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United States Patent Office Patented Sept 3,149,57 United States Patent Office Patented Sept. 15, 1964 2 Gold alloy plates may also be obtained from this bath 3,149,957 ACD GOLD PATNG by incorporating into the bath certain metal salts, com Edward A. Parker, Cransion, and James A. Powers, East plexes, or chelates, because such other metals plate out Providence, R.I., assignors to Technie, Inc., Providence, with the gold. Among such compounds may be men R.E., a corporation of Rhode island tioned antimony tartrate, nickel or cadmium ethylene No Drawing. Filed Apr. 27, 1959, Ser. No. 808,913 diamine tetra-acetic acid. Similar compounds of indium, 1. Claims. (C. 204-46) zinc, copper, iron may be used to obtain alloy plates. It is, therefore, a primary object of the present inven The present invention relates to the electrolytic de tion to provide an aqueous acid solution from which gold position of gold on surfaces and more particularly to the O may be electrodeposited in which the harmful conse electrodeposition of gold and gold alloys from solutions quences of the formation of free hydrogen cyanide are of the same onto objects to develop a gold or gold alloy eliminated. appearance. It is another object of this invention to provide a bath It has been customary for many years in the electro for the deposition of substantially 24 karat gold electro plating of gold and gold alloys to use a bath containing plate which is characterized by higher hardness, greater an alkali metal aurocyanide, with or without other soluble wear resistance, and increased speed of plating than that metal salts. In addition to the above compounds, certain obtained in a conventional 24 karat bath. other additives have been used to increase the conductivity It is another object of the present invention to provide of the bath; to increase throwing power; to maintain a solution from which gold and gold alloys may be elec color; or to provide a buffered pH region in which the 20 trodeposited in a clean, smooth fashion to give a gold or bath was to operate. A few examples of these additives gold alloy plate which is of high reflectivity and will pro are free alkali metal cyanide, alkali metal hydroxide, vide the mirror image of an object placed before it. alkali metal carbonate or bicarbonate, and, almost always, It is a further object of the present invention to pro a di-basic, or tri-basic alkali metal phosphate. In all in vide a solution from which gold and gold alloys may be stances, however, the pH of the aurocyanide bath in plat 25 electrodeposited in which the harmful effects of alkali ing operations is never reduced below pH=6, and is most and alkali producing materials are avoided. usually maintained in the pH range of 8 to 12. The present invention, accordingly, comprises a bath Gold baths of this conventional type containing free from which gold may be electrolytically deposited, the alkali cyanide are never allowed to operate on the acid bath essentially consisting of an aqueous solution of an side, because of the formation and elimination from the 30 alkali metal aurocyanide, with or without additional al bath of the extremely toxic hydrogen cyanide gas. An loying salts, the cobalt chelate of ethylenediamine tetra additional chemical reason for maintaining alkaline baths acetic acid, or a cobalt chelate of a chelating agent hav resides in the fact that where an alkali metal aurocyanide ing a stability constant of about the same order of mag solution is contacted with a strong mineral acid, one nitude, and at least one buffer material to maintain the cyanide radical is displaced from the aurocyanide complex pH between 3.0 and 4.5 during the deposition of the gold anion and there separates from the solution insoluble from the solution. aurous cyanide. The maintenance of the pH value between 3.0 and 4.5 Such 24 karat baths must be operated hot and are usu is most easily accomplished by incorporating into the ally limited to rather low current densities. It is also bath salts of moderately strong organic acids whose acid difficult to obtain bright deposits in heavy thicknesses, i.e. 40 dissociation constants are of such a nature that they serve greater than .000050'. It, therefore, has been common as buffers in restricting the pH of the solution to limits practice in the prior art to add to such a bath certain as defined during the electro-deposition of the gold or its soluble metal salts which are co-deposited with the gold alloy. Among these substances may be listed, as rep in order to impart certain qualities to the plate such as resentative, certain organic acids which have at least one color, hardness, wear resistance, etc. 45 ionization constant of such a value as to create a buffer We have found that the addition of the cobalt (II) region in the specified pH range, when properly neu chelate of ethylenediamine tetra-acetic acid, or other tralized with the appropriate amount of base. chelating agent giving with cobalt (II) a chelate having In general, the acids most useful are the soluble ali a stability constant of the same order of magnitude, to an phatic acids containing 2 to 8 C atoms, and also may alkali metal aurocyanide at a relatively low pH will pro 50 carry hydroxyl groups in functional relationship to the duce a bright, smooth electrodeposit of gold which is carboxylic groups. Typical useful acids are citric, citra better than 99.8% pure. The amount of cobalt chelate employed in the solution of gold to effect this result may conic, itaconic, lactic, gluconic, glutaric and glycollic. vary from 1 gram per liter up to 50 grams per liter. This list, while not all inclusive of the substances However, we prefer to operate in the region of 20-30 which may be used, is indicative of the general types of grams per liter. Heavy deposits may also be obtained 55 compounds which have been found to be useful. It is which are brilliant in character. By the addition of this also possible to use mixtures of the compounds, pro compound the speed of deposition for bright plating is vided the pH of the final gold bath lies in the range of immensely increased so that current densities of 60 ann 3.0 to 4.5. peres per square foot are possible. It has also been The electrolytic deposition can be carried out using in found that such 24 karat plates are of smaller grain size, 60 soluble anodes such as platinum, gold, stainless steel or increased hardness, less porous, and have better Wear carbon, and replenishment of the gold and its alloying resistance than similar plates obtained from a conven components can be accomplished during plating to main tional 24 karat bath. tain these materials at their original concentrations as It appears that the effect of having present in the solu the bath is used. The ratio of anode surface area to tion a cobalt salt holding the cobalt (II) in such stable 65 cathode surface area should be most preferably not less form that it does not plate out is that the cobalt inhibits than 1:1. The temperature of the bath should be held the growth of coarse crystallites of gold in the plated sur within the limits of 55° to 95° F. during the electro face. It thereby permits the development of bright plate, deposition. It has been found that mechanical agitation pure, at low voltage, high current density over a wide of the bath, such as with a stirrer, serves to facilitate range to assist in obtaining full coverage over all parts of 70 clean, smooth deposition, and at the same time permits the area. the use of higher current densities without smutting the 3. 4. plated surface. The electrical tension between the anodes gold. Plate thickness was 0.0002 inch indicating a cur and the wares is held between 2 and 4 volts. rent efficiency of 40 percent. The following examples illustrate typical composition Barrel plating of small items is readily consummated for use in plating operations in accordance with this in the type of solution illustrated in Examples I-V with invention: a still further improvement in appearance and uniformity Example even at current densities as low as 1-5 amperes per Square foot. The formulation given in Example V will give ex Into an amount of water sufficient to form 1 liter of cellent results. The gold content may be reduced to one solution is dissolved: half this value to reduce losses in dragout. 12 grams of pure gold in the form of potassium gold (I) O It will be evident from the specific examples given that cyanide . the preferred gold plating formula of this invention in 105 grams of citric acid cludes a very substantial molar excess of the organic acid 56 grams of potassium hydroxide over the amount of gold present. Generally it may be 12.6 milliiiters con. phosphoric acid said that at least two moles of the acid or mixture of 1 gram Co as cobalt dipotassium of ethylenediamine 5 acids being used per moie of gold is a preferred range. Within said range the gold is held in complex form, tetra-acetic acid hydrogen cyanide is not released, optimum pH in the This solution may be utilized for the electrodeposition range of 3-5 is achieved and gold plate of mirror-bright of gold in the usual manner with a current density of finish is consistently produced. 5 amperes per square foot (a.s.f.).
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