CA1081651A

CA1081651A - Non-cyanide electrodeposition of silver

Info

Publication number: CA1081651A
Application number: CA279,441A
Authority: CA
Inventors: John M. Deuber
Original assignee: Oxy Metal Industries Corp
Current assignee: Oxy Metal Industries Corp
Priority date: 1976-06-09
Filing date: 1977-05-30
Publication date: 1980-07-15
Also published as: CH632532A5; DE2725312A1; JPS52150744A; FR2354398A1; FR2354398B1

Abstract

NON-CYANIDE ELECTRODEPOSITION OF SILVER

Abstract of the Disclosure An aqueous electrolyte solution for the electro-plating conductive surfaces with silver contains a non-cyanide silver salt and a thiosulphate.

Description

Background of the Invention Silver and gold have been plated from alkaline cyanide baths since about 1800. In the late 1950~s, acid gold electrolytes were developed when it was discovered that alkali gold cyanide was stable at a pH as low as 3Ø
Sodium and potassium silver cyanide do not have the stability of alkali metal gold cyanides in acid solutions and, conse-quently, no corresponding acid silver plating solutions were developed until the mid-1960's. The Foulke (U.S. Patent 3,362,895) found that potassium silver cyanide, if buffered in the region of 6 to 7 would remain reasonably stable and ;~
that relatively small amounts of an alkali metal or ammonium thiocyanate would stabilize the plating baths.
It is obvious that it would be highly preferred not to use the cyanide in plating baths because the cyanide is poisonous, hazardous and creates waste dlsposal hazards.
Moreover~ the cyanide ~:ends to accumulate in crevices and recessed areas of the plated article and is not easily removed by rinsing.
It is the ob~ect of this invention to provide an acid silver plating bath in which cyanides are not used.
This and other ob~ects of the invention will become apparent to those of ordinary skill in this art from the following detalled description.

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Summ~y of thc Invention This invcntion relatcs to non-cyanide acid silver plating solutions and more ~articularly to a silver plating solution containing a non-cyanide silvcr salt and a thiosul-phate.

Description of the Prefcrred Em~)odiments .
In accordance Yith the present invention, a non-cyanide acid silver plating solution is provided and con-tains a non-cyanide silver salt and a t}-iosulphate. As the ~ilver salt, any one or a mixture of soluble non-cyanide salts can be employed. ~or example, silver halides such as silver chloride, silver bromide, silver iodide and silver flor-ide, silver sulphate, silver sulfamate, silver thiosulphate, silver floroborate a~d silver nitrate can be employed. The particular salt is chosen based on availability and cost considerations and it l~ill be thereore recognized that the silver salt of preference is silv~r chloride. The silver salt is used in sufficient cluantities to provide a concen-tration of about 5 to 50 grams of silver per liter and preferably about 20 to 40 grams of silver per liter. It is not pract;cal to use less than 5 or more than 50 grams per litcr.
~ Yithout being limited to theory, it is believed that the thiosulphate employed coordinates with the sil~er to form silver t~liosulphate and this is the reason that the electroplating bath remains rcasonably stable. The t]lioCya-nate should be used in an amouJIt SUC]I that there is at least about two molcs of thiosulpllate per mole of silver in the clectroplating bath. Althougll greater cluantities of thio-sulphate can be used, it is gencrally preferred to keep thethiosulpllate concentration at about 2 to 4 molcs pcr mole of silver. Tllc tlliosulphate is adcled to the electro~lating bath in the form of an al~al; mctal t}liosulphate. Although any of the al~ali metals can be uscd, tlle ~referred thiosul-phate is sodium thiosulphate.
The acid silver electroplating bath of the instant invention is buffered to a p~ of about 3.5-7.0 and preferably about 4.5-5.5. Any combination of acid and salt can be used as long as it buffers the electroplating bath to this p~l range.
Por example, a combination of acetic acid and sodium acetatc or a combination of sodium hydroxide and potassium hydrogen phthalate are appropriate buffer combinations. A preferred buffer combination is sulphuric acid and sodium b;sulphate The electroplating baths of the instant invention can also contain other additives such as bri~hteners if desired.
Although most commonly kno~n brighteners are acce~tal~le, it is preferable to employ an antimony bearing compound or complex such as antimony potassium tartrate or an aryl sulfonate such ~-as the sodium salt of condensed naphthalene su]fonic acidJ
either alone or in combination. Incorpor~tion into the solu-tion of the salts of nickel, cobalt, copper, iron, zinc, indium, arsonic, antimony and bismuth, can in some instances act as brighteners. Conventional conductivity salts ~hich may be added comprise the sulfates, sulfam~tes, formates, acetates, citrates, lactates, tartrates, fluohorates, ~orates and pllosphates, of the aforcsaid base meta~s Silver plating with the solutions of the instant inventioJI follo-~s conventional practice. Tllus, it is most i appropriate to use ambient temperature alt]~ough hi~her or lower tcm~eratures can be employed if desired. In general, currcnt densities of about 1 to 50 amps per square foot are employed and the solution can be agitated by use of a sparger or moving cathode, if desired. Any electroconducting surface which is desired to be ~lated can he used as the cathode and conventional anodes such as stainless steel can be used.

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108~l651 A particular advalltage of thc instant acid silver solutions is that, li~e tllc cyanide plating solution, a silver anode can bc utilized, This is economically advantageous because the anodc will furnish silver to the electrolyte and plating need not be halted intermittently to re~lenish the silver concentration.
Example 1.
A bath was prepared by dissolving 29 ~rams of silver chloride in one liter of an aqueous solution containing 100 grams per liter of sodium thiosulphate buffered to a p~I
of about 4.5. A stainless steel cathode was plated in this bath at 20C. and 10 amps per square foot using silver anodes.
The resulting electroplate was very smooth and white in appearance.
Example 2.
Example 1 was repeated additionally employing brighteners in the bath. O.S grams per liter of the sodium salt of a condensed naphthalene sulfonic acid and 3.5 grams per liter of antimony potassium tartrate were so employed and a bright deposit was obtained.
Example 3.
Following the procedure o Example 1, a good silver de~osit was obtained using as the electrolyte an aqueous solution containing 35 grams per liter of silver bromide and 120 grams per liteT of potassium thiosulpllate buffered to a pH of about 5Ø
Various changes and modifications can be made in the non-cyanide acid silver plating solutions of the instant invention without departing from the spirit and scope thereof.

The various embodiments set forth herein were for the purpose of further illustrating the invention, but were not intended to limit it~

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An aqueous electrolyte solution for electro-plating conductive surfaces with silver comprising a non-cyanide silver salt and a thiosulphate buffered to a pH of about 3.5-7Ø

2. The aqueos electrolyte solution of claim 1 wherein the silver salt is present in a concentration of 5 to 50 grams of silver per liter of solution.

3. The aqueous electrolyte of claim 2 wherein the thiosulphate is an alkali metal thiosulphate and is present in an amount of at least two moles per mole of silver.

4. The aqueous electrolyte of claim 3 wherein the silver salt is a silver halide.

5. The aqueous electrolyte of claim 4 wherein the alkali metal thiosulphate is sodium thiosulphate.

6. The aqueous electrolyte of claim 5 wherein the sodium thiosulphate is present in an amount of 2 to 4 moles per mole of silver.

7. The aqueous electrolyte of claim 6 wherein the solution is buffered to a pH of about 4.5-505.

8. The aqueous electrolyte of claim 1 wherein the solution is buffered to a pH of about 4.5-5.5.

9. The aqueous electrolyte of claim 1 further comprising at least one brightener.

10. The aqueous electrolyte of claim 9 containing two brighteners, one of which is the sodium salt of an antimony bearing compound or complex and the other of which is an aryl sulfonate.

11. The aqueous electrolyte of claim 10 wherein the two brighteners are antimony potassium tartrate and con-densed naphthalene sulfonic acid.