Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/16—Polishing
  • C25F3/22—Polishing of heavy metals
• • A—HUMAN NECESSITIES
  • A44—HABERDASHERY; JEWELLERY
  • A44C—PERSONAL ADORNMENTS, e.g. JEWELLERY; COINS
  • A44C27/00—Making jewellery or other personal adornments
  • A44C27/001—Materials for manufacturing jewellery
  • A44C27/002—Metallic materials
  • A44C27/003—Metallic alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C5/00—Alloys based on noble metals
  • C22C5/04—Alloys based on a platinum group metal

Definitions

• the present invention relates to a method for plasma polishing a timepiece or jewelry item comprising at least two pieces made of at least 750% gold or at least 800% platinum, which are assembled together by brazing.
• the invention also relates to a brazing composition, as well as to a timepiece or jewelry item having a braze whose material comprises such a composition.
• Electropolishing, or electrochemical polishing is a well-known technique that is used to remove material from the surface of a metal part.
• Electropolishing a part usually requires a current generator (direct or pulsed), an electrolyte, a cathode (negatively polarized) and an anode (positively polarized).
• the cathode is the seat of reduction reactions, while the anode is the seat of oxidation reactions.
• the best known electropolishing solutions are those dedicated to steels and copper metals. They are composed of a mixture of acids at different concentrations and sometimes organic additives.
• Electropolishing has a domain where the metal to be treated dissolves preferentially, called the electropolishing domain or electropolishing plateau, range or step. It is in this domain that a viscous film forms on the surface of the anode, which allows the polishing of the part.
• Electropolishing allows macro-smoothing and micro-smoothing of the surface, depending on the nature of the electrolyte used. Macro-smoothing is defined as the removal of roughness and defects greater than 1 ⁇ m in height, and micro-smoothing as the removal of roughness and defects less than 1 ⁇ m.
• Electropolishing does, however, have some drawbacks.
• the electrolyte used is often harmful to health or the environment.
• the degree of gloss of the treated surface is sometimes insufficient depending on the type of parts for which they are intended. Some micro-roughness may also still be visible after treatment.
• Plasma polishing or “Plasma Electrolytic Polishing” in English (acronym PEP), can at least partially resolve these drawbacks.
• This type of process makes it possible to smooth the surface of a metal part to be treated, by generating a plasma at the surface of the part.
• Plasma polishing involves placing the metal workpiece to be polished as an anode in an electrochemical cell, the nature of the electrolyte of which has been previously determined.
• a voltage usually between 100 V and 1000 V, is applied across the electrochemical cell to generate a plasma membrane on the surface of the metal workpiece to be treated, which acts to remove material from the surface of the workpiece to be polished.
• the plasma membrane is maintained for a period effective to cause satisfactory reduction in the roughness of the workpiece.
• the document FROM 10 2021 000850 describes an electrolyte for use in plasma polishing of precious metal objects.
• Document D2 RU 2 116 876 describes a solder material comprising platinum, silicon and palladium.
• the document JP 2000 052087 describes a brazing material based on platinum and boron.
• the document JP S58 1036 describes alloys and their use in making nozzles for spinning rayon or acrylic fibers.
• Plasma metal polishing has several advantages over electrochemical polishing. It uses electrolytes that are less hazardous to health and less harmful to the environment, particularly without cyanide derivatives. It also allows access to the entire exterior surface of the part, regardless of the complexity of its geometry, and leads to effective and uniform roughness reduction.
• the brazing is likely to deteriorate during the plasma polishing process. More specifically, this process can lead to uneven material removal between the metal elements and the brazing, particularly when the materials of the metal elements and the brazing are different, whether in their nature or in their contents.
• Brazing consists of assembling two metal parts by melting a filler metal whose melting point is lower than that of the metal parts.
• One aim of the invention is to provide a plasma polishing method for a watch or jewelry item, making it possible to overcome the aforementioned drawbacks (alteration of the solder and unsatisfactory visual appearance).
• the invention aims in particular to provide such a plasma polishing method for treating a watch or jewelry item comprising two metal parts assembled by brazing, leading to a homogeneous removal of material from the metal parts and the brazing.
• the method thus aims to avoid structurally and visually degrading the brazing.
• the plasma polishing process according to the invention is particularly suitable for the luxury industry by improving the aesthetics of the brazing while limiting material loss.
• the watch or jewelry item comprises two metal parts assembled by a platinum alloy brazing.
• the metal parts joined together by brazing are made of gold alloy or platinum alloy.
• the alloy when the alloy is gold-based (at least 750% by mass of gold or at least 18 carats), it may, for example, include at least 917% by mass of gold (22 carats). This may include yellow gold, white gold, pink gold or red gold.
• the alloy is based on platinum (at least 500 ⁇ by mass of platinum), it advantageously comprises at least 800 ⁇ by mass of platinum, more advantageously at least 950 % by mass of platinum, for example 953 % by mass of platinum.
• the alloy when the alloy is based on palladium (at least 500 ⁇ by mass of palladium), it advantageously comprises at least 800% by mass of palladium, more advantageously at least 950% by mass of palladium, for example 953% by mass of palladium.
• the two metal parts joined by brazing comprise the same alloy (gold or platinum or palladium), at least at the brazing level.
• one metal part is made of gold alloy and the other metal part is made of gold alloy of different color and/or different carat weight, at least at the brazing level.
• one metal part is made of a gold alloy and the other metal part is made of a platinum or palladium alloy, at least at the brazing point.
• one metal part is made of a platinum alloy and the other metal part is made of a palladium alloy, at least at the brazing point.
• the watch or jewelry item may include precious stones (diamonds, emeralds, rubies, sapphires).
• the watch or jewelry item may be a pendant, a ring, earrings, a necklace, a bracelet, etc.
• brazing is an operation of joining two metal parts by melting a filler alloy, without melting the alloys of the metal parts.
• the melting temperature of the filler alloy (braze) is lower than that of the alloys of the assembled metal parts, so that the alloy of the metal parts and the filler alloy (braze) differ at least by the nature or by the contents of the metals.
• Platinum alloy braze is able to resist structural degradation when used in the plasma polishing process.
• the platinum alloy braze comprises at least 500% by mass of platinum and at least one alloying element selected from the group consisting of: silicon, copper, germanium, gallium, silver, tin, gold, indium, bismuth and aluminum, the sum of the contents of platinum and alloying elements being equal to 1000 ⁇ .
• the platinum alloy used to form the braze has a melting temperature lower than that of the alloys of the two metal parts, at least at the braze.
• this alloy constituting the braze can have a eutectic composition, that is to say a composition based on platinum whose behavior, during its fusion, corresponds to that of a pure body.
• the platinum alloy has a melting temperature of less than 1000°C, advantageously less than or equal to 800°C. It is advantageously greater than or equal to 100°C, more advantageously greater than 400°C. These temperature ranges allow industrial optimization, while avoiding degrading the metal parts during brazing.
• the platinum alloy (braze) is an alloy comprising at least 500 ⁇ by mass of platinum, for example at least 800 ⁇ .
• the sum of the platinum and alloying element contents being equal to 1000 ⁇ .
• the platinum alloy (braze) used may in particular be chosen from the group consisting of: Pt 850.3 Si 23.2 Cu 99.8 Ge 26.5 and 0.2 %o impurity; Pt 849.3 Si 43.5 Ge 107.2 ; Pt 814.7 Si 13 Cu 76.2 Ge 15.7 Ga 80.4 ; Pt 807.6 Ga 192.4 ; Pt 823.7 Ge 176.3 ; Pt 805 Ga 95 Ag 100 ; Pt 850 Si 30 Ag 120 ; Pt 961 Si 39 ; Pt 968 Si 34 ; Pt 963 Si 37 ; Pt 941 Si 39 Cu 20 ; Pt 948 Si 34 Cu 20 ; Pt 953 If 34 Au 13 and Pt 950 If 37 Au 13 .
• the sum of the contents in thousandths ( ⁇ ) of the metals in the platinum alloy (braze) is equal to 1000 thousandths (1000% o).
• the present invention also relates to this brazing.
• the electrolyte is advantageously an aqueous solution of salts or organic compounds.
• it may be an aqueous solution comprising a compound selected from ammonium sulfate, ammonium halides such as ammonium fluoride or ammonium chloride, citric acid, urea, glucose, sodium saccharin, ammonium nitrate and mixtures thereof.
• the electrolyte comprises ammonium sulfate and ammonium fluoride when the metal parts are made of gold alloy.
• the electrolyte comprises ammonium chloride and ammonium nitrate when the metal parts are made of platinum alloy or palladium alloy.
• the electrolyte may also include media, such as beads.
• the watch or jewelry item corresponds to the anode of the electrochemical cell while the cathode is made of conventional metal (for example titanium).
• the generation of a plasma is achieved by applying an electrical voltage between the anode and the cathode, advantageously between 100 and 1000 V, more advantageously between 300 and 400 V, for example between 320 and 350 V.
• the electrical voltage is lowered after immersion of the watch or jewelry item in the electrolyte.
• the duration of the polishing treatment is advantageously between 1 minute and 120 minutes, more advantageously between 5 minutes and 120 minutes, more advantageously between 30 minutes and 60 minutes.
• the electrolyte is at a temperature advantageously between 50 and 95°C, more advantageously between 60 and 95°C, even more advantageously between 55 and 80°C, for example between 60 and 70°C.
• the electrolyte can be heated before or after applying the electrical voltage.
• Applying an electrical voltage in the electrochemical cell allows to generate a plasma membrane on a surface of the brazing, but also on the metal parts. More precisely, a viscous film forms on the surface of the anode and causes polishing by dissolution in a homogeneous manner.
• the viscous film does not form and the granular appearance may even be accentuated due to a possible attack on the grain boundaries of the metal parts.
• the plasma polishing process according to the invention makes it possible to optimize and control material removal over the entire part subjected to polishing.
• reaction hypotheses exist, including, at the anode, the following oxidation reactions: 2H 2 O (l) ⁇ 4H + (aq) + O 2 (g) + 4e - M (s) ⁇ M n+ (aq) + ne -
• M represents a metal (metal parts and brazing).
• the metal oxidizes, thus losing n electrons denoted e - , and becomes positively charged with a corresponding charge n+.
• the initially solid metal passes into solution in ionic form, leading to a removal of material from the surface of the metal parts and the brazing.
• O 2 (g) in the form of gas is observed.
• the cathode can bring into play the corresponding reduction reactions: 2H + (aq) + 2e - ⁇ H 2 (g) M n+ (aq) + ne - ⁇ M (s)
• the metal ions M n+ (aq) in solution pass into solid form M (s) by depositing on the surface of the cathode.
• a release of hydrogen H 2 (g) is observed in the form of gas.
• the polishing conditions and the nature of the gold and platinum alloys used make it possible to obtain a uniform polishing over the entire surface of the metal parts and the brazing.
• the brazing is also not degraded, and also has an excellent visual appearance, comparable to that of metal parts.
• the polishing process according to the invention makes it possible to reduce the surface roughness to a Ra of 0.22 ⁇ m or even less.
• the watch or jewelry item having been subjected to the plasma polishing process according to the invention, has a surface roughness of less than 0.5 ⁇ m, more advantageously less than 0.3 ⁇ m, for example between 0.01 ⁇ m and 0.25 ⁇ m.
• This article was then subjected to a plasma polishing process in an electrochemical cell comprising an electrolyte (aqueous solution of ammonium sulfate and ammonium chloride).
• a voltage of 365 V is applied between the anode (Pt 953 Cu 47 article) and the cathode (titanium) so as to generate a plasma membrane on the surface of the article. This voltage is applied for a period of 30 minutes while the temperature of the electrolyte is 65°C.
• the article thus polished has a homogeneous appearance and a surface roughness of between 0.01 ⁇ m and 0.3 ⁇ m.
• an identical article having undergone conventional electropolishing has a certain heterogeneity making its visual appearance unsatisfactory.
• Plasma polishing thus provides an alternative to manual polishing.
• the advantages of plasma polishing include its ease of implementation and low cost, while allowing industrial optimization of polishing, which is not possible for manual polishing.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Manufacturing & Machinery (AREA)
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• Organic Chemistry (AREA)
• Mechanical Engineering (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Adornments (AREA)

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Citations (4)

• 2023
  • 2023-08-24 EP EP23193168.4A patent/EP4512938A1/de not_active Withdrawn
• 2024
  • 2024-08-20 EP EP24195449.4A patent/EP4512939A1/de active Pending

Patent Citations (4)

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