers are deposited on components from chromium(VI)-containing electrolytes. Because the chromic acid used in this process is toxic and carcinogenic, it is included in the list of Substances of Very High Concern (SVHC) under the EU Chemicals Regulations (REACH). For this reason, attempts have been made for many years to replace chromium (Ⅵ)-containing electrolyte solutions with chromium (Ⅲ)-containing electrolyte solutions.

The fundamental problem here is the hexaaquachrome (Ⅲ) complex between the chromium (Ⅲ) ion and six water molecules in the aqueous electrolyte - this complex kinetically inhibits the reduction of chromium (Ⅲ), thereby inhibiting the formation of a chromium film. A stable complex called - is formed. Therefore, complexing agents such as formates, oxalates or glycidates are usually added to the electrolyte solution. When this chromium complex is formed, chromium can be deposited more quickly.

During reduction of chromium(III) ions, chromium(III) ions are first reduced to chromium(II) ions, and then chromium(II) ions are reduced to metallic chromium. As chromium(III) ions are reduced to chromium(II) ions, the charge of the chromium cation and thus the stability of the chromium aqua complex as well as the stability of most other chromium complexes decrease. Accordingly, the kinetic inhibition of chromium film formation is caused in particular by the upstream reduction of chromium(III) ions to chromium(II) ions.

Against this background, the present invention provides a device and It was based on the purpose of forming the method.

This object is solved by the device according to claim 1 and the method according to claim 5. Advantageous embodiments are explained in the independent claims and the description below.

The basic idea of the present invention is to reduce chromium (Ⅲ) ions to chromium (Ⅱ) ions and to reduce chromium (Ⅱ) ions to metallic chromium in two different electrochemical cells, and the electrolytes of the electrochemical cells are circulated in a circulation system. are exchanged with each other.

The device according to the invention has a non-split deposition cell in which the anode is located and is suitable for receiving cathodically connected components. Instead of a single anode, multiple anodes and additional smaller auxiliary anodes may be placed in the cell. This achieves in particular good uniformity of the coating of the components. The deposition cell may be an immersion bath. Due to the use of an immersion bath, one or several parts or even more parts can be immersed in a drum or in a rack. However, a deposition cell can also be a continuous cell in which material passes through a water bath past one or more vertically, horizontally or radially arranged anodes. Moreover, the deposition cell can also be combined with a reservoir and pump to pump the electrolyte into the circuit to ensure good mixing of the electrolyte, especially between the components and the anode. The deposition cell may also be a coating cell in which an electrolyte solution is supplied between the anode and the component surface or the semi-finished product surface in the absence of the component or semi-finished product and/or anode in the water bath. In this case, the cell is also combined with a pump and reservoir for the electrolyte.

The present invention provides the advantage that conventional coating cells can be used not only for the coating of individual parts but also for the coating of mass-produced goods and semi-finished products. For coating mass produced goods, the use of a rack and barrel process is preferred. For coating semi-finished products such as wire, strip and tube, it is desirable to use a continuous line. The invention can also be practiced by internal coating processes, for example for containers, tubes and bores. Here, electrolyte is charged into a container or tube and an anode is introduced.

The deposition cell is a non-divided cell. A non-divided deposition cell is understood to be a space in which a liquid is located, whereby said space is not divided by a membrane into sub-cells, which are connected to each other only via a membrane.

The deposition cell contains a liquid in which the chromium(II)-containing material is dissolved. Depending on the chromium salt used, the liquid contains, in addition to the dissolved chromium(II) ions, additional anions such as chloride, sulfate, hydrogen sulfate, fluoride, formate, oxalate, methanesulfonate, glycinate, citrate or acetate. It can be included. Additionally, the liquid may contain at least one solvent such as water, ethylene glycol, acetic acid, dimethyl sulfoxide, formamide, dimethyl formamide or ethylene carbonate. To some extent, chromium(III) ions may also be present in the liquid, particularly preferably during the deposition process, when chromium(II) ions are oxidized at the anode of the deposition cell. Moreover, the liquid may contain an acid buffer and/or one or more conducting salts such as sodium sulfate, sodium chloride, sodium methanesulfonate, potassium sulfate, potassium chloride, potassium sulfate, aluminum sulfate and boric acid or ammonia, glycine, thiosulfate, diethanolamine, thiourea urea. and additives such as polyethylene glycol.

According to the invention, the device additionally has an electrolytic cell. The electrolytic cell is divided into a cathode chamber where the cathode is located and an anode chamber where the anode is located by a membrane disposed in the electrolytic cell.

The electrolytic cell can be configured as a water bath. In a preferred configuration, the cathode and anode are geometrically configured and aligned such that the distance from each other is the same at all locations and the membrane is approximately centered. In a particularly preferred configuration, the cathode, membrane and anode are formed planarly parallel to each other and a short distance in the range of 2 mm to 5 cm is selected between the cathode and the membrane and the anode and the membrane. If a short distance is chosen, it is advantageous to configure both the cathode and anode chambers as perfusion cells. In this case, the electrolyte in the anode chamber can also be circulated by a pump. The electrolyte circuit may also be provided with an electrolyte reservoir. Multiple cells can also be combined to form cell stacks with separate inlets and outlets.

The membrane separating the cathode chamber from the anode chamber is preferably an anion exchanger. However, operation by cation exchangers or diaphragms is also conceivable. For example, anion exchange membranes with a polymer backbone based on polyetheretherketone, polysulfone, polyphenylene ether, polybenzimidazole, fluoropolymers or polystyrene copolymers can be used. Trimethylammonium, pyridinium, sulfonium, phosphonium, guanidinium, imidazolium or piperidinium can be bound to the polymer backbone as cationic functional groups.

The cathode located in the cathode chamber is preferably formed of an electrically conductive material characterized by a high overvoltage for cathode water splitting. Cathodes made of copper, lead, tin, titanium, lead-antimony alloys or carbon are particularly suitable. Solid porous electrodes such as metal foam or carbon tiles can be used. Coatings made of copper or carbon fleece, for example, can also be considered. For example, bismuth, indium, lead, bismuth-lead, silver-lead, gold-lead or copper-lead are suitable.

The anode located within 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 a solvent identical to the liquid in the cathode chamber and an acid whose anions are identical to the anions present in the electrolyte solution in the cathode chamber.

According to the invention, the cathode chamber is connected to the deposition cell via a line and a pump arranged in this line, in which case the pump can pump liquid from the cathode chamber to the deposition cell and/or from the deposition cell to the cathode chamber. According to the invention, a method that can be implemented by the device according to the invention is conceivable, in which the direction of flow through the line is changed. The method according to the present invention

In the first operating state, the liquid is transported from the cathode chamber to the deposition cell through the line by the pump,

In the second operating state, it can be carried out in such a way that the liquid is transferred from the deposition cell to the cathode chamber by means of a pump through a line.

The method may consist of a sequence of two operating states. However, other operating conditions in which no liquid is conveyed through the lines can also be arranged.

In another embodiment, only the line with the pump disposed therein is used to transport liquid in one direction. Embodiments in which the liquid is transported from the cathode chamber to the deposition cell through a line by a pump are also conceivable. In the above-described embodiment, the cathode chamber may be connected to the deposition cell via an additional return line, through which liquid flows from the deposition cell to the cathode chamber. Embodiments in which the liquid is transported from the deposition cell to the cathode chamber through a line by a pump are also conceivable. In the above-described embodiment, the deposition cell may be connected to the cathode chamber via an additional return line, through which liquid flows from the cathode chamber to the deposition cell.

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

A line can also be a channel.

The present invention allows high concentrations of chromium (II) ions to be maintained in a splitless film deposition cell. In this way, the strongly kinetically inhibited chromium deposition can be accelerated and the current requirement for chromium deposition in the deposition cell (charge in ampere-hours per kilogram mass of deposited chromium) is reduced. Since the split deposition cell can be omitted, parts of complex shape can also be coated in an electrolyte containing chromium(II), especially with the use of additional auxiliary anodes. Additionally, chromium-containing electrolytes can also be used in splitless deposition cells because, due to the lower oxidation potential of chromium(II) ions, only chromium(II) ions are converted to chromium(III) ions at the anode of the deposition cell. This is because it is oxidized and prevents chloride from being oxidized to toxic chlorine. For this reason, oxidation of chromium(III) ions to chromium(VI) is also prevented. It is also a special advantage that it becomes possible to pulse-current-deposit chromium from a chromium(II)-containing electrolyte solution at a high pulse current density in the deposition cell. It is conceivable that this method could also be used to deposit microcrack-forming chromium layers, which previously could only be produced from toxic chromium(VI)-containing electrolytes.

In a preferred embodiment, a chromium(III) containing liquid is present in the cathode chamber. Chromium(III)-containing liquids contain chromium(III) ions. The liquid in the cathode chamber may contain the same components as the liquid in the deposition cell, especially if the liquids are continuously exchanged. The concentration of chromium(II) ions may be slightly higher in the catholyte. Additionally, the pH (acid concentration) in the catholyte and the pH (acid concentration) in the film deposition cell may be different.

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 coupled to the electrolyte solution in the cathode chamber via a capillary, the so-called Haber-Luggin capillary. The opening of the capillary within the cathode chamber is preferably positioned in the immediate vicinity of the cathode surface. Suitable reference electrodes include a silver-silver chloride electrode, a calomel electrode, a lead sulfate electrode, or a mercury sulfate electrode.

In a preferred embodiment, the device has a connector that can be electrically connected to the component to be coated and that can apply a potential to the component. A connector may be a terminal, for example. Depending on the deposition process, electrical contact may also be made via racks with receptacles for the components (rack process), via discharge electrodes in drums (drum process), via current rollers in continuous processes, forming sliding contacts or other contacts. This can be achieved through discharge electrodes.

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 a first pole of a first voltage source. In a preferred embodiment, a connector is provided that is connectable to the component to be coated, is used to apply a potential to the component and is electrically connected to a second pole of the first voltage source.

In a preferred embodiment, the device comprises a (second) current or voltage source with 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 a second pole of a second voltage source.

In the method according to the invention for coating a component with a chromium layer,

· The component to be coated is immersed in a chromium(II)-containing solution present in the electrolytic cell of the device according to the invention,

· The component to be coated is connected cathodically, the anode is connected anodically,

· Chromium deposition occurs on the cathodically connected component from the liquid in the deposition cell,

· The liquid in the deposition cell is pumped into the cathode chamber through a line, or the liquid in the cathode chamber is pumped into the deposition cell through a line.

At the anode of the deposition cell, chromium(II) cations may be oxidized to chromium(III) cations. In a preferred embodiment, the electrolyte solution in the deposition cell depleted of chromium(II) cations and enriched with chromium(lll) cations can be pumped into the cathode chamber. Both constant liquid flow and intermittent supply or discharge of electrolyte are applicable. In a preferred embodiment, the liquid flow or amount of liquid exchanged can be dimensioned such that the concentration of chromium(II) ions in the deposition cell is only slightly lower than in the cathode chamber of the electrolytic cell.

For reduction of chromium (IIl) cations in the electrolytic cell, the cathode potential of the cathode can be measured and adjusted relative to the reference electrode. The cathode potential can be adjusted by controlling the cell voltage or current. Accordingly, the cathode potential can be set small enough to reduce chromium(III) ions to chromium(II) ions and large enough to prevent chromium film formation on the cathode of the electrolytic cell.

Depending on the chromium salt used as starting material, it may be appropriate to use a cation exchange membrane or an anion exchange membrane. This makes it easier to maintain constant electrolyte concentration and pH in the electrolyte circulation system or to prevent oxidation of toxic substances at the anode. Chromium(III) ions and chromium(II) ions usually exist in solution as complex chromium cations. Depending on the number of anions and the number of charges bound to the chromium cation, the charge of the complex is positive, neutral, or negative. If the chromium(II) and chromium(III) complexes are completely or predominantly positively charged, an anion exchange membrane may be used to prevent or minimize the transfer of chromium ions into the anode chamber. Cation exchange membranes may also be used if the chromium complexes exist primarily as negatively charged ions. If chloride-containing starting salts are not used, diaphragm electrolytic cells may also be used instead of membrane electrolytic cells. It is also possible to use a membrane electrolysis cell divided into three chambers by anion and cation exchange membranes. In cases where the negative ions of the electrolyte in the cathode chamber are not allowed to reach the anode in the anode chamber, where they can be oxidized to toxic substances, a three-chamber cell with a cathode chamber, an anode chamber and a central chamber without electrodes is suitable. do. For example, chloride in the electrolyte in the cathode chamber is oxidized to toxic chlorine gas when it reaches the anode in the anode chamber. This can be avoided by a three chamber cell where the central chamber is separated from the cathode chamber by an anion exchange membrane and the central chamber is separated from the anode chamber by a cation exchange membrane. In this way, chloride can pass through the anion exchange membrane into the central chamber, but transfer to the anode chamber is almost completely prevented by the use of the cation exchange membrane.

The present invention can also be applied to electrode deposition of chromium-containing alloys, such as zinc-chromium or chromium-iron electrode deposition. Additionally, the present invention can be used for electroplating iron or plating iron-containing alloys. The method can also be used for galvanic chromium and iron extraction or recovery of these metals from salt solutions. For this purpose, the electrolyte must contain an appropriate metal salt in the solvent:

· For zinc-chromium deposition: at least one additional salt containing zinc is required.

· For chromium-iron deposition: Additionally, at least one ferrous salt.

· For iron deposition: at least one ferrous salt should be used instead of a chromium-containing material.

· Ferrous alloys: Instead of chromium-containing substances, at least one ferrous salt and a salt of an alloying partner should be used.

The invention is explained below with reference to the drawings which merely illustrate exemplary embodiments of the invention in more detail.

Figure 1 is a schematic diagram of a device according to the invention.

The component to be chrome plated (1) is cathodically connected to an electrolyte solution containing chromium (II) ions by the current source (4) and anode (2) of the deposition cell (3), and chromium deposition occurs. Chromium(II) and Chromium(III) ions, which exist almost exclusively as complex chromium cations, are shown in simplified form in the exemplary embodiments only as Cr ²⁺ and Cr ³⁺ ions (cations), respectively. Negative ions of the electrolyte solution within the deposition cell and within the cathode and anode chambers of the electrolysis cell were also not included.

On the component 1 to be chrome plated, preferably, chromium(II) ions are converted to metallic chromium (Scheme 1) and, as a secondary reaction (Scheme 2), water reduction occurs with the evolution of hydrogen.

At the anode 2 of the deposition cell 3, mainly chromium(II) ions - these ions are easily oxidized - are oxidized to chromium(III) ions (Scheme 3).

Chromium salt is added to the electrolyte solution of the electrolytic cell (7) as chromium (Ⅱ) or chromium (Ⅱ) salt, and chromium (Ⅲ) cations are added to the cathode (8) of the electrolytic cell (7) divided according to the following Scheme 4. It is reduced to chromium (Ⅱ) cation.

The electrolyte solution from the cathode chamber of the electrolytic cell 7 is delivered by the pump 5 through a line to the deposition cell 3, and from there it is returned to the cathode chamber of the electrolytic cell 7 through the return line 6. . The concentration of chromium (II) cations required for reactions 1) and 3) can be maintained in the film deposition cell 3 by continuous or repeated recycling of the electrolyte solution. To ensure that chromium is not deposited on the cathode 8, the voltage Uz of the voltage source 12 is adjusted such that the voltage difference U _B between the cathode 8 and the reference electrode 11 is such that the chromium(III) It must be controlled to correspond to a target voltage that reduces chromium (ll) but does not reduce metallic chromium. The feasibility of such voltage control is due to the fact that the standard potential for the reaction in Scheme 1) is -0.913 V, and the standard potential for the reaction in Scheme 3) is only -0.41 V.

The membrane 10 in the electrolytic cell 7 can be a cation or anion exchange membrane, or a simple diaphragm is used. Depending on the choice of chromium salt, it may be appropriate to use a cation or anion exchange membrane. For example, when chromium(III) sulfate is used as a starting material, the sulfate anion supplied to chromium(III) sulfate is removed from the electrolyte circuit by the anion exchange membrane and transported to the anode chamber, so using an anion exchange membrane It is reasonable. In this way, the electrolyte concentration can be kept constant despite constant replenishment of chromium sulfate. At the same time, the chromium cations are almost completely retained by the anion exchange membrane and their oxidation to toxic chromium(VI) at the anode 9 is prevented. At the anode 9 in the anode chamber of the electrolytic cell 7, water can be oxidized by evolution of oxygen and an increase in acid concentration according to the following equation 5):

If chromium sulfate is used as the chromium salt for the process, it is convenient to introduce sulfuric acid into the anode chamber. The sulfuric acid produced within the anode chamber can then be used to adjust the pH of the chromium-containing electrolyte solution. In addition to chromium salts, complexing agents such as formates, glycinates or oxalates and other additives may be added to the coating electrolyte.

Pulsed current deposition of chromium layers is also possible in a coating cell. For this purpose, a simply pulsed current source or a pulsed countercurrent source must be used instead of the direct current source (4). Chromium deposition by intermittent or pulsed current reversal can also be performed by this device.

Claims (9)

A coating for coating the component (1) or semi-finished product with a chromium layer, having a non-split film deposition cell (3) suitable for receiving the component (1) or semi-finished product, in which the anode (2) is located and cathodically connected. As a device, a chromium (II)-containing electrolyte is located in a film formation cell, a cathode chamber in which a cathode 8 is located, and an anode 9 are located by a membrane 10 with an electrolytic cell 7 disposed therein. It is divided into an anode chamber, and the cathode chamber is connected to the film deposition cell 3 through a line and a pump 5 disposed in this line, and the pump 5 is used to transfer the film from the cathode chamber to the film deposition cell 3 and/or A coating device capable of pumping liquid from the cell (3) to the cathode chamber. 2. The coating device of claim 1, wherein a chromium(III) containing liquid is present in the cathode chamber. The coating device according to claim 1 or 2, wherein the reference electrode (11) is provided in the cathode chamber or is connected to the electrolyte solution in the cathode chamber via an electrolyte bridge. The coating device according to claim 1 , wherein the cathode chamber is connected to the deposition cell (3) via an additional return line (6). The coating device according to claim 1 , wherein a current source (4) is provided in the form of a rectifier or a pulsed current source or a pulsed countercurrent source. A coating 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 component to be coated (1) or semi-finished product is immersed in the chromium (II)-containing electrolyte solution present in the film forming cell (3),
· The component to be coated (1) or the semi-finished product is connected cathodically, and the anode (2) is connected anodically,
Chromium deposition takes place on the cathodically connected component (1) or semi-finished product from the liquid in the deposition cell (3),
· A coating method wherein the liquid present in the deposition cell (3) is pumped into the cathode chamber of the electrolytic cell (7) through a line, or the liquid present in the cathode chamber is pumped into the deposition cell (3) through a line. The coating method according to claim 6, wherein the potential of the cathode located in the cathode chamber is set in the cathode chamber with the help of a reference electrode (11) located in the cathode chamber. The coating method according to claim 6 or 7, wherein the chromium (III) ions are reduced to chromium (II) ions at the cathode (8) and the chromium (II) ions thus consumed are replenished. 9. A coating method according to any one of claims 6 to 8, wherein the coating is effected by direct current, or the coating is effected by pulsed current or intermittent or pulsed reverse current.

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