US20230015534A1

US20230015534A1 - Electroplating composition and method for depositing a chromium coating on a substrate

Info

Publication number: US20230015534A1
Application number: US17/785,114
Authority: US
Inventors: Simon PAPE; Jana Breitfelder; Anke WALTER
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 2019-12-18
Filing date: 2020-12-17
Publication date: 2023-01-19
Also published as: EP4077770A1; WO2021122932A1; JP2023507017A; KR20220116477A; CN114829677A

Abstract

The present invention refers to an electroplating composition for depositing a chromium coating on a substrate, the composition including:(i) trivalent chromium ions,(ii) at least one complexing agent for the trivalent chromium ions, and(iii) at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof.

Description

FIELD OF THE INVENTION

The present invention relates to an electroplating composition and method for depositing a chromium coating on a substrate. In particular the electroplating composition according to the present invention allows for the electrolytic deposition of functional chromium coatings, also called hard chromium coatings, on a substrate, in particular on a ferrous substrate, in particular on a nickel or nickel alloy coated ferrous substrate.

BACKGROUND OF THE INVENTION

Functional chromium coatings usually have a much higher average coating thickness, typically from at least 1 μm up to several hundreds of micro meters, compared to decorative chromium coatings, typically below 1 μm, and are characterized by excellent hardness and wear resistance.
Functional chromium coatings obtained from a chromium electroplating composition containing hexavalent chromium are known in the prior art and are a well-established standard.
During recent decades, hexavalent chromium-based methods are more and more replaced by trivalent chromium-based methods, which are much more health-and environment friendly.
Typical chromium-based electroplating methods are described in the following prior art.
WO 2015/110627 A1 refers to an electroplating composition for depositing chromium and to a method for depositing chromium on a substrate using said electroplating composition.
U.S. Pat. No. 2,748,069 relates to an electroplating solution of chromium, which allows obtaining very quickly a chromium coating of very good physical and mechanical properties. The chromium plating solution can be used for special electrolyzing methods, such as those known as spot or plugging or penciling galvanoplasty. In such special methods the substrate is typically not immersed into a respective electroplating solution.
WO 2018/185154 A1 discloses a method for electrolytically depositing a chromium or chromium alloy coating on a substrate.
U.S. Pat. No. 4,009,085 discloses lubricating compositions and a process for treating a metal sheet to impart lubricity and abrasion resistance thereto.
U.S. Pat. No. 3,432,408 A relates to the prevention of mist and spray in acidic hexavalent electroplating baths.
US 2016/068983 A1 refers to methods and plating baths for electrodepositing a dark chromium layer on a workpiece, the bath for example comprising a diol.
EP 0 100 133 A1 refers to zinc and nickel tolerant trivalent chromium plating baths and plating process, utilizing for example a betaine.
CN 108034969 A refers to a sulfate-based trivalent chromium electroplating bath comprising for example a diol or polyethylene glycol.
US 2018/245227 A1 relates to the use of ionic liquids in electroplating, and in particular for electroplating thick, hard chromium from trivalent salts.
However, when using trivalent chromium-based methods for electroplating according to the prior art, the cathodic current efficiencies (CCE) observed are typically smaller compared to the cathodic current efficiencies observed when using hexavalent chromium-based methods for electroplating.
The cathodic current efficiency (CCE) is based on Faraday's law and is described as the percentage of metal actually deposited on the substrate during electroplating compared to the theoretical ideal case, when all, i.e. 100%, of the metal present in the electroplating composition could be deposited on the substrate. In hexavalent chromium-based methods for electroplating according to the prior art typical CCEs are between 20% and 25%, while in trivalent chromium-based methods for electroplating according to the prior art typical CCEs can be as low as 10%.
An important factor, which can influence the CCEs in trivalent chromium-based methods for electroplating are inter alia the types and concentrations of complexing agents, which are used to stabilize the trivalent chromium ions in the electroplating composition. Other factors, which can influence the CCEs in trivalent chromium-based methods for electroplating are inter alia the types and concentrations of further additives, which can be added to the electroplating composition.

OBJECTIVE OF THE PRESENT INVENTION

It was therefore the first objective of the present invention to provide an electroplating composition comprising trivalent chromium ions and a respective method for depositing a chromium coating on a substrate with improved qualities of the coating (e.g. such as hardness and/or wear resistance).
It was therefore the second objective of the present invention to provide an electroplating composition comprising trivalent chromium ions and a respective method for depositing a chromium coating on a substrate resulting in an increased cathodic current efficiency (CCE).

SUMMARY OF THE INVENTION

The objectives mentioned above are solved according to a first aspect by an electroplating composition for depositing a chromium coating on a substrate, the composition comprising:

(i) trivalent chromium ions,
(ii) at least one complexing agent for the trivalent chromium ions, and
(iii) at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof.

By utilizing the at least one complexing agent for the trivalent chromium ions, a particularly effective stabilization of trivalent chromium ions in the electroplating composition can be achieved, which allows for an effective deposition of the chromium coating on the substrate during electroplating.
It has been surprisingly found that using at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof, a significant increase of the cathodic current efficiency (CCE) during electroplating is observed, while on the other hand a coating with high quality is obtained.
The objectives mentioned above are solved according to a second aspect by a method for depositing a chromium coating on a substrate, the method comprising the following steps:

(a) providing the substrate,
(b) providing an electroplating composition for depositing a chromium coating on the substrate, the composition comprising:
- (i) trivalent chromium ions,
- (ii) at least one complexing agent for the trivalent chromium ions, and
- (iii) at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof.
(c) contacting the substrate with said electroplating composition and applying an electrical current such that the chromium coating is deposited on at least one surface of said substrate.

By having the at least one additive as defined above in the electroplating composition, a comparatively high cathodic current efficiency (CCE) is obtained during step (c).
The objectives mentioned above are solved according to a third aspect by a substrate with a surface, wherein the surface of the substrate comprises a chromium coating obtained by a method for depositing according to the second aspect.
The objectives mentioned above are solved according to a fourth aspect by a use of at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof, for increasing the cathodic current efficiency in an electroplating composition for depositing a chromium coating on a substrate.

BRIEF DESCRIPTION OF THE TABLE

In Table 1, correlations between varying concentrations of different additives in respective electroplating compositions, represented as the total amount of additive in g/L, based on the total volume of the respective electroplating composition, in respect to the resulting cathodic current efficiency (CCE) are shown. Each bar (C1) to (18) represents a specific electroplating composition. Further details are given in the “Examples” section below in the text.

DETAILED DESCRIPTION OF THE INVENTION

In the context of the present invention, the term “at least one” or “one or more” denotes (and is exchangeable with) “one, two, three or more” and “one, two, three or more than three”, respectively. Furthermore, “trivalent chromium” refers to chromium with the oxidation number +3. The term “trivalent chromium ions” refers to Cr³⁺-ions in a free or complexed form. Furthermore, “hexavalent chromium” refers to chromium with the oxidation number +6.
The term C_X-C_yif used in the context of the present invention refers to a compound comprising a total number from “x” carbon atoms to “y” carbon atoms. For example, the term C₁-C₂₅diols refers to diols comprising a total number from 1 carbon atom to 25 carbon atoms.
The cathodic current efficiency (CCE) is determined as described in the text above and is based on gravimetric analysis (see examples below in the text).
The electroplating composition of the present invention comprises at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols and mixtures thereof.
In rare cases an electroplating composition of the present invention is preferred with the proviso that the at least one additive does not comprise polymeric glycols. Preferably, the electroplating composition is substantially free of or does not comprise polyethylene glycol, more preferably is substantially free of or does not comprise polyalkylene glycols, most preferably is substantially free of or does not comprise polymeric glycols. In such a case, the electroplating composition of the present invention preferably comprises at least one additive selected from the group consisting of betaines, monomeric diols, and mixtures thereof.
In very rare cases an electroplating composition of the present invention is preferred with the proviso that the at least one additive does not comprise monomeric diols. Preferably, the electroplating composition is substantially free of or does not comprise ethanediol, more preferably is substantially free of or does not comprise alkanediols, even more preferably is substantially free of or does not comprise monomeric diols. In such a case, the electroplating composition of the present invention preferably comprises at least one additive selected from the group consisting of betaines, polymeric glycols, and mixtures thereof
In the electroplating composition of the present invention no hexavalent chromium is intentionally added to the electroplating composition. Thus, the electroplating composition is substantially free of or does not comprise hexavalent chromium (except very small amounts which may be formed anodically).
Preferably, the electroplating composition of the present invention is an aqueous electroplating composition comprising trivalent chromium ions. Preferably, the electroplating composition comprises further additives and/or metal ions, more preferably iron ions, nickel ions, copper ions and/or zinc ions.
In the context of the present invention, the chromium coating comprises chromium alloys, i.e. a coating comprising not only chromium but also alloying elements. In very rare cases, metal alloying elements are preferred, preferably from metal ions as mentioned above. More typical and preferred are non-metal alloying elements, preferably carbon, nitrogen, and/or oxygen.
The electroplating composition of the present invention is preferably used more than one time for depositing a chromium coating on a plurality of different substrates, preferably during a continuous process. Preferably, the electroplating composition is repeatedly utilized during electroplating, preferably for a usage of at least 100 Ah per liter electroplating composition, preferably at least 150 Ah per liter, more preferably at least 200 Ah per liter, most preferably at least 300 Ah per liter.
Basically, preferred is an electroplating composition of the present invention, wherein the betaines comprise at least 5 carbon atoms, more preferably at least 10 carbon atoms, and even more preferably at least 15 carbon atoms. Preferable not having more than 50 carbon atoms.
Preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises at least one or more than one betaine, preferably independently comprising at least 5 carbon atoms, more preferably at least 10 carbon atoms, and even more preferably at least 15 carbon atoms. Preferable not having more than 50 carbon atoms.
More preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises at least one or more than one betaine independently comprising at least 10 carbon atoms, preferably at least 15 carbon atoms. Preferable not having more than 50 carbon atoms.
This all means that out of the selection from betaines, polymeric glycols, monomeric diols, and mixtures thereof, the electroplating composition of the present invention must at least comprise a betaine, preferably with the number of carbon atoms as defined above.
Preferred is an electroplating composition of the present invention, wherein the betaines independently comprise (or betaines at least denote):

- a positively charged quaternary nitrogen atom, and
- a negatively charged sulfonate group and/or negatively charged carboxylate group,
  with the proviso that the positive charge cannot be removed by deprotonation.

Preferred is an electroplating composition of the present invention, wherein the betaines have a neutral net charge.
Preferred is an electroplating composition of the present invention, wherein the betaines are linear betaines.
Preferred is an electroplating composition of the present invention, wherein the betaines do not comprise an aromatic ring structure, preferably do not comprise a ring structure.
Preferred is an electroplating composition of the present invention, wherein the positively charged quaternary nitrogen atom has substituents such that said positive charge results, with the proviso that the substituents are not hydrogen.
Preferably, said substituents are independently selected from the group consisting of alkyl, ester, and amide.
Preferably, alkyl comprises C₁-C₂₀alkyl, more preferably C₁-C₁₇alkyl, most preferably C₁-C₁₅alkyl.
Preferably, ester comprises C₈-C₂₀ester, more preferably C₉-C₁₇ester, most preferably C₁₀-C₁₆ester.
Preferably, ester comprises fatty acid esters, most preferably with a number of carbon atoms as defined above.
Preferably, amide comprises C₈-C₂₀amide, more preferably C₉-C₁₇amide, most preferably C₁₀-C₁₆amide.
Preferably, amide comprises fatty acid amides, most preferably with a number of carbon atoms as defined above for amide.
Most preferably, at least one, preferably two, substituent is alkyl, preferably as defined above, most preferably a C₁-C₅alkyl, even more most preferably a C₁-C₃alkyl, and in addition at least one substituent is ester, preferably as defined above, or amide, preferably as defined above.
Preferred is an electroplating composition of the present invention, wherein the betaines independently comprise a positively charged quaternary nitrogen atom and a negatively charged sulfonate group, with the proviso that the positive charge cannot be removed by deprotonation.
Preferred is an electroplating composition of the present invention, wherein the betaines are selected from the group consisting of N-substituted-ammonium sulfobetaines and N-substituted-ammonium carboxybetaines.
Preferred is an electroplating composition of the present invention, wherein the betaines are selected from the group consisting of N-substituted-N,N-Dialkyl -ammonium sulfobetaines, preferably N-substituted-N, N-Dialkyl-N-alkyl ammonium sulfobetaines.
Preferred is an electroplating composition of the present invention, wherein the N-substituted-N,N-Dialkyl-ammonium sulfobetaines and the N-substituted-N,N -Dialkyl-N-alkyl ammonium sulfobetaines, respectively, is N-substituted with a substituent selected from the group consisting of alkyl, and amidoalkyl, wherein amidoalkyl is preferably cocoamidopropyl.
Preferred is an electroplating composition of the present invention, wherein the betaines comprise one or more of N,N-Dimethyl-N-(3-cocoamidopropyl)-N-(2-hydroxy-3-sulfopropyl) ammonium betaine, N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Octyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Decyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Hexadecyl-N,N -dimethyl-3-ammonio-1-propanesulfonate, N-Octadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, and N,N-Dimethyl-N-dodecylglycine betaine.
By selecting the preferred betaines a particular advantageous cathodic current efficiency (CCE) is achieved, preferably along with a foam formation for mist suppression during electroplating. Advantageously, the concentration of betaines can be comparatively low (see examples).
Regarding the at least one additive, in some cases an electroplating composition of the present invention is preferred, wherein the electroplating composition comprises at least one betaine and preferably in addition one or more than one of polymeric glycols and/or monomeric diols.
In particular preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises at least one betaine and in addition one or more than one monomeric diol.
Preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises the betaines in a total concentration in a range from 0.0005 g/L to 1 g/L, based on the total volume of the electroplating composition, preferably from 0.001 g/L to 0.5 g/L, more preferably from 0.005 g/L to 0.3 g/L, and most preferably from 0.01 g/L to 0.2 g/L.
Preferred is an electroplating composition of the present invention, wherein the polymeric glycols are polyalkylene glycols, preferably polyethylene glycols. Preferably, the polyalkylene glycols, preferably the polyethylene glycols, have an average molecular weight in a range from 150 Da to 5000 Da, preferably from 200 Da to 2500 Da. Most preferred are polymeric glycols, selected from the group consisting of polyethylene glycol 200, polyethylene glycol 600, and polyethylene glycol 1500.
Preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises the polymeric glycols in a total concentration in a range from 0.01 g/L to 50 g/L, based on the total volume of the electroplating composition, preferably from 0.05 g/L to 35 g/L, more preferably from 0.1 g/L to 20 g/L, most preferably from 0.15 g/L to 25 g/L.
Also, when using polymeric glycols as a representative of the at least one additives a beneficial increase in the cathodic current efficiency (CCE) is obtained.
Preferred is an electroplating composition of the present invention, wherein the monomeric diols are C₁-C₂₅diols, preferably C₂-C₂₃diols, more preferably C₂-C₂₁diols, even more preferably C₂-C₁₉diols, most preferably C₂-C₁₈diols.
In some cases, preferred is an electroplating composition of the present invention, wherein the monomeric diols comprise one or more than one C₁-C₁₀diol, preferably one or more than one C₂-C₈diol, more preferably one or more than one C₂-C₆diol, even more preferably one or more than one C₂-C₅diol, most preferably one or more than one C₂-C₄diol, even most preferably the monomeric diol comprises 1,2-propane diol and/or 1,3-propane diol. Most preferably in combination with one or more than one betaine.
In some cases, preferred is an electroplating composition of the present invention, wherein the monomeric diols comprise, in addition to the C₁-C₁₀diols and its preferred variants, or alternatively to the C₁-C₁₀diols, and its preferred variants, preferably in addition, one or more than one C₁₁-C₂₅diol, preferably one or more than one C₁₂-C₂₃diol, more preferably one or more than one C₁₃-C₂₁diol, even more preferably one or more than one C₁₄-C₂₀diol, most preferably one or more than one C₁₅-C₁₉diol, even most preferably one or more than one C₁₆-C₁₈diol, most preferably in combination with one or more than one betaine.
Preferred is an electroplating composition of the present invention, wherein the one or more than one C₁₁-C₂₅diol, and its preferred variants, is selected from the group of saturated and unsaturated C₁₁-C₂₅diols, preferably saturated C₁₁-C₂₅diols.
Preferred is an electroplating composition of the present invention, wherein the one or more than one C₁₁-C₂₅diol, and its preferred variants, is branched.
Preferred is an electroplating composition of the present invention, wherein the one or more than one C₁₁-C₂₅diol, and its preferred variants, comprises one, two or more than two iso-propyl moieties.
Preferred is an electroplating composition of the present invention, wherein the one or more than one C₁₁-C₂₅diol, and its preferred variants, comprises an anti-foam compound. In many cases, such C₁₁-C₂₅diols have the potential to function as an anti-foam compound. Furthermore, in many cases C₁to C₁₀diols are often excellent solvents for aforementioned anti-foam compounds.
Preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises the monomeric diols in a total concentration in a range from 0.001 g/L to 60 g/L, based on the total volume of the electroplating composition, preferably from 0.1 g/L to 50 g/L, more preferably from 1.0 g/L to 40 g/L, even more preferably from 5.0 g/L to 35 g/L, most preferably from 15 g/L to 30 g/L.
In a few specific cases, preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises the monomeric diols in a total concentration in a range from 0.001 g/L to 10 g/L, based on the total volume of the electroplating composition, preferably from 0.01 g/L to 8.0 g/L, more preferably from 0.1 g/L to 6.0 g/L, even more preferably from 0.5 g/L to 4.0 g/L, most preferably from 1.0 g/L to 3.0 g/L. This most preferable selection preferably applies if the electroplating composition of the present invention comprises one or more than one betaine, most preferably if the electroplating composition of the present invention comprises one or more than one betaine and one or more than one C₁₁-C₂₅diol, and its preferred variants.
Preferred is an electroplating composition of the present invention, wherein the electroplating composition is substantially free of or does not comprise hexavalent chromium ions.
Preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises trivalent chromium ions in a total concentration from 10 g/L to 30 g/L, based on the total volume of the electroplating composition, preferably from 14 g/L to 27 g/L, and most preferably from 17 g/L to 24 g/L.
With the preferred selections of the concentration range of the trivalent chromium ions in the electroplating composition, a particular effective deposition of the chromium coating on the substrate can be achieved. If the total amount of trivalent chromium ions is significantly below 10 g/L in many cases an insufficient deposition is observed, and the deposited chromium is usually of low quality. If the total amount is significantly above 30 g/L, the electroplating composition is not any longer stable, which includes formation of undesired precipitates.
Preferred is an electroplating composition of the present invention, wherein the at least one complexing agent for the trivalent chromium ions is selected from the group consisting of organic complexing agents and salts thereof, preferably carboxylic acids and salts thereof, more preferably aliphatic carboxylic acids and salts thereof, most preferably aliphatic mono-carboxylic acids and salts thereof. Preferred aliphatic mono-carboxylic acids and salts thereof are C₁-C₁₀aliphatic mono-carboxylic acids and salts thereof, preferably C₁-C₈aliphatic mono-carboxylic acids and salts thereof, more preferably C₁-C₆aliphatic mono-carboxylic acids and salts thereof, most preferably C₁-C₃aliphatic mono-carboxylic acids and salts thereof.
Preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises the at least one complexing agent in a total concentration in a range from 50 g/L to 350 g/L, based on the total volume of the composition, preferably from 100 g/L to 300 g/L, even more preferably from 150 g/L to 250 g/L.
By utilizing in particular the above-mentioned preferred selection of complexing agents, the trivalent chromium ions can be efficiently stabilized in the electroplating composition by the complexing agents
Preferred is an electroplating composition of the present invention, wherein the electroplating composition has a pH in a range from 4.1 to 7.0, preferably from 4.5 to 6.5, more preferably from 5.0 to 6.0, and most preferably from 5.3 to 5.9.
The preferred acidic pH ranges are in particular beneficial for effectively depositing a chromium coating on the substrate having the desired qualities.
In particular preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises one or more than one betaine and one or more than one monomeric diol, with the proviso that the one or more than one monomeric diol comprises one or more than one C₁₁-C₂₅diol comprising one, two or more than two iso-propyl moieties.
Even more preferred is an electroplating composition of the present invention, wherein the electroplating composition comprises one or more than one betaine and more than one monomeric diol, with the proviso that

- at least one of the more than one monomeric diol comprises one or more than one C₁₁-C₂₅diol (or another preferred diol among the C₁₁-C₂₅diol as mentioned above) comprising one, two or more than two iso-propyl moieties, and
- at least one of the more than one monomeric diol comprises one or more than one C₂-C₈diol (or another preferred diol among the C₂-C₈diol as mentioned above).

Preferred is an electroplating composition of the present invention, wherein the electroplating composition is essentially free of or does not comprise boric acid, preferably is essentially free of or does not comprise boron-containing compounds.
Boron-containing compounds are not desired because they are environmentally problematic. When using boron-containing compounds, waste water treatment is expensive and time consuming. Furthermore, boric acid typically shows poor solubility and therefore has the tendency to form precipitates. Although such precipitates can be solubilized upon heating, a respective electroplating composition cannot be utilized for electroplating during this time. There is a significant risk that such precipitates facilitate a reduced chromium coating quality. Thus, the electroplating composition of the present invention is preferably essentially free of or does not comprise boron-containing compounds. Surprisingly, the electroplating composition of the present invention performs very well without boron-containing compounds, in particular in the above-mentioned preferred pH ranges.
Preferred is an electroplating composition of the present invention, wherein the electroplating composition is essentially free of or does not comprise organic compounds containing divalent sulfur, preferably is essentially free of or does not comprise sulfur-containing compounds with a sulfur atom having an oxidation number below +6.
Omitting organic compounds containing divalent sulfur from the electroplating composition is particularly advantageous when employing the electroplating composition for deposition of hard, functional chromium coatings.
The term “does not comprise” typically denotes that respective compounds and/or ingredients are not intentionally added to e.g. the electroplating composition. This does not exclude that such compounds are dragged in as impurities of other chemicals. However, typically the total amount of such compounds and ingredients is below the detection range and therefore is not critical in the various aspects of the present invention.
Preferred is an electroplating composition furthermore comprising one or more than one compound selected from the group consisting of

- one or more than one type of halogen ions, preferably bromide,
- one or more than one type of alkaline metal cations, preferably sodium and/or potassium,
- sulfate ions, and
- ammonium ions.

By adding one or more of the above-mentioned compounds the deposition of chromium during an electroplating process can be improved, most preferably during the method of the present invention.
Preferably, the electroplating composition of the present invention comprises one or more than one type of halogen ions, preferably bromide, in a concentration of at least 0.06 mol/L, based on the total volume of the electroplating composition, more preferably at least 0.1 mol/L, even more preferably at least 0.15 mol/L. In particular bromide anions effectively suppress the formation of hexavalent chromium species at the at least one anode.
Preferably, the electroplating composition comprises one or more than one type of alkaline metal cations, preferably sodium and/or potassium, in a total concentration ranging from 0 mol/L to 0.5 mol/L, based on the total volume of the electroplating composition, more preferably from 0 mol/L to 0.3 mol/L, even more preferably from 0 mol/L to 0.1 mol/L, and most preferably from 0 mol/L to 0.08 mol/L.
Typically, rubidium, francium, and caesium ions are not utilized in an electroplating composition comprising trivalent chromium ions. Thus, preferably the one or more than one type of alkaline metal cations includes metal cations of lithium, sodium, and potassium, mostly sodium and potassium.
Preferred is an electroplating composition of the present invention, wherein the trivalent chromium ions of the electroplating composition are obtained from a soluble, trivalent chromium ion containing source, typically a water-soluble salt comprising said trivalent chromium ions. Preferably, the soluble, trivalent chromium ion containing source comprises alkali metal cations in a total amount of 1 wt.-% or less, based on the total weight of said source. In some cases, preferably, such a source is utilized for replenishing trivalent chromium ions if the method is operated continuously. A preferred water-soluble salt comprising said trivalent chromium ions is alkali metal free trivalent chromium sulfate or alkali metal free trivalent chromium chloride. In some cases it is preferred that the electroplating composition of the present invention contains sulfate ions, preferably in a total amount in the range from 50 g/L to 250 g/L, based on the total volume of the electroplating composition.
Preferably, the soluble, trivalent chromium ion containing source comprises or is chromium sulfate, more preferably acidic chromium sulfate, even more preferably chromium sulfate with the general formula Cr₂(SO₄)₃and a molecular weight of 392 g/mol.
More preferably, for replenishing, a soluble, trivalent chromium ion containing source is preferred, wherein the anion is an organic anion, preferably an organic acid anion, most preferably formate and/or acetate.
The present invention according to the second aspect provides a method for depositing a chromium coating on a substrate, the method comprising the following steps:

(a) providing the substrate,
(b) providing an electroplating composition (preferably as described above, most preferably as above described as being preferred) for depositing a chromium coating on the substrate, the composition comprising:
- (i) trivalent chromium ions,
- (ii) at least one complexing agent for the trivalent chromium ions, and
- (iii) at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof.
(c) contacting the substrate with said electroplating composition and applying an electrical current such that the chromium coating is deposited on at least one surface of said substrate.

Preferably, the aforementioned regarding the electroplating composition of the present invention (preferably as described above as being preferred), applies likewise to the method of the present invention (preferably a method as described below as being preferred).
Preferred is a method of the present invention, wherein in step (c) the electrical current is a direct current.
Preferably, the direct current (DC) is a direct current without interruptions during the electroplating, wherein more preferably the direct current is not pulsed (non-pulsed DC). Furthermore, the direct current preferably does not include reverse pulses.
Preferred is a method of the present invention, wherein in step (c) the electrical current has a cathodic current density of at least 18 A/dm², preferably of at least 20 A/dm², more preferably of at least 25 A/dm², even more preferably of at least 30 A/dm², most preferably of at least 39 A/dm². Preferably, the cathodic current density is in a range from 18 A/dm²to 60 A/dm², more preferably from 25 A/dm²to 55 A/dm², most preferably from 30 A/dm²to 50 A/dm².
Typically, the substrate provided during the method of the present invention is the cathode during the electroplating process (i.e. in step (c)). Preferably, more than one substrate is provided simultaneously in step (c) of the method of the present invention.
Preferred is a method of the present invention, wherein in step (c) at least one anode is provided, wherein the at least one anode is independently selected from the group consisting of graphite anodes and mixed metal oxide on titanium anodes. Such anodes have shown to be sufficiently resistant in the electroplating composition of the present invention. Preferably, the at least one anode does not comprise any lead or chromium.
In step (c) of the method of the present invention a chromium coating is deposited, either a pure one or an alloy. Preferably, the chromium coating is an alloy. Preferred alloying elements are carbon, nitrogen, and oxygen, preferably carbon and oxygen. Carbon is typically present because of organic compounds usually present in the electroplating composition. In many cases preferred is a method of the present invention, wherein the chromium coating does not comprise one, more than one or all elements selected from the group consisting of sulphur, nickel, copper, aluminium, tin and iron. More preferably, the only alloying elements are carbon, nitrogen, and/or oxygen, more preferably carbon and/or oxygen, most preferably carbon and oxygen. Preferably, the chromium coating contains 90 weight percent chromium or more, based on the total weight of the chromium coating, more preferably 95 weight percent or more.
Preferred is a method of the present invention, wherein in step (c) the electroplating composition has a temperature in a range from 20° C. to 90° C., preferably from 30° C. to 70° C., more preferably from 40° C. to 60° C., most preferably from 45° C. to 58° C.
In the preferred temperature range (in particular in the most preferred temperature range) the chromium coating is optimally deposited in step (c). If the temperature significantly exceeds 90° C., an undesired vaporization occurs, which can negatively affect the concentration of the composition components. Furthermore, the undesired anodic formation of hexavalent chromium is significantly less suppressed. If the temperature is significantly below 20° C. the electrodeposition is insufficient.
Preferred is a method of the present invention, wherein step (c) is performed for a time period from 10 min to 100 min, preferably from 20 min to 90 min, more preferably from 30 min to 60 min.
Preferred is a method of the present invention, wherein in step (c) the electroplating composition is stirred, preferably with a stirring rate in a range from 100 rpm to 500 rpm, most preferably from 200 rpm to 400 rpm.
By performing the method step (c) in the abovementioned preferred temperature ranges and/or (preferably and) for the preferred time periods and/or (preferably and) with the preferred stirring rates, particularly advantageous electrodeposition kinetics during step (c) can be ensured.
Preferred is a method of the present invention further comprising after step (c) step

- (d) heat-treating the chromium-coated substrate obtained from step (c).

Preferred is a method of the present invention, wherein in step (d) the heat-treating is carried out at a temperature in a range from 100° C. to 250° C., preferably from 120° C. to 240° C., more preferably from 150° C. to 220° C., most preferably from 170° C. to 200° C.
Preferred is a method of the present invention, wherein in step (d) the heat-treating is carried out for a time period from 1 h to 10 h, preferably from 2 h to 4 h.
By preferably performing a heat-treatment of the substrate, more preferably at the preferred temperatures and/or for the preferred time periods, the properties of the chromium coating can be further improved in some cases (e.g. hardness).
Preferred is a method of the present invention, wherein in step (c) the cathodic current efficiency (CCE) is 11% or more, preferably 12% or more, most preferably 13% or more. This most preferably applies, if an identical method with the only exception that the electroplating composition does not comprise an additive, is carried out.
By significantly increasing the cathodic current efficiency (CCE) to 11% or more, the entire method is more effective and economical. Furthermore, less energy is wasted and less hydrogen gas is produced during step (c).
Preferred is a method of the present invention, wherein the substrate comprises a metal rod.
Preferred is a method of the present invention, wherein the substrate comprises a metal or metal alloy, preferably comprises one or more than one metal selected from the group consisting of copper, iron, nickel and aluminum, more preferably comprises one or more than one metal selected from the group consisting of copper, iron, and nickel, most preferably comprises at least iron.
In many cases preferred is a substrate comprising a pre-coating, the pre-coating preferably being a nickel or nickel alloy coating, most preferably a semi-bright nickel coating, on which the chromium coating is applied to during step (c) of the method of the present invention. In particular preferred is a steel substrate pre-coated with a nickel or nickel alloy coating. However, preferably other pre-coatings are alternatively or additionally present. In many cases such a pre-coating significantly increases corrosion resistance compared to a metal substrate without such a pre-coating. However, in some cases the substrates are not susceptible to corrosion due to a corrosion inert environment (e.g. in an oil composition). In such a case a pre-coating, preferably a nickel or nickel alloy pre-coating, is not necessarily needed.
Generally, preferred is a method of the present invention, wherein in step (c) the chromium coating has a thickness in a range from 1.1 μm to 500 μm, preferably from 2 μm to 450 μm, more preferably from 4 μm to 400 μm, even more preferably from 6 μm to 350 μm, yet even more preferably from 8 μm to 300 μm, and most preferably from 10 μm to 250 μm.
In some cases, preferred is a method of the present invention, wherein in step (c) the chromium coating has a thickness of 0.5 μm or more, preferably of 0.75 μm or more, more preferably of 0.9 μm or more, even more preferably of 1.0 μm or more, yet even more preferably of 1.5 μm or more, and most preferably of 2.0 μm or more. In some further cases a method of the present invention is preferred, wherein in step (c) the chromium coating has a thickness of 15 μm or more, preferably of 20 μm or more.
Preferred is a method of the present invention, wherein in step (c) the concentration of the at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof (preferably at least of the betaines), is continually or semi-continually monitored, wherein

- the monitored concentration is compared to a target concentration of said at least one additive (preferably of said betaines), and
- if the monitored concentration is below the target concentration than the at least one additive (preferably the betaines) is added to the electroplating composition.

In particular the aforementioned regarding the betaines in the electroplating composition applies likewise to the method of the present invention.
The present invention according to the third aspect provides a substrate with a surface, wherein the surface of the substrate comprises a chromium coating obtained by a method for depositing according to the second aspect.
Preferred is a substrate of the present invention, wherein the chromium-coated substrate comprises or is a metal rod.
The aforementioned regarding the electroplating composition of the present invention (preferably an electroplating composition as described as being preferred) and the aforementioned regarding the method of the present invention (preferably a method for depositing as described as being preferred) preferably applies likewise to the substrate of the present invention.
In particular, preferred embodiments of the electroplating composition of the first aspect and preferred embodiments of the method for depositing according to the second aspect are also preferred embodiments for the substrate according to the third aspect. This applies in particular and most preferably to the characteristics of the chromium coating.
The present invention according to the fourth aspect provides a use of at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof, for increasing the cathodic current efficiency in an electroplating composition for depositing a chromium coating on a substrate.
The aforementioned regarding the electroplating composition of the present invention (preferably an electroplating composition as described as being preferred) and the aforementioned regarding the method of the present invention (preferably a method for depositing as described as being preferred) preferably applies likewise to the use of the present invention.
The present invention is described in more detail by the following non-limiting examples.

EXAMPLES

For a number of experiments, respective test electroplating compositions were prepared (volume: appr. 850 mL) comprising 10 g/L to 30 g/L trivalent chromium ions (source: basic chromium sulfate), 50 g/L to 250 g/L sulfate ions, at least one organic complexing compound (an aliphatic mono carboxylic organic acid), ammonium ions, and bromide ions. The compositions did not contain boric acid nor any boron-containing compounds and no organic compounds with divalent sulfur. The pH was in a range from 5.4 to 5.7.
In a reference electroplating composition (C1) no additive was contained, defining the reference cathodic current efficiency (CCE). In a number of further experiments, various additives in a number of concentrations were tested (see Table 1 below).
In each test, the respective electroplating composition was subjected to electroplating in order to obtain a chromium coating on a substrate (mild steel rod with 10 mm diameter). As anodes a graphite anode was used. Electrodeposition was carried out at 40 A/dm²for 45 minutes at 50° C. under mild stirring.
CCE was determined based on the Faraday law and gravimetric analysis.

TABLE 1

Cathodic current efficiency determined
on the basis of various additives

Exp.	Additive	c [g/L]	CCE [%]

C1	none	—	10.7
1	1,3-propanediol	0.2	11.2
2	1,3-propanediol	2	11.2
3	1,3-propanediol	20	13.1
4	PEG 200	0.2	13.5
5	PEG 200	2	12.8
6	PEG 200	20	13.9
7	PEG 600	0.2	12.8
8	PEG 600	2	11.6
9	PEG 600	20	12.4
10	PEG 1500	0.2	12.8
11	PEG 1500	2	13.1
12	PEG 1500	20	13.9
13	N-Dodecyl-N,N-dimethyl-3-ammonio-1-	0.001	12.8
	propanesulfonate
14	N-Dodecyl-N,N-dimethyl-3-ammonio-1-	0.01	13.1
	propanesulfonate
15	N-Dodecyl-N,N-dimethyl-3-ammonio-1-	0.1	13.1
	propanesulfonate
16	N,N-Dimethyl-N-(3-cocoamidopropyl)-N-(2-	0.01	11.7
	hydroxy-3-sulfopropyl) ammonium betaine
17	N,N-Dimethyl-N-(3-cocoamidopropyl)-N-(2-	0.05	12.5
	hydroxy-3-sulfopropyl) ammonium betaine
18	N,N-Dimethyl-N-dodecylglycine betaine	0.05	15.0

Very similar results were obtained with 1,2-propanediol, diethyleneglycol and triethyleneglycol (data not shown), confirming the results shown in Table 1.
The results in Table 1 show that the tested compound increases the CCE up to 40% compared to the CCE of the reference experiment. Furthermore, according to experiments 1 to 12 a comparatively high additive concentration is required compared to the additive utilized in experiments 13 to 18. Thus, betaines allow to significantly increase the CCE at comparatively low concentrations. It was furthermore observed that experiments 13 to 18 showed foaming. In further experiments foaming was limited by adding an anti-foam compound (a tetramethyldecanediol comprising two iso-propyl moieties (a monomeric C₁₁-C₂₅diol), solubilized in propylene glycol (a monomeric C₃diol) prior to addition (total amount of diols in the electroplating composition below 4.0 g/L).
In further comparative examples and in view of U.S. Pat. No. 3,432,408, sulfobetaines as utilized in experiments 13 and 16 were tested in a hexavalent chromium electroplating composition (55° C., 50 A/dm²). The CCE was appr. 25% without additive and appr. 25% with 0.04 g/L additive. Thus, no significant increase in the CCE was observed, confirming that the positive effect of CCE increase is limited to electroplating compositions comprising trivalent chromium ions.

Claims

1. An electroplating composition for depositing a chromium coating on a substrate, the composition comprising:

(i) trivalent chromium ions,

(ii) at least one complexing agent for the trivalent chromium ions, and

(iii) at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof.

2. The electroplating composition according to claim 1, wherein the electroplating composition comprises at least one or more than one betaine independently comprising at least 10 carbon atoms.

3. The electroplating composition according to claim 1, wherein the betaines independently comprise:

a positively charged quaternary nitrogen atom, and

a negatively charged sulfonate group or negatively charged carboxylate group,

with the proviso that the positive charge cannot be removed by deprotonation.

4. The electroplating composition according to claim 1, wherein the betaines are selected from the group consisting of N-substituted-N,N-Dialkyl-ammonium sulfobetaines.

5. The electroplating composition according to claim 1, wherein the betaines comprise one or more of N,N-Dimethyl-N-(3-cocoamidopropyl)-N-(2-hydroxy-3-sulfopropyl) ammonium betaine, N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Octyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Decyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Hexadecyl-N,N -dimethyl-3-ammonio-1-propanesulfonate, N-Octadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, and N,N-Dimethyl-N-dodecylglycine betaine.

6. The electroplating composition according to claim 1, wherein the electroplating composition comprises at least one betaine and in addition one or more than one monomeric diol.

7. The electroplating composition according to claim 1, wherein the electroplating composition comprises the betaines in a total concentration in a range from 0.0005 g/L to 1 g/L, based on the total volume of the electroplating composition.

8. The electroplating composition according to claim 1, wherein the electroplating composition has a pH in a range from 4.1 to 7.0.

9. The electroplating composition according to claim 1, wherein

the electroplating composition comprises one or more than one betaine and one or more than one monomeric diol,

with the proviso that the one or more than one monomeric diol comprises one or more than one C₁₁-C₂₅diol comprising one, two or more than two iso-propyl moieties.

10. A method for depositing a chromium coating on a substrate, the method comprising the following steps:

(a) providing the substrate,

(b) providing an electroplating composition for depositing a chromium coating on the substrate, the composition comprising:

(i) trivalent chromium ions,

(ii) at least one complexing agent for the trivalent chromium ions, and

(iii) at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof,

(c) contacting the substrate with said electroplating composition and applying an electrical current such that the chromium coating is deposited on at least one surface of said substrate.

11. The method of claim 10, wherein in step (c) the cathodic current efficiency (CCE) is 11% or more.

12. The method of claim 10, wherein in step (c) the chromium coating has a thickness in a range from 1.1 μm to 500 μm.

13. The method of claim 10, wherein in step (c) the concentration of the at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof, is continually or semi-continually monitored, wherein

the monitored concentration is compared to a target concentration of said at least one additive, and

if the monitored concentration is below the target concentration than the at least one additive is added to the electroplating composition.

14. A substrate with a surface, wherein the surface of the substrate comprises a chromium coating obtained by a method for depositing according to claim 10.

15. (canceled)

16. The electroplating composition according to claim 1, wherein the electroplating composition comprises the betaines in a total concentration in a range from 0.01 g/L to 0.2 g/L, based on the total volume of the electroplating composition.

17. The method of claim 10, wherein in step (c) the chromium coating has a thickness in a range from 10 μm to 250 μm.