US2176668A - Silver plating process - Google Patents

Description

Patented Got. E7, 1939 UNETEE STATiE arrests earns arses m sses sravaa rna'rme raocass Jersey No Drawing.

Appliwtion February it, 1938, Serial No. 189,766

2 Claims. (or. roe-1e) This invention relates to ,electrodeposits of silver and similar metals, including silver alloys, and to a process of electrodeposition thereof.

This application is a continuation in part of our co-pending application Serial No. 70,396,

' filed March 23, 1936, now Patent No. 2,110,792.

In the present accepted commercial practice of electroplating silver coatings, the standard bath is composed of silver cyanide, an excess oi alkali cyanide over that required to dissolve the silver cyanide, alkali carbonate and water. Through this solution an electric current of suitable characteristics is passed, the current passing into the solution through silver anodes of high purity, through the solution depositing sll-.

ver on the articles to be plated which constitute the cathodes, and; then out of the bath through the cathodes. Under proper conditions of current, temperature, agitation and maintenance of a proper concentration of the electrolyte, a crystalline, non-lustrousmatte white deposit of.

silver may be secured at the cathode] Bumng or burnishing, sometimes both, of such a deposit .is, therefore, necessary to secure a bright .sur-

face. This bumng and burnishing is costly and wasteful and accordingly highly undesirable. It is, therefore, common practice to add to the above plating bath a brightener which will give the deposit, as it is made in the bath, a slightly lustrous appearance. For producing such a deposit, carbon-disulphide has lawn proposed and used commercially for nearly years. A number of other brighteners have been also proposed such as gums, sugars, sodium thiosulphate, tellurium and selenium compounds and others. The only one in extensive commercial use, however, is carbon-disulphide and the other proposed brighteners have never come into extensive commercial use.

Carbon-disulphide although known and used for nearly a century has certain well-recognized the quantity of carbon-disulphide present, the

current density, temperature, agitation and solution composition must all be adjusted and maint-ained within closely balanced limits. The carbon-disulphide limits are far narrower than required by the other components of the solution,

namely, the silver cyanide, alkali cyanide, carbonate and water. One dimculty in maintaining the proper quantity of carbon-disulphide present is that it is volatile at room temperature and this, in addition to its disappearance due to electrolysis, causes it to varnish rapidly from the solution making frequent additions necesssary. Consequently, the precautions which have to be taken in dealing with this material are numerous. plating solutions including carbon-disulphide are used, even with the best of control, there are always certain solutions which perform unsatisfactorily and from which dull deposits are ob-' tained, and even in solutions giving semi-lustrous deposits, there are always variations in appearance from vat to vat and even from section to section on the same article. In general, in employing this material, trial-and-error methods of plating have to be employed to secure the sulphide whatsoever. In addition, we have found with carbon-disulphide that only a relatively thin plating of semi-lustrous character can he obtained, in that, with increasing thickness of the deposit, the deposit changes from the semi-lustrous appearance to, eventually, a dull white appearance.

The principal object of the present invention accordingly is to provide a deposit or coating of silver that has as deposited an inherent, smooth, uniform, highly lustrous character even for the thickest commercial coatings. of the invention is to provide a process for the production of such a deposit or coating which will do away with at least some of the objections semi-lustrous deposits. Departure from condi- Another object to carbon-disulphide and other brighteners heren tofore. employed,

In accordance with our invention we have found that highly lustrous deposits or platings of silver may be produced under proper conditions of ingredient proportions and other char- 4 acteristics of the solution and current employed, by electrolyzing a cyanide silver bath containing silver cyanide, alkali cyanide, and alkali carbonate in the presence of a brightener, comprising any of the group of chemical compounds 5 represented by the formula It is a fact that where many individual 10 where X is a member of the group consisting of Nil-I2 and substituted NH: in which the substituents for hydrogen are methyl, ethyl, acetyl or NHz. When X is NH: the formula represents urea deposit is secured superior in luster to any such deposits .or coatings heretofore produced. The deposits and plating bath also have to a much less extent the disadvantages enumerated above for carbon-disulphide and in some cases these disadvantages are completely eliminated.

The following is a preferred example of a plating bath in accordance with the invention employing urea as a brightener, and a preferred method of using the same. The electrolytic bath contains Silver cyanide grams 25 Potassium cyanide do 45 Potassium carbonate-. do 20 Urea dn 45 Water liter 1 At a temperature of 80 F. lustrous silver deposits may be obtained at a current density of 6 amperes per sq. ft. of cathode surface.

Generally similar proportions and operating conditions are suitable where any of the other brighteners above noted are substituted for urea. The baths may be used immediately upon being prepared but we ordinarily prefer to allow them to age for two or three days before use.

The above mentioned brighteners are substantially non-volatile, and the deposits resulting from their use are substantially uniform and approach mirror brightness. Being of such high luster they require little or no polishing. With these substances the current density, temperature, agitation and solution composition may be varied to the same extent as these factors vary in ordinary plating practice and it is not necessary, as in the case of carbon-disulphide, to adopt any extreme precautions to produce, the deposit. These materials may be consumed during electrolysis to some extent but by no means as rapidly as carbon-disulphide, and in any case their replenishment is easy to take care of. The range of concentration of the materials as noted is not very critical and accordingly the control of the concentration is not difllcult.

We claim:

1. A process for the electrodeposition of silver which comprises: electrolyzing a silver cyanide plating bath containing a brightener having the chemical formula where X is a member of the group consisting of NH: and substituted NH: in which the substituents for hydrogen are methyl, ethyl, acetyl or NHz, to form 'a bright deposit of silver therefrom.

2. A process for the electrodeposition of silver which comprises electrolyzing a silver cyanide plating bath containing urea as a brightener to form a bright deposit of silver therefrom.

BIRGER EGEBERG. NATHAN PROMISE-L.