Corrosion resistant cr(III) coating

Abstract

The present invention relates to an electroplated article comprising a substrate and a coating, which includes a chromium layer, preferably a hard chromium layer, produced from a Cr(III) compound, characterized in that a layer is arranged between the surface of the substrate and the chromium layer, preferably the hard chromium layer, in which a metal is contained having a standard electrode potential that is lower than the standard electrode potential of the substrate. The electroplated article is characterized by high corrosion resistance. The present invention further relates to a method for producing the electroplated article.

Description

[0001] The present invention relates to a corrosion-resistant, electroplated article with a chromium layer made of a chromium(III) compound, and to a method for its production.

[0002] Coating objects with a layer of chromium is a process that has been carried out for many years. A distinction is made between so-called bright chrome coatings, up to about 2 µm thick, which primarily serve decorative purposes, and so-called hard chrome coatings with greater thicknesses. Hard chrome coatings give an object technical functionality and are used, for example, in the manufacture of tempering rollers or straightening rollers in the steel industry, or conveyor rollers for fibrous products.

[0003] EP-0 565 070 B1 and EP-0 722 515 B1 describe a process for electroplating surfaces, in which a chromium coating is electroplated onto the surface of a substrate under specific current conditions. This process is now established in the market as the TOPOCROM® process. With the TOPOCROM® process, a chromium coating can be easily applied in various configurations without the need for mechanical or chemical pre- or post-treatment of the surface. In particular, it eliminates the need for shot blasting or sandblasting of the surface to be coated, which results in a surface texture ("orange peel") that is unfavorable for processing carbon fibers.

[0004] In the current state of the art, hard chrome plating is commercially deposited from chromium electrolytes containing hexavalent chromium. However, in recent years the use of Cr(VI) electrolytes has been increasingly restricted by regulations, as Cr(VI) compounds are classified as highly hazardous to health and the environment.

[0005] One possible alternative is the use of Cr(III) electrolytes in electroplating processes. Cr(III) electrolytes are already used in decorative chrome plating processes to achieve layer thicknesses in the range of 200 nm to 1000 nm. However, for hard chrome plating with its considerably greater required layer thicknesses, there is currently no industrially viable process that can produce chromium layers with the necessary properties using Cr(III) electrolytes with a layer thickness of ≥ 3 µm.

[0006] In patent application EP23217110.8, the applicant describes a device and a method for the electroplating of a hard chrome layer made of a chromium(III) compound onto the surface of a substrate. The device comprises 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, where RH is a C1-10 alkyl group, characterized in that the anode is separated from the electroplating bath by a cation-selective membrane.

[0007] It has been shown that although the deposition of a hard chrome layer from a chromium(III) compound can be achieved very well in this way, the hard chrome layer obtained in this way is disadvantageous in terms of its corrosion resistance compared to conventional hard chrome layers made from Cr(VI) compounds.

[0008] The object of the present invention was therefore to provide an electroplated article with a chromium layer, preferably a hard chromium layer, made from a chromium(III) compound, and a method for its production, which has improved corrosion resistance.

[0009] This problem is solved by the present invention.

[0010] In detail, the present invention relates to an electroplated article comprising a substrate and a coating which includes a chromium layer produced from a Cr(III) compound, preferably a hard chromium layer, characterized in that a layer is arranged between the surface of the substrate and the chromium layer, preferably a hard chromium layer, in which a metal is contained which has a standard electrode potential which is below the standard electrode potential of the substrate.

[0011] According to the invention, it was found that the presence of a metal with a standard electrode potential below the standard electrode potential of the substrate in a layer between the substrate and a chromium layer produced from a Cr(III) compound, preferably a hard chromium layer, provides unexpectedly high corrosion protection for this chromium layer.

[0012] Preferably, for a substrate selected from the group consisting of iron, steel and a substrate with a standard electrode potential comparable to iron, the metal located in the interlayer is selected from the group consisting of zinc and tin.

[0013] According to the invention, corrosion resistance is determined in a neutral salt spray test according to DIN EN ISO 9227.

[0014] According to the invention, a "coating" is understood to be a system consisting of one or more layers which are electroplated onto a substrate surface.

[0015] Chromium coatings deposited in electroplating baths using a Cr(III) compound as the Cr source showed no corrosion protection whatsoever in this salt spray test. Distinct red rust stains were observable after only a few minutes / hours. Attempts to improve corrosion resistance through measures such as the application of a nickel layer (suitable for coatings produced from a Cr(VI) compound) between the substrate surface and the trivalent chromium layer did not yield sufficient improvement.

[0016] Without committing to a specific theory, the invention hypothesizes that the corrosion of trivalent chromium layers is not introduced from the outside, but rather is based on irregularities in the substrate surface (for example, induced by defects or liquid residues) that promote the formation of cracks in the applied chromium layer, which in the worst case extend through the entire applied chromium layer. In this case, the cracks can rupture through the surface of the applied chromium layer. Corrosion can then occur very rapidly, in the worst case even without external influences (such as the salt spray test).

[0017] It is suspected that the high susceptibility to corrosion of chromium layers electroplated from Cr(III) compounds is due to the fact that the layer build-up takes place at an angle of about 90° to the substrate surface, which could promote the formation of the cracks described above through the entire chromium layer.

[0018] According to the invention, it has been found that, in contrast to other metals such as nickel or copper, whose presence in such a coating does not sufficiently improve corrosion resistance, a proportion of a metal with a standard electrode potential below the standard electrode potential of the substrate in an intermediate layer of the coating leads to a surprisingly high corrosion resistance of a chromium layer electroplated from Cr(III) compounds in a neutral salt spray test according to DIN EN ISO 9227 of at least 72 h, preferably at least 120 h, more preferably at least 300 h and particularly preferably at least 500 h.

[0019] The corrosion resistance of a material or its corrosion protection coating can be determined using a salt spray test according to DIN EN ISO 9227 (NSS test). During the test, the specimens are placed in a chamber where a 5% saline solution with a controlled pH value is continuously atomized at a temperature of 35 °C. The mist settles on the specimens, coating them with a corrosive salt water film. The test duration depends on the expected corrosion resistance of the material system being tested. After the salt spray test, the specimens are rinsed with deionized water to remove any loosely adhering corrosion products. The corrosion attack on the tested material system is then assessed visually or using electrical and microscopic methods.

[0020] According to the present invention, the metal, having a standard electrode potential below that of the substrate, is contained in a layer located between the substrate surface and the chromium layer, preferably a hard chromium layer. In other words, a layer containing a metal with a standard electrode potential below that of the substrate is first applied to the substrate surface before the actual hard chromium plating with a Cr(III) compound is carried out.

[0021] According to a preferred embodiment, the layer arranged between the surface of the substrate and the chromium layer, preferably a hard chromium layer, is selected from the group consisting of a zinc layer, a zinc alloy layer such as a ZnNi alloy layer or a ZnFe alloy layer, a tin layer, and a tin alloy layer such as an SnNi alloy layer or an SnFe alloy layer.

[0022] Such layers can be applied using a conventional electroplating process with appropriate metal baths.

[0023] Depending on the type of coating to be applied, the electroplating bath is preferably selected from the group consisting of an acidic or alkaline zinc bath, an acidic or alkaline zinc-nickel bath, an alkaline zinc-iron bath, an acidic tin bath, an acidic tin-nickel bath, and an acidic tin-iron bath. Such electroplating baths are known and commercially available. The pH value of acidic electroplating baths is typically in the range of 5 to 6, while the pH value of alkaline baths is typically greater than 10, preferably greater than 12.

[0024] Preferably, the zinc-nickel bath or the zinc-iron bath contains a high proportion of zinc ions, preferably in the range of 80 to 99%, based on the total amount of all metal ions contained in the electroplating bath. In the case of a zinc-nickel bath, the proportion of zinc ions is preferably in the range of 80–95%, particularly preferably 85–90%, based on the total amount of all metal ions contained in the electroplating bath. In the case of a zinc-iron bath, the proportion of zinc ions is preferably in the range of 80–99.5%, more preferably 90–99.5%, particularly preferably 95–99.5%, based on the total amount of all metal ions contained in the electroplating bath.

[0025] Preferably, the tin-nickel bath or the tin-iron bath contains a high proportion of tin ions, preferably in the range of 50 to 99%, based on the total amount of all metal ions contained in the electroplating bath. In the case of a tin-nickel bath, the proportion of tin ions is preferably in the range of 50 to 95%, particularly preferably 55 to 90%, based on the total amount of all metal ions contained in the electroplating bath. In the case of a tin-iron bath, the proportion of tin ions is preferably in the range of 50 to 99.5%, more preferably 55 to 99.5%, particularly preferably 60 to 99.5%, based on the total amount of all metal ions contained in the electroplating bath.

[0026] The zinc ions can be provided in the form of conventional zinc compounds, such as ZnCl₂ or ZnSO₄. The tin ions can be provided in the form of conventional tin compounds, preferably tin(II) compounds, such as SnSO₄, Sn(BF₄)₂, or tin alkylsulfonic acids like tin(II) methanesulfonate.

[0027] Such electroplating baths can be used to form zinc-containing or tin-containing layers on the surface of a substrate, which preferably have a zinc content of 80 to 100 wt.% Zn or a tin content of 50 to 100 wt.% Sn, preferably 85 to 100 wt.% Zn or 55 to 100 wt.% Sn.

[0028] According to the invention, the electroplating of such a layer between the surface of the substrate and the chromium layer, preferably a hard chromium layer, is carried out under conditions (current density, temperature, type of substrate) as described below for the production of the chromium layer, preferably a hard chromium layer.

[0029] According to the invention, the layer arranged between the surface of the substrate and the chromium layer, preferably a hard chromium layer, preferably has a thickness in the range of 3 to 50 µm, preferably 5 to 15 µm.

[0030] It has been shown that such layer thicknesses provide high corrosion resistance.

[0031] In this embodiment, the formation of cracks as described above, which extend from the surface of the substrate to the surface of the chromium layer, preferably hard chromium layer, was not observed.

[0032] According to the invention, the electroplating of such a zinc-containing chromium layer, preferably a hard chromium layer, is preferably carried out under conditions (current density, temperature, type of substrate) as described below for the production of the pure hard chromium layer.

[0033] According to the invention, the electroplated article comprises a chromium layer, preferably a hard chromium layer, in its coating.

[0034] As described in the applicant's patent application EP23217110.8, for the large-scale deposition of hard chrome layers, the oxidation of carboxylate ions to CO₂ at the anode must be prevented or at least significantly limited. This can be achieved by separating the anode from the electroplating bath using an ion-selective membrane. This allows for stable process control over a longer period, as it prevents a rapid and significant increase in pH. In this way, the high layer thicknesses required for hard chrome plating can be deposited stably.

[0035] During electroplating from a bath containing Cr(III) ions, oxidation of Cr(III) ions to Cr(VI) ions can occur. This is generally undesirable and can completely prevent successful hard chrome plating. Therefore, the anode is preferably made of a material in which oxidation of Cr(III) ions to Cr(VI) ions does not occur. In principle, the hard chrome layer can be produced with any anode material suitable for electroplating from a chromium-containing electroplating bath. A mixed metal oxide (MMO) electrode is preferred.

[0036] 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 (for example, selected from the group consisting of ruthenium, iridium, and tantalum) or compounds thereof, such as their oxides, to give the anodes specific properties.

[0037] If electroplating is carried out using a metal mixed oxide (MMO) electrode as the anode, unwanted oxidation of Cr(III) ions to Cr(VI) ions can be avoided.

[0038] The anode is separated from the electroplating bath by a cation-selective membrane. Cation-selective membranes are well-known and commercially available. Perfluorinated cation exchange membranes are one example. Cation-selective membranes can be penetrated by cations, but not by anions.

[0039] In principle, the anode can be separated from the electroplating bath in any way by means of a cation-selective membrane. For example, the electroplating bath can be divided into two sections, preferably two halves, by placing a cation-selective membrane at a desired position within the electroplating bath.

[0040] 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, the anode being located in the interior space, and a medium entering 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.

[0041] For example, it could be a cuboid or cube-shaped container with one open side. This open side can be closed with a plate that has openings allowing the electrolyte solution to pass into the container. The openings must be sized to allow the molecules and ions contained in the electrolyte solution to pass through.

[0042] 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 the anode located in this interior.

[0043] The container is preferably dimensioned such that it has enough space to accommodate a conventional anode, but occupies as little volume as possible of the electroplating bath.

[0044] The anode, located inside the container, is connected to a power source, such as a battery, via an electrical connection like a power cable. This 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.

[0045] The device for electroplating according to the invention comprises an electroplating bath. Depending on the type of electroplating desired (layer between substrate and chromium layer, preferably hard chromium layer; pure chromium layer, preferably hard chromium layer), the electroplating bath has a different composition as described above.

[0046] The electroplating bath is provided in a container of the type commonly used in electroplating. In addition to the electroplating bath (i.e., the electrolyte solution), the container also contains the anode described above, the cation-selective membrane (in the case of deposition of a chromium layer, preferably a hard chromium layer), and a substrate to be coated. During electroplating, the substrate acts as the cathode.

[0047] In principle, any electrically conductive material suitable for electroplating can be used as a substrate. According to the invention, the substrate is preferably made of a metal such as iron or steel.

[0048] While not required, it can be advantageous in certain cases to subject the substrate to conventional pretreatment, such as applying a nickel layer, to make it suitable for electroplating. This eliminates the need for shot blasting or sandblasting, or for additional complex chemical and / or electrochemical pretreatment steps of the surface to be coated, which can lead to an undesirable surface texture ("orange peel"). According to the invention, a layer of metal with a standard electrode potential below that of the substrate, or a pure chromium layer from a Cr(III) electrolyte, can be applied directly to the substrate. As mentioned, however, such pretreatment may be advantageous or appropriate under certain circumstances.

[0049] Preferably, a layer of a polyhydroxy compound such as glycerin can be applied to the substrate before electroplating, in particular the electroplating of a chromium layer, preferably a hard chromium layer, as described in EP-3 000 918 A1.

[0050] The anode and the substrate acting as the cathode are connected to a power source by means of electrical conductors, for example, power cables. The power source, together with the anode and the substrate acting as the cathode, forms a circuit in which direct current flows during operation. Conventional direct current sources can be used. Preferably, the device can include a temperature control unit to enable electroplating at a temperature preferred according to the invention in the range of 35 to 75°C, preferably 40 to 60°C. Such temperature control units, for example, external heating elements, are well known.

[0051] For the deposition of a coating with several chromium layers, preferably hard chromium layers, a device and a process are preferred 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.

[0052] According to the present invention, the electroplating bath, i.e., the electrolyte solution, used for depositing a chromium layer, preferably a hard chromium layer, contains at least one Cr(III) compound in an aqueous solution for the electroplating of chromium onto the substrate acting as the cathode. Any Cr(III) compound used in the prior art for electroplating processes can be used. According to the invention, Cr(III) sulfate (Cr₂(SO₄)₃) is preferred.

[0053] Preferably, the electroplating bath used for depositing a chromium layer, preferably a hard chromium layer, i.e., the electrolyte solution, contains at least one carboxylate compound of the formula R-COOH, where RH is a C1-10 alkyl group, preferably H or a C1-4 alkyl group. The alkyl group may be linear or branched but does not have any further functional group. Formic acid or acetic acid are examples. A salt of the aforementioned carboxylate compound may also be used. Alkali metal salts, alkaline earth metal salts, or ammonium salts of the corresponding carboxylate compound are preferred. A preferred salt is ammonium formate as a formic acid salt.

[0054] According to a particularly preferred embodiment of the present invention, the electroplating bath used for depositing a chromium layer, preferably a hard chromium layer, contains no added foreign ions such as lead, copper, iron, or nickel ions. Such foreign ions can interfere with or influence the electroplating of chromium from a Cr(III) electrolyte. According to a preferred embodiment of the present invention, the electroplating bath used for depositing a chromium layer, preferably a hard chromium layer, consists of the Cr(III) compound and the carboxylate compound described above. Preferably, according to the invention, the electroplating bath used for depositing a chromium layer, preferably a hard chromium layer, has a concentration of Cr(III) ions in the range of 0.5 mol / l to 2.0 mol / l, preferably 0.6 mol / l to 1.5 mol / l.

[0055] According to the invention, the electroplating bath used for the deposition of a chromium layer, preferably a hard chromium layer, 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.

[0056] According to the invention, the electroplating bath used for the deposition of a chromium layer, preferably a hard chromium layer, preferably has a pH value in the range of 4.5 to 6.5, preferably 5.0 to 5.5.

[0057] The present invention further relates to a method for producing an electroplated article according to one of the preceding claims, comprising the step of producing an electroplated coating on a surface of a substrate, wherein a) in a first step, a metal having a standard electrode potential below that of the substrate is electroplated onto a surface of the substrate to obtain a corresponding layer, and b) in a next step, a Cr(III) compound from an electroplating bath containing a Cr(III) compound is electroplated onto the surface of the layer deposited in step a) to obtain a chromium layer, preferably a hard chromium layer.

[0058] Preferably, zinc or tin is deposited as the metal in step a).

[0059] Preferably, the zinc ions or the tin ions are electroplated from an electroplating bath selected from the group consisting of an acidic or alkaline zinc bath, an acidic or alkaline zinc-nickel bath and an alkaline zinc-iron bath, producing a layer which is selected from the group consisting of a zinc layer, a zinc alloy layer such as a ZnNi alloy layer or a ZnFe alloy layer, a tin layer, and a tin alloy layer such as an SnNi alloy layer or an SnFe alloy layer.

[0060] The chromium(III) compound chromium(III) sulfate used for depositing a chromium layer, preferably a hard chromium layer, is preferred. Particularly preferably, the electroplating bath containing the Cr(III) compound additionally contains a carboxyl compound of the formula R-COOH or a salt thereof, where RH is a C1-10 alkyl group.

[0061] The electroplating of a layer (layer with metal having a standard electrode potential below the standard electrode potential of the substrate, or chromium layer, preferably hard chromium layer) on a surface of a substrate according to the invention is preferably carried out in a device described above and comprises the steps: a) Inserting the substrate into the appropriate electroplating bath of the device, b) Applying a direct current to the anode of the device and the substrate acting as the cathode.

[0062] As described above, in the electroplating of a chromium layer, preferably a hard chromium layer, the separation of the anode or the anode compartment from the electroplating bath by means of a cation-selective membrane essentially prevents undesirable oxidation of the carboxyl compound contained in the electroplating bath at the anode. This has the advantage, among others, that this electroplating process can be carried out at a pH value in the range of 4.5 to 6.5, preferably 5.0 to 5.5, without a rapid, significant increase in pH value during the electroplating process. Furthermore, the carboxyl compound contained in the electroplating bath is thus protected from decomposition.

[0063] According to the invention, the electroplating of a layer (layer with metal having a standard electrode potential below the standard electrode potential of the substrate, or chromium layer, preferably hard chromium layer) is preferably carried out at a temperature in the range of 35 to 75°C, preferably 40 to 60°C, when a single layer is to be deposited.

[0064] In the case of the deposition of 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, the deposition preferably takes place in several process stages at different or, optionally, the same temperatures. In a first process stage, a base layer of chromium can be deposited with 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 with 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.

[0065] Finally, in a third process stage, a top chromium layer can be deposited with an electrolyte which again has a higher temperature in the range of 40 to 60°C, preferably 45 to 55°C.

[0066] As described above, this multilayer structure can be applied to a layer of metal, as described above, which has a standard electrode potential below that of the substrate, and which has been previously applied to a surface of the substrate of the article to be electroplated.

[0067] According to another embodiment, the deposition of a base layer described above can be dispensed with, so that only one or two chromium layers (structural and, if applicable, top layer) need to be deposited as described above.

[0068] It is further preferred that the electroplating of a layer (layer with metal having a standard electrode potential below the standard electrode potential of the substrate, or chromium layer, preferably hard chromium layer) is carried out with a current density in the range of 0.1 to 200 A / dm², preferably 0.5 to 150 A / dm² and particularly preferably 1 to 120 A / dm².

[0069] The chromium layers obtained according to the invention can be applied to untreated substrates as described above.

[0070] The coatings obtained according to the invention exhibit comparable properties, such as gloss characteristics, to chromium coatings electroplated from electrolytes containing Cr(VI) ions, but additionally possess exceptionally high corrosion resistance. In a neutral salt spray test according to DIN EN ISO 9227, the coatings according to the invention show no corrosion whatsoever over a period of 500 hours or more. Long-term tests also show no corrosion of the coatings according to the invention, even after a significantly longer test duration.

[0071] It is still possible to deposit another chromium layer onto an existing chromium layer using a Cr(III) electrolyte solution. In this way, greater layer thicknesses or multilayer chromium layers with sublayers exhibiting different properties can be obtained.

[0072] For example, as described in EP-0 722 515 B1 and EP-4 012 074 A1, a multilayer chromium coating system can be produced using the TOPOCROM® process. First, a DC base layer can be applied to a substrate, followed by a structural layer. In certain applications, an additional chromium layer (so-called finished chromium layer or topcoat) can be applied to the structural layer. This creates a structural layer with dome-shaped (hemispherical) protrusions, allowing for precise control of surface roughness and density (topography). This surface is completely free of sharp edges.

[0073] Preferably, the base layer has a thickness of preferably 1 to 500 µm, preferably 10 to 250 µm. According to the invention, the thickness of the structural layer is dependent on the roughness. An exemplary roughness value (average roughness 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 section) of a structural layer according to the invention is 0.1–15 µm, preferably 0.4–12 µm. The finished chromium layer for protecting the structural layer preferably has a thickness of preferably 2 to 20 µm, particularly preferably 3 to 15 µm, and especially 4 to 10 µm.

[0074] To produce a chromium base layer, current densities in the range of 30 to 50 A / dm² are preferably applied, preferably 35 to 45 A / dm² for a period of 5 to 360 min, preferably 5 to 60 min, preferably 30 to 50 min.

[0075] The so-called structural layer is then applied to this base layer. For example, in the TOPOCROM® process, the structural chromium layer formed comprises hemispherical domes. Preferably, the structural layer is produced using a direct current deposition process, wherein at least one initial pulse of electrical voltage and / or electrical current is applied to the surface to be coated to initiate nucleation of the deposit material, and subsequently, at least one follow-up pulse is applied to promote the growth of the deposit material nuclei through the deposition of further deposit material, with the electrical voltage and / or electrical current being increased or decreased in several stages during the nucleation phase.

[0076] 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.

[0077] According to another embodiment, the deposition of a base layer described above can be dispensed with, so that only one or two chromium layers (structural and, if applicable, top layer) need to be deposited as described above.

[0078] The present invention is explained in more detail with reference to non-limiting figures and examples. These show: Fig. 1 a microscopic (SEM) image of a hard chrome layer electroplated from Cr(III) Fig. 2 a microscopic image of a cross section through the electroplated article produced in Example 1. Example 1

[0079] An iron rod (coating length 100 mm, diameter 6 mm) was placed in an electroplating bath (with a volume of 800 ml) containing an acidic ZnNi electrolyte. The electroplating bath had a pH of 5 and a temperature of approximately 28°C.

[0080] The electroplating was carried out for 60 min at a current density of 0.7 A / dm².

[0081] The coated product was rinsed and then placed in an electroplating bath (with a volume of 800 ml) containing chromium(III) sulfate (corresponding to an amount of 20-25 g / l Cr(II) ions in the bath) and ammonium formate (in an amount corresponding to a molar ratio of Cr(III) ions to carboxylates in the range of 0.2 to 0.5). The electroplating bath had a pH of 5 and a temperature in the range of 55-60°C.

[0082] Electroplating was carried out for 30 minutes at 55°C under conditions of 10⁻¹² A, 10⁻¹² V, and a current density of 50–60 A / dm². Halfway through the plating time, additional chromium sulfate (corresponding to 5 g / L Cr(III) ions in the bath) was added. A layer with a thickness of 50 µm was produced.

[0083] As a comparison of Figures 1 and 2 It is evident that a hard chrome layer electroplated from a Cr(III) compound in the absence of Zn ions has visible openings of cracks in its surface and is therefore not corrosion-resistant.

[0084] The in Fig. 2The cross-section shown was etched with nitric acid (HNO₃) in ethanol for 2–3 seconds to better visualize the standard structures of the ZnNi layer. The iron substrate, on which the ZnNi layer (second layer from the right) is located, can be seen on the far right. The Cr layer (third layer from the right) is located on top of the ZnNi layer. Example 2

[0085] Example 1 was repeated using an alkaline ZnNi electrolyte (6 g / L Zn ions, 1 g / L Ni ions, 40 g / L complexing agent, 120 g / L NaOH). Electroplating was performed as described in Example 1. Subsequently, a Cr(III) layer was applied as described in Example 1. Example 3

[0086] Example 1 was repeated using an alkaline Zn electrolyte (6 g / L Zn ions, 120 g / L NaOH). Electroplating was performed as described in Example 1. Subsequently, a Cr(III) layer was applied as described in Example 1. Example 4

[0087] An iron rod (coating length 100 mm, diameter 6 mm) was placed in an electroplating bath (with a volume of 800 ml) containing a sulfuric acid tin electrolyte. The electroplating bath was at a temperature of 25°C.

[0088] The electroplating was carried out for 30 min at a current density of 1 A / dm².

[0089] Subsequently, a Cr(III) layer was applied as described in Example 1.

[0090] A corrosion resistance of more than 48 h was observed for the chromium layer electroplated from Cr(III) compounds in a neutral salt spray test according to DIN EN ISO 9227.

Claims

1. Electroplated article comprising a substrate and a coating comprising a chromium layer produced from a Cr(III) compound, preferably a hard chromium layer, characterized by the fact that A layer is arranged between the surface of the substrate and the chromium layer, preferably a hard chromium layer, in which a metal is contained that has a standard electrode potential which is below the standard electrode potential of the substrate.

2. Electroplated article according to claim 1, characterized by the fact that The metal that has a standard electrode potential below that of the substrate is selected from the group consisting of zinc and tin.

3. Electroplated article according to claim 1 or 2, characterized by the fact thatThe layer arranged between the surface of the substrate and the chromium layer, preferably a hard chromium layer, is selected from the group consisting of a zinc layer, a zinc alloy layer such as a ZnNi alloy layer or a ZnFe alloy layer, a tin layer, and a tin alloy layer such as an SnNi alloy layer or an SnFe alloy layer.

4. Electroplated article according to any one of the preceding claims, characterized by the fact that the layer arranged between the surface of the substrate and the chromium layer has a thickness in the range of 3 to 50 µm, preferably 5 to 15 µm.

5. Electroplated article according to any one of the preceding claims, characterized by the fact that the layer arranged between the surface of the substrate and the chromium layer, preferably a hard chromium layer, is a zinc-containing layer and has a zinc content of 80 to 100 wt.% Zn, preferably 85 to 100 wt.% Zn.

6. Electroplated article according to any one of the preceding claims, characterized by the fact that the layer arranged between the surface of the substrate and the chromium layer, preferably a hard chromium layer, is a tin-containing layer and has a tin content of 50 to 100 wt.% Sn, preferably 55 to 100 wt.% Sn.

7. Electroplated article according to any one of the preceding claims, characterized by the fact that The substrate is selected from the group consisting of iron and steel.

8. A method for producing an electroplated article according to one of the preceding claims, comprising the step of producing an electroplated coating on a surface of a substrate, wherein a) in a first step a metal having a standard electrode potential which is below the standard electrode potential of the substrate is electroplated on a surface of the substrate to obtain a corresponding layer, and b) in a next step a Cr(III) compound from an electroplating bath containing a Cr(III) compound is electroplated on the surface of the layer deposited in step a) to obtain a chromium layer, preferably a hard chromium layer.

9. Method according to claim 8, characterized by the fact that in step a) the metal zinc or tin is deposited.

10. Method according to claim 8 or 9, characterized by the fact thatIn the first step, the zinc ions or the tin ions are electroplated from an electroplating bath selected from the group consisting of an acidic or alkaline zinc bath, an acidic or alkaline zinc-nickel bath and an alkaline zinc-iron bath, producing a layer which is selected from the group consisting of a zinc layer, a zinc alloy layer such as a ZnNi alloy layer or a ZnFe alloy layer, a tin layer, and a tin alloy layer such as an SnNi alloy layer or an SnFe alloy layer.

11. Method according to any one of claims 8 to 10, characterized by the fact that the chromium(III) compound is chromium(III) sulfate and the electroplating bath containing the Cr(III) compound additionally contains a carboxyl compound of the formula R-COOH or a salt thereof, where RH is a C1-10 alkyl group.

12. Method according to any one of claims 8 to 11, characterized by the fact thatthe process with a current density in the range of 10 to 300 A / dm² 2 , preferably 25 to 200 A / dm² 2 and especially preferably 30 to 120 A / dm² 2 is carried out.

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