Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/04—Electroplating: Baths therefor from solutions of chromium
  • C25D3/06—Electroplating: Baths therefor from solutions of chromium from solutions of trivalent chromium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D17/00—Constructional parts, or assemblies thereof, of cells for electrolytic coating
  • C25D17/10—Electrodes, e.g. composition, counter electrode
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/10—Electroplating with more than one layer of the same or of different metals
  • C25D5/12—Electroplating with more than one layer of the same or of different metals at least one layer being of nickel or chromium
  • C25D5/14—Electroplating with more than one layer of the same or of different metals at least one layer being of nickel or chromium two or more layers being of nickel or chromium, e.g. duplex or triplex layers
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/60—Electroplating characterised by the structure or texture of the layers

Definitions

• 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.
• 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 ⁇ m 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.
• Cr(III) electrolytes are already used in decorative chrome plating processes to achieve layer thicknesses in the range of 200 nm to 1000 nm.
• layer thicknesses in the range of 200 nm to 1000 nm.
• the applicant has described a device and a method for the electroplating of a hard chrome layer from 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.
• 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.
• 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 the chromium layer, preferably a hard chromium layer, additionally comprises zinc.
• corrosion resistance is determined in a neutral salt spray test according to DIN EN ISO 9227.
• a “coating” is understood to be a system consisting of one or more layers which are electroplated onto a substrate surface.
• Chromium layers deposited from electroplating baths with a Cr(III) compound as the Cr source show in This salt spray test showed no corrosion protection whatsoever. Distinct red rust stains were visible after only a few minutes/hours. Attempts to increase corrosion resistance through measures such as providing a nickel layer (suitable for layers made from a Cr(VI) compound) between the substrate surface and the trivalent chromium layer did not yield sufficient improvement.
• 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.
• 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).
• a proportion of zinc in the chromium layer leads to a surprisingly high corrosion resistance of a coating made of Cr(III) compounds.
• 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% sodium chloride 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.
• the zinc in the chromium layer is preferably contained in an amount of 0.1 to 10 wt.% Zn, preferably 1 to 5 wt.% Zn.
• the zinc ions are preferably added in the form of a zinc compound such as Zn formate, a Zn salt of a carboxylic acid, ZnCl2 or ZnSO4 .
• the electroplating bath containing a Cr(III) compound has a proportion of zinc ions in the range of 1 to 800 ppm, preferably 50 to 600 ppm, particularly preferably 100 to 500 ppm.
• the chromium layer preferably a hard chromium layer, deposited from such an electroplating bath has a semi-shell structure with zinc layers arranged between the semi-shells.
• the semi-shells can be, for example, CrZn semi-shells or a semi-shell structure with zinc inclusions.
• the electroplating of such a zinc-containing chromium layer is preferably carried out under conditions (current density, temperature, type of substrate) as described below for the production of a pure hard chromium layer.
• the electroplated article comprises a chromium layer, preferably a hard chromium layer, in its coating.
• the anode is preferably made of a material in which oxidation of Cr(III) ions to Cr(VI) ions does not occur.
• 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.
• MMO electrodes are well known and commercially available.
• a substrate e.g., a titanium plate or a titanium grid
• a thin layer of other metals for example, selected from the group consisting of ruthenium, iridium, and tantalum
• compounds thereof such as their oxides
• MMO metal mixed oxide
• 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.
• the anode can be separated from the electroplating bath in any way by means of a cation-selective membrane.
• 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.
• 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.
• it could be a cuboid or cube-shaped container with an open side. This open side can be closed with a plate.
• the container must have openings to allow the electrolyte solution to pass through. These openings must be dimensioned to allow the molecules and ions contained in the electrolyte solution to pass through.
• 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.
• 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.
• the anode located inside the container, is connected to a power source, such as a battery, via an electrical connection like a power cable.
• a power source such as a battery
• 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.
• the device for electroplating according to the invention comprises an electroplating bath.
• the electroplating bath has a different composition as described above.
• the electroplating bath is provided in a container of the type commonly used in electroplating.
• the container also contains the anode described above, which is cation-selective.
• a membrane in the case of deposition of a chromium layer, preferably a hard chromium layer
• the substrate acts as the cathode.
• the substrate is preferably made of a metal such as iron or steel.
• a pure chromium layer from a Cr(III) electrolyte or a zinc-containing chromium layer from a Cr(III) electrolyte containing zinc ions can be applied directly to the substrate.
• pretreatment may be advantageous or appropriate under certain circumstances.
• a layer of a polyhydroxy compound such as glycerin can be applied to the substrate prior to electroplating, in particular the electroplating of a chromium layer, preferably a hard chromium layer, as described in the EP-3 000 918 A1 is described.
• 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.
• 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.
• a device and a process are preferred as described in the EP-3 000 918 A1 described, 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.
• the electroplating bath i.e., the electrolyte solution, used for depositing a chromium layer, preferably a hard chromium layer (pure or zinc-containing), contains at least one Cr(III) compound in an aqueous solution for the electroplating of chromium onto the substrate acting as the cathode.
• a chromium layer preferably a hard chromium layer (pure or zinc-containing
• Cr(III) compound used in the prior art for electroplating processes can be used.
• Cr(III) sulfate Cr2 ( SO4 ) 3
• the electroplating bath used for depositing a chromium layer preferably a hard chromium layer (pure or zinc-containing), i.e., the electrolyte solution, contains at least one carboxylate compound of the formula R-COOH, where R is H or a C1-10 alkyl group, preferably H or a C1-4 alkyl group.
• the alkyl group can 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 can 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.
• the electroplating bath used for depositing a chromium layer contains no added foreign ions such as lead, copper, iron, or nickel ions, apart from any zinc ions that may be present. Such foreign ions can interfere with or influence the electroplating of chromium from a Cr(III) electrolyte.
• the electroplating bath used for depositing a chromium layer preferably a hard chromium layer (pure or zinc-containing), consists of the Cr(III) compound and the carboxylate compound described above, and optionally a zinc compound.
• the electroplating bath used for the deposition of a chromium layer preferably a hard chromium layer (pure or zinc-containing), preferably 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.
• the electroplating bath used for the deposition of a chromium layer preferably a hard chromium layer (pure or zinc-containing), 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.
• the electroplating bath used for the deposition of a chromium layer preferably a hard chromium layer (pure or zinc-containing), preferably has a pH value in the range of 4.5 to 6.5, preferably 5.0 to 5.5.
• 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 this production comprises the electroplating of a Cr(III) compound from an electroplating bath, characterized in that zinc ions are added to the electroplating bath containing a Cr(III) compound and are electroplated together with the chromium to obtain a zinc-containing chromium layer, preferably a hard chromium layer.
• the zinc ions are added to the electroplating bath containing a Cr(III) compound and electroplated together with the chromium to obtain a zinc-containing chromium layer, preferably a hard chromium layer.
• the electroplating bath containing a Cr(III) compound preferably has a zinc ion content in the range of 1 to 800 ppm, preferably 50 to 600 ppm, and particularly preferably 100 to 500 ppm.
• the chromium(III) compound used for depositing the chromium layer preferably a hard chromium layer, is preferred. Chromium(III) sulfate.
• the electroplating bath containing the Cr(III) compound preferably additionally contains a carboxyl compound of the formula R-COOH or a salt thereof, where RH is a C1-10 alkyl group.
• 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 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.
• the carboxyl compound contained in the electroplating bath is thus protected from decomposition.
• the electroplating of a chromium layer is preferred. carried out at a temperature in the range of 35 to 75°C, preferably 40 to 60°C, if a single layer is to be deposited.
• the deposition preferably takes place in several process stages at different or, optionally, the same temperatures.
• 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.
• 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.
• a top chromium layer can be deposited with an electrolyte that again has a higher temperature in the range of 40 to 60°C, preferably 45 to 55°C.
• At least one chromium layer of the aforementioned multilayer structure shall contain zinc, which can be introduced as described above.
• the base layer described above contains zinc in the amounts described above.
• the deposition of a base layer described above can be omitted. so that only one or two chromium layers (structural and, if applicable, top layer) need to be deposited as described above (which may, if applicable, contain zinc as described above).
• the electroplating of a layer is carried out with a current density in the range of 10 to 300 A/ dm2 , preferably 25 to 200 A/ dm2 and particularly preferably 30 to 120 A/ dm2 .
• the chromium layers obtained according to the invention can be applied to untreated substrates as described above.
• the coatings obtained according to the invention exhibit exceptionally high corrosion resistance.
• the coatings according to the invention show no corrosion whatsoever over a period of 500 hours and more. Long-term tests also show no corrosion of the coatings according to the invention even after a significantly longer test period.
• the TOPOCROM® process describes how a multi-layered chromium coating system can be produced.
• a A DC base layer is applied to a substrate before a structural layer is applied to this base layer.
• an additional chromium layer can be applied to the structural layer. This creates a structural layer with dome-shaped (hemispherical) protrusions, which allow for the desired surface roughness and closure (topography) to be adjusted as needed. This surface is completely free of sharp edges.
• one or more of these layers may contain zinc.
• the base layer has a thickness of preferably 1 to 500 pm, preferably 10 to 250 pm.
• the thickness of the structural layer is roughness-dependent.
• 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 pm, preferably 0.4–12 ⁇ m.
• the finished chromium layer for protecting the structural layer preferably has a thickness of preferably 2 to 20 ⁇ m, particularly preferably 3 to 15 pm, and especially 4 to 10 pm.
• current densities in the range of 30 to 50 A/ dm2 , preferably 35 to 45 A/dm2 are preferably applied for a period of 5 to 360 min, preferably 5 to 60 min, preferably 30 to 50 min.
• the so-called structural layer is then applied to this base layer.
• this base layer for example, in the TOPOCROM® process, this includes...
• the structural chromium layer formed there has hemispherical domes.
• the structural layer is produced by means of a direct current deposition process, wherein nucleation of the deposit material is achieved on the surface to be coated by means of at least one initial pulse of the electrical voltage and/or the electrical current, and subsequently, growth of the deposit material nuclei is brought about by means of at least one subsequent pulse through the deposition of further deposit material, wherein the increase or decrease of the electrical voltage and/or the electrical current takes place in several stages during the nucleation phase.
• a surface coating 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.
• one of these sublayers can contain zinc, which can be introduced as described above.
• the deposition of a base layer described above can be dispensed with, so that only one or two chromium layers (structural and optionally top layer, optionally containing zinc as described above) need to be deposited as described above.
• An iron rod (coating length 100 mm, diameter 6 mm) was 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), 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), and 350 to 400 ppm zinc sulfate.
• the electroplating bath had a pH of 5 and a temperature in the range of 55–60°C.
• Electroplating was carried out for 50 minutes at 55°C under conditions of 10 ⁇ 12 A, 10 ⁇ 12 V, and a current density of 50–60 A/ dm2 .
• additional chromium sulfate (corresponding to 5 g/L Cr(III) ions in the bath) was added.
• a layer with a thickness of 50 pm was produced.
• the hard chrome layer produced in Example 1 contains approximately 3% zinc, based on the total amount of the deposited layer.
• the surface structure of the layer according to Example 1 is, as shown from Fig. 2 evidently, clearly different from the surface structure of the pure hard chrome layer according to Fig. 1 .
• FIG. 3 A microscopic (SEM) image of a cross-section of the article is shown, which was electroplated as described in Example 1.
• the substrate (iron) is visible at the bottom.
• the deposited hard chrome layer is located on top of it.
• a hemispherical structure of the chrome layer is clearly visible.
• Zinc layers (stained yellow) are arranged between some of the hemispherical sections.
• Example 1 In a neutral salt spray test according to DIN EN ISO 9227, the article produced in Example 1 showed no corrosion even after 500 h.

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Citations (10)

• 2024
  • 2024-05-29 EP EP24178793.6A patent/EP4656775A1/fr active Pending
• 2025
  • 2025-05-15 WO PCT/EP2025/063393 patent/WO2025247659A1/fr active Pending

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