Device and method for galvanic deposition of chromium

Abstract

The present invention relates to a device (1) for galvanic deposition of a hard chromium layer of a chromium (III) compound on a surface of a substrate (2), comprising an anode (3) and an galvanic bath (4) in which the substrate (2) acting as a cathode can be introduced, wherein the galvanic bath (4) contains a Cr (III) compound and a carboxyl compound of formula R-COOH or a salt thereof, wherein R is H or a C1-10 alkyl radical, characterised in that the anode (3) is separated from the galvanic bath (4) by a cation-selective membrane (5). The present invention further relates to a method for galvanic deposition of a hard chromium layer of a chromium (III) compound on a surface of a substrate (2), in said device (1), and to an article produced by said method comprising a chromium layer.

Description

[0001] Device and method for electroplating chromium

[0002] The present invention relates to an apparatus and a method for the electroplating deposition of a hard chromium layer made of a chromium ( III ) compound on a surface of a substrate .

[0003] Coating objects with a layer of chrome has been a practice for many years. A distinction is made between so-called bright chrome coatings of up to 2 μm thickness, which serve primarily decorative purposes, and so-called hard chrome coatings with greater layer thicknesses. Hard chrome coatings impart technical functionality to an object and are used, for example, in the manufacture of skin-pass rolls or straightening rolls in the steel industry, or conveyor rolls for fibrous products.

[0004] EP-0 565 070 B1 and EP-0 722 515 B1 describe a process for electroplating a surface by means of which a chromium coating is galvanically applied to the surface of a substrate under conditions of a specific current flow. This process has now become established on the market as the TOPOCROM® process. With the TOPOCROM® process, a chromium coating can be applied in various variations in a simple manner, without the need for any mechanical or chemical pre- or post-treatment of the surface. In particular, there is no longer any need for shotblasting or sandblasting of the surface to be coated, which leads to a surface texture ("orange peel") that is unfavorable for processing carbon fibers.

[0005] Hard chrome coatings are currently commercially deposited from chromium electrolytes containing chromium in the hexavalent state. In recent years, however, the use of Cr(VI) electrolytes has been increasingly restricted by regulations, as Cr(VI) compounds are considered to pose significant health and environmental risks.

[0006] A possible alternative is the use of Cr(III) electrolytes in electroplating processes. Cr(III) electrolytes are already used in decorative chromium plating processes. However, for hard chromium plating, with the considerably larger layer thicknesses to be deposited, there is currently no large-scale process that would allow the production of chromium coatings with the required properties using Cr(III) electrolytes with a layer thickness of > 3 pm.

[0007] In Bohnet's dissertation (Jens Bohnet, Development of a process for the deposition of technical chromium layers from a chromium (III) electrolyte, University of Stuttgart 2009), investigations were conducted into the deposition of hard chromium layers from Cr(III) electrolytes. It was found that a hard chromium layer appeared possible only under narrow, specific conditions (a special electrolyte solution containing ammonium chromium alum, glycine, and ammonium sulfate, a narrow process window of 38-42°C, and a pH of 2.1 to 2.3). These narrow process conditions pose significant challenges for large-scale implementation.

[0008] WO 2021 / 214389 A1 discloses a method for depositing a chromium layer on an article made from a Cr(III) electrolyte, in which heat treatment with the formation of chromium carbides is said to be unnecessary in order to achieve the desired hardness. Details of the coating method are not disclosed. However, the presence of iron and / or nickel cations is described as necessary for the deposition of a chromium-containing layer. In the examples, the substrate to be coated was pretreated, in particular a prior application of a nickel layer before chromium plating.

[0009] The required pretreatment of the substrate to be coated represents a considerable challenge for large-scale implementation.

[0010] It was the object of the present invention to provide a method for the galvanic deposition of a hard chromium layer with which the disadvantages of the prior art described above are overcome.

[0011] The above problem is solved by the subject matter of the patent claims.

[0012] The present invention relates to a device for the electroplating of a hard chromium layer from a chromium (III) compound on a surface of a substrate, comprising an anode and an electroplating bath into which the substrate acting as a cathode can be introduced, wherein the electroplating bath contains a Cr (III) compound and a carboxyl compound of the formula R-COOH or a salt thereof, wherein R is H or a Cl-10-alkyl radical, characterized in that the anode is separated from the electroplating bath by a cation-ion selective membrane.

[0013] It has been found that the electrodeposition of a hard chromium layer from an electrolyte solution comprising Cr(III) cations and a carboxyl compound can in principle be carried out without the restrictions described in the prior art (narrow process window, pretreatment of the substrate to be coated). However, it has been found that the electrodeposition of a hard chromium layer from such an electrolyte solution cannot be easily implemented on an industrial scale. Many common anode materials give off violent amounts of CO2. This is unacceptable for an industrial-scale process both in view of the significant decomposition of the electrolyte and the unacceptable formation of large quantities of the greenhouse gas CO2.

[0014] Furthermore, it was found that a constant process is not possible under these conditions. Due to the evolution of CO2 at the anode (instead of the electrolysis of water that normally occurs at the anode), a rapid increase in pH occurs due to the following reaction equations (for the example of using formate as a carboxyl compound): + H +

[0015] H2+ — ► H2+

[0016] While only one proton is produced per reaction at the anode, two hydroxyl ions are formed per reaction at the cathode. As a result, only very thin layers can be deposited before the sharply increased pH prevents further deposition of Cr. These problems have not been described in the prior art.

[0017] In GB-1 602 404, the use of a cation exchange membrane was proposed for electrodeposition from a bath containing Cr(III) sulfate to prevent possible oxidation of Cr(III) to Cr(VI). A carboxyl compound as in the present invention was not used, but an amino acid (glycine) was used instead. CCy evolution at the anode was not discussed.

[0018] In DE-10 2006 035 871 B3, the use of an anion exchange membrane was proposed to prevent the penetration of Cr(III) ions to the anode and their oxidation to Cr(VI).

[0019] It was found that for large-scale deposition of hard chrome coatings, the oxidation of carboxylate ions to CO2 at the anode must be prevented or at least significantly reduced.

[0020] It was found that this goal can be achieved by separating the anode from the galvanic bath by an ion-selective membrane.

[0021] Without being limited to this explanation, it is assumed that a neutral complex forms in the electroplating bath from the Or (III) ions and the carboxylate ions, which undergoes only partial oxidation at the anode to CO2 and a positively charged complex. This positively charged complex can pass through the cation-selective membrane and thus be separated from the anode.

[0022] The present invention enables stable process control over an extended period of time, as there is no rapid and significant increase in the pH value. This allows the high layer thicknesses required for hard chromium plating to be deposited stably. During electroplating from a bath containing Cr(III) ions, oxidation of Cr(III) ions to Cr(VI) ions may occur under certain circumstances. This is fundamentally undesirable and can completely prevent successful hard chromium plating.

[0023] According to a preferred embodiment of the present invention, the anode is therefore made of a material which does not oxidize Cr(III) ions to Cr(VI) ions.

[0024] The present invention can, in principle, be carried out with any anode material that can be used for electroplating from a chromium-containing electroplating bath. Preferably, an anode is used that is a mixed metal oxide (MMO) electrode.

[0025] Mixed metal oxide (MMO) electrodes are well known and commercially available. To produce these MMO electrodes, a substrate (e.g., a titanium plate or a titanium grid) is coated with a thin layer of other metals (e.g., selected from the group consisting of ruthenium, iridium, and tantalum) or compounds thereof, such as their oxides, to impart specific properties to the anodes.

[0026] If the present invention is carried out with a mixed metal oxide (MMO) electrode as anode, undesirable oxidation of Cr (III) ions to Cr (VI) ions can be avoided.

[0027] According to the present invention, the anode is separated from the electroplating bath by a cation-selective membrane. Cation-selective membranes are well known and commercially available. Examples include perfluorinated cation exchange membranes. Cation-selective membranes can be penetrated by cations, but not by anions.

[0028] In principle, the anode can be separated from the electroplating bath in any way using a cation-selective membrane. For example, the electroplating bath can be divided into two sections, preferably two halves, by a cation-selective membrane by providing a cation-selective membrane at a desired position within the electroplating bath.

[0029] According to the present invention, it is preferred that the anode and the cation-selective membrane are arranged in a container, preferably a plastic container, wherein the container comprises an inlet and an interior space, wherein the anode is located in the interior space and a medium passing through the inlet can only reach the interior space by passing through the cation-selective membrane. The shape of the container is freely selectable and can, for example, be in the form of a cube, cuboid, or cylinder.

[0030] For example, it can be a cuboid or cube-shaped container with an open side surface. This open side surface can be closed by a plate that has openings for the passage of the electrolyte solution into the container. The openings must be dimensioned such that the molecules and ions contained in the electrolyte solution can pass through.

[0031] Behind the plate, a cation-selective membrane is arranged in such a way that the molecules and ions passing through the plate must pass through the cation-selective membrane in order to reach the interior of the container and thus to the anode arranged in this interior.

[0032] The container is preferably dimensioned in such a way that it has sufficient space to accommodate a conventional anode , but takes up as little volume of the galvanic bath as possible .

[0033] The anode located in the container is connected to a power source, such as a battery, by an electrical connection such as a power cable. The electrical connection is routed through a surface, preferably the top surface, of the container in such a way that no molecules or ions of the electrolyte solution can enter the container at the point of passage.

[0034] The device according to the invention comprises a galvanic bath. The galvanic bath is provided in a container of the type commonly used in electroplating. In addition to the galvanic bath (i.e., the electrolyte solution), the container also contains the above-described anode, the cation-selective membrane, and a substrate to be coated. During the galvanic deposition of a chromium layer, the substrate acts as the cathode.

[0035] In principle, any material suitable for chromium plating and capable of conducting electricity can be used as the substrate. According to the invention, the substrate is preferably made of a metal such as iron or steel.

[0036] According to the present invention, it is not necessary to subject the substrate for electrodeposition to a conventional pretreatment, such as applying a nickel layer, in order to make it accessible for electrodeposition. In particular, there is no need for shotblasting or sandblasting or for additional complex chemical and / or electrochemical pretreatment steps of the surface to be coated, which lead to an unfavorable surface texture ("orange peel"). According to the invention, a chromium layer can be applied directly to the substrate from an Or(III) electrolyte.

[0037] According to the present invention, a layer of a polyhydroxy compound such as glycerol can be applied to the substrate prior to electrodeposition, as described in EP-3 000 918 A1.

[0038] The anode and the substrate acting as the cathode are connected to a power source via electrical wires, such as power cables. The power source, together with the anode and the substrate acting as the cathode, forms an electrical circuit through which direct current flows during operation. Conventional direct current sources can be used.

[0039] According to the present invention, the device can preferably have a temperature control unit in order to be able to carry out a galvanic deposition at a temperature preferred according to the invention in the range of 35 to 75 ° C, preferably 40 to 60 ° C, particularly preferably 50 to 60 ° C. Such temperature control units, for example external heating elements, are well known.

[0040] However, according to the invention, preference is given to a device and a method as described in EP-3 000 918 A1, in which the process stages are not realized by heating or cooling a single electrolyte contained in the reactor, but rather an electrolyte solution with a temperature T1 for the next process stage is replaced by an electrolyte solution with a temperature T2 V T1.

[0041] According to the present invention, the electroplating bath, ie, the electrolyte solution, contains at least one Cr(III) compound in an aqueous solution for the electrodeposition of chromium on the substrate acting as the cathode. Any Cr(III) compound used in the prior art for electroplating processes can be used. Cr(III) sulfate (Cr2(SO4)3) is preferred according to the invention.

[0042] According to the present invention, the electroplating bath, ie, the electrolyte solution, contains at least one carboxylate compound of the formula R-COOH, where R is H or a C 1-10 alkyl radical, preferably a C 1-4 alkyl radical. The alkyl radical can be linear or branched, but does not contain any further functional groups. Examples include formic acid or acetic acid.

[0043] According to the present invention, a salt of the above carboxylate compound can also be used. According to the invention, alkali metal salts, alkaline earth metal salts, or ammonium salts of the corresponding carboxylate compound can preferably be used. A preferred salt is ammonium formate as the formic acid salt.

[0044] According to a particularly preferred embodiment of the present invention, the electroplating bath contains no added foreign ions such as lead, copper, iron, or nickel ions. Such foreign ions can interfere with or influence the electroplating deposition of chromium from a Cr(III) electrolyte. According to a preferred embodiment of the present invention, the electroplating bath consists of the above-described Cr(III) compound and the above-described carboxylate compound.

[0045] According to the invention, the galvanic bath preferably has a concentration of Cr (III) ions in the range from 0.3 mol / l to 2.0 mol / l, preferably 0.5 mol / l to 2.0 mol / l, more preferably 0.6 mol / l to 1.5 mol / l.

[0046] According to the invention, the galvanic bath preferably has a concentration of carboxylate ions such that the molar ratio of Cr (III) ions to carboxylate ions is in the range of 0.05-0.9, preferably 0.1-0.9, more preferably 0.12 to 0.7 and particularly preferably 0.13 to 0.5.

[0047] According to a further preferred embodiment of the present invention, the electroplating bath consists of the above-described Cr (III) compound, the above-described carboxylate compound, and triethanolamine.

[0048] According to the invention, the electroplating bath preferably has a triethanolamine concentration in the range of 2-100 mL / L, preferably 5-50 mL / L. It has been shown that the addition of triethanolamine can increase the stability of the electroplating bath, and the electroplating bath can be operated even at ambient conditions (20-25 °C) without significant precipitation.

[0049] It has been found that a very simple electroplating bath is sufficient for the process according to the invention. The additional components present in the prior art, such as halogen salts, boric acid, and the like, can be omitted according to the invention, resulting in economic and ecological advantages.

[0050] According to the invention, the galvanic bath preferably has a pH value in the range from 4.5 to 6.0, preferably 5.0 to 5.5.

[0051] The present invention further relates to a method for the galvanic deposition of a hard chromium layer made of a chromium (III) compound on a surface of a substrate, in a device according to the invention as described above, comprising the steps: a) introducing the substrate into the galvanic bath of the device, b) applying a direct current to the anode of the device and the substrate acting as cathode.

[0052] As described above, the separation of the anode or anode compartment from the electroplating bath by means of a cation-selective membrane essentially prevents unwanted oxidation of the carboxyl compound contained in the electroplating bath at the anode. This has the advantage, among others, that the process in step b) can be carried out at a pH in the range of 4.5 to 6.0, preferably 5.0 to 5.5, without a rapid, significant increase in the pH during the electroplating deposition. Furthermore, the carboxyl compound contained in the electroplating bath is thus protected from decomposition.

[0053] According to the invention, the process in step b) is preferably carried out at a temperature in the range from 35 to 75°C, preferably 40 to 60°C, particularly preferably 50 to 60°C, if a single chromium layer is to be deposited. In the case of deposition of several chromium layers, as described in EP-0 565 070 B1 and EP-0 722 515 B1 and established on the market as the TOPOCROM® process, the deposition preferably takes place in several process stages at different or optionally identical temperatures. Thus, in a first process stage, a base layer of chromium can be deposited using an electrolyte which has a temperature in the range from 40 to 60°C, preferably 45 to 55°C. In a second process step, a structural chromium layer can be deposited using an electrolyte having a temperature in the range of 25 to 39°C, preferably 30 to 38°C, or alternatively in the range of 40 to 60°C, preferably 45 to 55°C.Finally, in a third process step, a top layer of chrome can be deposited using an electrolyte which again has a higher temperature in the range of 40 to 60°C, preferably 45 to 55°C.

[0054] According to the invention, the process in step b) is preferably carried out at a pH in the range from 4.5 to 6.0, preferably 5.0 to 5.5.

[0055] According to a further embodiment of the present invention, the deposition of a base layer as described above can be dispensed with, so that only one or two chromium layers (structural and optionally cover layer) have to be deposited as described above.

[0056] However, it has been shown that the layer stability or passivity of the deposited chromium layer can be further increased if the process additionally comprises a step c) in which a direct current is applied to the anode of the device and the substrate acting as the cathode at a pH in the range from 6.0 to 7.0. According to the invention, step c) can preferably be carried out by adding a substance that increases the pH. Step c), if carried out, is preferably carried out at the end of the galvanic deposition for a short period of time of 1 s to 60 min, preferably 1 min to 30 min.

[0057] It is further preferred according to the invention that the process in step b) and optionally step c) is carried out with a current density in the range of 10 to 300 A / dm 2 , preferably 25 to 200 A / dm 2 and particularly preferably 30 to 120 A / dm 2 is carried out .

[0058] It has been shown that, using the device and method according to the invention, hard chromium layers can be deposited from an electrolyte containing Cr(III) ions, which layers have the thickness required for hard chromium layers. This is achieved by the fact that, according to the invention, it is possible to carry out the deposition of chromium consistently over a long period of time.

[0059] The chromium layers obtained according to the invention can be applied to untreated substrates as described above.

[0060] The chromium layers obtained according to the invention have comparable properties, for example gloss properties, to chromium layers which are galvanically deposited from electrolytes containing Cr (VI) ions.

[0061] According to the invention, it is possible to deposit a further chromium layer from a Cr(III) electrolyte solution onto an existing chromium layer. In this way, greater layer thicknesses or multi-layer chromium coatings with partial layers having different properties can be obtained. For example, as described in EP-0 722 515 B1 and EP-4 012 074 A1, a multi-layer chromium coating system can be produced using the TOPOCROM® process. A direct current base layer can first be applied to a substrate before a structural layer is applied to this base layer. Furthermore, for certain applications, an additional chromium layer (so-called finished chromium layer or top layer) can be applied to the structural layer.A structural layer with dome-shaped (hemispherical) elevations is formed, which can be used to adjust the desired roughness and closed surface (topography) as desired. This surface is absolutely free of sharp edges.

[0062] According to the invention, the base layer preferably has a thickness of preferably 1 to 500 pm, preferably 10 to 250 pm. According to the invention, the layer thickness of the structural layer is roughness-dependent. An exemplary roughness value (mean roughness value Ra according to DIN EN ISO 4287: 2010, i.e. the calculated mean value of all deviations of the roughness profile from the mean line along the reference distance) of a structural layer according to the invention is 0.1-15 gm, preferably 0.4-12 gm. The finished chrome layer for protecting the structural layer preferably has a thickness of preferably 2 to 20 gm, particularly preferably 3 to 15 gm and in particular 4 to 10 gm.

[0063] To produce a chromium base layer, current densities in the range of 30 to 50 A / dm 2 , preferably 35 to 45 A / dm 2 for a period of 5 to 3600 min, preferably 5 to 60 min, preferably 30 to 50 min.

[0064] The so-called structural layer is then applied to this base layer. For example, in the TOPOCROM process, the structural chrome layer formed there comprises hemispherical domes. The structural layer is preferably produced by means of a direct current application process, wherein nucleation of the deposition material is achieved by means of at least one initial pulse of the electrical voltage and / or the electrical current on the surface to be coated and then growth of the deposition material nuclei is induced by means of at least one subsequent pulse by means of the deposition material nuclei being added, wherein during the nucleation phase the increase or decrease of the electrical voltage and / or the electrical current takes place in several stages, the time between the increases is between 0.1 and 120 seconds, preferably 0.1 and 30 seconds, wherein current density changes in stages from 0.5 to 50 A / dm 2, preferably 1 to 6 A / dm 2 take place .

[0065] Alternatively, as described in EP-4 012 074 A1, a surface coating can be applied, comprising a base layer and a structural layer applied thereto, wherein the base layer comprises at least two partial layers in which the deposited chromium is contained in different amounts.

[0066] According to a further embodiment of the present invention, the deposition of a base layer as described above can be dispensed with, so that only one or two chromium layers (structural and optionally cover layer) have to be deposited as described above.

[0067] The present invention is explained in more detail with reference to non-limiting figures and examples.

[0068] Shown are: Fig. 1 a schematic representation of an embodiment of the device according to the invention

[0069] Fig. 2 is a schematic representation of an embodiment of a container for an anode for the device according to the invention.

[0070] Fig. 1 shows a schematic representation of an embodiment of the device 1 according to the invention. The device 1 can have any shape. A cylindrical shape is preferred. The dimensions of the device 1 can be varied depending on the body to be coated.

[0071] Arranged in the device (1) are a substrate 2 acting as a cathode and an anode 3, for example, an MMO electrode. Substrate 2 and anode 3 are located in a galvanic bath 4, which represents the electrolyte solution required for the galvanic deposition. The galvanic bath contains Cr(III) ions, preferably in the form of chromium(III) sulfate, and a carboxyl compound, preferably ammonium formate.

[0072] The anode 3 is separated from the galvanic bath 4 by a cation-selective membrane 5.

[0073] Substrate 2 and anode 3 are connected to a direct current source 6 .

[0074] Fig. 2 shows a schematic representation of an embodiment of the device 1 according to the invention with an embodiment of a container 7 for an anode 3. The container 7 has dimensions adapted to the size of the anode 3 and is preferably cuboid-shaped. The container 7 has an inlet 7a and an interior space 7b. The interior space 7b contains the anode 3 and the anolyte 7c (i.e. the part of the electroplating bath 4 under the influence of the anode). The inlet is preferably a plate 7a with which a side surface of the container 7 can be closed in the usual way (for example by clamp closures). The plate 7a has openings through which the molecules and / or ions contained in the electroplating bath 4 can reach the interior space 7b.For this purpose, the molecules and / or ions contained in the electroplating bath 4, after passing through the plate 7a, must pass through the cation-selective membrane 5, which is arranged between the inlet 7a and the interior 7b in a conventional manner (e.g., by clamp closures). The anode 3 in the interior 7b of the container 7 is thus separated from the electroplating bath 4 by the cation-selective membrane 5.

Claims

Patent claims 1. Device (1) for the galvanic deposition of a hard chromium layer from a chromium (III) compound on a surface of a substrate (2), comprising an anode (3) and a galvanic bath (4) into which the substrate (2) acting as a cathode can be introduced, wherein the galvanic bath (4) contains a Cr (III) compound and a carboxyl compound of the formula R-COOH or a salt thereof, wherein R is H or a Cl-10 alkyl radical, characterized in that the anode (3) is separated from the galvanic bath (4) by a cation-selective membrane (5).

2. Device according to claim 1, characterized in that the anode (3) is made of a material on which oxidation of Or (III) ions to Cr (VI) ions does not occur.

3. Device according to claim 2, characterized in that the anode (3) is a mixed metal oxide (MMO) electrode.

4. Device according to one of claims 1 to 3, characterized in that the anode (3) and the cation-selective membrane (5) are arranged in a container (7), preferably a plastic container, the container comprising an inlet (7a) and an interior space (7b), the anode (3) being located in the interior space (7b) and a medium passing through the inlet (7a) being able to reach the interior space (7a) only by passing through the cation-selective membrane (5).

5. Device according to one of claims 1 to 4, characterized in that the galvanic bath (4) contains a chromium(III) compound and formic acid or a formic acid salt.

6. Device according to claim 5, characterized in that the chromium (III) compound is chromium (III) sulfate.

7. Device according to claim 5 or 6, characterized in that the galvanic bath (4) contains ammonium formate as formic acid salt.

8. Device according to one of claims 5 to 7, characterized in that the galvanic bath (4) is selected from the group consisting of a galvanic bath which consists of the Cr (III) compound, preferably chromium (III) sulfate, and the carboxylate compound, preferably ammonium formate, and a galvanic bath which consists of the Cr (III) compound, preferably chromium (III) sulfate, the carboxylate compound, preferably ammonium formate, and triethanolamine.

9. A method for the galvanic deposition of a hard chromium layer made of a chromium (III) compound on a surface of a substrate (2), in a device (1) according to one of claims 1 to 8, comprising the steps of: a) introducing the substrate (2) into the galvanic bath (4) of the device (1), b) applying a direct current to the anode (3) of the device (1) and the substrate (2) acting as the cathode.

10. The process according to claim 9, characterized in that the process in step b) is carried out at a temperature in the range of 35 to 75°C, preferably 40 to 60°C, particularly preferably 50 to 60°C.

11. The process according to claim 9 or 10, characterized in that the process in step b) is carried out at a pH in the range from 4.5 to 6.0, preferably 5.0 to 5.

5.

12. The method according to any one of claims 9 to 11, characterized in that the method additionally comprises a step c) in which the application of a direct current to the anode (3) of the device (2) and the substrate (2) acting as cathode is carried out at a pH in the range of 6.0 to 7.

0.

13. The method according to any one of claims 9 to 12, characterized in that the method in step b) and optionally step c) is carried out with a current density in the range of 10 to 300 A / dm 2 , preferably 25 to 200 A / dm2 and particularly preferably 30 to 120 A / dm 2 is carried out.

14. Galvanic bath for a process according to any one of claims 9 to 13, comprising chromium (III) sulfate and ammonium formate, wherein the bath contains no interfering metal ions.

15. Article comprising a chromium layer obtainable by the process according to any one of claims 9 to 13.

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