US3480523A - Deposition of platinum-group metals - Google Patents

Description

'us. Cl. 204-43 United States Patent DEPOSITION 0F PLATINUM-GROUP METALS Colin John Nelson Tyrrell, London, England, assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 436,319, Mar. 1, 1965. This application Apr. 12, 1966, Ser. No. 541,944

Claims priority, application Great Britain, Apr. 15, 1965, 16,256/ 65; Nov. 8, 1965, 47,295/65; Apr. 4, 1966,

Int. Cl. C23b 5/30, 5/24 14 Claims ABSTRACT OF THE DISCLOSURE A bath (and process) for plating iridium, palladium, platinum, rhodium and ruthenium and alloys thereof comprises a water solution of the bromide salt of the metal and excess hydrobromic acid.

This application is a continuation-in-part of my copending US. application, Ser. No. 436,319, filed Mar. 1, 1965, now abandoned.

The present invention is directed to the electrodeposition of iridium, platinum, palladium, rhodium and ruthenium and of alloys thereof from an aqueous solution and, more particularly, to a special aqueous bath and a process for the electrodeposition of iridium and the said other platinum-group metals and alloys thereof.

The properties of iridium have long suggested that it would be very desirable to provide a satisfactory means for producing iridium coatings. Thus, iridium is an extremely hard, white metal which demonstrates outstanding resistance to corrosion in aqueous media, including, for example, concentrated acids, aqua regia and most halogens. It'is also resistant to anodic corrosion in most aqueous solutions and has a lower over-voltage for some reactions than platinum. It is resistant to attack by a number of molten salts and molten metals, including the alkali metals, lead and mercury. In addition, iridium has a high melting point and is resistant to diffusion with base metals. These desirable properties of iridium indicate that if satisfactory means for producing electrodeposited coatings of this metal could be found, they would be useful in contact applications such as switches and the like, as diffusion barriers, as coatings for readily oxidizable metals such as molybdenum and tungsten, in the production of anodes for use in chlorine cells, etc. Electrodeposits of other platinum-group metals and alloys thereof are also desirable in the same and in other applications.

Processes for the electrodeposition of iridium have recently been reviewed by F. H. Reid in Metallurgical Reviews, 1963, volume 8, pages 196 to 199 and 207 and 2.08. None of the aqueous baths discussed in that paper is, however, wholly suitable for use on an industrial scale and the author concluded that the deposition of iridium from aqueous electrolytes does not at present seem practical.

I have now discovered a special aqueous bath which may be employed to produce hard, adherent electrodeposited coatings of iridium in thicknesses up to about microns.

It is an object of the present invention to provide an aqueous "bath useful as an electrolyte for electroplating iridium, platinum, palladium, rhodium and ruthemum.

It is a further object of the invention to provide a special process employing a special aqueous bath to produce electrodeposits of iridium, platinum, palladium, rhodium and ruthenium.

3,480,523 Patented Nov. 25, 1969 ice Other objects and advantages of the invention will become apparent from the following description.

Generally speaking, the iridium plating electrolyte provided in accordance with the present invention consists essentially of an acid aqueous solution of iridium bromide in hydrobromic acid (HBr). The iridium content of baths according to the invention may be as low as 0.5 gram per liter (g.p.l.) but, preferably, it is from 5 to 10 g.p.l. Higher iridium concentrations may be used if desired. Thus, iridium concentrations up to 20 g.p.l. or even higher may be employed but, at such higher concentrations, the stability of the bath is decreased while there is little increase in the plating rate. The acidity of the bath should be at least 0.1 mole per liter of free hydrobromic acid (HBr), and is preferably from 0.1 to 1.0 mole per liter in excess of the amount calculated to convert iridium present to IrBr The optimum acidity depends on the iridium Content, with greater amounts of free acid generally being employed at higher iridium contents. The stability of the preferred baths is good. The bath gives satisfactory electrodeposits of iridium on many metals and also may be employed to plate iridium on to nickel by immersion.

It has further been found that the metals platinum, palladium, rhodium and ruthenium can also be satisfactorily electrodeposited from acid aqueous hydrobromic acid solutions, and, furthermore, that such solutions containing two or more of iridium, platinum, palladium, rhodium and ruthenium can be used as baths for the electrodeposition of alloys of the metals contained therein. The solutions consist essentially of bromides of the metal or metals concerned and of free hydrobromic acid; that is to say, they contain no other radicals that form complexes with the metals. They should also be free from other halide ions. If desired, however, uncomplexed cations such as potassium, sodium and ammonium ions may be present to help carry the current, which leads to improvement of the current efiiciency and plating rate, particularly when the acidity of the solution is not high. Regard must, of course, be had to the possible formation of insoluble compounds of such cations with the platinum metals. Thus, potassium and ammonium cations should not be introduced into solutions containing platinum, in order to avoid the formation of insoluble potassium and ammonium bromoplatinates.

Iridium may be deposited electrolytically from baths according to the invention on to cathodes of various metals, including copper, brass, nickel, mild steel, molybdenum and titanium. Metals such as molybdenum, tungsten and titanium that are not attacked by the electrolyte may be coated directly, but metals that are so attacked must be protected, e.g., by a flash coating of palladium or preferably of gold. If this is not done, adherent deposits are not obtained.

The surface condition of the substrate is of great importance in the electrodeposition of iridium and careful preparation of the surface to be coated is necessary to obtain good adhesion. Highly polished surfaces are not altogether suitable for the direct reception of a thick iridium deposit and it is preferable to subject the metal surface to a light controlled etching or other roughening treatment prior to plating so as to remove the surface layer and destroy any passive film that: may exist. Such a pretreatment is desirable when the substrate is titanium, molybdenum or tungsten. However, too heavy etching leads to the formation of a loosely adherent powdery surface film during plating, possibly as a result of high local current densities at protrusions on the etched surface. To avoid this, titanium surfaces may advantageously be roughened by sand blasting.

In carrying out the electrodeposition, the cathode current density and the initial acidity of the solution are both important. The current density should be at least 0.14 ampere per square decimeter (a./dm. since below this the plating rate is very low and some attack on the substrate is observed. As the cathode current density is increased above 0.14 a./dm. the plating rate increases to a maximum and then decreases again, while the cathode efiiciency progressively decreases. For these reasons, the current density preferably does not exceed 0.35 a./dm. Nevertheless, it may be much higher and broadly may be varied in the range 0.14 to 12 a./dm. without greatly affecting the quality of the deposit. At current densities above 12 a./drn. the bath becomes unstable and black deposits are formed.

Unless there is at least 0.1 mole per liter of free hydrobromic acid, the bath slowly decomposes. On the other hand, if the bath contains more than 1.0 mole per liter of free hydrobromic acid, the plating rate decreases rapidly.

The effect of temperature is small between room temperature and 60 C., the plating rate being extremely low in this temperature range, but above 60 C. there is a rapid increase to a maximum at 75 0., followed by a rapid decrease at temperatures above 80 C. This decrease is probably caused by faster evolution of hydrobromic acid and subsequent slow decomposition of the bath, with change of the iridium from the tetravalent to the trivalent form. If the free hydrobromic acid is expelled by boiling, the bath decomposes. I have found the advantageous concentrations to be g.p.l. iridium and 0.1 mole per liter hydrobromic acid, and the advantageous operating conditions to be a temperature of 70 C. to 80 C., preferably 75 C., and a current density of 0.15 a./dm. In plating most base metals under such operating conditions, the cathode efiiciency is about 65%. In plating titanium, however, the current density calculated on the basis of the nominal surface area of the cathode is generally no more than 45%, but this is probably due to a marked increase in the actual surface area by the heavy etching employed.

While the coatings deposited from this bath are very satisfactory, the bath has practical disadvantages. The acidity is high, bromine is evolved at the anode, and if the iridium becomes reduced to the trivalent state, from which it is not satisfactorily deposited, it is difficult to oxidize back again to the tetravalent state.

I have now found that these disadvantages are partly or wholly avoided if the bath also contains ammonium bromide. The presence of ammonium bromide widens the pH range within which the complex iridium bromide is stable and thus enables the bath to be used at lower acidity up to pH 4, preferably in the range 2 to 3. It also reduces or entirely suppresses the evolution of bromine at the anode, and it enables plating to be performed at a high rate. In the presence of amomnium bromide, trivalent iridium may be more readily reoxidized to the tetravalent state by heating with hydrobromic acid and a trace of bromine.

The bath advantageously contains at least 1 g.p.l. but not more than about 20 g.p.l. of ammonium bromide. Excessive additions lead to precipitation of iridium from the bath, but subject to this the concentration may be up to the maximum at which the bath remains homogeneous. This will depend on the amounts of iridium and hydrobromic acid present.

Acid aqueous hydrobromic acid baths, which may also contain about 1 to about 20 g.p.l. of sodium, potassium or ammonium bromide, may also be used for the electrodeposition of other platinum metals and their alloys, and the invention includes the electrodeposition from such baths of platinum, palladium, rhodium and ruthenium and of alloys of two or more of iridium, platinum, palladium, rhodium and ruthenium. As already explained, potassium and ammonium should not be present in the platinum-containing baths.

The iridium plating baths provided in accordance with the invention can be made by dissolving anhydrous or hydrated iridium oxide, e.g., iridium dioxide, in aqeuous hydrobromic acid. The resultant solutions correspond to solutions of iridium bromide in hydrobromic acid, although the form in which the iridium and bromide in such a solution are bonded together is uncertain.

The solutions containing palladium, rhodium and ruthenium are most conveniently prepared by dissolving hydroxide or hydrated oxides of the metal or metals in aqueous hydrobromic acid, while the platimum-containing solutions are preferably made by converting sodium chloroplatinate to sodium bromoplatinate by means of hydrobromic acid and nitric acid or hydrobromic acid and bromine. Whichever method is employed, some potassium, sodium or ammonium ions will generally be present in the solutions containing palladium, rhodium, or ruthenium, since the hydroxides or hydrated oxides of these metals will usually be contaminated with small amounts of these ions as a result of its precipitation by means of potassium or sodium hydroxide or ammonia, and, in the case of the platinum solutions, sodium will be introduced with the chloroplatinate. If desired, larger amounts may be added separately to the solution, for example, as potassium, sodium or ammonium bromides or as alkali added to neutralize excess acid in the solutions. In the case of the metals other than platinum, it is preferred to employ potassium, since it is found that it tends to reduce the incidence of cracking of the deposits. Advantageously, such solutions contain from 3 to 15 g.p.l. of potassium bromide.

The concentration of palladium, platinum, rhodium or ruthenium in the solutions, like that of iridium, should be at least 0.5 g.p.l. and may be as high as 20 g.p.l. or even more, though at concentrations above 20 g.p.l. the deposits tend to be cracked. Preferably, however, it is from 5 to 10 g.p.l.

The optimum acidity of the solutions diflfers for the different metals. Thus, platinum and ruthenium, like iridium, are best deposited from strongly acid solutions, e.g., having a pH less than 2, while in the case of palladium and rhodium less strongly acid solutions are preferred, e.g., having a pH in the range 2 to 4. Some basis metals may therefor need protection against attack by the acid solutions: for example, copper may be protected from attack by the platinum and ruthenium solutions by a flash coating of gold.

It is found that acid aqueous hydrobromic acid baths containing platinum may be prepared in a particularly satisfactory manner by dissolving sodium hexahydroxyplatinate or hexahydroxyplatinic acid in aqueous hydrobromic acid. The use of either a hexahydroxyplatinate or the free acid has the advantage over the hexachloroplatinate salt that it avoids the need for an additional treatment to eliminate chloride, which is an undesirable constituent of the bath. It is desirable to form concentrated solutions, that is to say, solutions containing at least 20 g.p.l. in the aggregate of the metal or metals. Such a concentrated solution can be used either to form the initial bath by dilution or to replenish the bath.

The baths may contain other constituents. Thus, small quantities of brightening agents or conductivity improvers may be present without harm, although they have not been found to offer any advantage.

The iridium baths are preferably free from other halide ions. Thus, iridium baths prepared in a similar way by dissolving iridium dioxide in the other halogen acids are much inferior. Thus, chloride baths plate at a much lower efficiency and give inferior deposits, iodide baths give poor deposits and are unstable, and fluoride baths are excessively corrosive.

As mentioned hereinbefore, iridium in baths containing iridium with or without platinum metals, e.g., platinumiridium baths, can readily be reoxidized to the tetravalent state by adding hydrobromic acid, together with a small amount of bromine, and heating the bath to boiling. When large baths are employed, it is not always convenient to heat them to boiling. Another advantageous method of bringing about this reoxidation is to heat the bath to about 70 C., add sodium bromate solution in the amount theoretically required to oxidize all the iridium present to the tetravalent state, and then heat the bath at about 70 C. for about one hour until the iridium is reoxidized and excess bromine is expelled. Excess amounts of sodium bromate should not be added since sodium hydroxide may then be produced and precipitate platinum metals from the bath.

For the purpose of giving those skilled in the art a better understanding of the invention, the following 11- lustrative examples are given:

EXAMPLE I A bath containing 8.0 g.p.l. of iridium and 0.3 mole per liter of free hydrobromic acid was prepared by dissolving 4.66 grams of iridium dioxide in milliliters of aqueous hydrobromic acid (46% HBr by weight) and diluting to 500 milliliters. This bath was used to deposit iridium on a cleaned titanium cathode. The method used to clean the titanium cathode Was to immerse it in an aqueous solution containing 3% each of nitric and hydrofluoric acids, and then for 16 hours in concentrated hydrochloric acid (specific gravity 1.18). After wiping 01f the smut formed by this treatment, the titanium surface was given a brief cathodic cleaning treatment before it was put into the plating bath. An insoluble iridium anode was used in the deposition, with the bath at 75 C. and a cathode current density of 0.16 a./dm. Under these conditions, a deposit of iridium 2.5 microns in thickness was obtained in 2.5 hours at a cathode current efliciency of 45.3%. Increase in the current density to 0.65 a./dm. decreased the cathode efliciency to 15% and increased the time required to deposit a layer of iridium 2.5 microns thick to 4 hours.

EXAMPLE II TABLE I Deposition rate,

Bath temperature, C. 1 microns per hour 1 Bath became unstable.

EXAMPLE HI This shows the results obtained when the bath described in Example I was used to plate cathodes of five different metals at a temperature of 75 C. with the use of an insoluble iridium anode. 7

Four of the metals were copper, brass, nickel and mild steel, and the cathodes consisted of pieces of sheet of these metals which had been buffed, degreased, dipped in 5% sulfuric acid, washed with Water and given a flash coating of gold by-electrodeposition from an acid gold cyanide bath buffered to pH 4. The fifth metal was molybdenum, and the cathode consisted of a piece of molybdenum sheet prepared by degreasing and then etching for 7.5 minutes in an aqueous solution containing 10% sodium hydroxide and 10% potassium ferricyanide 'at 90 C. Table II below gives the results.

When baths according to the invention are used for plating nickel by immersion, a substrate consisting of or coated with nickel is simply immersed in the bath at a temperature between room temperature and C. An example is as follows:

EXAMPLE IV A piece of nickel sheet was cathodically degreased, dipped in 5% sulfuric acid, etched for 30 seconds in a solution of ferric chloride, and then immersed in the iridium bath described in Example I for 10 minutes at 75 C. without the passage of any current. A bright metallic deposit was obtained.

Passage of an electric current through the bath with the etched nickel sheet as the cathode leads to the formation of a nonadherent deposit partly by chemical replacement and partly by electrodeposition. To obtain an adherent deposit by electrodeposition, it is necessary, as described above, to protect the surface of the nickel cathode.

EXAMPLE V In order to demonstrate the beneficial effect of ammonium bromide in an iridium plating bath, a bath containing 5 g.p.l. of iridium was prepared by dissolving hydrated iridium dioxide (IrO ,2H O) in a 0.1 mole per liter excess of hydrobromic acid and adding 5 g.p.l. of ammonium bromide. The resulting bath had a pH of 2 to 3 and was used at a temperature of 75 C. to 80 C. with an insoluble iridium anode to deposit iridium on a cleaned titanium cathode at cathode current densities up to 4 a./dm. The cathode efficicncy was from 9% to 13% and the plating rate was 0.05 micron per minute. There was little or 110 evolution of bromine at the anode.

When the precautions described hereinbefore with regard to preparation of the metal surface to be plated are observed, iridium deposits up to 10 microns in thickness are obtained which have a hardness of 900 D.P.N. (diamond pyramid number) using a 10 gram load on a section of 5 micron coating. The deposits thus obtained have good adherence, are crack-free in thicknesses up to about 1 micron and are bright with a total reflectivity of about 61% (as compared to for silver) in coatings having a thickness up to 4 microns.

In operation, insoluble anodes, which can conveniently be of iridium, platinum or platinized titanium are employed and the required iridium concentration in the bath can be maintained by addition of iridium bromide dissolved in hydrobromic acid provided the overall requirements in regard to hydrobromic acid content are observed.

Some further examples will now be given pertaining to the plating of platinum metals other than iridium.

EXAMPLE VI A ruthenium plating solution substantially free from alkali metal was made by dissolving in an excess of concentrated hydrobromic acid, ruthenium hydroxide that had been precipitated from a ruthenium chloride solution by means of potassium hydroxide and purified by dialysis. After appropriate dilution with water, the solution contained 5 g.p.l. of ruthenium and had a pH of 1.5.

Ruthenium was electrodeposited from a portion of this solution at 70 C. on to a polished copper cathode protected by a flash coating of gold, using a cathode current density of 2 a./dm. and a platinum anode. The plating rate was 3 microns per hour and at a thickness of 2.5

microns, the deposit was smooth, bright and adherent, but was somewhat cracked.

To further portions of the solution varying amounts of potassium, sodium and ammonium were added as their bromides, and copper specimens were electroplated from each of them under the same conditions as before. It was found that potassium additions in amounts from 3 to 15 g.p.l. considerably reduced the cracking. 3 g.p.l. of sodium or ammonium gave some improvement, but was less effective than the same amount of potassium.

Increasing the acidity of a further portion of the alkali metal-free solution, so that it contained 0.3 mole per liter of free hydrobromic acid, reduced its stability, greatly reduced the plating rate and led to more severe cracking of the deposit.

Reduction of the current density during electrodeposition from the original solution below 2 a./dm. reduced the plating rate, e.g., to 0.5 micron per hour at 0.4 a. dm. while increasing it above 2 a./dm. did not appreciably increase the rate.

The plating rate also depends on temperature, 70 C. being the optimum. The plating rate becomes progressively slower at lower temperatures, while higher temperatures offer no advantage.

EXAMPLE VII Rhodium hydroxide was precipitated from a solution of rhodium chloride by means of potassium hydroxide, filtered off, washed with water, dried, and dissolved in excess of concentrated hydrobromic acid and diluted to give a rhodium plating solution containing g.p.l. of rhodium and having a pH of 3. The solution contained a small amount of potassium.

Rhodium was electrodeposited from this solution on to a copper cathode at 40 C. at the rate of 3 microns per hour using a cathode current density of 0.36 a./dm. At a thickness of 2.5 microns, the deposit was smooth, bright, adherent and substantially crack-free.

The acidity of the solution was found to be important. Addition of further hydrobromic acid to reduce the pH below 2 led to blackening of the deposit, while raising the pH above 4 steadily reduced the plating rate. At pH 5, the bath began to decompose.

Blackening of the deposit also occurred on raising the cathode current density above 2 a./dm.

The optimum plating temperature was 40 C. to 50 C. Below this, the plating rate progressively decreases, while it is not appreciably increased at higher temperatures,

EXAMPLE VIII A palladium plating solution was prepared as described for rhodium in Example VII using palladous chloride as the starting material. The solution contained 5 g.p.l. of paladium and traces of potassium, and had a pH of 3.

Palladium was electrodeposited from this solution on to a copper cathode at 40 C. at the rate of 7.5 microns per hour using a cathode current density of 0.36 a./dm. The deposit was smooth, bright and adherent, and was crack-free up to a thickness of at least 2.5 microns.

The effects of varying the acidity, current density and temperature were the same as those described for rhodium.

EXAMPLE IX A platinum plating solution was prepared as follows: sodium hexahydroxyplatinate was heated under reflux with an excess of concentrated hydrobromic acid and a small amount of bromine to replace the hydroxyl groups by bromine. After boiling off the residual bromine, the solution was diluted to give a plating solution containing 5 g.p.l. platinum and free hydrobromic acid and having a pH of 2.

Electrodeposition of platinum from this solution at 70 C. on to a copper cathode protected by a flash coating of gold proceeded at the rate of 2.5 microns per hour using a current density of 1.0 a./d m. The deposits were smooth, bright and adherent.

Reduction of the acidity led to deterioration in the quality of the plating and at pH above 3 it was poor. The optimum plating temperature was 70 C., the plating rate becoming less as the temperature fell below this and not increasing at higher temperatures. The optimum current density was 1.0 a./dm. but current densities up to 2 a./dm. or above could be used at the expense of slightly lower plating rates.

The use of acid aqueous hydrobromic acid solutions of two or more platinum metals to electrodeposit alloys of these metals is particularly advantageous, since known methods of electrodepositing such alloys require the use of a mixture of different electrolytes which may not be readily compatible. Alloys that may be electrodeposited in accordance with the invention include platinum-iridium, rhodium-iridium, palladium-ruthenium, rhodium-ruthenium, palladium-rhodium, platinum-ruthenium and rhodiu m-ruthenium.

EXAMPLE X This is an example of the electrodeposition of platinumiridium alloys.

A portion of a 5 g.p.l. platinum solution described in Example IX was mixed with an equal volume of a solution containing 5 g.p.l. of irridiu m and 0.3 mole per liter of free hydrobromic acid, prepared by dissolving precipitated iridium dioxide in an excess of concentrated hydrobromic acid and suitable dilution. The mixture was then further diluted with an equal volume of water to give a platinum-iridium plating solution containing 2.5 g.p.l. platinum, 2.5 g.p.l. iridium, 0.5 g.p.l. sodium and 0.15 mole per liter of free hydrobromic acid.

This solution was used at 75 C. to electrodeposit platinum-iridium alloy coatings on copper cathodes protected by a flash coating of gold using three diflFerent current densities. The composition of the deposits was found to vary with the current density as shown in the following Table III:

TAB LE III Cathode current density, a./dm. Percent iridium Percent platinum At a current density of 2.5 a./dm. the plating rate was 0.8 micron per hour. All the deposits were adherent, but as the current density increased their appearance changed from bright to matte.

It is further surprisingly discovered that in electrodepositing alloys from an acid bromide bath containing platinum and iridium the composition of the deposit is markedly affected by the temperature of the bath. Thus, the proportion of iridium in a platinum-iridium alloy deposit rapidly increases as the temperature is raised. Suitable solutions for the deposition of platinum-iridium alloys include those which contain up to a total of 20 g.p.l. platinum and iridium, e.g., from 5 to 10 g.p.l. and in which the ratio of platinum to iridium is from 10:1 to 1:10 by weight, e.g., from 4:1 to 1:4, and the pH is preferably not greater than 2. The cathode current density may be up to 2 or even 3 a./dm. and the bath temperature from room temperature up to 75 C. or even higher, e.g., about C.

In order to demonstrate the foregoing, the following Example XI is given.

EXAMPLE XI A series of baths with different platinum to iridium ratios and ditferent pH values, but each having a total platinum and iridium content of 5 g.p.l., were prepared from platinum and iridium concentrates made by dissolving sodium hexahydroxyplatinate Na Pt(OH)6 and bydrated iridium dioxide IrO ,2H O, respectively, in aqueous hydrobromic acid by heating under reflux. After filtering, appropriate amounts of the concentrates were diluted by 9- the addition of water to give the desired solution. The results of plating tests using iridium anodes and cleaned titanium cathodes are set out in the following Table IV.

While the theory underlying the present invention is not altogether clear, it seems possible that the superiority of the bromide baths of the invention may be due to the fact that the iridium is present in them in the form of TABLE IV a cationic complex IrBr whereas in the presence of other Cathode halide ions an anionic complex of the formula H IrA PHI ratio gsgg (where A is a halogen atom other than bromine) is ihimh pH ashr m. tL/dlIL in deposit formed. M Q9 22 M 4 F Iridium electrodeposits produced in accordance with 1.2 3 the, invention are useful as electrical contact surfaces, 45 3% 32 such as in switches and the like, for diifusion barriers,

' 112 22 as coatings for readily oxidizable metals such as molyb- 9' 5}; denum and tungsten, in the production of anodes for 2.2 32 use in chlorine cells,etc. Coating with. platinum-iridium L5 70 v 1 24 ll ys by the process of the invention is a particularly 7 1.2 26 useful way of making electrodes for electrolysis, e.g., of F 29 2 f? brine in chlorine production, since a platinum-iridium 0- 2 a coating, especially one having the composition 10% 7 iridium-90% platinum, has a low over-voltage. 50 g 22 Although the present invention has been described in 01s 40 conjunction with preferred embodiments, it is to be unf; h 33 derstood that modificationsrand variations may be re- 1;1 1.8 70 0.15 18 sorted to without departing from the spirit and scope 3 2 1% of the invention, as those skilled in the art will readily 7 0. 72 s 25 understand. Such modifications and variations are con- 1; 1 1.0 70 0:6 41 sldeled 10 be wlthln the purview and scope of the inven- 1.2 48 tion and appended claims. 3:7 .-1. 70 60 v I claim: I

. 1 1. A plating bath for the deposition of a platinum The following examples illustrate the electr'odePosiet l f th group consisting of i idi l i tion of other platinum metal alloys. palladium, rhodium, and ruthenium and alloys thereof which consists essentially of an acid aqueous solution of EXAMPLE X about 0.5 up to about 20 grams per liter of the. platinum metal as the bromide, about 0.1 to about 1.0 mole er Solutlons were prepare-d contammg liter free hydrobromic acid, and at least about 1 to ab ut (a) 2.5 g.p.l. of rhodium and 2.5 g.p.l. of ruthenium, 20 i grams per liter of a bromide material from the group (b) 2.5 g.p.l. of palladium and 2.5 g.p.l. of rhodium, f and (c) 2.5 g.p.l. of palladium and 2.5 g.p.l. of ruthenium. 2 potisslumh andthammonmm i In each of the solutions, the platinum metals were present b e i g i t e 1 .dcontams as their bromides together with 4 g.p.l. of ammonium 4 P a mum e ma l 15 $0 01111 form 6. bromide and 4 g.p.l. of potassium bromide and suflicient A Platlng P PfiP to Claim h r ru the excess hydrobromic acid to give the pH values indicated P13011111?! metal S lrldlum and the bromide material 15 in Table V. Portions of these solutions were employed ammomum P at different temperatures and cathode current densities A Plating P accordlng 61211111 1 rein the to electrodeposit alloy coatings respectively of rutheniurnplatlmfm meta1 15 selected P the group conslstlng 0f rhodium, palladiumrhodium and palladium-ruthenium palladium, l'hOdllJlIl and ruthenium and the bath contains on etched titanium cathodes. The compositions of the about 3 to about 15 grams p liter O potassium brocoatings are shown in Table V. mide.

TABLE V Bath Cathode tempercurrent ature, density, Percent Percent Percent pH 0. a./ Ru Rh Pd (3) Rhodium-ruthenium.- 0.68 70 0.3 70 2.0 (b) Palladium-rhodium--- 1.6 23 3 I 70 0.3 70 2.0 (c) Palladium-ruthenium- 0.73 70 0.3

EXAMPLE XIII A solution was prepared containing 1.6 g.p.l. each of palladium, rhodium and ruthenium as their bromides, together with 4 g.p.l. ammonium bromide and 4 g.p.l. of potassium bromide, and suflicient excess hydrobromic acid to give a pH of 0.6. This solution was used at a current density of 2.0 a./dm. and a temperature of 70 C. to deposit an alloy coating containing 20% ruthenium, 49% rhodium and 31% palladium on an etched titanium cathode. At a current density of 0.3 a./dm. the deposit consisted Wholly of palladium.

5. A process according to claim 4 wherein the bath contains at least about 0.5 up to about 20 grams per liter of said platinum metal and about 0.1 to about 1.0 mole per liter of free hydrobromic acid.

6. A process according to claim 5 wherein the bath contains about 1 to about 20 grams per liter of a bromide material from the group consisting of ammonium, sodium and potassium bromides, with the proviso that when the bath contains platinum the bromide material is sodium bromide.

7. A process according to claim 6 wherein the platinum metal is iridium and the bromide material is ammonium bromide.

8. A process according to claim 6 wherein at least two platinum metals are present in the bath and alloys containing at least two metals from the group consisting of iridium, platinum, palladium, rhodium and ruthenium are deposited.

9. A process according to claim 5 wherein the content of said platinum metal does not exceed about 10 grams per liter.

10. A process according to claim 4 wherein the bath contains platinum and iridium in a weight ratio of about 10:1 to about 1:10 and platinum-iridium alloys are deposited wherein the proportion of iridium to that of platinum in said alloy is controllably increased by increases in the bath temperature between room temperature and about 80 C.

11. A process according to claim 10 wherein the weight ratio of platinum and iridium in the bath is about 4:1 to 1:4.

12. A process according to claim 4 in which the cur rent density does not exceed about 0.35 ampere per square decimeter.

13. A process according to claim 5 wherein the bath contains platinum and iridium and a platium-iridium alloy is deposited.

14. The process for producing an iridium deposit upon an etched nickel substrate which comprises establishing an acid aqueous bath containing at least about 0.5 up to about 20 grams per liter of iridium and at least 0.1 up to about 1.0 mole per liter of free hydrobromic acid, and immersing an etched nickel article in said bath while said bath is at a temperature of between room temperature and about 80 C. to form upon the immersed por tion of said article an iridium immersion deposit.

References Cited UNITED STATES PATENTS 1,837,193 12/1931 Bart 204-47 1,981,715 11/1934 Atkinson 204-4'7 XR 1,990,277 2/1935 Feussner et a1. -172 XR 2,057,638 10/ 1936 Zimmermann et al. 204-47 2,416,949 3/ 1947 Perley et a1 204-47 XR 3,207,680 9/1965 MacNamara 204-47 OTHER REFERENCES Mellor, I. W., A Comprehensive Treatise on Inorganic and Theoretical Chemistry, p. 775, vol. 15, (1936).

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner US. Cl. X.R.

533 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,480,523 Dated Nov- 25 IQAQ Inventor(s) Colin John Nelson Tyrrell It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 8, line 18, change um-ruthenium." to umpal1adiumruthenium. J

Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attestingg Officer Commissioner of Patents