CH632532A5 - Method of producing an aqueous electrolyte solution for electroplating conducting surfaces with silver - Google Patents

Description

632532

PATENT CLAIMS

1. A process for the preparation of an aqueous electrolyte solution for the electrolytic deposition of silver on electrically conductive surfaces, characterized in that at least one cyanide-free solution is contained in an aqueous solution containing a thiosulfate and buffered to a pH of 3.5-7.0 Silver salt dissolves.

2. The method according to claim 1, characterized in that the silver salt is used in a concentration corresponding to 5-50 g / I of elemental silver.

3. The method according to claim 2, characterized in that an alkali metal thiosulfate is used as thiosulfate in a concentration of at least 2 mol per mol of silver.

4. The method according to claim 3, characterized in that a silver halide is used as the silver salt.

5. The method according to claim 4, characterized in that sodium thiosulfate is preferably used as a thiosulfate in a concentration of 2-4 mol per mol of silver.

6. An aqueous electrolyte solution prepared by the method of claim 1.

7. Solution according to claim 6, characterized in that it is buffered to a pH of 4.5-5.5.

8. Solution according to claim 6 or 7, characterized in that it also contains at least one gloss agent.

9. Solution according to claim 8, characterized in that it contains two brighteners, one of which is the potassium salt of an antimony-containing compound or complex and the other is an arylsulfonate.

10. Solution according to claim 9, characterized in that the one brightening agent is antimony potassium tartrate and the other is the sodium salt of a condensed naphthalenesulfonic acid.

Silver and gold plating from alkaline baths containing cyanide has been practiced since about 1800. In the late 1950s, when it was discovered

Acid gold electrolytes have been developed so that alkali gold cyanide is stable down to a pH of 3.0. Sodium and potassium silver cyanide do not show the stability of alkali metal gold cyanides in acidic solutions, and accordingly no corresponding acidic silver plating solutions were developed until the mid 1960s. It was then disclosed in U.S. Patent 3,362,895 that solutions of potassium silver cyanide buffered to a pH in the range of 6-7 are reasonably stable and that relatively small amounts of an alkali metal or ammonium thiocyanate stabilize such plating baths.

It is obvious that cyanide-free plating baths are highly desirable because the cyanide is poisonous and dangerous and its rejection is dangerous. In addition, the cyanide tends to accumulate in the cavities and depressions of plated objects and is not easily removed by rinsing.

It is an object of the present invention to provide a process for producing a cyanide-free acid bath for silver plating.

Any soluble, cyanide-free silver salt or a mixture of such salts can be used for the production of the electrolyte solution according to the invention, for example silver chloride, bromide, iodide, fluoride, sulfate, sulfamate, thiosulfate, fluoroborate, nitrate. The silver salt used in each case is generally chosen in view of its availability and price, from which it follows that silver chloride is preferred. The silver salt is expediently used in a concentration such that the electroplating bath contains 5-50 g / 1, preferably 20-40 g / 1, of elemental silver. In practice, it is impractical to use concentrations of less than 5 g / 1 or more than 50 g / 1 silver.

Without being restricted to this theory, it is believed that the thiosulfate used interacts with the silver to form silver thiosulfate and this is the reason that the electroplating bath remains reasonably stable. The thiosulfate should advantageously be used in such a concentration that at least 2 moles of thiosulfate per mole of silver are present in the electroplating bath. Although larger amounts of thiosulfate can also be used, it is generally preferred to keep the concentration at 2-4 moles of thiosulfate per mole of silver. The thiosulfate is conveniently added to the electroplating bath in the form of an alkali metal thiosulfate, with sodium thiosulfate being preferred, although any other alkali metal salt can be used.

The solution produced according to the invention is buffered to a pH value of 3.5-7.0, preferably 4.5-5.5, it being possible to use any combination of acid and salt which bring the solution to this pH range buffers. Suitable buffers are, for example, a combination of acetic acid and sodium acetate or of sodium hydroxide and potassium hydrogen phthalate, and a combination of sulfuric acid and sodium bisulfate is preferred.

In addition, an embodiment of the solution prepared according to the invention contains at least one, preferably two, gloss agents, e.g. the potassium salt of antimony-containing compounds or complexes, such as antimony potassium tartrate, and optionally an aryl sulfonate, such as the sodium salt of a condensed naphthalenesulfonic acid. Further addition of salts of nickel, cobalt, copper, iron, zinc, indium, arsenic, antimony and bismuth can act as a brightener in certain cases. Conventional salts for increasing the electrical conductivity that can be added are, for example, the sulfates, sulfamates, formates, acetates, citrates, lactates, tartrates, fluoroborates, borates, phosphates, of the aforementioned metals.

Electroplating using the solution prepared according to the invention can be carried out in a conventional manner. It is therefore most convenient to work at ambient temperature, although higher or lower temperatures can also be used. Generally the current density is 0.1-5.4 A / dm2 and the plating bath can be moved, if desired, by using a perforated whirlpool cathode or by moving the cathode. Any electrically conductive surface to be plated can be used as the cathode and conventional anodes, for example made of stainless steel, can be used as the anode.

A particular advantage of the solution described is that, just as with plating solutions containing cyanide, a silver anode can be used. This is economically advantageous since the anode releases silver into the electrolyte solution and the plating does not have to be interrupted continuously to supplement the concentration of the silver in the plating bath.

example 1

A plating bath was prepared by dissolving 29 g of silver chloride in 11 of an aqueous solution buffered to pH 4.5 and containing 100 g of sodium thiosulfate. A stainless steel cathode was placed in this bath at 20 ° C and a current density of 1 A / dm2 using

2nd

s

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IS

20th

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30th

35

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45

50

55

60

65

3rd

Turn of silver anodes electrolytically silvered. The electroplating obtained was very smooth and white in appearance.

5

Example 2

Example 1 was additionally used with 0.5 g / 1 of the sodium salt of a condensed naphthalene sulfonic acid and 3.5 g / 1 of antimony potassium tartrate as gloss

632532

medium repeated in the plating bath, a shiny silvering being obtained.

Example 3

According to the procedure described in Example 1, good silvering was obtained using an aqueous electrolyte solution buffered to a pH of 5.0, containing 35 g / 1 silver bromide and 120 g / 1 potassium thiosulfate.

B