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INVENTION
Patent of the Russian Federation RU2231579
A METHOD FOR OBTAINING A PROTECTIVE COATING ON THE SURFACE OF SILVER PRODUCTS
Applicant's name: OAO Ekaterinburg Non-Ferrous Metals Processing Plant (RU)
The name of the inventor: Timofeev NI (RU); Saltykova N.A. (RU); Studenok ES (RU); Ermakov A.V. (RU); Grokhovskaya L.G. (RU)
The name of the patent holder: JSC "Ekaterinburg Non-Ferrous Metals Processing Plant" (RU)
Address for correspondence: 620014, Yekaterinburg, Lenin Avenue, 8, JSC "Ez OTSM"
Date of commencement of the patent: 2003.01.05
The invention relates to methods of electrochemical preparation of protective coatings on the surface of articles made of silver and its alloys with copper and can be used in the manufacture of medals, cutlery, dishes, jewelry, etc. For their protection from tarnishing. The method includes cathodic treatment of products previously degreased in an electrolyte containing, g / l: potassium chromate 80-120 and sodium carbonate 60-80, at a cathode current density of 0.4-0.8 A / dm 2 for 2-10 Min, at a temperature of 5-60 ° C with an anode made of carbon-graphite material, subsequent washing and drying. The technical result: expansion of the functionality of the method due to the production of protective coatings that are visually indistinguishable on products from both silver and its alloys with copper, intensification and simplification of the method. In addition, the method is effective even in the case where the starting products have spots.
DESCRIPTION OF THE INVENTION
The invention relates to methods of electrochemical preparation of protective coatings on the surface of silver and its alloys with copper and can be used in the manufacture of medals, cutlery, dishes, jewelry, etc. To protect them from tarnishing.
A method for obtaining a protective coating on the surface of silver products is known, passivation with cathodic treatment in solutions containing (g / l): 100 potassium chromate and a small amount of chromic anhydride (up to pH 8-9) at 18-25 ° C, current density 1-6 A / dm 2 , the duration of 5-10 minutes. Stainless steel is used as the anode (S.Ya. Grihiles, Oxide and phosphate coatings of metals.) Galvanotechnics: Mechanical Engineering, 1978, p.83). With the passivation of small parts in the drum, the cathode current density is reduced to 1-1.5 A / dm 2 and processed for 30-40 minutes.
The Applicant has experimentally carried out a known method in which a silver article was used as a cathode in one case, a product of a silver alloy with copper of the grade CpM-925 was used as an anode, stainless steel as an anode, and the current density was 5 A / dm 2 , Its following shortcomings were revealed:
- On silverware, the coating had a yellowish tinge, and on the products of the CmM-925 alloy, the coating was uneven and had a dark color, which excludes its use for silver-copper alloys, and for silver worsens the product appearance of the products.
- The anode of stainless steel in the process of passivation dims, that is, there is a transition of the components of the anode in the electrolyte, which clogs it. Over time, this leads to a deterioration in the quality of the coating.
Common to the known and claimed methods is the cathodic treatment of silver articles in an aqueous electrolyte containing potassium chromate.
The closest to the claimed is the method of obtaining a protective coating on the surface of silver products - passivation - by cathodic treatment in an electrolyte containing (g / l): 100-150 potassium chromate (potassium chromate) and 1-2 sodium carbonate, at an electrolyte temperature of 18- 22 ° С, density of 1 A / dm 2 for 35 minutes. After the treatment, the products are washed and dried for 5-10 minutes at a temperature of 60-70 ° C (V.I. Marchenkov, Jewelcrafting, Moscow: Higher School, 1984, p. 177) .
The known method is intended for the application of protective coatings to articles of silver and is not suitable for the protection of silver alloys with copper. The inventor experimentally reproduced the indicated regime (electrolyte composition (g / l): 120 potassium chromate, 2 sodium carbonate, temperature 20 ° C, cathode current density 1 A / dm 2 , process time 35 min) and local darkening of the surface of passivated products was detected , Which is unacceptable. In addition, the method is not sufficiently intensive, as a result of which it has a considerable duration, and is complicated in implementation because of the need for temperature control and maintenance of the required process regime.
Common features for the known and claimed methods of obtaining a protective coating on the surface of silver products are: cathodic treatment of articles using an electrolyte containing potassium chromate and sodium carbonate, and subsequent washing of the products from the electrolyte and drying.
The invention is aimed at enhancing the functionality of the method by providing protection coatings not only on silver, but also on its alloys with copper, to obtain coatings that visually differ from the color of the original products, to intensify the process and simplify it.
This is achieved by the fact that in the process for obtaining a protective coating on the surface of silver articles including their cathodic treatment using an electrolyte containing potassium chromate and sodium carbonate, subsequent washing and drying, according to the invention, the electrolyte contains the components in the following ratio, g / l: chromate Potassium 80-120, sodium carbonate 60-80, while cathodic treatment of previously degreased and water-washed articles of silver and its alloys with copper is conducted at a cathode current density of 0.4-0.8 A / dm 2 for 2-10 min At a temperature of 5-60 ° C with an anode made of carbon-graphite material. In addition, the products before cathodic treatment are kept in a 10% by weight boiling aqueous solution of sulfuric acid, and then washed and polished.
THE METHOD IS PROVIDED AS FOLLOWS:
Products made of silver and its alloys with copper (4-8% by weight) - jewelry, medals, dishes, cutlery - are degreased, washed with water, then subjected to cathodic treatment in an electrolyte containing potassium chromate and sodium carbonate, with the following ratio , G / l:
Potassium Chloride 80-120
Sodium carbonate 60-80
Cathodic processing (passivation) of the articles is carried out at a temperature of 5-60 ° C, a current density of 0.4-0.8 A / dm 2 for 2-10 minutes. With a lower current density and a holding time, the process becomes ineffective, with a higher current density, the coating on the product acquires a darker color, with a longer duration, the thickness of the coating increases and the product has a blue hue, which changes its decorative properties, and this is not always permissible. The cathode is a silver or copper alloy product, and an anode is a carbon graphite material, for example in the form of a plate. Then the products are washed from the electrolyte in running water and dried (in a warm air stream or wiped with a soft cloth). The aqueous electrolyte works stably for one to two months.
In case the starting product has stains, for example, soldering under a flux, before the cathodic treatment the article is kept in boiling aqueous solution of 10 wt% sulfuric acid before the appearance of white plaque, then it is washed in running water and the white coating is removed by polishing. After degreasing and washing, the product is subjected to cathode treatment.
Tests of the samples with a protective coating were carried out over a 1 wt% aqueous solution of sodium sulfide (Na 2 S) with traces of hydrogen sulfide for 10 days. The surface of products made of silver and its alloys with copper, protected by coating, did not change in color, unlike samples of products that did not have a protective coating.
Example 1
Cathodic passivation of products made of the CPM-925 alloy (12 pieces of medals) with a total area of 260 cm 2 was carried out after they were degreased with gasoline and washed in running water in an electrolyte containing: 120 g of potassium chromate and 65 mg of sodium carbonate . The products served as a cathode. A plate made of glassy carbon was used as the anode. The processing was carried out at an electrolyte temperature of 25 ° C, a cathode current density of 0.6 A / dm 2, and lasted 10 minutes. Washing products from the electrolyte was carried out with running water, followed by drying in a stream of warm air. The surface of the products had a light appearance, visually did not differ from the color of the original products. When stored for three months, darkening of the surface of passivated products did not occur.
Example 2
Pre-degreased and washed silver plates of grade C9999 in the amount of 8 pieces with a total area of 40 cm 2 were subjected to cathode treatment. Passivation was carried out in an electrolyte containing, g / l: 90 chromate of potassium and 60 sodium carbonate at a temperature of 15 ° C, cathode current density of 0.8 A / dm 2 for 5 minutes. As an anode, a plate of glassy carbon was used. Then the products were washed in running water and dried, wiped with a soft cloth. The color of the silver plates did not change after passivation. When these plates were held above a 1 wt% Na 2 S aqueous solution in a sealed vessel, darkening was not observed for three days, unlike silver plates that had not undergone cathodic treatment.
Example 3
The cathode treatment was subjected to three glasses of the CpM-960 alloy with a total area of 270 cm 2 , having dark spots as a result of soldering under the flux. Before processing, the glasses were held in a boiling aqueous 10% by weight solution of sulfuric acid until a white coating appeared. After washing in running water, white coating was removed by polishing on the felt, degreased, washed with water and subjected to passivation in the electrolyte of the composition, g / l: 100 potassium chromate and 80% sodium carbonate at 50 ° C, cathode current density 0.4 A / dm 2 in For 3 minutes. A plate made of glassy carbon was used as the anode. Washed in running water and dried in air. When these products were stored for 6 months, darkening of the passivated surface did not occur while on darkened patches from the same batch dark spots from soldering became black, and the entire surface of the products acquired an uneven dark color.
As can be seen from the examples given, the proposed method has wider functionality as compared to the known one, since it allows obtaining qualitative, reliable and visually undetectable protective coatings both on silverware and on products made of its alloy with copper. The claimed method is easy to implement, since it does not require temperature control of the electrolysis process regimes, and has a high intensity, which makes it possible to shorten its duration. In addition, the claimed method is also quite effective for cases where the starting products have dark spots.
CLAIM
1. A method for producing a protective coating on the surface of articles of silver and its alloys with copper, including cathodic treatment using an electrolyte containing potassium chromate and sodium carbonate, subsequent washing and drying, characterized in that the electrolyte contains components in the following ratio, g / L:
Potassium Chloride 80 - 120
Sodium carbonate 60 - 80
While cathodic treatment of defatted articles is conducted at a cathode current density of 0.4-0.8 A / dm 2 for 2-10 minutes at a temperature of 5-60 ° C with an anode made of carbon graphite material.
2. A method according to claim 1, characterized in that the articles are kept in a 10% by weight boiling aqueous solution of sulfuric acid before the appearance of a white coating before cathodic treatment, and then washed and polished.
print version
Date of publication 10.12.2006гг
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