US3396092A - Method of electroplating - Google Patents

Description

United States Patent 3,396,092 METHOD OF ELECTROPLATING Robert 'C. Moyer, Garfield Heights, and Charles D. Stricker, Shaker Heights, Ohio, assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed June 7, 1965, Ser. No. 462,079 7 Claims. (Cl. 204-19) This invention relates to electroplating. More particularly, the invention relates to a method of electroplating a partially, metal-coated workpiece.

A well known expedient for improving the surface properties of articles, particularly metal articles is to apply a protective coating to the surface of the article which possesses properties superior to the base to which it is applied. Such products as galvanized steel, tin plate, etc., are well known examples of the use of protective metal coatings to improve the surface properties of steel. However, if for some reason it is necessary to re-coat a portion of the workpiece from which the coating has been removed or is inadequate, care must be exercised to provide a sound coating to the uncoated portion without harming the coating already on the work. The problem of re-coating frequently arises with coated metal articles which have been welded and wherein the welding operation exposes the base metal by removal of the protective coating. Similarly, metal-coated strands of wire, strip, etc. which are to be joined together into coi-l lengths are usually welded. The welding operation removes coating from the surface of the metal at the joint thereby exposing uncoated metal, and it is desirable to re-coat this section to restore the protective coating.

Among coating techniques, electroplating is regarded as providing the most uniform and desirable coating, and accordingly, would be the preferred approach for re-coating, if it were practical. However, if -a partially coated workpiece, such as the welded joint of the coated strands referred to above, is electrolytically coated, i.e. electroplated, in an acid electrolyte, the metal coating already on the strand may be injured. Before the plating potential is applied, the coating on the work may either be removed by electrolyte entirely or if not removed, it may begin dissolving and a deposit will form on the surface which will not permit the desired adherence of the plating to the core. For this reason, re-coating practices have typically avoided electroplating, relying instead on other methods of applying metal coatings such as painting and immersion in molten metal. Unfortunately, attempts at re-coating the joint by applying a protective coating of hot molten metal to the joint have proved unsatisfactory because of the non-uniformity of such a coating and the difiiculty in controlling coating thickness. Moreover, such coatings do not possess the desired ductility, adherence and tenacity. A method of coating the joint electrolytically without injuring the coating already on the wire and which provides a uniform ductile coating of controlled thickness has, therefore, long been desired.

The present invention provides a method of electroplating such partially coated metal workpieces which avoids the aforementioned difiiculties. The method according to the invention permits the uncoated portion to be electroplated without damage to the coating already present.

According to the invention, a partially plated workpiece is electroplated as the cathode in an acid electrolyte containing a salt of the metal to be plated. However, prior to contacting the partially coated metal workpiece with the electrolyte, a holding voltage is applied to the workpiece which is above the decomposition potential of the electrolyte. After application of the holding voltage, the workpiece is immersed in electrolyte to plate 3,396,092 Patented Aug. 6, 1968 it. Upon completion of electroplating, the holding voltage is maintained on the workpiece substantially until the workpiece is withdrawn from the electrolyte; that is to say, until withdrawing the workpiece from the electrolyte in the electroplating cell. The term decomposition potential as used herein refers to the voltage required to cause a sustained current flow through the electrolyte from an anode to the cathodic workpiece. If a partially, metal-coated workpiece is inserted in an acid electrolyte without a holding voltage previously applied thereto, the acid electrolyte will attack the coating on the work and will either dissolve the coating or create undesirable films or deposits which interfere with plating. By applying a holding voltage to a cathodically-connected, partially coated work prior to contacting it with the electrolyte, current immediately flows between the anode and the workpiece. Thus, when the workpiece is inserted into the acid electrolyte, the electrolyte is not able to attack the coating on the work, and/or form undesirable deposits.

An additional advantage of our method of electroplating is that it can be readily adapted to an automatic plating system which does not require constant attention by an operator. Thus, for example, a system can be employed in which a holding voltage potential is applied to the work prior to immersion in electrolyte, after which the voltage can be increased to increase the current density and thereby increase the electroplating rate. A timer may be included in this system to control the plating cycle and, upon completion of the plating cycle, to reduce the operating voltage to the holding voltage. The holding voltage can then be maintained on the workpiece until it is withdrawn from the electrolyte.

When electroplating a partially coated workpiece, it has been discovered that it is particularly desirable to vibrate the work during plating. The vibrating action action permits the plating to be carried out at a higher current density, minimizes pinholes and greatly improves adhesion of the coating to the base metal. By using high current densities, the work can be re-plated very rapidly. Vibrating the work during electroplating additionally prevents coating tree-formation which occurs when plating at high current densities onto rough surfaces by standard plating techniques. Accordingly, the preferred embodiment of the invention contemplates vibration of the workpiece during electroplating.

The amount of the holding voltage applied to the work depends upon the particular electrolyte employed and the electroplating conditions used. The electrolyte decomposition potential is affected slightly by the following: (1) the electrolyte concentration, (2) electrolyte agitation, (3) surface condition of the work and type of electrode material and, (4) electrolyte temperature.

The theoretical electrolyte decomposition potentials for various electrolytes are available in textbooks and handbooks. For example, the textbook entitled Principles of Physical Chemistry by S. H. Maron and C. F. Prutton, 3rd edition, contains on page 575, a list of decomposition potentials of 1 N solutions at room temperature using platinum electrodes. Some of these are:

Volts NiCl 1.85 CuSO, 1.49 CdSO, 2.03 ZnSO, 2.55

When plating the respective trolyte of the plating-metal salt, it has been found that to prevent the electrolyte from attacking the partially coated cathodic workpiece, 0.2 to 0.5 volt should be added to the theoretical decomposition potentials at standard conditions to insure a sustained flow of current to the cathode. If the holding voltage is less than 0.2 volt more than the decompositionpotential, it may not be suflicient to provide a current flow between the anode and cathode. If a correction of more than 0.5 volt is added to the decomposition potential, the plating rate increases too much to control plating thickness without supervision. As an illustration, where a partially zinc-coated steel article is to be re-plated with zinc, the holding voltage applied, to the coated steel work would be between 2.75 to 3.05 volts. This holding voltage is arrived at by adding 0.2 to 0.5 volt to the decomposition. potential for zinc sulfate of 2.55 volts.

As a further illustration, the welded joint of galvanized wire from which the zinc coating had been removed, can be re-coated by electroplating as follows:

Zinc-coated steel wire strands, e.g. 0.06 to 0.18-inch in diameter, are prepared for electroplating by removing oxides and soils from the surface in any conventional manner such as with the use of a wire brush. If oils or greases are present, additional cleaning with detergents or solvent may be required. After cleaning and removal metals froma id. Q .Ih ihighacid con e ra o is de r t n e the electrolyte conductivity and to permit the use of high current densities. The electrolyte temperature is desirably maintained between 80 and 110 F.

The wire strand is electrically connected so that it is cathodic to the anode;.within theelectrolyte. A holding voltagepotentialof between 2.8 and 3.0 volts is applied to the steel Wire strand-before insertionor suspension in the electrolyte. The applied holding voltage very slightly exceeds" the decomposition"potential and some plating onto the cathodic workpiece will occur. However, to accomplish the full re-coating within a reasonably rapid time, the voltage is increased to above 6 volts, preferably 6 to 7 /2 volts. In this way, current densities of from 600 to 1400 amp/sq. ft. are achieved and an adherent coating of approximately 1 to 1 /2 mils of zinc can be deposited in one minute; When the plating cycle has been completed, the strand can be removed from the electrolyte and the holding voltage potential can be discontinued.

' As discussed above, the holding voltage potential must be applied to the workpiece before contact with the acid electrolyte. Should the workpiece be immersed into the of detergents or solvents, if used, the wire is ready for plating.

To enable high current densities to be used and to improve the quality of the coating, the wires should be vibrated during electroplating. Any suitable vibrator may be used. The wires are clamped and mounted to the vibrator work-holder and the vibrator holder is connected to the negative side of a rectifier. A DC. rectifier is used which has two voltage positions: one position for the holding voltage (about 3 volts) and 'a second position for electroplating (about 6 /2 volts). Connected electrically to the rectifier is a timer which automatically establishes the plating cycle. At all times except during the plating cycle a potential of between 2.8 and 3 volts is maintained between the electrodes. The vibrator is preferably connected to the time so that it is operated throughout the plating cycle only and automatically shuts off when the cycle is completed. The timer mechanism can be set to the required time to obtain the desired coating thickness. The timer can be arranged to activate the plating cycle (by increasing the applied voltage) and the vibrator can be adjusted empirically to obtain the desired optimum amplitude and frequency. As an example, an optimum frequency range for zinc-coated wire of the type described above is 3000 to 4500 c.p.m. with an amplitude range of 0.1 to 0.3-inch. After complettion of the plating cycle, the vibrator is automatically disconnected and the system reverts from the operating voltage to the holding voltage. In this way, the workpiece can be allowed to remain in the electrolyte for prolonged times without appreciably changing the thickness or quality of the coating.

Any suitable electroplating unit may be used; however, a preferred system is a portable unit with a tank for electrolyte and adapted to receive small sections such as the welded joint of coated wire strands. A suitable anode, such as a lead or silver-bearing-lead anode, is positioned within the electrolyte tank so that the wire, clamped to the vibrator, may be inserted into the electrolyte with the area of the work to be plated centered over the anode. Electrode spacing of 1 to 3 inches is desirable. It is also desirable that the workpiece not rest on the anode holder since this will dampen the vibration. A zinc sulfate electrolyte is used for zinc coating which contains about 10 to 14 ounces per gallon of 96% by weight H 80 ,v and about 22 to 34 ounces per gallon of zinc sulfate (ZnSO; H 0) electrolyte without a positive current flow instantaneous from the anode to the workpiece, the metal coating on the work would begin dissolving and form a loose deposit on the strand surface. For example, it has been found that if a partially zinc-coated work is immersed into the electrolyte for as little as one second without a holding voltage applied thereto, the subsequent coating is spongy and non-adherent.

It is apparent from the above that various changes and modifications may be made without departing from the invention; For example, Where a timer is used as in the preferred embodiment, they device can be set to increase the voltage automatically to the desired value for a predetermined plating period. After completion of plating, the timer can be used to cause the voltage to be reduced to the holding voltage value. With this arrangement, it is not necessary for an operator to maintain the system under surveillance at all times. Instead, the operator can be free to discharge other duties after the electroplating unit has been set.

We claim 1. A method of electroplating a partially-coated metal workpiece in an acid electrolyte containing a salt of the metal to be plated ,onto the work, comprising the steps of applying :a holding voltage potential to the partiallycoated workpiece prior to electroplating which is above the decomposition potential of said electrolyte, contacting said workpiece with said electrolyte to electroplate same and increasing the voltage applied thereto above said holding voltage to increase the electroplating rate, maintaining said increased voltage for a period of time corresponding, to the desired amount of plating to beaccomplished, reducing the voltage applied to the amount of said holding voltage and maintaining said holding voltage. on said workpiece until withdrawing same from said electrolyte. v

2. A method according to claim 1 wherein said workpiece is vibrated during electroplating thereof.

.3. A method, of electroplating a partially Zinc-coated steel workpiece in an 'acid electrolyte containing a zinc salt comp'rising the steps 0f applyinga holding voltage potentialto the workpiece prior to electroplating which is above the decomposition potential of the zinc salt electrolyte, suspending the workpiece as a cathode in said electrolyte to electroplate same, increasing the" voltage a'niountof plating to be accomplished, reducing the voltage applied to"th e amount of the holding voltage upon completion of the plating cycle and maintaining said holding voltage on said workpiece until it is withdrawn from said electrolyte.

4. A method according to claim 3 wherein a holding voltage of between 2.8 and 3.0 volts is applied to the metal workpiece.

5. A method according to claim 4 wherein the voltage is increased to from about 6 to about 7.5 volts to electroplate same.

6. A method according to claim 3 wherein the Workpiece is vibrated during electroplating thereof.

7. The method of claim 3 wherein said holding voltage potential to the workpiece prior to contacting it with electrolyte is from 0.2 to 0.5 volt above the theoretical decomposition potential of the metal salt measured in a 1 N solution at room temperature using platinum electrodes.

References Cited UNITED STATES PATENTS 1,942,356 1/1934 Fink et 'al. 20429 2,453,668 11/1948 Marisic et a1. 204-40 XR 2,744,860 5/1956 Rines 20445 2,951,018 8/1960 Seaborg et a1. 204228 XR 3,336,658 8/1967 Husni 204-45 XR JOHN H. MACK, Primary Examiner. G. KAPLAN, Assistant Examiner.

Claims (1)

1. A METHOD OF ELECTROPLATING A PARTIALLY-COATED METAL WORKPIECE IN AN ACID ELECTROLYTE CONTAINING A SALT OF THE METAL TO BE PLATED ONTO THE WORK, COMPRISING THE STEPS OF APPLYING A HOLDING VOLTAGE POTENTIAL TO THE PARTIALLYCOATED WORKPIECE PRIOR TO ELECTROPLATING WHICH IS ABOVE THE DECOMPOSITION POTENTIAL OF SAID ELECTROLYTE, CONTACTING SAID WORKPIECE WITH SAID ELECTROLYTE TO ELECTROPLATE SAME AND INCREASING THE VOLTAGE APPLIED THERETO ABOVE SAID HOLDING VOLTAGE TO INCREASE THE ELECTROPLATING RATE, MAINTAINING SAID INCREASED VOLTAGE FOR A PERIOD OF TIME CORRESPONDING TO THE DESIRED AMOUNT OF PLATING TO BE ACCOMPLISHED, REDUCING THE VOLTAGE APPLIED TO THE AMOUNT OF SAID HOLDING VOLTAGE AND MAINTAINING SAID HOLD VOLTAGE OF SAID WORKPIECE UNTIL WITHDRAWING SAME FROM SAID ELECTROLYTE.