US2530967A - Bright alloy plating - Google Patents

Description

Nov. 2l, 1950 G. w. JERNsTED-r BRIGHT ALLOY PLATING Filed Sept. 9, 1947 j-Rnode Composed of Tel/'nar' Copper', T'zn and Zinc. @la e 14 E ecrolyze Combosed of Cojalber Cyanide Zinc Cyanide, .5tannaze, :Sodium Carbonate and lcali wlTNEssEs: i lNvENToR M d Gear-9e W Jefnsdt Patented Nov. 21, '1950 BRIGHT ALLOY PLATING George W. JernStedt, Pittsburgh, Pa., assigner to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 9, 1947, Serial No. 772,977

2 Claims.

This invention relates to electro-plating and, more particularly, to electro-plating of bright ternary alloy coatings upon metal surfaces to provide for appearance and corrosion-resistance.

This application is a continuation-impart of my copending patent applications Serial No. 458,399, led September 15, 1942, and entitled Bright Alloy Plating, now abandoned, Patent No. 2.435,96?, issued February 17, 1948, entitled Bright Alloy Plating, and Serial No. 591,073, filed April 30, 1945, now abandoned.

In applying electro-plated coatings to metal surfaces in order to provide for attractive appearance and predetermined color and corrosionresistance, -only a limited number of metals are ordinarily available. Of these, some are costly and their use is limited for this reason. At the rpresent time, certain of the more usual electroplating metals are scarce, and the number of electro-plating metals available to the industry for general use has been sharply reduced. It is quite important to employ metals obtainable at low prices which are not so scarce for plating purposes, and a feature of this invention is to provide desirable bright plate by using metals obtainable in relative abundance.

As is well known to those skilled in the art, no bright electro-plated metal coating is satisfactory to a high degree in meeting to a high degree all the major plated coating requirements such as high corrosion-resistance, low cost, abrasion-resistance, and bright, attractive surface color. Some metals, for instance chromium, when applied by plating do not confer any sig'- nicant degree of corrosion-resistance to the base metal, and it is necessary to apply protective coatings such as nickel plating or copper plating between the chromium plate and the base metal. Other metals such as silver, are subject to rapid tarnishing or oxidation and, therefore, are unn desirable for many commercial purposes.

A feature of my invention is the economical electro-deposition of a bright silvery coating of a ternary alloy of copper, tin and zinc having as its outstanding properties high corrosion-resistance and high abrasion resistance. This optimum combination of characteristics is not available in any other known metal or alloy.

Ternary alloy plating has been practiced in some cases but, in general, the electro-plating of alloys consisting of three or more components is ordinarily quite dicult. Not only are the conditions of the electrolyte extremely critical as a rule,'but changes in voltage and amperage per unit area of anode or cathode are reflected in disproportionate changes in the results. However, according to this invention, an easily controlled ternary alloy plating method is an important feature. The ternary alloy is plated directly from anodes of the alloy, thereby greatly reducing the cost of the plating operation. Relatively great variations in the bath and other plating conditions are tolerable without undue detrimental results following.

The object of this invention is to provide a process for producing an electro-plated ternary alloy coating upon the surfaces of metal which coating is characterized by a silvery bright, attractive surface appearance, good abrasionresistance, and an extraordinarily high degree of corrosion-resistance without the use of other protective coatings.

Another object of this invention is to provide for electro-plating a bright silvery ternary alloy coating from anodes composed of the alloy.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. For a fuller understanding of the nature and objects of this invention, reference should be had to the following detailed description and drawing, the single gure of which is a cross-section of an electro-plating apparatus functioning according to the invention.

Pursuant to this invention, a ternary alloy composed of copper, tin, and zinc is electro-plated upon metal members with highly satisfactory results. By varying the composition of anodes of the alloy and the operating conditions of the apparatus a bright silvery electroplated coating, similar to chromium or silver plate, is produced. In either case, the electro-plating may be conducted within a relatively wide range of operating conditions with complete success.

It has been proposed heretofore to plate copper. zinc, and tin as a bright silvery alloy from a bath or electrolyte containing suitable salts of these metals using insoluble anodes. However, the preparation of the metal in the form of salts and the fact that the salts produce a much smaller amount of plated metal than the weight of the salts renders the cost of the prior art practice excessive. A further disadvantage present in such prior art practice is due to the fact that the bath containing only metal salts continually varies as the electroplating, proceeds, producing an increasing excess proportion of the anion of the metal salt as well as depleting the metal content thereby producing a continually varying bath. This leads to non-uniform plating, besides requiring continual maintenance of the bath.

From a commercial standpoint, the prior art practice is not satisfactory or economical.

The ternary alloy which is electro-deposited in practicing the present invention preferably consists of from 50% to 75% copper, 15% to 30% tin, and to 20% zinc, al1 parts by weight. The presence `of minor amounts of other metals or impurities is believed not to affect the nature of the electro-plated product. Particularly good results are obtained if the copper content is from 55% to 65%, the tin from 20% to 30% and the zinc from to 20%. The tin content .of the anodes may be two or three per cent more than the plated alloy, since a small fraction of the tin precipitates out of the electrolyte. Therefore, the anodes may contain from to 35% tin, the copper being from 50% to '75% andthe zinc from 5 to 20%.

In making the anodes, it has been found that the ternary alloy is conveniently prepared by melting the required weight of copper in a crucible with the predetermined amount of Atin and zinc added just prior to the casting Vof the vmelt into suitable anodes. Alternately, a brass of suitable composition may be melted initially and tin added in producing the ternary alloy. Permanent molds or, in some cases, sand molds may be used in casting anodes. operation, .supporting hooks -of a conducting metal such as steel may be cast into the anodes .to provide for supporting them from the anode conductor bar in the electro-plating apparatus. In case a conductor anode frame or holder is to be employed, the ternary alloy may be cast into any desired shape, such as balls, suitable for putting into the anode holder.

.Referring to the single figure of the drawing, there is illustrated one form of electro-plating apparatus IB .suitable for the practice of the invention. The apparatus I0 comprises a tank l2 of wood, steel, or other chemically resistant material capable of retaining a cyanide electrolyte lil therein. A suitable source of heat such, for example as a steam coil 28, is provided for the .tank l2 since the preferred operating temperature is above room temperatures. The electrolyte I4 consists of the following constituents:

Ounces per gal.

Free cyanide 0.5 to 5.0 Copper 0.2 to0.50 Tin V0.05 to 0.20 Zinc 0*.1 to 0.5 Sodium carbonate 2.0 to 12.0

Sodium hydroxide, about 0.25 lto 0.8 ounce per gallon to give a pH of from about 12 to 13 Parts Copper cyanide 2 to 5 Sodium cyanide 10 to 80 Zinc cyanide l to 6 Sodium stannate 9%; to 'l Sodium carbonate 15 to90 and suicient sodium hydroxide (roughly 3 to 6 parts) to give a pH of from 12 to 13.5.

An electrolytethat has been satisfactory for During the casting 4 both barrel and still-tank plating has the following composition:

Ounces per gallon Copper cyanide 0.5 Zinc cyanide 0.27 Sodium stannate (Na2SnO3-3H2O) 0.22 Sodium cyanide 3.75 Sodium carbonate 4.0 Sodi-um hydroxide 0.6

The electrolyte, when maintained in a pH range of from about 12.6 to 13.5, produces excellent bright silvery electroplate. The anodes were composed of from 52% to 65% copper, 25% to 35% tin and 10% to 20% zinc. Analysis of the ternary alloy electro-plate produced by this last electrolyte in combination with the anodes shows copper from 55% to 60%, tin 25% to 28% and zinc 14% to 18%.

In all of the above compositions zinc may be added as zinc sulfate. Other zinc, tin and copper salts, which are soluble in a cyanide solution may be employed in preparing the electrolyte. It will be understood that potassium salts may be used instead of sodium salts, allowance rbeing made for the difference in molecular Weight. The electrolyte is prepared by dissolving the finely powdered salts in the predetermined quantity of Water. The solution may be ltered `before introducing it into the tank l2.

The anodes i8 prepared as indicated previously are suspended or supported from the anode bars i6. From a cathode bar 20 a cathode supporting member 22 is suspended. In plating small articles, such as shown at 24 in the ngure, it may be desirable to provide the cathode member 22 with a plurality of hooks or supports 26, only a portion of which comprises exposed metal for ow of electrical current to the articles supported thereon. The remainder of the cathode support may be coated with a suitable electrically insulating material to prevent wastage of current. When members 24, which are to be plated, are suspended on the supports 25, electrical contact will be made between the members and the exposed metal portions on the support. In other cases where the members being plated are of sufficiently large size, they may be suspended directly from the cathode bar 20 by using suitable conducting hooks or the like.

The electrolyte I4 is extremely reactive with the metal of the anodes I8. The anode eciency is about while the cathode eiciency is about 35 thereby the metal content of the bath tends to approach the limits of concentration set forth in the rst table. Accordingly, it is necessary under most conditions to employ one or more relatively insoluble steel or carbon anodes in combination with a number of ternary alloy anodes I8. Satisfactory results have been obtained with from 15% to 30% of the total anode area consistingoi the alloy anodes, the remaining anode area being provided by the insoluble anodes.

In order to plate bright silvery deposits on the members 24 in the apparatus l0, it has been found necessary to apply a voltage between the anodes I8 and members 24 critically correlated with the separation between centers. In ordinary plating tanks where the anode to cathode distance is standardized at from 6 to 8 inches, the voltage should be maintained within the limits of from 3 to 5 volts. For this 6 to 8 inch spacing. if the voltage is substantially above 5 volts milky deposits are produced, while with a voltage of subthe proportion of 1 volt for each 4 inches increase in sp-acing and decreased in the proportion of 1 volt for each 4 inches decrease in spacing. At a one inch separation, about the minimum practically feasible, from 11A@ to 2 volts will electrodeposit a bright silvery alloy plate.

The alloy plating baths operated as disclosed 'herein have outstanding throwing power. It compensates for the uneven distances between bulky articles and the anodes. The throwing power en- 'ables crevices of great depth to plate as brightly as the surface. In many cases sections of tubing will plate bright entirely over both inside and outside surfaces.

Eample I Steel pole blocks in shape being approximately cubes with a, concave hollow in one face, of a dimension of approximately 11/2 inches on each edge, previously plated with 0.0005 inch of copper, were mounted on a rack in groups of 48 and placed in a still tank containing an electrolyte of the following composition:

Ounces per gal.

Cyanide (free) Metal:

Copper 0.32 Zinc 0.13 Tin 0.08 Sodium carbonate 5.0

Sodium hydroxide to give a pH of 12.8 Temperature 150 F.

Example II A plating barrel was loaded with threaded instrument terminals composed of copper. The terminals were from 1 inch to 11A; inches long and of a diameter of 1A; inch. About 50 pounds of the terminals were placed in the barrel. The barrel was rotated at an oblique angle to the vertical in a tank containing the following electrolyte:

Ounces per gal.

Cyanide (free) 3.0 Metal:

Copper 0.35

Zinc 0.15

Tin 0.10

Sodium carbonate 5.0

Sodium hydroxide to give a pH of 12.8 Temperature 155 F.

The anodes were disposed at the outside of the barrel at an average distance of nine inches from the surface of the mass of the terminals in the barrel. 'Ihe anodes comprised 75% insoluble anodes and soluble anodes of the same composition as in Example I. Current at 51/2 volts was applied. In an hour the parts were plated with 0.0002 inch of a bright ternary alloy having the samecomposition as the plate in Example I.

In producing bright smooth deposits of the silvery plate, it has been found that derivatives of alpha, beta, and gamma betaines containing at least one non-cyclic hydrocarbo-n radical composed of from 10 to 20 carbon atoms may be added to the electrolyte I4 as anti-pitting and -brightening agents. The amount of betaine may be added in amounts ranging from 0.01 to 1 ounce per gallon to produce the desired results.

The apparatus may be operated at a temperature of F. to 160 F. over a wide range of amperes Vper unit area. Still tank plating has been accomplished at temperatures of 140 F. to F., while barrel plating operates well at 150 F. to F. It is possible to obtain ternary alloy coatings 0.0001 to 0.0005 inch thick in a short time, about 12 minutes at a current density of 15 amperes per square foot is required per 0.0001 inch thicknessv of deposit. For most purposes, coatings of this thickness have adequate covering power and corrosion-resistance. In some cases where it is desirable to have a thicker coating, plates varying from 0.001 to 0.002 inch thick and even heavier and which are quite bright have been obtained by electro-plating for greater periods of time.

The electro-plated alloy is surpassed only by freshly plated silver in reflectivity. Tests indicate that its reflectivity is approximately 85% of that of freshly plated silver. However, an advantage over silver plate is that the ternary alloy plate deteriorates in reilectivity very slowly, whereas silver becomes tranislied rapidly under normal circumstances.

The corrosion-resistance of the ternary alloy electro plate is superior to nickel, tin, chromium,

and silver when these latter metals are plated ony copper or copper alloys. For example, 0.001 inch thickness of nickel on copper when subjected to salt-spray tests will fail in less than 100 hours. The ternary alloy 0.0002 inch thick plated on copper regularly passes a 200 hour salt-spray test without any visible corrosion. Very few electroplated metals have the corrosion-resistance that the ternary alloy of this invention has.

An additional advantage of the ternary alloy applied to copper, in particular, is that the alloy can be soldered to as effectively and easily as to tinned parts. This feature is exceedingly important in the fabrication of instruments, meters, and other electrical apparatus, for example, where contacts and other parts must be soldered in assembling the apparatus. It is essentially non-magnetic and therefore is useful in instrument manufacture.

The alloy may be plated upon many metals used asa base. Steel and ferrous metals may be directly plated; though, to secure maximum corrosion resistance, steel is initially copper or brass plated and then ternary alloy plated. Copper, brass and other copper alloys are excellent bases for the alloy plate.

Hardness tests indicate that the ternary alloy electro-plate is much harder and more resistant `to abrasion than electro-plated coatings of nickel, tin, or silver.

A particular advantage of the alloy plating bath described herein over the usual bright plating baths is due to the greatly superior throwing power of the ternary alloy. The alloy has a greater throwing power than nickel, tin, silver, or chromium. For this reason, the ternary alloy coatingsl need only be about one-fifth as thick as a nickel, tin, or silver plate to give a good coverage and corrosion-resistance to metal surfaces.

Since certain-changes incarrying out' the above processes and certain modifications in the compositions which embody the .invention may be made without departing from its scope, it is intended that all the matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1.. 'The method of electrodepositing bright silvery corrosion-resisting coatings of a ternary alloy composed of from 50% to 75% copper, 15% to 30% tin and 5% to 20% Zinc upon a member which comprises making the member the cathode in -an aqueous electrolyte comprising essentially 0.5 to 5 oz. of free cyanide per gallon, 0.2 to 0.5 oz. of copper per gallon, 0.05 to 0.2 oz. of tin per gallon, v0.1 to 0.5 oz. of zinc per gallon, alkali metal hydroxide and 2 to l2 oz. of alkali metal carbonate per gallon, the electrolyte being maintained at a temperature of between approximately 140" F. and 160 F. and at a pH of from 12 to 13.5 the anode comprising a ternary `alloy ccmposed oi from 50% to 75% copper, 15% to 35% of Yanode and cathode.

to A30% tin, 4and,l0% .to 20% zinc upon a'member which comprises making the member the cathode in an aqueous electrolyte comprising essentially 0.5 to 5 oz. of free cyanide per gallon, 10.2 1200.5 oz. of copper per gallon, 0.05 to 0.2 oz. of tin per gallon, y0.1 to 0.5 oz. of zi-nc per gallon, alkali metal hydroxide and 2 to 1'2 oz. of alkali metal carbonate per gallon, the electrolyte being main-- tained at Va temperature of between approximately 140 F. and 160 F. and at a pH of 'above 12, .the anode comprising a ternary alloy composed of from 52% to 65% copper, 25% to 35% tin and from 10% to 20% zinc and passing electrical current through the electrolyte at a potential proportional to the distance between the anode and cathode, the potential being 'from 3 to 5 volts between the anode and cathode when they are spaced 5 to 8 inches apart, the Voltage being increased or decreasw one `Jolt for each ll inches increase or decrease, respectively, spacing between GEORGE W. JERNSTDT.

REFERENCES CITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Number Name Date 2,181,773 Wernlund Nov. 28, 1939 2,198,305 Cinamon Apr. 2.3, 1940 2,255,057 Holt Sept. 9, 1941 2,435,967 Jelnstedt Feb..l'?, 1948 FOREIGN PATENTS Number Country Date 22,002 Great Britain of 1005

Claims (1)

1. THE METHOD OF ELECTRODEPOSITING BRIGHT, SILVERY CORROSION-RESISTING COATINGS OF A TERNARY ALLOY COMPOSED OF FROM 50% TO 75% COPPER, 15% TO 30% TIN AND 5% TO 20% ZINC UPON A MEMBER WHICH COMPRISES MAKING THE MEMBER THE CATHODE IN AN AQUEOUS ELECTROLYTE COMPRISING ESSENTIALLY 0.5 TO 5 OZ. OF FREE CYANIDE PER GALLON, 0.2 TO 0.5 OZ. OF COPPER PER GALLON, 0.05 TO 0.2 OZ. OF TIN PER GALLON, 0.1 TO 0.5 OZ. OF ZINC PER GALLON ALKALI METAL HYDROXIDE AND 2 TO 12 OZ. OF ALKALI METAL CARBONATE PER GALLON, THE ELECTROLYTE BEING MAINTAINED AT A TEMPERATURE OF BETWEEN APROXIMATELY 140*F. AND 160*F. AND AT A PH OF FROM 12 TO 13.5 THE ANODE COMPRISING A TERNARY ALLOY COMPOSED OF FROM 50% TO 75F% COPPER, 15% TO 35% TIN AND 5% TO 20% ZINC AND PASSING ELECTRICAL CURRENT THROUGH THE ELECTROLYTE AT A POTENTIAL PROPORTIONAL TO THE DISTANCE BETWEEN THE ANODE AND THE CATHODE, THE POT;ENTIAL BEING FROM 3 TO 5 VOLTS BETWEEN THE ANODE AND CATHODE WHEN THEY ARE SPACED 6 TO 8 INCHES APART, AND THE VOLTAGE BEING INCREASED OR DECREASED ONE VOLT FOR EACH 4 INCHES INCREASE OR DECREASE, RESPECTIVELY, IN THE SPACING BETWEEN ANODE AND CATHODE.

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