JP2012504701A - Method for depositing a palladium-rhodium layer with improved whiteness - Google Patents

Abstract

  The invention particularly relates to an electrochemical process for depositing a coating comprising an alloy of platinum and rhodium on a decorative article. The method of the present invention is characterized in that the prescribed condition is utilized that the electrodeposited layer has a high whiteness, which is unexpectedly close to the appearance of silver.

Description

  The invention particularly relates to a method for electrodeposition of a coating comprising an alloy of platinum and rhodium on a decorative article. The method according to the invention is characterized in that the electrodeposited layer has a high whiteness which, unexpectedly, is very close to the appearance of silver.

  Silver products are known to discolor and become unsightly, especially over time. Therefore, the use of silver products, especially in the decorative sector, is limited. In particular, frequent cleaning of the article is important to maintain its appearance and aesthetics. For this reason, it is advantageous that the silver product can be coated with a protective coating that has a similar appearance and is inert under prevailing conditions. This is particularly interesting for tableware and jewelry coatings.

The coating can be applied to such articles in a variety of ways. Electrolytic coating is one of the options for conductive articles. Various coating processes are typically used in the electrical industry depending on the type and nature of the part being coated. These processes differ, among other things, in the current density that can be utilized. There are substantially three different coating processes described.
1. Drum coating of bulk materials and mass produced parts:
In this coating process, a relatively low working current density is used (order unit: 0.05 to 0.5 A / dm ² ).
2. Rack coating of individual parts:
In this coating process, a medium working current density is used (order unit: 0.2-5 A / dm ² ).
3. High speed coating of strips and wires in a continuous plant:
In this coating field, very high working current densities are used (order units: 5-100 A / dm ² ).

  The coating of metals with rhodium and platinum is already known in the prior art. DE-B 1229816 describes a coating comprising a metal alloy of a platinum group metal and containing for the anode at least 50% by weight platinum together with at least 50% by weight platinum, at least 50% by weight rhodium or rhodium. ing. This document merely describes that platinum alloys were obtained, inter alia, by electroplating. The coating formed in this way is intended to protect the anode against corrosion.

  Methods for depositing platinum-rhodium alloys from basic baths are also known. In U.S. Pat. No. 4,427,502, polyamine ligands are used to complex noble metals in solution. Alloys such as rhodium are obtained which can contain at least 10 mol% platinum together with other noble metals. GB 348919 discloses the electrodeposition of a platinum-rhodium alloy containing 90% by weight platinum and 10% by weight rhodium from an ammonia electrolyte containing nitrile ions, paradium chloride and rhodium chloride.

Furthermore, such alloys can also be obtained from acidic electrolytes. Thus, US Pat. No. 3,671,408 describes how platinum-rich or rhodium-rich platinum and rhodium alloys can be deposited on transition metals from a bath containing sulfuric acid and sulfamic acid. . An acid 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 the coating of silver products, since very white deposits can be achieved here. Alloys deposited in this way contain more than 90% by weight of rhodium. Deposition is carried out, inter alia, from a bath containing about a 10-fold excess of rhodium.

U.S. Pat. No. 3,748,712 describes the coating of silver products with an alloy having a thickness of 1-50 microinches and containing 91-99% by weight rhodium and 1-9% by weight platinum. The electrolytic bath is said to contain 0.5-20 g / l of a noble metal sulfate compound and to be operated at a pH of 1-3.5. The current density applied is reported to be about 0.5~4.3A / dm ^2.

  What is common to both publications is that a very rhodium-rich alloy is proposed as a coating in order to be able to achieve the required whiteness. However, the price of rhodium is prohibitively high compared to platinum. Unfortunately, rhodium is not only the whitest white metal, but also the most expensive. Thus, the cost of 1 gram of rhodium is about 5 times the cost of 1 gram of platinum. For this reason, many gem manufacturers are trying to avoid expensive rhodium and replace it with less expensive platinum. However, this is only effective if the difference in color and gloss from the prescribed silver is not too clear.

  It is therefore desirable to develop a method that uses less rhodium to obtain a coating with a brightness comparable to that of the silver to be produced. This must be superior to the methods known from the prior art. In particular, the coating obtained according to the present invention must be further advantageous in order to provide a substantially intact or comparable good or further improved adhesion, discoloration resistance and color and color stability.

  These and further problems, which are not described in the prior art, but have been recognized in a manner apparent to the person skilled in the art, are achieved by the method having the features of the present claim 1. Advantageous embodiments of the method of the invention are defined in the claims dependent on claim 1.

The stated problem is particularly surprisingly advantageous in spite of its simplicity by electrolysis, in particular in the electrodeposition process of platinum and rhodium alloys on decorative articles, the rhodium content of the alloys being at least 40 mass. % To 85% by weight, preferably from 45% to 80% by weight, particularly preferably from 48% to 70% by weight, even more preferably from 49% to 60% by weight, Very advantageously about 50% by weight,
As an electrolytic bath, electrolysis at a pH of ≦ 1 and a current density of ≧ 2 A / dm ²
a) 0.4 to 5.0 g / l of platinum ions,
b) 1.0 to 5.0 g / l rhodium ion and optionally c) selected from the group consisting of supporting electrolytes, brighteners, surfactants, wetting agents, ligands that complex noble metals and stabilizers. This is achieved in an electrodeposition process carried out in an aqueous solution containing one or more additives. The method of the present invention makes it possible to obtain a tightly adhered and stable coating that looks like silver, even if attractive in suitable products and articles, with less rhodium in the platinum-rhodium coating. To do. The coating is generally two lightness units (according to Cielab system-http: //www.cielab.de/) that are brighter than theoretically expected from the proportion of pure rhodium.

  Furthermore, the throwing power of the platinum-rhodium electrolyte used in the method of the present invention is particularly good. In electroplating, throwing power is the ability of the electrolyte to achieve an improved coating distribution on the workpiece to be coated even with a non-uniform current distribution. In particular, a distinction is made between throwing power between macro and micro. Macro throwing power refers to the ability of the electrolytic bath to achieve a substantially uniform layer thickness over the entire surface of the workpiece, including the lower region. Micro throwing power refers to the ability of the electrolytic bath to deposit metal in the 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. They are advantageously pyrophosphate, carbonate, carbonate hydroxide, bicarbonate, sulfite, sulfate, phosphate, nitrite, nitrate, halide, hydroxide, hydroxide, It is preferably introduced in the form of a water-soluble salt selected from the group consisting of oxides and mixtures thereof. Embodiment wherein the metal is used in the form of a salt having an ion selected from the group consisting of pyrophosphate, carbonate, sulfate, bicarbonate salt, hydroxide, hydroxide and bicarbonate Is even more advantageous. The type and amount of salt introduced into the electrolyte is also critical to the color of the resulting decorative coating and can be set according to consumer requirements. The metal to be deposited, as indicated, is present in the electrolyte in an ion-dissolved form for the application of decorative layers to gems, consumer products and industrial products. 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 rhodium ion concentration is 1.0 to It can be set in the range of 5.0 g / l electrolyte, preferably 1.5-2.5 g / l electrolyte. In improving the quality of the product, it is particularly advantageous to introduce the metal to be deposited as sulfate or phosphate, carbonate or bicarbonate, so that the resulting ion concentration is in each case the electrolyte 1 A range of 0.5 to 1.0 grams of platinum and 1.0 to 2.0 grams of rhodium per liter.

The electrolyte can contain one or more of the indicated additives. Unlike the metal salts to be deposited, further additives are organic additives that act as supporting electrolytes (for example H / Na / K / NH ₄ sulfate, phosphate, sulfonate or mixtures thereof ( Handbuch der Galvanotechnik, Carl Hanser Verlag, 1966)), brighteners (including aromatic or heterocyclic sulfonic acids, selenite, aluminum, magnesium) (Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2 + 4 , Volume 91, 2000)), wetting agents (eg polyfluorinated sulfonic acids, aliphatic sulfates (A. v. Krustenstjern, Edelmetallgalvanotechnik 1970, Eugen G. Leuze Verlag, Saulgau))) or stabilizers (eg sulfonic acids, Sulfites (A. v. Krustenstjern, Edelmetallgalvanotechnik 1970, Eugen G. Leuze Verlag, Saulgau)), ligands complexing noble metals (eg sulfates, phosphates, methanesulfonates or mixtures thereof (Edelmetalltaschenbuch Degus ^{sa, 2 nd edition, 1995,} Huethig Verlag, Heidelberg)) or a surfactant (anionic, cationic, a non-ionic surfactant (A. v. Krustenstjern, Edelmetallgalvanotechnik 1970 , Eugen G. Leuze Verlag, a Saulgau)) Including.

  The addition of brighteners and wetting agents is particularly advantageous only if the appearance of the decorative layer to be deposited has to meet special requirements. These adjust the gloss of the layer at all tones between matte and high gloss silk, in addition to the color of the coating, which mainly depends on the proportion of metal to be deposited (Figure 3). Is possible. It is also advantageous to add one or more compounds selected from the group consisting of monocarboxylic and dicarboxylic acids, alkane sulfonic acids, betaines, sulfamic acids, sulfites, selenic acids and aromatic nitro compounds. . These compounds act as electrolytic bath stabilizers or brighteners. It is particularly advantageous to use oxalic acid, alkanesulfonic acid, in particular methanesulfonic acid, or nitrobenzotriazole or mixtures thereof. Suitable alkane sulfonic acids can be found in EP 1001054. Possible carboxylic acids are, for example, citric acid and its Na / K salt (Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2 + 4, Volume 91, 2000). The betaines to be used are preferably those that can be found in WO 2004/005528. In particular, it is advantageous to use what is described in EP 636 713. In this context, it is even more advantageous to use 1-3 (3-sulfopropyl) pyridinium betaine or 1- (3-sulfopropyl) -2-vinylpyridinium betaine.

  Furthermore, a supporting electrolyte can be added to the electrolyte in the method of the present invention. Possible supporting charges include pyrophosphate, carbonate, carbonate hydroxide, bicarbonate, sulfite, sulfate, phosphate, nitrite, nitrate, halide, hydroxide or carboxylate anion, phosphone. An alkali metal or alkaline earth metal salt having an anion such as an acid anion and a sulfonate anion. In this context, mention may be made in particular of sulfuric acid and phosphoric acid, which have been added in excess, for example to the reaction of producing rhodium sulfate or rhodium phosphate from rhodium oxide hydrate (Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2 + 4, Volume 91, 2000).

  Also, surfactants (eg, anionic, cationic and / or nonionic surfactants with or without polyfluorinated substituents that are resistant to very low pH over time (electroplating chemicals, TIB Chemicals AG , Mannheim)) can also be added.

  Application of coatings to decorative articles, consumer goods and industrial products using the electrolyte according to the invention is carried out as shown in an electrochemical process. Here it is important that the metal to be deposited is kept in solution at all times during the process, regardless of whether electroplating is carried out in a continuous or batch process. In order to ensure this, the electrolyte according to the invention can contain a complexing agent. The ligands that complex the noble metal may include those having sulfur or phosphorus atoms, such as sulfuric acid and phosphoric acid, which generate, for example, rhodium sulfate or rhodium phosphate from rhodium oxide hydrate. (Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2 + 4, Volume 91, 2000).

  The amount of the compound complexing the noble metal in the electrolyte can be adjusted by methods intended by those skilled in the art. This is limited by the fact that the concentration in the electrolyte should exceed the minimum amount in order to obtain the effect to a sufficient extent.

  The pH of the electrolyte is in the range of ≦ 1 required for this electroplating application. The lower limit is set by the fact that electrolytes tend to be unstable at pH values that are too low. Accordingly, the range of 0 to 0.8, and even more preferably about 0.2 is advantageous. The acidification of the electrolyte can generally be carried out using an inorganic acid. For this reason, it is particularly advantageous to use sulfuric acid. In an even more advantageous embodiment, the aqueous electrolytic bath is acidified with sulfuric acid at a concentration of 100 ml / l or less.

  The method of the present invention can be operated at a temperature selected by those skilled in the art based on general technical knowledge. Advantageously, the electrolytic bath is maintained in the range of 20-70 ° C. during electrolysis. It is further advantageous to select a range of 30-50 ° C. This process is particularly preferably carried out at a temperature of about 45 ° C.

  When using the method of the present invention, various anodes can be used. An insoluble anode is advantageous. The insoluble anode is preferably a material selected from the group consisting of platinum-plated titanium, graphite, iridium transition metal mixed oxides and certain carbon materials (“diamond-like carbon”, DLC) or combinations of these anodes. A configured anode is used. Also advantageous are mixed oxide anodes (MMO) composed of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide. Additional materials can be found in Cobley, A.J. et al. (The use of insoluble Anodes in Acid Sulphate Copper Electrodeposition Solutions, Trans IMF, 2001, 79 (3), pp. 113-114). It is even more advantageous to use Platinode® 177 MMO (obtained from Umicore Galvanotechnik GmbH).

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

Platinum ions can also be used conventionally in the form of complexes in the process of the present invention. A commercially available compound of this kind is, for example, platinum ammonia complex [Pt (NH ₃ ) ₄ SO ₄ ] or [Pt (NH ₃ ) ₂ SO ₄ ]. Since nitrogen-containing ligands can be present in the electrolyte, they are advantageously introduced into the electrolyte in the form of the corresponding platinum complexes. Thus, platinum ions are advantageously used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines. The use of multidentate ligands, in particular ligands based on diamine, triamine or tetraamine, is advantageous here. Ligands having 2 to 11 carbon atoms are particularly advantageous. It is even more advantageous to use a ligand selected from the group consisting of ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 1,2-propylenediamine, trimethylenetetramine, hexamethylenetetramine. It is. For this reason, ethylenediamine (EDA) is most advantageous.

  The method according to the invention makes it possible in particular to deposit a white platinum-rhodium layer on a decorative metal article made of silver in an economically advantageous manner. The whiteness (brightness) of the approximately 50:50 alloy is significantly improved relative to the theoretically expected value and approaches that of alloys containing> 90% rhodium metal (Summary 1) .

  It has been found that the color and optical appearance, for example the glossiness of the layer, is not significantly worse than that of at least a pure rhodium layer. This can save a dramatic proportion of expensive rhodium. This was unexpected from the prior art.

FIG. 1 shows the brightness (whiteness) as a function of the proportion of rhodium in the alloy. FIG. 2 shows lightness (whiteness) as a function of current density. FIG. 3 shows the brightness (whiteness) as a function of the proportion of rhodium in the alloy. FIG. 4 shows the alloy as a function of current density and temperature.

Example:
Electrodeposited platinum layer with improved whiteness and electrolyte therefor 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

Operating conditions:
Temperature: 45 ° C
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: 2.0 A / dm ² (0.25-5.0 A / dm ² )
Anode: MMO (Platinode® 177 type; commercially available from Umicore Galvanotechnik GmbH, titanium anode coated with mixed metal oxides for rhodium or platinum electrolytes of strong acids)
Deposition rate: about 0.084 μm / min (at 2.0 A / dm ² )
Deposit yield: about 7.1 mg / amine (at 2.0 A / dm ² )
Alloy (about): Pt: Rh = 50: 50 (at 2.0 A / dm ² )

White, glossy layers can be produced using electrolytes operated with these parameters. These colors (according to CIEL ^* a ^* b ^* ) were measured using a colorimeter from Xrite (model SP 62). Here, the L ^* value determines the brightness of the layer (this corresponds to the percentage of reflection of light incident on the layer, L ^* = 0 means perfect black, L ^* = 100 is the light intensity It means complete reflection (http://www.cielab.de/).

The sharp increase in brightness (whiteness) is noticeable when the current density increases from 0.25 A / dm ² to about 2.0 A / dm ² . For current densities> 2.0 A / dm ² , the brightness is slightly improved or remains constant (FIG. 2).

Description of drawings:
FIG. 1 The brightness (whiteness) of the coating is shown here as a function of the proportion of rhodium in the alloy. It can be seen that the lightness of the rhodium and alloy in proportion exceeding 40% is improved above the theoretically expected value.
FIG. 2 The lightness (whiteness) of the coating when the applied current density changes is almost constant as long as a current density of ≧ 2 A / dm ² is selected.
FIG. 3 The brightness (whiteness) of the alloy no longer improves beyond the proportion of rhodium of about 50%. Thus, small differences in this range of alloy composition are of little importance.
FIG. 4 The alloy composition remains nearly constant over a wide current density range of ≧ 2 A / dm ² .

Claims (6)

In the electrodeposition process of platinum and rhodium alloys, in particular on decorative articles, wherein the rhodium content of the alloy is in the range of at least 40% to 85% by weight
As an electrolytic bath with electrolysis at a pH of ≦ 1 and a current density of ≧ 2 A / dm ² ,
a) 0.4 to 5.0 g / l of platinum ions,
b) 1.0 to 5.0 g / l rhodium ion and optionally c) selected from the group consisting of supporting electrolytes, brighteners, surfactants, wetting agents, ligands that complex noble metals and stabilizers. An electrodeposition method, characterized in that it is carried out in an aqueous solution containing one or more additives.   The process according to claim 1, characterized in that the aqueous electrolytic bath is acidified with sulfuric acid at a concentration of 100 ml / l or less.   The method according to claim 1 or 2, characterized in that the temperature is in the range of 20-70 ° C during the deposition.   4. A process according to claim 1, wherein a mixed metal oxide anode is used as the anode. The method according to claim 1, wherein the current density is 7.0 A / dm ² or less.   6. The process as claimed in claim 1, wherein platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.

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