Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/50—Electroplating: Baths therefor from solutions of platinum group metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
  • C25D3/567—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of platinum group metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated

Definitions

• 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.
• 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.
• 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.
• 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
• HBr free hydrobromic acid
• 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.
• 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.
• 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.
• 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.
• a pretreatment is desirable when the substrate is titanium, molybdenum or tungsten.
• 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.
• titanium surfaces may advantageously be roughened by sand blasting.
• 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.
• 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.
• 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.
• 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 cathode efiiciency is about 65%.
• 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.
• the bath also contains ammonium bromide.
• 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.
• 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.
• 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.
• such solutions contain from 3 to 15 g.p.l. of potassium bromide.
• the concentration of palladium, platinum, rhodium or ruthenium in the solutions 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.
• 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.
• 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.
• 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.
• iridium baths prepared in a similar way by dissolving iridium dioxide in the other halogen acids are much inferior.
• chloride baths plate at a much lower efficiency and give inferior deposits
• iodide baths give poor deposits and are unstable
• fluoride baths are excessively corrosive.
• iridium in baths containing iridium with or without platinum metals 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.
• hydrobromic acid e.g., acetylcholine
• 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.
• 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).
• 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 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
• 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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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
• the deposit was smooth, bright and adherent, but was somewhat cracked.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Example IX 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the compositions of the about 3 to about 15 grams p liter O potassium brocoatings are shown in Table V. mide.
• 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.
• a process according to claim 10 wherein the weight ratio of platinum and iridium in the bath is about 4:1 to 1:4.
• cur rent density does not exceed about 0.35 ampere per square decimeter.
• 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.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Electroplating Methods And Accessories (AREA)
• Electrolytic Production Of Metals (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=27255322

Family Applications (1)

Country Status (9)

Cited By (29)

Families Citing this family (1)

Citations (6)

• 0
  • NL NL127936D patent/NL127936C/xx active
  • NL NL135500D patent/NL135500C/xx active
• 1965
  • 1965-02-24 NL NL6502321A patent/NL6502321A/xx unknown
  • 1965-02-25 FR FR7046A patent/FR1425229A/fr not_active Expired
  • 1965-03-03 DE DEJ27615A patent/DE1278797B/de active Pending
  • 1965-03-04 SE SE2799/65A patent/SE314569B/xx unknown
  • 1965-03-04 CH CH304865A patent/CH436906A/fr unknown
  • 1965-03-04 BE BE660626A patent/BE660626A/xx unknown
  • 1965-04-15 GB GB16256/65A patent/GB1108051A/en not_active Expired
• 1966
  • 1966-04-12 US US541944A patent/US3480523A/en not_active Expired - Lifetime
  • 1966-04-12 AT AT342366A patent/AT267991B/de active
  • 1966-04-14 SE SE05078/66A patent/SE338477B/xx unknown
  • 1966-04-14 DE DEJ30604A patent/DE1280014B/de active Pending
  • 1966-04-15 CH CH548366A patent/CH516647A/fr not_active IP Right Cessation
  • 1966-04-15 BE BE679602D patent/BE679602A/xx unknown
  • 1966-04-15 NL NL6605066A patent/NL6605066A/xx unknown
  • 1966-05-06 BE BE680663D patent/BE680663A/xx unknown
  • 1966-06-03 NL NL6607715A patent/NL6607715A/xx unknown
  • 1966-07-08 AT AT655766A patent/AT271127B/de active
  • 1966-10-06 DE DEJ31938A patent/DE1279424B/de active Pending

Patent Citations (6)

Also Published As

Similar Documents