US2802782A

US2802782A - Anode

Info

Publication number: US2802782A
Application number: US602820A
Authority: US
Inventors: Bayes Ross; Otto W Langhans; Greenspan Lawrence
Original assignee: American Platinum Works
Current assignee: American Platinum Works
Priority date: 1956-08-08
Filing date: 1956-08-08
Publication date: 1957-08-13
Anticipated expiration: 1974-08-13

Description

States lPtuentO Y ANoDE Ros's Bayes, Basking Ridge, and Otto W. Langhans, Madison, N. J., and Lawrence Greenspan, Bronx, N. Y.,

assignors to The American Platinum Works, Newark, N. J., a corporation of New Jersey Application August 8, 1956, Serial No. 602,820

' s claims. (cl. 2414-292) This invention relates to the art of electroplating and is concerned with the electrodeposition of vsilver and similar metals, including silver alloys.4 The invention is more particularly concerned with the design Yof an anode for use in electroplating.

ThisI application is a continuation-in-part of our prior application, now abandoned Serial No. 351,645, led on April 28, 1953.

y Of utmost importance in the plating art in the design of anodes is the factor ofV current density. In order to assure longer life of the anode and uniformlyv plated products it is desirable to obtain uniform current density distribution over the anode. It is also important to maintain the anode area over the life of the anode by ensuring even corrosion across the entire surface of the anode. By doing this, a more efficient plating is obtained with less scrap anode remaining to be reprocessed.

The previously known anodes have generally consisted of a ilat plat having square edges. Such plates have been formed by casting or by rolling to a desired thickness. Such plates, furthermore, exhibit the undesirable properties of shedding and uneven dissolution of metal to such an extent that useful life ofthe anode is curtailed. Y u u It is an object of this invention, therefore, to provide an anode for electroplating purposes which shall approach a more uniform current density distribution during operation in an electrolytic bath. It is aA further object of the invention to provide an anode which exhibits the property of substantially uniform dissolution of metal across its surface in operation. y It isa still further object of this invention to provide a properly shaped anode which Willumaintain its area Yover a significantly greater period in ya plating bath than the `commonly shaped ilat plate anode. y

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: i

Figure l is a graph representing the current distribution on the side of a very thin at strip. A Y Figure 2 is a cross-sectional view through an anode constructed according to the invention. 'l

Figure 3 is a frontal view of the surface area'of ka conventional anode superimposed over the anode of the Apresent invention after both were subjected to a 75% weight loss. 1 Y Figure 4 is a view similarrto that of Figure 3 but lwhere the anodes were subjected to a weight loss of '80%. Figure 5 is a view also similar to that of Figure 3 where the anodes `were subjected to a weight loss of 86%.

Figure 6 is a schematic view of the anode of the invention immersed in an electrolytic cell. In the electroplating art the solution in the tank is considered as a conductor'of ,electric current, ,the two electrodes generally occupying. the end spaces .of-.the tank, wherein the sides, bottom and top are insulators.

For purposes of analysis the electrolytic cellimay be as sumed to be a rectangular box with the ends being the anode and the cathode and the box being filled ywith an electrolyte of uniform composition. `The resistancelR of the solution is then directly propertional to thelength L of the box (the distance between the electrodes) and inversely proportional to its cross-sectional area A, i. e., to the area of the electrodes. If the specific conductivity of the electrolyte be taken as the constant k then:

L R-, (o applying Ohms law with the Equation 1, it isreadily seen that the current I flowing in the electrode,is: g

KaV Y L l n (2) If the current density C be dened as the total current divided by the area of the'electrodes, then Equation 2 becomes: ,A

CFA-T Y It is thus seen that the current density is proportional to the voltage drop per unit length of solution. 'This applies to all arrangements of electrodes and is not limited to the rectangular box analysis supra. In considering the current distribution on silver a odes as commercially used, the anode approximates a thin plate and for purposes of analysis, it is assumed that the anode is infinitely thin and flat.

Moreover, 1t is also assumed that the anode is infinitely faraway from the cathode, so that puttingit still further away would not materially alter its current distribution'. `With reference to Figure l, the height of the graphtabove the X axis is proportional to the current density at the corresponding place on the anode shown immediately underneath, along the X-axis. The Width of the anode extends from X=l to X=|l along the X axis. Equation 3 then becomes: f

Figure l is a plot of this equation for all values vof X between -I-l and -l. It is thus seen that this theoretical formula makes the current density infinite at the edges of the anode, i. e., for X=1 and X=-|1.

In Figure 1 the average current density on the anode is represented by a line parallel to the X-axis through the ordinate 1/2'.

If the current were uniformly distributed overl the entire at plate, this line, instead of the curved* line, would represent the current-density curve. Thetotal area under this straight line is II units, which is ,the totalicurrent passing through one side of the strip, and therefore also the area under the curve representing the actual current density. in Figure 1 is equal to the cross hatched area' on the left and right sides of the curve. Thus, the-edges can be said to steal the current from the center portion.

In actual practice, however, one does not encounter true at strips with infinite current densities at the' edges. However, the current density distribution shown in Figure l indicates very closely the situation occuring in practice where the current density at the edgesof the anode is very much higher than that on the remainder of the anode;

It is thus shown that an `anode of the ordinary rectangular square edge shape corrodes at a much greater .tate along the edges than at the center portion of thev plate because of the much higher current density distribution along the edges. As the anode dissolves during use, the

The shaded area under the curve edges assume a knife-like appearance due to the uneven dissolution of metal. This sharpening of the edges further intensies this effect. The result is that the effective surface of the anode diminishes unduly rapidly. Sharpening of the edges also increases the tendency toward shedding of loose particles of the anode metal causing roughness in the electro-deposit. Thus the overall effect is a reduction in the general efficiency of the plating process. Moreover, When the anode is not totally immersed, the area decreases very markedly adjacent the top of the anode where it is immersed in the electrolyte. As a result the anode detaches itself at this point and drops off before the end of its useful life.

According to the invention the edges of the silver anode are bulbous, which may most economically be brought about by Aextruding a iiat plate to the configuration shown, for example in Figure 2. We have determined that the thickness of the bulbous end is related to the thickness of the flat plate itself, and that the relationship between dimensions b and a (as shown in Figure 2) should be within the range of 2:1 to 4:3. We have also determined that the width of the enlarged end bears a relation to the Width of the flat center' portion, and that the relationship between dimensions c and rl (as shown in Figure 2) should be within the range of lzl to 4zl0. if the ratio of bza goes beyond the limit of 4:3, or if the ratio of cza' goes below 1:10, then the anode approximates a square edge flat rectangular plate and no appreciable compensation for undesirable current density distribution is obtained. On the other hand, if the ratio of bza goes beyond the limit of 2:1, then the center portion of the anode wears out before the edges, i. e. premature thinning at the center would occur. However, the prime requisite is that the ends be substantially bulbous and rounded or curved or any other shape corresponding to a substantially bulbous enlarged end. It is not necessary that smooth curves be used, the term substantially bulbous enlarged end as used hereinafter is meant to include such variations from rounded or curved bulbous enlarged ends as serrated or rectangular enlarged ends with chamfered edges, and is intended to include the shapes shown in the drawings.

Figures 3, 4 and 5 illustrate an experiment showing the advantage of an anode S, shaped according to the teachings of the instant invention over a conventional flat rectangular anode R. In this experiment a standard silver plating solution was used and both anodes had the same width and the same weight per linear inch. The upper portion of the anode was painted with stop-off lacquer so that the lower 8 of each anode was exposed to the action of the plating current. The current density used was l0 amperes per sq. ft. average, representing common silver plating practice.

After approximately 75% of the weight of the immersed portion of the anode was dissolved, the area of the anode S was about 5% greater than the regular anode R, as shown by theshaded area in Figure 3. The latter anode had worn to a sharp knife edge whereas the anode S still had rounded edges. As the electrolysis proceeded from this point on there was a progressive relative improvement in the area of the anode S over the plain anode R.

At 80% loss in weight (Figure 4) the shaped anode S had an area 15% greater than the plain anode. At 86% loss in weight (Figure 5) the shaped anode area was 22% greater than the plain anode. At this `point in common plating practice the plain anode would be scrapped. On the other hand, the shaped anode made according to the present invention can still be used effectively for an appreciable period.

In Figure 6 there is illustrated a typical plating bath for carrying out the present invention. The plating tank 1 having a liner 2 of a material resistant to the action of the electrolyte carries a supply of conventional electrolyte 4 for depositing the desired metal. A conductor bar 6 carries the support 8 for the base member 10 to be electroplated. A second conductor bar 12 carries the anode 14 of Figure 2 of the invention of the metal to be plated. A suitable source of current is connected to the conductor bar 6 and to the conductor bar 12.

The invention thus provides an anode that for a large portion of its useful life gives a significantly greater area resulting in generally more eicient plating. Moreover, an anode made according to the present invention can be used an appreciably longer time than an equivalent conventional flat anode resulting in less scrap to be refined.

In combination with the particular anode shape described above, it is essential that the anode is an extruded silver anode.

The physical structure of the anode, particularly the grain size, strongly influences the manner of its electrolytic corrosion. Where the anode is used continuously for long periods of time, and where the grain size is large, the surface becomes rough during anodic dissolution, the corrosion is accentuated along the grain boundaries and tree-like dendritic structures form on the anode and shed off. Furthermore, metallic impurities which normally are concentrated in the grain boundaries have insufficient time to dissolve and remain suspended as fine particles in the plating bath.

Fine silver anodes usually contain small amounts of metallic impurities (of the order of several hundredths of a percent) most of which are difcultly soluble in the silver plating electrolytes commonly used. If these impurities are exposed during anodic corrosion at the receding anode surface at a rate faster than dissolution can occur they are shed into the solution as finely divided particles which remain suspended in the electrolyte and are deposited on the cathode causing rough and inferior electrodeposits. The manner in which these impurities are distributed thruout the mass of the silver anode strongly influences the amount of fine particles released into the electrolyte. Since in the casting and cooling of the ingot the impurities segregate in the grain boundaries the larger the grain boundary area the smaller the concentration of impurity in the grain boundary and the more favorable the conditions for solution of the impurity to occur during anodic dissolution. This optimum condition is reached where the grain size is small since a much larger grain boundary area is available for dispersion of the impurity. On the other hand with an anode containing predominantly larger crystals the precipitated film of irnpurities between the larger grains would be thicker and consequently take a longer time to dissolve anodically in the plating solution. In the latter case the result is, as explained above, the release into the plating bath of finely divided particles which deposit on the cathode causing rough and inferior electrodeposits. l

Even in an anode of exceptional purity we have observed that large grain size is detrimental to good plating results. In this case during electrolytic corrosion the grain boundary is attacked at a greater rate than the *grain itself. There is a strong tendency for dissolution of the grain boundary to occur before the crystal is completely dissolved with :the result that small crystalline particles are shed from the anode surface and are carried over to the cathode causing rough and inferior plating.' In the case of very large grain size intercrystalline attack .occurs at such a rate as to expose tree-like dendritic structures which fall off the anode and actually lcan be seen to gather in a mass in the bottom of the plating tank directly ,below the anode. This phenomenon is known in the electro, plating trade as shedding. In extreme cases it has been observed that intercrystalline attack can cause such irregular anodic dissolution as to actually perforate a substantial thickness of anode and cause relatively large masses of silver to fall off the anode. Obviously this can be extremely wasteful not only from the standpoint of the cost of reclaiming the silver, but making necessary frequent shutdowns of the tank to clean it out.

A number of samples of fine silver anodes produced by conventional methods of casting, rolling and annealing were examined for the determination of grain size. The usual method of preparing metallurgical specimens was used in which a small piece is cut from the sample, mounted in Bakelite or other plastic material and this surface ground by means of successively finer grinding media until a at, highly polished surface is obtained. Since silver is a relatively soft metal, the surface of the metal is disturbed in the mechanical polishing operations so that the ordinary methods of supercially attacking the surface by means of the proper etching agent was not used because it would not give a true picture of the actual grain structure. Instead the specimen after mechanical polishing was electrolytically polished to remove the disturbed amorphous layer and then carefully etched electrolytically to reveal the true grain structure. Photomicrographs were taken at 100 diameter magnication.

The grain size was arrived at first by using the ASTM grain size standard as a criterion with which the photomicrograph is compared. This approximate count was then checked by the intercept method in which the number of grains intercepted by a line of denite length is counted on the projected image or on the photomicrograph. With a given magnification, the average grain diameter can be calculated and, by squaring the grain diameter the average grain area can be obtained and the grains per sq. mm. calculated. This latter method assumes that the grains approach a square shape. This method is considered by metallurgists to be close enough for all practical purposes.

Rolled silver anodes, manufactured by conventional methods, and examined in the manner outlined above, showed a variation in grain size within the limits of about 1 micron and 600 microns. However the grains `appeared to occur either as relatively small or large grains wherein the small grains varied within the limits of about one and three microns, averaging about 2 microns in size, and the large grains showed a variation within the limits of about 200 and 600 microns with an average of about 380 microns. Furthermore, the photo-micrographs showed about 75% of the area to be occupied by the large grains and 25% of the area by small grains. This indicated that the mass of silver composed of large grains was approximately three times the metal mass occupied by small grains. Plating tests performed with such conventional anodes gave -electrodeposits which were much rougher than those obtained with .anodes of the instant invention.

We have now determined that to obtain a smooth plated product and to overcome the objections indicated above with the use of conventional anodes, the grain size of the anode must be controlled and must be made substantially uniform. Moreover, when the anode is manufactured so as to have a uniform grain diameter substantially within the range of about to about 100 microns, optimum smooth plated products are obtained. We further found that anodes of such small grain size could be most economically manufactured by the process of extrusion.

The following examples illustrate how the invention may be carried out in practice, but are in no way to be construed as limiting the invention to the details given in the examples:

Example 1.--Silver anodes 2 inches wide and 1A inch thick were extruded at a temperature of 1450 F. and at a pressure of 325 to 400 tons. ined according to the procedure outlined above. The

They were `then exam- 55 grain size was found to vary within the limits of about l0 to 100 microns, the greatest distribution occurring within the range of about 30 to 60 microns averaging about 45 microns.

Example 2 Silver anodes 2 inches wide and 1/z inch thick were extruded at a temperature of 1050 F. and a pressure of 370 to 600 tons. Examination of the anodes showed a grain size Variation within the limits of 20 and microns with the greatest distribution within the range of 55 to 70 microns, averaging about 60 microns.

Plating tests made with anode specimens manufactured as above described resulted in smooth electrodeposits of silver free from objectionable roughness. Surface examination of the cathodes on electrolytic dissolution showed the surface to be smooth and uniform.

We thus provide an anode for silver plating which during electrolytic corrosion produces a minimum of undissolved metallic particles and impurities and which corrodes smoothly and uniformly without shedding or flaking, allowing for the production of smooth electrodeposits.

We claim:

1. In an electroplating apparatus including a plating tank containing a supply of electrolyte, a homogeneous imperforate extruded silver anode having substantially bulbous enlarged edges, wherein the ratio of the thickness of the enlarged edges to the thickness of the plate is Within the range of about 2:1 to about 4:3.

2. The apparatus of claim 1 wherein the ratio of the width of the enlarged edges to the width of the central plate-like portion is within the range of about 1:10 to about 4: 10.

3. A homogeneous imperforate extruded silver anode consisting of a flat plate having substantially bulbous enlarged edges extending the full length thereof, wherein the ratio of the thickness of the enlarged edges to the thickness of the anode is within the range of about 2:1 to about 4:3.

4. A homogeneous imperforate anode of extruded silver, consisting of a at plate with enlarged bulbous edges, wherein the ratio of the thickness of the enlarged edges to the thickness of the anode is within the range of about 2:1 to about 4:3, and wherein the ratio of the width of the enlarged edges to the width of the central plate-like portion is within the range of about 1:10 to about 4:10.

5. An extruded homogeneous imperforate anode of extruded silver, consisting of a at plate with substantially enlarged edges extending the full length thereof, wherein the ratio of the thickness of the enlarged edges to the thickness of the anode is Within the range of about 2:1 to about 4:3, wherein the ratio of the width of the enlarged edges to the width of the central platelike portion is within the range of about 1:10 to about 4:10, said anode having a uniform grain size Within the range of about 10 to about 100 microns.

References Cited in the file of this patent UNITED STATES PATENTS 2,274,056 Geiger Feb. 24, 1942 FOREIGN PATENTS 133,882 Great Britain Oct. 23, 1919

Claims

1.IN AN ELECTROPLATING APPARATUS INCLUDING A PLATING TANK CONTAINING A SUPPLY OF ELECTROLYTE, A HOMOGENEOUS IMPERFORATE EXTRUDED SILVER ANODE HAVING SSUBSTANTIALLY BULBOUS ENLARGED EDGES, WHEREIN THE RATIO OF THE THICKNESS OF THE ENLAGRED EDGES TO THE THICKNESS OF THE PLATE IS WITHIN THE RANGE OF ABOUT 2:1 TO ABOUT 4:3.