US 2575712 A
Description (OCR text may contain errors)
Nov. 2o, 1951 G. w. JERNsTEDT 2,575,712
ELECTROPLATING Filed Sept. 29, 1945y George h/.Jrp/edf.
Patented Nov. 20, 1951 ELECTROPLATING George W. Jernstedt, Belleville, N. J., assigner to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 29, 1945, Serial No. 619,404
This invention relates to electroplating, and more particularly to a process for producing electrodeposits of superior brightness and smoothness by applying a predetermined current during the plating operation.
In electrodepositing metal from an electrolyte upon a base member, it is well known to those skilled in the art that great care is required to secure reasonably good deposits of metal. Many factors must be taken into account by the operator of a plating plant in order to secure acceptable electroplating. The shape and size of the base member is quite critical. If the base member is quite critical. If the base member has recesses, cavities or sharp projections, it
requires considerable skill and much investigation to produce an acceptably uniform coating over the entire surface thereof. In producing relatively thick coatings of plated metal, for example, coating of ten mils thickness, it frequently happens that unless the most painstaking precautions are taken that the electrodeposits tend to become more nodular and are characterized by a coarse, crystalline or rough condition.
Even with the greatest of care it is generally necessary to grind, buff and polish electrodeposited coatings or metal on a base member in order to secure an acceptably smooth and bright plating of metal. This greatly increases the cost of the final product and requires a great deal of hand labor and the use of special equipment.
It has been discovered that metal may be electrodeposited from an electrolyte by employing a particular current cycle during plating which will improve the plated metal remarkably to attain unusual smoothness and brightness. Furthermore, base members of complex shapes may be plated much more successfully with less care being necessary to accommodate the anodes to the member, modifying the electrolytes and other features of the plating bath.
The object of this invention is to provide a process for electrodepositing bright, smooth and homogeneous coatings on a base member from an electrolyte.
A further object of this invention is to provide for applying metal to a base member by electrodeposition employing direct current in combination with a condenser discharge at predetermined intervals.
Other objects of the invention will in part be obvious and will in part appear hereinafter. For a better understanding of the nature and objects of the invention. reference should be had to the following detailed description and drawing in which:
Figure 1 is a vertical cross section of an apparatus for carrying out the invention.
Fig. 2 is a chart of the electrical current applied to the base member during plating.
Claims. (Cl. 204-44) Fig. 3 is a greatly enlarged and somewhat exaggerated illustration of plating accomplished according to conventional methods.
Fig. 4 is a greatly enlarged cross section of electroplating on a member as effected by the present invention; and
Figure 5 is a perspective view of a magnet plated according to the present invention.
The electroplating process of the present invention consists in first applying a direct current to the base member to render the base member cathodic whereby an increment of plated metal is disposed and thereafter discharging a condenser through the base member, the base member being anodic during the discharge, and repeating the cycle until a predetermined thickness of plated metal has been produced. Unusual and unexpected results have been obtained by discharging a condenser through the base member in the cycle. The plated metal is rendered much smoother, brighter and denser by the treatment. Furthermore, members of complex surfaces have been found to be plated more uniformly over the entire surface using conventional anodes Without any special effort such as specially placed anodes or thieves to accommodate the corners, recesses, .projections and other irregular portions of the members.
In Fig. l of the drawing, there is illustrated a typical plating bath for carrying out the present invention. The plating tank l0 having a liner |2 of rubber or other material resistant to the action of the electrolyte carries a supply of conventional electrolyte i4 for depositing the desired metal. A conductor bar I6 carries the support I8 for the base member 20 to be electroplated. A second conductor bar 22 carries an anode 24 either of the` metal to be plated or else an inert electrode of graphite, platinum or the like. A suitable source 25 of direct current, such, for example, as a bat,- tery, rectifier or direct current generator is connected by the lead 28 to the conductor bar 22 and the lead 30 to the conductor bar I6. A suitable switch or circuit breaker 32 is interposed in the circuit to make and break the ilow of current from the lead 30 to the conductor bar I6.
A condenser or capacitor 34 is provided with a lead 36 to be connected to conductor bar I6 by the switch 32 at the time when the circuit from the source 26 has been broken. The other lead 38 from the capacitor is connected to the conductor 22. The capacitor 34 is so charged from a source 40 of high voltage direct current that discharge current from the lead 36, when connected to the base member 20, renders the base member anodic. A high resistance 42 is introduced between the source 40 and the condenser 34 to prevent any detrimental effect on the source 40 when the condenser 34 is discharged. A variable resstance 44 is inserted in circuit with the` 3 condenser in order to provide for varying the current oW and the time of the condenser discharge.
While eurrentis iioWing from the direct current source 26 to the base member 20, metal is plated on the base member. At the same time, high voltage from source 40 is applied to the condenser 34 to charge the condenser. Upon moving the switch 32 from the V.lead 3D to Vbreak the plating current and then 'to make contact with lead 36, the electrical charge built .up in the condenser 34 is discharged through the base member 2D. The switch 32 may be operated manually or it may be operated from a suitable mechanically timed operating mechanism. A .Referring to Fig. 2 of the drawing, there is plotted a curve .of time against the current density applied to member 2.!!.in the apparatus of Fig.V 1. At `the .point O the base member is supplied with current .from the .source 2B at a current density A, the base member is rendered cathodic, and plating occurs during the time interval A to B. The time .from A to .B may vary fromabout 2 to 40 seconds, a period of from 4 to 8 seconds was used with success. The current density at A to B may be of any suitable value providing burning .or other adverse effects do not occur. In actual practice, it has been found that the current densityfrom A to B may be considerablv higher than the current density which is employed for any given electrolyte using continuous-direct current. Thus when copper plating is carried out with direct current only, ordinarily the current densities .are from to 25 amperes per scuare foot; while according to the present invention, the current density may vary :from 50 to 150 ani-peres lper souare foot (for copper) at A to B without burning. gassing or other undesirable results. This is believed to be due to the fact that .much higher lcurrent densities may be successfully employed for intervals of from 2 to 40 .seconds than would be possible with continuousdirect current applied for a few minutes or more. for example.
. Ihe direct current Vduring .the interval A to B isV shown as beine' constant. However rippled, pulsating or non-uniform vdirect current may be employed with good results. VThus the current density at time A may be much higher than at time B.
f .At point B when the switch 32 is moved to disconnect the direct current from the base Vmember, the current density .drops to .the value of zero vat C. Immediately thereafter, the condenser 34 is discharged through the base member .720., rendering the .base member anodic, and the current `density reaches a value much greater than with the cathodic plating `current A to B. As shown, a peak anodic value .at D is reached and thereafter the condenser current decays to avvalue of substantially zero at the point E. .In most cases, the time `from 1C to D is one-quarter second or less. This time interval may be regulated by varying the resistance 44. Thereafter, the current cycle is repeated.
The condenser employed in the practice of the present invention should be charged to a potential of above approximately 100 volts in order to secure outstanding results. Better results yet have :been obtained when the condenser is charged to about 300 volts and higher. The voltagemay be -a-s much as 10.00 volts, -but the problems of handling such high voltages make it desirable to use the lower voltages. While some benets `are secured by employing a condenser charged at as low as 30 volts, the improvement in plating is not particularly outstanding. The throwing power of the condenser discharge is much better at volts .and higher. The capacity of the condenser plays an important part .in the process. A condenser of about 10 to 0.1 microfarad capacity is suitable for each square foot of base member surface on which metal is being plated. The required capacity of the condenser will vary 'directly with the area of the members being plated and the coulombs per square foot of member applied during interval A vto B, and linversely to the voltage applied thereto. The prime requirement is that the condenser discharge apply enough coulombs of current to deplate unsound and inferior metal from the previous increment deposited on the metal surface. .A few per cent of the vprevious plated increment may be depleted by the condenser discharge with advantage in many cases. As .much as 25% or more of the plated metal may be deplated with considerable benefit. The coulombs available on discharge or the condenser may be in the range from 0.003 coulombs to l0 coulombs per square foot depending on the amount of metal plated by the direct current in the time A to B.
It has been observed that the metal plated by continuous direct current varies in quality, being of a ymuch higher quality with regard to smoothness, density and uniformity during the initial portion of the plating period and thereafter the plated metal becomes progressively less desirable. This later plated metal tends to become more crystalline and to build up at projections and other locations favored by the tendencyl of the electrical current to concentrate at sharp points and by the presence of more metallic ions in the adjacent electrolyte. This undesirabie effect is counteracted during the discharge of the condenser which builds up a much greater deplating current density at the high spots in the plated coating which are deplated to Ia greater extent than the cavities and low spots in the plated coating. It has been observed that after the discharge of the condenser the entire plated coating is rendered smoother and brighter' than previous to the discharging of the condenser. Also any metal deplated from the base member by the condenser discharge is driven into the solution immediately adjacent the base member so that a relatively great and abnormal concentration of metal ions is present when plating is resumed during the next interval of direct cathodic current. The plating during the nexty direct current intervalV is greatly improved since.
it is applied to a much smoother and brighter surface. The efciency of plating, likewise, is improved due to the presence of the usual concentration of metal ions near the metal surfaces.4
Even scratches, recesses and the like are plated almost as eiiciently as any other portion of the base member during the succeeding cathodic interval. In this way, successive increments ofthe highest quality of electrodeposited metal are-- smoother than the original surface of .the
Inspection of members 'plated in the apparatus base of Fig. l, employing the current cycle shown in;
Fig. '2, shows that the plated metal Vis far superior to any plated metal produced heretofore by con- In practice the .surface of `the plated metal is much ventional direct current methods. Members plated by the practice of this invention are so smooth and bright that buffing and polishing are not required. The electrodeposits are homogeneous and much more uniform in thickness over the entire base member surface even when applied to the complex shaped bodies. These advantages are obtained in addition to the greater speed with which plating may be carried out. In many cases, members have been plated a given thickness of metal in less than half the time the same members could have been plated using the most rapid and eicient electroplating systems available commercially at the present time. In some cases, using the process of this invention plating to a given thickness from a given electrolyte has been applied in from 20% to 50% of the time required using ordinary direct current.
Referring to Figs. 3 and 4 of the drawing, there are illustrated certain physical diierences between conventional direct .current plating and the plating of the present invention upon base members having various imperfections such as are practically always encountered. The base mcrnberlEtl in each case has a burr 52, a scratch or crack 54 and an inclusion 56 of metal oxide, carbon particle, sand, or the like. When the base member is subjected to ordinary direct current plating, an electrodepositedV metal coating 58 is deposited as shown in Fig. 3. Over the burr 52 a projection or nodule 60 is formed. The projection or nodule 60 grows much faster than flat portions due to the concentration of current at the burr 52 and this eifect becomes progressively more exaggerated as the nodule increases. The contrary result occurs at the scratch 5l! where the current density is less than that of the average over the metal surfaces and plating occurs at a much lower rate than the average thickness of the plated metal. A further predisposing cause to the lower rate of plating within the scratch 54 arises from the fact that the electrolyte adjacent the crack must supply more than the average amount of metal thereto. This defect is further aggravated in that instead of the metal ions reaching the bottom of the crack, they tend to form ridges on each side of the scratch 54 thereby exaggerating the imperfection in the metal surface. Since the inclusion 55 conducts current less effectively than the base metal, plating does not occur above the inclusion and a pore or bare spot occurs. It will be apparent from the inspection of Fig. 3 that the plated metal obviously tends to become more and more rough as more metal is plated. The electrodeposited metal 58 is relatively non-uniform in thickness and when subjected to corrosion will fail at the pore 55 and scratch 54 before the remainder of the plated metal would fail.
By comparison, metal plated by means of the current cycle of the present invention on the base member 50 results in the electrodeposit 62 shown in Fig. 4. At the burr 52 the plated metal is actually deposited at a somewhat lesser rate than the average of the member, and is of a greater radius than the radius of the burr. Thus, the surface of the plate is proportionately less rough than that of the base metal below. At the scratch 54 the plated metal iills up faster than the average build-up in the rest of the plated metal 52. Thus, the scratch becomes less and less apparent. With respect to the inclusion 56, investigation shows that the plated metal soon bridges any pores that are reasonably small and eventually a complete coating of plated metal is applied thereover. The entire surface of the base member therefore becomes smoother. These differences in results obtained by the practice of the present invention have actually been found to occur.
Numerous metals have been electrodeposited by the practice of the present invention. Copper, silver, brass and zinc electrodeposits have been produced with highly desirable uniformity and brightness. Copper has been electroplated from conventional cyanide baths. Likewise, silver has been plated from cyanide baths. For silver the current density at A to B may be from to 150 amperes per square foot. Brass has been plated from a bath containing copper and zinc cyanides. In plating brass current densities as high as amperes per square foot during the interval A to B may be employed. Numerous other metals may be plated by employing the current cycle of the present invention with advantage. In all cases plating is more easily accomplished with a more uniform coverage. The sole exception is chromium in which case the invention is not particularly eiective for its Yintended purpose since the base metal tends to def plate when subjected to the condenser discharge instead of the chromium being deplated.
Referring to Figure 5 of the drawing, illustrated is a magnet 54 that is plated with approximately 0.018 inch thickness of copper. A smooth, bright copper electrodeposit of copper was produced over the entire surface thereof by the current cycle of this invention from an aqueous electrolyte composed of:
Ounces per gallon Copper 7.35 Free potassium cyanide 1.5 Potassium hydroxide 2.5
A brightening agent may be added to the electrolyte with benefit, though its use is not necessary. The thickness of the plating on the magnet E4 was more uniform over the entire surface as compared to direct Current plating. Furthermore the density of the electrodeposited copper was greater than that of copper plate secured by conventional plating processes.
The process of the present invention may be applied to any type of electroplating. Various metals or alloys, alone or in various combinations with one another may be plated on base members. The base member may be of metal or a conducting non-metal such as graphite. A great variety of articles may be given superior electrodeposits of metals by the process. Automobile hardware, household appliances, print'- ing plates, and numerous other objects can be electroplated to advantage.
The metal electrodeposited by the process of the present invention has the properties and characteristics of the metal electrodeposited in accordance with the process or" my copending patent application Serial No. 610,107 led August 10, 1945, now Patent 2,451,341, dated October l2, 1948.
Although the invention has been described by reference to particular embodiments thereof, it is, of course, not to be limited thereto except insofar as is necessitated by the scope of the appended claims.
I claim as my invention:
1. In the process of electrodepositing a metal selected from the group consisting of copper, brass, silver and zinc upon a base member from an aqueous cyanide electrolyte suitable for the electrodeposition of the metal, the steps comprising applying to the base member the aqueous electrolyte .containing cyanide, passing an electrical current through the base member and electrolyte to render the base member cathodic for a period of time of from 2 to 40 seconds to electrodeposit metal upon the base member, discharging in `about 1/4 of va second through the base member and electrolyte a condenser charged to a potential of at least 30 volts and not in excess of 1000 volts, the condenser having a sumcient capacity to discharge from about 0.003 to 10 coulombs of current per square foot of base member surface being plated, the base member being anodic during the discharge, to deplate unsound and .inferior electrodeposited metal, and repeating the cathodic current and anodic condenser discharge cycle until ka desired thickness i metal has 'been electrcdeposited on the base member.
12. In the process of electrodepositing abright and smooth coating of copper upon a base member from an electrolyte, the steps comprising applying to the base member an aqueous alkaline electrolyte containing copper cyanide, passing an electrical current 'through the base member and electrolyte to render the base member catliodic for a period of time ofV irom about v2 to 40 seconds to electrodeposit copper upon the base member, discharging in about 1/4 of .a second through the base member and the electrolyte a condenser Yhaving a capacity of from 0.1 to microfarads per square foot of base member surface being plated charged to a potential of at least 100 volts and not exceeding 1000 volts, the 4base member being anodic during the condenser discharge, to deplate unsound and inferior electrodeposited copper, and repeating the cathodic current and anodic condenser discharge until a desired thickness of copper has been electrodeposited on the base member.
3. In the process of electrodepositing a bright and smooth coating of silver upon a base member from an electrolyte, the steps comprise ing applying .to the base member an aqueous alkaline electrolyte containing silver cyanide, passing an electrical current through the base member and electrolyte to render the base member cathodic for a period of time of from about 2 to 40 seconds to electrodeposit silver upon the .base member, discharging in about 1/4 of a second through the base member and the electrolyte a condenser having a capacity of from about 0.1 to 10 microfarads per square foot of base member surface being plated charged to a potential of at least 100 volts and not exceeding 1,000 volts, the base member being anodic during the condenser discharge, to depl-ate unsound and interior electrodeposited silver, and repeating the cathodic current and anodic condenser discharge until a desired thickness of silver has been electrodeposited on the base member.
4. In the process of electrodepositing a bright and smooth coating of brass' upon a base member froman electrolyte, the steps comprising applying to the base member an aqueous alkaline electrolyte containing copper cyanide 4and zinc cyanide, passing an electrical current through the base member and electrolyte to render the base member cathodic for a 'period of time of from about 2 to ll0 seconds to electrodeposit brass upon the base member, .discharging in about ,1/4 of a second through the base member and the electrolyte a condenser having a capacity of from about 0.1 to 10 microfarads per square foot of base member surface being plated charged to a potential of at least volts and not exceeding 1,000 volts, the base member being anodic during the condenser discharge, to deplate unsound and inferior electrodeposited brass, and repeating the cathodi; .current and anodic condenser discharge until a desired thickness of brass has .been electrodeposited on the base member.
5. The process of electroplating on a base member a metal selected from the groupconsisting of copper, brass, 4silver and Zinc .fromV a liquid electroplat-ing electrolyte having dissolved therein as the sole platable metal a metal from said group that may be plated on the base member when the base member is the Vcathode and readily deplatedfrom the base member and put into solution in the electrolyte when the base member is the anode, comprising the steps of connecting the base member into an electroplating circuit including the electrolyte, causing electric current to ovv in the electroplating circuit to make the base member cathodic to electroplate metal from the electrolyte on the base member for a period of time of between 2 and 40 seconds, then at the end of the period of time discharging through the base member and the electrolyte a condenser having a capacity of from 0.1 to 10 microfarads charged to a potential of from 30 to 1000 volts to render the base member anodic during the discharge whereby to deplate a portion of the electroplated metal, and continuing to make the base member cathodic by connecting it into the electroplating circuit and to make the base member anodic by discharging the condenser therethrough until :a desired thickness of metal has been electroplated on the base member.
GEORGE W. JERNSTEDT.
REFERENCES CITED The followingv references are o record in the iile of this patent:
UNTTED STATES PATENTS Jernstedt c Oct. 12, 1948