US2335821A - Palladium plating bath - Google Patents

Description

Patented Nov. 30, 1943 2,335,821 PALLADIUM PLATING BATH Edmund Merriman Wise, Westfield, and Raymond Francis Vines, Garwood, N. J., assignors to The International Nickel Company, Inc., New York, N. Y., a corporation of Delaware -No Drawing.

Application June 25, 1940, Serial No. 342,286. In Canada May 20, 1940 a Claims.

The present invention relates to the electrodeposition of palladium including the electrorefining thereof and, more particularly, to the production of smooth, ductile, heavy electrodeposits of palladium.

Heretofore the electrodeposition of palladium has been limited to the production of thin electrodeposits of the order of about 0.00001 inch to about (rarely) 0.001 inch thick. These thin electrodeposits are useful for decorative effects such as for protecting silver from tarnish. However, the prior art baths and processes were not satisfactory for producing heavy, ductile platings.

The prior art baths may be divided into 4 main types depending upon the type of palladium salt used. Pilets bath is an example of the first type in which the essential constituent is a complex ammonium palladium salt. Phosphates are usually present and it is assumed enter into the palladium complex. The bath may also contain conducting salts, bufiers and brighteners or grain refiners. Baths of this type, as, for example, thoseof Wise mentioned in U. S. Patent No. 1,991,995, have also been employed where extremely low metal ion concentrations are desired and have contained ammonium plus cyanide complexes. The baths of this type have a low palladium ion concentration and, are generally operated in the approximatelyneutral or somewhat alkaline condition at relatively low platingrates. Insoluble anodes of platinum, graphite and the like'are usually employed since palladium does not dissolve to any appreciable extent in the electrolyte. Consequently, the bath must be replenished by the addition of palladium salts to replace the palladium removed from the electrolyte by deposition at the cathode. These additions of replenishing salt lead .to the accumulation of undesirable salts in the bath which eventually becomes so great that the bath must be discarded. Thin deposits from this type of bath are bright but heavier depositsare darkand brittle and are likely to peel from the basis metal.

A second type of palladium plating bath is typified by that described in the Keitel et a1. U. S. Patents Nos. 1,779,436 and 1,779,457. These baths are quite similar to the Type 1 baths in all respects except that the essential palladium salt which is also used for replenishing the bath forms gaseous products and palladium on electrolysis. In this manner, the replenishment of the pal1adium content of the electrolyte is accomplished without causing the accumulation of other undesirable salts. These baths are operated under conditions similar to those of Type 1 baths and the deposits are likewise bright when thin but dark and brittle and likely to peel from the basis metal when heavy.

A third type likewise contains as its essential constituent a complex ammonium palladium salt and is typified by Atkinsons bath described in British Patent No. 381,931. However, these baths differ in operation from those of Type 1 in that a porous cell surrounds the catholyte and permits the migration of undesirable salts to the anolyte where they may be removed. In this manner the accumulation of undesirable salts introduced in the replenishing solutions is avoided. Atkinson and Raper in discussing the results obtainable by the employment of baths of the third type have stated that deposits up to about 0.00002 inch thick are bright but that thicker deposits are milky. While Atkinson's bath and process may be used for building up thick deposits of the order of about 0.01 inch thick, nevertheless these relatively thick deposits have a tendency to be brittle. (Jnl, Electrodepositors Tech. Soc. volume 8, 1933.)

A fourth type of bath has been found to give satisfactory thin palladium deposits. Typical of this type of bath employing complex alkali metal palladium nitrite is that disclosed by Wise (U. S.

1,970,950). These baths containing complex palladium nitrites may also contain chlorides or bromides as disclosed byRaper (U. S. 1,993,623). In the presence of chlorides or bromides, palladium anodes dissolve quantitatively and consequently the bath is self-replenishing. However, in the absence of chlorides or bromides palladium salts must be added to the bath to replace the palladium plated out and this likewise resultsin an accumulation of undesirable salts in the bath. These baths likewise may be classified as low palladium ion baths and are operated at approximately neutral or slightly alkaline pH. According to Atkinson and Raper (loc. cit.) these baths are only useful for depositing a layer of palladium about 0.0001 inch or less in thickness since thicker deposits are very often unsatisfactory due to the fact that fine cracks appear in the deposit.

From the foregoing brief rsum of the prior art processes it is apparent that none of the prior art processes is suitable for depositing ductile, heavy electrodeposits of palladium.

Furthermore, all the prior art baths are of the low palladium ion concentration type resulting from the use of complex palladium compounds as the source of the palladium ion. All the prior art baths of necessity are operated at practically neutral or slightly alkaline pH to prevent decomposition or precipitation of the complex palladium salt. As a result of the experience gained the art has taught that these conditions were necessary for successful plating and for this reason palladium plating baths have been restricted to those of the complex compound type. In fact, Atkinson and Raper (loc. cit.) state the simple salts of the metal (palladium) as for example the chloride (PdClz) and the sulfate (PdSOl) are not completely stable in aqueous solution, and hydrolysis also occurs to some extent. Even in acid solutions, such electrolytes are not suitable for electroplating purposes, since the deposits are dark and probably contain much basic matter.

In contrast to this definite statement by those experts and in contrast to the teachings of the prior art, we have discovered that smooth ductile heavy deposits of palladium may be obtained from a high palladium ion concentration low pH bath.

It is an object of the present invention to provide a bath having high palladium ion concentration and suitable for the electrodeposition of palladium.

It is another object of the present invention to provide a bath for the electrodeposition of smooth, ductile, heavy deposits of palladium at a relatively rapid rate.

It is a further object of the present invention to provide a bath for the electrodeposition of palladium wherein the palladium is present as a chloride and the pH of the bath is low.

The present invention likewise contemplates the inclusion in the bath of conducting salts, buffering agents, grain refiners and thelike.

The invention likewise contemplates the electrowinning or refining of palladium from baths containing the palladium as a chloride and the production of high purity palladium suitable for rolling and annealing without melting.

It is also within the contemplation of the present invention to employ the baths and the process of the present invention in producing mirrors as described in our copending application, U. S. Serial No. 320,850, in producing or protecting of dental restorations and particularly for protecting tooth pins and the like, in producing thin walled tubing and in surfacing electrical contacts.

Other objects and advantages will become apparent from the following description.

Broadly speaking, the present bath employs palladium as a chloride at a pH of about 2 or below. The pH may be maintained by the addition of hydrochloric acid at intervals to replace that lost by evaporation, etc. Conducting salts or buffering agents such as chlorides of ammonia, alkali metals or alkaline earth metals, bromides,

- phosphates, acetates, borates, etc., may be used.

We do not consider sulphates in considerable quantities desirable additions as they increase the tendency to hydrolyze and precipitate a basic salt. Moderate additions of nitrates cause the deposit to assume a very undesirable needle-like form when the bath is operated at high temperatures. Fluoride additions, although initially not exercising any notable effect, later caused the deposit to become extremely brittle, probably due to reaction with the glass container employed. Depositable metal ions other than palladium should of course be absent unless an alloyed deposit is desired. The presence of salts of tin, copper, lead, silver, aluminum and chromium generally should be avoided as these elements tend to produce brittle deposits. A simple palladium chloride-hydrochloric acid bath has been found to give satisfactory results. However, the addition of ammonium chloride to the aforesaid simple bath appears to improve the ductility of deposits obtained from certain lots of palladium anodes which otherwise produce rather less ductile plates.

We have found that the purity or quality of the palladium used in preparing the bath and anodes affects the character of the deposit. Thus, we have found that ductile deposits are best produced from vacuum melted palladium heated to high temperatures such as about 1600 C. to about 2000 C. or from palladium melted under oxidizing conditions. When normal commercial palladium is used for preparing the bath and for anodes, thick electroplates can be obtained but in some cases they have a tendency to be of moderately low ductility. Furthermore, palladium melted under reducing conditions, especially reducing conditions produced by carbonaceous re.-

' ducing agents such as city gas, generally yields plates with low ductility. Under such conditions, that is, with normal commercial palladium or palladium melted under reducing conditions, ammonium chloride additions to the bath improve the ductility of the plates produced from such palladium.

In contrast to the prior art baths our new palladium plating bath does not give entirely satisfactory results by direct plating on nickel, copper, iron, silver and the like. When it is necessary to plate such less noble metals, a preliminary strike from a low metal ion concentration bath of palladium, gold, platinum, rhodium or other noble'inetal is preferably deposited first and then the final ductile thick plate ofpalladium is deposited from our novel bath.

The palladium concentration of the novel bath may be varied within wide limits, for example from about 10 grams per liter to about grams per liter, i. e. a palladium ion concentration great er than about 10- the higher concentrations permitting the use of higher current densities. At

' these higher concentrations and higher current densities a deposit of given thickness may be secured with considerably shorter plating times. However, with highly concentrated solutions, there is an increase in the initial cost of the bath and the drag-out losses are higher. Therefore, concentrations of about 25 toabout 50 grams of palladium per liter are preferable. The hydrochloric acid concentration may also be varied.

within wide limits as baths containing as little as 50 cc. and as much as 700 cc. of concentrated hydrochloric acid per liter have given good deposits. It is of interest that increasing the hydrochloric acid concentration within this range first causes a decrease in the pH of the bath from about +1.0 to below 0 and then an increase to about +2.0 as measured with a glass electrode. Baths with low hydrochloric acid concentrations are preferably operated at a pH below 1.0 but higher pHs may be used with baths of higher acid content. To the simple palladium chloride hydrochloric acid bath, ammonium chloride may be added in amounts of about 2% grams to about 50 grams per liter. However, high concentrations of ammonium chloride are to be avoided in baths having high hydrochloric acid concentration as it is relatively insoluble in the presence of strong hydrochloricacid.

The character of the deposits obtained is dependent upon such factors as palladium concentration, pH, temperature, current density and agitation and these factors are in turn interdependent as those skilled in the art know. Temperatures from 20 C. to 90 C. have been used with good results but since the loss of hydrochloric 1 acid at high temperatures is objectionable, a temperature of about 50 C. is preferred. In addition; current densities up to 50 amperes per square foot have given good results but a current density or about 10 amperes per square foot is generally preferred. With baths of high palladium content operated at high temperature and violently agitated much higher current densities can be employed, and under these circumstances the use of insoluble anodes'may be preferred. In that case, the palladium content canbe maintained by dissolving palladium anodically in a separate cell equipped with a diaphragm to prevent the deposition of palladium on the cathode of said separate cell and causing the anodically dissolved palladium to flow into the plating cell. If the palladium anodes employed be free from deleterious impurities no purification of the resulting solution is required, but if impure palladium be used it may be necessary to purify the resulting solution prior to permitting it to flow into Alternatively palladium may be Constituent Range Preferred Palladium as Pd C12. 25-175 gms./liter.. 50 gins/liter. Ammonium chloride -50 gms/liter 20-50 gmsJlitct. pH (glass electrode) 0.5 to +2.0 1 0O.5.

Temperature 25 C. to 85 C. 50 C.

Current densit 5-50 amps/sq. it... amps/sq. it.

Anodes Melted under andizing conditions.

Agitation Air lift Air lift.

1 With low hydrochloric acid concentrations this pH is preferably kept below +1.0 but with higher hydrochloric concentrations higher pH values may be used. Under the preferred conditions, palladium may be deposited at a rate of 0.00075 inch per hour or about 80 minutes per 0.001 inch thickness. 7

While very satisfactory plates have been obtained from unbufiered baths, there are a number of buffers which may be incorporated in our palladium baths. Thus for example, boric acid, oxalic acid, trichloracetic acid, monochloracetic acid, phosphoric acid, citric acid, and acetic acid have been found suitable for buffering our new palladium plating bath.

Example N0. 1

324 grams of palladium are dissolved anodically in hydrochloric acid using a porous cell around the cathode to prevent deposition of the palladium. After solution is complete the palladium chloride solution is boiled to remove excess H01 and chlorine and made up to about 2 liters with distilled water. The pH of the solution is generally below 0. This bath when heated to 50 C., and operated using a current of about 1.1 amperes will deposit about 0.025 inch of palladium on a 3 inch by' 1 inch specimen in about 16 /2 hours. The deposit is smooth, White, dense and fine grained. The edges are slightly heavier as in Example 2 or in any similar manner.

than the center cf-the plate and the composite sheet can be bent double with only slight edgecracking, that is to say, slight" cracking at the edges where the deposit isjsubstantially heavier than on the rest of the plate. I

Example No. 2

92 grams of palladium are dissolved in aqua regia and the solution so obtained evaporated to dryness. The residuev is preferably taken up with hydrochloric acid and water and evaporated to dryness to expel the nitric acid and other nitrogenous compounds. It has been found that three evaporations are generally 'sufficient to accomplish this. 'The final residue is then taken up with hydrochloric acid and water, 100 grams oi! ammonium chloride added, the solution boiled and diluted to 2000, cc. The palladium concentration is about 46 grams per liter and the hydrochloric concentration suflicient to give a pH of about 0.4. A three inch' by one inch sample plated in such a bath is suflicientlyductile to be I swaged cold as plated'or after annealing.

Example No. 3

grams of palladium are dissolved in aqua regia, the solution "evaporated to dryness and taken up with hydrochloric acid and water and the nitric acid and nitrogenous bodies expelled about 19 /2 hours about 20 grams of palladium are deposited providing a plate about 0.015 inch in thickness with a deposit which is smooth,

white, fine grained and very ductile.

The cathode current emciencies in these new baths are very close to and the anodes corrodepractically quantitatively. Therefore, the baths maintain their palladium concentration during use. Consequently additions of costly palladium salts are not necessary, and the accumulations oi! undesirable salts concomitant with the. operation of many prior art baths is avoided.

In electrowinning palladium from impure palladium content anodes it may be desirable to separate the anolyte and catholyte by a permeable diaphragm and to withdraw the anolyte (preferably from the bottom) and purify it prior to returning it to the cathode compartment, so as to permit the production of a highly pure palladium deposit.

Although the present invention has been described in conjunction with certain preferred embodiments thereof it is to be understood that variations and modifications may be made as those skilled in the art will readily understand.

Such variations and modifications are to'be considered within the purview of the present specification and the scope of the appended claims. Thus, calcium chloride and the like may be added to the bath to obtain'grain refinement in a de- The The palposit. Buffering salts such as boric acid, etc, may also be added. Oxalic acid, trichloracetlc acid, monochloracetic acid, phosphoric acid, citric acid, and acetic acid have been found suitable for bufiering our new palladiumbath.

We claim:

1. A process for electrodepositing ductile thick coatings of palladium which comprises electrolyzing an aqueous bath having a palladium-ion content greater than and consisting of about 25 to about 50 grams of palladium per liter as the chloride, about 50 cc. to 700 cc. of concentrated hydrochloric acid per liter, and about 2.5 to about 50 grams of ammonium chloride per liter at a.

temperature of about C. to about 90 C. whereby thick ductile electrodeposits of palladium are obtained.

2. An aqueous bath suitable for the electrodeposition of ductile thick coatings of palladium which consists of about 10 to about 175 grams of palladium as the chloride per liter, at least-about 2.5 grams of ammonium chloride per liter of bath, but less than the amount of ammonium chloride necessary to saturate the bath, and about 50 cc. to about 700 cc. of concentrated hydrochloricacid per liter, said bath being devoid of depositable metal ions other than palladium and having a palladium-ion concentration greater than 10-.

3. A process for electrodepositing ductile thick coatings of palladium which comprises electrolyzing an aqueous bath having a palladium-ion concentration greater than 10 and consisting of about to about 50 grams of palladium per liter as the chloride, about 50 to about 700 cc. concentrated hydrochloric acid per liter and about 2.5

grams to about grams of ammonium chloride per liter at a temperature of about 20 C. to about C. and at a current density not greater than about 50 amperes per square root whereby thick deposits of palladium up to about 0.025 inch in thickness are obtained.

4. A bath for the electrodeposition of thick ductile deposits of palladium employing as anode commercially pure palladium which consists'of an aqueous solution containing palladium chloride in sufiicient amount to provide about 10 grams to about grams of palladium per liter, about 50 to about 700 cc. of concentrated hydrochloric acid per liter and about 2 /2 grams to about 50 grams of ammonium chloride per liter.

5. A bath for the electrodeposition of thick ductile deposits of palladium employing as anode commercially pure palladium which consists of an aqueous solution containing palladium chloride in sufficient amount to provide about 10 grams to about 175 grams of palladium per liter, about 50 to about 700 cc. of concentrated hydrochloric acid per liter, about 2 /2 grams to about 50 grams of ammonium chloride per liter and at least one of the salts capable of increasing the conductivity of said bath selected from the group consisting of bromides, phosphates, acetates and borates of ammonia, alkali metals and alkali earth metals and chlorides of alkali metals and alkali earth metals in amounts insufficient to appreciably embrittle palladium electrodeposited v therefrom.

. EDMUND MERRIMAN WISE. RAYMOND FRANCIS VINES.