Device and method for the electrodeposition of chromium

Abstract

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

Description

[0001] The present invention relates to an apparatus and a method for the electroplating of a hard chromium layer made of a chromium(III) compound on a surface of a substrate.

[0002] 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 primarily serve 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 fiber-like products.

[0003] EP-0 565 070 B1 and EP-0 722 515 B1 describe a process for electroplating a chromium coating onto the surface of a substrate under conditions of a specific current flow. This process has since become established on the market as the TOPOCROM ®< process. With the TOPOCROM ®< process, a chromium coating can be easily applied in various variations without the need for mechanical or chemical pre- or post-treatment of the surface. In particular, it eliminates the 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.

[0004] 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.

[0005] A possible alternative is the use of Cr(III) electrolytes in electroplating processes. Cr(III) electrolytes are already used in decorative chrome plating processes. However, for hard chrome plating, with the considerably larger layer thicknesses required, there is currently no large-scale process that can produce chrome coatings with the required properties using Cr(III) electrolytes with a layer thickness of ≥ 3 µm.

[0006] 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 chrome layers from Cr(III) electrolytes. It was found that a hard chrome 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.

[0007] WO 2021 / 214389 A1 discloses a method for depositing a chromium layer on an object made of a Cr(III) electrolyte, in which heat treatment with the formation of chromium carbides is said to be unnecessary to achieve the desired hardness. Details of the coating process 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 prior to chromium plating.

[0008] The required pretreatment of the substrate to be coated represents a considerable challenge for large-scale implementation.

[0009] 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.

[0010] The above problem is solved by the subject matter of the patent claims.

[0011] 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 a C1-10 alkyl radical, characterized in that the anode is separated from the electroplating bath by a cation-ion selective membrane.

[0012] It has been found that the galvanic deposition 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 limitations described in the prior art (narrow process window, pretreatment of the substrate to be coated). However, it has been found that the galvanic deposition of a hard chromium layer from such an electrolyte solution is not easily feasible on an industrial scale. Many common anode materials result in vigorous evolution of CO2. This is unacceptable for a large-scale process, both in view of the significant decomposition of the electrolyte and the unacceptable formation of large quantities of the greenhouse gas CO2.

[0013] Furthermore, it was found that a constant process control is not possible under these conditions. Due to the evolution of CO2 at the anode (instead of the electrolysis of water that would otherwise occur 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 2 + -< → H 2 + -<

[0014] While only one proton is formed 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 Cr deposition. These problems have not been described in the prior art.

[0015] GB-1 602 404 proposed the use of a cation exchange membrane 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. CO2 evolution at the anode was not discussed.

[0016] 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).

[0017] It was found that for large-scale deposition of hard chrome coatings, the oxidation of carboxylate ions to CO 2 at the anode must be prevented or at least significantly reduced.

[0018] It was found that this goal can be achieved by separating the anode from the galvanic bath by an ion-selective membrane.

[0019] Without being limited to this explanation, it is assumed that a neutral complex forms in the electroplating bath from the Cr(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.

[0020] The present invention enables stable process control over an extended period of time, as there is no rapid and significant increase in pH. This allows the high layer thicknesses required for hard chrome plating to be deposited stably.

[0021] 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 chrome plating.

[0022] 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.

[0023] The present invention can, in principle, be carried out with any anode material that can be used for electrodeposition from a chromium-containing electroplating bath. Preferably, an anode is used that is a mixed metal oxide (MMO) electrode.

[0024] Mixed metal oxide (MMO) electrodes are well known and commercially available. To manufacture 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] For example, it can be a cuboid or cube-shaped container with an open side surface. This open side surface can be closed with 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.

[0030] 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.

[0031] 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 electroplating bath as possible.

[0032] 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 penetration.

[0033] 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.

[0034] In principle, any material suitable for chrome plating and capable of conducting electricity can be used as a substrate. According to the invention, the substrate is preferably made of a metal such as iron or steel.

[0035] According to the present invention, it is not necessary to subject the substrate to a conventional pretreatment, such as applying a nickel layer, to make it accessible for electrodeposition. In particular, the need for shot 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"), is eliminated. According to the invention, a chromium layer can be applied directly to the substrate from a Cr(III) electrolyte.

[0036] 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.

[0037] 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.

[0038] According to the present invention, the device may preferably comprise 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.

[0039] However, according to the invention, preference is given to a device and a process 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 is replaced for the next process stage by an electrolyte solution with a temperature T2 ≠ T1.

[0040] 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 (Cr 2 (SO 4 ) 3 ) is preferred according to the invention.

[0041] 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 a C1-10 alkyl radical, preferably a C1-4 alkyl radical. The alkyl radical can be linear or branched, but does not contain any further functional group. Examples include formic acid or acetic acid.

[0042] 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.

[0043] 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.

[0044] According to the invention, the galvanic bath preferably has a concentration of Cr(III) ions in the range from 0.5 mol / l to 2.0 mol / l, preferably 0.6 mol / l to 1.5 mol / l.

[0045] According to the invention, the electroplating 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 0.1-0.9, preferably 0.12 to 0.7 and particularly preferably 0.13 to 0.5.

[0046] 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.

[0047] 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) Inserting the substrate into the electroplating bath of the device, b) Applying a direct current to the anode of the device and the substrate acting as the cathode.

[0048] As described above, the separation of the anode or anode compartment from the electroplating bath by means of a cation-selective membrane essentially prevents undesired 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.

[0049] According to the invention, the process in step b) is preferably carried out at a temperature in the range of 35 to 75°C, preferably 40 to 60°C, particularly preferably 50 to 60°C, if a single chromium layer is to be deposited.

[0050] When depositing multiple 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, deposition preferably takes place in multiple 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 having a temperature in the range of 40 to 60°C, preferably 45 to 55°C. In a second process stage, 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 chrome layer 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.

[0051] 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.

[0052] 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.

[0053] 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 cathode at a pH value in the range of 6.0 to 7.0.

[0054] According to the invention, step c) can preferably be carried out by adding a pH-increasing substance. Step c), if carried out, is preferably carried out at the end of the electrodeposition for a short period of time of 1 second to 60 minutes, preferably 1 minute to 30 minutes.

[0055] 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 from 10 to 300 A / dm 2< , preferably 25 to 200 A / dm 2< and particularly preferably 30 to 120 A / dm 2<.

[0056] It has been shown that, with 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 layer 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.

[0057] The chromium layers obtained according to the invention can be applied to untreated substrates as described above.

[0058] The chromium layers obtained according to the invention have comparable properties, for example gloss properties, to chromium layers which are electrodeposited from electrolytes containing Cr(VI) ions.

[0059] According to the invention, it is possible to deposit an additional chromium layer from a Cr(III) electrolyte solution onto an existing chromium layer. This allows for larger layer thicknesses or multilayer chromium coatings with sublayers having different properties to be obtained.

[0060] 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, via which the desired roughness and closedness of the surface (topography) can be adjusted as required. This surface is absolutely free of sharp edges.

[0061] According to the invention, the base layer preferably has a thickness of 1 to 500 µm, preferably 10 to 250 µm. 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 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 µm, preferably 0.4-12 µm. The finished chrome layer for protecting the structural layer preferably has a thickness of 2 to 20 µm, particularly preferably 3 to 15 µm, and in particular 4 to 10 µm.

[0062] 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< are preferably applied for a period of 5 to 3600 min, preferably 5 to 60 min, preferably 30 to 50 min.

[0063] 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.Preferably, the structural layer is produced by means of a direct current application method, 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 brought about by the deposition of further deposition material by means of at least one subsequent pulse, 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 take place in stages of 0.5 to 50 A / dm 2< , preferably 1 to 6 A / dm 2<.

[0064] 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 sub-layers in which the deposited chromium is contained in different amounts.

[0065] 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.

[0066] The present invention is explained in more detail with reference to non-limiting figures and examples.

[0067] They show: Fig. 1 a schematic representation of an embodiment of the device according to the invention Fig. 2 a schematic representation of an embodiment of a container for an anode for the device according to the invention.

[0068] In Fig. 1 A schematic representation of an embodiment of the device 1 according to the invention is shown. 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.

[0069] The device (1) contains 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.

[0070] The anode 3 is separated from the galvanic bath 4 by a cation-selective membrane 5.

[0071] Substrate 2 and anode 3 are connected to a direct current source 6.

[0072] In Fig. 21 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 influenced by 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

1. Apparatus (1) for the electroplating of a hard chromium layer made of 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, where R is a C1-10 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 which does not oxidise Cr(III) ions to Cr(VI) ions.

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 thatthe 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) comprises 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. 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 7, 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.

9. Method according to claim 8, 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.

10. Method according to one of claims 8 or 9, characterized in that the process in step b) is carried out at a pH in the range of 4.5 to 6.0, preferably 5.0 to 5.

5.

11. Method according to one of claims 8 to 10, characterized in thatthe 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.

12. Method according to one of claims 8 to 11, characterized in that the process in step b) and optionally step c) 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.

13. A galvanic bath for a process according to any one of claims 8 to 12, comprising chromium(III) sulfate and ammonium formate, the bath containing no interfering metal ions.

14. Article comprising a chromium layer obtainable by the process according to any one of claims 8 to 12.

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