US3215610A - Method and bath for electrodepositing bright silver - Google Patents

Description

United States Patcnt O 3,215,610 METHOD AND BATH FOR ELECTRO- DEPOSITING BRIGHT SILVER Willi Skaliks, Berlin-Zehlendorf, Germany, assignor to Schering, A.G., Berlin, Germany No Drawing. Filed May 8, 1962, Ser. No. 193,302

Claims priority, application Germany, May 19, 1961, Sch 29,728

14 Claims. (Cl. 204-46) This invention relates to a method of forming a bright silver deposit by electrolysis of an aqueous bath, and to a bath suitable for performing the method.

It is known in the art that additions of small amounts of tetravalent or divalent selenium to an otherwise conventional alkaline plating bath containing a complex silver salt such as silver cyanide improves the brightness of the electrodeposit formed when the article is to be plated at the cathode in such a bath. However, it has been found that the known silver plating baths containing selenium compounds as brighteners are not entirely satisfactory. The known baths containing quadrivalent selenium have no leveling effect, and actually may enhance pre-existing surface defects such as grinding marks.

The known divalent selenium brighteners are free from this shortcoming, but the bright plating range available with divalent selenium compounds as brighteners in the electrodeposition of silver is substantially limited to cath ode current densities of 1 ampere per square decimeter or more. Deposits produced at lower current densities have a hazy or milky appearance.

It has been found that the brightness of silver deposits electrolytically produced in the presence of compounds of selenium, which selenium has the valence of minus 2, can be greatly improved, and that the bright plating range can be extended to a lower limit which is a small fraction of one ampere per square decimeter, and that other valuable properties may be imparted to the silver electrodeposit by employing with the selenium brightener a condensation product of naphthalene-alpha-sulfonic acid with formaldehyde. The formula of this product is wherein X is a cation which permits the condensation product to be dissolved in the alkaline electrolyte, and does not give rise to undesirable decomposition products in the bath, and n is an integer from 4 to 6.

The compounds of the above formula in which- X is either sodium or potassium arereadily available, but other alkali metal salts and the ammonium salt are equally usable since the cation does not appear to participate in the brightening effect if it is stable in the solution.

It has also been found that the afore-described condensation product is an excellent silver brightener when emloyed in the absence of a selenium compound. Whether used with or without an auxiliary selenium brightener, the condensation product is employed in concentrations of 0.02 to 0.5 gram per liter of the plating bath. The actual concentration chosen will depend upon the concentration of the other conventional constituents of the bath, on the deposition rate, on the result desired, and on other variables in a manner well known in this art.

The condensation product of naphthalene-alpha-sulfonic acid and formaldehyde is available as an article of 3,215,610 Patented Nov. 2, 1965 commerce, and is being sold for example, under the trade names and trademarks Solegal A, Daxad 11, and Compound 8-Alpha in the United States and abroad.

Suitable selenium compounds include the selenides of the alkali metals and of ammonium, selenocyanides, and numerous organic selenium compounds in which selenium is divalent and negatively charged, and which are well known to have some brightening effect when added to complex silver salt solutions in the absence of the condensation product of naphthalene sulfonic acid and formaldehyde.

The electrolytes of this invention are surprisingly resistant to contamination by dust and other airborne contaminants. It has also been found that excellent bright silver deposits can be obtained from solutions used over a long period of time without forming a sludge of cyanide decomposition products and of other impurities in the bath. The silver deposits formed have a distinct leveling effect so that surface irregularities of the article coated with the silver deposit tend to be obscured as the thickness of the silver layer increases. Deposits formed in the presence of approximately 0.001 to 0.1 gram per liter of divalent selenium are mirror bright when deposited at current densities ranging at least from 0.1 to 5.0 amperes per square decimeter.

The silver deposit is dense and free from pores even when formed in the absence of the wetting agents normally employed to prevent gas bubbles from adhering to the plated surface. The full brightness of the silver deposit is apparent as soon as the first visible layer of silver is deposited. Silver layers as thin as 0.2 micron show uniform mirror brightness. Because of the wide bright plating range of the electrolytes of the invention, the control of cathode current density is not critical, and uniform bright deposits are readily formed on articles of complex shape without the use of auxiliary anodes or similar expedients.

. The deposits have an initial hardness of to kilograms per square millimeter on the Vickers scale. Hardness drops after some time to approximately 90 kilograms per square millimeter. In contrast to other methods used, this hardness is still unusually high, and makes the silver deposits of the invention eminently suitable for use on articles subject to mechanical wear such as cutlery, switches, and other electrical contacts.

Also, it is to be noted that the electroplating baths of the invention are relatively insensitive to variations in op erating conditions. Neither temperature nor current density is particularly critical, and they do not unfavorably affect the properties of the silver plating produced when held within reasonable limits readily maintained without complex controls. .The fact that wetting agents are not necessary to control porosity increases the useful life of the solutions which are not contaminated by decomposition products of the surface-active agents. Periodic or continuous filtration of the baths and purification with activated charcoal are not necessary with the silver plating baths of the invention.

Silver is commercially deposited from complex silver cyanide solutions almost to the exclusion of other complex salts of the metal. In the following illustrative specific examples of the method of the invention, and of the baths employed for performing the method, complex silver cyanide baths therefor have been described. However, it is to be understood that the invention is not to be limited to such examples. but the invention is not limited to any particular complex silver salt, and those skilled in the art may substitute other alkaline complex silver solutions and employ ade quate voltage to decompose silver complexes other than the cyanide complex.

Such baths are preferred,-

3 Example 1 An electrolyte is prepared by dissolving in one liter of water under ordinary conditions, the following:

40.0 grams silver cyanide 130.0 grams potassium cyanide 20.0 grams potassium carbonate 0.04 gram selenium as K Se 0.07 gram condensation product of formaldehyde and naphthalenesulfonic acid in the form of the sodium salt Mirror bright silver deposits are obtained over a current density range from 0.1 to 4.0 amperes per square decimeter when either the electrolyteor the cathode is agitated. The brightness of the deposit is virtually unaffected by temperature variations between and 30 C.

Example 2 An electrolyte is prepared from one liter of water under ordinary conditions and 30.0 grams silver cyanide 120.0 grams potassium cyanide 30.0 grams potassium carbonate 0.06 gram selenium as KCNSe 0.10 gram condensation product of formaldehyde and naphthalene-alpha-sulfonic acid in the form of the potassium salt Mirror bright silver deposits are. obtained at current densities between 0.1 and 4.0 amperes per square decimeter and at temperatures between 10 and 30 C. when either the electrolyte is agitated or the article to be plated is suspended from a reciprocating cathode rod in the usual manner.

Example 3 An aqeuous electrolyte is prepared from one liter of water and 25.0 grams silver cyanide 130.0 grams potassium cyanide 30.0 grams potassium carbonate 0.2 gram condensation product of formaldehyde and naphthalenesulfonic acid in the form of the potassium salt Semi-bright silver deposits are obtained at cathode current densities of 0.2 to 3.0 amperes per square decimeter, and at temperatures between 10 and 30 C. when relative movement of electrolyte and cathode is providedby either agitating the bath or moving the article which is being plated.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not limited thereto, but is to be construed broadly, and is restricted solely by the scope of the appended claims.

I claim:

1. A bath for the electrolytic deposition of silver comprising an aqueous alkaline solution of a complex silver salt and an effective amount of a brightener dissolved in said solution and having the formula SOaX wherein n is an integer from four to six; and X is a cation stable in'said solution.

2. A bath as set forth in claim 1, wherein X is an alkali metal.

3. A bath as set forth in claim 2, wherein X is sodium.

4. A bath as set forth in claim 2, wherein X is potassium.

5. A bath as set forth in claim 1, wherein the concentration of said brightener is between substantially 0.02 and 0.5 gram per liter.

6. A bath as set forth in claim 1, further comprising as an auxiliary brightener a compound of selenium dissolved in said solution, said selenium compound containing selenium in the divalent and negatively charged form.

7. A bath as set forth in claim 6, wherein the concentration of said selenium is substantially between 0.001 and 0.1 gram per liter.

8. Abath as set forth in claim 1 wherein said complex silver salt is a complex cyanide.

9. A method of electrodepositing silver on a conductive article which comprises making said article the cathode in an aqueous alkaline solution of a complex silver salt containing as a brightener an effective amount of a compound dissolved in said solution and having the formula SIOQX wherein n is an integer from four to six; and X is a cation stable in said solution.

10. A method as set forth in claim 9, wherein X is an alkali metal selected from the group consisting of sodium and potassium.

11. A method as set forth in claim 10, wherein the concentration of said brightener is between substantially 0.02 and 0.5 gram per liter.

12. A method as set forth in claim 9, wherein said solution further contains as an auxiliary brightener a dissolved compound of selenium, said selenium compound containing selenium in the divalent and negatively charged form.

13. A method as set forth in claim 12 wherein the concentration of said selenium in said solution is substantially between 0.001 and 0.1 gram per liter.

14. A method of electrodepositing silver on a conductive article which comprises making said article the cathode at a predetermined voltage in an aqueous alkaline solution of a complex silver salt containing as a brightener 0.02 to 0.5 gram per liter of a compound dissolved in said solution and having the formula SOaX wherein n is an integer from four to six, and X is an alkali metal selected from the group consisting of sodium and potassium and 0.001 and 0.1 gram per liter of selenium in the divalent and negatively charged form, said selenium being present as a compound of selenium dissolved in said solution, said voltage being sufficient to deposit silver from saidsolution on said conductive article.

References Cited by the Examiner UNITED STATES PATENTS 2,195,409 4/ 40 Flett 20446 2,777,810 l/ 57 Ostrow 20446 JOHN H. MACK, Primary Examiner.

JOHN R, SPECK, Examiner.

Claims (2)

1. A BATH FOR THE ELECTROLYTIC DEPOSITION OF SILVER COMPRISING AN AQUEOUS ALKALINE SOLUTION OF A COMPLEX SILVER SALT AND AN EFFECTIVE AMOUNT OF A BRIGHTENER DISSOLVED IN SAID SOLUTION AND HAVING THE FORMULA

6. A BATH AS SET FORTH IN CLAIM 1, FURTHER COMPRISING AS AN AUXILIARY BRIGHTENER A COMPOUND OF SELENIUM DISSOLVED IN SAID SOLUTION, SAID SELENIUM COMPOUND CONTAINING SELENIUM IN THE DIVALENT AND NEGATIVELY CHARGED FORM.