UNITED STATES PATENT OFFICE 2,555,375 PROCESS of PLATING BRIGHT SWER ALLOY Waldemar P

Patented June 5, 1951 2,555,375 UNITED STATES PATENT OFFICE 2,555,375 PROCESS OF PLATING BRIGHT SWER ALLOY Waldemar P. Ruemmler, Calumet City, Ill., as signor, by mesne assignments, to Battelle De velopment Corporation, Columbus, Ohio, a cor poration of Delaware No Drawing. Application November 13, 1948, Serial No. 59,963 4 Claims. (C. 204-43) 1. 2 This invention relates to a method of plating deposited antimony and silver. Wherever, due bright silver alloy and to an electrolyte useful to a change in the amount of bath constituents in the process of producing a bright silver alloy or in operating conditions, an increase in the plate. In particular, it relates to the coelectro antimony content of the alloy plate occurs, there deposition of silver and antimony to form an will be a corresponding decrease in the silver Con adherent alloy plate having a smooth, brilliant tent, so that there are always only two elements appearance and image reflecting properties and present, i. e., silver and antimony. Likewise, which is Scratch and tarnish resistant. where there is a decrease in the antimony coin The technical and commercial advantages of tent, there will be an increase in the Silver Con bright metal plating are well known to those 10 tent of the plate. Very minor amounts of in skilled in the art. For example, a pure silver purities are probably present in the electrode plate is relatively dull and must be polished. A posited plate of this invention, as will be obvious bright plate on the other hand, is a saving in to those skilled in the art. However, they do metal, because there is no loss occasioned by not appear to materially effect the resulting the buffing and coloring operations that are re 15 bright silver-antimony alloy plate. quired in making a pure silver plate bright and As little as about 0.08 per cent by weight of pleasing in appearance. This is particularly antimony in the electrodeposited silver alloy Significant in the case of a costly metal like silver. plate causes a brightening effect. Appreciably The irrecoverable loss, as of silver dust in buffing less than this amount of antimony does not ap and Silver loss in coloring operations, although 20 pear to materially brighten the appearance of removing only a few ten-thousandths of an inch the plate, and the Scratch and tarnish resistance of the Silver plate, amounts to many thousands characteristics of such an electrodeposited plate of dollars per year. Obviously, then, a bright are approximately the same as those of the pure Silver plate possesses great economic advantages. Silver plate. Heretofore, no truly bright silver plate, in the Plates containing from 2.5 to 3.0 per cent by Sense of possessing mirror-like reflectivity, Was Weight of antimony are bright and have mirror known. like reflectivity. By “mirror-like reflectivity' is Furthermore, the pure silver plate tarnishes meant the ability to reflect an image. Very readily and is rather soft and easily scratched. bright plates have been produced containing Additional loss of silver occurs, because the pure 30 from 1 to 12 per cent of antimony. Silver metal must be polished frequently to re Since the plate becomes more brittle as the move the tarnish. Due to this soft nature of the antimony content increases, a choice of bright pure Silver, the basis metal must be replated from Silver-antimony plate composition is determined time to time to restore its finish, causing the by the application intended. For the best ductil user considerable expense. 35 ity with good brightness, it has been found that It is, therefore, an object of this invention to the antimony content should not exceed about provide a process for coelectrodepositing silver 7% by weight. Hence, the bright silver alloy and antimony as an alloy plate having the afore plates of this and lower antimony contents can mentioned improved qualities. be classified as Sterling in quality. Plates con It is a further object of this invention to pro 40 vide an electrolytic bath capable of producing taining from 7 to 12% antimony still possess an electrodeposited silver-antimony alloy plate Sufficient ductility to have many uses, but where having the aforementioned improved qualities. the antimony content appreciably exceeds 12% It has been found that silver and antimony by Weight, the plate becomes brittle and is eas can be coelectrodeposited from an electrolytic 45 ily chipped. Thus, the commercial and indus bath to form an adherent alloy plate having a trial applications of plates having high anti brilliant appearing surface which is scratch and mony contents are limited. Furthermore, flat tarnish resistant. It has been found that this Ware having thereon an electrodeposited coating bright alloy plate may be deposited on a cathode of bright silver containing over 12% antimony from a strongly alkaline, aqueous, silver cyanide 50 Would be departing so much in actual silver con antimonyl tartrate bath which is operated at tent from the pure silver plated flatware that relatively low current densities and tempera its Sales value would be considerably reduced. tures. The aqueous, alkaline electrolytic bath used The plate, as deposited from the above electro in this process contains silver cyanide, potassium lytic bath, is essentially a pure alloy of electro 55 cyanide, the double tartrate of antimony and 2,555,875 3 4. potassium, potassium tartrate, potassium cal to 13.5. It is preferred, however, that the pH bonate, and potassium hydroxide. of the solution remain in the range of from 11.5 In the bath solution, it is believed that the to 12 as this provides better control of the proc silver exists as the soluble silver cyanide COXin ess and is sufficient to enable the silver and anti plex while the antimony exists as the Soluble mony to codeposit on the cathode. The pH can antimony tartrate complex. The composition be controlled within these ranges, of course, by of the resulting plate can be varied by varying adjusting the concentrations of potassium hy the proportion of the silver complex to the anti droxide, potassium carbonate, and potassium mony complex in the bath. Generally, the cyanide. other factors like current density and tempera 10 In the electrolytic bath used in this process ture etc., remaining unchanged, an increase in of electrodepositing a bright silver-antimony the ratio of the Silver to antimony in the bath alloy equivalent sodium salts may be used where will result in an increase in the ratio of silver potassium salts are shown. Equivalent ammo to antimony in the plated alloy. On the other nium salts, i. e., salts containing the ammonium hand, a decrease in the ratio of silver to anti 5 radical, NH4+, may also be used without signifi mony in the bath, will result in an increase in cantly altering the results obtained. Emount of antimony in the bright silver alloy The current density in the electrolytic bath plate. should range from 5 to 90 amperes per Square The Soluble silver cyanide complex is formed foot. It is preferred to use current densities of on the addition of silver cyanide to the bath. 20 from 20 to 70 amperes per square foot. It has About 20 to 150 grams per liter of silver cyanide been found, for example, that a current density is Sufficient. It is preferred, however, to use as low as 20 amperes per square foot in a bath to about 30 grams per liter of silver cyanide. This which were added equal portions by weight of Will fix the Silver relative to the antimony and silver cyanide and potassiun antimonyl tar permit easier control of the plate composition 25 trate in the bath above described produced a as only one prime component has to be varied. bright silver alloy plate having 1.89 per cent by To aid in the formation of the soluble silver weight of antimony therein. On the other hand, cyanide complex in the solution and in render where the antimony tartrate compound was ing the Solution alkaline, about 20 to 100 grams added to the bath slightly in excess of the Silver per liter of potassium cyanide is added to the 30 cyanide, a current density of 70 amps per Square electrolyte. Excellent results have been ob foot produced a very bright alloy plate having tained using a concentration of about 30 to 50 from 11 to 12 per cent by weight of antimony. grams per liter which represents the preferred For producing mirror-like, bright plates con range. taining about 3% antimony where there is also The Soluble antimony tartrate complex is ob 35 only a slight excess of the potassium antimonyl tained from the double tartrate of antimony and tartrate over the silver cyanide in the bath, a potassium. From about 3 to 100 grams per liter current density of about 30 amps per Square foot of this compound should be added to the bath. was found to be sufficient. Excellent alloy plates have been obtained by The temperature of the bath during the plat adding to the bath about 5 to 36 grams per liter 40 ing operation has not been found to be very of potassium antimonyl tartrate so that this critical. These baths can be operated at tem forms a preferred range. peratures of about 70° F. to 140° F. and produce Potassium tartrate, is necessary in the bath good plates. Excellent results have been ob to prevent hydrolysis of the potassium anti tained when the electrolytic bath was main monyl tartrate. Hydrolysis results in the loss of 45 tained at temperatures of from about 75° F.

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