Electroplating bath for electrochemical deposition of a Cu-Sn-Zn-Pd alloy, method for electrochemical deposition of said alloy, substrate comprising said alloy and uses of the substrate

The invention provides an electroplating bath for electrochemical deposition of a novel Cu-Sn-Zn-Pd alloy on a substrate. The novel alloy is characterized by exceptional corrosion resistance. The commonly used precious metal intermediate layer (e.g. a Pd-layer) between the substrate and the finishing layer is no longer necessary which allows a substantial reduction of the production costs of the plated substrates. Moreover, the novel alloy can be provided free of toxic metals (e.g. free of nickel) which makes it hypoallergenic and not prone to cause skin irritation. Finally, the plating of the inventive alloy between a substrate and its finishing layer prevents discolouration or colour fading of the finishing layer over time. All these advantages render the novel alloy particularly suitable for plating it on items of the fashion industry. Nickel has been highlighted as an allergenic metal and its use in consumer products is strongly restricted. Prior to these restrictions, a decorative galvanic layer sequence comprised a nickel layer to reach the bright aspect of the final article, but also to optimize the corrosion resistance properties and to function as a copper diffusion barrier. A high robustness is required for the final object in order to resist the aggressive media created by environmental pollution. A specific nickel-phosphorus alloy was also proposed in order to protect articles produced for the Asian market, where the atmosphere tends to contain high nitrogen and sulphur oxide concentrations.

In a large number of research projects it is presently attempted to substitute nickel by non-allergenic alternatives. As one of the results, the introduction of ternary bronze alloys (copper-tin-zinc) for replacing the nickel layer has been proposed to producers and has become the currently most common solution to the technical problem.

Unfortunately, however, bronze as a protective under-layer does not provide the provision of a corrosion resistance like the one previously achieved with nickel. These copper alloys are also less efficient as copper diffusion barriers. In order to improve the performance of the galvanic sequence of layers by substituting a nickel layer with a bronze layer, most variants require the use of a precious metal under-layer like palladium which is commonly applied between the bronze layer and the final decorative finishing layer. This additional under-layer considerably increases the production costs and can lead to a lack of adhesion of the finishing layer due to palladium passivity.

Tin and other tin alloys as barrier layers with high tin content have also been developed, but are not really efficient regarding the high brightness required by the fashion market or the high resistance necessary to pass pertinent corrosion tests.

EP 1 930 478 Bl presents a quaternary bronze alloy where the fourth metal is gallium, indium or thallium. Thallium was introduced into the decorative market as a grain refining agent to substitute lead previously present in typical cyanide bronze electrolytes. However, the use of thallium does not raise the bronzes corrosion resistance i.e. the alloy is still highly sensitive to acidity generated by nitrogen and sulphur oxides present ubiquitously in polluted atmospheres. In addition, thallium is highly toxic. Gallium and indium alloys have the disadvantage that they are poorly resistant to aggressive media such as synthetic sweat or saline humidity.

EP 2 035 602 Bl proposes the introduction of a palladium, ruthenium, rhodium or cobalt layer between the copper-tin layer and the finishing layer. These metals raise considerably the production costs of the final article.

Moreover, the passivity of these electroplated layers results in poor adhesion of the final layer and in poor performance regarding corrosion resistance.

EP 2 757 180 Al recommends the use of tin alloys with a precious metal, ruthenium in this particular case. The ruthenium content needs to be high and this does not allow reducing the production costs due to the high price of ruthenium. Moreover, the process does not yield products with the bright aspect required by the decorative and fashion industries.

CN 1 175 287 A discloses the deposition of white ornamental surfaces built on a base material covered with copper with a thickness of 1 micron as an under- layer. Said layer is followed by a layer of a Sn-Cu-Pd alloy in a thickness of 0.2 microns or higher, comprising 10-20 wt.-% Sn, 10-80 wt.-% Cu and 10-50 wt.- % Pd as the essential components. Owing to the lack of zinc in this alloy, it does not give the required performance regarding the efficiency as a copper migration barrier. This ternary copper-tin-palladium alloy is not suitable as a nickel substitute since the deposit is not bright and shows only poor corrosion resistance.

To date, the prior art does not provide a suitable unique under-layer to substitute nickel for its specific applications regarding brightness, corrosion- resistance and metal diffusion barrier.

It is therefore the objective of the present invention to substitute nickel by providing an under-layer with outstanding corrosion resistance in an economic way, wherein the under-layer is supposed to form an efficient metal migration barrier. The problem is solved by the electroplating bath according to claim 1, the method according to claim 5, the substrate according to claim 10 and the use of the substrate according to claim 15.

According to the invention, an electroplating bath for electrochemical deposition of a Cu-Sn-Zn-Pd alloy (preferably a quaternary Cu-Sn-Zn-Pd alloy) on a substrate is provided, the bath comprising or consisting of

a) water;

b) a source of copper ions;

c) a source of tin ions;

d) a source of zinc ions; and

e) a palladium salt and/or a palladium complex;

characterized in that the electroplating bath has an alkaline pH.

The inventive electroplating bath allows the provision of a substrate having an alloy layer which comprises the precious metal palladium. The novel alloy resists aggressive atmospheric and other environmental conditions and considerably increases the shelf and usage life of substrates (plated articles). Even without an intermediate precious metal under-layer (e.g. a palladium under- layer) between the substrate and the finishing layer, excellent corrosion protection is provided (pertinent standardized corrosion tests are successfully passed). Furthermore, without disadvantages related to corrosion protection, the use of the inventive alloy allows a substantial reduction of the production costs compared to the use of a pure precious metal underlayer. In addition, the final article can be provided free of toxic metals (e.g. free of nickel) which renders it hypoallergenic and not prone to cause skin irritation. Finally, the new alloy provides a smooth coating to the article and prevents diffusion of metallic components from the lower layers to the finishing layer and vice versa. Thus, a colour fading or discolouration of the final aspect is prevented.

In summary, the new bronze alloy layer has lower production costs, very high brightness, very high corrosion resistance and excellent ageing behaviour.

By adjusting the copper and/or tin content, the final colour (yellow or white bronze) may be adjusted. For efficient copper migration barrier properties, it has been discovered that a concentration range of 1 to 20 % wt.-% zinc in the final alloy is sufficient. The palladium content of > 0.25 wt.-% in the alloy was found sufficient for providing the required corrosion resistance. Production costs can be minimized by keeping the palladium concentration≤ 5 wt.-% in the final alloy while corrosion protection performance is maintained. It was found that a palladium content higher than 5 wt.-% in the alloy considerably raises the production costs without significantly improving corrosion resistance.

In the inventive electroplating bath, the concentration of

a) copper in the electroplating bath may be between 2.5 g/L and 25 g/L, preferably 3.5 g/L to 20 g/L; and/or

b) tin in the electroplating bath may be between 5 g/L to 35 g/L, preferably 9.75 g/L to 26.25 g/L; and/or

c) zinc in the electroplating bath may be between 0.25 g/L to 5 g/L; and/or d) palladium as palladium salt and/or palladium complex in the electroplating bath may be between 5 to 200 mg/L.

In a preferred embodiment of the invention,

a) the source of copper ions is selected from the group consisting of copper sulphate, copper oxide, copper hydroxide, copper chloride, copper nitrate, copper acetate, copper carbonate and copper cyanide, or a mixture thereof, preferably copper cyanide; and/or

b) the source of tin ions is a tin(ll) and/or tin(IV) compound, preferably a tin(IV) salt, more preferably potassium stannate; and/or

c) the source of zinc ions is zinc acetate, zinc chloride, zinc cyanide, zinc sulphate and/or an alkali zincate; and/or

d) the palladium salt and/or palladium complex is selected from the group consisting of palladium chloride, palladium bromide, palladium cyanide, palladium nitrite, palladium nitrate, palladium sulphate, palladium thiosul- phate, palladium acetate, palladium hydrogencarbonate, palladium hydroxide and palladium oxide, with or without ligands selected from the group of ammonia and amines, most preferably complexes selected from the group consisting of palladium diamino dichloride, palladium diamino sulphate, palladium diamino dinitrate, tetramine palladium chloride, tetramine palladium sulphate, tetramine palladium nitrate, tetramine palladium hydrogencarbonate, palladium ethylenediamine chloride, palladium ethylenediamine sulphate, palladium potassium thiosulphate, and mixtures thereof.

In a further preferred embodiment, the electroplating bath does not comprise a source of nickel ions, preferably no source of nickel and silver ions, optionally no source of nickel, silver and indium ions.

The electroplating bath may further comprise

a) a complexing agent, preferably potassium cyanide and/or sodium cyanide, preferably at a concentration of 20 to 80 g/L, more preferably, 25 to 60 g/L; and/or

b) a base, preferably potassium hydroxide and/or sodium hydroxide, preferably at a concentration of 1 to 60 g/L, more preferably 2 to 40 g/L; and/or c) a conductive salt, preferably Rochelle salt, potassium carbonate and/or sodium carbonate, preferably at a concentration of 10-100 g/L; and/or d) a surfactant, preferably an amphoteric, anionic and/or non-ionic surfactant, more preferably selected from the group consisting of betaines, sul- fobetaines, alkyl sulphates, alkyl ether sulphates, alkyl ether phosphates, alkyl sulfonates, alkyl sulfosuccinates, alkyl benzene sulfonates, alcohol po- lyglycol ethers, polyethylene glycols, and mixtures thereof, wherein the surfactant concentration is preferably 0.05 g/L to 1 g/L, more preferably 0.15 g/L to 0.5 g/L; and/or

e) an inorganic brightening agent, preferably selected from the group consisting of a salt of bismuth, antimony and/or selenium, more preferably bismuth nitrate, bismuth acetate, bismuth citrate, bismuth chloride, potassium antimony hexahydroxide, antimony chloride, antimony nitrates, sodium selenite, selenium dioxide, selenium tetrachloride, selenium sulphide and/or mixtures thereof; and/or

f) an organic brightening agent, preferably selected from the group consisting of reaction product of an amine and epihalohydrine derivatives.

In a preferred embodiment, the electroplating bath comprises a complexing agent and a surfactant, preferably a complexing agent, a surfactant and a brightening agent (inorganic and/or organic), more preferably a complexing agent, a surfactant, a brightening agent and a base. Furthermore, a method for the electrochemical deposition of a Cu-Sn-Zn-Pd alloy on a substrate is provided, the method comprising the steps

a) forming an electrical contact between a substrate and a negative electrode of a power source;

b) contacting the substrate with the inventive electroplating bath;

c) contacting at least a part of a positive electrode of the power source with the inventive electroplating bath; and

d) applying a voltage between the positive and negative electrode of the

power source until a deposit of a Cu-Sn-Zn-Pd alloy has formed on the substrate.

The method may be characterized in that a substrate is used that comprises or consists of a metal or an alloy selected from the group consisting of bronze, brass, Zamack, alpaca, copper alloy, tin alloy, steel and mixtures thereof and/or the substrate used is a metal-plated object of plastic and/or an alloy- plated object of plastic.

In a preferred embodiment, a positive electrode may be used that comprises or consists of an insoluble anode material, preferably graphite, mixed metal oxides, platinated titanium and/or stainless steel.

In a further preferred embodiment, the applied voltage is adjusted to provide a current density of 0.05 to 5 A/dm ² , preferably 0.2 to 3 A/dm ² .

The temperature of the electroplating bath may be kept at between 20 and 80 °C, preferably at between 40 to 70 °C. At temperatures below 20 °C, the coating is less bright, not homogeneous and not uniform in its colour. Above 80 °C, the electroplating results in too many break-down products which results in a quick build-up of potassium carbonate as well as a rapid ageing of the electrolyte. The optimum temperature range was discovered to be between 40 to 70°C.

Moreover, according to the invention, a substrate comprising an electro- chemically deposited Cu-Sn-Zn-Pd alloy layer is provided, the alloy layer comprising or consisting of

a) 30 to 90 % wt.-% of copper; b) 5 to 60 % wt.-% of tin;

c) l to 20 wt.-% of zinc; and

d) > 0.25 to≤ 5 wt.-% palladium. The Cu-Sn-Zn-Pd alloy layer electrochemically deposited on the inventive substrate is free of cracks, bright and provides the substrate with excellent corrosion resistance. Moreover, the inventive substrate is characterized by an excellent ageing behaviour i.e. it does not show discolouration or colour fading over time.

In a preferred embodiment of the invention, the alloy comprises

a) 40 to 85 % wt.-% of copper, optionally 45 to 80 wt.-%; and/or

b) 10 to 50 % wt.-% of tin, optionally 15 to 45 wt.-%; and/or

c) 2 to 15 wt.-% of zinc, optionally 3 to 10 wt.-%; no silver; and/or d) no indium; and/or

e) no nickel; and/or

f) no mercury.

A concentration of zinc between 2 and 15% wt.-%.in the alloy was discovered to give the most effective copper diffusion barrier.

In a preferred embodiment, the alloy layer is free of nickel, preferably free of nickel and silver, optionally free of nickel, silver and indium or free of nickel, silver, indium and mercury.

The thickness of the electrochemically deposited Cu-Sn-Zn-Pd alloy layer may be 1 nm to 25 μιη, preferably 10 nm to 20 μιη, more preferably 0.1 μιη to 15 μιη, even more preferably 1 μιη to 10 μιη, most preferably 2 μιη to 5 μιη. The inventive substrate may be characterized in that it comprises additionally a) an electrochemically deposited layer comprising or consisting of acidic copper, wherein said layer is preferably located between the substrate and the electrochemically deposited Cu-Sn-Zn-Pd alloy layer; and/or b) an electrochemically deposited finishing layer comprising or consisting of a noble metal, wherein the electrochemically deposited Cu-Sn-Zn-Pd alloy layer is preferably located between the substrate and the electrochemically deposited finishing layer.

Optionally, the electrochemically deposited layer comprising or consisting of acidic copper has a thickness of 1 nm to 1 mm, preferably 10 nm to 500 μιη, more preferably 0.1 μιη to 100 μιη, even more preferably 1 μιη to 50 μιη, most preferably 5 μιη to 20 μιη or even 10 μιη to 15 μιη.

The electrochemically deposited finishing layer may optionally have a thickness of 0.01 μιη to 20 μιη, preferably 0.02 to 10 μιη, more preferably 0.05 to 5 μιη, most preferably 0.1 μιη to 3.0 μιη or even 0.2 μιη to 0.5 μιη.

In a preferred embodiment, the substrate is producible with the inventive method.

Finally, the use of the inventive substrate as fashion item is suggested, preferably as an article selected from the group consisting of jewellery, fashion, leather article, watch, eyewear, trinket, lock and/or perfume packaging application. In fact, the inventive substrate fulfils all requirements of the fashion industry (especially the one for jewellery and leather goods articles), namely:

- no alterations by long synthetic sweat contact (standard NFS-80772 - from 12 to 24 hours);

- resistance to interactions with leather under severe conditions for 96 hours (standard ISO-4611 in humid heat atmosphere);

- resistance to aqueous nitric acid simulating the atmosphere polluted with nitrogen oxides; and

- stability to exposure to an atmosphere containing both nitrogen and sulphur oxides simulating common atmospheric pollution (not standardized).

With reference to the following examples, the subject according to the present invention is intended to be explained in more detail without wishing to restrict said subject to the special embodiments shown here.

In all examples, the electroplating method for depositing an alloy on a substrate (brass or Zamack) comprised the following plating sequence:

- copper layer on substrate: 10 - 15 microns layer thickness - bronze layer on copper layer: > 2 microns layer thickness

- gold finishing layer on bronze layer: 0.2 - 0.5 microns layer thickness

Example 1: Electrodeposition of a quaternary white bronze Cu-Sn-Zn-Pd deposit

The deposit was obtained using the following electrolyte solution

- copper as CuCN: 6 g/L

- tin as K ₂ Sn0 ₃ : 30 g/L

- zinc as Zn(CN) ₂ : l g/L

- palladium as Pd(NH ₃ ) ₄ S0 ₄ : 50 mg/L

- free potassium cyanide: 50 g/L

- free potassium hydroxide: 25 g/L

- surfactant solution: 3 mL/L

- brightening agent solution: 3 mL/L

The electrodeposition was performed at 60 °C since this temperature turned out to be the best compromise for spreading the (white) brightness range to its maximum and obtaining a homogeneous alloy throughout the current density range. The copper plated substrate is introduced into the electrolyte after proper cleaning and activation, with a current density of 1 A/dm ² applied for 20 minutes in order to raise the Cu-Sn-Zn-Pd bronze layer thickness to 5 microns.

The final aspect of the ternary Cu-Sn-Zn-Pd bronze layer is bright and presents a white colour.

Example 2: Electrodeposition of a quaternary yellow bronze Cu-Sn-Zn-Pd deposit

The deposit was obtained using the following electrolyte solution:

- copper as CuCN: 15 g/L

- tin as K ₂ Sn0 ₃ : 12 g/L

- palladium as Pd(NH ₃ ) ₄ CI ₂ : 30 mg/L

- zinc as Zn(CN) ₂ : l g/L - free potassium cyanide: 35 g/L

- free potassium hydroxide: 15 g/L

- surfactant solution: 3 mL/L

- brightening agent solution: 5 mL/L

The electrodeposition was performed at 50°C since this temperature turned out to be the best compromise for spreading the (yellow) brightness range at its maximum and obtain a homogeneous alloy through the current density range. The copper plated substrate is introduced into the electrolyte after proper cleaning and activation with a current density at 1.5 A/dm ² applied for 15 minutes in order to raise the Cu-Sn-Zn-Pd bronze layer thickness to 5 microns.

The final aspect of the quaternary Cu-Sn-Zn-Pd bronze layer is bright and presents a pale yellow colour.

Example 3: Electrodeposition of quaternary white bronze Cu-Sn-Zn-Pd deposit with claimed Zinc content (1 - 20 wt.-%)

The deposit was obtained using the following electrolyte solution:

copper as CuCN : 6 g/L

tin as K ₂ Sn0 ₃ : 30 g/L

zinc as ZnCN ₂ : 2 g/L

palladium as Pd(NH ₃ ) ₄ S0 ₄ : 50 mg/L

free Potassium Cyanide: 50 g/L

free Potassium Hydroxide: 25 g/L

surfactant solution: 3 mL/L

brightening agent solution: 3 mL/L The quaternary bronze alloy electrodeposition is performed at 60°C, since this temperature turned out to be the best compromise to obtain a bright and homogeneous alloy on the whole range of current density. The copper plated substrate is introduced into the electrolyte after proper cleaning and activation, current density was fixed at 1 A/dm ² and applied for 15 minutes in order to obtain a bronze layer thickness of 3 microns. The final aspect of the quaternary Cu-Sn-Zn-Pd bronze layer is bright and presents a white colour.

Reference example 1: Electrodeposition of a ternary white bronze Cu-Sn-Zn deposit

A deposit was obtained using the following electrolyte solution:

- copper as CuCN: 6 g/L

- tin as K ₂ Sn0 ₃ : 30 g/L

- zinc as Zn(CN) ₂ : l g/L

- free potassium cyanide: 50 g/L

- free potassium hydroxide: 25 g/L

- surfactant solution: 3 mL/L

- brightening agent solution: 3 mL/L

The electrodeposition is performed using the same conditions as in Example 1.

The final aspect of the ternary Cu-Sn-Zn bronze layer is bright and presents a white colour.

Reference example 2: Electrodeposition of a ternary white bronze Cu-Sn-Pd deposit

The deposit was obtained using the following electrolyte solution:

- copper as CuCN: 6 g/L

- tin as K ₂ Sn0 ₃ : 30 g/L

- palladium as Pd(NH ₃ ) ₄ S0 ₄ : 50 mg/L

- free potassium cyanide: 50 g/L

- free potassium hydroxide: 25 g/L

- surfactant solution: 3 mL/L

- brightening agent solution: 3 mL/L

The electrodeposition is performed using same conditions as in Example 1. The final aspect of the ternary Cu-Sn-Pd bronze layer is hazy and presents a grey colour. The aspect of the deposit is not homogeneous.

Reference Example 3: Electrodeposition of a ternary yellow bronze Cu-Sn-Zn deposit

The deposit was obtained using the following electrolyte solution:

- copper as CuCN: 15 g/L

- tin as K ₂ Sn0 ₃ : 12 g/L

- zinc as Zn(CN) ₂ : l g/L

- free potassium cyanide: 35 g/L

- free potassium hydroxide: 15 g/L

- surfactant solution: 3 mL/L

- brightening agent solution 2: 5 mL/L

The electrodeposition is performed using the same conditions as in Example 2.

The final aspect of the ternary Cu-Sn-Zn bronze layer is bright and presents a pale yellow colour.

Reference Example 4: Electrodeposition of a ternary yellow bronze Cu-Sn-Pd deposit

The deposit was obtained using the following electrolyte solution:

copper as CuCN: 15 g/L

tin as K ₂ Sn0 ₃ : 12 g/L

palladium as Pd(NH ₃ ) ₄ CI ₂ : 30 mg/L

free potassium cyanide: 35 g/L

free potassium hydroxide: 15 g/L

surfactant solution: 3 mL/L

brightening agent solution: 5 mL/L

The electrodeposition is performed using the same conditions as in Examph 2.

The final aspect of the ternary Cu-Sn-Pd bronze layer is bright and presents yellow colour.

Reference Example 5: Electrodeposition of a nickel and nickel phosphorus layer sequence

This nickel layer sequence is used as a reference to highlight the comparable behaviour of the new quaternary Cu-Sn-Zn-Pd alloy regarding corrosion resistance of final articles. - substrate: copper alloys (brass or Zamack)

- copper: 15 microns

- bright nickel: 10 microns

- nickel phosphorus: 3 microns

- finishing: gold

Reference Example 6: Electrodeposition of a white ternary Cu-Sn-Zn alloy and a precious metal underlayer sequence

The layer of ternary bronze and palladium as underlayer sequence is used as a reference to highlight the advantages of the nickel-free quaternary Cu-Sn-Zn-

Pd alloy regarding corrosion resistance and the savings in production costs in comparison of the actual hypoallergenic solution.

- substrate: copper alloys (brass or Zamack)

- copper: 15 microns

- ternary Cu-Sn-Zn alloy: 5 microns

- palladium alloy: 0.3 microns

- gold finishing: 0.5 microns Reference Example 7: Electrodeposition of quaternary white bronze Cu-Sn-

Zn-Pd deposit with low Zinc content (0.5 wt.-%)

The deposit was obtained using the following electrolyte solution:

- copper as CuCN : 8 g/L

- tin as K ₂ Sn0 ₃ : 25 g/L

- zinc as ZnCN ₂ : 0.5 g/L palladium as Pd(NH ₃ ) ₄ S0 ₄ :

free Potassium Cyanide:

free Potassium Hydroxide:

surfactant solution:

brightening agent solution

The quaternary bronze alloy electrodeposition is performed at 60°C, since this temperature turned out to be the best compromise to obtain a bright and homogeneous alloy on the whole range of current density. The copper plated substrate is introduced into the electrolyte after proper cleaning and activation, current density was fixed at 1 A/dm ² and applied for 15 minutes in order to obtain a bronze layer thickness of 3 microns.

The final aspect of the quaternary Cu-Sn-Zn-Pd bronze layer is slightly hazy and presents a white colour.

Reference Example 8: Electrodeposition of quaternary white bronze Cu-Sn- Pd deposit with high Zinc content (> 25 wt.-%)

The deposit was obtained using the following electrolyte solution:

copper as CuCN : g/L

tin as K ₂ Sn0 ₃ : 16 g/L

zinc as ZnCN ₂ : 6 g/L

palladium as Pd(NH ₃ ) ₄ S0 ₄ : 100 mg/L

free Potassium Cyanide: 45 g/L

free Potassium Hydroxide: 6 g/L

surfactant solution: 3 mL/L

brightening agent solution: 3 mL/L The quaternary bronze alloy electrodeposition is performed at 60°C, since this temperature turned out to be the best compromise to obtain a bright and homogeneous alloy on the whole range of current density. The copper plated substrate is introduced into the electrolyte after proper cleaning and activation, current density was fixed at 1 A/dm ² and applied for 15 minutes in order to obtain a bronze layer thickness of 3 microns. The final aspect of the quaternary Cu-Sn-Zn-Pd bronze layer is slightly hazy and presents white colour.

Example 3 - Evaluation of the electroplated products Table 1: EDS analysis for determination of alloy composition

The electroplated products obtained in Examples 1 to 3 and Reference Examples 1 to 4, 7 and 8 were subjected to EDS analysis to obtain the alloy compositions. The result expressed as weight percentage is shown in Table 1.

Table 1 Table 2: Performance tests of electroplated layers

The electroplated products obtained in Examples 1 to 3 and Reference Examples 1 to 8 were subjected to corrosion resistance tests. Salt spray tests were performed according to the ISO 9227 standard. Synthetic sweat resistance tests were conducted following NFS 80722 requirements, and leather interaction resistance was evaluated in accordance with ISO 4611 testing conditions. The resistance to a S0 ₂ /NO _x atmosphere was tested in a close container with high S0 ₂ and NO _x gas concentrations. The results are shown in Table 2.

Table 2 (n/a: data not available)

A comparison of the results of the corrosion tests of the selected embodiments of the invention and of the Reference Examples demonstrates the improvements reached with the quaternary Cu-Sn-Zn-Pd alloy.

Regarding Example 3 and Reference Examples 7 and 8, an additional synthetic sweat resistance tests has been performed, namely an ageing with TURBULA for 3 minutes has been performed. After said ageing, Example 3 still shows no sign of oxidation or change of colour. On the contrary, in Reference Example 7, green salt and exfoliations from the copper layer were observed and in Reference Example 8, white rust and exfoliations from the copper layer were observed. The comparison of the data obtained with Example 3 with the data obtained with Reference Examples 7 and 8 demonstrates that the zinc content in the alloy is important to ensure a sufficient corrosion protection. Indeed, it has become evident that if the final alloy comprises zinc below or above the range of 1 to 20 wt.-%, the corrosion resistance is lost. Table 3: Evaluation of the copper migration barrier properties

The electroplated products obtained in Examples 1 to 3 and Reference Examples 1 to 8 were subjected to copper diffusion tests. The copper migration barrier properties are evaluated by heating the final articles for 48 h at 180°C. Under these conditions, the precious metal layer aspect must not be altered. The results are shown in Table 3.

Table 3 A comparison of the results of the copper migration tests for the selected embodiments of the invention and for the Reference Examples demonstrates the improvements reached with the quaternary Cu-Sn-Zn-Pd alloy.

The copper diffusion tests results regarding Example 3 and Reference Examples 7 and 8 highlight the importance of zinc content in the electroplated alloy to ensure a copper diffusion barrier. Indeed, it has become clear that if the final alloy comprises zinc below or above the range of 1 to 20 wt.-%, the copper migration barrier property is lost.

Table 4: Nitric acid resistance tests

The nitric acid resistance tests are conducted by dipping the plated article into a 65 % aqueous solution of HN0 ₃ . The results are shown in Table 4.

Table 4

A comparison of the results of the nitric acid resistance tests demonstrates the improvements reached with the quaternary Cu-Sn-Zn-Pd alloy.