Electro-galvanic gold plating process
United States Patent [191 Wright et al.  
 [ Apr. 29, 1975 ELECTRO-GALVANIC GOLD PLATING PROCESS [75] Inventors: Ralph R. Wright, Oak Ridge; Calvin C. Wright, Lenoir City; George S. Petit, Oak Ridge, all of Tenn.  
 [22] Filed: Mar. 21, 1974 [21] Appl. No.: 453,480  
 [56] References Cited UNITED STATES PATENTS 5/1957 Pigeon 204 243 6/l968 Greene 204/248 X FOREIGN PATENTS OR APPLICATIONS 931,638 7/1963 United Kingdom 204/46 G 15,383 1899 United Kingdom 204/249 Primary Examiner-G. L. Kaplan Attorney, Agent, or Firm-Dean E. Carlson; David S. Zachry; Irving Barrack [57] ABSTRACT A gold plating arrangement is described which consists essentially of a plating bath consisting of 0.5 to 10 grams per liter lithium or potassium aurocyanide and 15 to 150 grams per liter ammonium chloride dissolved in water, and a base metal in contact with a second metal which occupies a higher position in the EMF series than gold or said base metal. The resultant plate is much denser than that obtainable by conventional electrolytic plating, thus serving as an effective corrosion protective surface against acid media.  
 3 Claims, N0 Drawings ELECTRO-GALVANIC GOLD PLATING PROCESS BACKGROUND OF THE INVENTION The present invention relates to a gold plating process. It was made in the course of, or under, a contract with the US. Atomic Energy Commission.  
  It is a principal object of this invention to provide a novel gold plating bath composition as well as a method for utilizing said composition to achieve a thin, uniform, dense, and adherent gold plate deposited on a selected metal substrate in a method which requires no external source of electricity.  
  Because gold is virtually impervious to chemical attack in highly acidic media, it is ideally suitable for use as a protective coating or cladding on less corrosionresistant metals such as copper, brass, monel (coppernickel), nickel, and bronze. Electrolytic plating processes have routinely been used to apply a gold layer on metal substrates. Experience has shown, however, that the density of the gold plate is frequently insufficient to effectively insulate the metal substrate from corrosive media. In addition, the throwing power of electrolytic processes cannot reach inaccessible areas to guarahtee uniform plating. The same objections apply to the technique of mercurial gilding in which a gold amalga is painted or brushed on a work surface and then he ted to volatilize the mercury. A uniform gold plate ca be obtained by the so-called immersion method w ich simply involves immersion ofa base metal in a suit&#39; ble gold cyanide solution. Plating occurs by virtue of the electrochemical action resulting from proximity of two dissimilar metals, i.e., the plating metal (gold) and the base metal. However, as soon as a thin film of gold is deposited on the base metal, further plating stops because there is then no longer any dissimilar metal surface available to generate the required electrochemiical action.  
  The present invention is similar to the immersion method in that it requires no external source of electricity to effect plating. However, it differs in that electrochemical action can be sustained over a long peri d of time to permit deposition of a thin, dense. adher nt gold film of a desired thickness.  
 SUMMARY OF THE INVENTION According to the invention, a sustained electrogalvanic action is obtained in a selected gold plating solution between a metal workpiece and a metal electrode in contact with said workpiece, in which said metal electrode occupies a higher position in the electromotive series than gold or the metal workpiece, with reference to Lunges Handbook of Chemistry, ninth Edition, p. 1,218, in a selected gold plating solution which has the capacity to dissolve the electrode metal and maintain its ions in solution. The gold plating bath in which sustained electro-galvanic and hence plating action takes place to permit practice of the process aspect of this invention is composed of 0.5 to grams/liter lithium or potassium aurocyanide and -150 grams/- liter ammonium chloride.  
  In practice, a given workpiece to be plated is immersed in the plating solution in contact with a selected metal electrode. The metal electrode serves as an anode while the surface of the workpiece serves as a cathode. Among metals which may serve as anode are nickel, magnesium, cobalt, iron, zinc, and aluminum. Among metals which may be effectively plated with the plating bath are copper, nickel, brass, monel, and silver. The ammonium chloride dissolves the anode metal and maintains the resultant ions in solution, apparently as a chloride complex, over a pH range from 4.5 to 6.5. If the pH decreases below 4.5, hydrogen cyanide is released. The plating rate drops to almost zero at higher pH values.  
  An effective plating rate for producing an adherent, dense, protective gold plate occurs at a bath temperature of from 90 to 100 C. with a gold salt concentration in the range 0.5 to up to its maximum solubility, l0 grams/liter. A temperature lower than 90C. or a lower gold salt concentration leads to impractically low plating rates for producing an effective corrosion protective plate but is suitable to form an ultra-thin plate useful as a decorative finish. When the bath temperature exceeds 100C., hydrogen pitting of the plate occurs. For best results, the solution should be maintained at a temperature of 98C. at a pH of 5.3 at a potassium aurocyanide concentration which does not fall below about 1 gram/liter. During plating, pH may be lowered with a HCl-NH Cl mixture or may be raised with a mixture of NH,OHNH CI, and, as the gold in solution is consumed, make-up gold should be added. Under these conditions, plating rates will range from 0.25 to 100 microinches per hour to deposit a continuous, adherent, dense gold plate on all cathode surfaces of the base metal in contact with the bath. Although the plating rate is low in comparison to rates obtainable with electrolytic plating, the quality of the plating in terms of density is much higher to provide effective protection of the base metal from chemical attack by corrosive reagents. The quality of the plate in terms of corrosion protection is not a function of plate thickness so much as it is plate density.  
 The utility and advantages of this invention are illustrated in the following representative example.  
 EXAMPLE A brass workpiece was immersed in a solution containing 1 gram/liter potassium aurocyanide and grams/liter ammonium chloride in contact with a nickel anode, the nickel anode being in a cloth anode bag. Plating was performed at a temperature from 98 to 100C. at a pH ranging from 5.3 to 5.5. After 20 hours, a uniform, continuous gold deposit 0.0003 inch thick formed on the workpiece. The plated part was immersed in a circulating solution of 15 percent hydrofluoric acid at a temperature of C. After 27 hours no apparent corrosion was noted. A part which was electrolytically plated to 0.001 inch thickness and exposed to the same conditions was found to have been extremely corroded.  
 What is claimed is:  
  l. A method for depositing metallic articles with gold which comprises immersing said article in a plating bath maintained at a temperature in the range -100C and at a pH in the range 4.5-6.5, said bath consisting essentially of 0.5 to 10 grams/liter lithium or potassium aurocyanide and 15l50 grams/liter ammonium chloride, while said article is in electrical contact with a dissimilar metal which renders the article cathodic in said bath.  
  2. The method according to claim 1 in&#39;which the pH of the bath is maintained within the range 4.5-65 by addition of either HCINH,C1 mixture or a mixture of NH OH-NH CL 3. The method according to claim 1 in which the dissimilar metal occupies a higher position in the electromotive series than gold or the metal article.