EP4259853A2

EP4259853A2 - Electroplating bath for depositing a black chromium layer, method for depositing, and substrate comprising such a layer

Info

Publication number: EP4259853A2
Application number: EP21823927.5A
Authority: EP
Inventors: Berkem Özkaya; Philipp Wachter; Michael Jonat
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 2020-12-11
Filing date: 2021-12-10
Publication date: 2023-10-18
Also published as: WO2022123008A3; CN115777030A; TW202231928A; JP7538321B2; US20230193497A1; JP2023531316A; JP3242416U; WO2022123008A2

Abstract

The present invention relates to a very specific electroplating bath for depositing a black chromium layer, a respective method thereof, and a respective substrate with said black chromium layer thereon. The substrate comprising said black chromium layer is excellently suited for decorative purposes.

Description

Electroplating bath for depositing a black chromium layer, method for depositing, and substrate comprising such a layer

Field of the Invention

Background of the Invention

From the very beginning of chromium coatings, a high interest in black chromium coatings was observable due to its great appeal for optical applications.

Beginning with even black hexavalent chromium coatings, the focus today significantly shifted to trivalent chromium coatings due to a higher environmental acceptance. Since some years the demand for dark, even neutral deep dark (also named neutral black), trivalent chromium coatings, is more and more increasing, for example for decorative automotive parts. However, since such a neutral black color tone might be perceived as too cold in some cases, a slight color tone modification is often demanded, which does not compromise the deep black tone itself but is adding a tiny warmth to it to create a more warm black color tone. In principle, both the neutral black color tone as well as the warm black color tone have strong demand in industry although being very similar.

However, the degree of black varies significantly and depends on deposition parameters as well as bath ingredients.

In many cases, the black color obtained from trivalent chromium coatings was not black enough to meet either a neutral black or a warm black color tone, for example to meet requirements for decorative parts in the automotive industry. In other cases, darkness was met but the overall optical impression is not sufficient. In even other cases an insufficient color stability over time was obtained.

WO 2012/150198 A2 refers to a method and plating bath for electrodepositing a dark chromium layer.

WO 2017/053655 A1 refers to a method for adjusting the lightness L* by means of an activated carbon filter as well as a dark electroplated trivalent chromium layer on a workpiece.

CN 107099824 B refers to a black chromium electroplating solution, a composite plating layer and a preparation method thereof. The trivalent black chromium coating formed by this black chromium electroplating solution has a deep black and strong uniform coverage.

JP 2014 100809 A refers to a vehicular decorative component having black plating film and method for manufacturing the same.

US 2020/094526 A1 refers to a black plated resin part comprising a black chromium plating layer exhibiting an b* value of 3.0 or less.

According to US’526 significant progress was made regarding a neutral black color tone and a warm black color tone. However, in all these attempts further improvements are needed for an industrial utilization. For example, in many cases the desired color tone is only obtained through unacceptably long idle times to initiate a natural color ageing, which finally results in the desired color tone. In other cases, the desired color tone is obtained rather quickly but the deposition is not possible on geometrically sophisticated substrates due to haze formation, burnings and skip plating.

Thus, there is a great demand to further improve the available methods and plating baths in order to overcome said problems.

Objective of the Invention

It was therefore the objective of the present invention to provide an electroplating bath and a respective method for electroplating, which allows on the one hand a quick and stable color tone formation (for both a neutral black color tone as well as warm black color tone) and on the other hand allows deposition on a broad variety of substrate geometries without plating defects and thus, providing an excellent optical appearance including an excellent gloss. Furthermore, both color tones should be specifically and easily targeted and thus obtained.

Detailed Description of the Invention

This objective is solved by means of the present invention, an electroplating bath and a respective method for electroplating.

Therefore, the present invention refers to an electroplating bath for depositing a black chromium layer, the electroplating bath comprising:

(A) trivalent chromium ions;

(B) one or more than one complexing agent for said trivalent chromium ions;

(C) optionally, one or more than one pH buffer compound for said electroplating bath;

(D) thiocyanic acid, salts, esters, and/or isoforms thereof, in a total amount ranging from 100 mmol/L to 250 mmol/L, based on the total volume of the electroplating bath;

(E) one or more than one compound of formula (I), salts, and/or sulfoxides thereof

R¹-S-(CH₂)n-CH(NH₂)-R²

(I), wherein

- R¹ is a branched or unbranched C1 to C4 alkyl,

- R² is selected from the group consisting of COOH, salts thereof, and (CH₂)_m-OH,

- n is an integer ranging from 1 to 4, and

- m is an integer ranging from 1 to 4.

Own experiments have shown (see examples below) that above mentioned problems strongly relate to the concentration of (D) and can be solved by maintaining the concentration of (D) in the above mentioned concentration range. The desired color tones (either neutral black or warm black) with an excellent gloss are quickly formed and can be specifically targeted. Furthermore, they can be formed even on substrates with sophisticated geometries such that a very uniform deposition quality is provided without plating defects. However, it furthermore turned out that these excellent results are obtained only if the comparatively narrow concentration range as defined above is kept for (D). Very preferably, the black chromium layer is a decorative chromium layer. Typical applications are automotive parts, most preferably for the interior of a car. The electroplating bath of the present invention is very suitable in order to obtain such a black chromium layer, most preferably such a black chromium layer as defined throughout the present text.

The black chromium layer in the context of the present invention is very much preferably defined by the L*a*b* color system, preferably as introduced in 1976 by the Commission Internationale de I'Eclairage, if not stated otherwise.

Generally preferred is an electroplating bath of the present invention, wherein the black chromium layer has an L* value of 50 or below, preferably of 49 or below, more preferably of 48 or below, even more preferably 47 or below, yet even more preferably 46 or below, further more preferably 45 or below, most preferably 43 or below. An L* value of 50 or below is typically well perceived as black and dark. Generally, the lower the L* value (preferably as defined above) the stronger the impression of a black/dark color tone.

Preferred is an electroplating bath of the present invention, wherein the black chromium layer has an a* value ranging from -1.5 to +3, preferably ranging from -1 to +2.5, most preferably ranging from -0.5 to +2. Preferably, the a* value is at least positive. Most preferably this applies to the neutral black color tone and to the warm black color tone.

A distinction between a neutral black color tone and a warm black color tone is typically based on slightly different b* values.

In some cases, preferred is an electroplating bath of the present invention, wherein the black chromium layer is a neutral black chromium layer. This more preferably means that an electroplating bath of the present invention is preferred, wherein the black chromium layer has an b* value ranging from -2.5 to +2.9, preferably ranging from -2 to +2, more preferably ranging from -1.5 to +1.5, most preferably ranging from -1 to +1.

For a neutral black color tone, most preferably the L* value is 45 or below, more preferably 44 or below, even more preferably 43 or below, yet even more preferably 42 or below, most preferably 41 or below.

In other cases, preferred is an electroplating bath of the present invention, wherein the black chromium layer is a chromium layer having a warm black color tone. This more preferably means that an electroplating bath of the present invention is preferred, wherein the black chromium layer has an b* value ranging from +3 to +6, preferably ranging from +3.5 to +5.8, most preferably from +4 to +5.5.

In the context of the present invention, both the neutral black color tone as well as the warm black color tone can be specifically targeted and thus, obtained, which is of great benefit.

Compound (A) and general bath compounds:

The electroplating bath of the present invention is preferably aqueous, i.e. it 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.

Preferred is an electroplating bath of the present invention, wherein the electroplating bath is acidic, preferably having a pH ranging from 1.5 to 5.0, more preferably from 2.0 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 electroplating bath of the present invention comprises (A) trivalent chromium ions.

Preferred is an electroplating bath of the present invention, wherein 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.

Preferably, 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.

Preferred is an electroplating bath of the present invention, wherein the electroplating bath comprises sulfate ions, most preferably from a trivalent chromium salt. Preferred is an electroplating bath of the present invention, wherein the electroplating bath is substantially free of, preferably does not comprise, a compound comprising chromium with an oxidation number +6. Thus, the electroplating bath is substantially free of, preferably does not comprise, hexavalent chromium.

Preferred is an electroplating bath of the present invention, wherein the electroplating bath is substantially free of, preferably does not comprise, cobalt ions. Preferably, the black chromium layer is substantially free of, preferably does not comprise, cobalt, or only in amounts as defined in the text below. Only in very rare cases it is also preferred that the electroplating bath and the black chromium layer, respectively, comprise cobalt, although this is less preferred. However, comparatively low amounts of cobalt can be tolerated although they are not necessary for the black appearance. In such a case, preferred is an electroplating bath of the present invention, wherein cobalt ions are present in a range from 1 mg/L to 500 mg/L, based on the total volume of the electroplating bath and based on the element cobalt, preferably from 2 mg/L to 400 mg/L, more preferably from 3 mg/L to 300 mg/L, even more preferably from 4 mg/L to 200 mg/L, yet even more preferably from 5 mg/L to 100 mg/L, most preferably from 6 mg/L to 50 mg/L.

Preferred is an electroplating bath of the present invention, wherein the electroplating bath is substantially free of, preferably does not comprise, nickel ions. In some cases, a typical Ni-contamination of up to 150 ppm is observed, which is basically acceptable and therefore considered as substantially free of nickel ions. Thus, in some cases the electroplating bath of the present invention preferably comprises nickel ions in a concentration ranging from 0 ppm to 200 ppm, based on the total weight of the electroplating bath, preferably from 1 ppm to 150 ppm, most preferably from 2 ppm to 100 ppm. Preferably, the black chromium layer is substantially free of, preferably does not comprise, nickel.

It is generally preferred to avoid such environmental questionable nickel and cobalt ions. This generally leads to less complicated wastewater treatment and bath disposal. In addition, neither nickel nor cobalt is needed to obtain the neutral black color tone and the warm black color tone.

Preferred is an electroplating bath of the present invention, wherein the electroplating bath is substantially free of, preferably does not comprise, fluoride ions. Preferably, the black chromium layer is substantially free of, preferably does not comprise, fluorine. Preferred is an electroplating bath of the present invention, wherein the electroplating bath is substantially free of, preferably does not comprise, compounds containing fluorine. This most preferably comprises fluorine-containing surface-active compounds. They are in particular not desired due to increased environmental limitations.

Preferred is an electroplating bath of the present invention, wherein the 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. Preferably, the black chromium layer is substantially free of, preferably does not comprise, phosphorous. However, this does not exclude phosphorous in a subsequent layer deposited onto the black chromium layer, e.g. a passivation layer.

Preferred is an electroplating bath of the present invention further comprising halogen anions, preferably chloride anions. In the context of the present invention this is preferred and respective electroplating baths are named chloride-containing baths. More preferred is an electroplating bath of the present invention comprising chloride ions and sulfate ions.

Preferred is an electroplating bath of the present invention, wherein the chloride ions have a concentration ranging from 50 g/L to 200 g/L, based on the total volume of the electroplating bath, preferably ranging from 60 g/L to 185 g/L, more preferably ranging from 70 g/L to 170 g/L, even more preferably ranging from 80 g/L to 155 g/L, most preferably ranging from 90 g/L to 140 g/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. Typically, chloride ions are present as the anion of a conductivity salt as preferably mentioned before. A very preferred conductivity salt is ammonium chloride, sodium chloride and potassium chloride, ammonium chloride being preferred most.

Preferred is an electroplating bath of the present invention further comprising bromide anions. This typically avoids an anodic formation of undesired hexavalent chromium species. Preferably, the bromide ions have a concentration ranging from 3 g/L to 20 g/L, based on the total volume of the 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.

More preferred is an electroplating bath of the present invention comprising chloride ions, bromide ions, and sulfate ions, most preferred with concentrations as defined throughout the present text as being preferred.

In some cases an electroplating bath of the present invention is preferred further comprising Fe(ll) 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 0.6 mmol/L to 6 mmol/L, even more preferably from 0.8 mmol/L to 5 mmol/L, most preferably from 1 mmol/L to 4 mmol/L. This is in particular preferred if the electroplating bath of the present invention comprises chloride ions. Thus, most preferred is an electroplating bath of the present invention comprising chloride ions, bromide ions, sulfate ions, and Fe(ll) ions, most preferred with concentrations as defined throughout the present text as being preferred for them. Fe(ll) ions are preferably from a respective iron salt, preferably from an iron (II) sulfate salt. Typically, iron ions have several beneficial effects on the electroplating performance and on the deposited black chromium layer obtained by the present invention. In many cases, an increased electroplating rate is observed which allows a thicker layer thickness. Preferably, the black chromium layer comprises iron, more preferably as defined throughout the text.

Furthermore very preferred is an electroplating bath of the present invention, wherein the trivalent chromium ions and the Fe(ll) ions (if present) are the only transition metals in the plating bath, most preferably chromium ions and iron ions (if present) are the only transition metals in the plating bath. An exception are Ni contaminations as mentioned already above, which are generally acceptable and therefore preferably included.

In some cases, an electroplating bath of the present invention is preferred further comprising at least one sulfur-containing compound being different from (D) and (E).

In some cases, an electroplating bath of the present invention is preferred further (i.e. in addition to (D) and (E)) comprising saccharin and/or salts thereof.

In some cases, an electroplating bath of the present invention is preferred further (i.e. in addition to (D) and (E)) comprising a sulfur-containing diol, most preferably in addi- tion to above mentioned saccharin and/or salts thereof.

Preferred is an electroplating bath of the present invention further comprising at least one surface-active compound. A preferred surface-active compound comprises a cationic or an anionic surface-active compound, preferably an anionic surface-active compound. A preferred anionic surface-active compound comprises sulfosuccinates, alkyl benzene sulfonates having 8 to 20 aliphatic carbon atoms, alkyl sulfates having 8 to 20 carbon atoms, and/or alkyl ether sulfates. Preferably, the at least one surface-active compound is free of fluorine atoms.

Preferred sulfosuccinates comprise sodium diamyl sulphosuccinate.

Preferred alkyl benzene sulfonates having 8 to 20 aliphatic carbon atoms comprise sodium dodecyl benzene sulfonate.

Preferred alkyl sulfates having 8 to 20 carbon atoms comprise sodium lauryl sulfate.

Preferred alkyl ether sulfates fatty alcohols comprise sodium lauryl polyethoxy sulfates.

Preferred is an electroplating bath of the present invention, wherein the at least one surface-active compound has a total concentration ranging from 0.001 g/L to 0.05 g/L, based on the total volume of the electroplating bath, preferably from 0.005 g/L to 0.01 g/L.

In contrast, in some cases an electroplating bath of the present invention is preferred, wherein the electroplating bath is substantially free of, preferably does not comprise, chloride ions, preferably does not comprise halogen anions. In the context of the present invention this is less preferred and respective electroplating baths are named chloride-free baths. In such a case, the electroplating bath of the present invention preferably comprises sulfate ions to compensate the missing chloride ions. Even more preferred, the electroplating bath of the present invention comprises sulfate ions in addition to the sulfate ions from the chromium salt, most preferably by means of a conductive salt. A very preferred conductive salt is potassium sulfate, sodium sulfate, ammonium sulfate, or mixtures thereof. In this particular case, the electroplating bath is preferably in some cases substantially free of, preferably does not comprise, bromide ions. However, it is preferred in some rare cases that such an electroplating bath comprises iron ions, preferably Fe(ll) ions, most preferably in the concentrations as defined above.

The electroplating bath of the present invention comprises (B) one or more than one complexing agent for said trivalent chromium ions. Such compounds keep the trivalent chromium ions in solution. Preferably, the one or more than one complexing agent is not a compound of (D) and (E) and is therefore preferably different from (D) and (E).

Preferred is an electroplating bath of the present invention, wherein 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 (preferably also the organic carboxylic acid comprising one, two, or three carboxylic groups and/or salts thereof) are preferably substituted with a substituent or unsubstituted. A preferred substituent comprises an amino group and/or a hydroxyl group. Preferably, the substituent does not comprise a SH group.

More preferably, the organic carboxylic acid and/or salts thereof (preferably also the organic carboxylic acid comprising one, two, or three carboxylic groups 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. Preferably, the amino carboxylic acids (preferably alpha-amino carboxylic acids, respectively) is not a compound according to (E), more preferably is not a sulfur-containing amino carboxylic acid (preferably is not a sulfur-containing alpha-amino carboxylic acid, respectively), most preferably is not methionine. It is preferred that the one or more than one complexing agent is distinct from the compound of formula (E).

More preferred is an electroplating bath of the present invention, wherein 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. This most preferably applies, if the electroplating bath of the present invention comprises chloride ions. In contrast, if the electro- plating bath of the present invention is chloride-free, preferably the one or more than one complexing agent comprises oxalic acid, tartaric acid, malic acid, citric acid, and/or salts thereof, most preferably malic acid and/or salts thereof.

Preferred is an electroplating bath of the present invention, wherein 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 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. This most preferably applies if the electroplating bath comprises chloride ions but generally also applies to a chloride-free electroplating bath.

If the electroplating bath of the present invention is particularly chloride-free, the one or more than one complexing agent has a total concentration ranging from 5 g/L to 100 g/L, based on the total volume of the electroplating bath, preferably ranging from 5.5 g/L to 75 g/L, more preferably ranging from 6 g/L to 50 g/L, even more preferably from 6.5 g/L to 25 g/L, yet even more preferably ranging from 7 g/L to 18 g/L, most preferably ranging from 7.5 g/L to 13 g/L. This preferably applies to oxalic acid, tartaric acid, malic acid, citric acid, and salts thereof, most preferably to malic acid and salts thereof.

Preferred is an electroplating bath of the present invention, wherein (B)/(A) forms a molar ratio ranging from 1 to 1.5, preferably ranging from 1.1 to 1.4, most preferably ranging from 1.2 to 1.3.

The electroplating bath of the present invention comprises (C) optionally, one or more than one pH buffer compound for said electroplating bath. Most preferably, the electroplating bath of the present invention comprises (i.e. not optionally) one or more than one pH buffer compound. In the latter case, an electroplating bath of the present invention is preferred, wherein the one or more than one pH buffer compound for said electroplating bath is distinct (i.e. different) from (B). In such a case, the one or more than one pH buffer compound does not comprise a carboxylic acid, preferably does not comprise an organic acid. In such a case they are counted to (B).

In many cases an electroplating bath of the present invention is preferred, wherein 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.

Very preferred is an electroplating bath of the present invention, wherein 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 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. This even more preferably applies to said boron-containing compound, yet even more preferably to said boric acid together with said borate, most preferably to said boric acid. Most preferably the one or more than one pH buffer compound comprises boric acid but no borate. Thus, most preferred is an electroplating bath of the present invention, wherein (C) comprises boric acid, preferably in a total amount ranging from 35 g/L to 90 g/L, based on the total volume of the 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.

In some other cases the electroplating bath of the present invention 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.

Compound (D):

The electroplating bath of the present invention comprises (D) thiocyanic acid, salts, esters, and/or isoforms thereof, in a total amount ranging from 100 mmol/L to 250 mmol/L, based on the total volume of the electroplating bath. Our own experiments show that this compound is essential along with the aforementioned concentration range. Preferably, the concentration above is based on SCN; i.e. based on monobasic thiocyanate. The term “acid” in “thiocyanic acid” includes its deprotonated/dissociated form.

According to own experiments (see in the text below under “Examples”) if the concentration is significantly lower than 100 mmol/L no sufficiently black chromium layer is obtained. Most important, if the concentration of (D) is below 100 mmol/L, the heat- treating in the method of the present invention does not significantly affect the final color formation. Furthermore, an undesired high gloss is obtained, which starts to nega- tively impair the black optical appearance of the black chromium layer.

Furthermore, if the concentration of (D) is significantly exceeding 250 mmol/L the number of plating defects is increasing, in particular on substrates with sophisticated surface geometry. Own hull cell experiments have shown that with a too high concentration of (D) an unacceptable small range of current densities is available. However, this is not acceptable in view of technical demands. Furthermore, if the concentration of (D) is significantly exceeding 250 mmol/L a too low gloss is obtained, given an unacceptable dull perception of the black chromium layer.

In contrast, these problems are overcome with the present invention.

Preferred is an electroplating bath of the present invention, wherein (D) comprises thiocyanic acid, salts, and/or esters thereof in said total amount, most preferably thiocyanic acid and/or salts thereof in said total amount. Preferably the salt comprises potassium thiocyanate and/or sodium thiocyanate, most preferably in said total amount.

Preferred is an electroplating bath of the present invention, wherein the bath comprises (D) in a total amount ranging from 105 mmol/L to 245 mmol/L, based on the total volume of the electroplating bath, preferably from 110 mmol/L to 240 mmol/L, more preferably from 115 mmol/L to 230 mmol/L, even more preferably from 120 mmol/L to 220 mmol/L, yet even more preferably from 125 mmol/L to 210 mmol/L, most preferably from 130 mmol/L to 200 mmol/L. Also, these preferred concentration ranges above are based on SCN; i.e. are based on monobasic thiocyanate, as well as all further mentioned concentration ranges below. Furthermore, it is explicitly preferred that above concentrations can be freely combined to form not explicitly disclosed concentration ranges. This includes most preferably further combinations of lower limits with higher limits not explicitly mentioned throughout the text.

In some cases an electroplating bath of the present invention is preferred, wherein said bath comprises (D) in a total amount ranging from 130 mmol/L to 250 mmol/L, based on the total volume of the electroplating bath, preferably from 150 mmol/L to 245 mmol/L, more preferably from 175 mmol/L to 240 mmol/L, most preferably from 191 mmol/L to 235 mmol/L. These are particularly preferred concentration ranges for a neutral black color tone.

In some other cases an electroplating bath of the present invention is preferred, wherein said bath comprises (D) in a total amount ranging from 100 mmol/L to 205 mmol/L, based on the total volume of the electroplating bath, preferably from 105 mmol/L to 200 mmol/L, more preferably from 110 mmol/L to 190 mmol/L, even more preferably from 115 mmol/L to 180 mmol/L, most preferably from 120 mmol/L to 174 mmol/L. These are particularly preferred concentration ranges for a warm black color tone.

The electroplating bath of the present invention comprises (E) one or more than one compound of formula (I), salts, and/or sulfoxides thereof

R¹-S-(CH₂)n-CH(NH₂)-R²

(I), wherein

- R¹ is a branched or unbranched C1 to C4 alkyl,

- n is an integer ranging from 1 to 4, and

- m is an integer ranging from 1 to 4.

In the context of the present invention “sulfoxides” thereof denotes an oxygen chemically linked via a double bond to the sulphur atom, i.e. the compound of formula (I) above includes a R¹-S(=O)-(CH₂)_n-CH(NH₂)-R² structure.

Preferred is an electroplating bath of the present invention, wherein said bath comprises (E) in a total amount ranging from 100 mmol/L to 950 mmol/L, based on the total volume of the electroplating bath, preferably from 135 mmol/L to 800 mmol/L, more preferably from 150 mmol/L to 650 mmol/L, even more preferably from 165 mmol/L to 550 mmol/L, yet even more preferably from 180 mmol/L to 500 mmol/L, most preferably from 195 mmol/L to 450 mmol/L.

In some cases preferred is an electroplating bath of the present invention, wherein said bath comprises (E) in a total amount ranging from 100 mmol/L to 450 mmol/L, based on the total volume of the electroplating bath, preferably from 120 mmol/L to 400 mmol/L, more preferably from 140 mmol/L to 350 mmol/L, even more preferably from 150 mmol/L to 310 mmol/L, yet even more preferably from 170 mmol/L to 270 mmol/L, most preferably from 190 mmol/L to 230 mmol/L. These are particularly preferred con- centration ranges for a warm black color tone.

In other cases preferred is an electroplating bath of the present invention, wherein said bath comprises (E) in a total amount ranging from 200 mmol/L to 950 mmol/L, based on the total volume of the electroplating bath, preferably from 220 mmol/L to 750 mmol/L, more preferably from 250 mmol/L to 580 mmol/L, even more preferably from 280 mmol/L to 530 mmol/L, yet even more preferably ranging from 311 mmol/L to 490 mmol/L, most preferably from 330 mmol/L to 420 mmol/L. These are particularly preferred concentration ranges for a neutral black color tone.

Preferred is an electroplating bath of the present invention, wherein the molar ratio of (E) : (D) is 0.9 or more, preferably 1.0 or more, more preferably 1.1 or more, most preferably 1.2 or more.

More preferred is an electroplating bath of the present invention, wherein the molar ratio of (E) : (D) is ranging from 0.9 to 2.65, preferably from 1 to 2.6, more preferably from 1.05 to 2.55, even more preferably from 1.1 to 2.5, yet more preferably from 1.15 to 2.45, most preferably from 1.2 to 2.40.

In some cases preferred is an electroplating bath of the present invention, wherein the molar ratio of (E) : (D) is ranging from 0.9 to 2.5, preferably from 1 to 2, more preferably from 1.1 to 1.8, yet even more preferably from 1.15 to 1.5, most preferably from 1.2 to 1.3. These are in some cases preferred molar ranges for a warm black color tone.

In other cases preferred is an electroplating bath of the present invention, wherein the molar ratio of (E) : (D) is ranging from 1.6 to 2.65, preferably from 1.9 to 2.6, more preferably from 2.05 to 2.55, even more preferably from 2.1 to 2.5, yet even more preferably from 2.15 to 2.45, most preferably from 2.2 to 2.4. These are in some cases preferred molar ranges for a neutral black color tone.

Preferred is an electroplating bath of the present invention, wherein R¹ is methyl, ethyl, n-propyl, or /so-propyl, preferably methyl or ethyl, most preferably methyl.

Preferred is an electroplating bath of the present invention, wherein R² is COOH and/or salts thereof. As mentioned, COOH includes also the deprotonated/dissociated form thereof.

Preferred is an electroplating bath of the present invention, wherein n is 1 or 2, preferably 2. Preferred is an electroplating bath of the present invention, wherein m is 1 or 2.

Preferred is an electroplating bath of the present invention, wherein in (E) the compound of formula (I) comprises methionine. Most preferably, the compound of Formula (E) is methionine.

Method for electroplating:

The present invention furthermore relates to a method for electroplating a black chromium layer on a substrate, the method comprising the steps

(a) providing the substrate,

(b) contacting the substrate with an electroplating bath according to the present invention, preferably an electroplating bath as described as being preferred throughout the present text,

(c) applying an electrical current such that the black chromium layer is electroplated onto the substrate,

(d) heat-treating the substrate obtained from step (c) at a temperature ranging from 30°C to 100°C.

The aforementioned features regarding the electroplating bath of the present invention, including the preferred variants thereof, preferably apply likewise to the method for electroplating of the present invention, most particular to step (b) of said method. Furthermore, the aforementioned regarding the L*a*b* values most preferably applies to the black chromium layer electroplated in step (c).

In step (a) the substrate is provided.

In some cases, a method of the present invention is preferred, wherein the substrate comprises a plastic substrate, preferably is a plastic substrate. In other cases, a method of the present invention is preferred wherein the substrate comprises a metallic substrate, preferably is a metallic substrate.

In many cases a method of the present invention is preferred, wherein in step (a) the substrate comprises a thermoplastic substrate, preferably an amorphous thermoplastic substrate and/or a semi-crystalline thermoplastic.

More preferred is a method of the present invention, wherein in step (a) the substrate comprises butadiene moieties, preferably polybutadiene. Also preferred is a method of the present invention, wherein in step (a) the substrate comprises nitrile moieties.

Also preferred is a method of the present invention, wherein in step (a) the substrate comprises acryl moieties.

Very preferred is a method of the present invention, wherein in step (a) the substrate comprises polymerized styrene.

Most preferred is a method of the present invention, wherein in step (a) the substrate comprises acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene - polycarbonate (ABS-PC), polypropylene (PP), polyamide (PA), polyetherimide (PEI), a polyetherketone (PEK), or mixtures thereof, preferably acrylonitrile butadiene styrene (ABS) and/or acrylonitrile butadiene styrene - polycarbonate (ABS-PC). Such plastic substrates are typically used in decorative applications such as automotive parts, in particular ABS and ABS-PC.

Preferred is a method of the present invention, wherein the polyetherketone (PEK) comprises polyaryletherketone (PAEK), poly ether ether ketone (PEEK), poly ether ether ether ketone (PEEEK), poly ether ether ketone ketone (PEEKK), poly ether ketone ether ketone ketone (PEKEKK), poly ether ketone ketone (PEKK), and/or mixtures thereof, preferably poly ether ether ketone (PEEK), polyaryletherketone (PAEK), and/or mixtures thereof.

In some cases, 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.

Preferred is a method of the present invention further comprising prior to step (b) at least one metal plating step to deposit at least one metal layer, most preferably at least one nickel plating step to deposit at least one nickel layer. In many cases two or even three such metal plating steps are preferably involved.

Most preferably, the at least one nickel 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.

More preferred is a method of the present invention, wherein the at least one nickel 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.

Also preferred is a method of the present invention, wherein the at least one nickel 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. In the context of the present invention MPS denotes that the MPS nickel layer comprises non-conductive micro-particles, which cause micro-pores in a subsequent chromium layer, preferably in the black chromium layer. The at least one MPS nickel layer is preferably optionally.

In some cases, a method of the present invention is preferred, wherein the MPS nickel layer is adjacent to the black chromium layer.

In other cases a method of the present invention is preferred wherein the black 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.

Preferably, the black chromium layer is part of a layer stack.

In step (b) the substrate, preferably with the at least one nickel layer (preferably as defined above as being preferred) is contacted with the electroplating bath of the present invention, preferably by dipping.

Preferred is a method of the present invention, wherein the contacting during step (c) ranges from 1 minute to 30 minutes, preferably from 2 minutes to 20 minutes, more preferably from 3 minutes to 15 minutes, even more preferably from 4 minutes to 10 minutes, most preferably from 5 minutes to 8 minutes.

Preferred is a method of the present invention, wherein in step (c) the electroplating bath has a temperature in a range from 25°C to 60°C, preferably from 28°C to 50°C, more preferably from 30°C to 47°C. This most preferably applies if the electroplating bath comprises chloride ions. In some cases a method of the present invention is preferred, wherein in step (c) the electroplating bath of the present invention has a temperature in a range from 35°C to 65°C, preferably from 40°C to 63°C, more preferably from 45°C to 61 °C, most preferably from 50°C to 59°C. This most preferably applies if the electroplating bath is a chloride-free electroplating bath.

In step (c) an electrical current is applied.

Preferred is a method of the present invention, wherein the electrical current is a direct current, preferably in a range from 3 A/dm² to 30 A/dm², more preferably from 4 A/dm² to 25 A/dm², even more preferably from 5 A/dm² to 20 A/dm², most preferably from 6 A/dm² to 18 A/dm².

In some cases a method of the present invention is preferred, wherein the electrical current is a direct current, preferably in a range from 3 A/dm² to 20 A/dm², more preferably from 4 A/dm² to 15 A/dm², most preferably from 5 A/dm² to 10 A/dm². This most preferably applies if the electroplating bath is a chloride-free electroplating bath.

Preferred is a method of the present invention, wherein in step (c) at least one anode is utilized. The at least one anode is 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.

Preferred is a method of the present invention, wherein the electroplated black chromium layer has a layer thickness ranging from 0.05 pm to 1 pm, preferably from 0.1 pm to 0.8 pm, more preferably from 0.125 pm to 0.6 pm, most preferably from 0.15 pm to 0.5 pm.

Most important in the context of the method of the present invention is step (d), the heat-treating. It allows a quick and direct formation of the desired black color tone. The temperature applied in step (d) is not the temperature utilized in step (c) for the electroplating bath. Steps (c) and (d) are distinct steps.

Preferred is a method of the present invention, wherein the heat-treating is at a temperature ranging from 32°C to 99°C, more preferably ranging from 45°C to 92°C, even more preferably ranging from 52°C to 88°C, most preferably ranging from 60°C to 84°C.

More preferred is a method of the present invention, wherein in step (d) the heat- treating is carried out in water, preferably having a temperature ranging from 32°C to 99°C, more preferably ranging from 45°C to 92°C, even more preferably ranging from 52°C to 88°C, most preferably ranging from 60°C to 84°C.

As mentioned, the heat-treating is preferably in water. This preferably means that this step is carried out in a treatment compartment comprising a treatment composition. Preferably, the treatment composition is aqueous, more preferably comprising as solvent only water, most preferably essentially consisting of water. Essentially consisting of water means that besides tiny contaminations from previous method steps, the main component of the treatment composition is and remains water. Typically, said contaminations are tolerable for the purpose of this step.

Even more preferred is a method of the present invention, wherein in step (d) the heat- treating is a hot water rinse, most preferably by dipping, even most preferably by dipping into the treatment composition.

Preferred is a method of the present invention, wherein in step (d) the heat-treating is carried out without an electrical current. This means that this step is preferably electroless.

The method of the present invention does not exclude further steps, preferably such as additional rinsing, cleaning, pre-treating, and/or post-treating. Preferably, steps as defined in the examples below apply likewise to the general method described throughout the present text. A preferred post-treating step comprises a sealing step, preferably with an inorganic and/or organic sealer, and/or a contacting step with an anti-fingerprint composition.

Substrate comprising a black chromium layer:

The present invention furthermore relates to a specifically defined substrate comprising a black chromium layer, wherein the black chromium layer has, according to the L*a*b color system, an L* value of 50 or less, and a gloss value ranging from 150 to 230, referenced to a measuring angle of 60°, preferably from 160 to 220, more preferably from 165 to 210, most preferably 170 to 200. The aforementioned regarding the electroplating bath of the present invention and the method of the present invention preferably applies likewise to the specifically defined substrate of the present invention (if technical applicable). This most preferably applies to the L*a*b* values defined for the electroplating bath of the present invention and the method of the present invention. However, the following features of the substrate of the present invention preferably also apply to the method of the present invention (if not already mentioned in the text above).

Preferred is a substrate of the present invention, wherein the black chromium layer has an L* value of 49 or less, preferably 48 or less, more preferably of 47 or less, even more preferably of 46 or less, yet even more preferably of 45 or less, most preferably of 44 or less.

Preferred is a substrate of the present invention, wherein the black chromium layer has an a* value ranging from -1.5 to +3, preferably ranging from -1 to +2.5, most preferably ranging from -0.5 to +2, and/or (preferably and) an b* value ranging from -2.5 to +6, preferably ranging from -2 to +5.6, most preferably ranging from -1.5 to +5.

Preferably, the black chromium layer is substantially free of, preferably does not comprise, cobalt, or comprises cobalt, wherein in the black chromium layer more chromium is present than cobalt. The latter preferably means that the atom ratio of chromium to cobalt (i.e. Cr : Co) is more than 1 , preferably 2 or more, more preferably 3 or more, most preferably 4 or more. This is most preferably based on the total amount of chromium and cobalt atoms in the black chromium layer.

Preferably, the black chromium layer is not the only metal layer between the black chromium layer and the substrate.

Preferred is a substrate of the present invention, wherein the substrate comprises at least one nickel layer under the black chromium layer.

Preferred is a substrate of the present invention, wherein the at least one nickel layer comprises at least one bright-nickel layer or at least one satin nickel layer. This is most preferred.

Preferred is a substrate of the present invention, wherein the at least one nickel 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.

Preferred is a substrate of the present invention, wherein the at least one nickel 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. In the context of the present invention MPS denotes micro porous.

Preferred is a substrate of the present invention, wherein said at least one MPS nickel layer faces on the one side the black chromium layer and on the other side said at least one bright-nickel layer or said at least one satin nickel layer.

Preferred is a substrate of the present invention, wherein said at least one semi-bright nickel layer is under said at least one bright-nickel layer or said at least one satin nickel layer.

Preferred is a substrate of the present invention, wherein the black chromium layer is part of a layer stack, the layer stack comprising, along a direction from the black chromium layer to the substrate (adjacent or not adjacent):

(i) the black chromium layer,

(ii) optionally, at least one MPS nickel layer,

(iii) at least one bright-nickel layer and/or (preferably or) at least one satin nickel layer, and

(iv) optionally, at least one semi-bright nickel layer.

This preferably allows a very good gloss in the context of the present invention, most preferably ranging from 150 to 230, referenced to a measuring angle of 60°. This very most preferably applies if (iii) is a bright-nickel layer.

Preferred is a substrate of the present invention, wherein the black chromium layer comprises iron (i.e. is more than zero at.-%), preferably up to 15 at.-%, based on all atoms in the black chromium layer, more preferably up to 12 at.-%, even more preferred up to 10 at.-%, yet even more preferably up 8 at.-%, most preferably up to 6 at.- %.

In some cases, the layer stack preferably comprises a sealer layer and/or an antifingerprint layer, most preferably on the black 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.

Below, the present invention is illustrated by the following non-limiting examples.

Examples

In the following, Hull Cell electroplating was performed to evaluate the optical appearance of the black chromium layer depending on the current density distribution.

General procedure:

As a substrate, copper panels (99 mm x 70 mm) were used.

In a first step, the copper panels were cleaned by electrolytic degreasing with Uniclean® 279 (product of Atotech Deutschland GmbH), 100 g/L at room temperature (RT). Afterwards the substrates were rinsed with water, pickled with 10% H2SO4 by volume, and rinsed with water.

In a second step, the cleaned substrates were deposited with a bright nickel layer (10 min, 4 A/dm², UniBrite 2002, product of Atotech) such that a nickel-plated substrate was obtained and rinsed with water.

In a third step, the black chromium layer was deposited by utilizing the following electroplating bath:

(A) ca. 20 to 25 g/L Cr³⁺ ions (provided as basic chromium sulfate),

(B) ca. 30 g/L Formic acid,

(C) ca. 60 g/L Boric acid,

(D) Tbl. 1 Potassium thiocyanate, (E) Tbl. 1 Methionine, ca. 10 g/L Ammonium bromide, ca. 100 g/L Ammonium chloride, ca. 100 g/L Potassium chloride, and ca. 0.5 g/L Fe SO₄ • 7 H₂O

The electroplating baths further comprised small amounts (up to 4 g/L) of saccharin and between 5 g/L and 50 g/L of a S-containing diol. No cobalt ions and no nickel ions were present. Thus, the black chromium layer did not comprise cobalt and nickel. However, further experiments indicate that relatively small amounts of cobalt can be tolerated (not shown).

The pH value was adjusted to 3.2.

Compounds (D) and (E) were utilized in various concentrations as summarized in Table 1 below.

If not stated otherwise, each electroplating bath was tested in a Hull cell having a graphite anode and the nickel-plated substrate was installed as the cathode. An electrical current of 5 A was passed through for 3 minutes at temperatures ranging from 35°C to 45°C (see Table 1 for further details).

After plating, the substrates were rinsed with water and dried for a first color measurement (abbreviated as “CM1” in Table 1). After “CM1”, the substrate was subjected to a hot water rinse for 10 minutes at 70°C and 80°C, respectively, dried and a second color measurement was carried out (abbreviated as “CM2” in Table 1).

In a first set of experiments (abbreviated as Examples E1.1 to E1.7 in Table 1), a warm black color tone (b* ranging from ca. +3 to +6) was immediately created after the hot water rinse, wherein in a second set of experiments (abbreviated as Examples E2.1 to E2.3 in Table 1) a neutral black color tone (b* about or less than zero) was immediately created after the hot water rinse. Color measurements according to the L*a*b* color space system were carried out with a colorimeter (Konica Minolta CM-700d; measuring mode: SCI; Observer angle: 10°; Light source: D65) and at a position of ca. 3.5 cm from the left edge of the substrate and 2 cm from the lower edge (representing a typical medium current density (MCD) of approximately 10 A/dm² to 12 A/dm²). Comparative Examples are abbreviated as “CE”. Besides above mentioned color measurement, the substrates were also optically inspected depending on the locally present current densities (abbreviated as “ASD range” in Table 1). For that, the area of defect-free black chromium layer (i.e. having a homogeneous black chromium layer without haze and burnings) was determined and re-calculated as a corresponding current density range (“ASD range”). A comparatively broad range of re-calculated current densities is considered better because it shows that from low to high current densities a defect-free black chromium layer is obtained.

Gloss was determined with a reflectometer (REFO 3-D, Dr. Lange), referenced to a measuring angle of 60°.

Table 1, electroplating bath compositions and results

* denotes 35°C electroplating bath temperature

** denotes 45°C electroplating bath temperature

*** denotes rinse water temperature

Under “Rating” the over-all performance is evaluated as follows: + denotes that either the color measurement (i.e. CM1 and CM2) or the ASD range are meeting the requirements; however, this is not sufficient and therefore not desired;

++ denotes that both the color measurement (i.e. CM1 and CM2) and the ASD range are meeting the requirements; this is desired;

+++ denotes that both the color measurement (i.e. CM1 and CM2) and the ASD range are excellently meeting the requirements; this is very much desired

- CE1 shows that a certain concentration of (D) is needed in order to obtain a sufficiently black/dark color tone at all. The concentration of (D) is significantly below 100 mmol/L. It furthermore shows that a lack of (D) cannot be compensated by comparatively high concentrations of (E).

Additional Comparative Examples CE2 to CE4 were carried out based on US 2020/094526 A1 :

- CE2 corresponds to Sample No. 5 in Table 1 (which represents all samples ranging from 5 to 13 in Table 1), wherein the total amount of thiocyanic acid is again below 100 mmol/L (i.e. 100 ml/L Trichrome Graphite Makeup and 30 ml/L Trichrome Graphite Maintenance result in less than 100 mmol/L thiocyanic acid); the color measurement CM1 showed an L*; a*; b* of 54; 0.5; 3.8 immediately after plating and an ASD range from 7 to 50. Although the ASD range is relatively broad, the color is not sufficiently black/dark immediately after plating.

As shown in US’526, Table 1 , a really dark (and neutral black) color tone is obtained only in Example No. 7 with L*; a*; b* of 44; 0.8; 0.4 (measured at high current density) with an “Acceleration test”, which includes a waiting time under pre-determined conditions for 18 days (see [0064] ion US’526). Furthermore, a sufficiently warm black color tone was obtained only in Example No. 6 (allowed to stand at ambient air for 18 days) and Example No. 13 (again with “Acceleration test for 18 days). Own experiments show that a 10 minutes hot water rinse has no significant effect on CE2 to obtain a warm black color tone or a neutral black color tone. Examples 5 to 13 in US’526 therefore have at least the following disadvantages according to our own observations: First, an idle time (or also called aging time) of 18 or 19 days is considered inacceptable in view of industry requirements. Second, the same chemical composition provides different optical results depending on the idle time and conditions applied. Both is not desired. It is desired to quickly obtain a well-defined black color tone (either a neutral black color tone or a warm black color tone). As shown above, this can be achieved with the present invention.

Thus, our own experiments clearly show that a total concentration of (D), e.g. of thiocyanate, of below 100 mmol/L is too low to be significantly affected by a hot water rinse.

- CE3 corresponds to Sample No. 14 in Table 2 in LIS’526, wherein the total amount of thiocyanic acid is 15 g/L (i.e. 254 mmol/L), which is significantly higher than in Sample No. 5 of LIS’526. The color measurement CM1 showed an L*; a*; b* of 47; 1.0; 5.7 immediately after plating at ca. 10 A/dm², which is in good agreement with what is disclosed in LIS’526, Table 2, No. 14, “Initial”. However, irrespective of any hot water rinse, our experiment also showed that the ASD range is unacceptably narrow (only ca. 1 ASD) in the Hull Cell setup. In contrast, the examples of the present invention clearly show that a thiocyanic acid concentration below 250 mmol/L allows not only the desired color tone but additionally broadens the ASD range. Thus, a total amount significantly above 250 mmol/L very negatively affects the possible current density range on a substrate and therefore strongly increases the possibility of plating defects. This can be surprisingly solved by the present invention by not exceeding 250 mmol/L.

- CE4 corresponds to Sample No. 17 in Table 2 of US’526, wherein the total amount of thiocyanic acid is 40 g/L (i.e. 677 mmol/L). Compared to CE3 the total amount of thiocyanic acid was significantly further increased. Although a deposition is possible, a strong and undesired white haze is covering major parts of the hull cell substrate indicating that the range of an acceptable current density range is even smaller compared to CE3. Thus, CE4 confirms the finding of CE3 and supports the conclusion that CE4 is not suitable for electroplating sophisticated or complex substrates which require a significantly broader current density range.

The examples according to the invention show that a either a neutral black color tone or a warm black color tone can be specifically addressed and obtained within an acceptable short time. The concentration of (D) is high enough to obtain an effect from the hot water rinse but is not exceeding a critical concentration such that a relative broad current density range is still guaranteed.

In addition, for examples E1.1 to E1.7 the gloss was determined after the hot water rinse with the following results: E1.1: in range from >210 to 230; E1.2 to E1.6: in a range from 185 to 210; E1.7: in a range from 160 to <185. These results show that the gloss correlates indirectly with the concentration of (D). According to the results, a very low concentration of (D) results in a comparatively high gloss, e.g. more than 210 (see E1.1). In contrast, a concentration of (D) in a range as in examples E1.2 to E1.6 results in a very much desired gloss ranging from 185 to 210. In contrast, higher concentrations of (D), such as in E1.7, show already a still acceptable but already reduced gloss ranging from 160 to <185. This indicates that significantly higher concentrations of (D), in particular above 250 mmol/L, strongly impair the gloss of a respective black chromium layer by lowering the gloss significantly below 150, which leads to an increased (and undesired) impression of a matt/dull surface. In contrast, if the concentration of (D) is too low, i.e. (significantly) below 100 mmol/L, gloss is comparatively high (e.g. >230) but impairs the optical perception in such a way that it leads to an impression that the black color tone is not sufficiently dark or black. This perception is undesired.

Further examples according to the invention were carried out, wherein the electroplating bath was modified in such a way that no chloride ions were included (specific data not shown). In these experiments no hull cell experiments were carried out but rather electroplating trials in a beaker with the same hull cell substrates but at a specific current density of 10 A/dm². In these examples a warm black color tone with an L*; a*; b* of 45; 1.3; 3.8 was obtained.

Claims

C L A I M S An electroplating bath for depositing a black chromium layer, the electroplating bath comprising:

(A) trivalent chromium ions;

(B) one or more than one complexing agent for said trivalent chromium ions;

R¹-S-(CH₂)n-CH(NH₂)-R²

(I), wherein

- R¹ is a branched or unbranched C1 to C4 alkyl,

- n is an integer ranging from 1 to 4, and

- m is an integer ranging from 1 to 4. The electroplating bath according to claim 1 further comprising halogen anions, preferably chloride anions. The electroplating bath according to claim 1 or 2, wherein the bath comprises (D) in a total amount ranging from 105 mmol/L to 245 mmol/L, based on the total volume of the electroplating bath, preferably from 110 mmol/L to 240 mmol/L, more preferably from 115 mmol/L to 230 mmol/L, even more preferably from 120 mmol/L to 220 mmol/L, yet even more preferably from 125 mmol/L to 210 mmol/L, most preferably from 130 mmol/L to 200 mmol/L.

29

4. The electroplating bath according to any of the aforementioned claims, wherein said bath comprises (E) in a total amount ranging from 100 mmol/L to 950 mmol/L, based on the total volume of the electroplating bath, preferably from 135 mmol/L to 800 mmol/L, more preferably from 150 mmol/L to 650 mmol/L, even more preferably from 165 mmol/L to 550 mmol/L, yet even more preferably from 180 mmol/L to 500 mmol/L, most preferably from 195 mmol/L to 450 mmol/L.

5. The electroplating bath according to any of the aforementioned claims, wherein the molar ratio of (E) : (D) is 0.9 or more, preferably 1.0 or more, more preferably 1.1 or more, most preferably 1.2 or more.

6. The electroplating bath according to any of the aforementioned claims, wherein the molar ratio of (E) : (D) is ranging from 0.9 to 2.65, preferably from 1 to 2.6, more preferably from 1.05 to 2.55, even more preferably from 1.1 to 2.5, yet more preferably from 1.15 to 2.45, most preferably from 1.2 to 2.40.

7. The electroplating bath according to any of the aforementioned claims, wherein R¹ is methyl, ethyl, n-propyl, or /so-propyl, preferably methyl or ethyl, most preferably methyl.

8. The electroplating bath according to any of the aforementioned claims, wherein R² is COOH and/or salts thereof.

9. The electroplating bath according to any of the aforementioned claims, wherein n is 1 or 2.

10. The electroplating bath according to any of the aforementioned claims, wherein in (E) the compound of formula (I) comprises methionine.

11. A method for electroplating a black chromium layer on a substrate, the method comprising the steps

(a) providing the substrate,

(b) contacting the substrate with an electroplating bath according to one of claims 1 to 10,

30 (d) heat-treating the substrate obtained from step (c) at a temperature ranging from 30°C to 100°C. The method of claim 11 , wherein in step (d) the heat-treating is carried out in water, preferably having a temperature ranging from 32°C to 99°C, more preferably ranging from 45°C to 92°C, even more preferably ranging from 52°C to 88°C, most preferably ranging from 60°C to 84°C. A substrate comprising a black chromium layer, wherein the black chromium layer has, according to the L*a*b color system, an L* value of 50 or less, and a gloss value ranging from 150 to 230, referenced to a measuring angle of 60°, preferably from 160 to 220, more preferably from 165 to 210, most preferably 170 to 200. The substrate of claim 13, wherein the black chromium layer has an a* value ranging from -1.5 to +3, preferably ranging from -1 to +2.5, most preferably ranging from -0.5 to +2, and/or an b* value ranging from -2.5 to +6, preferably ranging from -2 to +5.6, most preferably ranging from -1.5 to +5. The substrate of claim 13 or 14, wherein the black chromium layer is part of a layer stack, the layer stack comprising, along a direction from the black chromium layer to the substrate:

(i) the black chromium layer,

(ii) optionally, at least one MPS nickel layer,

(iii) at least one bright-nickel layer and/or at least one satin nickel layer, and

(iv) optionally, at least one semi-bright nickel layer. The substrate of one of claims 13 to 15, wherein the black chromium layer is substantially free of, preferably does not comprise, cobalt, or comprises cobalt, wherein in the black chromium layer more chromium is present than cobalt.