US2792341A - Process for electrodeposition of platinum - Google Patents

Description

May 14, 1957 2,792,341

PROCESS FOR ELECTRODEIPOSITION OF PLATINUM R; H. ATKINSON Filed Dec. 2, 1955 m M? 1 a a M 5 0 y w 11 ll 0 a IIIIII II a. l. w I l l 2 a? 4 s I n a ,c 0 z z amr A P 1 004 M 0 5 s P 6 FES w G I wfi n PF MM 1 I n S 0 I 0: Z flew TaIwi l PROCESS FOR ELECTRUDEPOSITION F PLATINUM Ralph Hall Atkinson, Westfield, N. J., assignor to The International Nickel Company, Inc.,New Yorlt,.N. Y., a corporation of Delaware Application December 2, 1955, Serial No. 550,632

4 Claims. (Cl. 204-47) The present invention relates to the electrodeposition of platinum andmore particularly to the production, of thick, smooth, commercially acceptable electrodeposits of platinum from an acid chloride electrolyte- It has been well known that thin, adherent. electroe deposits of platinum may be obtainedby electrolyzing a variety of baths. These thin deposits, as obtained-db rectly from the baths, have been of .the orderof. 0.00001 inch to rarely 0.001 inch in thickness. Thereareseveral main classes of baths from which these decorativeplates are obtained. Roseleurs bath is anexample of .oneclass. This bath contains an aqueous solution. of a complex platino phosphate usually modified with suitable buffering and brightening agents. The bath,v whenusedwith low cathode current density and when suitably modified, gives bright adherent thin plates which do notsrequire subsequent polishing. Another. classof thesedecorative plating baths is the type using ammoniacal dinitro-diammino platinum as a source of the metal to betdeposited. Good throwing power together with bright thin deposits are characteristic of this type of.bath.. At the same. time, the cathode current density may be higher thanthatused with Roseleurs bath. The third main. class of these baths is the alkali platinatetype. The-platinate .issusu-i ally buifered and stabilizedby the addition. of. walks.- line oxalate. Under the best conditions, thisbath achieves a cathode efliciency, with respect to platinum, of almost 100%. The typical deposits are very bright when thin and .the throwingpower of the solutionistgood- Eachofi the aforementioned baths may be usedlinxadiaphragm. cell.

The aforementioned prior platinum. platinglbathssutfer from certain commondisadvanta-ges. ThlIS,-..th-1CHZO lytes areusually unstable, inert salts accumulateduriug operation, and the cells need constant attention in order to maintain the requiredconcentration of platinum in the electrolyte. None of the prior processes :usesasoluble. massive anode and, most important, thick. ductile deposits cannot be produced directly-from thebaths. By. using a procedure consisting of plating a. thin layer of metal, followed by scratch brushing or polishing thedeposit, and then repeating the process as oftenas required, a thick smooth deposit may be built up. Without this. procedure thick deposits are uneven andundesirably crystalline in nature. However, when suclr repetition procedures are employed, the resulting-thick deposits are usually not ductile enoughto. withstand cold.

Working without cracking.

There have been references to the:deposition ofrplati num from acid chloride media. G.enerally,,only color and adherence were used as. criteria for. determining whether or not the deposits were. satisfactory. Oneadvantage of the acid chloride baths is that under the proper conditions the electrolyteis corrosive to massive plati-1 num. When. properly employed, this eiiectmaybesused to. advantage by providing; a soluble anodeprocess. This eliminates the necessity for frequent adjustment of 'the Patent 6 a ice metal concentration inv the electrolyte. However, the metal which is deposited under the appropriate conditions: forv the solution of the anode is, according to the priorrarhgrossly crystalline or blistered. Kushner (Metal Industry, 19-39, page 131) declares that deposits from acidchloride baths are spongy, dark or non-adherent. To my knowledge, no satisfactory ductile platinum electrodepositshave been obtained when using the acid chloride bathunder conditions which will efficiently corrode massiveplatinum anodes.

Althoughattempts were made to. overcome the: foregoing difiiculties. andother disadvantages, none,. as far as I am aware, was entirely successful whencarried into practice commercially on an industrial scale;

It'has. now been discovered that thick,-smoo'th, ad herent electrodeposits of platinum may beobtained by specially; controlling the process when usingan electrolyte containinga high concentration of hydrochloricacid.

It is.anobject of the present invention to'provide a process by means of which thick, smooth electrodeposits of. platinum. may be directly produced in-a singleplating operation.

Another. object of the invention is to provide a process bymeans of which thick, smooth, ductile electrodeposits of platinum may be made'without intermediate'brushing or polishing operations.

The invention also contemplates providing a process utilizing a stable'bath'suitable for the production of thick, smooth, ductile-'- electrodeposits of platinum wherein the concentration; of. the platinum in the electrolyte will remainiwithin operating specifications.

It is a further object of the invention to provide a. process using a stable acid chloride bath from which platinum may be deposited in a form suitable for mechanical working without a prior-annealing step.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing which shows a special'relationship between the concentration of'hydrogen chloride ir'rthe electrolyte and the rateof deposition of the metal Orr-the cathode which is required according to thepresent invention in order to obtain the results contemplated by the'invention, including electrodeposits of platinum free from cracks as plated.

Generally speaking, the present invention'contemplates a process for the production of thick, smooth electrodepositsiof platinum from an aqueous bath which containsinsolution' between about 180 and about 300 grams perliter (g. p. l.) of hydrogen chloride and between about lO and about 50 g. p. l. of platinum. Platinum is presentin'the bath chiefly in the form ofthe complex anion -PtCl6= with small amounts of PtClE-being usually co-prescnt; Metal is electrodeposited at a cathode while the'sbathis maintained at temperatures between about 45 C. and about C. it has unexpectedly been.dis-. covered .that' there'is a relationship between the desired physical characteristics of the deposit and a combination of the factors of rate of deposition ofmetal on the cathode and the concentration of hydrogen chloride in the bath. The accompanying drawing depicts thisrelatiom ship and shows the particular combinations. of rate of deposition" and concentration of hydrogen chloride which arevapplicable to this invention when the bath is main-. tained within the proper temperature range. When deposits'are'm-ade under the conditions represented by the area FGHEF'bounded by lines FGHE and PE in the accompanying drawing, these deposits are smooth and adherent when appreciable thicknesses are attained and are free from cracks in the as-plated condition. Deposits which are made under the conditions. represented by the more restricted area ABCDEA bounded" by the lines ABCDE and AE in the drawing will have the additional feature of being sufficiently ductile in the as-plated condition to withstand appreciable cold working without cracking or separating from the base metal. On the other hand, metal deposited 'under conditions represented by the areas outside the aforementioned areas of the drawing will be rough, spongy, semiburnt or cracked as plated. For example, if ductility is not an important consideration, thick, smooth adherent deposits of platinum may be obtained with a minimum rate of deposition about 026x10" inch per hour when the hydrogen chloride concentration of the bath is 300 g. p. 1. If the concentration of hydrogen chloride is lowered to about 260 g. p. 1., the minimum rate of deposition with which thick, smooth adherent deposits may be obtained is about ().."*l8 l0- inch per hour. Again, if the concentration of hydrogen chloride is lowered still further, to say 180 g. p. 1., then a minimum rate of deposition of about 0.59X 10- inch per hour is necessary in order to obtain thick, smooth adherent deposits. At all of these aforementioned concentrations of hydrogen chloride, the maximum rate of deposition with which one can obtainthick, smooth adherent deposits is about 1.0O l inch per hour. Table I illustrates the permissive maximum and minimum values for the rate of deposition which may be used at selected concentrations of hydrogen chloride if one 4 plenishment thereof either from the anolyte or from some external source. In the absence of the diaphragm, the concentration of platinum metal in the electrolyte will remain reasonably uniform due to the similar rates of solution of the anode and deposition of metal on the cathode which are achieved in employing the bath contemplated in accordance with the invention. Only infrequent additions of hydrogen chloride and water are then necessary to replace incidental losses from evaporation and the like.

As was mentioned hereinbefore, the bath from which the metal is deposited is an aqueous acid chloride-chloroplatinate bath which must contain in solution between about 10 and about 50 g. p. l. of platinum. Ordinarily, the platinum is added to the bath initially in the form of chloroplatinic acid. If platinum metal is used as an anode, the concentration of the platinum will be maintained substantially constant during the plating operation due to solution of the metal from the anode. The initial addition of platinum in the form of chloroplatinic acid. is recommended because of the convenience of the preparation of this compound. However, if one wishes, the platinum may be added in any form which will produce a solution which is free from materials which will have desires to obtain deposits which are not only thick, smooth and adherent but are also ductile.

TABLE I Minimum Maximum Rate of Rate of 00110. of Hydrogen Chloride in g./I. Den. in Den. in

lnchXlO-l InchXIO- I' hr. vhr.

It must be noted that the factor of rate of deposition is not equivalent to the cathode current density which is the usual criterion of the electrodepositing art. There is no simple relationship between these two factors when depositing platinum metal from the electrolytev contem plated in this invention. When an article or a series of similar articles are to be plated under a particular set of conditions (e. g., bath composition, temperature, Gil culation, electrode separation, potential, etc.), the proper rate of deposition must be initially determined by measurement of the change in thickness of the deposit with respect to time. When the proper rate of deposition of metal is obtained in accordance with the inventiomthe rate may then be maintained by control of the particular conditions (e. g., bath composition, temperature, circulation, electrode spacing, potential, cathode current density or its equivalent cathode current, etc.). Any change in the selected conditions will usually affect the rate of deposition and necessitate another determination of the rate of deposition or a prior determination of the influence of the change upon the rate of deposition.

\ The cell may be constructed of any material resistant to corrosion by the electrolyte. It may be of a diaphragm or a non-diaphragm type. The diaphragm may be of the usual type and, as those skilled in the art know, it must be resistant to the action of the electrolyte but must permit the passage of the electrical current. It is within the scope of the invention to use either soluble platinum metal anodes or insoluble anodes made from material inert to the action of the electrolyte (e. g., rhodium). When a diaphragm cell is used with soluble platinum anodes, the concentration of platinum metal in the catholyte may be kept within the proper working range-by re-- crease this ratio.

a detrimental effect on the bath. For conveniences sake. these materials are called subversive materials.

There are several different kinds of these subversive materials, e. g., strong oxidizing and reducing agents should be absent from the bath. The platinum should be predominately in the valence state of four with the ratio of platinic to platinous ions being greater than about twenty to one. Additions of strong reducing agents, particularly organic matter, would tend to de- On the other hand, an excess of strong oxidizing agents will tend to reduce the cathode efiiciency. Metals which will hydrolyze in high concentrations of hydrochloric acid, (e. g., titanium, zirconium, etc.), should be absent from the bath because the hydrolysis products of these metal ions may be insoluble substances, colloidal in nature, which would tend to co-deposit with the platinum thus impairing the adherence, continuity and ductility of the deposit. Tungsten and molybdenum should be absent from the bath. These metals represent a class of metallic elements which will form insoluble acids in strongly acidified solutions, e. g., tungstic and molybdic acids. Potassium, ammonium, cesium, rubidium, etc., ions should be substantially 7 absent from the bath because of the well known tendency of these materials to cause the precipitation of platinum as the chloroplatinate, e. g., potassium chloroplatinate. Metals which may co-deposit under the process conditions should be absent from the bath in appreciable quantities. For example, nickel, iron, copper, silver, gold etc., should be absent. It must be understood, however, that impurities normally associated with commerical plati'num in quantities normally occurring in commerical grades of platinum do not have any noticeable elfect on the character of the deposits.

In view of the foregoing remarks, platinum may be added to the bath not only in the preferred form of chloroplatinic acid but also in the form of any one or a plurality of compounds including sodium chloroplatinate, lithium choloroplatinate, alkaline earth chloroplatinates, hydrated platinic oxide and mixtures of chloroplatinitic acid and chloroplatinous acid containing less than about 5% of chloroplatinous acid. When platinum is added to the bath in any of the forms noted herein,

it is obvious that there is necessarily added a stoichioaireagsm platinumrtojthebath. In addition to;the:inertrsubstances mentioned? iniconnectioni with theaddition of: platinum, the--bath may tolerate up to, about 50, g. ,p; l.. in the aggregateof inertingredients, (e; g.,.the:soluble-sulfates and acetates. of ,sodium, lithium, alkaline earth; metals, magnesium, aluminum, etc.). Included: in these inert ingredients and .in the aggregate permissive upperlimits of. concentration are free sulfuric. and; acetic acids. As far;as is:known, the'addition ofmoderate. amounts, e. g., upto about.5.0 g'. p; .1. in.the aggregate, of these soluble substances does noteffect the working characteristics of the" bathas contemplated. in this-invention. In general, it'shouldbe statedthat it is: preferred" to avoid the addition' of substantial. amounts ofanions other than chloride-and. chloroplatinate becauseof the complex chemical reactionswhiclt mayoccurtin mixtures of this type.

Theelectrolyte' which is employed in accordance-with the invention is corrosive to=many= base metals, such as irom- 'copper, nicke1,1zinc; silver; etc. When it is' desiredJtopIate. onsuch base metals, they should'be given a preliminary strike ofa resistant noble metal from a suitable high pH bath in whichthe; concentration of the noble metal ion islow. Satisfactory strike deposits'include gold, platinum, palladium or rhodium. Those skilled in the artwill recognize. that any one of a num ber of baths designed to deposit these metals-may be used. Particular attention should be paid to the surface preparation of the cathode prior to plating these strikes: in order to obtain optimum adherence ofthe strike. The strike should be of such a thickness as to effectively reduce the corrosionrateeven in theabsence oficathodic protection;

In carrying theinvention into practice, it'is desirable to maintainthe conditions of electrodepositing within the-ranges'in Table II always keeping in mind that the factors of rate of deposition and concentration of hydro gen chloride must be related to each other as shown in the drawing.

TABLE II Broad Preferred Platinum in Bath, grams per liter 10 to 50 15m 25 Hydrogen chloride, grams per liter. 180 to 300 240 to 260 Rateoi Deposition, in./hr. l.0- 0. 3' to 1; 0 M0 0.8 Temperature, C 45 to 90 65 to 75 Example I A plating solution was prepared containing 4 parts by weight ofplatinnm.as-chloroplatinic acid, -150 parts by weight of concentrated hydrochloricacid and 76 parts by weight of-distilled water. The concentration ofhydrogen chloride in this electrolyte was about 260 g. p. l. and the platinum concentration was about 20 g. p. l. The electrolyte was placed in a glass container, center portion of the bottom of which was heated, and the temperature was brought to and maintained at 70 C. The cathode was a pure platinum wire 2 inches in length and 0.044 inch in diameter which had been previously cleaned with alkaline cleaning solution and then rinsed thoroughly with distilled water.

'6 Thea anodes were: two: strips: of pure platinum, each 5 inches :byi 0;5:.-inch :by :0.0.l0 inch in dimension with about 1.75; inches: immersed in the: electrolyte. A potential was applied. to the system. in order to efiect a rateof deposition approximating, 0.5.)(107 in./hr.. The current which wasrused was..0.05-5. ampere. (amp), giving a cath ode:current density of 3.11amperes per square decimeter (ampddmfi); After one hour it was determined that thermetal depositedwas 0.5)(10- inch thickand: was smooth, adherent and gray in color.- A seconddeposit from the same bath on-.-the, samecathode was made with a rate of deposition of about 0.7)(10- in./hr. using a current'of 0.060 amp., the cathode current density being 3.4 amp./dm: Atthe end of the second hour, the deposithad' increased in thickness to=1.2' l0- inch. The entire deposit: was smooth, adherent and gray in color and was ductile as-indi'cated by a-cold rolling test with 20% cold" reduction as the criterion. The. deposit, as plated, hadian-average-Knoop'Hardness Number (K. N.)' of 220 using a 25' gram load. The plating solution was stable and had aneffective life of at least 57 plating hours-with only infrequent additions of hydro;- clilorieacid"and-waternecessary to replace losses dueto evaporation:

Example -II A. bath, with the same. hydrogen chloride concentra: tion as in Example I and having. a. concentration 01321.8 g..p. l. of platinum, as. chloroplatinic,acidwas,used to deposit platinum, onv a. platinum. strip cathode. measuring 1 inch by 0.25,. inch by.0.0,10 inch, Anodes, similar to those. in. Example I were also used. Using a rate of deposition of .about.0,6 10- in./hr. metal-was electrodeposited and. after, six hours at, an operatingv tempera; tureof 70 C. a deposit 3,.63X 1.0.? inch thick was .oh tained. The. current used was 0.080 ampand'the cathodecurrent density was 2.4 amp./dm. The resulting deposit was uniform, ductile,adherent and hadan, average-.hardnessofi 214 K. H. N. witha 25 gram load.

Example 111' A glass container was separated into anodicand cathodic compartments through the use of a porous porcelain diaphragm. Asolution similar to that used in Example I was placed in the cathodic compartment. A hydrochloric acid electrolyte was used as the anolyte. The anodes and cathode were similar to those used in Example I and the operating temperature was 70 C. A rate of deposition ofapproximately 0.7)(10- in./hr. was established and after one hour, 0.6,'7 l.0- inch of metal was deposited usinga current of 0.045 amp. and :1 current density of; 2.6- amps./dm. Thedeposit was light gray in color, smooth, adherent and free from cracks as plated. A further deposit at approximately the same rate of deposition was made on the same cathode, and after another hour the deposit was found to have increased in thickness to l.35j l0'- inch. A current of 0.040 amp. with a cathode current density of 2.3 amps./ dm was used forthis secondhour. The total deposit was smooth and ductile and had an average hardness of 194 K. H. N. with a 25 gram load. Chemical analysis of the catholyte revealed that its platinum content was reduced from 23 down to 15 g. p. l. in the. course. of two hours plating in. the absence of replenishment.

Example IV A, plating solution, was prepared. having a platinum concentration. of.22.g. p. l. and ahydrogen chloride concentration of 219 g. p. 1. It was placed in a glass con: tainer similar to the one used in Example I and heated to about 70 C. in the same manner as in Example I. The cathode was a platinum wire having the same di mensions as the cathode in Example I. This cathode was previously cleaned in alkaline cleaning solution and thoroughly rinsed in distilled water. The anodes were the same as used in Example I. A potential was applied to the system in order to effect a rate of deposition approximating 0.9X10" in./ hr. The current which was used was 0.063 amp. giving a cathode current density of '3.5 amps./dm. After one hour, it was determined that the metal deposited was 0.88 10- inch thick and was a smooth gray mat showing no cracks as plated when examined under the microscope. The deposit had an average Knoop Hardness Number with a 25 gram load of about 274. It was found to be brittle when cold rolled for a 20% cold reduction.

Example V A plating solution was prepared having a platinum concentration of 23.6 g. p. l. and a hydrogen chloride concentration of about 260 g. p. l. The cathodes, anodes and cell were similar to those used in Example I. A temperature of about 45 C. was attained and maintained. A potential was applied to the system in order to effect a rate of deposition approximating 0.5X10" in./hr. After one hour, it was determined that the metal deposited was 0.49 10'- inch thick. The current employed was 0.035 amp. and the cathode current density was 2.0 amps/dmfi. Visual examination revealed that the plate was a light gray mat. On microscopic examination the plate was found to be free from cracks. The average Knoop Hardness Number with a 25 gram load was 241 and the plate was found to be ductile upon a 20% cold rolling reduction.

The following test was conducted to illustrate the efiect of using a rate of deposition which, when plotted against the hydrogen chloride concentration, results in a point that lies outside the area FGHEF in the drawing. In conducting this test, a plating solution was prepared con taining 222 g. p. l. of hydrogen chloride and 22 g. p. l. of platinum. The plating run was made at a temperature of 70 C. for three hours with a current of 0.045 amp. and a cathode current density of 2.5 amps./dm. The rate of deposition was 0.27 10- in./hr. Upon superficial examination, the deposit 0.80 10- inch thick was found to be smooth, dark gray with slight sheen, hard and brittle. Microexarnination revealed that the deposit was cracked as plated.

The data set forth in Table III illustrate the fact that cathode current density and rate of deposition are not equivalents when depositing from the acid chloridechloroplatinate baths at a standard temperature (70 TABLE III Cathode C. D. ampsJdmJ Rate of Dep. lu./hr.

It is to be observed that the present invention provides a process for the production of thick, smooth ductile electrodeposits of platinum. In carrying out the invention, deposits up to about 0.02 inch in thickness are readily obtained. The process involves electrolyzing the bath under stated conditions of rate of metal deposition correlated to hydrogen chloride concentration. The essential ingredients of the bath are an aqueous solution of hydrogen chloride and a source of chloroplatinic ion. The platinum in this bath is associated with chlorine such that the ratio of platinum in the chloroplatinic form to platinum in the chloroplatinous form is at least about 20:1. The balance of the bath is essentially water. However, the inclusion of 0 to about 50 g. p. l. in the aggregate of soluble non-subversive inert substances, e. g., sodium salts, lithium salts, sulfuric acid, acetic acid, alkaline earth metal salts, etc. is contemplated within themeaning of the term balance essentially.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A process for the production of thick, smooth, adherent electrodeposits of platinum free from cracks in the as-plated condition which comprises establishing an aqueous acid chloride-chloroplatinate bath containing in solution about 180 to about 300 grams per liter of hydrogen chloride, about 10 to about 50 grams per liter of platinum to provide a source of chloroplatinic ion, the balance being essentially water and electrodepositing platinum from said bath while maintaining the bath at a temperature between about 45 C. and about 90 C. at such a rate of deposition that the rate of deposition of the metal on the cathode when plotted against the concentration of hydrogen chloride in the bath will be represented by a point lying within the area FGHEF in the accompanying drawing.

2. A process as set forth and described in claim 1 wherein the bath is maintained at a temperature between about C. and about C.

3. A process for the production of thick, smooth adherent, ductile electrodeposits of platinum free from cracks in the as-plated condition which comprises establishing an aqueous acid chloride-chloroplatinate bath containing in solution about 215 to about 300 grams per liter of hydrogen chloride, about 10 to about 50 grams per liter of platinum to provide a sorce of chloroplatinic ion, the balance being essentially water and electrodepositing platinum from said bath while maintaining the bath at a temperature between about 45 C. and about C. at such a rate of deposition that the rate of deposition of the metal on the cathode when plotted against the concentration of hydrogen chloride in the bath will be represented by a point lying within the area ABCDEA in the accompanying drawing.

4. A process as set forth and described in claim 3 wherein the bath is maintained at a temperature between about 65 C. and about 75 C.

References Cited in the file of this patent Kishner: Metal Industry, volume 37 (-1939), page 131.

Claims (1)

1. A PROCESS FOR THE PRODUCTION OF THICK, SMOOTH, ADHERENT ELECTRODEPOSITS OF PLATINUM FREE FROM CRACKS IN THE AS-PLATED CONDITION WHICH COMPRISES ESTABLISHING AN AQUEOUS ACID CHLORIDE-CHLOROPLATINATE BATH CONTAINING IN SOLUTION ABOUT 180 TO ABOUT 300 GRAMS PER LITER OF HYDROGEN CHLORIDE, ABOUT 10 TO ABOUT 50 GRAMS PER LITER OF PLATINUM TO PROVIDE A SOURCE OF CHOROPLATINIC ION, THE BALANCE BEING ESSENTIALLY WATER AND ELECTRODEPOSITING PLATINUM FROM SAID BATH WHILE MAINTAINING THE BATH AT A TEMPERATURE BETWEEN ABOUT 45* C. AND ABOUT 90* C. AT SUCH A RATE OF SEPOSITION THAT THE RATE OF DEPOSITION OF THE METAL ON THE CATHODE WHEN PLOTTED AGAINST THE CONCENTRATION OF HYDROGEN CHLORIDE IN THE BATH WILL BE REPRESENTED BY A POINT LYING WITHIN THE AREA FGHEF IN THE ACCOMPANYING DRAWING.

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