Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/48—Electroplating: Baths therefor from solutions of gold
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D1/00—Electroforming
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
  • C25D3/62—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of gold

Definitions

• the present invention relates to a solution and a process for surface plating of pieces and parts with gold and gold alloy and for electroforming items in gold alloys.
• Plating solutions are known in the art and are used to electrodeposit a layer of a gold alloy on a substrate, e.g. electronic contacts, or to electroform pieces such as earrings and jewelry in general.
• a widely known and used solution for electroforming comprises Au, Cu and Cd ions and free CN ions that are used as complexing agents.
• plastic and low melting models are made conductive and eventually immersed in the solution.
• a current is then passed through the solution bath to provide a deposition of Au on the models. This process has several drawbacks.
• the quantity of Au deposited with respect to the other species present in the solution is greatly dependent on the operating conditions, i.e. shape of the pieces and their position with respect to the anode(s), current distribution, flow conditons etc. .
• the manufacturer has to stop the deposition process and weigh the cathode.
• a further problem is due to presence of free cyanide ions in the solution; this makes the bath dangerous in more than one way, e.g. when the solution pH is adjusted a small error could result in cyanide gas being freed from the solution.
• the complexing agent is EDTA and the solution is substantially free from cadmium ions.
• the use of tri and, preferably, tetradentate chelating agents and salts thereof, namely EDTA and its salts, also results in a solution that is less subject to changes of operating conditions. It is therefore possible to operate closer to the required content of gold in the final product, with considerable savings, and it is no longer necessary to interrupt the process to weigh the products for control purposes.
• the solution contains Au + and Cu + and the plated or formed product will comprise an alloy Au/Cu, free from cadmium. It was also found that the alloy deposition is substantially homogeneous and that accordingly the product is also homogeneous and substantially free from dramatic discontinuities in the composition of Au-containing product.
• the other conditions of the electroplating process are as follows:
• the products are degassed, preferably vacuum degassed, to improve their resistance by removing hydrogen possibly entrapped within the deposited material.
• degassing can be carried out by heating of the products.
• the ceramic material is preferably a carbide and most preferably is Boron carbide (B 4 C).
• the ceramic material is such that it is compatible with the complex of Au, i.e. the ceramic material is such that the electroactive complex containing gold (i.e. the gold complex that will deposit from the solution) can adsorb onto the surface of the ceramic material.
• the preferred concentration of Boron carbide in the solution is within 0.5 to 60 g/l, preferably from 2 g/l to 50 g/l; the particle size of the ceramic material - and in particular for boron carbide - is preferably within the range of 0.1 to 10 ⁇ m.
• the adsorbed electroactive complex will then be electrodeposited from the solution together with the particle of ceramic material.
• the resulting product has improved physical and mechanical properties, especially wear resistance, with respect to the traditionally obtained products, and a better ageing behaviour.
• the thickness of a product obtained according to the invention can be approximately half the thickness of a corresponding product obtained according to known processes, while giving the same performances.
• boron carbide or other corresponding suitable ceramic material can impart improved characteristics also to products obtained from Au/Cu/Cd solutions containing also free CN-ions (i.e. to known solutions).
• a further object of the invention is therefore a gold electroplating solution according to claim 6.
• Another object of the invention is a process according to claim 9.
• the concentration range of EDTA is equimolar to twice the moles of Cu and that of boron carbide is within 0.5 to 60 g/l.
• the products are degassed, preferably vacuum degassed, to improve their resistance by removing hydrogen possibly entrapped within the deposited material.
• degassing can be carried out by heating of the products.
• a further feature of the process is that the solution is stirred in order to impart a correct flow to the solution. More specifically, it was found that in order to obtain good results, especially if the solution contains a ceramic material, the flow of the solution should be substantially perpendicular to the surface of the model. In other words, the flow should be substantially perpendicular to the surface area on which the gold or gold alloy should be deposited. In order to do this, on the average every portion of the surface is subjected to a substantially perpendicular flow of solution for a same amount of time, so as to give a substantially uniform deposition of the gold alloy.
• the reactor was a cylinder provided with temperature control.
• Three basic solutions were prepared, namely Composition (g/l) A B C Au (as KAu(CN) 2 ) 7.5 7.5 7.5 Cu (as CuO) 2.5 2.5 2.5 EDTA disodic salt monohydrate 17 19.5 21 citric acid 60 40 30 ammonium citrate 20 40 60
• B 4 C (1500 mesh) at 2 g/l; 10 g/l and 50 g/l.
• the deposition substrate (cathodic material) was a plurality of hollow roundish supports made of copper, previously obtained by lost-wax electroforming.
• the supports were mounted on a rotatable carrousel.
• the anode was platinated titanium.
• the solution was stirred by the combined action of a magnetic stirrer and by rotation of the cathode at about 80 rpm.
• the bath temperature was of about 50 °C and its pH was within the range of 6.5 to 6.7.
• the current density was 12-14 mA/cm 2 and deposition speed was 22/28 ⁇ m/h.
• the deposition time was set to obtain, in the above operating conditions, deposits 120-200 ⁇ m thick.
• the amount of carbon boride particles incorporated within the deposit of gold alloy was within the range of 18% to 35% by volume and the specific weight of the products was within the range of 14.7 to 12.3 g/cm 3 .
• the matrix alloy title was not checked during the deposition process; nevertheless, the title of all the products was within the range of 750/1000 to 780/1000, i.e. consistent with the planned value of 750/1000 (18 carats).
• the hardness (Vickers microhardness - HV 0.025 ) of products with and without ceramic material was determined: products incorporating the ceramic material showed hardness values that were 20-25% higher than the values of corresponding products without the ceramic material.
• vacuum degassing is preferable to degassing only by heating: vacuum degassed samples have better values of fracture toughness.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electroplating And Plating Baths Therefor (AREA)

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Citations (4)

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• 1998
  • 1998-12-23 EP EP98124576A patent/EP1013799A1/de not_active Withdrawn
• 1999
  • 1999-12-22 WO PCT/EP1999/010279 patent/WO2000039367A2/en not_active Application Discontinuation
  • 1999-12-22 EP EP99966991A patent/EP1149190A2/de not_active Withdrawn

Patent Citations (4)

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