US3753874A - Method and electrolyte for electrodepositing a gold-arsenic alloy - Google Patents

Abstract

AN AQUEOUS ELECTROLYTE SOLUTION HAVING A PH FROM 5.5 TO 8 FOR DEPOSITING HARD, DUCTILE AND BRIGHT ARSENIC CONTAINING GOLD HS BEEN PROVIDED, BESIDES THE VARIOUS BUFFERS AND ALKALI GOLD CYANIDE, COMPLEX AA THIO SULFATE IS ADDED TO FACILITATE THE PROPER INCLUSION OF ARSENIC IN THE GOLD DEPOSIT, SALT COMPOSITION SUITABLE FOR OBTAINING AQUEOUS ELECTROLYTES, A MEHTOD FOR DEPOSITING THE ARSENIC GOLD, TE GOLD ALLOY, AND ELECTRICAL DEVICES HAVING THE ARSENIC GOLD DEPOSIT ON A SURFACE OF THESE DEVICES HAVE ALSO BEEN DISCLOSED.

Description

United States Patent Office 3,753,874 Patented Aug. 21, 1973 3,753,874 METHOD AND ELECTROLYTE FOR ELECTRO- DEPOSITING A GOLD-ARSENIC ALLOY Richard Henry Zimmerman, Palmyra, and Richard Lee Brenneman, Camp Hill, Pa., assignors to AMP Incorporated, Harrisburg, Pa.

No Drawing. Continuation-impart of abandoned application Ser. No. 807,105, Mar. 13, 1969. This application Dec. 21, 1971, Ser. No. 210,609

Int. Cl. C23b /42 US. Cl. 204-43 G 28 Claims ABSTRACT OF THE DISCLOSURE An aqueous electrolyte solution having a pH from 5.5 to 8 for depositing hard, ductile, and bright arsenic containing gold has been provided; besides the various buffers and the alkali gold cyanide complex, a thio sulfate is added to facilitate the proper inclusion of arsenic in the gold deposit; salt compositions suitable for obtaining aqueous electrolytes, a method for depositing the arsenic gold, the gold alloy, and electrical devices having the arsenic gold deposit on a surface of these devices have also been disclosed.

This application is a continuation-in-part of application Ser. No. 807,105, filed Mar. 13, 1969, and now abandoned.

This invention relates to an improved method of electro depositing gold on electrical devices, such as gold plating electrical conductors, a novel electrolyte used in practicing the method of electro depositing gold as well as a novel salt composition for use in an electrolyte solution or bath. Electrical devices improved with electro deposited gold thereon according to the present invention are within the scope of this invention.

A wide variety of electrolytes are used as solutions for plating gold therefrom. As it is well known, electrolyte solutions used for this purpose are generally classified in three types based on a pH scale. The first type is an alkaline electrolyte solution. This electrolyte contains gold as potassium gold cyanide in solution and additional amounts of potassium and sodium cyanide. Electroplating .from a bath operating with this solution is carried out at a pH of 8 and higher.

As a second type, an acid gold electrolyte solution is well known. It contains gold in a form of complex cyanides, and it has additional salts dissolved in the solution. These salts are derived from organic acids such as citrates, acetates, lactates, etc. Electro depositing from this bath is carried out at a pH below 6.

As a third type of electrolyte solution, a neutral gold electrolyte is employed. It is a buffered solution whose pH level is kept from 6 to 8. Although it is not truly a neutral" bath over this pH range, it has been designated as such in the art.

A number of variations are known of these three basic electrolyte bath compositions. For example, additives used as complexing agents are often employed. While these additives oifer a solution to certain problems, these same additives, together with the gold salts, cause other problems.

A very complex and as yet poorly understood electrochemical interaction of the additives with each other is the reason for a wide variation in results. Further, not only dissolved gold containing salts which interact with the additives but also the variation in the anodic and cathodic reactions, as these occur in the bath, produce different results. These various phenomena render the chemistry of electroplating gold from an electrolyte solution an art with predictability being very difiicult, if not impossible.

Hence, when the electrolyte components are changed or substituted, many balancing considerations apply where sacrifices in one variable must be made .for a gain in another variable. However, these balancing considerations are invariably empirical. As an electrolyte bath consists of a number of components, the permutations and combinations and the effects of each additive on the electrolyte behavior and the obtained product are manifold; and consequently, the outcome is unpredictable in the method of plating and the final product. Hence, the plating outcome, while often acceptable in one respect, is completely unacceptable in some other respect.

Further, as it is well known, various electrolytes or baths based thereon can only be operated over a narrow temperature, pH, and set dissolved salt composition range, as the variations in these may cause gold to be reduced and precipitated from the bath solution. Still further, in operating the bath, unwanted products are formed from trace impurities in the electrolyte; these impurities interact with the electrolyte and/or the gold being electro deposited such as forming alloys or becoming occluded in the electro deposited gold.

These impurities often interact with a base metal or the electroformed gold in the operating environment to which the gold plated device is to be subjected. As a consequence, the end product, while initially acceptable in appearance, fails in one or more of the basic requirements which are hardness, ductility, wear, continuity of film or porosity, oxidation resistance, along with other characterizing properties used to evaluate electro deposited gold and which measure the quality of the deposited layer on an electrical conductor and/ or deposit such as conductivity of the composite, coloration, density, smoothness, purity, nobility, cycling ability, etc.

When gold in various compositions is deposited from an electrolyte such as in a standard electrolyte bath such as when employing a rack or a barrel method, or when gold is deposited selectively such as when employing a strip line, jet or belt method, generally it deposits in two crystalline forms. In the so-ealled alloyed gold, i.e. containing co-deposited additives, the form is typically laminar with the laminae or striations of the crystals parallel to the base on which it is deposited and visible at magnifications of about to 200 times. With high purity golds a columnar form is more typical with the crystals forming columns perpendicular to the base on which it is deposited, visible at about the same magnification. More rarely, gold deposits in a random crystalline form and still more rarely in a random, fine grained crystalline form.

Laminar and columnar gold crystals are formed from the conventional electrolytes, and these forms are fairly well known; however, it is impossible to predict when fine grained, bright, randomly oriented crystals will be formed from "an electrolyte solution and what electrolyte solution will produce bright, ductile, hard, randomly oriented, fine grained crystals. Moreover, it is substantially impossible to predict when the fine grained crystals will form bright gold.

For the above reasons, the compounding of suitable salts and the use of these in an electrolyte bath in terms of the bath eiiiciency and final product properties is still an art beyond routine results achievable with conventional electrolytes suitable for gold plating, e.g. the fine grained gold electrodeposit. Thus, the evaluation of any novel electroplating bath composition must be based on the electrolyte composition, the method of employing the novel electrolyte bath, and physical properties of the electroformed gold and products made therefrom, any of which may result in a novel combination in itself.

In accordance with the present invention, a novel electrolyte salt composition has been found. This salt composition consisting of the herein defined components or ingredients is suitable for formation of an electrolyte solution which, in turn, is suitable for depositing gold from an electrolyte solution. This electrolyte salt composition when dissolved and used in an electrolyte solution within the defined neutral pH range of 5.5 to 8 under electro deposition conditions has resulted in a novel method for depositing gold, characterized by high efficiencies, fast plating times, and absence of detrimental side reactions; and more importantly, the salt composition when so used produces a novel electro deposited or formed gold product, novel articles of manufacture based on the novel electro deposited or formed gold, which articles possess exceptionally advantageous properties, such as brightness, hardness, e.g. up to 260 Knoop units, foil strength, ductility, wear, smoothness, solderability, etc. These properties in the final product are achieved despite the presence of a heretofore unwanted component, i.e. arsenic, in the plating bath which component in fact is now incorporated in the electro deposited gold layer by means of a heretofore not employed complexing agent.

Thus, according to the invention, an electro deposited or formed gold has been obtained which is bright, hard and ductile and which consists of randomly oriented, fine grain crystals. This gold displays brightness, hardness, and ductility properties, to name only a few, which properties are contributed by the co-deposited arsenic, the last from the electrolyte solution under electro deposition conditions of particular kind via a particular complex, e.g. thio-arsenic (III) complexes and/or tris (thio sulfato) arsenic (III).

The above-mentioned arsenic complexes contribute to the unexpected results in a manifold manner. First, these complexes elevate the reduction potential with all the attend-ant advantages, and second, the complexes prevent the oxidation of As+++ to As+++++. Moreover, when using these complexes, the electroplated or electrodeposited gold possesses the exceptional properties previously mentioned.

Suitable thio or thio sulfato arsenic complexes are those formed from arsenic and a thio compound such as thio urea; thio chrysine, etc.; further tris (thio sulfato) arsenic complexes formed from precursors such as alkali thio sulfates, e.g., sodium thio sulfate and arsenic, etc. Of these arsenic complexes, the tris (thio sulfato) group, such as alkali thio sulfate, is by far the most preferred one and is one which produces eminently superior results. The arsenic complexes suitable for use must be soluble or solubilizable in the electrolyte at the operating conditions.

Without espousing any theory, these complexes contribute to the electrolytic deposition reaction by possibly some catalytic action, perhaps by providing arsenic in a form which does not escape as an arsine gas but rather provides arsenic as an alloy forming component under proper temperature and current density conditions. It is believed that 0.1 to 0.9% of weight of arsenic in the gold obtained from an electrolyte solution by electro deposition or electroforming in the specified manner produces a novel form of crystalline gold as evidenced by many of the physical and chemical characteristics of this product as further descussed herein.

For example, the hardness of a gold foil produced according to the novel method when measured in Knoop units at 25 gram load shows a range of 190 to 250, which hardness, together with the exceptional brightness and ductility has heretofore not been obtained.

Further, the Tafel slope of the novel electro deposited or formed gold is almost equivalent to pure gold when measured against pure gold electrodes in a 0.1 molar ammonium chloride solution used in this test.

The above properties such as brightness are exceptionally well displayed when the amount of arsenic in the electro deposited gold is 0.1 to 0.5 percent by weight and when the deposition is properly carried out. In the deposit, the balance is gold with trace impurities normally associated with electro deposition of gold. As the amount of arsenic increases in the gold deposit, the properties also vary; for example, at concentrations above 0.9% by weight, the grain structure starts to change, and at about 1.0% by weight of arsenic, crystal structure tends towards the columnar type.

Consequently, on the basis of the amount of arsenic by weight and the crystalline structure, the present electro deposited gold can be delineated from the prior art electro deposited gold such as by grain structure, brightness, hardness and ductility. Although other fine grained, crystalline gold forms have been observed, these are neither based on arsenic nor as bright and hard nor ductile at the hard ness levels herein nor as noble when compared for polarization behavior in respect to slope characteristics based on Tafel equation, i.e. Tafel slope.

Surface smoothness of the novel gold deposits is also exceptional. In an electron microscope at a magnification of 32,000 times and a photographic enlargement of up to 80,000 times, these deposits appear exceptionally smooth based on the definition employed in the art.

Additional, exceptional properties of the novel gold deposits which are based mostly on performance are abrasive resistance, wearability in use of a gold plated article, oxidation resistance, easy wettability for soldering, capability of forming thick electro deposits, and good throwing power, e.g. 37% as determined in a Haring cell.

In reference to electrical contacts having on the contact surfaces the novel electro deposited gold, the prod uct specifications govern these devices for their acceptability. Specific geometries employed for the specific contact devices have characteristic wear cycles. When electrical devices are compared which have contact surfaces plated with the novel gold deposits to the devices plated with gold having a similar hardness produced from a commercially available electrolytic formulation containing gold, cobalt, and indium, it has been found that on statistical bases the same connectors which have previously been barrel plated with the novel gold deposit have outlasted and outperformed the similarly plated prior art connectors after repeated work cycling (insertionwithdrawal), e.g. when measured at initial lower level contact resistance and rated current resistance and after 250 to 500 work cycles at 1 and 2.5 milliamps DC. and 5 amps A.C. At the same time the novel gold plated connectors have out-performed the prior art plated connectors based on the same test after repeated work cycling and when measured as final contact resistance for durability and corrosion after repeated temperature cycling.

Improved results based on these tests bespeak the ability of the connectors to perform especially in low voltage level circuitry for extended periods of time. Other equally important properties which the novel gold plated connectors possess are: crimpability, i.e. it relates to ductility; scratch resistance; gold deposit porosity; chemical resistance; excellent solderability, etc. The last property is measured by means of a dip method comparing solder covered area and solder bare area; it can also be measured by ease of spreading of a certain amount of solder and the surface area which a certain amount covers and the contact angle of this solder with the plated surface.

If the novel electro deposited gold is used to produce heavy deposits, it still displays the fine crystalline structure which is rather unexpected. It is also rather unexpected that while commonly employed hardening and brightening agents normally tend to produce an oxide film TABLE I.-ELECTROLYTE BATH COMPOSITION Weight of components per unit volume of water Electrolyte bath components G./l. Oz./gal.

Monobasic otassium phosphate 60 to 120 8 to 16.

(KHzPOfi.

Potassium citrate (K CaH O H20) 60 to 120 Do sequestering agent; such as disodium .25 to 1.0 0.033 to 0.132.

ethylenediaminetetraacetate dihydrate (NaZEDTAQH O).

Gold as potassium gold cyanide [KA11(CN)2] (the amount used is on basis of elemental gold).

10.7 to 14 1.3 to 1.5 tr.

Trivalent arsenic as sodium arsenite .02 to .2 .0026 to .026.

(NaAsOz) (the amount used is on basis of elemental arsenic).

As complexing agent therefor, sodium 5 to 0.66 to 1.32.

In a bath solution used herein, the total quantity of the specified amounts of each component is added per unit volume of the water according to the units employed. The sequestering agent is an optional additive and is employed as a safeguard against contamination.

Generally, a number of sequestering agents may be used. Of these agents ethylene diamine tetraacetate is most often used; and as a result of the prevalent use of this compound, any of the other sequestering agents and the amounts of the same in electrolyte bath may be expressed in terms of equivalent activity which these compounds display on weight basis under identical conditions to ethylene diamine tetraacetate. Consequently, specifying an equivalent activity of a sequestering agent in terms of ethylene diamine tetraacetate also defines the amount of the sequestering agent.

As mentioned before, the bath is maintained at a pH from 5.5 to 8, thus the above potassium phosphate and/ or potassium citrate are employed to obtain the desired pH conditions. In respect to the ranges for the active components, gold in the form as defined above is used ranging from 10 to 15 grams per liter or more particularly 10.7 to 14 grams per liter or 1.3 to 1.5 troy ounce per gallon, and these are the desired ranges, with the preferred amount being about 12.3 g./l. Arsenic is desirably used ranging from 0.02 to 0.04 gram per liter or 0.0026 to 0.0052 ounce per gallon, preferably about 0.03 g./l. As the electrolyte salt composition, the above designated composition is representative, however, without reference to the unit volume of water. Further, the electrolyte salt composition, as a combination, in its broadest generic aspect consists of gold in the form of a gold cyanide, arsenic and the complexing agent for the arsenic as listed in Table 1 above, again without reference to the unit volume of water. In a more narrow sense, the salt composition contains gold in the form of a gold cyanide, e.g. potassium gold cyanide, arsenic such as sodium arsenite, one of the recited complexing agents for arsenic added in an excess to the stoichiometric requirement for complexing arsenic in order to suppress arsine and to assure that no arsine escapes from the bath, a buffering agent, e.g. potassium phosphate and/or potassium citrate to obtain the range of pH mentioned above when the salt is compounded to be dissolved in some unit measure in a unit volume of water.

The potassium phosphate and/or potassium citrate mentioned above may also be added to give an approximate pH when the above-recited salt composition is added to water, and the final adjustment may be made by appropriate addition of the buffering agent. In the narrowest sense the salt composition consists essentially of all the above-recited components and, in addition, for suppression of impurities a sequestering agent such as disoduim ethylene diamine tetraacetate dihydrate. As it can be appreciated, a sequestering agent is added to safeguard the formed deposit from inclusion of unwanted, impurityoriginated co-deposits during the deposition process. Hence, for purposes of compounding the precursor salts for the electrolyte solution, a compounder may include the essential salts and any other salts recited above or give directions on how the salts may be supplemented to produce the necessary electrolyte solution.

In operating an electrolyte bath, it has been found that temperature ranging from F. to F. can be usefully employed. An optimum temperature is 140 F. If lower temperatures are used, then it has been found that the brightness of the heavy deposits decrease without loss of hardness and etficiency. Thus, at a temperaure of 80 F. the deposits become softer (about 155, Knoop hardness number) and burnt. As the present electrolyte bath is of the so-called neutral bath type previously discussed above, a conveniently suitable pH range is from 6.0 to 6.5. A pH of 6.2 represents the optimum condition. Specific gravity of this bath is at least 10 B. The sodium (thiosulfate)-As (III) complex is replenished in the electrolyte bath by adding a solution of 100 g./l. of sodium thio-sulfate ('Na S- O -5H O) and 17.4 g./l. of sodium arsenite (NaAsO and bringing up the bath to the specified electrolyte composition at the specified pH conditions, the last most conveniently by means of the buffering agent (although the addition of potassium gold cyanide will affect the pH, the adjustment of pH is most conveniently achieved with the buffering agents).

As the non-consumed anode material, carbon or platinum is used. Equally inert materials may 'be also employed.

According to the invention, rack, basket, strip or selective plating methods are suitable for obtaining the novel gold form. The use of a strike is recommended, since the electrolyte bath will immersion deposit gold on nickel, copper or brass in less than one minute.

The above-described gold is commonly deposited on a metal base. Metals suitable for this purpose and also useful as electrical conductors are such as copper and its alloys, iron and its alloys, nickel and its alloys, aluminum and its alloys, etc.

Current density for a conventional rack plating method is up to about 10 amperes per square foot (a.s.f.); at higher current densities in a rack plating method, the grain structure may change.

At a rack current density of 5 a.s.f. heavy deposits are dull, and at 15 a.s.f. heavy deposits are burnt; there is negligible change in efficiency or hardness of the deposit. For a barrel plating method, current density is about 3 a.s.f. Higher current densities are obtainable when plating methods other than rack and barrel methods are employed, e.g. up to 100 a.s.f. and higher if the previously mentioned arsenic complex formers, which also function as arsine suppressants, are employed in the bath.

Agitation of the bath during the plating operation is carried out by mechanical means in a vigorous fashion.

The density of the deposit obtained, when employing a bath solution of the composition given in Table I, was 19.0 grams per cubic centimeter or 31.2 milligrams per 0.0001 cubic inch. A hardness of to 250 Knoop hardness units with a 25-gram load was obtained.

In the gold depositing art, the surface appearance of a deposit is classified commonly as either bright, semibright, or matte. In accordance with this invention, rack plating work is bright to a thickness of at least 1.5 mils, while barrel plated work is bright to a thickness of at least 0.2 mil.

As mentioned before, the novel method of operating a bath is characterized by excellent efiiciency either on percentage basis or deposition rates. Representative efficiency figures for the electrolyte bath of the above composition are illustrated below:

The efiiciency in percent is defined as the amount of gold deposited in comparison with the theoretical efiiciency for pure gold which is 123 mg./ amp minute.

When leaf type electrical connector articles were plated in the same bath, a plating rate to achieve 0.1 mil deposition was achieved in a rack plating method in 3.6 to 4.0 minutes at 10 a.s.f. at the above-identified optimum conditions while vigorously stirring the electrolyte bath by mechanical means; in barrel plating, the same rate for the same deposit thickness at 3 a.s.f. was achieved in 14.5 to minutes.

Electrical devices which can be suitably electroplated with the novel gold composition according to a method disclosed herein are items such as crimpable or solderable electrical conductor, a coaxial connector, microminiature connections for integrated circuits, solid state devices, etc.

The structure of the novel gold deposit changes when certain impurities are found in the electrolyte. For example, copper, iron and nickel are commonly encountered impurities. While copper, iron and nickel as an impurity(ies) do not adversely affect bath etiiciency or properties of the deposit, iron does effect a change in the crystalline structure of the deposit at levels as low as 10 mg./l. of electrolyte solution. At this concentration, the structure may be partially of randomly fine crystals and partially of columnar crystals. Further, while copper and nickel at low current density values, i.e. when rack plating at 5 a.s.f., and concentrations up to 100 mg./l., do not change the crystalline structure and properties at high current density values, e.g. at 10 a.s.f. or above, the struct-ure may be laminar for copper or columnar for nickel. These observations were made when plating with the composition in Table I at 140 F. and at 10 a.s.f. However, these impurities do not reduce gold from the electrolyte bath at the operating conditions.

For high speed plating operation, the following electrolyte composition has been found to be very suitable. Composition and operating conditions of an electrolyte bath and properties of the gold deposit are illustrated below.

TABLE IIL-ELECTROLYTE COMPOSITION, OPERATING CONDITIONS AND PROPERTIES OF ELECTRO DEPOS- I'IED GOLD IN A HIGH SPEED PLATING OPERATION Specific Components and conditions bath Suitable range Gold AsKAu(CN)1 31.1 g./l. Monobasie potassium phosphate 90 g./l 60 to 240. Potassium citrate 90 g./l 60 to 240.

From lsltolehzigiat-I met 0 to gig thiosulfate m -flitg l poun o e e- Arsenic al mental arsenic by weight. pH of electrolyte- 6.2 5.5 to 8. Specific gravity 19 Be- Temp, F 140 100 to 160. Density, g./ec. of deposit. 18.5-.-- Cathode e11, percent- 88 Current density, a.s.f Plating rate (min.) .000 Knoop Hardness HK (25 g. load) Appearance Wt. percent As in deposit l3 l Each salt or mixture 01 same to assure a pH from 5.5 to 8.

In summary, as it is evident from the above discussion and data, the proportions of components in the electrolyte solution for high speed and rack and barrel plating range from 10 to about 32 grams per liter gold; arsenic from 0.02 to .2 gram per liter, the thio complexing agent from stoichiometric ratio to a 250 or even 400 fold excess, on weight basis, the thio compound to elemental arsenic; and the buffering agent or mixtures of same from 60 to 240 grams per liter and suflicient to impart the desired pH in the range from 5.5 to 8.

The above-described properties of the novel gold form were determined according to the procedures for gold plated products set out in ASTM-B4886S.

The term an electro deposited gold article is meant to cover both an electroplated article and an electroformed gold article of which the latter is an article which is built up to the desired dimensions by gold deposition from an electrolyte. The term a heavy gold deposit signifies deposited gold of a thickness in excess of one mil. In reference to a rack plating method, high current density signifies values above 10 a.s.f., i.e., up to 50 a.s.f. A low-stress gold deposit is defined herein as being ductile according to the ASTM test indicated above; and wherein an electro deposited gold foil, when removed from a base metal upon dissolution of the base is characterized by absence of failure by bending and unbending over a 0.32 dia. wire through 180 angular degrees.

What is claimed is:

1. An aqueous electrolyte solution for depositing hard and bright arsenic containing gold, said solution having a pH range from 5.5 to 8, comprising an alkali metal containing butter for imparting said pH range to said solution, an alkali metal gold cyanide complex, and a tris (thio sulfato) arsenic (III) complex or thio arsenic (III) complex to harden and brighten the gold by inclusion of arsenic in a gold deposit.

2. An aqueous electrolyte solution according to claim 1 and wherein the tris (thio sulfato) arsenic (III) complex is derived from sodium arsenite and sodium thio sulfate.

3. An aqueous electrolyte solution for electrolytic deposition of arsenic gold from a dissolved electrolyte in a water medium, said solution comprising: as a buffering agent, monobasic potassium phosphate, potassium citrate, or mixtures of same; an arsenic complex added to said solution as sodium arsenite and sodium thio sulfate to yield 0.02 g./1iter of solution to .2 g./liter of solution of elemental arsenic in a ratio of said thio compound to elemental arsenic of up to 250 fold excess; elemental gold, added as [KAu(CN) from 10 g./liter of solution to 32 g./liter of solution, the pH of said aqueous solution being in the range from 5.5 to 8 imparted thereto by said buffering agent and whereby an arsenic gold having from .1 to 1% by Weight of arsenic in said gold deposition is obtained from said electrolyte solution under electro deposition conditions.

4. An electrolyte solution according to claim 3 and wherein the solution comprises on basis of one liter of water:

Grams Monobasic potassium phosphate or potassium citrate as K C H O .H O or mixtures of said phosphate and citrate 60 to Arsenic complex:

Arsenic, as elemental arsenic and added as sodium arsenite .02 to .04 Sodium thiosulfate, as Na S O .5H O 2 to 8 Elemental gold, added as [KAu(CN) 10 to 15 and wherein the pH of the solution is from 6.0 to 6.5 imparted thereto by said phosphate or citrate, or mixtures thereof.

5. An electrolyte solution according to claim 4 and wherein a sequestering agent is included in an amount based upon equivalent activity of .25 g./liter of solution of dissodium ethylenediamine tetraacetate.

6. An aqueous electrolyte according to claim 3 and including a sequestering agent based on equivalent activity of about .25 g./l. of dissodium ethylenediamine tetraacetate dihydrate.

7. An aqueous electrolyte soltuion according to claim 3 suitable as a high speed plating electrolyte and wherein the solution comprises:

and wherein the solution upon palting on a metallic base is to yield a gold deposit containing at least 0.1% to 1% by weight of arsenic.

8. An electrolyte solution according to claim 3 and wherein said solution contains trace amounts of metallic impurities.

9. A salt composition suitable for deposition of gold from an electrolyzed aqueous solution thereof comprising: gold as potassium gold cyanide; arsenic as a trivalent arsenic compound; and as a complexing agent for said arsenic, an alkali metal thio sulfate.

10. A salt composition suitable for deposition of gold from an electrolyzed aqueous solution of said salt composition at a pH from 5.5 to 8, said salt composition comprising: monobasic potassium phosphate, potassium citrate, or mixtures of same; gold as [KAu(CN) arsenic as sodium arsenite; and an alkali metal thio sulfate as a complexing agent for said arsenic and as an arsine suppressant.

11. The salt composition according to claim 10 comprising: 60 g. of monobasic potassium phosphate; 60 g. of potassium citrate monohydrate; from 10 to 15 g. of gold on elemental basis as potassium gold cyanide; 5 g. of sodium thio sulfate as sodium thio sulfate pentahydrate; and from 0.02 g. to 0.04 g. of elemental arsenic as sodium arsenite.

12. The salt composition according to claim 11 and including .25 g. of dissodium ethylene diamine tetraacetate dihydrate.

13. The salt composition according to claim and including as a sequestering agent ethylene diamine tetraacetate dihydrate.

14. A method for depositing a hard, ductile and bright electro deposited gold ha-ving incorporated therein arsenic comprising the steps of:

introducing into an aqueous electrolyte solution a base coupled to a cathode, said electrolyte being a buifered solution of potassium phosphate, potassium citrate or mixtures of same as a bulfering agent and comprising an alkali metal gold cyanide complex having from 10 to 32 g./liter of solution gold expressed as elemental gold, a tris (thio-sulfato) arsenic (III) complex, a thio arsenic (IH) complex, or a mixture of said complexes including an excess of the thio compound and wherein said thio compound is up to a 400 fold by weight on basis of elemental arsenic, said electrolyte solution being maintained at a pH from 5.5 to 8;

impressing a voltage between said cathode and an anode to obtain a current therebetween; and

depositing, at a temperature up to 160 F. on said base from the electrolyte solution, arsenic and gold wherein said gold contains from .1% to 1% of arsenic to form a hard, bright and ductile gold deposit.

15. A method for depositing arsenic containing gold according to claim 14 and wherein said electrolyte comprises on basis of one liter of water:

Grams As a buffering agent:

Monobasic potassium phosphate 60 to 120 Potassium citrate as K C H O -H 'O 60 to 120 Arsenic complex:

Arsenic, as elemental arsenic and added as sodium arsenite .02 to .04 Sodium thiosulfate, as Na S O -5H O 5 to 8 Elemental gold, added as [KAu(CN) 10 to 15 and wherein the pH of the solution is from 6.0 to 6.5 imparted thereto by said buffering agent or mixtures of same.

16. The method for depositing gold according to claim 15 and wherein a sequestering agent is used to safeguard against trace impurities.

17. The methodaccording to claim 15 and wherein a sequestering agent is used to safeguard against incorporation of trace amounts of metallic impurities, said sequestering agent being used in an amount equivalent to, in activity, on a one liter basis of the solution of .25 g./liter, of the same solution of disodium ethylene diamine tetraacetate dihydrate.

18. A method for depositing arsenic containing gold according to claim 14 and wherein said electrolyte is for high speed belt plating and comprises:

Elemental gold, added as [KAu(CN) 31.1

Monobasic potassium phosphate Potassium citrate 90 Arsenic complex as:

Elemental arsenic added as sodium arsenite .10

Sodium thio sulfate as arsenic complexing agent 10 and wherein the solution for said belt plating is to yield a gold deposit containing 0.1% to 1% by weight of arsenic.

19. The method according to claim 18 and wherein as a sequestering agent ethylene diamine tetracetate is used.

20. A method for depositing arsenic containing gold according to claim 14 wherein the arsenic complex is derived from an arsenic compound and sodium thio sulfate, thio urea, or thio chrysine.

21. A method for depositing arsenic containing gold according to claim 20 wherein the arsenic complex is formed in situ and wherein the arsenic is introduced as sodium arsenite.

22. A method for depositing gold according to claim 14 and wherein said electrolyte solution comprises: as a buffering agent, monobasic potassium phosphate, potassium citrate or mixtures of same; an arsenic complex added to said solution as sodium arsenite and sodium thio sulfate to yield 0.02 g./liter of solution to .70 g./ liter of solution of elemental arsenic; elemental gold, added as KAu(CN) in an amount from 10 g./l. to 32 g./l. of solution, the pH of said aqueous solution being in the range of 5.5 to 8 and the deposited gold having from .1 to 1% by weight of arsenic in said gold.

23. The method for depositing arsenic containing gold according to claim 14 and wherein said aqueous electrolyte comprises:

Elemental gold, added as [KAu(CN)2] 31.1 Arsenic complex, as:

Elemental arsenic, added as sodium arsenite .10

Sodium thio sulfate 10 24. A method for depositing arsenic containing gold according to claim 14 and wherein a 250 fold excess by weight of sodium thio sulfate in reference to elemental arsenic is used as an arsine suppressing agent.

11 25. An aqueous electrolyte solution for depositing hard and bright arsenic containing gold, said solution comprising:

G./l. Monobasic potassium phosphate 60 Potassium citrate 60 Gold as potassium gold cyanide 10.7 to 14 Arsenic as sodium arsenite .2 to .4

As complexing agent for said arsenite, sodium thiosulfate Elemental gold, added as [KAu(CN) 31.1

Potassium citrate 90 Monobasic potassium phosphate 90 Arsenic complex as:

Elemental arsenic added as sodium arsenite .10

Sodium thio sulfate as arsenic complexing agent impressing a voltage between said cathode and a nonconsumable anode to obtain a current therebetween; and

depositing, at a temperature up to 160 F. on the metal base from the electrolyte solution, arsenic and gold to form a hard, bright and ductile gold deposit.

27. An aqueous electrolyte solution for depositing hard and bright arsenic containing gold, said solution having a pH range from 5.5 to 8, comprising an alkali metal containing buffer for imparting said pH range to said solution, an alkali metal gold cyanide complex, and a tris (thio-sulfato) arsenic (HI) complex or thio arsenic (III) complex to harden and brighten the gold by inclusion of arsenic in a gold deposit, wherein the arsenic complex is derived from an arsenic compound and sodium thio sulfate, thio urea, or thio chrysine.

28. A salt composition suitable for deposition of gold from an electrolyzed aqueous solution thereof comprising: gold as potassium gold cyanide; arsenic as a trivalent arsenic compound; and as a complexing agent for said arsenic, thio urea, thio chrysine, or sodium thio sulfate.

References Cited UNITED STATES PATENTS 3,666,640 5/1972 Smith 204-43 X 3,423,295 1/1969 Greenspan 204-43 3,475,292 10/1969 Shoushanian 204-43 X 3,520,785 7/1970 Duva 20443 GERALD L. KAPLAN, Primary Examiner US. Cl. X.R.