CA1048964A

CA1048964A - Gold plating solutions and method

Info

Publication number: CA1048964A
Application number: CA74215619A
Authority: CA
Inventors: Lawrence Greenspan
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-12-13
Filing date: 1974-12-10
Publication date: 1979-02-20
Also published as: US3902977A; FR2254658A1; IT1024416B; FR2254658B1; JPS5092833A; DE2458913A1; CH606503A5; GB1453386A; AU7622274A

Abstract

ABSTRACT OF THE DISCLOSURE
An aqueous gold plating solution for use in depositing bright gold plating is provided, which includes an alkali gold cyanide as the gold source, a metal chelate of an iminodiacetic acid, such as the indium or nickel chelate of iminodiacetic acid, which metal codeposits in small quantities with the gold, one or more conducting salts and a buffer, to maintain said solution at a pH within the range of from about 3 to about 4.5. The aqueous gold plating solution may optionally include a metal chelate of an aminopolycarboxylic acid, such as the nickel chelate of ethylenediamine tetraacetic acid.
In addition, a method of electrodepositing gold on a substrate employing the gold plating solutions described above and gold-plated substrates or articles produced thereby are also provided.
A synergistic chelate combination employed in the aforedescribed gold plating solutions to produce mirror bright gold finishes is also provided, which combination includes the nickel chelate of iminodiacetic acid, the nickel chelate of ethylenediamine-tetraacetic acid and the cobalt chelate of iminodiacetic acid.

Description

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~ 10~8964 ~ ~ The present invention relates to gold platinq solutions. In its more specific aspect, this inven~tion relates to gold plating solutions and to the method of gold plating using these solutions to obtain bright gold finishes and to gold-plated articles produced thereby.
Gold plating by electrodeposition employing a gold plating bath or electrolyte is a long known and widely used technique.
However, in recent years, the price per ounce of gold has s~ared making the cost of gold practically prohibitive and resulting in a concomitant increase in the price of gold-plated articles.
Nevertheless, the demand for such gold-plated articles is increasing.
This is particularly true of gold-plated articles which for all practical and aesthetic purposes take on the appearance of pure gold articles, that is articles which have a hard, pale-colored, smooth, mirror- ;
bright surface.
Over the years, gold plating has been carried out primarily employing an electrolytic bath containing an alkali metal aurocyanide as the gold source, which may also include other soluble metal salts. One such type of electrolytic bath is operated on the alkaline side, and i5 usually maintained at a pH in the range of 8 to 12 so as to avoid formation of the extremely toxic hydrogen cyanide gas as well as the possible formation of insoluble aurous cyanide.
An efficient electroplating operation utilizing the alkaline electrolytic baths described above, must operate at relatively high temperatures and usually at rather low current densities. Under these conditions, it is difficult to obtain thick bright gold deposits, that is, a deposit having a thickness greater than 50 microinches.
It, therefore, has been common practice to add to such an alkaline bath certain soluole metal salts which are codeposited with the gold to impart certain desirable qualities to the plate such as color, hardness and wear resistance.

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4~3,964 It is also known to employ gold plating baths on the weak acid side, for example, at a pH ranging from 3 to 5, wherein alkali metal aurocyanide is employed as the source of gold. Such a technique and bath are disclosed in U.S. Patent No. 2,905,601 to Rinker et al.
The problem of hydrogen cyanide gas formation is overcome by employing with such baths a weak stable organic acid, such as acet _ acid, citric acid, tartaric acid, lactic acid or similar acids which may be neutralized with ammonium hydroxide to give the desired pH. These baths also include one or more soluble base metal salts, a portion of the metal of which codeposits with the gold to produce a plate of the desired color and thickness.
Unfortunately, it has been found that where weak organic acids are employed in gold plating baths such as disclosed in U.S. Patent No. 2,905,601, oxidation at the insoluble anode occurs producing organic decomposition products which discolor the bath and adversely affect the resulting plate.
British Patent No. 919,463 which relates .o the alectropl t-ng of bright gold, discloses an improvement over the technique and bath disclosed in U.S. Patent No. 2,905,601. The improvement resides in the inclusion of a metal compound in the form of a soluble metal compound or chelate (in place of the base metal salts of 2,905,601! which ionizes to produce metal ions in the electrolytic bath in sufficient quantity to codeposit with the gold to give bright metal alloy deposits. The only examples of the solubie metal compound or chelate disclosed are nickel dimethylethanolamine and cobalt triglycollamate.
U.S. Patent No. 3,149,057 to Parker et al discloses acid gold plating baths which include an alkali metal aurocyanide as the gold source, with or without additional alloying salts, the cobalt chelate of ethylenediamine tetraacetic acid, or a cobalt chelate of a chelating agent having a stability constant of about the same order of magnitude, and at least one buffer material to maintain the pH between 3 and 4.5 during the deposition of the qold from the solution.

~ 94~964 This patent indicates that the cobalt chelate is such that cobalt is not deposited under plating conditions, but that the cobalt chelate is employed to form a complex with the alkali metal gold cyanide so that hydrogen cyanide will not be released even when the pH of the bath is maintained between 3 and 5. The bath may also include metal salts chelates or complexes, wherein the metal component thereof will plate out; examples of such compounds disclosed are antimony tartrate, nickel or cadmium ethylenediamine tetraacetic acid and similar compounds of indium, zinc, copper and iron.
U.S. Patent No. 3 ,149,058 to Parker et al discloses a gold plating bath which includes an alkali metal aurocyanide, with or without additional alloying components, the nickel chelate of an appropriate polycarboxylic acid, having a stability constant of the order of magnitude of about 10, and at least one buffer material to maintain the pH between 3.5 and 5.5 during the deposition of the gold from the solution. As indicated in working Example II thereof, only about 0.18 percent nickel is present in the gold plate.
It has been found that where alkali metai gold cyanide is employed in a gold plating bath with nickel or cobalt chelatesOf aminopolycarboxylic acids, with or without other alloying components, as disclosed in the aforedescribed British Patent 919,483 and U.S.
Patents Nos. 3,149,057 and 3,149,05~,the gold alloy deposit is not sufficiently thick to exhibit the desired brightness and color of the resulting plate, especially for decorative purposes such as in jewelry.
It is theorized that the nickel, cobalt or other base metals are complexed to too high a degree to enable sufficient metal ion con-centration to be available for plating purposes to produce the desired thickness and color.
In accordance with the present invention, an aqueous gold plating bath or electrolytic solution having a pH ranging from ~ ' :
1(~i4~964 about 3 to about 4.5 is provided which is not subject to the oxidation and complexing problems associated with prior art acid gold plating baths, and produces bright gold alloy deposits of thicknesses of 200 microinches or more, possessing exce~lent metallurgical properties such as ductility, hardness and wear properties. The gold plating solution of the invention is a stable solution which is easily maintained, and which will provide a constant color and uniform deposition even when the temperature, pH, current density, agitation, metallic contact or anode to cathode ratio vary. The gold alloy deposits produced in accordance with the invention require no buffing or other mechanical operations to further enhance their brightness and have a pleasing range of colors that are acceptable to the jewelry trade and are of a sufficiently high Karat to be non-tarnishing.
Thus, in accordance with the present invention, there is pro-vided an aqueous electrolytic bath for use in electroplating gold alloys, which bath has a pH within the range of from about 3 to about 4.5 and comprises a gold cyanide as the gold source, at least one metal chelate of an iminodiacetic acid, at least a portion of the metal of which ionizes and codeposits with the gold, at least one conducting salt, and a buffer to maintain said bath at the desired pH.
In a preferred embodiment of the invention, the bath also includes one or more metal chelates of aminopolycarboxylic acids.
The gold cyanide, employed in the aqueous electrolytic bath as the gold source, is desirably an alkali metal gold cyanide or ammonium double cyanide of gold. Examples of such compounds include potassium aurocyanide, sodium aurocyanide and ammonium aurocyanide, and the preferred gold cyanide is potassium aurocyanide. The gold in the form of the cyanide complex present in the bath ranges from about 1 to about 24 grams/liter, and preferably from about 3 to about 8 ~ grams/liter, depending upon the thickness of gold deposit desired.
~r Where gold eereYrtYoei-~ of less than 1 gram/liter are employed, low current densities are employed and cono~mitant long plating times are required to form a plate of desired thickness. Thus, the use of such low concentrations would not be commercially feasible. Although gold concentrations of more than 24 grams/liter may be employed, the re-sulting plate is usually too yellow in color for decorative purposes - such as in jewelry.
The metal helate of an iminodiacetic acid employed in the aqueous electrolytic bath should have a stability constant of the order of magnitude of less than about 10 so that it will ionize to produce sufficient metal ions which codeposit with the gold.
The bath contains about-S to about lOgrams/litermetal in the form of metal chelate of an iminodiacetic acid, and preferably from about 0.5 to about 3 grams/liter. The concentration and stability constant of these metal chelates determine the amount of metal codeposited with the gold and enhances the gold plate. If less than 0.05 grams/liter of such metal in the form of the chelate of an iminodiacetic acid is employed, the plating solution will not contain a sufficient ion concentration of such metal for codeposition with the gold to produce *he desired alloy deposit. Where more than 10 grams~liter of such metal in the form of the above chelate is employed, the concentration of such metal codepositing with gold will be unduly I high and may produce a plate of undesirable color. Furthermore, the specific gravity of the plating solution may be so high to cause crystallization of some of the metal chelates present therein.
Suitable metal chelates include those in which the metal portion is selected from the group consisting of indium, nickel, cobalt, zinc and cadmium.
The ~minodiacetic acid portion of the metal chelate has the formula ~=C~2Coo C~2COOH
wherein R is hydrogen or an organic radical selected from the group con~isting of lower alkyl, acetamide, cyano-lower alkyl,ttri-lower alkylammoniw~ lower alkyl, N-lower alkoxy-lower alkyl, hydroxy-lower alkyl, N-carbethoxy-p-aminoethyl, N-mercapto-lower tB ~5~
.. . . .
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10~8964 ....~", alkyl and N-lower alkylthio-lower alXyl. The term "lower alkyl" as employed in the radicals refers to straight chain or branched chain radicals of up to and including eight carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, s-butyl, t-butyl, pentyl, hexyl, heptyl, octyl and the like. The term "lower alkoxy" includes straight and branched chain radicals of the structure RlO wherein Rl includes any of the above lower alkyl groups.
Examples of metal chelates which may be employed in the aqueous electrolytes ofthe invention include the cobalt, indium, nickel, zinc or cadmium chelates of iminodiacetic acid, the cobalt, indium, nickel, zinc or cadmium chelates of N-methyliminodiacetic acid, the cobalt, indium, nickel, zinc or cadmium chelates of N-acetamidoiminodiacetic acid, the cobalt, indium, nickel, zinc or cadmium chelates of ~-(N-trimethylammonium)-ethyliminodiacetic acid, the cobalt, indiu~, nickel, zinc or cadmium chelates of N-cyanomethyl-iminodiacetic acid, the cobalt, indium, nickel, zinc or cadmium chelates of N-methoxyethyliminodiacetic acid, the cobalt, indium, nickel, zinc or cadmium chelates of N-hydroxyethyliminodiacetic acid, the cobalt, indium, nickel, zinc or cadmium chelates of N-3-hydroxy-propyliminodiacetic acid, the cobalt, indium, nickel, zinc or cadmiumchelates of N-carbethoxy- ~ -aminoethyliminodiacetic acid, the cobalt, indium, nickel, zinc or cadmium chelates of ~-mercaptoethyl-iminodiacetic acid, and the cobalt, indium, nickel, zinc or cadmium chelates of N-methythioethyliminodiacetic acid.
The preferred chelates are the indium or nickel chelates of iminodiacetic acid, such as potassium nickel iminodiacetate and potassium indium iminodiacetate.
As indicated, the electrolytic bath may optionally include one or more metal chelates of aminopolycarboxylic acids which should have a stability constant ranging from about 6 to about 25. Where the metal chelate has a stability constant of less than about 6, the free ion concentration of such metal will be excessively high 1al48~64 resulting in an uneven plate. A metal chelate having a stability con-stant greater than 25 will not provide a sufficient ion concentration available for codeposition. These chelates are employed to impart desired tint to the gold alloy plate. For example, where the metal chelate is an indium chelate, a portion ofthe indium will codeposit with the gold to provide a greenish-yellow tint to the plate.
Where the metal chelate is a nickel chelate, a portion of the nickel will codeposit with the gold to provide a light or pale yellow tint to the plate. Where the metal chelate is cobalt chelate, a portion of the cobalt will codeposit with the gold to provide an orange tint to the plate. Furthermore, use of the cobclt chelate allows for use of increased current density without causing dulling of the plate.
The concentration of metal chelates of the aminocarboxylic acids in the electrolytic bath ranges from o to about 15 grams/liter, pre-ferably from about 0.25 to about 10 grams/liter, depending upon the tint desired in the electroplate or coating. Higher concentrations of such metal chelates do not appear to provide any additional beneficial effect. Examples of the metal chelates of aminopolycarboxylic acids suitable for use herein include, but are not limited to, nickel cobalt, indium, zinc, and cadmium chelates of ethylenediamine tetraacetic acid, hydroxyethyl ethylenediamine triacetic acid, hydroxypropyl ethylenediamine triacetic acid, ethylenediamine diacetic acidand the like. The preferred chelates are the nickel indium and cobalt chelates of ethylenediamine tetraacetic acid.
The conducting salt added to the aqueous electrolytic bath may include ammonium or alkali metal sulfamates, as well as any of the sulfates, sulfamates, formates, acetates, citrates, lactates~
- tartrates, fluoborates, borates, phosphates, carbonates and bicarbonates O~ metals such as nickel, zinc, cobalt, indium, iron, manganese, anti-mony, copper, alkali metals such as potassium or sodium and the like.

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The bath may include one, two or more of such conducting salts as long as the added salts are soluble and compatible with all other bath ingredients and will not cause undesired precipitation. For -example, where an indiu~ chelate is employed in the bath, such as the indium chelate of iminodiacetic acid, phosphate or other phosphorus containing salts should be avoided. Normally, such conducting salts will be employed in an amount within the range of from about 10 to about 150 grams/liter and preferably within the range of from`
about 15 to about 40 grams/liter. The concentration of conducting salts should be maintained within the above-mentioned broad range inasmuch as if the concentration thereof is below 10 grams/liter the plating solution will not be sufficiently conductive, while if the concentration thereof is above 150 grams/liter the specific gravity of the plating solution will be undesirably high with the result that one or more of the metal chelates may crystallize out of the solu-tion.
A suitable buffer is added to the aqueous electrolyte to main-tain the bath within the desired pH range of from about 3 to about 4.5. Preferably, the buffer comprises one or more organic acids which has at least one ionization constant of such a value as to create a buffer region in the above pH range. Examples of such organic acids -suitable for use herein include, but are not limited to, aminoacetic acid~ sulfamic acid, citric acid, citraconic acid, itaconic acid, lactic acid, gluconic acid, glutaric acid, glycollic acid, tartaric acid, acetic acid, kojic acid, formic acid and the like. In addition, inorganic acids such as boric acid, sulfuric acid, phosphoric acid and the like may be employed. The above acids may be employed in the form of salts as well.
In order to bring the electrolytic bath to the proper pH, the bath may be neutralized with an appropriate amount of base.
Examples of such bases suitable for use herein include ammonium hydroxide, ammonia, alkali metal hydroxides such as potassium or sodium hydroxide and the like.

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`: ~0~3964 The buffer will ordinarily be employed in an amount within the range of from about 5 to about 200 grams/liter to maintain the electro~ytic bath at the desired pH of from about 3 to about 4.5 and preferably from about 3.5 to about 4.2. If the plating solution is maintained at a pH below 3, some of the gold cyanide may preci-pitate. Furthermore, if the plating solution is maintained outside of the above pH range it will be difficult to obtain the codeposition of metals and the desired color in the plate. It will be appreciated that a smaller amount of the buffer may be required than expected where other components of the electrolytic bath also provide bufering aotion, such as the nickel chelate of ethylenediamine tetraacetic acid.
A plating exhibiting a mirror bright finish is achieved by adding to the electrolytic bath of the invention the nickel chelate of iminodiacetic acid, the nickel chelate of ethylenediamine tetraacetic acid and the cobalt chelate of iminodiacetic acid. This combination of nicXel chelates and cobalt chelatehasa synergistic effect and produces a coating superior to one produced when the nickel chelates ~r~ and the cobalt chelate are used a~one, as well as the additive effect ' C0~77 0~ ~7 thereof. A bath using this ir.~ir~t*~= of chelates produces an ~0 intense brightness and mirror bright finish.
The nickel in the form of the nickel chelate of ethylenediamine tetraacetic acid is employed in a weight ratio to the nickel in the form of nickel chelates of iminodiacetic acid of within the range of from about 10:1 to about 0.2:1, preferably from about 5:1 to about O.S:l. Cobalt in the form of the cobalt chelate is employed in a weight ratio to the nickel in the form of the nickel chelate of iminodiacetic acid ofwithin the range of from about 10:1 to about 0-o5:l~preferably from about S:l to about 0.2:1~ The ratios of chelates should be maintained within the above ranges in order to obtain the desired synergism.
The electroplating process is carried out at a current density within the range of from about 1 to about 40 amperes per square .

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1~48964 foot, preferably 5 to 20, while maintaining the bath at a temperature within the range of from about 20 to about 80C. and preferably from about 25 to about 40C. If the current density is maintained below 1 ampere per square foot, an inordinate amount of plating time will be required in order to obtain a plate of desired thickness. On the other hand, if the current density is maintained above 40 amperes per square foot, the resulting plate will be undesirably dull and may be burnt. Furthermore, where the temperature of the plating solution is maintained at below about 20C, the resulting plate will not have the desired brightness. Where the temperature is maintained above 80C, the decomposition of one or more of the components of the plating solution may result.
The electrodes used may include such materials as platinum, gold, stainless steel, platinized titanium or carbon as the anode and such materials as steel, copper, nickel, brass and the like as the cathode (the wares plated). The ratio of cathode to anode sur-face area, whiie not critical, should be within the range of from about 1:1 to about 1:4 and preferably about 1:2. It is customary to agitate the bath, such as with a stirrer, or to move the work (pieces to be plated) as with a cathode oscillating rod to facilitate uniform and smooth deposi~ion. The voltage between the anode and the wares to be plated as the cathode will usually be between 2 and 6 volts.
My invention provides a heavy plate onto a base metal or ware such as stainless steel, brass, copper, which is bright, stain resistant and corrosion resistant. The plate or coating will have a thickness of from about 5 to about 200 microinches or more and may have a thickness of 500 microinches or more. Furthermore, where the afore-described synergistic combination of chelates is employed, the resulting plate will be mirror bright and will be substantially brighter than deposits produced employing baths containing one or two of the individual components of such synergistic combination. The gold alloy plate r~

will comprise from about 90 to about 99.8% gold and from about 10 to about 0.2% of the alloying compohents such as indium, nickel, cobalt, zinc or~cadmium or mixtures thereof. Such plate will have a Knoop hardness of at least 120 Knoop and in some cases as high as 250 Knoop.

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16~4~3~64 The following working examples represent preferred embodi-ments of the present invention:

E~AMPLE 1 In an amount of water to form 1 liter of solution was dissolved:

4 grams of gold in the form of potassium gold cyanide 1 gram of nickel a~ potassium nickel iminodiacetate 20 grams ammonium phosphate 20 grams ammonium sulfamate 10 grams aminoacetic acid The pH wasadjusted to 3.5-3.8 with phosphoric acid.
polished brass test panel l"x3" was plated using a current density of S amperes per square foot at a temperature of 35C., for 50 minutes. A mirror bright pale yellow deposit 2.5 microns (100 microinches) was obtained, having a Knoop hardness of approximately 250. Above this thickness,a slight haziness tends to develop.
Also where the current densitywas increased to 10 amperes per square foGt the deposit tended to become dull around the edges of the panel.

~X~PLE 2 In an amount of water to form 1 liter of solution was dissolved:

4 grams of gold in the form of potassium gold cyanide 1 gram of nickel as potassium nickel iminodiacetate 3 grams of nickel as potassium nickel ethylenediamine-tetraacetate 20 grams ammonium phosphate 20 grams ammonium sulfamate 10 grams aminoacetic acid The pHwasadjusted to 3.5-3.8 with phosphoric acid.
A polished brass test panel lnx3" was plated using a current density of 10 amperes per square foot at a temperature of 35C., for 25 minutes. A mirror bright pale yellow deposit 2.5 microns (100 microinches) was obtained (Knoop hardness about 250). Above this thickness the deposit tended to become slightly hazy. Where the current densitywas increased to 15 amperes per square foot,the deposit tended to become dull around the edges of the panel.

0~3964 In an amount of water to form 1 liter of solution was dissolved:
4 grams of gold in the form of potassium gold cyanide 1 gram of nickel as potassium nickel iminodiacetate 3 grams of nickel as potassium nickel ethylenediamine-tetracetate 0.1 gram cobalt as potassium cobalt iminodiacetate 20 grams ammonium phosphate 20 grams ammomium sulfamate 10 grams aminoacetic acid - The pH WaSadjusted to 3.5-3.8 with phosphoric acid.
A polished brass panel lnx3~ was plated using a current density of 10 amperes per square foot at a temperature of 35C for 50 minutes. A mirror bright pale yellow deposit 5 microns (200 microinches) was obtained. Above this thickness the deposit remained mirror bright.
Another panel was plated using a current density of 15 amperes per square foot for 33 minutes yielding a plate of the same thickness and color and was mirror bright.

EXA*IPLE 4 In an amount of water to form 1 liter of solution was dissolved:
4 grams of gold in the form of potassium gold cyanide 1 gram of nickel as potassium nickel iminodiacetate 3 grams of nickel as potassium nickel ethylenedi~ine tetraacetate 1 gram of indium as sodium indium ethylenediamine-tetraacetate 0.1 gram cobalt as potassium cobalt ethylenediamine tetraacetate 40 grams ammonium sulfamate 10 grams ammonium citrate 10 grams aminoacetic acid The pH wasadjusted to 3.5-3.8 with phosphoric acid.
A polished brass panel l~x3" was plated using a current density of lS amperes per square foot at a temperature of 35 for 33 minutes.
A mirror bright deposit 5 microns (200 microinches was obtained).
The color of the deposit was pale orange yellow. This color is a standard color, widely acceptable commercially in the jewelry trade.

11~)48964 In an amount of water to form l liter of solution was dissolved:
4 grams of gold in the form of potassium gold cyanide l gram of indium as potassium indium iminodiacetate 10 grams ammonium citrate 10 grams glycine (aminoacetic acid) 40 grams ammonium sulfamate The p~ was adjusted to 3.5-3.8 with sulfamic acid. A
polishsd brass panel l"x3" was plated using a current density of 5 amperes per square foot at a temperature of 35C for 15 minutes. A
mirror bright greenish yellow deposit 2 V2 microns (lO0 microinches) was obtained.

In an amount of water to form 1 liter of solution was dissolved:
; 4 grams of gold in the form of potassium gold cyanide l gram of zinc as potassium zinc iminodiacetate 20 grams ammonium phosphate 40 grams ammonium sulfamate 20 grams aminoacetic acid The pHwas adjusted to 4.0-4.2 with phosphoric acid. A
polished brass panel l"x3" was plated using a current density of 5 amperes per square foot for 15 minutes. A mirror brioht yellowish white deposit 2 1/2 microns (lO0 microinches) was obtained.

In an amount of water to form 1 liter of solution was dissolved:
4 grams of gold in the form of potassium gold cyanide l gram of zinc as potassium zinc iminodiacetate l gram of nickel as potassium nickel iminodiacetate 20 grams ammonium phosphate 40 grams ammonium sulfamate 20 grams aminoacetic acid The p~was adjusted to 3.8-4.2 with phosphoric acid. A
polished brass panel lHx3" was plated using a current density of lO amperes per square foot. A mirror bright white gold deposit of 5 microns (200 microinches) was obtained.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An aqueous electrolytic bath for gold plating comprising a gold cyanide as a gold source in an amount sufficient to provide a gold concentration within the range of from 1 to 24 grams/liter, a metal chelate of an iminodiacetic acid in an amount to provide a concentration of said metal within the range of from 0.05 to 10 grams/
liter, at least one conducting salt in an amount within the range of from 10 to 150 grams/liter, and a buffer to maintain said electrolytic bath at a pH within the range of from 3 to 4.5.

2. The electrolytic bath in accordance with claim 1 wherein said metal iminodiacetic acid chelate has a stability constant of at most about 10.

3. The electrolytic bath in accordance with claim 2 wherein said metal of said metal iminodiacetic acid chelate is indium.

4. The electrolytic bath in accordance with claim 2 wherein said metal of said metal iminodiacetic acid chelate is nickel.

5. The electrolytic bath in accordance with claim 2 wherein said metal of said metal iminodiacetic acid chelate is cobalt.

6. The electrolytic bath in accordance with claim 2 wherein said metal of said metal iminodiacetic acid chelate is cadmium.

7. The electrolytic bath in accordance with claim 2 wherein said metal of said metal iminodiacetic acid chelate is zinc.

8. The electrolytic bath in accordance with claim 1 wherein said iminodiacetic acid portion of said metal chelate has the formula:

wherein R is hydrogen or an organic radical selected from the group consisting of lower alkyl, acetamide, cyano-lower alkyl, (tri-lower alkylammonium) lower alkyl, N-lower alkoxy-lower alkyl, hydroxy-lower alkyl, N-carbethoxy-p-aminoethyl, N-mercapto-lower alkyl and N-lower alkylthio-lower alkyl.

9. The electrolytic bath in accordance with claim 8 wherein R is hydrogen.

10. The electrolytic bath in accordance with claim 8 wherein R is methyl.

11. The electrolytic bath in accordance with claim 1, including in addition, a metal chelate of an aminopoly-carboxylic acid having a stability constant ranging from 6 to 25.

12. The electrolytic bath in accordance with claim 11 wherein said metal chelate of an aminopolycarboxylic acid is a metal chelate of ethylenediamine tetraacetic acid.

13. The electrolytic bath in accordance with claim 12 wherein said metal chelate of ethylenediamine tetraacetic acid is the nickel chelate of ethylenediamine tetraacetic acid and the metal chelate of an iminodiacetic acid is the nickel chelate of iminodiacetic acid.

14. The electrolytic bath in accordance with claim 13, including, in addition, the cobalt chelate of iminodiacetic acid.

15. The electrolytic bath in accordance with claim 13, including, in addition, the indium chelate of ethylenediamine tetraacetic acid.

16. The electrolytic bath in accordance with Claim 1 wherein said gold cyanide is an alkali gold cyanide selected from the group consisting of alkali metal gold cyanides and ammonium gold cyanide.

17. The electrolytic bath in accordance with Claim 16 wherein said alkali gold cyanide is potassium aurocyanide.

18. The electrolytic bath in accordance with Claim 16 wherein said metal chelate of animinodiacetic acid is the nickel chelate of iminodiacetic acid, said electrolyte further comprising the nickel chelate of ethylenediamine tetraacetic acid, and a chelate selected from the group consisting of the indium chelate of ethylenediamine tetraacetic acid and the cobalt chelate of imino-diacetic acid and mixtures thereof.

19. The electrolytic bath in accordance with Claim 18 wherein said conducting salt comprises a water-soluble ammonium salt or alkali metal salt.

20. The electrolytic bath in accordance with Claim 18 wherein said buffer comprises a weak organic acid.

21. The electrolytic bath in accordance with Claim 1, wherein said gold cyanide is present in an amount within the range of from about 1 to about 24 grams/liter, said metal chelate of an iminodiacetic acid is present in an amount within the range of from about 0.25 to about 10 grams/liter and said conducting salt is present in an amount within the range of from about 10 to about 150 grams/liter.

22. The electrolytic bath in accordance with Claim 21 wherein said buffer is present in an amount within the range of from about 5 to about 200 grams/liter.

23. The electrolytic bath in accordance with claim 1, wherein said gold cyanide comprises ammonium aurocyanide or potassium aurocyanide in an amount within the range of from about 1 to about 24 grams/liter, said metal chelate of an iminodiacetic acid comprises the nickel chelate of imino-diacetic acid in an amount within the range of from about 0.25 to about 10 grams/liter, the conducting salt comprises ammonium or alkali metal sulfamate in an amount within the range of from about 10 to about 150 grams/liter and the buffer comprises aminoacetic acid in an amount within the range of from about 5 to about 200 grams/liter, and further including the nickel chelate of ethylenediamine tetraacetic acid in an amount within the range of from about 0.25 to about 10 grams/liter.

24. The electrolytic bath in accordance with claim 23, further including the indium chelate of ethylenediamine tetraacetic acid in an amount within the range of from about 0.25 to about 15 grams/liter.

25. The electrolytic bath in accordance with claim 23, further including the cobalt chelate of iminodiacetic acid in an amount within the range of from about 0.05 to about 10 grams/liter.

26. A method of electroplating gold which comprises electrolyzing an aqueous solution having a pH within the range of from about 3 to about 4.5 comprising the aqueous electrolytic bath as defined in claim 1, to form an electro-plated coating of gold and a small amount of said metal of said metal chelate of an iminodiacetic acid.

27. The method in accordance with claim 26 wherein said metal chelate of an iminodiacetic acid is selected from the group consisting of the nickel and indium chelates of iminodiacetic acid and mixtures thereof.

28. The method in accordance with claim 26 wherein said metal chelate of an iminodiacetic acid is selected from the group consisting of the cobalt, zinc and cadmium chelates of iminodiacetic acid and mixtures thereof.

29. The method in accordance with claim 26 wherein said aqueous electrolytic bath further includes up to 15 g/l of a metal chelate of an aminopolycarboxylic acid having a stability constant ranging from 6 to 25.

30. The method in accordance with claim 29 wherein said metal chelate of an aminopolycarboxylic acid comprises the nickel chelate of ethylenediamine tetraacetic acid.

31. The method in accordance with claim 30 wherein said electrolytic bath further includes a chelate selected from the group consisting of the cobalt chelate of imino-diacetic acid and the indium chelate of ethylenediamine tetraacetic acid, and mixtures thereof.

32. A gold-plated article prepared from the electro-lytic bath in accordance with claim 1 comprising a substrate electroplated with a coating of gold alloy, said alloy comprising from 90 to 99.8% by weight and from 10 to 0.2%
by weight of a metal selected from the group consisting of nickel, indium, cobalt, zinc and cadmium and mixtures thereof, said metal codeposited from its chelate thereof with said gold, said coating having a thickness within the range of from 5 to 500 microinches.