US2588566A - Electrolytic process for stripping copper - Google Patents

Description

March 11, 1952 R. PEASLEE 2,588,566

ELECTROLYTIC PROCESS FOR STRIPPING COPPER Filed Feb. 4, 1948 ALTERNATING l0 cunazm INVENTOR RDEERT L. F'EAELEE.

BY Y m AGML ATTORNEY Patented Mar. 11, 1952 1 1" ELECTROLYTIC PROCESS FOR STRIPPING COPPER Robert L. Peaslee, Packanack Lake, N. J assignor to Curtiss-Wright Corporation, a corporation of Delaware Application February 4, 1948, Serial No. 6,159

2 Claims. (01. 204-146) This invention relates to the removal of copper and other metals from the surfaces of steel parts or other bodies and is particularly directed to a novel method for effecting such removal. The present invention constitutes an improvement over that disclosed in co-pending application Serial Number 590,644, filed April 27, 1947, in the name of R. L. Peaslee and T. K. Mooney (now abandoned).

In order to harden portions of the surfaces of steel parts, it is conventional practice to carburize said surface portions by exposure of said surface portions to a carbonaceous atmosphere at elevated temperatures. The surface portions of said steel parts which are not to be carburized are protected against this heat treatment by plating copper over said surface portions. After the carburizing treatment, the copper plate is stripped or removed from the steel parts. It is an object of this invention to provide a novel and simple method for stripping or removing copper plate from steel parts without injuring the steel parts in any way.

Specifically the method of the present invention comprises the steps of immersing a pair of copper plated steel or other metallic parts in a bath of an ammonium salt and passing an alternating electric current through said bath with said parts comprising opposite electrodes for said current. Said bath preferably comprises a solution of ammonium carbonate. However, various other ammonium salts may be used such as ammonium acetate, ammonium sulfate, ammonium phosphate, ammonium oxalate, ammonium nitrate, and ammonium bifluoride. In fact the use of a solution of almost any ammonium compound, whether organic or inorganic is within the scope of this invention. A bath of ammonium carbonate is preferred because of its fast removal of copper and because it does not etch or otherwise attack the steel. With a bath of ammonium carbonate, an excess of oxygen and ammonia are preferably maintained within the bath.

Other objects of the invention will become apparent upon reading the annexed detailed description in connection with the drawing in which:

Figure 1 is a schematic view of the apparatus employed in carrying out the novel stripping process; and

Figure 2 is a sectional view taken along line 2--2 of Figure 1.

Referring to the drawing, a tank l0, illustrated by dot-and-dash lines, is filled with a bath comprising a solution of ammonium carbonate, (NHOzCOa. The steel or other metallic parts,

ed across the tank In on a pair of terminals 20 mounted on said tank but electrically insulated therefrom. Similarly the rod I6 is adapted to be supported across the tank on a pair of terminals 22 mounted on said tank but electrically insulated therefrom.

When copper plated steel parts, such as gears I2, are immersed in an ammonium carbonate solution, said solution acts to remove copper from said steel parts. This action takes place even though no electric current is passed through the solution. As disclosed in said aforementioned application, if direct current is passed through the solution with a copper plated steel part comprising the positive electrode or anode for said direct current, the rate of removal of the copper from said part is greatly increased. In using direct current, it was found that copper does not plate out or deposit on the negative electrode or cathode as might be expected. This fact indicated that alternating current could be used instead of direct current. Upon trying conventional cycle alternating current, applicant discovered that for a rate of alternating current flow equal to a given rate of direct current fiow, copper was removed twice as fast with alternating current as compared with direct current. In addition, upon suspending a pair of copper plated steel parts in an ammonium carbonate solution and connecting said parts to opposite terminals of a source of alternating current, it was further discovered that copper was removed approximately twice as the periodical reversal of alternating current prevents the formation of some sort of insulating or inactive layer around the copper plated steel workpieces, so that although with alternating current copper is being removed electrolytically from a workpiece only during the periods that said workpiece is the anode, its rate of removal is so fast during said periods that its average rate of removal is twice the rate of removal with direct current of the same magnitude as said alternating current.

As illustrated in Figure 1, the rods and I6. are connected to the opposite terminals of asource of conventional alternating current 24, having a frequency of 60 cycles per second, by a switch 26 and wires 21. When the switch 28 is closed, copper is stripped or removed from the workpiece I2 at the aforementioned fast rate. That is, with alternating current, copper is removed from both workpieces 12 at twice the rate it would be removed from one of said workpieces if the rods l4 and It were connected to a source of direct current with said direct current having the same rate of flow as the alternating current.

The rate of removal of copper from the workpieces l2, increases with increase of the rate of alternating current flow. As long as the workpieces are not being burned by arcing of the electric current, there is no upper limit to the rate of current flow. Preferably the range of the density of the current flow through the bath is from 10 to 25 amperes per square foot.

The removal of copper is facilitated byaerating the bath with air. In addition, in order to check depletion of the bath, ammonia gas, NHs, is introduced therein. As illustrated, air and ammonia gas are introduced into the bath at the bottom of the tank l through a suitable aerator 28. The aerator 28 comprises a plurality of pipes 30 extending across the bottom of the tank l0 and each of the pipes 30 is provided with a plurality of holes 32 along its length. Air and ammonia gas are supplied to the aerator through a pipe 34 which in turn is connected to air and ammonia supply pipes 36 and 38 respectively. In this way, during operation of the process, the bath is aerated with fine bubbles of air and ammonia. The holes 32 should be quite small and closely spaced in order to obtain a large amount of aeration with as small an amount of gas as possible. It has been found satisfactory to use a number seventy-one size drill for forming the holes 26.

The air supply pipe 36 is provided with a valve 40 having a valve stem 42 comprising a plunger of a solenoid 44. When energized, the solenoid 44 moves the valve stem 32 to open the valve. Upon de-energization of said solenoid, the valve 40 is arranged to closeas for example by a spring. The electric circuit for the solenoid 44 includes a source of electric energy 46 and a pair of serially connected electric switches 48 and 50. The switch 48 is normally open and. is adapted to be closed by the rod [4 when said rod is placed in position across its terminals 58. Similarly the switch 50 is normally open and is arranged to be closed by the rod it when said rod is placed in position across its terminals 22. With this construction, unless both electrode rods 14 and I6 are supported in position across the tank ID, the air valve 40 is closed. When both electrode rods I4 and I6 are supported in position across the tank It], both switches 48 and 50 are closed thereby effecting energization of the solenoid 44 and opening the air valve 40. In this way, when the tank I0 is not in use, the air valve 40 is automatically closed thereby preventing removal of the free ammonia within the solution in the tank l0 by the drag-out effect of the air passing through the solution within the tank.

Ammonia gas is very soluble. Accordingly, if the air flow through the pipe 34 were completely shut off, the ammonia remaining in this pipe and in the pipe 38 would go into solution and cause a suction in these pipes thereby drawing the solution up into the ammonia pipe 38. To prevent this, a by-pass pipe 52 is connected around, the air valve 40 and a restricted orifice 54 is disposed in said by-pass. The purpose of this restricted by-pass around the valve 40 is to provide for a continuous small flow of air into the tank bath from the aerator 28 when the valve 40 is closed, thereby preventing the formation of a partial vacuum in the aerator 28 and the pipe 34 as a result of the ammonia going into solution.

If the temperature of the bath exceeds F., there will be an excessive loss of the volatile constituents of the bath and there will be fluctuations of the ammonia content of the bath. Accordingly, cooling means (not shown) may be provided to maintain a desired bath temperature. However, the bath is still quite effective at temperatures as high as F., but for a minimum loss of volatile constituents, the bath should be maintained between 60 F. and 70 F. The temperature of the bath may be maintained below 60 F. but the cost of cooling then becomes excessive. In fact, the temperature of the bath may be reduced to approximately 40 F. without any appreciable detrimental effect other than the increased cost of cooling, but if the temperature drops below 40 F., some of the constituents of the bath will freeze out.

A fresh bath is made by dissolving ammonium carbonate (NI-102003 in water. The permissible range of concentration of ammonium carbonate in the bath is quite large and can vary from 2 ounces per gallon to saturation. If the minimum concentration of 2 ounces per gallon is not maintained, the time required to remove a given amount of copper becomes quite long. It should be noted that there may be some slight etching of the steel workpieces if they are allowed to remain in a bath which contains considerably less than this minimum concentration of 2 ounces per gallon of ammonium carbonate. It is desirable to maintain a minimum ammonium carbonate concentration of at least 8 ounces per gallon and preferably the ammonium carbonate concentration should be at least 12 ounces per gallon. This concentration is maintained by periodically adding ammonium carbonate to the bath. Excessive amounts of ammonium carbonate in the bath have no detrimental effect but if the ammonium carbonate concentration exceeds 16 ounces per gallon, there is a waste of chemicals. Instead of periodically adding ammonium carbonate to the bath, it is possible to maintain its concentration therein by introducing carbon dioxide CO2 and ammonia NH: into the bath.

It has also been found that the time required for the removal of a given amount of copper plate is appreciably longer in a fresh bath of ammonium carbonate, as compared to a bath having a substantial copper concentration, as exists in a bath which has previously been used. Accordingly, when a new bath is made up it is desirable also to use a portion of the old bath in order to provide the new bath with an initial copper concentration. In addition, or in lieu of using a portion of an old bath, copper electrodes may be hung in the bath to helpbuild up an initial copper concentration. The bath will operate with a range of copper concentration from 0 ounce per gallon to saturationthat is, to approximately 20 ounces per gallon. However, a minimum copper concentration of at least the 0.2 ounce per gallon is desirable in order to increase the rate at which copper is removed from the copper-plated workpieces. When the copper concentration exceeds 18 ounces per gab.

lon, there is a possibility of precipitating a copper ammonium compound on the steel workpieces and the efllciency of the bath is reduced. Preferably, the range of concentration of copper in the bath should be between 0.5 and ounces per gallon.

The aforedescribed process, forv stripping cop per from steel parts, is very simpleand does not require complicated apparatus. I'hus, the electric and chemical control is verysimple since it is not necessary to maintain the, electric current or the concentration of any of thesjconstituents of the bath within any small orinarrow range. Another advantage of the aforede'scribed process is that the copper removed-from the steel parts can readily be recovered 10m the bath as copper oxide CuO by heating said;,bath.

While I have described my invention in detail in its present preferred embodiment, it will be obvious to those skilled in thejar-t; after understanding my invention, that various changes and modifications may be made therein'without departing from the spirit or scope thereof. I aim in the appended claims to coverall such modifications. I

I claim as my invention:

1. The process of removing copper from the surfaces of a pair of metallic bodi'estfsaid process comprising the steps of immersin'ggsaid bodies in a solution consisting of ammonium carbon-ate in water having 0.5 to 15 ounces of copper dissolved therein per gallon of solution; and passing an alternating current through said solution with said bodies comprising opposite electrodes for said current, said alternating current comprising conventional symmetrical alternating current having a frequency of 60 cycles per second.

2. The process of removing copper from the surfaces of a pair of metallic bodies: said process comprising the steps of immersing said bodies in a solutibn consisting of ammonium carbonate in water having 0.5 to 15 ounces of copper dissolved therein per gallon of solution; passing an alterhating. current through said solution with said bodies comprising opposite electrodes for said current, said alternating current comprising conventional symmetrical alternatinigj current having aij frequency of cycles per second; and introdu'cing ammonia gas into said solution.

ROBERT L; PEASLEE.

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

UNITED STATES nTE Ts OTHER REFERENCES Transactions of The Electrochemical Society," vol. 7'1 1940) pages 207-209.

Metal Cleaning and Finishing, November 1933, pages 465, 466.

Claims (1)

1. THE PROCESS OF REMOVING COPPER FROM THE SURFACES OF A PAIR OF METALLIC BODIES: SAID PROCESS COMPRISING THE STEPS OF IMMERSING SAID BODIES IN A SOLUTION CONSISTING OF AMMONIUM CARBONATE IN WATER HAVING 0.5 TO 15 OUNCES OF COPPER DISSOLVED THEREIN PER GALLON OF SOLUTION; AND PASSING AN ALTERNATING CURRENT THROUGH SAID SOLUTION WITH SAID BODIES COMPRISING OPPOSITE ELECTRODES FOR SAID CURRENT, SAID ALTERNATING CURRENT COMPRISING CONVENTIONAL SYMMETRICAL ALTERNATING CURRENT HAVING A FREQUENCY OF 60 CYCLES PER SECOND.

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