WO2005047570A2

WO2005047570A2 - Surface finishing method for silver and its alloys

Info

Publication number: WO2005047570A2
Application number: PCT/EP2004/012761
Authority: WO
Inventors: Antonio Fochesato; Marco Calegaro
Original assignee: F.Lli Calegaro Di Luigi Di Francesco Calegaro S.P.A.
Priority date: 2003-11-14
Filing date: 2004-11-10
Publication date: 2005-05-26
Also published as: ITMI20032218A1; EP1685279A2; WO2005047570A3

Abstract

A method is described for the surface finishing or plating of silver and/or its alloys characterized in that it comprises the following steps: - a washing pre-treatment of a silver object to be subjected to finishing, which comprises: a. washing in an ultrasound bath having frequencies higher than 40,000 Hertz; b. subsequent washing of the object in a bath containing a solution consisting of perchloro ethylene and dichloro propane in a weight ratio ranging from 70/30 to 50/50: c. washing in a bath with distilled water; - immersion in a galvanic bath containing a solution of a metal belonging to groups 8, 9, 10 and 13 of the periodic table.

Description

SURFACE FINISHING METHOD FOR SILVER AND ITS ALLOYS The present invention relates to a surface finishing or plating method for silver and its alloys. More specifically, it relates to a physico-chemical finishing method in an electro-galvanic bath, which provides a new surface finishing or plating of silver and its alloys. One of the main problems to be faced with the use of silver and its alloys, is the formation of silver sulphide (Ag₂S) on the surface of silver and its alloys, when in contact with air, with the consequent browning of the silver. Objects made of silver and its alloys do, in fact, be- come brown when in contact with air, due to the formation of Ag₂S as a result of the presence of hydrogen sulphide (H₂S) in the air. Even minimum traces of hydrogen sulphide are enough to start the Ag/H₂S reaction, leading to the formation of silver sulphide also called "silver glance" which has a characteristic dark colour. Once formed, silver sulphide is extremely stable and insoluble in water and in all acids, with the exception of nitric acid. Finishing treatments based on electro-plating or plat- ing in galvanic baths of silver objects are already known, but these forms of treatment produce an end- product in which the finishing has the following drawbacks: poor duration with time and consequent attack on the part of the hydrogen sulphide present in the air, with the formation of a dark surface patina. The objective of the present invention is consequently to solve the above-mentioned problem by means of a new finishing method which allows the drawbacks of the known art to be overcome. A particular finishing method has been surprisingly found, which allows the production of silver and its alloys which do not react with H₂S and are therefore permanently rustproof. An object of the present invention consequently re- lates to a method for the surface finishing or plating of silver and/or its alloys, characterized in that it comprises the following steps: a washing pre-treatment of a silver object to be subjected to finishing, which comprises: a. washing in an ultrasound bath having frequen- cies higher than 40,000 Hertz; b. subsequent washing of the object in a bath containing a solution consisting of perchloro ethylene and dichloro propane in a weight ratio ranging from 70/30 to 50/50: c. washing in a bath with distilled water; immersion in a galvanic bath containing a solution of a metal belonging to groups 8, 9, 10 and 13 of the periodic table. The perchloro ethylene/dichloro propane ratio is preferably equal to 60/40. In particular, the metal belonging to groups 8, 9, 10 and 13 is selected from rhodium, palladium, osmium, ruthenium and indium. Step a. of the washing pre-treatment is carried out over a period of 60 to 120 seconds, at a temperature ranging from 20 to 60°C. Step b. of the washing pre-treatment is carried out over a period of 30 to 150 seconds, at a temperature ranging from 20 to 60°C. Step c. of the washing pre-treatment is carried out over a period of 30 to 120 seconds, at a temperature ranging from 20 to 60°C. The main advantage of the method according to the present invention consists in the fact that it allows the production of manufactured products made of silver or its alloys, i.e. silver having a titre of 1000, 925 and 800 and its alloys, permanently rust- and scratch-proof. A further advantage is that the method according to the present invention allows manufactured products to be obtained, which, depending on the type of solution used in the galvanic bath, have surface finishings which are permanently rustproof and with a different optical appearance. In particular, the finishing treatment according to the present invention by means of plating with a rhodium solution, allows an end-product to be obtained, having a shiny white surface with a pale blue shading, whereas the use of a solution of palladium produces an end-product having a white surface with a light grey shading. When using a solution of osmium, an end-product is obtained, with a very shiny white surface, whereas the use of a solution of ruthenium produces an object with a surface appearance whose colour varies from dark grey to black ac- cording to the concentration of metal used. White surface does not refer to a white-coloured surface (of the Ti0₂ type) but a silver coloured surface with a white shading. The washing pre-treatment phase of the end-product is essential for obtaining a perfect silver plating. In the method according to the present invention, the washing is carried out in three steps with: a) an ultrasound bath with frequencies higher than 40,000 Hertz which effects a first removal of the particles forming the "dirt" deposited on the surface of the metal; b) a bath in a solution of perchloro ethylene/dichloro propane for the definite removal of the dirt particles; c) a bath with distilled water for washing the degreas- ing solution so that not even the slightest trace remains, of products containing chlorine on the surface of the metal. The selection of electrodes in the electro-galvanic bath is also important for obtaining a plated end-product with optimum characteristics. In particular, for finishing or plating with rhodium, it has been observed that the use of platinized niobium electrodes gives better results in terms of yield and finishing quality with respect to the use of platinized titanium electrodes. Niobium electrodes allow a better equalization of the cathode/anode current compared to titanium electrodes. The same electrodes are also used in plating with palladium. For plating with ruthenium and indium, the best electrodes are those made up of a mixed oxides, in particular a mixture of iridium dioxide and tantalum pen- toxide, coated with titanium which are preferred for the lower oxidation potential with respect to electrodes coated with platinum. In a first embodiment, the finishing method according to the present invention, when effected with a solution of rhodium, provides an end-product with the characteristics described below. The finishing method according to the present invention, effected by means of silver plating with a rhodium- based solution, gives the silver object a shiny white surface with permanent rustproof properties and a much higher surface hardness than that of silver. Rhodium-plated silver does in fact have a Mohs hardness of 4.3 against 3.75 of 925 titre silver. Rhodium- plated silver consequently has much higher scratch resistance and punch resistance properties. An electro-galvanic bath, modified with platinum electrodes coated with niobium, is used for silver finishing with rhodium. The solution used is made up of rhodium sulphate or rhodium phosphate in a solution of phosphoric or sulphuric acid. The metal concentration ranges from 0.8 to 5.0 g/1, whereas the concentration of acids ranges from 25 to 50 ml/1. The current intensity ranges from 20 to 100 A/m², over a time of 20 to 60 seconds. The plating thickness ranges from 0.05 to 1.5 microns. In a second embodiment, the finishing method according to the present invention, effected with a palladium solution, allows a manufactured product to be obtained, having the characteristics specified below.. The finishing method according to the present inven- tion, effected by means of silver plating with the palladium solution, gives the silver end-product a white-grey surface appearance and a very high resistance to oxidation and scratching. When it is not necessary to produce a manufactured product with a particularly shiny surface appearance, palladium represents a valid alternative to rhodium, as its cost is much lower than the cost of rhodium. Palladium chloride is normally used as plating salt, with a palladium concentration ranging from 1 to 9 g/1 in acid baths, or a concentration of 10/15 g/1 in alkaline baths . Palladium has a Mohs hardness of .8 against 3.75 of 925 titre silver. Palladium consequently allows an end- product to be obtained, having a better scratch resis- tance with respect to that with rhodium-plated silver. The electro-galvanic baths used for palladium finishing are chelated basic (ammonia solution) baths or acid baths. The ammonia baths contain from 10 to 15 g/1 of pal- ladium ammonium nitrate or palladium ammonium chloride with a current intensity ranging from 1 to 25 A/m², over a period of 15 to 60 seconds. The acid baths contain palladium chloride at a concentration varying from 1 to 9 g/1 of palladium in hydro- chloric acid, with a current intensity ranging from 1 to 10 A/m², for a time of 15 to 60 seconds. The plating thickness ranges from 0.2 to 1.25 microns . In a third embodiment, the finishing method accord- ing to the present invention, when effected with a ruthenium solution, allows a manufactured product to be obtained, with the following characteristics. The finishing method according to the present invention, effected by means of silver plating with a ruthe- niu solution, gives the silver manufactured product a surface appearance whose colour varies from grey to black depending on the amount of ruthenium; plating with ruthenium is in fact similar to black nickel plating, but the ruthenium characteristics are much higher. Plating with ruthenium gives the silver manufactured product extremely high scratch resistance properties as Mohs hardness of ruthenium is 6.5 and therefore higher both than that of rhodium and palladium. Ruthenium sulfamate or nitrosyl sulfamate is used as plating salt at concentrations ranging from 5.0 to 5.5 g/1 in sulfamic acid with an acid concentration ranging from 6 to 8.0 g/1, or ruthenium chloride in water at concentrations ranging from 1 to 10% by weight. The current density ranges from 108 to 320 A/m² for a time of 25 to 60 seconds, whereas the plating thickness ranges from 0.25 to 1.5 microns. Ruthenium chloride in water can also be used as plating salt, with a ruthenium concentration ranging from 1 g/1 to 10 g/1. The current intensity ranges from 100 to 320 A/m² for a time of 15 to 60 seconds. In a fourth embodiment, the finishing method according to the present invention, when carried out with an indium solution, allows a manufactured product to be obtained, having the following characteristics. The finishing method according to the present invention, effected by means of silver plating using an indium solution, gives the silver manufactured product a surface appearance having a characteristic extremely shiny silver white colour (gleaming white) . Indium is a soft and low-melting (156.2 °C) metal with a Mohs hardness of 1.2. Indium is much softer than silver and therefore does not confer scratch resistance to the silver manufactured product, but gives a characteristic gleaming-white colour to silver and makes it rustproof. Three types of electro-galvanic baths are used for the plating, based on cyanide, sulfamate or fluoro bo- rate. Indium is the only trivalent metal which can be rap- idly deposited by means of an electro-galvanic bath in a hydrocyanic solution. An indium cyanide bath is used a high plating layer must be obtained. In this case, the electro-galvanic baths contain from 30 to 40 g/1 of metal indium and from 90 to 120 g/1 of total cyanide. The current density ranges from 162 to 216 A/m², for a time ranging from 20 to 60 seconds. Indium sulfamate is extremely stable and relatively easy to control, and is characterized by a high yield at the cathode. The baths normally contain indium sulfamate having an indium concentration ranging from 80 to 110 g/1 in sulfamic acid from 20 to 40 g/1. The current density ranges from 108 to 1080 A/m², for a time ranging from 20 to 60 seconds. In the case of indium fluoro borate, the electro- galvanic baths contain from 200 to 310 g/1 of indium in boric acid and the acid concentration ranges from 20 to 40 g/1. The current density ranges from 160 to 280 A/m², for a time of 20 to 40 seconds. Some examples are now provided, which should be considered as being illustrative and non-limiting of the present invention. The finishing method according to the present invention was commercially called "Neoargentum" . EXAMPLE 1.

SILVER plating with RHODIUM The manufactured product to be subjected to finishing first undergoes a washing pre-treatment which includes: washing in an ultrasound bath having a frequency higher than 40,000 Hertz, which provides a first removal of the particles of the "dirt" deposited on the metal surface, for a period of about 60 seconds, at a temperature of about 25°C; a subsequent bath in a perchloro eth- ylene/dichloro propane 60/40 solution for the final re- moval of the dirt particles, for about 30 seconds, at a temperature of about 30°C; finally, a bath with distilled water for washing away the degreasing solution, so that no traces of products containing chlorine remain on the metal surface, for about 30 seconds, at a temperature of approximately 40 °C; the plating procedure is then started. A solution was prepared, containing rhodium III sulphate Rh₂(S0₄)3 in an H₂S0₄ solution, as plating agent, having a concentration of 0.8 to 5 g/1 of rhodium sul- phate - H₂S0. In particular, in the present example a solution was prepared consisting of 1 g/1 of rhodium III sulphate in a ratio 1:50 (1 part by weight of rhodium for every 50 parts by weight of H₂S0₄) to which a further 15% of sul- phuric acid at 23°C was added. The electrodes are platinum coated with niobium (niobium-plated platinum) . The process has a duration of 30 seconds with a current intensity equal to 30 A/m². The thickness of rhodium deposited on the silver is 1.2 microns. Cathode efficiency > 90%. The rhodium deposition on silver is perfect, with no stains, smudges and false reflections. EXAMPLE 2. SILVER plating with PALLADIUM The manufactured product to be subjected to finishing first undergoes a washing pre-treatment following a procedure analogous to that described in example 1 and the plating treatment is then started. Palladium tetrachloride H₂PdCl₄ is used as plating agent, in an alkaline solution (ammonia solution) . The solution used is typically H₂PdCl₄ at 37% in an ammonia salt. Process temperature 25°C. Process duration 40 seconds with a current intensity equal to 20 A/m². The thickness of the deposited palladium layer is 1.1 microns. Cathode efficiency > 80%. The deposition of palladium on silver is perfect, with no stains, smudges and false reflections. EXAMPLE 3. SILVER plating with RUTHENIUM. The manufactured product to be subjected to finish- ing first undergoes a washing pre-treatment following a procedure analogous to that described in example 1, and the plating treatment is then started. Ruthenium chloride RUCI₃Η₂O soluble in water is used as plating agent, in the form of a solution contain- ing 25% of ruthenium chloride. Process temperature 40°C. Process duration 50 seconds with a current intensity equal to 35 A/m². The thickness of the deposited ruthenium layer is 1.25 microns. Cathode efficiency > 25%. The deposition of ruthenium on silver is perfect, with no stains, smudges and false reflections. Plating with RuCl₃ confers a solid grey-black col- ouring to the silver, with a shiny appearance, with no stains and smudges. EXAMPLE 4. SILVER plating with INDIUM. The manufactured product to be subjected to finish- ing first undergoes a washing pre-treatment following a procedure analogous to that described in example 1 and the plating treatment is then started. Indium sulfamate is used as plating agent, with dextrose and triethanol a ine present in the bath, as growth inhibitors of the metal granules and for the control of the particle size of the spherulites, respectively. Contrary to rhodium, palladium and ruthenium, indium is in fact a soft metal (Mohs 1.2) and during the plating operation is deposited on the silver in the form of mi- cro-granules (visible under a microscope) . Deposit in the form of micro-granules is a common characteristic of all low-hardness metals . In order to have a smooth, opaque surface with a frozen effect, very small granules must be obtained, and it is therefore necessary to operate with a low intensity current of about 10 A/m² and to use a bath with vigorous stirring. A solution containing 105 g/1 of indium metal was used. Process temperature 20°C. Cathode efficiency > 90%. Plating with rhodium gives the silver an opaque surface with a frozen effect, with no stains and smudges. EXAMPLE 5. Comparative. The procedure described in example 1 was repeated, without effecting the washing pre-treatment . It has been observed that the finishing or plating treatment effected with rhodium sulphate, without the washing pre-treatment, gives a poor quality surface fin- ishing. The surface of the manufactured product does in fact appear to be scattered with small stains and irregular clovos. The rhodium surface layer is irregular, with varia- tions of +5%/-5% with respect to the rhodium surface layer deposited on the manufactured product subjected to the washing pre-treatment.

Claims

1. A method for the surface finishing or plating of silver and/or its alloys characterized in that it comprises the following steps: - a washing pre-treatment of a silver object to be subjected to finishing, which comprises: a. washing in an ultrasound bath having frequencies higher than 40,000 Hertz; b. subsequent washing of the object in a bath con- taining a solution consisting of perchloro ethylene and dichloro propane in a weight ratio ranging from 70/30 to 50/50; c. washing in a bath with distilled water; immersion in a galvanic bath containing a solu- tion of a metal belonging to groups 8, 9, 10 and 13 of the periodic table.

2. The method according to claim 1, characterized in that the perchloro ethylene/dichloro propane weight ratio is equal to 60/40.

3. The method according to claim 1, characterized in that the metal is selected from rhodium, palladium, osmium, ruthenium and indium.

4. The method according to claim 1, characterized in that step a. of the washing pre-treatment is carried out for a time ranging from 60 to 120 seconds, at a tempera- ture ranging from 20 to 60°C.

5. The method according to claim 1, characterized in that step b. of the washing pre-treatment is carried out for a time ranging from 30 to 150 seconds, at a tempera- ture ranging from 20 to 60°C.

6. The method according to claim 1, characterized in that step c. of the washing pre-treatment is carried out for a time ranging from 30 to 120 seconds, at a temperature ranging from 20 to 60 °C.

7. The method according to claim 1, characterized in that the manufactured product to be finished is immersed in a galvanic bath having niobium-plated platinum electrodes and the solution used consists of rhodium sulphate or rhodium phosphate in a solution of sulphuric or phos- phoric acid.

8. The method according to claim 7, characterized in that the rhodium concentration ranges from 0.8 to 5.0 g/1 whereas the acid concentration ranges from 25 to 50 ml/1.

9. The method according to claim 7, characterized in that the intensity of the current applied ranges from 20 to 100 A/m² for a time ranging from 20 to 60 seconds.

10. The method according to claim 1, characterized in that the manufactured product to be finished is immersed in a galvanic bath having niobium-plated platinum elec- trodes and the solution used consists of palladium chlo- ride in a solution of hydrochloric acid, with a palladium concentration ranging from 1 g/1 to 9 g/1.

11. The method according to claim 10, characterized in that the current intensity applied ranges from 1 to 10 A/m² for a time of 15 to 60 seconds.

12. The method according to claim 1, characterized in that the manufactured product to be finished is immersed in a galvanic bath having niobium-plated platinum electrodes and the solution used consists of palladium chlo- ride or palladium nitrate in an ammonia solution, with a palladium concentration ranging from 10 to 15 g/1.

13. The method according to claim 12, characterized in that the intensity of the current applied ranges from 1 to 25 A/m² for a time ranging from 15 to 60 seconds.

14. The method according to claim 1, characterized in that the manufactured product to be finished is immersed in a galvanic bath having electrodes consisting of titanium-plated mixed oxides and the solution used consists of ruthenium sulfamate or ruthenium nitrosyl sulfamate in a solution of sulfamic acid.

15. The method according to claim 14, characterized in that the ruthenium concentration ranges from 5.0 to 5.5 g/1, whereas the acid concentration ranges from 6 to 8.0 g/i.

16. The method according to claim 14, characterized in that the current intensity applied ranges from 108 to 320 A/m² for a time of 25 to 60 seconds.

17. The method according to claim 1, characterized in that the manufactured product to be finished is immersed in a galvanic bath having electrodes consisting of titanium-plated mixed oxides and the solution used consists of ruthenium chloride in water, with a ruthenium concentration ranging from 1 to 10 g/1.

18. The method according to claim 17, characterized in that the current intensity applied ranges from 100 to 320

A/m² for a time of 15 to 60 seconds.

19. The method according to claim 1, characterized in that the manufactured product to be finished is immersed in a galvanic bath having electrodes consisting of tita- nium-plated mixed oxides and the solution used consists of indium cyanide in a cyanurate solution, with a concentration of indium ranging from 30 to 40 g/1 and of total cyanide ranging from 90 to 120 g/1.

20. The method according to claim 19, characterized in that the current intensity applied ranges from 162 to 216

A/m², for a time of 20 to 60 seconds.

21. The method according to claim 1, characterized in that the manufactured product to be finished is immersed in a galvanic bath having electrodes consisting of tita- nium-plated mixed oxides and the solution used consists of indium sulfamate in sulfamic acid, with a concentration of indium ranging from 80 to 110 g/1 and of sulfamic acid ranging from 20 to 40 g/1.

22. The method according to claim 21, characterized in that the current intensity applied ranges from 108 to

1080 A/m², for a time of 20 to 60 seconds.

23. The method according to claim 1, characterized in that the manufactured product to be finished is immersed in a galvanic bath having electrodes consisting of tita- nium-plated mixed oxides and the solution used consists of indium fluoro borate in boric acid, with a concentration of indium ranging from 200 to 310 g/1 and of acid ranging from 20 to 40 g/1.

24. The method according to claim 23, characterized in that the current intensity applied ranges from 160 to 280

A/m², for a time of 20 to 40 seconds.

25. The method according to any of claims 19, 21 or 23, characterized in that the galvanic bath contains growth inhibitors of the metal granules such as dextrose and triethanol amine.