US3450605A - Minimization of anode passivation in electroplating processes - Google Patents

Description

United States Patent US. Cl. 204-14 Claims ABSTRACT OF THE DISCLOSURE There is disclosed herein a method of minimizing anode passivation in electroplating processes, utilizing a pair of anodes which are alternatively made cathodic and anodic in the electrolyte taking advantage of the reversibility of the passivation mechanism to effect depassivation of the anodes without interruption of the plating cycle.

This invention relates in general to electroplating processes utilizing non-corroding anodes and contemplates novel methods and apparatus for use in such processes whereby the detrimental effects of anode passivation are substantially minimized.

Various articles of manufacture are plated, in a common practice, bysuspending them as the cathode in an electrolytic plating solution and applying a direct current voltage thereto. In these processes, particularly those wherein a large number of identical articles or surfaces are to be plated and, consequently, long anode life is desired, the anodes are frequently fabricated from the precious metals, such as platinum, which are inert chemically and non-corroding in the electrolyte at the conditions encountered in the plating operations. For the more precise plating applications, the anodes are placed in confrontation with the cathode at the areas to be plated and are usually formed to special configurations, each individual to the particular plating problem presented.

In the short run processes, such as those encountered in component salvage operations where the components are encountered in a wide variety of geometric forms, the necessity for resort to the precious metals imposes a severe economic burden which influences the feasibility of the process. It will be remembered that the anodes are formed according to the shapes to be plated and, hence, new anodes are generally dictated for each new component. The use of lower cost anode materials, such as titanium, is immediately suggested. However, depending to some extent uponvthe particular plating process involved, titanium and many materials similar thereto, rapidly develop dielectric surface coatings which interfere with the passage of current to the cathode and, hence, in the normal plating processes are rendered unsuitable as anode materials even for the short run operations.

It is a principal object of the present invention to provide methods of minimizing anode passivation in electroplating processes.

Another object is to provide apparatus for use in electroplating processes wherein the lower cost anode materials may be utilized.

These and other objects and advantages of the invention will be discussed in the detailed description which follows or will be evident therefrom or from practice of the invention.

In the detailed description which folows reference will be made to the attached drawing on which:

FIG. 1 is a somewhat schematic illustration of a particular preferred embodiment of the present invention;

FIG. 2 is a graphic illustration of the response of titanium to alternating power in a nickel plating bath;

Patented June 17, 1969 FIG. 3 is a graph demonstrating the response of titanium in a nickel plating bath to alternating power with a superimposed direct voltage.

Passivation of an electrode in an electroplating process, which tends to block the current flow, occurs as the electrode becomes anodic in the electrolyte in the face of an applied direct current voltage. Fortunately, in the case of many materials which are suitable for use as anodes except for the passivation problem, the passivation process is reversible. Hence, depassivation of the electrode will occur if the same electrode is made cathodic in the electrolyte. And even more fortunately, the depassivation reaction can frequently be caused to proceed more efiiciently than the passivation process. The present invention takes advantage of the reversibility of the passivation process.

In FIG. 2, the line segment 6K is representative of the polarographic response of titanium polarized anodically, and represents the region of electrical activity for this material. Point A corresponds to the Flad potential, or that voltage over which anode passivation occurs. Region E indicates the area of increasing anodic passivity which gives away with increasing voltage along segment EC to a point of anode corrosion and oxygen evolution. The curve OABC, therefore, is that generated in a process commencing with a clean titanium anode wherein the voltage is increased to and beyond the point where anode corrosion is initiated.

In the reverse process, or with decreasing power at the anode, the curve mm is generated. This occurs since the dielectric oxides generated on the anode surface during the power increase phase do not degrade readily, the segment 1 36, then, expressing the resistivity of the exposed titanium anode.

If a direct voltage is superimposed on the alternating current function, at various levels 0 O 0 the existing direct current forces the anode to cycle, due to the alternating wave, from an initial operating level different from that shown and described in connection with FIG. 2. Under these conditions, the'only current that can be drawn from the anode is that lying along the segment T5 5; O )BO, and without corrosion or gas evolution along O 73 0 B. The excess of current, resultant from the alternating wave, over that imposed by the direct voltage is observed as a side transient O A B, O A B, etc. As a greater and greater direct voltage is superimposed, the side transient is correspondingly curtailed until, at a location between A and B, the transient excess current falls to zero.

Since the anode is exposed to an alternating current, there is a portion of the cycle wherein the titanium electrode in fact becomes cathodic in the plating electrolyte. This is illustrated by the line segment O D'. As will be seen from FIGS. 2 and 3, there is a current-voltage asymmetry in the titanium anode system wherein the voltage in a direction reverse to that used in plating gives rise to a greater current flow than that wherein the flow is in the plating direction. Of course, the reverse voltage would tend to be opposed by the direct voltage impressed. In view of the direct current component, the alternating power will undergo some measure of self-rectification as seen by the anode, power for this neutralization effect being taken from the direct current plating circuit. This represents an undesirable and unnecessary burden for the direct current supply source.

During the period when the electrode is cathodic in the electrolyte as a result of the current reversal, the freshly formed dielectric oxides on the surface thereof are rapidly reduced, and the positive side transient is accordingly increased. Hence, it is undesirable tohave this current reversal opposed by the direct voltage. A blocking device is, therefore, positioned in the electrical system whereby the reverse current is kept out of the direct current power system and the anode is permitted to cycle through positive and negative phases.

According to the present invention, those electrodes which are subject to passivation are caused to sequentially assume both an anodic and a cathodic state in the plating bath. Hence, any surface layer which forms thereon during the anodic sequence is removed during the cathodic phase and the overall efifect is the maintenance of the electrode in a state of activation. In the case of those materials, such as titanium, wherein the depassivation reaction can be caused to proceed more efficiently than the reverse process, the detrimental elfects of passivation can be prevented for long periods of time.

To prevent any significant disruption with the deposition of the plate on the cathode, while the anode is alternatively and sequentially made anodic and cathodic in the electrolyte, a second anode is provided in the system, as seen in FIG. 1. At all times, at least one of the provided anodes is held anodic with respect to the cathode so that the plating process may proceed essentially uninterrupted, and the respective anodes are electrically interconnected so that when the first anode is caused to be anodic in the electrolyte, the second anode is cathodic with respect thereto, and vice versa.

To provide the desired sequencing, and as further illustrated in FIG. 1, the respective anodes 2 and 4 are connected in an electrolyte to a source of alternating current, shown in the drawing as a center-tapped current transformer 6. Anode 2 is connected to one output terminal 8 of the transformer secondary ill), and anode 4 is connected to another output terminal 12 of opposite polarity. Accordingly, as an alternating current is impressed on the transformer primary 14, anode 2 alternatively is made positive and negative, and anode 4 is similarly made positive and negative in phase with anode 2 but in cooperative opposition therewith.

A direct current supply 16 is provided, the positive terminal 18 of which is connected to the transformer at the center tap 20, and hence to anodes 2 and 4, the negative terminal 22 being connected to the article to be plated which acts as the cathode 24.

The plating solution and container 26 therefor are outlined in phantom in the drawing.

When the alternating current RMS voltage is less than the average voltage provided by the direct current source, passivation of the anodes will occur, and no significant plating of the cathode will occur. This arrangement may be used where a surface treatment of the anodes is the desired reaction. However, when the alternating current voltage equals or exceeds the direct voltage, then advantage may be taken of the efficient depassivation reaction and plating current can be drawn from the anodes. The response of the system in this situation, with respect to a single anode, resembles plating with "a single phase, half wave direct current voltage supply. If the appropriate geometric arrangement is made so that both anodes confront the cathode at areas to be plated, then single phase full wave conditions, usually a minimum essential in plating processes, are made to prevail.

A current diode 30, or other similar device, is required to prevent a direct current feed into the plating circuit from the alternating current supply which has a polarity opposite to that of the plating current. This means for preventing a current reversal in the direct current network is essential to prevent unnecessary work for the direct current source, since otherwise as previously indicated the direct current source must necessarily act to offset this reverse current flow. The alternating power supply is thus permitted to perform a switching function with,

respect to the two anodes.

The most desirable anode geometries and arrangements in a given system will be evident to those skilled in the art. However, as hereinbefore indicated, it is generally preferable to position both anodes in confrontation with the cathode at areas to be plated. Furthermore, coplanar anode systems will generally be avoided, especially when large areas are to be plated and, accordingly, large anode areas are required, to avoid overworking near edges and isolated portions of the cathode surfaces and reducing the depassivation effects on theanode at areas remote from the principal anode working surfaces.

EXAMPLE Various nickel base alloy salvage parts were plated according to the present invention utilizing titanium anodes. The nominal composition of the plating bath was as follows:

Oz./ Gallon NiCl -6H O 2 H BO 4 The bath was maintained at a pH of 5.8-6.2 and a temperature of 130 F. and plating was elfected at cathode current densities of approximately 20-100 amps per square foot. The direct current was supplied from a variable power supply limited to 10 volts and 5 amperes, and the alternating current was provided by a variable supply limited to 30 volts and 5 amperes at 60 cycles per second.

Substantial quantities of nickel, e.g., 0.001-0.003 inch per hour were deposited, the nickel having the same physical characteristics as that deposited by more conventional means. Furthermore, no corrosion of the titanium anodes was observed. The original operating parameters were maintained for periods in excess of four hours without necessity for correction of either the AC or DC power settings, thus establishing the stability of the system and its suitability to plating and electroforming operations of this general nature.

While the invention has been discussed in connection with particular preferred embodiments and examples for the purposes of the description, it will be seen to be applicable to other plating systems where anode passivaiton is a problem. In other systems, of course, different plating solutions and materials of construction will necessarily be utilized and the operating parameters of the process will vary accordingly. In other plating solutions, the suitable polarograms will, of course, appear different, particularly in terms of the permissible current densities. Further, in other systems utilizing highly oxidizing plating media, resort to the techniques taught herein may not be necessary. In the more conventional chromium plating processes, for example, the leadlead dioxide anode materials usually employed are in themselves cheap and inert. The greatest utility for the subject invention, therefore, will be found in those processes having characteristics normally dictating the use of the precious metal anodes. In these areas, however, the significant advantages of this invention will be evident.

Those skilled in the art will have no difficulty in adapt ing the teachings herein to their own particular purposes or requirements and such changes and modifications are contemplated within the true spirit and scope of the invention as set forth in the appended claims.

What is claimed is: 1. In an electroplating process wherein an anode and a cathode are connected in a direct current circuit and immersed in an electrolyte, the method of minimizing anode passivation comprising the steps of:

connecting a second anode in parallel with the first anode, connecting the respective anodes to the opposite terminals of a source of alternating current having a voltage exceeding the direct current voltage so that when the first anode becomes anodic in the electrolyte the second anode becomes cathodic with respect thereto and vice versa,

and interposing between the respective power sources means for preventing a current reversal in the direct current circuit.

2. The method of plating articles provided in a variety of geometric shapes comprising the steps of connecting each article as the cathode in a direct current circuit,

forming a pair of anodes in complementary geometric form and positioning the anodes to confront the cathode at areas to be plated,

connecting the respective anodes in parallel in the direct current circuit,

connecting the respective anodes to the opposite terminals of a source of alternating current having a voltage exceeding that of the direct current circuit,

providing a diode in the direct current circuit to the anodes to prevent a current reversal therein,

immersing the anodes and cathode in a plating electrolyte,

and plating each article to the desired thickness.

3. The method of claim 2 wherein the anodes are fabricated of titanium.

4. Apparatus for forming a deposit on a cathode in an electrolytic process comprising:

a source of alternating current,

a first anode connected to one terminal of said alternating current source,

a second anode connected to said alternating current source at a terminal of opposite polarity,

a source of direct current having a voltage less than that of said alternating current source, said anodes being connected in parallel to the positive pole of said direct current source,

a cathode connected to the negative pole of said direct current source,

and means to prevent a current reversal in the direct current circuit.

5. Apparatus for forming a deposit on a cathode in an electrolytic process comprising:

a center-tapped transformer,

a first anode connected to one output terminal of the transformer secondary,

a second anode connected to a second output terminal of the transformer secondary,

a source of direct current having a voltage less than that across said transformer output terminals,

an electrical connection between the positive pole of the direct current source and the center tap of the transformer secondary,

means in said electrical connection to prevent a current reversal therein,

and a cathode connected to the negative pole of said direct current source.

References Cited UNITED STATES PATENTS 1,529,249 3/ 1925 Gue 204231 1,735,509 11/1929 Setoh et al. 204231 2,696,466 12/1954 Beaver 204231 3,349,016 11/1967 Carlin 204231 FOREIGN PATENTS 198,274 5/ 1923 Great Britain.

JOHN H. MACK, Primary Examiner. T. TUFARIELLOR, Assistant Examiner.

US. Cl. X.R. 204228, 231

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