EP4245893A1

EP4245893A1 - Gold electroplating solution and its use for electrodepositing gold with an aged appearance

Info

Publication number: EP4245893A1
Application number: EP22305294.5A
Authority: EP
Inventors: Jonathan SCHOENLEBER; Jean-Yves Hihn; Marie-Pierre Gigandet; Jérôme HUSSON
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite de Franche-Comte
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite de Franche-Comte
Priority date: 2022-03-15
Filing date: 2022-03-15
Publication date: 2023-09-20
Also published as: WO2023174871A1

Abstract

The invention relates to a gold electroplating solution comprising gold in the form of a complex with a compound of formula (I) or a salt thereof:

R¹S-Ar-CO₂H (I)

Wherein :
R¹ is H, a C₁-C₆ alkyl group or -S-Ar-CO₂H,
Ar is, at each occurrence, independently, selected from a C₆-C₁₀ arylene or a 5 to 10 membered heteroarylene group, notably a phenylene, a pyridinylene, a furanylene, a pyrrolylene, or a thienylene group.

Description

Technical Field

The present invention relates to a gold electroplating solution, its use for electroplating a substrate as well as a method for electroplating a substrate. The invention also relates to the use of particular specific gold complexing agents in a method of electroplating gold or gold alloys onto a substrate.

Background of the invention

Gold deposits are commonly used as coatings for functional applications (microelectronics, printed circuit boards, ...) or decorative applications as a corrosion resistant conductive material or simply for its aesthetic decorative appearance in different shades. They are mainly produced by electrodeposition from a gold electroplating bath.
Gold electroplating baths mainly contain one or more gold salts as well as complexing agents to stabilize the bath and promote deposition, and additives to improve the properties and aesthetics of the deposits.
Due to their stability, cyanide gold baths represent the vast majority of electrolytic baths on the market.
An example of a bath is shown in document CA 846796 . A cyanide gold salt is coupled to a phosphorus complexing agent (and another metal salt for alloyed deposition) in an acidic medium. This bath operates at a pH of between 3.0 and 7.5 and a temperature ranging from 20 to 80°C depending on the complexing agent used.
Another example of this type of bath is given in EP 0 150 439 . A cyanide or sulphate gold salt is complexed in an acid medium (3<pH<5) with one or more polyamines to improve the brightness of the deposits. Unfortunately, the addition of this type of additive leads to instability of the baths. Another example of a similar bath is also presented in GB 1 294 309 .
Another cyanide bath is described in GB 2 039 532 . A cyanide gold salt is coupled with a nickel salt, a carboxylic acid and thioglycolic acid to produce white gold deposits. The operating pH of the bath is between 3.5 and 4.2 at a temperature of 40-60°C.
An original bath is presented in document WO 00/39367 . A cyanide gold complex is coupled with one or more complexing agents (EDTA, PTDA and their salts), a copper salt, a ceramic of the boron carbide family and cetylpyridinium chloride.
However, due to the high toxicity of the cyanide ions that are used and of the hydrocyanic acid that is liberated under certain conditions, the use of this commonly used process is associated with considerable safety problems.
Therefore, cyanide free gold electroplating baths, mainly sulphite-based alternatives have been developed.
An example of sulphite-based bath is given in GB 1 526 215 . The gold salt used is an aurosulphite complex and in order to maintain the stability of the bath it may contain various complexing agents, additives, buffers and conducting salts. The bath operates at a pH of between 5 and 11 and a temperature of between 20 and 55°C.
A similar example of a bath is described in US 5 277 790 . The bath used is composed of an aurosulphite complex, another source of sulphite ions to be in excess, one or more polyamines and an aromatic compound functionalized by NO₂ to operate at a pH between 4.0 and 6.5.
Another example of such a bath is given in EP 1 198 623 . The gold salt is chlorauric acid HAuCl4 coupled to an amino acid: cysteine. The bath is completely free of cyanide and chloride.
Finally, a bath for obtaining low-stress gold deposits for lithography applications is described in EP 0 582 353 . The aurosulphite salt used is coupled with arsenic to reduce stress and increase the brightness of the deposit made at a pH of 8.5-9.
However, gold sulphito-complexes are generally not sufficiently stable, and when the solution stands for a long time, elementary gold is formed, even with a very large excess of free sulphite ions, with the result that the solution becomes unusable.
There is a need for alternative stable electroplating baths, advantageously cyanide-free stable electroplating baths, enabling gold or gold alloy electrodeposition.

Summary of the invention

The invention now proposes a gold and/or gold alloy electroplating solution which is stable, without requiring the use of highly toxic cyanide compounds.
Thus, it is possible, by a relatively non-toxic electrodeposition process to obtain gold or gold alloy deposits having good technological and decorative properties, and at the same time, a high degree of purity.
Characteristically, these deposits have an appearance which varies from bronze to aged bronze and are thus particularly interesting for the galvanic coating of decorative parts in sectors such as watchmaking, jewellery, leather goods and eyewear.
More specifically, it has been discovered that the use of one or more complexing agents of formula (I) both enables advantageously to obtain a stable electrodeposition bath, and also to achieve the bronze color of the gold or gold alloy coating.
Thus, in a first aspect, the invention relates to a gold electroplating solution comprising gold in the form of a complex with a compound of formula (I) or a salt thereof:

R¹S-Ar-CO₂H (I)

Wherein :

R¹ is H, a C₁-C₆ alkyl group or -S-Ar-CO₂H,
Ar is, at each occurrence, independently, selected from a C₆-C₁₀ arylene or a 5 to 10 membered heteroarylene group, notably a phenylene, a pyridinylene, a furanylene, a pyrrolylene, or a thienylene group.

In the following, further advantageous features of the gold electroplating solution of the invention are specified. Thus, in embodiments, the gold electroplating solution as defined above is a solution :

further comprising a base and/or a conductive salt;
characterised in that it does not comprise any cyanide compound;
wherein the compound of formula (I) is selected from nicotinic compounds of formula (Ia) :
wherein the compound of formula (I) is selected from 2,2'-dithiobenzoic acid, 2-mercaptobenzoic acid, 2-mercaptonicotinic acid, 2-(methylthio)nicotinic acid 4-mercaptobenzoic acid, 6,6'-dithionicotinic acid, 6-mercaptonicotinic acid, 2-(methylthio)benzoic acid, or 3-mercaptobenzoic acid, and/or mixtures thereof;
wherein the pH is in the range of from 6 to 8, notably from 7 to 7.5 ;
further comprising an additional complexing agent selected from hydantoin, hypoxanthine and derivatives thereof;
wherein the molar ratio of gold relative to the compound of formula (I) is from 0.5 to 3 ;
wherein the conductive salt is an halide salt, notably an alkali or alkaline earth metal salt, especially a sodium or potassium salt;
further comprising a soluble species of an alloyable metal for the purpose of producing an alloyed gold electrodeposit, wherein the alloyable metal is notably selected from a salt of copper, silver, iron, indium, palladium, ruthenium, or a combination of two or more thereof;
further comprising one or more additives selected from a levelling agent, a brightening agent, a surfactant and/or a mixture thereof;
wherein the concentration of gold ranges from 0.1 to 10 g/L, preferably from 1 to 5 g/L;
wherein the concentration of the compound of formula (I) may range from 0.1 to 10 g/L, preferably from 1 to 5 g/L ;
wherein the concentration of the base ranges from 0.1 and 25 g/L, preferably from 1 to 10 g/L;
wherein the concentration of the conductive salt ranges from 1 to 50 g/L, preferably from 5 to 25 g/L.

In a second aspect, the invention relates to a use of a gold electroplating solution as defined above for producing an electrodeposit of gold or gold alloy on a substrate.
In a third aspect, the invention relates to a method of electroplating gold or gold alloy on a substrate, comprising the steps of:

a) Immersing the substrate in an electroplating solution as defined above, a surface of said substrate having an electrically conductive portion in contact with said solution ;
b) Passing an electric current through said conductive portion and said solution to cause electrodeposition of gold on said electrically conductive portion of said substrate ; and
c) recovering the obtained gold or gold alloy electroplated substrate.

In a further aspect, the invention relates to the use of a compound of formula (I) as defined above, as a complexing agent in a method of electroplating gold or gold alloy onto a substrate.
In still a further aspect, the invention relates to a gold or gold alloy electroplating substrate obtainable by the method as defined above.

Detailed description

The invention is now described in more detail and in a non-limiting manner in the following description.
The following terms and expressions contained herein are defined as follows.
As used herein, the terms "electroplating solution" means a electrolytic solution which is suitable for use as an electrodeposition bath.
As used herein, the term "alkyl" means a saturated, linear or branched, hydrocarbon radical of formula -C_nH_2n+1 having 1 to 6 carbon atoms. Examples of alkyl groups include methyl, ethyl, propyl, n-butyl etc ...
As used herein, the term "aryl" refers to a mono- or bicyclic hydrocarbon aromatic ring system having 6 to 10 ring carbon atoms. Examples include phenyl and naphthyl. Preferred aryl groups include unsubstituted phenyl and naphthyl groups.
As used herein, the term "heteroaryl" refers to an aromatic group containing 5 to 10 ring carbon atoms in which one or more ring carbon atoms are replaced by at least one hetero atom such as -O-, -N-, or -S-. Examples of heteroaryl groups include pyrrolyl, furanyl, thienyl, pirazolyl, imidazolyl, thiazolyl, isothiazolyl, isoxazolyl, oxazolyl, oxathiolyl, oxadiazolyl, triazolyl, oxatriazolyl, furazanyl, tetrazolyl, pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, triazinyl, indolyl, isoindolyl, indazolyl, benzofuranyl, isobenzofuranyl, purinyl, quinazolinyl, quinolyl, isoquinolyl, benzoimidazolyl, benzothiazolyl, benzothiophenyl, thianaphthenyl, benzoxazolyl, benzisoxazolyl, cinnolinyl, phthalazinyl, naphthyridinyl, and quinoxalinyl.
As used herein, the term "heteroarylene" or "arylene" refers to a divalent heteroaryl or aryl group. Heteroarylene and arylene groups are preferably unsubstituted, which means that they are not substituted by more than two groups different from H.
As used herein, the term "phenylene" means a divalent benzene aromatic ring of formula -C₆H₄- :
As used herein, the term "pyridinylene" means a divalent pyridine aromatic ring of formula :
All other terms used in the description of the present invention have their meanings as is well known in the art.

Gold electroplating solution

A first aspect of the present disclosure relates to a gold electroplating solution comprising a gold electroplating solution comprising gold in the form of a complex with a compound of formula (I) or a salt thereof:

R¹S-Ar-CO₂H (I)
Wherein :

R¹ is H, a C₁-C₆ alkyl group or -S-Ar-CO₂H,
Ar is at each occurrence, independently, selected from a C₆-C₁₀ arylene or a 5 to 10 membered heteroarylene group, notably a phenylene, a pyridinylene, a furanylene, a pyrrolylene, or a thienylene group.

The gold implemented in the electroplating solution is preferably in the form of gold ions, notably auric (III) or aureous (I) ions, provided by the dissolution of a gold salt, such as chloroauric acid (III), sodium tetrachloroaurate (III), or ammonium aurosulphite (I).
The complex of gold with the compound of formula (I) may be generated in situ in the electroplating solution, notably in the conditions of gold electrodeposition.
The gold electroplating solution comprises gold in the form of a complex of a compound of formula (I) or a salt thereof, generally in an aqueous medium.
Advantageously, the gold electroplating solution is a cyanide-fee electroplating solution, which means that it does not comprise any cyanide compound.
As a further advantage, the electroplating solution does not comprise ammonia.
In the compounds of formula (I), Ar is preferably the same at each occurrence. Ar may be notably a phenylene or a 5 membered heteroarylene.
The compound of formula (I) may be selected from nicotinic compounds of formula (Ia) :
The compound of formula (I) may be selected from thiobenzoic acid compounds of formula (Ib) :
In the compounds of formula (I), R¹ is preferably H, methyl or -S-Ar-CO₂H.
At each occurrence, Ar may be identical or different.
Preferably Ar is identical, which means that :

for compounds of formula (Ia), if R¹ is S-Ar-CO₂H, then Ar is pyridinylene ;
for compounds of formula (Ib), if R¹ is S-Ar-CO₂H, then Ar is phenylene.

The gold electroplating solution may comprise one or more compounds of formula (I) as a gold complexing agent.
In a preferred embodiment, the compound of formula (I) is selected from 2,2'-dithiobenzoic acid, 2-mercaptobenzoic acid, 2-mercaptonicotinic acid, 2-(methylthio)nicotinic acid 4-mercaptobenzoic acid, 6,6'-dithionicotinic acid, 6-mercaptonicotinic acid, 2-(methylthio)benzoic acid, or 3-mercaptobenzoic acid, and/or mixtures thereof.
The molar ratio of gold relative to the compound of formula (I) may be from 0.5 to 3, notably from 0.5 to 2, in particular from 0.9 to 1.5.
The gold electroplating solution may further comprise a base for pH regulation. The base may be an inorganic base, notably an alkaline or alkaline earth metal hydroxide, such as sodium or potassium hydroxide, or carbonate.
The pH-value of the electroplating solution may be in the range of from 6 to 8, notably from 7 to 7.5.
The electroplating solution may further contain a buffer to regulate the pH value of the electroplating solution. The buffer may notably be selected from the usual organic and/or inorganic buffer mixtures such as disodium phosphate, alkali metal carbonate.
The gold electroplating solution may further comprise a conductive salt, in order to improve the conductivity of the electroplating solution. The conductive salt may notably be an inorganic salt, such as sodium, potassium, calcium chloride.
The electroplating solution may be used as an electrodeposition bath in a manner in itself known.
The gold electroplating solution may further comprise an additional complexing agent selected from hydantoin, hypoxanthine and derivatives thereof.
The gold electroplating solution may further comprise a soluble species of an alloyable metal for the purpose of producing an alloyed gold electrodeposit. The alloyable metal may be useful to modify functional properties of the gold coating such as hardness or colour. The alloyable metal may be selected from a salt of copper, silver, iron, indium, palladium, ruthenium, or a combination of two or more thereof.
The gold electroplating solution may further comprise one or more additives selected from a levelling agent, a brightening agent, a surfactant and/or a mixture thereof. As an example, the surfactant may be sodium laurylsulfate.
The concentration of gold in the electroplating solution may range from 0.1 to 10 g/L, preferably from 1 to 5 g/L.
The concentration of the compound(s) of formula (I) in the electroplating solution may range from 0.1 to 10 g/L, preferably from 1 to 5 g/L.
The concentration of the base in the electroplating solution may range from 0.1 to 25 g/L, preferably from 1 to 10 g/L.
The concentration of the conductive salt in the electroplating solution may range of from 1 to 50 g/L, preferably from 5 to 25 g/L.
In a preferred embodiment, the gold electroplating solution is characterised in that it comprises :

a) gold ions, at a concentration of between 0.1 and 10 g/L, preferably between 1 and 5 g/L,
b) at least one compound of formula (I) at a concentration of between 0.1 and 10 g/L, preferably between 1 and 5 g/L,
c) a base at a concentration of between 0.1 and 25 g/L, preferably between 1 and 10 g/L, and
d) a conductive salt,at a concentration of between 1 and 50 g/L, preferably between 5 and 25 g/L.

Use of a gold electroplating solution

A second aspect of the present disclosure relates to a use of a gold electroplating solution as defined above, for producing an electrodeposit of gold or gold alloy on a substrate.
The substrate notably comprises at least an electrically conductive portion. This portion may be notably the surface of an electrically conductive substrate or an electrically conductive layer deposited on the substrate, such as a metal layer coating.
The substrate may be made of metal or metal alloy.
It may also include an electrically conductive layer, so-called "undercoating", which may be a metal or metal alloy layer coating, located between a substrate's surface and the gold or gold alloy electrodeposit.
The substrate may notably be selected from stainless steel, tin, brass, nickel.
The undercoating may be selected notably from white brass, palladium, nickel, palladium-nickel alloys.

Method of electroplating gold or gold alloy on a substrate

A third aspect of the present disclosure relates to a method of electroplating gold or gold alloy on a substrate, said method comprising the steps of:

a) Immersing the substrate in an electroplating solution as defined above, a surface of said substrate having an electrically conductive portion in contact with said solution ;
b) Passing an electric current through said conductive portion and said solution to cause electrodeposition of gold or gold alloy on said electrically conductive portion of said substrate ; and
c) recovering the obtained gold or gold alloy electroplated substrate.

Conventionally, the electroplating method is carried out in a deposition tank comprising an anode and a cathode which form the substrate to be coated. The device is provided with temperature control and heating means, bath stirring and circulation means and a bath filtration system.
It is advantageous to perform step b) at a temperature in the range of 10 to 80°C, preferably 20 to 55 °C, more preferably from 40 to 60°C and/or at a cathodic current density from 0.1 to 1 amperes per dm² (A/dm²).
In step b), the electric current may be a "pulsed current", that is a current wherein the polarity of the wave varies. This so-called "pulsed current" mode avoids competition between the diffusion of the reactive species (gold ions) and the reduction of the solvent. This avoids an increase in the concentration of certain species such as hydroxyl ions and therefore an increase in the pH near the electrode. This mode also makes it possible to influence the structural properties of the gold coating (crystalline organization, grain size) and thus the functional properties (hardness, adhesion). The bath can be used with single pulse sequences (alternating t_on where current is applied and t_off where current is zero), or reverse pulses which can include a current reversal time within the sequence.
The method may further comprise a surface treatment step d) of the gold or gold alloy electrodeposit. In particular, this treatment may be useful to completely (for a bronze appearance) or partially (for aged or patinated bronze appearance) remove any thin black deposit which may be formed on the surface of the obtained gold layer.
Appropriate surface treatment step d) may include abrasion, notably a mechanical brushing or polishing of the surface.
It can be carried out by various means, such as brushes, polishing wheels with or without abrasive paste, multi-axis agitation systems with abrasive elements and cleaning solutions (Turbula type), or high-powered ultrasound systems.
The method may further comprise a step e) of applying a transparent protective layer to the gold or gold alloy electrodeposit after step c) or d), such as a varnish coating. The protective layer may be applied by conventional techniques, such as by immersion, spray, for thick coatings, or electrophoretic process, for thin coatings.

Use of a compound of formula (I) as a complexing agent

In a further aspect, the invention relates to the use of a compound of formula (I) as defined above, as a complexing agent in a method of electroplating gold or gold alloy onto a substrate.

A gold or gold alloy electroplating substrate obtainable by the method of the invention

In yet a further aspect, the invention relates to a gold or gold alloy electroplated substrate obtainable by the method as defined above.
The electroplated substate may be notably a decorative part (watch cases, bracelets, glasses, belt buckles, palettes, bag chains, etc.).
The gold or gold alloy coatings deposited from the electroplating solution of the invention may have a thickness ranging from 0,3 à 5 µm.

Complex of gold together with a compound of formula (I)

In still a further aspect, the invention relates to a complex of gold together with a compound of formula (I) as defined above or a salt thereof, said compound of formula (I) being selected from 2,2'-dithiobenzoic acid, 2-mercaptobenzoic acid, 2-(methylthio)nicotinic acid 4-mercaptobenzoic acid, 6,6'-dithionicotinic acid, 6-mercaptonicotinic acid, 2-(methylthio)benzoic acid, or 3-mercaptobenzoic acid, and/or mixtures thereof.
The following examples illustrate the invention.

Examples

Example 1

The bath composition and process parameters are given in Table 1. The tests were carried out at constant current on a stainless steel substrate. Table 1

Operating conditions Values

Gold ions 4-5 q/L

Compound of formula (I) 1,5-2 q/L

Base 5 q/L

pH 7

Temperature 50°C

Average current density 0.25-0.5 A/dm²
The deposits obtained are adherent with a high covering power, with a thin black layer on the surface. After mechanical brushing, a patinated appearance is obtained with a bronze colour (L=50, a=8, b=20 measured by Konica-Minolta spectrophotometer CM-2600d) of the coating and black residues in the less accessible areas to give an aged appearance, quite original.
After application of an electrophoretic resin, the durability and wear resistance are very good.

Example 2

The bath composition and process parameters are given in Table 2. The tests were carried out at constant current on a stainless steel substrate.

Operating conditions	Values
Gold ions	4-5 q/L
Compound of formula (I)	1,5-2 q/L
Additional complexing agent	0-0,5 g/L
Base	5 q/L
pH	7,3
Temperature	55°C
Average current density	0.25-0.5 A/dm²

The deposits obtained are adherent with a high covering power, with a thin black layer on the surface. After thorough mechanical brushing and an ultrasonic treatment, a homogeneous bronze appearance (L=54, a=9, b=30) is obtained, which is also original. After application of an electrophoretic resin, the durability and wear resistance are very good.

Claims

A gold electroplating solution comprising gold in the form of a complex with a compound of formula (I) or a salt thereof:

R¹S-Ar-CO₂H (I)

Wherein :
R¹ is H, a C₁-C₆ alkyl group or -S-Ar-CO₂H,

Ar is, at each occurrence, independently, selected from a C₆-C₁₀ arylene or a 5 to 10 membered heteroarylene group, notably a phenylene, a pyridinylene, a furanylene, a pyrrolylene, or a thienylene group.
The gold electroplating solution of claim 1, further comprising a base and/or a conductive salt.
The gold electroplating solution as claimed in claim 1 or 2, characterised in that it does not comprise any cyanide compound.
The gold electroplating solution according to any of the preceding claims, wherein the compound of formula (I) is selected from nicotinic compounds of formula (Ia)
The gold electroplating solution according to any of the preceding claims, wherein the compound of formula (I) is selected from 2,2'-dithiobenzoic acid, 2-mercaptobenzoic acid, 2-mercaptonicotinic acid, 2-(methylthio)nicotinic acid 4-mercaptobenzoic acid, 6,6'-dithionicotinic acid, 6-mercaptonicotinic acid, 2-(methylthio)benzoic acid, or 3-mercaptobenzoic acid, and/or mixtures thereof.
The gold electroplating solution according to any of the preceding claims, further comprising an additional complexing agent selected from hydantoin, hypoxanthine and derivatives thereof.
The gold electroplating solution according to any of the preceding claims, further comprising a soluble species of an alloyable metal for the purpose of producing an alloyed gold electrodeposit.
The gold electroplating solution according to any one of the preceding claims, further comprising one or more additives selected from a levelling agent, a brightening agent, a surfactant and/or a mixture thereof.
Use of a gold electroplating solution as defined in any of the preceding claims for producing an electrodeposit of gold or gold alloy on a substrate.
The use according to claim 9, wherein the substrate comprises at least an electrically conductive portion.
The use according to claim 10, wherein the substrate is selected from a metal or a metal alloy, such as stainless steel, tin, brass, nickel.
A method of electroplating gold or gold alloy on a substrate, comprising the steps of:
a) Immersing the substrate in an electroplating solution

b) according to any of claims 1 to 8, a surface of said substrate having an electrically conductive portion in contact with said solution ;

c) Passing an electric current through said conductive portion and said solution to cause electrodeposition of gold on said electrically conductive portion of said substrate ; and

d) recovering the obtained gold or gold alloy electroplated substrate.
The method according to claim 12, further comprising a surface treatment step d) of the gold alloy electrodeposit and/or a step e) of applying a transparent protective layer to the gold or gold alloy electrodeposit after step c) or d), such as a varnish coating.
Use of a compound of formula (I) as defined in any of claims 1 to 8, as a complexing agent in a method of electroplating gold or gold alloy onto a substrate.
A gold or gold alloy electroplating substrate obtainable by the method of claim 12 to 13.