|Publication number||US3725219 A|
|Publication date||Apr 3, 1973|
|Filing date||May 4, 1965|
|Priority date||May 4, 1965|
|Publication number||US 3725219 A, US 3725219A, US-A-3725219, US3725219 A, US3725219A|
|Original Assignee||Allis Chalmers Mfg Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (4), Classifications (11)|
|External Links: USPTO, USPTO Assignment, Espacenet|
April 1973 H. J. WESOLOSKI 3,725,219
PROCESS FOR PLATING ARTICLES WITH SILVER-PALLADIUM ALLOYS Filed May 4, 1965 Sim/OWE United States Patent Ofiee 3,725,219 Patented Apr. 3, 1973 3,725,219 PROCESS FOR PLATING ARTICLES WITH SILVER-PALLADIUM ALLOYS Henry J. Wesoloslri, Milton, Masa, assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis. Filed May 4, 1965, Ser. No. 453,009 Int. Cl. C23b 5/52 U.S. Cl. 204-37 R 1 Claim ABSTRACT OF THE DISCLOSURE A process for plating an article with a silver-palladium alloy comprises the steps of: providing the article with a conventional silver strike; electroplating the article in a silver cyanide solution at a current density of about amperes per square foot for about minutes; electroplating the article in an agitating palladium chloride and ammonium chloride acidic solution at a current density of about 4 amperes per square foot for about 8 hours; and heating the article at a temperature of about 300 to 400 C. for about 8 hours to alloy the silver and palladium thereon.
This invention relates to articles plated with silver-palladium alloys and processes for plating the same.
Silver-palladium alloys have the property of absorbing certain gases and are presently available in sheet and ingot form for use as getters in evacuated electrical devices such as vacuum switches and electron tubes and for use as roll-on 0r dipped protective coatings for base and precious metals. When using this alloy as a getter to remove traces of free hydrogen and other gases in evacuated electrical devices, it was formerly the practice to cut a small piece of the alloy from sheet stock and mechanically mount it on a support within the evacuated device. This practice is undesirable because of the risk of the piece breaking loose and causing a short circuit or mechanical failure. More elaborate mounting means to prevent this risk are to be avoided because additional costs would be imposed on an already expensive component. When used as a roll-on or dipped protective coating to provide a tarnish resistant coating of good appearance on tableware and jewelry, expensive machinery and processes are required to bond the sheet alloy to the base member and dip coating results in a coating of uncontrolled thickness.
It is desirable from the standpoint of cost reduction to electroplate silver-palladium alloys on articles for gettering and other gas absorption purposes. It is also desirable to electroplate silver-palladium to provide tarnish resistant coatings having certain desirable qualities as regards texture and appearance. However, it is not known at present how to electroplate such an alloy directly since dissimilar metals cannot ordinarily be plated simultaneously.
Accordingly, it is an object of the present invention to provide articles plated with silver-palladium alloys and to provide improved processes for making the same.
Another object is to provide silver-palladium plated articles which exhibit desirable characteristics depending on the silver-palladium ratio.
Another object is to provide improved getters for use in vacuum type devices, particularly getters which take the form of a silver-palladium alloy plated on parts within such devices, which parts already serve some other function within the device.
Another object is to provide a tarnish resistant plate for metal articles, such as silver.
Other objects and advantages of the invention will hereinafter appear.
The drawing illustrates a preferred embodiment of the invention but it is to be understood that the embodiment illustrated in susceptible of modifications with respect to details thereof without departing from the scope of the appended claims.
In accordance with the present invention an article is first provided with a plating of silver of desired thickness, then with a plating of palladium of desired thickness over the silver, and then the article so plated is subjected to heat treatment at a temperature below the fusion points of the article, the silver and the palladium to elfect diffusion between the silver and palladium to provide a plating of a true silver-palladium alloy on the article. For purposes of illustration, the invention is hereinafter described as applied, first, to provide a getter for a vacuum switch and, secondly, to provide a tarnish resisting plating on an article.
Referring to the drawing, there is shown a portion of a vacuum switch which is used in the high voltage electrical transmission art for switching high voltage, high amperage circuits. The vacuum switch has a glass envelope 10 which is understood to be sealed at each end by means of metal plates 12 and 14 and has a high vacuum maintained therein and an internal volumetric capacity of approximately 65 cubic inches. Plate 12 affords mechanical support for an electrical connection to a stationary contact support rod 16 on which is mounted a stationary contact 18. Plate 14 is provided with aperture 20 for accommodating a movable contact support rod 22 on which is rigidly mounted a movable contact 24. A flexible metal bellows 26 is connected in sealed relationship as by brazing to the inner surface of plate 14 around aperture 20 and to the exterior or movable contact support rod 22 to permit relative movement of contacts 18 and 24 to open and closed position as movable contact support rod 22 is moved. A hollow cylindrical sleeve 28 surrounds bellows 26 and a portion of contact rod 22 and is rigidly secured as by brazing to plate 14. Sleeve 28 mechanically supports a washer 30, preferably of copper, which is brazed as at 31 and crimped in place as at 33 near one end of the sleeve. Washer 30 has an aperture 32 for accommodating movable contact support rod 22 and serves to support the rod against axial misalignment as it moves back and forth.
In the vacuum switch hereinbefore described it is necessary to maintain a high vacuum and an uncontaminated environment to assure a minimum of arcing and rapid arc extinguishment as the contacts 18 and 24 separate under load. Most impurities are removed during various steps in manufacture and assembly of the switch and substantially all impurities normally in gaseous form are removed during the establishment of the vacuum within glass envelope 10. However, trace impurities still remain after the tube is sealed and some impurities appear as a result of gases released from the contact materials during arcing. Accordingly, means are usually provided to getter the impurities which are present in gaseous form, such as hydrogen and methane.
Formerly, a small piece of silver-palladium alloy cut from sheet stock was rigidly secured to one end of a small piece of wire as by brazing and the other end of the wire was secured as by brazing to a suitable member within glass envelope 10 of the vacuum switch. However, such an assembly was rather delicate because of its necessarily small size and was subject to detachment which created a risk of damage to or shorting of the vacuum switch.
In accodance with the present invention, it is proposed to provide a member already within glass envelope 10 of the vacuum switch and normally serving another function with a silver-palladium plating to serve as a getter. In the embodiment shown washer 30 which supports movable contact support rod 22 is provided with a silver-pal ladium plating to also serve as the getter in the following manner.
In the particular embodiment shown, only one side of washer 30 needs to be plated since it has sufficient surface area with respect to the envelope volume to absorb impurities which might be present. To plate only one side of washer 30, the other side is coated with a substance to resist deposition of plating, such as Microstop lacquer.
Washer 30 which is, for example, made of copper is chemically cleaned by dipping in a suitable cleaning solution such as trichloroethylene or tetrachloroethylene in a still or ultrasonic bath. It is then thoroughly water rinsed in cold running water before putting into a solution such as 50 percent HNO by volume for ten seconds.
It is preferable at this stage to provide washer 30 with a prior silver strike to improve the adhesion of the subsequent silver plating. This is accomplished by passing a very high current, on the order of 40 to 50 amperes per square foot, for 30 seconds while the washer is submerged in a weak silver plating solution. Washer 30 is then submerged in an aqueous silver cyanide platin bath and connected as a cathode with a source of silver for deposition submerged in the same bath and connected as an anode. A plating current having a current denstiy of about amperes per square foot of the surface area to be plated is then passed through the cathode, bath and anode to deposit a plating of silver on washer 30. For each minute of plating time at the current density specified, a layer of silver millionths of an inch thick will be deposited on the surface of washer 30. A layer of silver of an inch thick deposited after 20 minutes on a washer about 2 /8 inches in diameter and having a hole 7 inch in diameter was found to be suitable for use in a getter for a vacuum switch rated at 15.5 kv. and 12,000 amperes.
After the silver is plated on washer 30, the washer is submerged in an aqueous plating bath comprising palladium chloride in a ratio of 50 grams per liter of distilled water, ammonium chloride in a ratio of 50 grams per liter of distilled water and an acid for maintaining the acidity of the bath below 0.5 pH. The tank for the bath is, for example, made of glass or plastic lined steel. Washer is connected as a cathode with an anode made of platinum (such as Platanode) or stainless steel (such as 316 stainless steel) in the same bath. A suitable anode to cathode surface area ratio was found to be 2 to 1.
A plating current having a current density of about 4 amperes per square foot of the surface area to be plated is then passed through the cathode, bath and anode to deposit a plating of palladium on the plating of silver already on washer 30. During plating of the palladium the plating bath should be agitated vigorously as by means of a mechanical impeller. For each minute of plating time at the current density specified, a layer of palladium 12.5 millionths of an inch thick will be deposited on the silver plated surface of washer 30. A layer of palladium 1 /2 thousandths of an inch thick deposited after eight hours on one side of washer 30 having the dimensions described was found to be suitable for use as a getter. In practice, then, a ratio of silver and palladium of 1 to 3 is suitable.
Since the palladium plating on the silver plating of washer 30 is porous, the washer is then subjected to heat treatment to cause diffusion of the silver into the palladium and alloying of them. Thus, the plated Washer is heated for eight hours at a temperature between 300 and 400 C. Since the temperature at which washer 30 is heated is below the fusion points of the copper, the silver plating and the palladium plating, it is apparent that solid state diffusion occurs to produce the desired alloy rather than actual melting. Thus, desired tolerances can be maintained by the method disclosed which may be a critical factor in some devices.
An alloy as specified hereinbefore (25% silver and palladium) has excellent properties as a getter when applied in the amount specified in the vacuum switch described particularly in absorbing hydrogen.
Experiment has shown that by changing the ratio of silver to palladium to 75 silver and 25% palladium (by weight) produces an alloy or plating which resembles silver in appearance and texture but is extremely tarnish resistant in that sulphides are not readily absorbed. Heretofore, it has been the practice to add chromium oxide or chromium trioxide to silver cyanide electroplating baths to plate out an alloy of silver chrome but the resultant product does not have the soft color of silver and tends to resemble chromium. Furthermore, this product has a relatively high electrical resistance and is not desirable as an electrical conductor. Chromate conversion coatings are used to keep silver from tarnishing before customer use but are very thin. A rhodium flash coating over silver has antitarnishing qualities but has a hard, cold appearing surface which could tend to peel.
In accordance with the present invention an article is provided with 75% silver plate and 25% palladium thereover (by weight of coating) in the manner hereinbefore described. The resultant product is then heated for eight hours at between 300 to 400 C. to effect solid state diffusion and alloying between the silver and palladium.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. A process for plating an article with a silver-palladium alloy comprising the steps of:
(a) submerging the article in a first bath containing a silver plating solution and a suitable anode and passing a current of a density of about 40 to 50 amperes per square foot through said anode, first bath and article for about 30 seconds to provide the article with a silver strike,
(b) submerging the article in a second bath containing a silver cyanide solution and a second suitable anode and passing a current of a density of about 10 amperes per square foot through said second anode, second bath and article for about 20 minutes to provide the article with a silver plating,
(c) submerging the article in a third aqueous bath containing about 50 grams of palladium chloride per liter of water and about 50 grams of ammonium chloride per liter of water and having an acidity of about 0 to 0.5 pH and having a third suitable anode and passing a current of a density of about 4 amperes per square foot through said third anode, third bath and article for about 8 hours to provide the article with a plating of palladium,
(d) agitating said third bath while plating is carried on therein,
(e) heating the article at a temperature of about 300 to 400 C. for about 8 hours to alloy the silver and palladium thereon by effecting solid state diffusion therebetween.
References Cited UNITED STATES PATENTS 923,864 6/1909 Levy 204-46 X 3,150,065 9/1964 Fatzer 204-47 FOREIGN PATENTS 5,600 1895 Great Britain 204-37 454,017 9/1936 Great Britain 204-19 GERALD L. KAPLAN, Primary Examiner W. I. SOLOMON, Assistant Examiner US. Cl. X.R. 20419; 313-174
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US4309461 *||May 15, 1980||Jan 5, 1982||Chugai-Duki Kogyo Kabushiki-Kaisha||Method of compounding decorative precious metal alloy selectively onto austenite stainless steel article|
|US4475991 *||Mar 25, 1983||Oct 9, 1984||Chugai Denki Kogyo K.K.||Method of diffusion cladding a Fe-containing base material for decorative articles and ornaments with precious metal constituents including Ag|
|US5149420 *||Jul 16, 1990||Sep 22, 1992||Board Of Trustees, Operating Michigan State University||Method for plating palladium|
|US6034472 *||Feb 26, 1998||Mar 7, 2000||Siemens Aktiengesellschaft||Vacuum tube having a getter apparatus|
|U.S. Classification||205/148, 205/170, 313/553, 205/228|
|International Classification||C25D5/10, H01H33/66, H01H33/668|
|Cooperative Classification||C25D5/10, H01H33/6683|
|European Classification||H01H33/668B, C25D5/10|