US 2891309 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
United States Patent 2,891,309 Patented June 23, 1959 ELECTROPLATING ON ALUMINUM WIRE Henry Fenster, Wyndmor, Pa., ass'ig'nor to American Leonie Manufacturing Company, Philadelphia, Pa., a corporation of New Jersey No Drawing. Application December 17, 1956 Serial N0. 628,565
8 Claims. (Cl. 29*528) This invention relates to a process for producing an electroplated aluminum wire, and more particularly, to a novel process for providing an aluminum wire with an electrodeposit'ed metal plate which does not peel, flake or blister upon further drawing of the wire.
Aluminum is frequently employed as an electrical conductoi, as for example in coaxial cable, because aluminum is less expensive than other conducting materials such as copper and the like. In addition, aluminum is relatively lightweight, and thus finds application where Weight is an important factor and the employment of a heavier conducting material would be undesirable. Unfortunately, all aluminum products, as for example aluminum wire, have an ever-present natural oxide film. This oxide film provides the surface of an aluminum conductor with high contact resistance. Because of this oxide film, an aluminum conductor may be joined, as for example by soldering to the same or other conducting materials, only with great difiiculty. It has, therefore, been the practice to electroplate various metals on aluminum to provide the aluminum surface with improved physical and electrical properties. For example, silver is applied to an aluminum surface to decrease contact resistance and to improve surface conductivity; copper, nickel or tin to provide a surface more adherent to solder, and chromium to reduce friction and increase resistance to wear.
Since aluminum wire may be drawn at a rate which is many times the rate at which one or more of the above-mentioned metals may be electrodeposi'ted on the surface of aluminum wire, it is highly advantageous in providing electroplated aluminum wire to electroplate a metal, such as copper, on an aluminum wire of relatively large diameter and then rapidly draw the electroplated aluminum wire to the desired size, thereby substantially increasing the rate of production of electroplated aluminum wire of the desired dimensions. However, it was found that the metal plate obtained by means of conventional processes for electroplating on aluminum, as for example the zinc-immersion method, tends to peel, flake or blister during further wire drawing operations, resulting in an unsatisfactory product. Thus, the undesirably slow procedure of electroplating an aluminum wire which has previously been drawn to the desired size is often resorted to in order to provide a commercially acceptable product. I
It is the primary object of this invention-to provide an aluminum wire with an electrodepos'ited metal plate which does not peel, flake or blister on further drawing and annealing of the wire by an improvement in the zinc-immersion process for electroplating on aluminum.
Another object of the present invention is the provision of a process for high speed production of electroplated aluminum wire of high quality at low cost.
These and other objects of this invention will become apparent from a consideration of this specification and claims.
According to the present invention, there is provided an improvement in the zinc-immersion process for preparing an electroplated aluminum wire involving successively immersing an aluminum wire in an etch-type alkaline cleaner, an acid conditioning agent and a sodium zincate solution, applying a copper strike to the wire surface by means of a copper cyanide solution, and electrodepositing a metal plate thereon, the improvement whereby there is provided an aluminum wire with an electrodeposited metal plate which does not peel, flake or blister upon further drawing of the plated wire, which comprises the step of immersing the aluminum wire in said sodium zincate solution while said solution is at a temperature of from about 95 to about 110 F. According to a preferred form of the invention, the sodium zincate solution is at a temperature of from about 95 to about 100 F. and the aluminum wire is immersed in the sodium zincate solution for a period of from about 10' to about 20 seconds; a current density of from about 60 to about 100 amp. per sq. ft. and a treatment time of from about 15 to about 120 seconds are employed in applying the copper strike. The preferred electrodeposited metal plate comprises copper.
As stated above, on aluminum a natural oxide film is always present or starts to re-form instantly after removal in one way or another. This oxide film undesirably provides aluminum the undesirable properties of high contact resistance and poor adhesion to solder. It is, thus, desirable to electroplate a metal on the surface of the aluminum in order to produce surface contact resistance and to present a surface more readily receptive to soft soldering. However, this same natural oxide film interferes with the plating operation and must be removed, modified, or replaced by another film.
A method for plating on aluminum which has been widely adopted commercially in recent years is the process known as the zinc-immersion process. According to the zinc-immersion process, the natural oxide film is removed from the aluminum surface to be plated and is replaced by a thin and adherent layer of metallic zinc. This provides the aluminum with a surface that Will respond to most of the plating procedures for depositing other metals. However, it was found that when aluminum wire is electroplated with a metal, such as copper, according to the conventional zinc-immersion method and the aluminum wire is then drawn to a finer size, the plating of copper or other metal blisters and peels or flakes oif. It has now been found, however, that in the zincimmersion method, by maintaining the sodium zincate solution in which the aluminum wire is immersed to replace the oxide film with a layer of metallic zinc at ele- H vated temperatures in the range of from about 95 to about 110 F., and preferably from about 95 to about F., the electrodeposited metal plate on the aluminum wire does not peel, flake or blister when the aluminum wire is subjected to further drawing and annealing. Entirely unexpectedly, elongations of as high as 15% have been obtained without the plated metal surface peeling, flaking or blistering during the drawing operation. Thus, according to the process of this invention, an aluminum wire of low porosity having a smooth, bright plated surface free from unplated areas may be rapidly and inexpensively produced.
The Well known Zinc-immersion process for electroplating on aluminum involves successively treating aluminum with an etch-type alkaline cleaner, an acid conditioning agent and a sodium zincate solution. After each of these treatments, the aluminum work piece should be thoroughly rinsed, preferably with cold running water, to substantially completely remove all traces of each agent with which the aluminum has been treated prior to further treatment of the metal. After the aluminum oxide film has been removed from the surface to be plated and replaced by a thin adherent layer of metallic zinc by treatment with the sodium zincate solution, a copper strike is applied by means of a copper cyanide solution, overwhich copper strike the metal plate is deposited in a conventional manner.
In electroplating aluminum wire by the zinc-immersion process, the wire is taken from a let-off spool and is successively treated with an etch-type alkaline cleaner, an acid conditioning agent and a sodium zincate solution generally by being immersed in a bath of the particular treating agent. A constant speed capstan may be employed to pull the wire through the several treating baths. Immediately subsequent to treatment with each agent and before treatment with another, the wire is immersed in a bath of cold running water to remove any residual traces of the prior treating material. After treatment with the sodium zincate solution followed by immersion in cold running water, the wire is made cathodic by being passed over an electrical contact roller, generally of brass, and then passed through the copper cyanide strike bath having anodes of sheet copper in the lower portion of the bath, wherein a copper strike is applied to the aluminum wire. After removal from the copper strike bath, the wire is passed over another brass contact roller and then through a bath of cold running water. The aluminum wire bearing a copper strike is then passed over a brass contact roller by which it is again made cathodic and passed through a plating bath containing the anode for plating the wire with the desired metal. After the plating operation, the plated wire is again passed over a contact roller and washed with cold running water. The plated wire, after being dried is wound on a take-up spool provided with a traversing mechanism to more evenly spool the wire. The take-up spool is provided with a friction clutch to maintain constant tension on the wire. Of course, there may be a number of modifications in the method and apparatus for handling of the wire in the above-described Wire conditioning and plating operation. The process of the instant invention is not to be limited to any particular method of handling the aluminum wire or any particular apparatus for this purpose.
The aluminum wire employed in the process of this invention should have an aluminum content of not less than about 99%. Accordingly, the terms aluminum and aluminum wire appearing in this specification and claims are intended to include within their meaning aluminum wire having this high percentage of aluminum. The other constituents of which the aluminum wire is comprised may be any of those materials generally found to be present with aluminum, whether intentionally included or in the nature of impurities, as for example copper, silicon, iron, manganese, and the like. Typical examplesof aluminums containing not less than about 99% aluminum are electrical conductor aluminum, commonly referred to as EC aluminum, which normally has an aluminum content of about 99.45%; 25-0 aluminum, presently referred to as 1100 aluminum, which has a minimum aluminum content of 99.00%, and 1197 aluminum, which has a minimum aluminum content of 99.97%. The size of the aluminum wire which may be electroplated according to the process of this invention may vary widely. Aluminum wire ranging in size from about mils to 0.325 in. may generally be employed. However, somewhat larger or smaller wire may be used to advantage. Preferably, however, aluminum wire not greater than about .2 in. and not less than about 20 mils is utilized. The temper of the wire may be hard or soft, but the wire should be relatively smooth and free of pits or slivers.
In order to obtain electrodeposits of satisfactory adhesion and uniformity, it is essential that the surface of the aluminum wire be free of grease, oil, drawing compound, and other forms of soil. Solvent and/or vapor degreasing can be employed to remove soils of this nature. A very satisfactory solvent for removing drawing compound is trichloroethylene. Emulsion type cleaners may also be used with good results.
The aluminum wire from which drawing compound and similar surface contaminants have been removed is then subjected to conventional treatment with an etchtype alkaline cleaner. The etch-type alkaline cleaner comprises generally an aqueous solution of caustic soda or potassium hydroxide. The purpose of the alkaline cleaner is'to obtain a maximum cleaning effect with uniform etching of the surface of the aluminum wire. Accordingly, various conditions of treatment well known to those skilled in the art, such as concentration of alkali, bath temperature, and immersion time, are selected to obtain this result. Generally, a concentration of from about 15 to about 70 grams per liter of NaOH (commercial, 76 percent Na O) is employed. The bath temperature is maintained in the neighborhood of from about 130 to about 210 F., and an immersion time of from about 5 to about 60 seconds, and preferably from about 10 to about 20 seconds is used. By subjecting an aluminum wire to an etch-type alkaline cleaner under the above conditions, approximately 0.0001 to about 0.0005 in. of aluminum are removed. The alumi num wire after treatment with alkaline cleaner is thor oughly washed, preferably with cold running water, to re move substantially all traces of alkaline cleaner.
Cleaning of the aluminum wire with an etch-type alkaline cleaner is followed, after washing with water, by conventional treatment of the wire surface with an acid conditioning agent. For this conditioning agent to be effective, it should accomplish two things: (1) remove any oxide film; (2) remove any microconstituentwhich may interfere with the formation of a continuous zinc layer or which may react with subsequent plating. Accordingly, acid concentration, immersion time, bath temperature, and the like are selected to produce these results. These conditions of treatment are also well known to those familiar with the zinc-immersion process. Generally, an acid concentration of from about 360 to about 540 grams per liter of nitric acid is employed, and the bath of acid conditioning agent is generally maintained at about room temperature. An immersion time of from 5 to about 60 seconds, and preferably from about 10 to about 20 seconds is utilized. In addition to nitric acid, the acid conditioning agent may also include other acids such as hydrofluoric acid, which do not interfere with the acid conditioning treatment. After the acid conditioning agent has been removed from the aluminum wire by washing of the wire with water at about room temperature, the wire surface is properly conditioned for the zinc-immersion step.
In the zinc-immersion step, where a properly condi: tioned aluminum wireis immersed in the zincate S0111, tion, the thin natural oxide film that is present on the surface of the aluminum wire dissolves and, as soon as any underlying aluminum is exposed, it also starts to dis: solve and is immediately replaced by an equivalent weight of zinc. When the aluminum surface is completely covered with a thin layer of zinc, action in this solution virtually ceases. The adhesion and quality of the sub: sequent electrodeposits obtained according to this invention are dependent upon obtaining a relatively thin, uniform, adherent metallic zinc film. It has now been found that the temperature of the zincate solution is of primary importance in obtaining a zinc deposit to which subsequent electrodeposits adhere in such manner that the aluminum wire may be subjected to further drawing and annealing operations without these electrodeposits flaking off, peeling or blistering from the surface of the aluminum wire. Thus, the temperature of the zincate bath according to the process of this invention is critical to the obtaining of the required type of electroplate and is within the range of from about 95 to about F., and preferably within the range of from about 95 to about 100 F. Onthe other hand, the temperature of the zincate solution employed is less than about 95 or greater than about 110 F., the subsequent electrodeposits are found to flake, peel oif or blister when an aluminum wire is subjected to further drawing and annealing.
In addition to the critical condition of temperature of the sodium zincate solution, the quality of the zinc coating is also influenced by those conditions heretofore Well known to the art. A highly-alkaline zincate solution containing from about 400 to about 600 grams per liter of sodium hydroxide (commercial 76 percent Na O), and from about 80 to 100 grams per liter of zinc oxide (technical grade) is generally employed. As can be seen from the .high concentration of sodium hydroxide and zinc oxide in the sodium zincate solution, the solution is very viscous, and losses occur largely from dragout. Although this is in a sense advantageous in that it limits the accumulation of impurities resulting from attack on the aluminum, the specific gravity of the solution should be checked periodically, and any loss made up by adding more of the components. Loss of volume by drag-out should be corrected by the addition of more solution of the specified composition. Generally, the solution should be maintained at a specific gravity of from about 1.38 to about 1.41, the preferred specific gravity being about 1.4. Immersion time is also important in obtaining the required type of zinc coating, and generally an immersion time of from about to about 60 seconds, and preferably from about 1.0 to about 20 seconds, is employed.
Recently, modified sodium :zincate solutions which give somewhat better results have been developed. One modified solution which may be employed to advantage in the process of this invention is prepared by dissolving the zinc oxide in a sodium hydroxide solution in the usual way but before the bath is diluted to volume a water solution of ferric chloride crystals and Rochelle salt is added. The bath should be stirred while the ferric chloride-Rochelle salt solution is added. A typical modified sodium zincate solution made in this manner comprises about '525 g. per .1. of sodium hydroxide (76% Na 'O); 100 g. per 1. zinc oxide; 1 g. per 1. ferric chloride crystals, and g. per 1. of Rochelle salt.
A preferred modified sodium zincate solution is de scribed in US. Patent 2,142,564, and comprises the conventional sodium zincate solution characterized in that the solution contains univalent copper salts. A typical preferred sodium zincate dip solution containing univalent copper salts comprises about 4.00 g. per 1. of caustic soda; about 80 g. per 1. of zinc oxide; about 2 g. per 1. of copper (e.g. in the form of potassium-copper cyanide), and about 25 g. per 1. of sodium sulphite. Various other sodium zincate solutions, not specifically described herein, but well known to those familiar with the sodium zincate process, may also be employed in the process of this invention.
After immersion in the zincate solution, substantially all traces of zinca'te solution should be removed from the aluminum wire, as for example by immersing the wire in a bath of running water maintained preferably at roomtemperature.
The zinc film obtained by means of the zinc-immersion step of this process will generally be of a thickness having a weight from about 0.1 to about 0.3 mg. per sq. in.
A copper strike is applied to the aluminum wire which has been provided with an adherent layer of metallic zinc by immersion in the above-described sodium zincate solution. For applying the copper strike prior to plating the aluminum Wire with other metals, a conventional Rochelle type copper cyanide strike solution may be employed; The copper strike solution generally comprises from about 25 ,to about 50 grams per liter of copper cy nide; from about 0.75 to about .3.75,grams .per liter of free sodium cyanide; from about 7.5 to about 45 grams per liter of sodium carbonate, and from about 15 to about 60 grams per liter of Rochelle salt. A preferred copper cyanide strike solution comprises 413 g. per 1. of copper cyanide; total sodium cyanide 48.8 g. per 1.; sodium carbonate 30 g. per 1.; Rochelle salt 60.0 g. per 1., and free sodium cyanide, max, 3.8 g. per 1. The copper strike solution is generally maintained at a temperature of from about to about 140 F., and preferably from about to about F. and a pH of from about 10.0 to 10.5. The use of a higher pH, such as that of high-speed copper plating baths, results in attack on the base aluminum, and an adherent copper strike is not obtained. The aluminum wire may be made cathodic by being passed over an electrically charged contact roller prior to and subsequent to passage through the tank of copper strike solution. Anodes of sheet copper may be placed in the bottom of the tank and made anodic in order to complete the electric circuit. Immersion time and current density are, of course, interrelated. Generally, a current density from as low as about 12 amp. per sq. ft. to as high as about 100 amp. per sq. :ft. may be employed with varying immersion times for the wire. According to the conventional copper strike pro- .cedure, an initial current density of about 24 amp. per sq. ft. is employed for a period of about 2 minutes in order to obtain relatively rapid coverage with deposited copper, after which time the current density is reduced to about 12 amp. per sq. ft. and deposition continued for an additional 3 to 5 minutes, depending upon thickness of the strike desired. .According to a preferred form of the process of this invention, a current density of from about 60 to about 100 amp. per sq. ft. is employed for a period of from about 15 to about 120 seconds, and preferably for about 60 seconds, in order to obtain the desired copper strike. Current densities above about 100 amp. per sq. ft. generally do not provide an adherent copper strike and thus are preferably avoided. There is also no advantage to be gained by using a current density below about 12 amp. per sq. ft. for the rate of deposition of copper at such low current densities is ex- .tremley low and the strike obtained is not appreciably more adherent than the strike obtained with higher current densities. By employing such a higher current density. an adherent copper strike is obtained, and process times are substantially decreased, thereby increasing the rate of production of electroplated aluminum wire. The wire emerging from the copper strike solution has a bright copper plate of a thickness of from about 0.00005 to about 0.0001 in. Variations in the process conditions for applying a copper strike are Well known and will present no ditficulties to those skilled in the art.
Subsequent to the application of the copper strike, the aluminum wire should again be Washed with :water to remove substantially all traces of the copper strike solution.
The aluminum wire bearing a base deposit of copper or copper strike may now be electroplated with the particular metal desired. A wide variety of metals may be electrodeposited in the form of a metal plate on the aluminum wire to which the copper strike has been applied. Generally, the electrodeposited metal plate comprises a metal which is below aluminum in the electromotive series of metals, as for example copper, nickel, tin, chromium, silver, and gold or an alloy formed of one or more of these metals, as for example brass. 'In the process of this invention, copper, tin or silver .mostfrequently are electrodeposited in the aluminum, wire the preferred plate comprising copper.
The thickness of the metal plate deposited on the aluminum wire depends upon the degree to which the wire is to be drawn, and the determination of the thickness of the plate and the degreelto which :a wire having a plate of any given thickness may be .drawn are iactors as' igsoa 7 well known to those skilled in the art of drawing plated wire.
' The particular method which may be employed to electrodeposit the metal plate on an aluminum wire bearing a copper strike, will, of course, vary with the particular metal plate desired. Generally, any of the well known electroplating methods may be employed in electrodepositing the metal plate, and the resulting electroplated aluminum wire may be further drawn and annealed without the plate peeling oif. In the plating step, electrical contact may be made with the aluminum wire in the same manner as that employed in applying the copper strike. For example, the wire prior to entering the plating bath, may be passed over an electrically energized contact roller, generally of bronze, which makes the wire cathodic. An anode, which generally comprises a sheet of metal to be plated, may be located in the bath solution.
A preferred bath for electrodepositing a copper plate on an aluminum wire bearing the above-described copper strike comprises a solution of copper fluoborate to which fluoboric acid is added to maintain a highly acidic pH. Generally, the bath will contain from about 350 to about 450 g. per 1. of copper fluoborate, and from about 1 to about 3 g. per 1. of fluoboric acid to maintain the pH of the solution at no higher than about 0.3. A bath temperature in the neighborhood of from about 140 to about 160 F. generally may be employed. Very high current densities in the neighborhood of about 1500 amp. per sq. it. are usually employed with an immersion time of from about 15 to about 60 seconds to plate the copper on the aluminum wire. Current density and immersion time may, of course, be varied, depending upon the thickness of the coating desired. Generally, a copper coating of about 1 mil is obtained with a current density of about 1000 amp. per sq. ft. and an immersion time of about 60 seconds.
Silver may be deposited on an aluminum Wire bearing a copper strike by first applying a silver strike followed by a silver plating step. In applying the silver strike, a silver cyanide strike solution comprising about 5 g. per 1. of silver. cyanide, and about 67 g. per 1. of sodium cyanide, maintained at a temperature of about 80 F. is employed. A current density of from about 15 to about 25 amp. per sq. ft. and a treatment time of about seconds will generally produce a very satisfactory silver strike. Following application of the silver strike, silver may be plated on the aluminum Wire by passing the wire through a silver plating solution comprising about 30 g. per 1. of silver cyanide, about 55.5 g. per 1. of potassium cyanide, and about 45 g. per 1. of potassium carbonate, the free potassium cyanide of the solution being about 41.3 g. per 1. The solution generally is maintained at a temperature of about 80 F., and a current density of about 5 amp. per sq. ft. is employed for the time necessary to provide the silver plate of the desired thickness.
Tin, likewise, may be plated on an aluminum wire to which a copper strike has been applied according to the above procedure. Either a stannate or a fluoborate-type tin plating solution, both of which are well known in the art, may be used.
Various other metals, such as chromium and the like, may, of course, be deposited on the aluminum wire bearing the copper strike, and methods for their deposition are well known to those skilled in the electroplating art.
After the aluminum wire has undergone the plating step, substantially all traces of plating solution should be removed from the wire, preferably by water rinsing.
As stated above, the electroplate obtained on aluminum wire by the process of this invention tightly adheres to the aluminum base, and upon subsequent drawing of the aluminum wire to finer wire size according to conventional wire drawing procedures, the ductile plate does not peel, flake off or blister, but rather remains tightly adhered to. the aluminum-wire... 'Thus, by the method of this invention an electroplated aluminum wire of fine diameter size may be rapidly produced at low cost. In addition, the drawn wire has a smoother, brighter plated surface. Wire elongation as high as 15% is possible with an electroplated aluminum wire produced according to the process of this invention, which indicates that increased rates of production of electroplated aluminum wire are obtainable according to the process of this invention as compared to a process wherein the wire must be plated after it has previously been drawn to the desired size and rate of production is limited by the relatively slow rate of electrodeposition of the metal coating.
The plated aluminum wire after having been drawn to the desired size may be annealed if desired. A preferred method of annealing the electroplated aluminum wire of this invention is to heat a strand of the wire to a temperature of from about 600 to about 750 F. in an atmosphere of dissociated ammonia gas.
The advantages of this invention will become further apparent from a consideration of the follow-ing specific examples.
Example I Hard drawn 20 mil EC aluminum wire is unwound from a spool, pulled through a plurality of processtanks by a constant speed capstan, and spooled on a take-up spool provided with a clutch mechanism for maintaining constant tension on the wire. The take-up spool is provided with a traversing mechanism to uniformly spool the wire.
The aluminum wire, which is free of surface soil, such as drawing compound and the like, is first passed through a tank containing an etch-type alkaline cleaner comprising an aqueous sodium hydroxide solution having a concentration of 60 g. of NaOH (commercial, 76 percent Na O) per 1. The wire is immersed for a period of 12 seconds in the alkaline cleaner, which is maintained at a temperature of 160 F., and then immersed into a second tank containing flowing rinse water maintained at about room temperature. From the rinse tank the aluminum wire is passed into a third tank containing an acid conditioning agent comprising an aqueous solution of nitric acid having a concentration of about 325 grams of nitric acid per liter. The tank containing acid conditioning agent is maintained at about room temperature and the treatment time employed is 12 seconds. The wire leaving the acid conditioning agent bath is passed to a fourth tank containing rinse Water to remove substantially all traces of the acid agent, and, is then passed to a fifth tank containing a solution of sodium zincate. The solution of sodium zincate is of the type described in US. Patent 2,142,564 and comprises 400 g. per 1. of sodium hydroxide; 80 g. per 1. of zinc oxide; 2 g. per 1. of potassium-copper cyanide, and 25 g. per 1. sodium sulphite, which solution has a specific gravity of about 1.4. The sodium zincate solution is maintained at a temperature of 98 F. and the immersion time employed is 15 seconds. The aluminum wire leaving the zinc-immersion bath has a silver-gray appearance due to the presence on the surface of the wire of a zinc film of about 0.2 mg. per sq. in.
After substantially all traces of the sodium zincate solution are washed from the aluminum wire in a sixth tank, the wire is passed over an electrically charged bronze contact roller by which the wire is made cathodic, and then into a copper cyanide strike solution containing an anode in the form of a copper plate. The cop'- per strike solution comprises 41.3 g. per 1. of copper cyanide, 48.8 g. per 1. total sodium cyanide, 30 g. per 1. sodium carbonate, 60 g. per l. Rochelle salt, and the solution has a maximum free sodium cyanidecontent of about 3.8 g. per 1. The strike solution is maintained at a temperature of A current density" of 80 amp. per sq. ft. is employed with a treatment time of 60 seconds.
Upon leaving the copper cyanide bath, the aluminum wire bearing the copper strike passes over another brass contact roller, and passed into an eighth tank containing flowing water at room temperature to wash away substantially all traces of the copper strike solution which may have remained on the wire. The thickness of the copper strike is about 0.00005 in.
The aluminum wire is passed over another bronze contact roller by which the wire is again made cathodic and into the ninth tank provided with an anode and containing a copper fluoborate solution for plating cop per on the wire. The copper fluoborate plating solution comprises about 400 g. per 1. of copper fluoborate and about 2 g. per 1. of fluoboric acid which is added to maintain the pH of the solution at not more than about 0.3. The temperature of the fluoborate plating solution is 150 F. A current density of about 1000 amp. per sq. ft. and an immersion time of 30 seconds is employed to provide the aluminum wire with a coating of copper of about 0.5 mil in thickness. After leaving the copper plating bath, the wire passes over another bronze contact roller and then through a bath containing running water at about room temperature to remove copper plating solution retained on the surface of the aluminum wire.
The electroplated wire is drawn through a number of dies on conventional wire drawing machines to a size of about 10 mil, and the drawn wire is annealed at a temperature of 700 F. in an atmosphere of dissociated ammonia gas. The wire is examined and found to have a smooth, bright copper coat of 0.25 mil in thickness which tightly adheres to the aluminum wire. There is no evidence of any peeling, flaking off of the copper plate from the drawn and annealed aluminum wire, nor is the copper plate blistered.
Example [I the process of this invention without departing from the scope thereof.
1. A process for making a coated aluminum wire having a substantially continuous, tightly adhering, blister-free, non-flaking electrodeposited metal coating which comprises successively immersing an aluminum wire comprising at least about 99 percent aluminum in an etchtype alkaline cleaner, an acid conditioning agent and a sodium zincate solution having a temperature from. about to about 110 F., applying a copper strike to the wire surface by means of a copper cyanide solution, electrodepositing a metal on the wire, and drawing the coated wire to a smaller diameter.
2. The process of claim 1 wherein the sodium zincate solution is at a temperature of from about 95 to about 100 F.
3. The procms of claim 2 wherein the electrodeposited metal comprises a metal selected from the group consisting of copper, tin and silver.
4. A process for making a coated aluminum wire having a substantially continuous, tightly adhering, blister-free, non-flaking electrodeposited metal coating which comprises successively immersing an aluminum wire comprising at least about 99 percent aluminum, and having a diameter of from about 20 mils to about 200 mils, in an etch-type alkaline cleaner, an acid conditioning agent, and a sodium zincate solution having a temperature of from about 95 to about F., applying a copper strike to the wire surface by electrodeposition from a copper cyanide solution at a current density of from about 60 to about 100 amp. per sq. ft. and a contact time of from about 15 to about seconds, thereafter electrodepositing a metal on the wire, and drawing the coated wire to a diameter of about 10 mils.
5. The process of claim 4 wherein the sodium zincate solution is at a temperature of from about 95 to about 100 F.
6. The process of claim 4 wherein the electrodeposited metal is selected from the group consisting of copper, tin and silver.
7. The process of claim 6 wherein the electrodeposited metal comprises copper.
8. The process of claim 7 wherein a contact time of about 60 seconds and a current density of about 80 amp. per sq. ft. are employed in applying said copper strike.
References Cited in the file of this patent UNITED STATES PATENTS 2,142,564 Korpiun Jan. 3, 1939 2,654,701 Calderon et a1. Oct. 6, 1953 2,100,258 Larson Nov. 26, 1953 OTHER REFERENCES Halls: Metal Treatment and Drop Forging, March 1951, PP- --130, and April 1951, pp. 177-182.