US 3226309 A
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United States Patent 3,226,309 METHOD 9F ELECTRO-DEPOSITION DE A PALLADiUM-SELVER ALLOY James E. Daley, New York, N.Y., assignor to Leesona Corporation, Cranston, R1, a corporation of Massachusetts No Drawing. Filed Jan. 10, 1962, Ser. No. 165,293
Claims. (Cl. 204-39) This invention relates to the preparation of alloys by electro-deposition. More specifically, the invention relates to the electro-deposition of surfaces of palladiumsilver alloys from a fused salt electrolyte.
In the prior art, electro-deposition is used extensively in industry to impart a decorative finish to a metal surface, or to lay down a metal surface which is highly protective against destructive environments. More recently, highly conductive surfaces of copper, silver, or gold have been applied by electro-deposition onto metals, such as steel or the like which are not good conductors of electricity. However, it has not been possible to codeposit silver and palladium to form an alloy by conventional electro deposition methods, i.e., from aqueous solutions of complex ions. Co-deposition, instead of providing an alloy, has invariably given rise to a two phase structure.
Accordingly, it is an object of the instant invention to provide a method of electro-depositing an alloy of palladium and silver.
It is another object of this invention to deposit a surface of palladium and silver in controlled ratios from a fused salt bath.
These and other objects of the instant invention will become more apparent from the following detailed description with particular emphasis being placed on the illustrative examples.
In general, the objects of the invention are accomplished by co-depositing palladium and silver from molten alkali halide electrolytes, such as a sodium chloride-potassium chloride eutectic mixture. Preferably, the electrolysis is carried out at relatively high temperatures in the absence of air and organic materials which possibly may have a deleterious effect upon the alloy. A wide potential span is operable for carrying out the reaction.
It is not completely clear why an alloy can be deposited from a fused alkali halide bath, however, it is theorized that temperature, current density and the difference between working potential and decomposition potential influences whether an alloy will be formed. Thus, it is noted that raising the temperature has the effect of increasing the mobility of metal atoms as well as the metal-metal diffusion coefiicients. Therefore, a metal will tend to deposit in a more random fashion with less tendency for the deposition to fall into potential energy wells, i.e., as would occur for single metal crystal growth or dendrite formation. The phenomenon of dendrite formation is governed by current density and is closely related to nucleation, and it is believed that the amount of nucleation is closely related to whether an alloy will be formed during co-deposition. At high current densities, a relatively large amount of nucleation will occur. Thus, the number of discrete particles formed is greater than at low current densities where the opposite occurs and a newly discharged atom seeks a similar atom on which to build. By working at a potential considerably more cathodic than the deposition potential, the same results are obtainable as when one Works at a high current density. The metal surface is at a relatively lower potential and the electrode will act as a deeper and relatively more homogeneous potential energy well, making it unlikely that a discharged atom would seek any site preferentially. Therefore, one would not obtain a preferential building up of either silver crystals or palladium crystals. Further-more, by carrying out the electrolysis at a potential more cathodic than the deposition potentials for silver and palladium, one can work in the limiting current region and thus, assure exact control of the alloy composition by selection of the silver and palladium. ion concentrations. It appears that the above theoretical considerations are met, at least to a large extent, when co-depositing palladium and silver metals from molten alkali halides.
In the electro-deposition of the instant alloys from a fused salt, the temperature range should preferably be as high as possible or until the fused salt starts to evaporate. However, it is possible to carry out the deposition at lower temperatures, it only being necessary that the salt is in the molten state. Thus, when working with a eutectic of potassium chloride and sodium chloride which melts 695700 C. the deposition can be carried out in the temperature range of from about 700950 C., whereas when working with molten sodium chloride which melts at 800 C., the electro-deposition would be carried out at a temperature of from about 800-900 C. If sodium bromide is employed which has a melting point of 755 C., the electro-deposition can be carried out at lower temperatures. Thus, by a judicious selection of the fused salt bath from which the deposition is to be performed, variations in the deposition temperature can be obtained.
A wide potential span can be employed in carrying out instant deposition. Although, as stated hereinbefore, it has been theorized that a greater amount of nucleation will bccur at high current densities, thus, increasing the number of discrete particles formed, substantially the same result can be obtained at low current densities when working with a potential which is more cathodic. Thus, in order to obtain an even coating of palladiumsilver, the process is carried out at a voltage of from about 1.0 to about 2.5 volts versus the Pt-Pt reference electrode. At voltages of less than about 1.0 only minor amounts of silver will be deposited, if any, whereas at voltages above about 2.5, complications may be encountered due to the deposition of alkali metals. For practical purposes, the optimum range is from about 1.3 to 2 volts. Current density of the electro-deposition of the alloy is directly proportional to the concentration of the silver and palladium ions .in the molten alkali halide bath and thus, depends both on the concentration of the ions in the bath and the voltage used. Under normal conditions, the current density will range from about 1 to about ma./cm The proper selection of these constants in the above ranges depends upon the end result desired.
The instant deposition can be performed from any of the fused alkali metal salts. Thus, the deposition can be performed satisfactorily employing molten potassium chloride-sodium chloride, potassium bromide-sodium bromide eutectic mixtures, and from the simple halide salts such as potassium chloride, sodium chloride, potassium bromide and from the iodates. As noted hereinbefore, by judicious selection of the salt, it is possible to vary the reaction temperature.
The instant electro-deposition process is carried out employing procedures whereby the potential of the system is controlled. Thus, palladium and silver salts are dissolved in the alkali metal halide bath, and the silverpalladium alloy plated upon a suitable sample by applying a potential difference across the electrolytic bath to carry out the deposition. Thus, one operable system comprises a working electrode, a counter electrode and a reference electrode in conjunction with a potentiostat to accurately control the potential in the system. Vari- 3 ous counter and reference electrodes can be used, however, the more noble metal anodes are preferred since they are more resistant to the corrosive influences of the molten salt. Suitable anodes include palladium, platinum, gold and silver.
In order to obtain good adhesion of the palladium alloy film onto the sample, it is necessary to chemically clean the surfaces upon which the alloy is to be plated in order that the surface is free from oxides and other impurities. The preparation of the surfaces is accom plished by methods known in the art.
A palladium-silver film can be applied to most metals or to a plastic membrane, obtaining good adhesion. Suitable metals upon which the palladium-silver alloy can be deposited include copper, nickel, platinum, gold and silver. Operable plastics must be able to withstand the deleterious effects of the molten alkali halide. In addition, before the coating can be applied, it is necessary to lay down a conductive coating of glass or metal upon the plastic substrate.
Having described the invention in general terms, the following examples are set forth to more particularly illustrate embodiments of the invention. Parts are by weight unless otherwise specified.
EXAMPLE 1 A carbon covered silver gauze is prepared by carbonization of a thin film of polyvinyl chloride on the silver gauze. The gauze is immersed in a molten sodium chloride-potassium chloride (1:1)eutectic bath containing palladium chloride and silver chloride in solution each at a concentration of 10 molar. A PzPz reference electrode is employed together with a counter electrode in conjunction with a potentiostat to accurately control the potential of the system. Plating is carried out at a voltage of 1.5 volts and a temperature of 700 C. The deposition is completed in approximately 5 minutes providing a thin alloy film which exhibits good adhesion to the carbon covered silver gauze.
EXAMPLE 2 A sintered porous nickel matrix having a porosity of 85% and an average pore size of eight microns is coated on one surface with a non-porous palladium-si1ver film as follows: palladium bromide and silver bromide are admixed in a molten bath of potassium chloride each at a concentration of 5 10 molar. The porous nickel sample is immersed in the solution and plating carried out at a voltage of 1.9 volts and at a temperature of 800 C. using a PtPt reference electrode and a counter electrode in conjunction with a potentiostat to accurately control the potential of the system. The deposition of a thin non-porous palladium-silver alloy film having good adhesion to the nickel surface is completed in about minutes.
For Examples 1 and 2, the porous metal matrix can be replaced by a metal such as silver, platinum, palladium, rhodium, iron, copper, zirconium, or ruthenium. The proper selection of these metals depends upon the end use of the coated material and is within the ability of one skilled in the art.
The palladium and silver can be dissolved in the alkali halide bath as any of the soluble salts, including the chlorides, sulphates and nitrates. Further, the concentrations of the metals can be varied depending upon the ratio of each metal desired in the final film. However, it has been found that enhanced results are obtained when the ion concentration of each metal is in the range of from about 5 1O to 5X10 molar. However, other concentrations can be employed depending upon the final film which is desired.
The thickness of the film which is deposited will depend upon the duration of the plating operation, the concentration of palladium and silver ions, the voltage employed (if a limiting current is not reached) and also whether or not the bath is stirred. Ordinarily, the electro-deposition will be completed in from about one to fifty minutes, and in the absence of mechanical stirring.
While various embodiments of the instant invention are set forth, the invention is not to be construed as being limited thereby, it being possible to produce still other embodiments without departing from the inventive concept herein disclosed. Such embodiments are within the ability of one skilled in the art.
What is claimed is:
1. The method of electro-depositing a palladiumsilver alloy onto an article comprising the steps of (1) dispersing palladium and silver ions in a molten alkali halide bath, (2) immersing the article to be coated in the solution, said article to be coated being the cathode and (3) passing an electrical current from an anode to said cathode through the solution and plating out said alloy.
2. The method of claim 1 wherein the molten alkali halide is a potassium chloride-sodium chloride eutectic mixture.
3. The method of claim 1 wherein the molten alkali halide is potassium chloride.
4. The method of claim 1 wherein the palladium and silver ions of said solution are each at a concentration of from about 5 X10 to 5 10 molar and the electrical current is passed at a voltage of from about 1.0 to about 2.5 volts versus a PtPt electrode.
5. The method of claim 4- Wherein the electrical current is passed through the solution at a voltage of from 1.3 to 2.0 volts.
References Cited by the Examiner UNITED STATES PATENTS 1,426,683 8/1922 Stalhane et al 204-39 XR 2,093,406 9/1937 Atkinson 20439 2,222,544 11/1940 Spanner et al 172 2,773,561 12/1956 Hunter 75-172 FOREIGN PATENTS 286,457 3/1928 Great Britain.
OTHER REFERENCES Aten et al.: Journal of the Electrochemical Society 47 1925, pages 265-274.
JOHN H. MACK, Primary Examiner.
MURRAY A. TILLMAN, Examiner.