KR20110071106A - Process for the deposition of platinum-rhodium layers having improved whiteness - Google Patents

Abstract

The present invention relates to an electrochemical method for depositing a coating comprising an alloy of platinum and rhodium, in particular on an ornament. The method of the present invention is characterized by the use of limited conditions in which the electrodeposited layers have a high whiteness, which is unexpectedly very close to the appearance of silver.

Description

Process for the deposition of platinum-rhodium layers having improved whiteness}

The present invention relates to an electrolytic method for depositing a coating comprising an alloy of platinum and rhodium, in particular on a decorative article. The process of the present invention is characterized in that the electrolytically deposited layer has a high degree of whiteness, which is unexpectedly very close to the appearance of silver.

Silver products are known to discolor, especially over time. Therefore, the use of silver products in particular in the decorative field is limited. Among other things, frequent cleaning of products is important to maintain their appearance and aesthetics. For this reason, it is advantageous to be able to coat silver products with protective coatings that have a similar appearance and are inert under normal conditions. This is especially important for the coating of eating utensils and jewelry.

Coatings can be applied to these products in various ways. The coating by the electrolytic method is selected for the electrically conductive product. Different coating methods are commonly used in the electroplating industry as a function of the type and nature of the parts to be coated. The methods differ among them in terms of current density that can be used. Essentially three different coating methods can be mentioned.

1. Drum coating for loose material and mass-produced parts:

In this coating method, relatively low working current densities are used.

[Order of magnitude: 0.05-0.5A / dm ² ]

2. Rack coating for individual parts:

In the coating method, a medium working current density is used.

(Size Accuracy: 0.2-5A / dm ² )

3. High speed sheaths for strips and wires in continuous plants:

In this field of coating, very high working current densities are used.

(Size degree: 5-100A / dm ² )

The coating of metals with alloys of rhodium and platinum is already known in the prior art. German patent DE-B 1229816 comprises a coating containing a metal alloy of platinum group metal and containing at least 50% by weight platinum, at least 50% by weight of rhodium or at least 50% by weight of platinum with rhodium for the anode. It is described. The document only mentions that platinum alloys can be obtained in particular by electroplating. The coating formed in this way is to protect the anode from corrosion.

Methods of depositing platinum-rhodium alloys from basic baths are likewise known. In U.S. Pat.No.4427502, polyamine ligands are used to complex noble metals in solution. With other precious metals, such as rhodium, alloys are obtained which can contain at least 10 mol% platinum. British Patent 348919 describes electrolytic deposition of a platinum-rhodium alloy comprising 90% by weight of platinum and 10% by weight of rhodium from an ammoniacal electrolyte containing nitrite ions, palladium chloride and rhodium chloride.

Such alloys may also be obtained from acidic electrolytes. Thus, US Patent 371408 describes how platinum-rich or rhodium-rich alloys of platinum and rhodium can be deposited on transition metals from a bath containing sulfuric acid and sulfamic acid. An acidic bath containing a platinum complex consisting of Pt (NH ₃ ) ₂ (NO ₂ ) ₂ and rhodium sulfate is used in this process. It is stated that rhodium-rich baths are particularly suitable for coating silver products because very white deposits are achieved in rhodium-rich baths. The alloy deposited in this way contains at least 90% by weight of rhodium. Deposition is performed, among others, from a bath containing about 10 times excess of rhodium.

U. S. Patent No. 3748712 describes a coating of silver products with an alloy having a thickness of 1 to 50 microinch and containing an alloy containing 91 to 99 weight percent rhodium and 1 to 9 weight percent platinum. Electrolytic baths contain 0.5 to 20 g / l of sulfate compounds of precious metals and are said to operate at a pH of 1 to 3.5. The applied current density is reported to be about 0.5 to 4.3 A / dm ² .

It is common in the two publications to propose a very rhodium-rich alloy as a coating to achieve the required whiteness. However, the price of rhodium is incredibly high compared to platinum. Unfortunately, rhodium is the most expensive platinum group metal as well as the most expensive. Thus, the cost of 1 g of rhodium is about 5 times the cost of 1 g of platinum. For this reason, many jewelry manufacturers try to avoid expensive rhodium and replace it with cheaper platinum. However, this is possible only if the difference in color and brightness is not so obvious from ordinary silver.

Therefore, there is a need to develop a method that can produce a film having a whiteness comparable to that of silver with less rhodium. This should be superior to methods known from the prior art. In particular, the coatings obtained according to the invention are more advantageous for manufacturing with significantly intact or comparably good or even improved adhesion, improved tarnishing resistance and also improved color and improved color stability. Should be.

These objects and further objects which are not mentioned in the prior art but can be recognized by one skilled in the art in an obvious manner are achieved by a method having the features of claim 1. Preferred embodiments of the method of the invention are defined in the claims citing claim 1.

The stated purpose is particularly surprisingly achieved in a simple and advantageous manner by electrolysis of an alloy of platinum and rhodium, in particular in an electrolytic deposition method on an ornament, wherein the rhodium content of the alloy is from 40% to 85% by weight, It is preferably 45% to 80% by weight, particularly preferably 48% to 70% by weight, very particularly preferably 49% to 60% by weight, very preferably about 50% by weight, pH ≤ 1 and At current density ≥ 2 A / dm ²

As an electrolytic cell

a) 0.4-5.0 g / l platinum ion,

b) rhodium ions 1.0-5.0 g / l and

Optionally c) at least one additive selected from the group consisting of a supporting electrolyte, a brightening agent, a surfactant, a wetting agent, a ligand complexing a noble metal and a stabilizer

It is carried out in an aqueous solution containing. The process of the invention makes it possible to obtain a firmly attached stable coating which is carried out with less rhodium in the platinum-rhodium coating and nevertheless still looks attractive and silver on appropriate goods and products. The coating is generally brighter by two lightness units (according to the Cielab system-http://www.cielab.de/) than would be theoretically expected from the ratio of pure rhodium.

In addition, the uniform throwing power of the platinum-rhodium electrolyte used in the process of the invention is particularly excellent. In electroplating, uniform electrodeposition is the ability of the electrolyte to achieve an improved coating distribution on the workpiece to be coated, even with a nonuniform current distribution. Specifically, it is classified into macro and micro uniform electrodeposition. Macro uniform electrodeposition is the ability of an electrolytic cell to achieve a substantially uniform layer thickness over the entire surface of the workpiece, including the bottom region. Microhomogeneous electrodeposition is the ability of an electrolytic cell to deposit metal in pores and scratches.

In the electrolyte of the present invention, the metal rhodium and platinum to be deposited are present in dissolved form as their ions. These are preferably groups consisting of pyrophosphates, carbonates, hydroxide carbonates, hydrogencarbonates, sulfites, sulfates, phosphates, nitrites, nitrates, halides, hydroxides, oxide hydroxides, oxides and combinations thereof It is introduced in the form of a water-soluble salt preferably selected from. Very particular preference is given to the embodiment in which the metal is used in the form of salts with ions selected from the group consisting of pyrophosphates, carbonates, sulfates, hydroxidecarbonates, oxide hydroxides, hydroxides and hydrogencarbonates. The type and amount of salt introduced into the electrolyte can likewise determine the color of the resulting decorative coating, which can be determined according to consumer requirements. The metal to be deposited is present in ionic dissolved form in the electrolyte to apply the decorative layer onto jewelry, consumer goods and industrial goods, as indicated. The ion concentration of platinum can be set in the range of 0.4 to 5.0 g / L electrolyte, preferably 0.5 to 4.0 g / L electrolyte, and the ion concentration of rhodium is 1.0 to 5.0 g / L electrolyte, preferably 1.5-2.5 g /. It can be set in the electrolyte l range. Upon upgrading the product, the metal to be deposited is introduced as sulfate or phosphate, carbonate or hydroxide carbonate such that the resulting ion concentrations in each case range from 0.5 to 1.0 g of platinum and 1.0 to 2.0 g of rhodium per liter of electrolyte. Is particularly preferred.

The electrolyte may contain one or more of the additives described above. In addition to the metal salts to be deposited, supporting electrolytes (e.g., H / Na / K / NH ₄ sulphate, phosphates, sulfonates or mixtures thereof (see Handbuch der Galvanotechnik, Carl Hanser Verlag, 1966)), polishes (Including aromatic or heterocyclic sulfonic acids, selenic acid, aluminum, magnesium (see Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2 + 4, Volume 91, 2000)), wetting agents (e.g., Polyfluorinated sulfonic acids, aliphatic sulfates (see literature; A. v. Krustenstjern, Edelmetallgalvanotechnik 1970, Eugen G. Leuze Verlag, Saulgau)) or stabilizers (e.g., sulfonic acids, sulfites (see literature; A. v. Krustenstjern, Edelmetallgalvanotechnik 1970, Eugen G. Leuze Verlag, Saulgau), ligands complexing precious metals (eg sulfates, phosphates, methanesulfonates or mixtures thereof (see Edelmetalltaschenbuch Degussa, 2 ^nd edition, 1995). , Huthig Verlag, Heidelberg)) or organic additives that function as surfactants (including anionic, cationic, nonionic surfactants (see literature; A. v. Krustenstjern, Edelmetallgalvanotechnik 1970, Eugen G. Leuze Verlag, Saulgau)). Additional additives included.

The addition of brighteners and wetting agents is particularly preferred only if the appearance of the decorative layer to be deposited meets certain requirements. This can control the brightness of the layer with any gradation between matt silk and high gloss, in addition to the color of the coating (FIG. 3), which mainly depends on the proportion of metals to be deposited. It is also preferred to add one or more compounds selected from the group consisting of monocarboxylic acids and dicarboxylic acids, alkanesulfonic acids, betaines, sulfamic acids, sulfites, selenic acids and aromatic nitro compounds. These compounds act as electrolytic cell stabilizers or as brighteners. Particular preference is given to using oxalic acid, alkanesulfonic acid, in particular methanesulfonic acid, or nitrobenzotriazole or mixtures thereof. Suitable alkanesulfonic acids can be found in EP1001054. Possible carboxylic acids are, for example, citric acid and its Na / K salts (see Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2 + 4, Volume 91, 2000). Betaines to be used are preferably those found in WO2004 / 005528. Particular preference is given to using those described in EP636713. In this context, very particular preference is given to using 1-3 (3-sulfopropyl) pyridinium betaine or 1- (3-sulfopropyl) -2-vinylpyridinium betaine.

In addition, a supporting electrolyte can be added to the electrolyte in the method of the present invention. Possible supporting electrolytes include pyrophosphate, carbonate, hydroxide carbonate, hydrogencarbonate, sulfite, sulfate, phosphate, nitrite, nitrate, halide, hydroxide or carboxylate anion, phosphonate anion, sulfonate anion. Alkali metal or alkaline earth metal salts with the same anion. In this context in particular, mention may be made of sulfuric acid and phosphoric acid which are added in excess to the reaction, for example, in the preparation of rhodium sulfate or phosphate from rhodium oxide hydrate (see Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue). No. 2 + 4, Volume 91, 2000).

Surfactants (eg, anionic, cationic and / or nonionic surfactants with or without polyfluorinated substituents that withstand very low pH for long periods of time (electroplating chemicals, manufacturer; TIB Chemicals AG, Mannheim) ) Can also be added.

Application of the coating to ornaments, consumer goods and industrial articles using the electrolyte according to the invention is carried out in an electrochemical process, as indicated. Here, it is important to keep the metals to be deposited permanently in solution during the process, regardless of whether the electroplating is carried out in a continuous process or in a batch process. To ensure this, the electrolyte according to the invention may contain complexing agents. As ligands for complexing noble metals, mention may also be made of sulfuric acid and phosphoric acid having sulfur or phosphorus atoms, such as sulfuric acid and phosphoric acid added in excess in the reaction in the preparation of rhodium sulfate or phosphate from rhodium oxide hydrate (see literature). ; Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2 + 4, Volume 91, 2000).

The amount of compounds that complex the noble metals in the electrolyte can be set in a manner targeted by those skilled in the art. This is limited by the fact that the concentration in the electrolyte must be at least above the minimum amount to achieve the relevant effect to a sufficient degree.

The pH of the electrolyte is required to be in the range ≦ 1 for this electroplating application. The lower limit is set by the fact that electrolytes tend to be unstable at too low pH values. Therefore, it is preferably in the range of 0 to 0.8, very particularly preferably about 0.2. Acidification of the electrolyte can generally be carried out using inorganic acids. It is preferable to use sulfuric acid among these for this purpose. In a very particularly preferred embodiment, the aqueous electrolyzer is acidified with up to 100 ml / l concentrated sulfuric acid.

The method of the present invention can operate at a temperature that those skilled in the art select based on their general technical knowledge. A range of 20 to 70 ° C. is preferred and the electrolyzer is maintained at this temperature during electrolysis. It is more preferable to select the range of 30-50 degreeC. The method is particularly preferably carried out at a temperature of about 45 ° C.

When the method of the invention is used, various anodes can be used. Insoluble anodes are preferred. As an insoluble anode, a material selected from the group consisting of platinum titanium, graphite, iridium-transition metal mixed oxides and certain carbon materials ("diamond-like carbon", DLC) or combinations of these anodes It is advantageous to use an anode consisting of. Preference is also given to a mixed oxide anode (MMO) consisting of an iridium-ruthenium mixed oxide, an iridium-ruthenium-titanium mixed oxide or an iridium-tantalum mixed oxide. Additional substances are Cobly A.J. Cobley, AJ et al., See The use of insoluble Anodes in Acid Sulphate Copper Electrodeposition Solutions, Trans IMF, 2001, 79 (3), pages 113 and 114. Very particular preference is given to using MMOs of the Platinode ^® 177 type (available from Umicore Galvanotechnik GmbH).

An important advantage of the present invention is that the deposition of the alloy composition does not change significantly over a wide range of current densities of ^{2 A} / dm ² or greater (FIG. 2). This also results in a surface quality that looks sufficiently uniform even at relatively high current densities for rack applications. Those skilled in the art will select the current density ranges to obtain results that are considered optimal based on economic and technical boundary conditions. 7.0A / dm ^2, and preferably it is advantageous to select a current density of 6.0A / dm ² or less, that the current density in the range of 3.0A / dm ² to 4.0A / dm ² is particularly preferred.

Platinum ions can also be used in a precomplexed form in the process of the invention. Commercial compounds of this type are, for example, ammonia complexes of platinum [Pt (NH ₃ ) ₄ SO ₄ ] or [Pt (NH ₃ ) ₂ SO ₄ ]. Since nitrogen-containing ligands may be present in the electrolyte, they are preferably introduced into the electrolyte in the form of the corresponding platinum complexes. Thus, platinum ions are preferably used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines. The use of polydentate ligands, in particular ligands based on diamines, triamines or tetramines, is advantageous here. Particular preference is given to ligands having from 2 to 11 carbon atoms. Very particular preference is given to using ligands selected from the group consisting of ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 1,2-propylenediamine, trimethylenetetramine, hexamethylenetetramine. Ethylenediamine (EDA) is most preferred for this purpose.

The process of the present invention allows the white platinum-rhodium layer to be deposited in an economically advantageous manner, especially on metal ornaments made of silver. The whiteness (brightness) of the about 50:50 alloy is significantly increased beyond what is theoretically expected, and is similar to the whiteness of alloys containing> 90% rhodium metal (Overview 1).

Outline 1:

Wear behavior and savings potential of alloy system Rh / Pt:

Rhodium-Platinum Alloy:

It can be seen that the color and optical appearance of the layer, for example shininess, is at least not significantly worse than those of the pure rhodium layer. This makes it possible to reduce the dramatic proportion of expensive rhodium. This could not be expected from the prior art.

Example

Electrolytically deposited platinum layers with improved whiteness and electrolyte for this purpose

Electrolyte Composition:

Platinum: 0.6g / ℓ

Rhodium: 1.5 g / l

Sulfuric acid, concentration: 40 ml / l (= about 70 g / l)

Total acid: 80 g / ℓ

Working condition:

Temperature: 45 ℃

pH: 0.25 (at 45 ° C) or 0.2 (at 25 ° C)

Density: 1.046 g / cm ³ (at 45 ° C.) or 1.051 g / cm ³ (at 25 ° C.)

Current Density: 2.0A / dm ² (0.25-5.0A / dm ² )

Anode: MMO (Type Platinoid ^® 177; Titanium Anode Coated with Mixed Metal Oxide for Strong Acid Rhodium or Platinum Electrolyte, Umicore Galvanotechnik GmbH)

Deposition rate: about 0.084 μm / min at 2.0 A / dm ²

Deposition rate: about 7.1 mg / Amin (at 2.0 A / dm ² )

Alloy (approx): Pt: Rh = 50:50 (at 2.0 A / dm ² )

Electrolytes operating at these parameters can be used to produce white glossy layers. These colors (according to CIEL ^* a ^* b ^* ) were measured using a color measuring instrument from Xrite (model SP 62). Here, the L ^* value determines the brightness of the layer (corresponds to the percent reflectance of light incident on the layer). L ^* = 0 means entirely black, and L ^* = 100 means complete reflection of light (http://www.cielab.de/).

When the current density increases from 0.25 A / dm ² to about 2.0 A / dm ² , a sharp increase in brightness (whiteness) is evident. At current densities> 2.0 A / dm ² , the brightness is only slightly increased or constant (FIG. 2).

BRIEF DESCRIPTION OF THE DRAWINGS Fig.

1; The brightness (whiteness) of the coating is shown as a function of the proportion of rhodium in the alloy. It can be seen that at a ratio of 40% or more of rhodium, the brightness of the alloy increases above the theoretically expected value.

2; When the applied current density is changed, the brightness (whiteness) of the film is approximately constant as long as a current density of ≧ ² A / dm ² is selected.

3; The brightness (whiteness) of the alloy no longer increases at a ratio of about 50% or more of rhodium. Therefore, small differences in alloy composition in this range are not very important.

4; The alloy composition is approximately constant over a wide current density range of ≧ ^{2 A} / dm ² or greater.

Claims (6)

Electrolysis,
at pH ≤ 1 and current density ≥ 2 A / dm ²
As an electrolytic bath
a) 0.4-5.0 g / l platinum ion,
b) rhodium ions 1.0-5.0 g / l and
Optionally c) at least one additive selected from the group consisting of a supporting electrolyte, a brightening agent, a surfactant, a wetting agent, a ligand complexing a noble metal and a stabilizer
Electrolytically depositing an alloy of platinum and rhodium, wherein the rhodium content of the alloy is greater than or equal to 40% by weight to less than or equal to 85% by weight, in particular on a decorative article Way. The method of claim 1 wherein the aqueous electrolyzer is acidified with up to 100 ml / l of concentrated sulfuric acid. The method according to claim 1 or 2, wherein the temperature is in the range of 20 to 70 ° C during the electrolytic deposition. The process according to claim 1, wherein a mixed metal oxide anode is used as the anode. The method of claim 1, wherein the current density is less than 7.0 A / dm ² . The process according to claim 1, wherein the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.

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