US2315607A - Anode - Google Patents

Description

Patented Apr. 6, 1943 AN ODE Eric w. Ferm, Clarksburg, w. Va., and Richard Hull, Lakewood, Ohio, assignors to E. I. du Pont de Nemours & Company, Wilmington, DeL, a

corporation of Delaware No Drawing. Application March 30, 1938, Serial No. 198,830

3 Claims.

This invention relates to anodes for use in the electrodeposition of metals and is particularly directed to anodes which have a desired predetermined rate of solubility in an alkaline plating bath.

In processes for the electrodeposition of metals such as zinc, cadmium, and copper it is customary to maintain the metal content of the plating baths by the use of anodes made of the metal being plated. The use of such an anode is supposed to replenish the plating bath by dissolving metal in the bath from the anode simultaneously with its deposition at the cathode.

Considerable difliculty is experienced in practice by reason of the fact that the anode eificiency in plating baths is ordinarily greater than the cathode efficiency. In other words, the metal of the anode is dissolved in the bath more rapidly than it is deposited from the bath on to objects being plated. As a result of the rapid solution of the anode the bath soon contains excessive amounts of metal compounds.

For the best results it is desirable that plating baths remain as near as possible to a definite formula and changes in bath composition resulting from too rapid solution of metal anodes cause considerable difflculty. In recently developed processes for the electrodeposition of bright zinc deposits for instance it is particularly important that the bath composition be held rigidly within limits if the best possible results are consistently to be obtained. In the deposition of cadmium and copper, likewise, the best results can be ob tained only if the bath composition is maintained within some predetermined optimum range.

In addition to the diiiiculties arising from the too rapid solution of metal anodes in plating baths further difliculties result from the fact that soluble metal anodes do not dissolve evenly and from the fact that sludge is formed which interferes with the production of highest quality deposits. It is probable that a considerable proportion of the sludge formation encountered is attributable to the presence of impurities in the metal anodes.

Zinc anodes used in cyanide plating baths ordinarily dissolve quite unevenly and a black sludge is formed which floats in the bath and on the surface of the bath. The presence of black sludge is particularly disadvantageous in processes for the deposition of bright zinc from cyanide baths since this black sludge interferes with the production of a smooth and bright plate.

Cadmium anodes similarly dissolve unevenly they are used in cyanide plating baths. Copper anodes likewise dissolve unevenly as well as dissolving more rapidly than is desired.

When the metal content of a plating bath builds up to a point such that it becomes necessary to correct the bath composition this can be done by removing some of the metal compound or, as is more customarily the practice, the excess of metal compounds may be diminished by discarding a portion of the solution and correcting the composition of the remaining solution by the addition of bath constituents other than metal compounds. It will be apparent that such procedures are uneconomical and time-consuming.

It has been proposed to compensate for the high anode efiiciency of metal anodes in cyanide baths by replacing some of the soluble metal anodes with insoluble iron or steel anodes. This expedient results in a. diminution of the amount of metal dissolved per unit of electrical current passed into the bath and in effect reduces the anode efficiency.

This type of procedure is not entirely satisfactory with zinc because when the bath is standing idle the iron or steel anodes accelerate the rate of corrosion of the zinc anodes thus causing the soluble metal to dissolve rapidly and unevenly during periods when the bath is not in operation.

The use of iron or steel anodes is particularly objectionable in the widely used Rochelle saltscyanide copper solution for in these baths the iron is attacked by the solution.

It is also been proposed to include small amounts of a different soluble metal in soluble metal anodes to produce an alloy more slowly soluble. For instance, it has been proposed to use zinc anodes containing small amounts of dissolved aluminum. Such alloys are slightly less soluble than the pure metals and the anode efficency is slightly decreased. This type of anode has found some usefulness in connection with zinc-aluminum alloy anodes for acid-zinc plating baths.

Unfortunately, these alloy anodes do not solve the problem of maintaining plating baths at a constant composition. The effect of the added alloying metal is too small to secure a balance of anode and cathode eificiencies. This is particularly true when anodes are used in cyanide baths. With the zinc-aluminum anodes mentioned the decrease in anode efliciency in cyanide baths is quite ineffective.

The use of such alloy anodes is further disand an objectionable sludge is also formed when advantageous by reason of the fact that the i added lloyin metal goes into solution and exercises an effect on the character of the deposit being produced. This is especially true when mercury is used in Zinc anodes. I

It is an object of this invention to provide anodes which have a range of efiiciency extending as low as required to balance the cathode efiiciency of alkaline plating baths. It is a further object of this invention to provide plating processes wherein anodes are used having an efficiency predetermined with respect to the cathode efficiency so that a substantially constant bath composition is maintained. Still further objects of my invention will become apparent hereinafter.

The objects of my invention are accomplished by including in a soluble anode a small, predetermined amount of a metal which instead of dissolving forms a bath-insoluble compound.

According to our preferred practice, for instance,-

a small amount of magnesium is alloyed with a zinc, cadmium, or copper anode to control the solubility of the anode in alkaline baths.

By selecting a proper amount of a metal which forms bath-insoluble compounds, anodes having an eificiency corresponding closely to the cathode eificiency of the baths in which they are to be used may readily be produced. By the use of such anodes it is possible to maintain the balance of bath constituents for a much longer period than has heretofore been practicable, particularly with cyanide baths.

Not only are the anodes of our invention advantageous by reason of their maintaining a bath composition accurately but additionally the anodes dissolve uniformly and with the production of a minimum amount of sludge.

The action of a metal which forms bath-insoluble compounds is entirely'difierent from the action of metals which have heretofore been included in soluble anodes to decrease their solubility. The metals heretofore used, such as aluminum, are themselves soluble in cyanide plating baths and soluble metal anodes such as zinc alloys including such added metals are likewise soluble though they dissolve slightly less rapidly than pure soluble anodes. The effectiveness of such alloying metals as have heretofore been used depends upon the formation of an alloy of somewhat decreased solubility.

The inclusion in an anode of a metal which instead of dissolving forms a bath-insoluble compound has an effect on anode solubility entirely disproportionate to the amounts used if judged by the action of metals as heretofore employed.-

Metals added to soluble anodes according to our invention apparently cause the formation of a film of an alkali insoluble compound on the surface of the anode when it is in use. This film polarizes the anode, in a measure resisting the passage of current between the anode and the bath. The film of insoluble compound has the further effects of making anode corrosion more uniform and of preventing objectionable sludge formation and distribution in the bath. Any sludge which forms is carried to the bottom of the bath by the insoluble compound and is thus rendered innocuous.

The efiect of a metal which forms bath-insoluble compounds in alkaline solution is directly proportional to the amount of such metals present in an anode. By the selection of a suitable amount of thealloying metal, soluble anodes of specific character of the bath and the cathode efficlency which it is desired to match.

Our invention will be found particularly advantageous in connection with the plating of metals from alkaline solutions and more specifically to the plating of zinc, cadmium, copper, and brass from cyanide baths.

Considering now the application of the principles of our invention to an illustrative use we shall first discuss zinc anodes of improved solubility characteristics and their use.

Electrolytic zinc anodes for use in cyanidezinc plating baths normally have an anode efflciency'of one hundred to one hundred and five per cent while the normal cathode efilciency in similar baths is around eighty to ninety-five per cent. The anode efiiclency of such electrolytic zinc anodes may be reduced to fall within the range of cathode efliciencies by the addition of a metal which instead of dissolving forms a bathinsoluble compound.

The use of our preferred alloying metal, magnesium, to the extent of 0.18 per cent, was surficient under a specific set of conditions to give an electrolytic zinc-magnesium alloy anode having an anode efficiency of eighty-five per cent. It is particularly desirable to be able to reduce the anode eiiiciency of electrolytic zinc anodes because the high purity of this type of anode renders it desirable for bright zinc processes.

Many zinc anodes are not made from electrolytic zinc but are made from somewhat less pure zinc obtained by distillation methods. These anodes are much used for zinc plating and they have found considerable application in the plat ing of bright zinc from cyanide-zinc plating baths. These less pure anodes dissolve somewhat more slowly than do the electrolytic anodes, the usual anode efficiency being around one hundred per cent for commercial grade zinc anodes.

Considerably smaller amounts of magnesium are sufficient to reduce the anode efiiciency of zinc anodes to the required amount when commercial zinc from distillation processes is used. For instance, 0.11 per cent of magnesium was sufiicient to give a zinc-magnesium anode having an eficiency of eighty-five per cent when the zinc was of a commercial grade obtained by distillation.

The use of magnesium i particularly advantageous with the use of anodes which are less pure than the electrolytic zinc because the magnesium tends to reduce the amount of black sludge formed and further causes this sludge to drop to the bottom of the tank where it cannot interfere with the bright plates. As a result, it is entirely feasible to produce brilliant zinc plates from cyanide-zinc plating baths using commercial grades of zinc obtained by distillation if magnesium is incorporated in the metal according to the teachings of our invention.

While it is desirable to use as pure zinc as is economically available it will be understood that various metals may be included in the zinc anodes in conjunction with magnesium in accordance with the practices already known in the art. We may for instance use magnesium jointly with aluminum or mercury, or we may use all three to obtain their joint effects.

It will further be apparent that since the impurities present in commercial grade zinc cooperate with magnesium to produce anodes of low emciency it may conceivably be advantageous under some circumstances to add such impuri ties Jointly with magnesium to zinc metal which,

by reason of its higher purity, dissolves too rapidly and would require an excessive amount of magnesium to obtain the desired anode efliciency.

The following specific examplesillustrate the application of our invention to zinc-magnesium 6 anodes.

Example I Molten zinc obtained in a zinc distillation process was transferred in the molten state to a gasfired graphite crucible and the temperature raised from about 450 C. up to a temperature of about 600 0. Eight hundreths per cent of magnesium was then added to the zinc by introducing finely divided magnesium into the molten zinc and maintaining it below the surface until it was dissolved in the zinc. After the magnesium was alloyed with the zinc, the mixture was further heated and the alloy was then cast into a form suitable for use as anodes. 20

The zinc used was of commercial grade frequently employed for the manufacture of zinc anodes. The zinc contained about 0.07 per cent of cadmium and 0.005 per cent of iron. It also contained 0.10 per cent of lead and the cast anodes, accordingly, contained these impurities in addition to the added magnesium. For testing the anodesa cyanide-zinc plating bath of the type recently developed for the deposition of brilliant zinc was made up as follows: 30

Grams per liter Zinc cyanide (Zn(CN)2) 60 Sodium cyanide (NaCN) 52.5 Sodium hydroxide (NaOH) 78 Molybdic acid.(MOa) 7.5

Using the above bath and the zinc magnesium alloy anodes above produced, excellent deposits were obtained. The anodes dissolved evenly and without the formation of deleterious black sludge. Th small amount of sludge which formed sank to the bottom of the bath and did not interfere with the plating operation.

The anode efliciency was determined and it was found that the anodes made as above described and containing 0.08 per cent magnesium had an efliciency of about eighty-nine per cent in the above bath. A ZOO-gallon plating installation using such anodes was operated commercially for a period of four weeks and the zinc content of the the bath was substantially unchanged. In a similar period but using zinc anodes not containing magnesium, the zinc metal content of a similar bath increased by thirty grams per liter of zinc metals.

A number of zinc-magnesium alloy anodes were made up according to the procedure shown above and with the commercial zinc above described. When used with a plating bath having a composition as indicated above, the following tabulated efficiencies were obtained with anodes containing the indicated amounts of magnesium:

Anode Per cent of magnesium emciency The anodes above shown, like the one first mentioned in this example were uniformly dissolved in the plating bath. The amount of sludge became increasingly smaller with increasing 7 amounts of magnesium until at 0.21 per cent of magnesium there was substantially no black sludge formed. At 0.109 per cent of magnesium there was very little black sludge and, of course, what black sludge did form settled to the bottom of the tank.

Example II A zinc-magnesium alloy anode was made up according to the procedure described in the above Example I, but using 0.11 per cent of magnesium. The zinc used was somewhat purer than that of the above example, containing only 0.05 per cent of lead. In making the alloy 0.05 per cent of lead was added to decrease the solubility of the anodes.

After the alloy was cast into a suitable form, the anodes were tested in a bath such as that shown in Example I and were found to have an anode efliciency of about eighty-six per cent.

Since the presence of lead is so undesirable in baths for the deposition of bright zinc it does not at first appear practical to add this impurity to the zinc metal. The magnesium, however, is so eflicacious in preventing the dele terious action of lead and other such impurities that it is entirely satisfactory to use metal containing them.

I As indicated above, the magnesium probably acts by forming an alkali-insoluble magnesium hydrate or cyanide film which surrounds the particles of lead and drags them to the bottom of the bath where they can do no harm. It will be observed, of course, that there must be sufficient magnesium present since otherwise the lead would be free to exercise a deleterious influence.

Example III made up with electrolytic zinc and containing the below indicated amounts of magnesium:

Anode P oer t of m i I er 1 agnes u n oflimency While the foregoing specific examples show certain illustrative Ways of making zinc-magnesium alloy anodes, it will be understood that these may be made in various ways according to known metallurgical practices. It is to be observed, however, that after the magnesium is added to the zinc, the mixture should be heated to a temperature not substantially lower than about 700 C. In making up one set of electrolytic zinc anodes this precaution was not observed, the metal being poured at a temperature somewhere slightly above 600 C. and the alloy anodes so produced had an efficiency only slightly lower than that of electrolytic zinc itself. The specific temperature required to effect the required change in alloy properties can readily be determined in each specific instance by a few simple trials. The electrolytic zinc alloy anodes mentioned were re-melted and heated to a temperature of 700 C. and it was then found that the magnesium exercised its usual profound effect upon the anode efficiency.

From the foregoing Examples I, II, and III it will be evident that widely varying amounts of magnesium may be used in zinc anodes depending upon the effect desired and upon the character of the zinc which it is desired to modify. By the use of suitable amounts of magnesium it is entirely possible to adjust the anode efficiency to correspond to the cathode efiiciency of substantially any commercial cyanide-zinc electroplating bath and it is principally in this connection that the invention will be found valuable. It will be understood, however, that the beneficial effects of magnesium may be obtained by the use of small amounts of magnesium which will prevent the deleterious influence of metal impurities in the anode even without obtaining a very great reduction in the anode efiicienoy. Very small amounts of magnesium may therefore be used to advantage.

Larger amounts of magnesium may of course be used, though it will be understood that it will not be commercial to use excessively large amounts. In general, of course, the upper limit on the amount of magnesium used is determined by the cathode efiiciency of the baths in which the anodes are to be employed.

While, as above indicated, widely varying amounts of magnesium may be used, it will generally be found desirable to use from .01 per cent to about 1.0 per cent of magnesium. More specifically. it will usuall be found that the desired anode eificienoy and character can be obtained using from about .05 per cent to 0.3 per cent of magnesium with electrolytic zinc or about .05 per cent to 0.2 per cent of magnesium with commercial zinc such as that shown in Examples I and II and obtained by distillation processes.

Instead of using magnesium in soluble zinc anodes for alkaline baths a above discussed, another metal which forms a bath-insoluble compound may be used. Calcium, for instance, may be used in the manner above described in connection with magnesium. It will be found that the effect of calcium on soluble anodes for use in alkaline plating baths is about the same as theeffect of magnesium but that larger amounts of calcium are required.

The following example will illustrate the use of calcium in zinc anodes according to our invention.

Example IV Molten zinc obtained in a zinc distillation process was transferred in the molten state to a gasfired graphite crucible and the temperature raised from about 450 C. up to a temperature of about 600 C. Five-tenths per cent of calcium was then added to the zinc by introducing calcium turnings into the molten zinc and maintaining them belowrthesurface until they were dissolved in the zinc. After the calcium was alloyed with the zinc, the mixture was further heated to about 800 C. and the alloy was then cast into a form suitable for use as anodes.

The zinc used was of commercial grade frequently employed for the manufacture of zinc anodes. The zinc contained about 0.07 per cent of cadmium and 0.005 per cent of iron. It also iii) aangeov contained 0.10 per cent of lead and the cast anodes, accordingly, contained these impurities in addition to the added calcium. For testing the anodes, a cyanide-zinc plating bath of the type recently developed for the deposition of brilliant zinc was made up as follows:

Grams per liter Zinc cyanide (Zn(CN)2) 60 Sodium cyanide (NaCN) 52.5 Sodium hydroxide (NaOH) '78 Molybdic acid (M003) 7.5

Using the above bath and the zinc-calcium alloy anodes above produced, excellent deposits were obtained. The anodes dissolved fairly evenly and without the formation of deleterious black sludge. The small amount of sludge which formed sank to the bottom of the bath and did not interfere with the plating operation.

The anode efficiency was determined and it was found that the anodes made as above described and containing 0.5 per cent of calcium had an efficiency of about eighty-five per cent in the above bath.

Example V A zinc-calcium alloy anode was made up according to the procedure described in the above Example IV, but using 0.6 per cent of calcium. The zinc used was somewhat purer than that of the above example, containing only 0.05 per cent of lead. In making the alloy 0.05 per cent of lead was added to decrease the solubility of the anodes.

After the alloy was cast into a suitable form, the anodes were tested in a bath such as that shown in Example IV and were found to have an anode eficiency of about eighty-four per cent,-

Since the presence of lead is so undesirable in baths for the deposition of bright zinc it does not at first appear practical to add this impurity to the zinc metal. The calcium, however, is so eificacious in preventing the deleterious action of lead and other such impurities that it is entirely possible to have them present and still obtain good results.

As indicated above, the calcium probably acts by forming an alkali-insoluble calcium hydrate or carbonate film which surrounds the particles of lead and drags them to the bottom of the bath where they can do no harm. It will be observed, of course, that there must be sufiicient calcium present since otherwise the lead would be free to exercise a deleterious influence.

Example VI A zinc-calcium alloy anode was made up using electrolytic zinc and 1.07 per cent of calcium. The alloy was made following the procedure used in the above Example IV, the electrolytic zinc being melted directly in the graphite crucible. The zinc-calcium alloy anodes so produced were tested in a bright zinc bath such as that shown above in Example IV and it was found that the anode efficiency was about eightytwo per cent.

While we have shown certain specific conditions in the foregoing for the production of zinccalcium alloy anodes, it will be understood that these like zinc-magnesium anodes may be made in various ways according to known metallurgical practices.

With these as with the zinc-magnesium anodes care must be observed as to the temperatures of melting. After the calcium is added to zinc, the mixture should be heated to a temperature not substantially lower than about 800 C. In mali'ng up one set of electrolytic zinc anodes this precaution was not observed, the metal being poured at a temperature somewhere slightly above 600 C. and the alloy anodes so produced had an efficiency only slightly lower than that of electrolytic zinc itself. The specific temperature required to effect the required change in alloy properties can readily be determined in each specific instance by a few simple trials. The electrolytic zinc alloy anodes mentioned were remelted and heated to a temperature of 800 C. and it was then found that the calcium exercised its usual profound effect upon the cathode efficiency.

While calcium, like magnesium, may be used in widely varying amounts it will generally be found desirable to use about 0.05 per cent to about 5.0 per cent of calcium. More specifically. it will usually be found that the desired anode efficiency and character can be obtained using from about 0.25 per cent to 1.5 per cent of calcium with electrolytic zinc or about 025 per cent to 1.0 per cent of calcium with commercial zinc such as that shown in Examples IV and V and obtained by distillation processes.

Cadmium anodes of decreased solubility for use in alkaline plating baths may be prepared in the manners above described with reference to zinc anodes. Employing magnesium for instance, as a metal according to our invention which does not dissolve but forms a bath-insoluble compound. a cadmium anode of decreased solubility may be prepared.

The amount of an alloying metal according to our invention required to impart the desired characteristics to a cadmium anode can be determined following the considerations above discussed with reference to zinc anodesj The effects of these metals on cadmium seem approximately the same as for zinc and the ranges of magnesium. for instance. given for zinc are equally applicable for cadmium.

The following example illustrates a cadmium anode prepared according to our invention:

Example VII To molten cadmium there was added magnesium in sufiicient quantity that the final alloy contained 0.15 per cent Mg. The cadmium-magnesium alloy was cast into a iorm suitable for use as an anode.

The cadmium-magnesium alloy of this example was tried in a cyanide-cadmium plating solution and the anode efliciency was found to be about sium or calcium which form compounds insoluble in the bath is much greater in copper anodes than in zinc or cadmium anodes and much smaller quantities of such metals suffice to effect a desired reduction in anode solubility. In general it will be found that from about .0005 per cent to .5 per cent of magnesium will effect any desired reduction in anode efficiency while more specifically we ordinarily use from about .001 to .2 per cent of magnesium. Calcium would be used in proportionately larger amounts.

In the following example there is illustrated the use of magnesium for lowering the rate of solubility of a copper anode.

Example VIII Magnesium was dissolved in molten copper in such amounts that a final alloy containing 0.13 per cent of magnesium was obtained. The copper was cast into a form suitable for use as an anode.

The copper-magnesium alloy thus prepared was used in a cyanide-copper plating solution and it was found that it had an anode efficiency of 23.8 per cent. Pure copper in the same solution had an efiiciency of 78.5 per cent. The copper-magnesium anode'had too low an anode efiiciency to serve as the sole source of soluble metal for a copper plating solution. Such anodes could advantageously be used, of course, instead of iron anodes when it is desired to lower the metal content of a copper bath.

A copper-magnesium alloy was made up containing 0.0012 per cent magnesium and this was found to have an anode efllciency of sixty-three per cent. A pure copper anode in the same bath had an anode efficiency of ninety per cent. The anode efiiciency of this copper-magnesium anode is about that required in the copper cyanide solution used to maintain the copper content of the solution at a fairly constant value. With copper anodes, as with cadmium and zinc anodes, the amount of an alloying metal used will need to be adjusted for the specific type of bath in which they are to be used.

The anodes of this example were used with the greatest of success in a Rochelle salts-cyanide copper solution. This use is particularly advantageous because iron anodes cannot satisfactorily be used in such solutions.

It will be understood that the alloyanodes of this invention may be fabricated in any desired way to make anodes. Generally the alloys of our invention will be cast on a metal spine as in the Blouch Patent 2,097,508 or in the form of balls.

The metals employed for anodes according to our invention are preferably as pure as possible in accordance with prior art practices. The inclusion of metals which do not dissolve but form insoluble compounds increases the deleterious effects of impurities. and metals of somewhat lower purity can successfully be used for the anodes of our invention.

The addition of various other metals such as aluminum or mercury may be practiced with the anodes of our invention for whatever joint benefits result.

While we have shown certain illustrative alloys and processes it will be understood that we do not intend to be limited thereby as one skilled in the art may readily prepare numerous anodes containing a metal which forms an insoluble compound in a plating bath without departing from the spirit of our invention.

Reference is made to Patent 2,243,696 of coapplicant Farm and to Patent 2,214,331 of coapplicant Hull.

We claim:

1. A cast copper anode for use in a cyanide plating bath, the anode containing from about 0.0005 to 0.5 per cent of magnesium, the coppermagnesium alloy having been heated prior to casting to a temperature not substantially lower than 700 C.

2. A soluble metal anode cast of a metal selected from the group consisting of brass and copper for use in a cyanide plating bath, the anode containing a metal selected from the group consisting of 0.05 to 5.0 per cent calcium and 0.0005 to 0.5 per cent magnesium and the alloy having been heated prior to casting to a temperature not substantially lowe than about 800 C. for calcium and not lower than about 700 C. for magnesium whereby the rate of solution of the anode is decreased by reason of the formation of a bath-insoluble film on the surface 01 the anode.

3. In a process for the electrodeposition of metal from a cyanide bath, the step comprising replenishing the metal content of the bath with