Title:
Process for electrodeposition of silver and products obtained therefrom
United States Patent 2110792


Abstract:
This invention relates to electrodeposits of silver and similar metals, including silver alloys, and to a process of electrodeposition thereof. In the present accepted commercial practice of electroplating silver coatings, the standard bath is composed of silver cyanide, an excess of alkali...



Inventors:
Birger, Egeberg
Nathan, Promisel
Application Number:
US7039636A
Publication Date:
03/08/1938
Filing Date:
03/23/1936
Assignee:
INTERNAT SILVER COMPANY
Primary Class:
Other Classes:
205/238, 205/263, 428/687, 428/927, 428/935
International Classes:
C25D3/46
View Patent Images:



Description:

This invention relates to electrodeposits of silver and similar metals, including silver alloys, and to a process of electrodeposition thereof.

In the present accepted commercial practice of electroplating silver coatings, the standard bath is composed of silver cyanide, an excess of alkali cyanide over that required to dissolve the silver cyanide, alkali carbonate and water. Through this solution an electric current of suitable chario acteristics is passed, the current passing into the solution through silver anodes of high purity, through the solution depositing silver on the articles to be plated which constitute the cathodes, and then out of the bath through the cathodes.

Under proper conditions of current, tempera1ture, agitation and maintenance of a proper concentration of the electrolyte, a crystalline, nonlustrous matte white deposit of silver may be secured at the cathode. Buffing or burnishing, Ssometimes both, of such a deposit is, therefore, necessary to secure a bright surface. This buffing 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 been proposed and used commercially for nearly 100 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 carbondisulphide 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 disadvantages which may be briefly summarized as follows: Even under the best conditions the deposit is not satisfactory and. frequently lacks any appreciable luster. It is, moreover, usually not u.niform over the entire plated surface unless extreme precautions are taken. The deposit is sensitive to plating conditions in that the quantity of carbor-disulphide present, the current density, temperature, agitation and solution composition must all be adjusted and maintained within closely balanced limits. The carbondisulphide limits are far narrower than required 60 by the other components of the solution, namely, the silver cyanide, alkali cyanide, carbonate and water. One difficulty 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, 6 causes it to vanish rapidly from the solution making frequent additions necessary. Consequently, the precautions which have to be taken in dealing with this material are numerous. It is a fact that where many individual 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 obtained, and even in solutions giving semi-lustrous deposits, there are always variations In appearance from vat to vat and even from section to ýsection on the same article. In general, in employing this material, trial-and-error methods of plating have to be employed to secure the semi-lustrous deposits. Departure from conditions so determined 20 may result in deposits the luster of which is actually inferior to those obtained from solutions containing no carbon-disulphide whatsoever. In addition, we have found with carbon-disulphide that only a relatively thin plating of semi-lus- 25 trous character can be 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 30 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. Another object of the inventiop 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 to carbon-disulphide and other brighteners heretofore 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 characteristics 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 several groups of chemical compounds, including derivatives of thio-carbahic acids, particularly thio-carbamates, thio-ureas, thio-cyanates, urea or alkali metal cyanates, these substances, particularly the thio-cyanates, being such as not to produce ammonia since we find that the production of ammonia in our electrolytic baths interferes with the proper luster.

Certain of the compounds which constitute brighteners in accordance with the invention may be represented by the chemical formula 15 where X is NH2 or substituted NH2 in which one or both hydrogen atoms are replaced by the substituent radicals methyl, ethyl, acetyl or NH2, and where Y is NH2, SNHaR, SNH2(R)2 or SM in which R is the methyl or ethyl radical, and 20 M an alkali metal.

The proportions of the brightener may vary from minute amounts for example 0.01 g/1 to large quantities for example 100 g/l, depending upon the particular substance and the attendant 25 conditions. By proper adjustment of the plating conditions a silver 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 30 enumerated above for carbon-disulphide and in some cases these disadvantages are completely eliminated.

The following are preferred examples of baths and methods for carrying out the invention as 35 we now prefer to practice it. It is understood that the invention is not to be considered as limited to these examples except as indicated in the appended claims.

Example 1 An electrolytic bath employing thio-urea NH, CS N.NH as a brightener is as follows: Silver cyanide ------------------- grams-Potassium cyanide .---------------do-50 Potassium carbonate --------------- do .-Thiourea --------------- ----- do.---Water -----------------------.liter-At a temperature of 80* F. lustrous silver de55 posits may be obtained at a current density of 6 amperes per sq. ft. of cathode surface.

Example 2 Another example employing acetyl-thio-urea NH.COCHs C/ cs NH, is as follows: The electrolytic bath contains: Silver cyanide-------------------- grams- 25 Potassium cyanide -----------------do . 45 Potassium carbonate.------------- do---- 20 70 Acetyl-thiourea-------------------do --- 45 Water------ -------------------- liter-. 1 At a temperature of:80° F. lustrous silver deposits may be obtained at a current density of 6 IT amperes per sq. ft. of cathode surface.

Example 3 Another example employing mono-methylammonium salt of mono-methyl-thiol-thionocarbamic acid NH.CHi / Cs s .NHI(CHs) is as follows: The electrolytic bath contains: Silver cyanide------------------grams-_ 25 Potassium cyanide ---------------do . 50 Potassium carbonate ---------------do 50 Mono-methyl ammonium salt of monomethyl-thlol-thiono-carbamic acid gram-_ 0.5 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.

Example 4 An example employing potassium thiocyanate CNSK is as follows: The electrolytic bath contains: 26 Silver cyanide -------------- --grams- 25 Potassium cyanide------------------do -- 45 Potassium carbonate-----------------do..-- 20 Potassium thiocyanate ---------------do ... 20 Water -----------------------------liter-. 1 30 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.

Example 5 35 An example employing urea NHi CO NHs is as follows: The electrolytic bath contains: Silver cyanide -------------------grams-. Potassium cyanide ----------------do . Potassium carbonate---. ------- - do--Urea---------------------------do-Water ----------- ------------- liter-25 45 20 45 45 1 At a temperature of 80" F. lustrous silver deposits may be obtained at a current density of 6 50 amperes per sq. ft. of cathode surface.

Other substances which may be satisfactorily employed coming under the formula mentioned includeThio-smi-cearbazide NH.NHs C/ Cs NHt Di-methyl-ammonium salt of di-methyl-thlol-thiono-carbamic acid 0 N.(CH,)s Cas SS.NH-(CHa)s Mono-ethyl-ammonium salt of mono-ethyl-thlol-thiono-catbmo acid NHCiHu CB.

S.NHV.C1HE Mono-methyl-ammonium salt or potassium salt of thiol-thiono- TO carbamic acid NHi Co a ý" flu.

23H1 o·nurv~ 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 thio-cyanates, thio-carbamates, thio-ureas and urea described above may be prepared by known methods.

Certain of the above mentioned brighteners, for example, thiourea and urea, 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 in any case their replenishment is easy to take care of. No more frequent additions of these materials are necessary than required for the silver cyanide, alkali cyanide or carbonate of the bath. The range of concentration of the materials as noted may be varied and accordingly the control of the concentration is not difficult.

We claim: 1. A process for the electrodeposition of silver which comprises electrolyzing a silver cyanide bath containing a thio-urea as a brightener to form a bright deposit of silver therefrom.

2. A process for the electrodeposition of silver which comprises electrolyzing a silver cyanide 5 bath containing acetyl-thio-urea as a brightener to form a bright deposit of silver therefrom.

3. A process for the electrodeposition of silver which comprises electrolyzing a silver cyanide bath containing a mono-methyl-ammonium salt of mono-methyl-thiol-thiono-carbamic acid as a brightener to form a bright deposit of silver therefrom.

4. A process for the electrodeposition of silver which comprises electrolyzing a silver cyanide bath containing a thio-semi-carbazide as a brightener to form a bright deposit of silver therefrom.

5. As a new article of manufacture, a metal base having a surface electrodeposit of silver characterized by high and substantially uniform luster as deposited and requiring thereby a minimum of polishing, said deposit being obtained from a silver cyanide plating bath containing thiourea as a brightener.

65 6. As a new article of manufacture, a metal base having a surface electrodeposit of silver characterized by high and substantially uniform luster as deposited and requiring thereby a minimum of polishing, said deposit being obtained from a silver cyanide plating bath containing acetyl-thio-urea as a brightener.

7. As a new article of manufacture, a metal base having a surface electrodeposit of silver characterized by high and substantially uniform luster as deposited and requiring thereby a minimum of polishing, said deposit being obtained from a silver cyanide plating bath containing a mono-methyl-ammonium salt of monomethyl-thiol-thiono-carbamic acid as a brightener.

8. 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 NHa and substituted NHa in which the substituents for hydrogen are methyl, ethyl, acetyl or 20 NH2, and where Y is a member of the group consisting of NHa, SNH3R, SNH2(R)2 and SM in which R is methyl or ethyl and M an alkali metal, said substituting groups being such as not to make the resulting compound insoluble in 25 the cyanide plating bath and not forming a substance substantially alkaline in water or producing ammonia during the process of electrolysis.

9. As a new article of manufacture a metal base having a surface electrodeposit of silver 80 characterized by high and substantially uniform luster as deposited and requiring thereby a minimum of polishing, said deposit being obtained from a silver cyanide plating bath containing a brightener having the chemical formula 35 x cs \ were X is a umemuer of me group consisting of NH2 and substituted NH2 in which the substituents for hydrogen are methyl, ethyl, acetyl or NH2, and where Y is a member of the group consisting of NH2, SNH3R, SNH2(R)2 and SM in 45 which R is methyl or ethyl, and M an alkali metal, said substituting groups being such as not to make the resulting compound insoluble in the cyanide plating bath and not forming a substance substantially alkaline in water or produc- 50 ing ammonia during the process of electrolysis.

10. As a new article of manufacture, a metal base having a surface electrodeposit of silver characterized by high and substantially uniform luster as deposited and requiring thereby a mini- I5 mum of polishing, said deposit being obtained from a silver cyanide plating bath containing thio-semi-carbazide as a brightener.

BIynR~ G ,.nrrn NATHAN PROMISEL. hU ER EGEBERG.