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
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/12—Electroplating: Baths therefor from solutions of nickel or cobalt
• • 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
  • C25D3/22—Electroplating: Baths therefor from solutions of zinc
• • 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
  • C25D3/30—Electroplating: Baths therefor from solutions of tin
• • 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
  • C25D3/565—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of zinc
• • 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
  • C25D3/60—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of tin
• • 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

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 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.
• 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.
• a substrate selected from the group consisting of iron, steel, and a substrate with a standard electrode potential comparable to iron is used.
• Interlayer metal selected from the group consisting of zinc and tin.
• 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 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.
• 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 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.
• the corrosion resistance of a material or its corrosion protection coating can, in principle, 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 and coats them with a corrosive salt water film. The test duration depends on the expected corrosion resistance of the material system being tested. After completion of the salt spray test, the specimens are rinsed with deionized water to remove any loosely adhering corrosion products. The corrosion resistance is then assessed visually or using electrical instruments. The corrosion attack on the tested material system is then assessed using microscopic methods.
• 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.
• 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.
• the layer arranged between the surface of the substrate and the 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.
• Such layers can be applied using a conventional electroplating process with appropriate metal baths.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Zinc ions can be provided in the form of conventional zinc compounds, such as ZnCl2 or ZnSO4 .
• Tin ions can be provided in the form of conventional tin compounds, Preferably tin(II) compounds are provided. Examples include SnSO4 , Sn(BF4) 2 , or tin alkylsulfonic acids such as tin(II) methanesulfonate.
• 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.
• 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.
• 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 ⁇ m, preferably 5 to 15 ⁇ m.
• the electroplating of such a zinc-containing chromium layer is carried out. preferably carried out under conditions (current density, temperature, type of substrate) as described below for the production of the pure hard chrome 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, wherein the anode is located in the interior space and a medium entering through the inlet can only pass through it.
• the cation-selective membrane allows the substance to enter the interior.
• the shape of the container is freely selectable and can, for example, be a cube, cuboid, or cylinder.
• 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.
• 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 is composed differently 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, the cation-selective membrane (in the case of depositing a chromium layer, preferably a hard chromium layer), and a substrate to be coated.
• the substrate acts as the cathode.
• the substrate is preferably made of a metal such as iron or steel.
• 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.
• 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 material used for depositing a chromium layer preferably a hard chromium layer
• the electroplating bath i.e., the electrolyte solution
• Any Cr(III) compound used in the prior art for electroplating processes can be used.
• Cr(III) sulfate ( Cr2 ( SO4 ) 3 ) is preferred.
• the electroplating bath used for depositing a chromium layer preferably a hard chromium layer, i.e., the electrolyte solution
• 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. 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, consists of the Cr(III) compound and the carboxylate compound described above.
• the electroplating bath used for the deposition of a chromium layer preferably a hard chromium layer, 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, 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, preferably has a pH value in the range of 4.5 to 6.5, preferably 5.0 to 5.5.
• zinc or tin is deposited as the metal in step a).
• 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.
• 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
• the chromium(III) compound chromium(III) sulfate used for depositing a chromium layer, preferably a hard chromium layer, is preferred.
• the electroplating bath containing the Cr(III) compound additionally contains a carboxyl compound of the formula R-COOH or a salt thereof, where R is H or 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 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.
• 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 which again has a higher temperature in the range of 40 to 60°C, preferably 45 to 55°C.
• 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.
• 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.
• the electroplating of a 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 .
• 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 comparable properties, for example gloss properties, to chromium coatings made from electrolytes with Cr(VI) ions. These coatings are electroplated and also exhibit exceptionally high corrosion resistance.
• 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 and more. Long-term tests also show no corrosion of the coatings according to the invention even after a significantly longer test duration.
• the TOPOCROM® process describes how a multilayer chromium coating system can be produced.
• a DC base layer can be applied to a substrate, followed by a structural layer.
• an additional chromium layer (so-called finished chromium layer or topcoat) can be applied to the structural layer.
• the base layer has a thickness of preferably 1 to 500 ⁇ m, preferably 10 to 250 ⁇ m.
• 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 ⁇ m, preferably 0.4–12 ⁇ m.
• the finished chromium layer for protecting the structural layer preferably has a thickness of 2 to 20 ⁇ m, more preferably 3 to 15 ⁇ m, and more preferably 4 to 10 ⁇ m.
• 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 structural layer is then applied to this base layer.
• the structural chromium layer formed comprises hemispherical domes.
• 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.
• 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.
• 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.
• 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.
• the electroplating was carried out for 60 min at a current density of 0.7 A/ dm2 .
• 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 It had a pH value of 5 and a temperature in the range of 55-60°C.
• 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 .
• additional chromium sulfate (corresponding to 5 g/L Cr(III) ions in the bath) was added.
• a layer with a thickness of 50 ⁇ m was produced.
• the in Fig. 2 The 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 Cr layer (third layer from the right) is located on top of the ZnNi layer.
• 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 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.
• 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.
• the electroplating was carried out for 30 minutes at a current density of 1 A/ dm2 .

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Citations (7)

• 2024
  • 2024-05-29 EP EP24178796.9A patent/EP4656776A1/fr active Pending

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