EP2196563A1 - Process for inhibiting tarnishing of silver coatings - Google Patents

Abstract

The method comprises applying a metal layer on a silver layer with a layer thickness of 5-100 nm, where the metal layer contacts the silver layer using an electrolyte containing metal ions, and applying a layer made of self-organized molecules on the metal layer. The silver layer is contacted with acidic electrolyte. A current is applied for applying the metal layer between the silver layer and a counter electrode in short-time. The electrolyte is contacted with a treatment solution, which has a reduction agent or metal reducible by the colloid metal. The method comprises applying a metal layer on a silver layer with a layer thickness of 5-100 nm, where the metal layer contacts the silver layer using an electrolyte containing metal ions, and applying a layer made of self-organized molecules on the metal layer. The silver layer is contacted with acidic electrolyte. A current is applied for applying the metal layer between the silver layer and a counter electrode in short-time. The electrolyte is contacted with a treatment solution, which has a reduction agent or metal reducible by the colloid metal. The electrolyte is present in a concentration of 1-10 g/l. A current density of 0.5-3 A/dm 2> is adjusted between the silver layer and the counter electrode. The current is applied for a time period of 1-10 seconds, where the time period is fixed with the stipulation in dependent of the current density and the concentration of the metal in the electrolyte, so that a layer thickness of the metal to be deposited is reached to 5-100 nm. An independent claim is included for a silver coating.

Description

The present invention relates to a method for preventing the tarnishing of silver layers and silver layers with significantly improved tarnish protection.

Objects made of silver, or objects silvered with corresponding silver layers tend to darken on contact with air. Depending on the duration of exposure, yellowish-brown to black deposits develop on the silver surfaces. The main cause of this problem is the formation of sulfides and oxides of silver by reaction with airborne organic sulfur compounds, or sulfur compounds derived from organic sources, such as food and beverages or body fluids and body fats.

The sulfur compounds introduced into the environment through various causes, such as those caused by the combustion of fossil fuels, can lead to tarnishing of silver layers.

Especially with silver jewelery or silver cutlery, such tarnishing occurs through intensive contact with food and drink or even body fat. In the case of silver flatware, tarnishing when cleaning the cutlery in dishwashers with the addition of commercial detergents is enhanced, so that a post-treatment of the silver after polishing by polishing is often necessary.

In the field of electrical engineering many silvered contact surfaces are used for electrical connectors. However, these are also subject to environmental influences a corresponding start, which significantly reduces the conductivity and thus can lead to contact difficulties.

While it is known in the field of silver jewelry and silverware to remove the formed sulfide and / or oxide layers by polishing with sodium thiosulfate, thiourea or ammonia-containing agents, this is often not possible at the contact surfaces of electrical connectors.

Particularly in the automotive industry, however, due to the ever-increasing automation of motor vehicle technology, ever lower currents are used in the electronic systems of motor vehicles, so that safe and highly conductive contacts are required here. For this purpose, silver contacts or silvered contact surfaces offer, but due to the exposure to organic sulfur compounds, especially in the engine compartment tend to tarnish and form correspondingly low-conductivity sulfide layers on the surface.

With this in mind, it is the object of the present invention to provide a method with which it is possible to protect silver surfaces against tarnishing in the long term or at least to significantly reduce the tendency to tarnish without sustainably influencing the appearance of the silver surface or its electrical properties. Moreover, it is the object of the present invention to provide a corresponding silver layer, which has a low tendency to tarnish.

This object is achieved with respect to the method by a method for preventing the tarnishing of silver layers, which is characterized in that on the silver layer, a layer of a metal from the group consisting of palladium, platinum, osmium and rhenium with a layer thickness of 5 nm to 100 nm is applied.

In one embodiment of the method according to the invention, the layer of a metal of the group consisting of palladium, platinum, osmium and rhenium is applied to the silver layer by contacting the silver layer with an electrolyte containing ions of a corresponding metal.

Frequently, silver layers are deposited on substrate surfaces of cyanide-containing silver electrolytes. In an advantageous embodiment of the method, the electrolyte, with which the silver layer for depositing a metal of the group consisting of palladium, platinum, osmium and rhenium is brought into contact, an acidic electrolyte, since acidic electrolytes against the eventual entry of cyanide by the upstream Silvering are insensitive.

The deposition of a corresponding palladium, platinum, osmium or rhenium layer or a layer containing at least one of these metals can be carried out both galvanically and de-energized.

In the case of the electrodeposition of such a layer, the current between the silver layer and a counterelectrode is at least briefly applied. In such a case, according to the invention, the concentration of the metal of the group consisting of palladium, platinum, osmium and rhenium in the electrolyte may be between 0.5 g / l and 20 g / l, preferably between 1 g / l and 10 g / l and still more preferably between 1 g / l and 5 g / l. The electrolyte which can be used according to the invention for the electrodeposition of such a layer furthermore has a conductive salt in a concentration of between 10 g / l and 200 g / l and, for improving the substrate surface, a wetting agent in a concentration of between 1 g / l and 10 g / l , Suitable conductive salts are, for example, alkanesulphonates, ammonium chloride, ammonium sulphate, ammonium sulphamate, as well as sulphates, citrates or phosphates of sodium or potassium. A suitable wetting agent is, for example, the wetting agent marketed under the name TamoINM and based on naphthalene sulphonic acid condensation products.

Between the silver layer and a suitable counterelectrode, such as an electrode of platinized titanium, a current density between 0.1 A / dm ² and 4 A / dm ² , preferably between 0.5 A / dm ² and 3 A / dm ^{2 is} set.

Depending on the concentration of the metal to be deposited on the silver layer in the electrolyte and the current density set, the contact time, ie the time for which a corresponding current density is set between the silver layer and the counterelectrode, varies between 1 second and 60 seconds, preferably between 1 second and 10 seconds.

In the case of the electrodeposition of a layer of a metal of the group consisting of palladium, platinum, osmium and rhenium with a layer thickness of 5 nm to 100 nm, the silver layer is contacted with the corresponding electrolyte at a temperature between 20 ° C and 80 ° C.

The preferred temperature for contacting is in a range between 40 ° C and 60 ° C.

In the case of the electroless deposition of a thin anti-tarnish layer according to the invention, the substrate surface having a silver layer is contacted with an electrolyte comprising a metal from the group consisting of palladium, platinum, osmium and rhenium in a concentration between 0.5 g / l and 5 g / l has. Preferably, the metal to be deposited in the form of a thin layer of the group consisting of palladium, platinum, osmium and rhenium in the form of a salt, such as sulphate, chloride or sulphonate, is added to the electrolyte. Furthermore, such an electrolyte for the inventive autocatalytic deposition of a thin anti-tarnish layer proves a conductive salt in a concentration between 5 g / l and 50 g / l. Suitable conductive salts are, for example, sodium borate, ammonium chloride, ammonium sulfate, ammonium sulfamate, as well as sulfates, citrates or phosphates of sodium or potassium. For autocatalytic deposition of the aforementioned metals, such an electrolyte has a reducing agent in a concentration between 5 g / l and 20 g / l. Among others, particularly suitable reducing agents are hydrogen peroxide, hypophosphite, hydrazine, formaldehyde or borohydrocarbon compounds.

An electrolyte as described above for autocatalytic deposition according to the invention may have a pH in the range between pH 5 and pH 9, with a range between pH 6 and pH 8 being preferred. The temperature of the electrolyte can be varied in the autocatalytic deposition in a range between 50 ° C and 80 ° C, with a range of 60 ° C to 70 ° C is preferred.

To improve the wetting of the substrate, the electrolyte may have a wetting agent in a concentration between 0.1 ml / l and 2 ml / l. Suitable wetting agents in this case are, for example, sodium lauryl ether.

In a further embodiment of the method according to the invention, the silver layer is brought into contact with an electrolyte which is a metal of the group consisting of palladium, platinum, osmium and rhenium in the form of a metal / metal colloid with the metal of said group as the core metal and a Has core surrounding colloidal metal. In this case, suitable colloidal metals according to the invention are, for example, iron, tin, lead, cobalt or germanium. Such metal / metal colloid systems are for example from EP 0 538 006 A1 known and are used for the metallization of plastic surfaces.

Following the treatment of the silver layer with the aforementioned electrolytes, it is contacted with a treating solution comprising a reducing agent or a metal reducible by the colloidal metal.

According to the invention, for example, hydrogen peroxide, hypophosphites, hydrazine, formaldehyde or hydrobromic compounds can be used here as reducing agents.

Suitable metals which are reducible by the colloidal metal are, for example, silver, gold, nickel, palladium, platinum, bismuth or mixtures of these. These metals, which can be reduced by the colloidal metal, are ionogenic in the treatment solution to be used according to the invention. By contacting the silver layer previously treated with a metal / metal colloid-containing solution with a treatment solution comprising a reducing agent or a metal reducible by the colloidal metal, the metallo-metal colloid core metal is exposed and deposited on the silver layer.

In the embodiment of the method according to the invention, in which the treatment solution has a metal which can be reduced by the colloidal metal, the metal reducible by the colloid metal can advantageously also be deposited on the silver layer.

In such a case, the treatment solution to be used according to the invention also has suitable complexing agents to prevent precipitation of the released colloidal metals.

In a particular embodiment of the method according to the invention, the metal / metal colloid-containing electrolyte has a metal / metal colloid with palladium as the core metal and tin as the colloidal metal, and the treatment solution has palladium in ionic form as metal reducible by the colloidal metal.

Regardless of how the layer of a metal of the group consisting of palladium, platinum, osmium and rhenium has been applied to the silver layer, it can be provided according to the invention to apply to such a metal layer a layer of self-assembling molecules (so-called SAMs). Particularly suitable self-assembling molecules (SAMs) are, for example, hexadecanethiol, octadecanethiol and / or alkanephosphoric acids and derivatives thereof.

These self-organizing molecules then form on the silver layer a further layer which prevents or at least reduces the tarnishing of the silver layer.

In a further embodiment of the method according to the invention, another metal, such as copper and / or nickel, can be deposited with the metal of the group consisting of palladium, platinum, osmium and rhenium. As a result, the mechanical or electrical properties of the deposited as tarnish protection metal layer can be influenced in an advantageous manner. For example, the co-deposition of copper leads to higher ductility of the coatings and to a cost reduction by mitabeschiedenen copper content. The deposition of nickel also leads to a higher ductility of the layer, but at the same time reduces pore formation and improves corrosion resistance.

The co-deposited metal may be contained by the process of the invention in electrolytes in a concentration between 0.1 g / l and 5 g / l, preferably between 0.5 g / l and 3 g / l.

In a further embodiment of the method according to the invention, at the beginning of the deposition of a metal of the group consisting of palladium, platinum, osmium and rhenium, a current pulse is applied between the silver layer and a counterelectrode, which serves to initialize a subsequent electroless deposition of said metal on the silver layer.

With regard to the silver coating, the object according to the invention is achieved by a silver coating, which is characterized in that the silver coating also has a layer comprising a metal of the group consisting of palladium, platinum, osmium and rhenium with a layer thickness of 5 nm to 100 nm is.

In one embodiment of the silver coating according to the invention, in addition to the metals of the group consisting of palladium, platinum, osmium and rhenium, a metal of the group consisting of copper, nickel, gold, platinum, bismuth or mixtures thereof can be deposited.

In one embodiment of the invention, a layer of a self-organizing molecule, such as, for example, hexadecanethiol and / or octadecanethiol, can be applied to the silver coating according to the invention.

Fig. 1 FIG. 12 shows the relationship between the selected concentration of the metal of the group consisting of palladium, platinum, osmium and rhenium in an electrolyte to be used in the present invention and the current density to be set and the contact time to obtain a silver layer tarnish prevention layer according to the present invention. As can be seen, at a deposition metal concentration of 8 g / l in the electrolyte and a set mean current density of 1.5 A / dm ^2, a layer thickness of 5 nm to 100 nm according to the invention can be achieved within a contact time of 1 second. The lower the set average current density, the longer the contact time must be. With a correspondingly reduced concentration of the deposition metal in the electrolyte, such as 1 g / l deposition metal, at a relatively high average current density of 2.5 A / dm ^2, a contact time of at least 5 seconds to achieve a layer thickness of 5 nm to 100 nm according to the invention provided.

The present invention is illustrated by way of example with reference to exemplary embodiments, but without the invention being limited to these exemplary embodiments.

example 1

A previously silver-coated substrate is contacted with an electrolyte containing 1 g / L of palladium sulfate, 100 g / L of an alkanesulfonate, and 3 g / L of a surfactant type TamolNM at a temperature of 40 ° C. A current density of 0.5 A / dm ^{2 is set} for a time of 5 seconds. The substrate surface obtained under these conditions is mirror-polished and shows no tendency to run even after a long time.

Example 2

On a substrate previously coated with a silver layer, a tarnish protection according to the invention is applied autocatalytically. For this purpose, the substrate with an electrolyte, which 1 g / l of palladium sulfate, 8 g / l sodium borate, 10 g / l of sodium hypophosphite and 0.5 ml / l of a wetting agent (sodium lauryl ether) at a temperature of 65 ° C and a set pH of 7.5 for 3 minutes in contact.

Under these conditions, a mirror-like substrate surface is obtained, which shows no tendency to tarnish even after a long time.

In a further embodiment of the invention, a previously coated with a silver layer substrate with an as in this example previously described electrolyte was brought into contact, wherein at the beginning of the deposition, a short current pulse of about 1 second at a set current density of about 0.1 A / dm ^{2 was} created. Through the so initiated deposition, the contact time could be shortened to 1 minute. Also in this case a mirror-like substrate surface was obtained, which shows no tendency to tarnish even after a long time.

Example 3

Following the deposition of the thin palladium layer, the substrate obtained in Example 1 and in Example 2 was contacted with an alkanephosphonic acid or an alkali phosphonic acid derivative (eg hexadecyl-, octadecylphosphonic acid, or mixtures thereof). After the layer had dried, an improved start-up behavior and increased corrosion protection were achieved compared to the substrates obtained from Examples 1 and 2. Thus, the treated layers showed after one Treatment with Sulfur Liver (Universal Oxide ECTH B-48) did not show any discoloration after exposure for 1 s or for 10 s exposure time, whereas untreated silver layers showed a spontaneous intense black discoloration even with a short exposure time.

Claims (20)

Method for preventing the tarnishing of silver layers,
characterized in that
a layer of a metal of the group consisting of palladium, platinum, osmium and rhenium with a layer thickness of 5 nm to 100 nm is applied to the silver layer. The method of claim 1, wherein the layer of a metal of the group consisting of palladium, platinum, osmium and rhenium is made by contacting the silver layer with an electrolyte containing ions of a metal of the group consisting of palladium, platinum, osmium and rhenium. The method of claim 2, wherein the silver layer is contacted with an acidic electrolyte. A method according to any one of the preceding claims, wherein for applying the layer of a metal of the group consisting of palladium; Platinum, osmium and rhenium at least briefly a current between the silver layer and a counter electrode is applied. A method according to any one of the preceding claims wherein the silver layer is contacted with an electrolyte comprising a metal of the group consisting of palladium, platinum, osmium and rhenium in the form of a metal / metal colloid with the metal of said group as the core metal and a metal oxide comprising colloidal metal surrounding this core. A method according to claim 5, wherein the silver layer is brought into contact with an electrolyte having as colloidal metal iron, tin, lead, cobalt or germanium. A method according to any one of claims 2 to 6, wherein the silver layer after contacting with an electrolyte of the one metal of the group consisting of palladium, platinum, osmium and rhenium in contact with a treatment solution which comprises a reducing agent or a metal reducible by the colloidal metal. A method according to claim 7, wherein the silver layer is brought into contact with a treating solution having as a reducing agent hydrogen peroxide, a hypophosphite, hydrazine, formaldehyde, or a borane compound. A method according to claim 6, wherein the silver layer is contacted with a treating solution having as the metal reducible by the colloidal metal silver, gold, nickel, palladium, platinum, bismuth or mixtures thereof. A method according to claim 4, wherein the electrolyte comprises the metal of the group consisting of palladium, platinum, osmium and rhenium in a concentration between 0.5 g / l and 20 g / l, preferably between 1 g / l and 10 g / l. A method according to claim 10, wherein between the silver layer and a counter electrode a current density between 0.1 A / dm ² and 4 A / dm ² , preferably between 0.5 A / dm ² and 3 A / dm ^{2 is} set. Process according to claims 4, 10 or 11, wherein the current is applied for a period between 1 s and 60 s, preferably between 1 s and 10 s. A method according to any one of claims 10 to 12, wherein the period of time in which a current is applied depending on the current density and the concentration of the metal of the group consisting of palladium, platinum, osmium and rhenium in the electrolyte is determined with the proviso that a layer thickness of the To be deposited metal is achieved in a range between 5 nm and 100 nm. Method according to one of the preceding claims, wherein on the layer of a metal of the group consisting of palladium, platinum, osmium and rhenium, a further layer is applied. A method according to claim 14, wherein as a further layer of a self-assembling molecules (SAM) is applied. A method according to claim 14 or 15, wherein a layer of hexadecanethiol and / or octadecanethiol is applied. A method according to any one of the preceding claims, wherein a metal of the group consisting of palladium, platinum, osmium and rhenium is deposited together with copper and / or nickel. Silver coating, characterized in that on the silver coating, a layer which comprises a metal of the group consisting of palladium, platinum, osmium and rhenium, is applied with a layer thickness of 5 nm to 100 nm. A silver coating according to claim 18, characterized in that a layer of a self-assembling molecule is applied to the layer comprising a metal of the group consisting of palladium, platinum, osmium and rhenium. A silver coating according to claim 19, wherein the self-organizing molecule is hexadecanethiol and / or octadecanethiol and / or an alkane phosphoric acid or a derivative thereof.

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