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
  • C25D15/00—Electrolytic or electrophoretic production of coatings containing embedded materials, e.g. particles, whiskers, wires
• • 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
  • C25D3/08—Deposition of black chromium, e.g. hexavalent chromium, CrVI
• • 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
  • C25D5/627—Electroplating characterised by the visual appearance of the layers, e.g. colour, brightness or mat appearance

Definitions

• the present invention relates to a method for electrodepositing a dark chromium layer on a substrate, a respective electroplating bath for depositing such a dark chromium layer, and a respective substrate comprising said dark chromium layer.
• the electroplating bath comprises colloidal particles containing the chemical element aluminum.
• the substrate comprising said dark chromium layer is primarily suited for decorative purposes.
• US 2011/155286 A1 refers to a composition for a chemical conversion treatment comprising trivalent chromium ions and optionally inorganic silica sol or an alumina sol.
• JP 5890394 B2 refers to a chromium plating solution containing trivalent chromium and ceramic particles.
• RU 2231581 C1 refers to a chromium electrolyte containing trivalent chromium ions and AI2O3 powder.
• US 2012/312694 A1 refers to an aqueous acidic trivalent chromium electrolyte comprising trivalent chromium ions and colloidal silica.
• the aqueous trivalent chromium electroplating bath comprises a combination of (iii) and (iv).
• (iii) is formed of particles
• (iv) is preferably a soluble compound.
• (iii) can be adapted in its concentration by either adding such colloidal particles or by partially (or fully) removing them from the electroplating bath. Most preferably such a removal is accomplished by physical/mechanical separation means, for example filtration. This typically allows a fine-tuning of the brightness L*, depending on the amount present.
• the dark chromium layer is a decorative chromium layer.
• Typical applications are automotive parts, most preferably for the interior of a car.
• the electroplating bath utilized in the method of the present invention is very suitable in order to obtain such a dark chromium layer, most preferably such a dark chromium layer as defined throughout the present text.
• the dark chromium layer in the context of the present invention is defined by the L*a*b* color system, preferably as introduced in 1976 by the Commission Internationale de I'Eclairage, if not stated otherwise.
• step (a) of the method of the present invention a substrate is provided.
• the substrate comprises a non-metallic substrate, preferably comprises a plastic substrate.
• the non-metallic substrate preferably the plastic substrate
• the non-metallic substrate comprises acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene - polycarbonate (ABS-PC), polypropylene (PP), polyamide (PA), polyurethane (PU), polyepoxide (PE), polyacrylate, polyetherimide (PEI), a polyetherketone (PEK), mixtures thereof, and/or composites thereof; preferably acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene - polycarbonate (ABS-PC), polyamide (PA), polyurethane (PU), polyepoxides (PE), polyacrylate, mixtures thereof, and/or composites thereof.
• plastic substrates are often used in decorative applications such as automotive parts, in particular ABS and ABS-PC.
• the substrate preferably the non-metallic substrate, more preferably the plastic substrate, comprises at least one metal layer (most preferably in addition).
• the at least one metal layer comprise a copper or copper alloy layer and/or a nickel or nickel alloy layer.
• a method of the present invention is preferred, wherein the substrate is a metallic substrate, preferably comprising iron, copper, nickel, aluminum, zinc, mixtures thereof, and/or alloys thereof.
• a very preferred metallic substrate comprising iron is steel.
• a mixture thereof preferably includes composites.
• step (a) pre-treating the substrate, preferably cleaning the substrate, most preferably degreasing and/or pickling the substrate.
• the degreasing comprises an electrolytic degreasing.
• the pickling comprises a contacting with an acid, preferably an inorganic acid.
• Step (a1) is preferably followed by a water rinse.
• step (b) of the method of the present invention said aqueous trivalent chromium electroplating bath is provided.
• Said electroplating bath comprises water, preferably at least 55 vol.-% or more is water, based on the total volume of the electroplating bath, more preferably 65 vol.-% or more, even more preferably 75 vol.-% or more, yet even more preferably 85 vol.-% or more, still more preferably 90 vol.-% or more, most preferably 95 vol.-% or more. Most preferably, water is the only solvent.
• the aqueous trivalent chromium electroplating bath is acidic, preferably having a pH ranging from 1.5 to 5.0, more preferably from 2.1 to 4.6, even more preferably from 2.4 to 4.2, yet more preferably from 2.7 to 3.8, most preferably from 3.0 to 3.5.
• the pH is preferably adjusted with hydrochloric acid, sulfuric acid, ammonia, potassium hydroxide, and/or sodium hydroxide.
• the aqueous trivalent chromium electroplating bath comprises (i), trivalent chromium ions.
• the trivalent chromium ions have a total concentration ranging from 5 g/L to 35 g/L, based on the total volume of the electroplating bath, preferably from 6 g/L to 32 g/L, more preferably from 7 g/L to 29 g/L, even more preferably from 8 g/L to 26 g/L, yet even more preferably from 9 g/L to 23 g/L, most preferably from 10 g/L to 22 g/L.
• the trivalent chromium ions are from a trivalent chromium salt, preferably from an inorganic chromium salt and/or an organic chromium salt, most preferably from an inorganic chromium salt.
• a preferred inorganic chromium salt comprises chloride and/or sulfate anions, preferably sulfate anions.
• a very preferred inorganic chromium salt is basic chromium sulfate.
• a preferred organic chromium salt comprises carboxylic acid anions, preferably formate, acetate, malate, and/or oxalate anions.
• the aqueous trivalent chromium electroplating bath comprises (ii), one or more than one complexing agent for said trivalent chromium ions.
• the one or more than one complexing agent is not a compound of (iv) and (v) is therefore preferably different therefrom.
• the one or more than one complexing agent comprises an organic acid and/or salts thereof, preferably an organic carboxylic acid and/or salts thereof, most preferably an organic carboxylic acid comprising one, two, or three carboxylic groups and/or salts thereof.
• the organic carboxylic acid and/or salts thereof are preferably substituted with a substituent or unsubstituted.
• a preferred substituent comprises an amino group and/or a hydroxyl group.
• the substituent does not comprise a SH moiety and/or a SCN moiety.
• the one or more than one complexing agent for said trivalent chromium ions is not a sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below.
• the organic carboxylic acid and/or salts thereof comprise amino carboxylic acids (preferably alpha-amino carboxylic acids), hydroxyl carboxylic acids, and/or salts thereof.
• Preferred (alpha-) amino carboxylic acids comprise glycine, aspartic acid, and/or salts thereof.
• the amino carboxylic acids preferably alpha-amino carboxylic acids, respectively
• the one or more than one complexing agent is distinct from (iv) und (v).
• the one or more than one complexing agent comprises formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, citric acid, glycine, aspartic acid, and/or salts thereof, preferably formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, citric acid, and/or salts thereof, more preferably formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, and/or salts thereof, even more preferably formic acid, acetic acid, and/or salts thereof, most preferably formic acid and/or salts thereof.
• the one or more than one complexing agent has a total concentration ranging from 5 g/L to 200 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably ranging from 8 g/L to 150 g/L, more preferably ranging from 10 g/L to 100 g/L, even more preferably from 12 g/L to 75 g/L, yet even more preferably ranging from 15 g/L to 50 g/L, most preferably ranging from 20 g/L to 35 g/L.
• the aqueous trivalent chromium electroplating bath comprises (iii), colloidal particles containing the chemical element aluminum.
• aqueous trivalent chromium electroplating bath is a colloidal suspension, preferably a sol. This is due to the presence of said colloidal particles, which are preferably correspondingly small.
• the colloidal particles containing the chemical element aluminum have a particle size of less than 1000 nm, preferably of less than 500 nm, more preferably at least 90% of the colloidal particles have a particle size of less than 500 nm, most preferably at least 90% of the colloidal particles have a particle size of less than 150 nm.
• colloidal particles have an average particle diameter D 50 of 100 nm or less, preferably of 80 nm or less, more preferably of 60 nm or less, even more preferably of 50 nm or less, most preferably of 40 nm or less, very most preferably of 30 nm or less, even most preferably of 25 nm or less, based on volume.
• colloidal particles comprise at least particles with a particle size of 100 nm or less, preferably of 80 nm or less, more preferably of 60 nm or less, even more preferably of 50 nm or less, most preferably of 40 nm or less, very most preferably of 30 nm or less, even most preferably of 20 nm or less.
• colloidal particles comprise aluminum oxide.
• colloidal particles comprise Al 2 O 3 , preferably substantially consist of Al 2 O 3 . Most preferably, these are the only colloidal particles in the aqueous trivalent chromium electroplating bath.
• the aqueous trivalent chromium electroplating bath comprises (iv), a first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below.
• a first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below.
• acids, salts, isoforms, and betaines thereof are included.
• sulfate anions are not counted among (iv) and (v).
• the aqueous trivalent chromium electroplating bath comprises (v), a second sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below being different from (iv).
• this preferably includes acids, salts, isoforms, and betaines thereof.
• a method of the present invention is preferred, wherein the aqueous trivalent chromium electroplating bath comprises (iv) and (v).
• (v) is not optional.
• the first and the optional second sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below comprises a divalent sulfur atom.
• They are preferably inorganic or organic.
• the first sulfur-containing compound comprises one or more than one organic sulfur-containing compound (preferably as described throughout the text), wherein preferably the second sulfur-containing compound comprises one or more than one inorganic sulfur-containing compound (preferably as described throughout the text).
• first and the optional second sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below is selected from the group consisting of (including its salts)
• a method of the present invention wherein (iv) and (v) together have a total concentration ranging from 16 mmol/L to 1150 mmol/L, based on the total volume of the electroplating bath, preferably from 45 mmol/L to 1000 mmol/L, more preferably from 80 mmol/L to 900 mmol/L, even more preferably from 120 mmol/L to 800 mmol/L, yet even more preferably from 150 mmol/L to 700 mmol/L, most preferably from 180 mmol/L to 650 mmol/L.
• the aqueous trivalent chromium electroplating bath comprises (iv) in a total concentration ranging from 15 mmol/L to 750 mmol/L, based on the total volume of the electroplating bath, preferably from 40 mmol/L to 650 mmol/L, more preferably from 70 mmol/L to 600 mmol/L, even more preferably from 100 mmol/L to 550 mmol/L, yet even more preferably from 120 mmol/L to 500 mmol/L, most preferably from 140 mmol/L to 470 mmol/L.
• the aqueous trivalent chromium electroplating bath comprises (v) in a total concentration ranging from 1 mmol/L to 400 mmol/L, based on the total volume of the electroplating bath, preferably from 5 mmol/L to 350 mmol/L, more preferably from 10 mmol/L to 300 mmol/L, even more preferably from 20 mmol/L to 250 mmol/L, yet even more preferably from 30 mmol/L to 200 mmol/L, most preferably from 40 mmol/L to 180 mmol/L.
• a method of the present invention wherein (iv) has a higher molar concentration than (v).
• a method of the present invention wherein in the aqueous trivalent chromium electroplating bath the molar ratio of (iv) to (v) is more than 1, preferable 1.1 or more, most preferably 1.2 or more.
• said first sulfur-containing compound has the highest concentration of sulfur containing compounds having a sulfur atom with an oxidation number of +5 or below.
• a method of the present invention wherein in the aqueous trivalent chromium electroplating bath the molar ratio of (iv) to (v) is ranging from 1.05 to 15, preferably from 1.10 to 12, more preferably from 1.15 to 10, even more preferably from 1.20 to 9, most preferably from 1.25 to 8.
• said first sulfur-containing compound has the highest concentration of sulfur containing compounds having a sulfur atom with an oxidation number of +5 or below.
• the first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below and being different from (iv) comprises a nitrogen atom, more preferably comprises an amino group, most preferably comprises an amino acid.
• said amino acid comprises an alpha-amino acid having a sulfur atom with an oxidation number of +5 or below, most preferably a proteinogenic amino acid having a sulfur atom with an oxidation number of +5 or below. Most preferably this comprises methionine and cysteine.
• the first sulfur-containing compound comprises methionine.
• the aforementioned molar ratios most preferably apply, if the first sulfur-containing compound comprises methionine.
• a method of the present invention is preferred, wherein the second sulfur-containing compound comprises methionine instead of the first one.
• methionine has a total concentration ranging from 100 mmol/L to 500 mmol/L, based on the total volume of the electroplating bath, preferably from 110 mmol/L to 450 mmol/L, more preferably from 120 mmol/L to 400 mmol/L, even more preferably from 130 mmol/L to 350 mmol/L, yet even more preferably from 140 mmol/L to 300 mmol/L, most preferably from 150 mmol/L to 250 mmol/L.
• methionine even has a lower total concentration, preferably ranging from 15 mmol/L to 100 mmol/L, more preferably from 20 mmol/L to 80 mmol/L.
• the second sulfur-containing compound comprises an inorganic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below, preferably comprises thiocyanate anions.
• the first sulfur-containing compound comprises thiocyanate anions instead of the second one.
• thiocyanate anions i.e. SCN -
• organic compounds comprising a thiocyanate moiety are considered to be organic thiocyanates.
• thiocyanate anions are present through a thiocyanate salt (e.g. potassium, sodium, ammonium thiocyanate) and/or through thiocyanic acid.
• thiocyanate anions have a total concentration ranging from 1 mmol/L to 400 mmol/L, based on the total volume of the electroplating bath, preferably from 3 mmol/L to 350 mmol/L, more preferably from 5 mmol/L to 300 mmol/L, even more preferably from 8 mmol/L to 250 mmol/L, yet even more preferably from 12 mmol/L to 200 mmol/L, most preferably from 15 mmol/L to 180 mmol/L.
• the aqueous trivalent chromium electroplating bath comprises further compounds or preferably does not contain particular compounds as outlined in the following.
• the aqueous trivalent chromium electroplating bath further comprises Fe(II) ions, preferably in a concentration ranging from 0.1 mmol/L to 10 mmol/L, based on the total volume of the electroplating bath, preferably from 0.4 mmol/L to 8 mmol/L, more preferably from 0.8 mmol/L to 6 mmol/L, even more preferably from 1.2 mmol/L to 5 mmol/L, most preferably from 1.5 mmol/L to 4.5 mmol/L.
• said Fe(II) ions positively affect the electroplating performance.
• the dark chromium layer comprises iron.
• the aqueous trivalent chromium electroplating bath further comprises sulfate anions, preferably in a concentration ranging from 0.2 mol/L to 1.3 mol/L, based on the total volume of said electroplating bath, more preferably from 0.3 mol/L to 1.1 mol/L, even more preferably from 0.4 mol/L to 1.0 mol/L, yet even more preferably from 0.5 mol/L to 0.9 mol/L, most preferably from 0.6 mol/L to 0.8 mol/L.
• sulfate ions are present due to the source of trivalent chromium ions, e.g. basic chromium sulfate. Sulfate ions excellently contribute to the conductivity of said electroplating bath.
• the aqueous trivalent chromium electroplating bath further comprises halogen anions, preferably halogen anions in a total concentration ranging from 0.1 mol/L to 6 mol/L, based on the total volume of said electroplating bath, more preferably in a total concentration ranging from 0.5 mol/L to 5 mol/L, even more preferably from 1 mol/L to 4.5 mol/L, yet even more preferably from 1.5 mol/L to 4.2 mol/L, most preferably from 2 mol/L to 3.9 mol/L.
• halogen anions preferably halogen anions in a total concentration ranging from 0.1 mol/L to 6 mol/L, based on the total volume of said electroplating bath, more preferably in a total concentration ranging from 0.5 mol/L to 5 mol/L, even more preferably from 1 mol/L to 4.5 mol/L, yet even more preferably from 1.5 mol/L to 4.2 mol/L, most preferably
• halogen anions comprise chloride anions, preferably in a total concentration ranging from 0.5 mol/L to 5 mol/L, based on the total volume of said electroplating bath, more preferably from 0.8 mol/L to 4.7 mol/L, even more preferably from 1.3 mol/L to 4.5 mol/L, yet even more preferably from 1.8 mol/L to 4 mol/L, most preferably from 2.3 mol/L 3.7 mol/L.
• Chloride ions are preferably from a chloride salt and/or hydrochloric acid, preferably from sodium chloride, potassium chloride, ammonium chloride, chromium chloride (at least as a part of all chloride ions), and/or mixtures thereof.
• chloride ions are present as the anion of a conductivity salt.
• a very preferred conductivity salt is ammonium chloride, sodium chloride and potassium chloride, ammonium chloride being preferred most.
• halogen anions comprise bromide anions, in some cases preferably in addition to chloride anions.
• Bromide ions typically avoid an anodic formation of undesired hexavalent chromium species.
• the bromide ions have a concentration ranging from 3 g/L to 20 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably ranging from 4 g/L to 18 g/L, more preferably ranging from 5 g/L to 16 g/L, even more preferably ranging from 6 g/L to 14 g/L, most preferably ranging from 7 g/L to 12 g/L.
• Bromide ions are preferably from a bromide salt, preferably from sodium bromide, potassium bromide, ammonium bromide, and/or mixtures thereof.
• the bromide ions are also present if sulfate ions are utilized in the aqueous trivalent chromium electroplating bath.
• step (b) the aqueous trivalent chromium electroplating bath further comprises ammonium ions.
• the aqueous trivalent chromium electroplating bath further comprises one or more than one pH buffer compound.
• the one or more than one pH buffer compound is distinct (i.e. different) from (ii). This preferably means that the one or more than one pH buffer compound does not comprise a carboxylic acid, preferably does not comprise an organic acid.
• the one or more than one pH buffer compound comprises a boron-containing compound, preferably boric acid and/or a borate, most preferably boric acid.
• a preferred borate is sodium borate.
• the one or more than one pH buffer compound has a total concentration ranging from 30 g/L to 250 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably ranging from 35 g/L to 200 g/L, more preferably ranging from 40 g/L to 150 g/L, even more preferably ranging from 45 g/L to 100 g/L, most preferably ranging from 50 g/L to 75 g/L.
• the one or more than one pH buffer compound comprises boric acid but no borate.
• the aqueous trivalent chromium electroplating bath comprises boric acid, preferably in a total amount concentration ranging from 35 g/L to 90 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably from 40 g/L to 80 g/L, more preferably from 50 g/L to 70 g/L, most preferably from 56 g/L to 66 g/L.
• the aqueous trivalent chromium electroplating bath does not explicitly comprise a distinct pH buffer compound. Rather, the one or more than one complexing agent for said trivalent chromium ions are present in such an amount and selected in such a way that they do not only serve as complexing agent for the trivalent chromium ions but additionally serve as pH buffer compound. In the context of the present invention this is less preferred but possible.
• the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, ions and/or compounds comprising zinc.
• the dark chromium layer is substantially free of, preferably does not comprise, zinc.
• the aqueous trivalent chromium electroplating bath is not a conversion treatment composition.
• the aqueous trivalent chromium electroplating bath is not suitable for conversion coatings and/or for applying on a zinc or zinc alloy layer.
• the method of the present invention is not a conversion coating method.
• the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, fluoride ions.
• the dark chromium layer is substantially free of, preferably does not comprise, fluorine.
• the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, phosphate anions, more preferably is substantially free of, preferably does not comprise, phosphorous-containing compounds.
• the dark chromium layer is substantially free of, preferably does not comprise, phosphorous.
• aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, sulfite anions.
• the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, a compound comprising chromium with an oxidation number +6.
• said electroplating bath is substantially free of, preferably does not comprise, hexavalent chromium. This in particular means that hexavalent chromium is at least not intentionally added to the aqueous trivalent chromium electroplating bath.
• the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, ions and/or compounds comprising cobalt.
• the dark chromium layer is substantially free of, preferably does not comprise, cobalt.
• the aqueous trivalent chromium electroplating bath comprises ions and/or compounds comprising cobalt.
• the dark chromium layer comprises cobalt.
• aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, a soluble aluminum compound (including salts thereof), preferably does not comprise dissolved aluminum ions.
• the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, nickel ions.
• a typical Ni-contamination of up to 150 ppm is observed, which is basically acceptable and therefore considered as substantially free of nickel ions.
• the nickel ions have a concentration ranging from 0 ppm to 200 ppm, based on the total weight of the aqueous trivalent chromium electroplating bath, preferably from 1 ppm to 150 ppm, most preferably from 2 ppm to 100 ppm.
• the aqueous trivalent chromium electroplating bath is free of nickel.
• the dark chromium layer is substantially free of, preferably does not comprise, nickel.
• aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, a sulfamic acid and salts thereof.
• step (c) of the method of the present invention the substrate is contacted with the aqueous trivalent chromium electroplating bath and an electrical current is applied such that a dark chromium layer is electrolytically deposited on the substrate.
• step (c) is a direct current, preferably in a range from 3 A/dm 2 to 30 A/dm 2 , more preferably from 4 A/dm 2 to 25 A/dm 2 , even more preferably from 5 A/dm 2 to 20 A/dm 2 , most preferably from 6 A/dm 2 to 18 A/dm 2 .
• step (c) at least one anode is utilized.
• the at least one anode is preferably selected from the group consisting of graphite anodes, precious metal anodes, and mixed metal oxide anodes (MMOs).
• Preferred precious metal anodes comprise platinized titanium anodes and/or platinum anodes.
• Preferred mixed metal oxide anodes comprise platinum oxide coated titanium anodes and/or iridium oxide coated titanium anodes.
• the dark chromium layer electrolytically deposited in step (c) has a layer thickness ranging from 0.05 ⁇ m to 1 ⁇ m, preferably from 0.1 ⁇ m to 0.8 ⁇ m,more preferably from 0.125 ⁇ m to 0.6 ⁇ m, most preferably from 0.15 ⁇ m to 0.5 ⁇ m.
• step (c) is a method of the present invention, wherein in step (c) the contacting is carried out for 1 minute to 20 minutes, preferably for 2 minutes to 15 minutes, more preferably from 3 minutes to 10 minutes.
• step (c) is a method of the present invention, wherein in step (c) the contacting is carried out at a temperature ranging from 20°C to 60°C, preferably from 25°C to 52°C, more preferably from 30°C to 45°C.
• a dark chromium layer is electrolytically deposited (i.e. it is an electroplated chromium layer) on the substrate, preferably with a lightness value L* according to the L*a*b* color-space system of 60 or less, more preferably of 58 or less, even more preferably of 56 or less, yet even more preferably of 53 or less, most preferably 51 or less.
• step (c) the dark chromium layer has, according to the L*a*b color system, a L* value ranging from 45 to 59, preferably from 47 to 55, most preferably from 49 to 53.
• the dark chromium layer comprises the chemical element aluminum. This means that said colloidal particles are preferably incorporated into the dark chromium layer.
• step (c) the dark chromium layer has a a* value, according to the L*a*b color system, ranging from -0.5 to +3.0, preferably from 0 to 2.5, more preferably from +0.3 to +2, most preferably from +0.5 to 1.5.
• the dark chromium layer has a b* value, according to the L*a*b color system, ranging from +3.1 to +7, preferably from 3.5 to 6.5, more preferably from +4 to +6, most preferably from +4.5 to 5.5.
• the at least one nickel or nickel alloy layer comprises at least one bright-nickel layer and/or (preferably or) at least one satin nickel layer, most preferably at least one bright-nickel layer.
• the at least one nickel or nickel alloy layer comprises at least one semi-bright nickel layer, preferably at least one semi-bright-nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer.
• the at least one semi-bright nickel layer is preferably optionally. Most preferably (if applied) the at least one semi-bright nickel layer is deposited prior to said at least one bright-nickel layer and/or said at least one satin nickel layer.
• the at least one nickel or nickel alloy layer comprises at least one MPS nickel layer, preferably at least one MPS nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer, most preferably at least one MPS nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer, and further to said at least one semi-bright nickel layer.
• MPS denotes that the MPS nickel layer comprises non-conductive micro-particles, which cause micro-pores in a subsequent chromium layer, preferably in the dark chromium layer.
• the at least one MPS nickel layer is preferably optionally.
• a method of the present invention is preferred, wherein the MPS nickel layer is adjacent to the dark chromium layer.
• the dark chromium layer is adjacent to the at least one bright-nickel layer and/or the at least one satin nickel layer, which is in many cases preferred, most preferably in combination with the at least one bright-nickel layer.
• the dark chromium layer is part of a layer stack.
• the present invention furthermore relates to an aqueous trivalent chromium electroplating bath comprising
• the aforementioned regarding the method of the present invention in particular relating to the aqueous trivalent chromium electroplating bath utilized in the method of the present invention, applies likewise to the aqueous trivalent chromium electroplating bath of the present invention, most preferably features described as particularly preferred.
• the present invention also relates to a substrate comprising a dark chromium layer, wherein the dark chromium layer comprises the chemical element aluminum and has, according to the L*a*b color system, a L* value of 60 or less, preferably of 58 or less, more preferably of 56 or less, even more preferably of 53 or less, most preferably 51 or less.
• the aforementioned regarding the method of the present invention in particular relating to the dark chromium layer obtained by the method of the present invention, applies likewise to the substrate of the present invention, most preferably features described as particularly preferred. This most preferably applies to the appearance and ingredients of the dark chromium layer mentioned before, explicitly or implicitly, in relation to the method of the present invention.
• the dark chromium layer comprises the chemical element aluminum and has, according to the L*a*b color system, a L* value ranging from 45 to 59, preferably from 47 to 55, most preferably from 49 to 53.
• the dark chromium layer has a a* value ranging from -0.5 to +3.0, preferably from 0 to 2.5, more preferably from +0.3 to +2, most preferably from +0.5 to 1.5.
• the dark chromium layer has a b* value ranging from +3.1 to +7, preferably from 3.5 to 6.5, more preferably from +4 to +6, most preferably from +4.5 to 5.5.
• the dark chromium layer is substantially free of, preferably does not comprise, cobalt. In contrast, in other cases it is preferred, wherein the dark chromium layer comprises cobalt.
• the dark chromium layer is substantially free of, preferably does not comprise, nickel.
• the dark chromium layer is not the only metal layer between the dark chromium layer and the substrate.
• At least one nickel layer or nickel alloy layer comprises at least one bright-nickel layer or at least one satin nickel layer. This is in many cases most preferred.
• said at least one nickel layer or nickel alloy layer comprises at least one semi-bright nickel layer, preferably at least one semi-bright-nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer.
• the at least one semi-bright nickel layer is preferably optionally. Most preferably the at least one semi-bright nickel layer is the nickel layer (of all nickel layers) closest to the substrate.
• said at least one nickel layer or nickel alloy layer comprises at least one MPS nickel layer, preferably at least one MPS nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer, most preferably at least one MPS nickel layer in addition to said at least one bright-nickel layer, and/or said at least one satin nickel layer, and further to said at least one semi-bright nickel layer.
• MPS denotes micro porous.
• the dark chromium layer is part of a layer stack, the layer stack comprising, along a direction from the dark chromium layer to the substrate (adjacent or not adjacent):
• the layer stack preferably comprises a sealer layer and/or an anti-fingerprint layer, most preferably on the dark chromium layer. If both is applied, preferably the sealer layer is applied first, followed by the anti-fingerprint layer, which preferably forms the very outermost layer.
• copper panels 99 mm x 70 mm were used as a substrate, primarily for mimicking plastic substrates deposited with a copper layer.
• the substrate was cleaned by electrolytic degreasing with Uniclean ® 279 (product of Atotech), 100 g/L at room temperature (RT). Afterwards the copper panels were pickled with 10 % H 2 SO 4 by volume and rinsed with water.
• Uniclean ® 279 product of Atotech
• the cleaned and rinsed substrate was subjected to nickel plating to obtain a bright nickel layer on top of the copper panel (parameters: 10 min at 4 A/dm 2 ; Makrolux ® NF electrolyte; product of Atotech).
• aqueous trivalent chromium base electroplating bath 128 g/L Basic chromium sulfate 46 g/L Formic acid 60 g/L Boric acid 12 g/L Ammonium bromide 100 g/L Ammonium chloride 110 g/L Potassium chloride 25-45 g/L Methionine 0.1 g/L Sodium diamyl sulphosuccinate
• the final pH value was about 3.
• Electroplating was carried out in a Hull Cell having a graphite anode and the substrate installed as the cathode. An electrical current of 5 A was applied to the aqueous trivalent chromium electroplating bath for 3 minutes, the bath having a temperature of about 35 °C for all examples (including comparative examples).
• the color according to the L*a*b* color space system was determined with a colorimeter (Konica Minolta CM-700 D spectrophotometer). Calibration was done with black and white standard. Color determination was done at an area in the center of the substrate. The measuring area is located 1 cm from the left edge and 2 cm from the lower edge (the left edge pointing to the anode).
• Table 1 Summary of compositions and results according to the invention No.
• Comparative examples CE1 and CE5 to CE7 show conditions without any particles but rather only containing organic coloring agents such as methionine. Interestingly, the presence of silica nanoparticles does not significantly lower the L* value (compare CE1 vs. CE2 to CE4); at least not for a methionine-containing plating bath. In these cases, the L* value remained more or less at 55. Nevertheless, all obtained chromium layers are already dark.
• the a* value remains basically constant.
• adding colloidal particles containing the chemical element aluminum significantly affects the b* value, which basically always increases.
• CE1 (b* +3.2) E1 to E3 (b* about +5.6);
• CE5 (b* 0) E4 to E6 (b* +2.6 to +3.1);
• CE6 (b* +0.4) E7 to E9 (b* +1.9 to +2.8);
• CE7 (b* +1.3) E10 and E11 (b* +4.0 to +4.7).
• the dark chromium layer obtained with the invention is glossy.

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• 2021
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