US2929766A - Plating of iridium - Google Patents

Description

March 22, 1960 J. c. WITHERS EI'AL 2,929,766

PLATING OF IRIDIUM Filed June 13, 1958 2 Sheets-Sheet. 1

Heat NaCN, KCN or Combination Above its Melting Point at Inert Atmosphere install lridium Electrodes in the Melt 2 Introduce AC'Current to lridium Electrodes 3 lridium Dissolved into Melt Forming lri dium Cyanide Complex Dry and Store A Use Object to be Plated B lridium Complex as Cathode Introduce DC Current Through 6 Electrolyte lridium Metal Plated on Object 7 f I g l INVENTORS JAMES C. WITHERS PAUL E. R/TT ATTORNEY March 22, 1960 J. c. WITHERS ET AL PLATING 0F IRIDIUM 2 Sheets-Sheet 2 Filed June 13, 1958 Electroplured Iridium Electropluted Iridium Electropluted lrid ium INVENTORS d4 MES C. W/THERS PAUL E. R/TT BY V A ORNEY Sits PLATING F IRIDIUM Application June 13, 19 58, Serial-No. 741,848 1 Claim. ((11. 204*32) The present invention relates to plating a metal from the platinum group and more particularly to plating iridium on a metal such as molybdenum or stainless steel.

Unfortunately, a majority of the metals that have high temperature oxidation resistance cannot be electroplated. However, iridium, which is a high temperature oxidation resistant metal, has been electrodeposited. The prior art indicates that many attempts have been made to plate an oxidation high temperature resistant metal such as iridium on molybdenum or steel, but there is no convincing evidence that any success has been achieved that is commercially feasible. Some attempts to protect molybdenum or steel against high temperature oxidation with a diffusion alloy prepared by heat treating alternate layers of nickel and chromium have achieved somemejasure of success. I

It is the principal object of the invention to provide an improved technique of plating iridium: on a metal selected from the group consisting of molybdenum, steel, tungsten, titanium, niobium, tantalum and alloys thereof. Other objects and advantages will appear as the description of the invention proceeds.

There are many advantages in being able to electroplate iridium on a metal such as molybdenum in that much more protection is alforded with thinner plates, of iridium than comparable thickness of other metals. Iridium has been plated. by applicants on molybdenum in thickness of 0.5 mil and this has afforded excellent pro tection at 1000? C. When unprotected molybdcnum'is heated to a high temperature, it. will completely disintegrate within a short time. To obtain the same protection that 0.5 mil of iridium offers molybdenum, to 30 mils of the diffusion alloy of chromium and nickel is needed. Therefore, it is seen that very thin coatings of iridium oifer excellent oxidation protection at high temperatures. But, this protection is. not limited to molybdenum, it can be afforded to steel, tungsten, titanium, niobium, tantalum, and other metals by depositing a thin plating of iridium. Also, where weight is an important factor, iridium plating would be highly advantageous.

A method of plating iridium from a melt of sodium and potassium cyanide has been used in the invention described. The method consists of forming an iridium cyanide complex in the melt with the passage of alternating current and then plating iridium from the melt with the passage of direct current. 1

Thus, in the method described, the technician'is' able to produce the salt-tin a concentrated form from which iridium can be plated- After it is produced, the technician need only dilute the salt, insert his electrodes: and he is able to plate iridium.

Applicants have studied diligently the oxidation resistance of the plated iridium. Results indicate that thin coatings ofier good protection to molybdenurnand. stainless steel. When molybdenum is heated to 1000 C. for circa thirty minutes, it will completely disintegrate; however, when given a chromium strikefollowed by a nickel and gold strike andplated with 0.0005 inch of iridium,

asserts hatented Mar. 22, 1960 2 the oxidation is greatly reduced and weight change is practically negligible.

Further, from an electrical resistivity standpoint it is advantageous to plate iridium in that its electrical resistance is very low. It is only 4.9 microhm centimeter at 20 C. compared with a range of 7.8 to 13 for nickelchromium coating. Iridium deposited on interior metals in very thin coatings (0.5 to 1 mil) imparts a majority of its characteristics thus resembling a solid section of iridium at a much reduced cost.

The iridium deposit obtained has low stress, it is bright and ductile, and may be plated in thick deposits. Iridium has a high melting point, is stable at high temperatures and yields a bright deposit. Therefore, it may be plated on other metals for high temperature application or on jewelry to produce a decorative finish. Since the operating temperature of the bath is 500 C., argon is introduced just above the surface of the melt to prevent oxidation of the salts and electrodes.

It has been noted that solid sections of platinum and iridium are being considered for high temperature structures, but these are very expensive. The use of iridium plated steels or other less costly metals as described herein gives higher strength and comparable corrosive resistance at a much lower cost. As the amount of iridium deposited on the cathode is extremely thin, it has been found that by following the method herein featured the coating of large articles at a cost which is not prohibitive for commercial practice is possible.

It is not possible tocondense the statement of this nevi/inventive concept into any few words but an understanding of the invention should be gained by a consideration of the specification as a whole, and it should be understood that some of the steps of the method which are brief and simple have beenfound to be of very great importance in the production. of an iridium on-stainless steel or molybdenum article.

At various places in the description of the invention, reference is made to proprietary trade names, which have been particularly included because, while they generally refer to a particular mode offunction or operation which is elsewhere described in general terms, the particular trade name is on the market, has been found to be satisfactory, and furnishes a ready source of the necessary products. v

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawing in which:

Figure 1 is a flow diagram of the process, the various steps of which are included in numbered boxes for ready reference and correlation in the specification;

Figure 2 illustrates the plated article of the present invention as a flat sheet member;

Figure 3 illustrates the plated article of the present invention as a round rod member; and

Figure 4 illustrates the plated article of the present invention as a round hollow rod member that has been plated externally and internally thereof.

In order to describe more fully the inventive method recited herein, the following introductive material is deemed helpful in understanding the details of the invention.

In the classification of metals, the platinum group consists of platinum, iridium, osmium, palladium, rhodium and ruthenium. These metals have high melting points ranging between 1773 C. for platinum to 2700 C. for osmium. They all appear to exhibit an inert chemical nature even at high temperatures. The very high melting 7 resisting applications. In addition, iridium is a hard metal, measuring 170 Vickers hardness number in an annealed state. Many of the properties of iridium are undetermined at present because of the difliculty of chemically and mechanically working the metal. While the plating of platinum, rhodium, and to a lesserextent palladium, is at present feasible, little information is available on the electrodeposition of iridium. For many years, the electrodeposition of iridium was believed impossible because of its high electropositive nature.

In the native state, iridium is most often found alloyed with osmium and with platinum and gold. Though irdiurn is not in the rare-earth group, it is very rare; one ton of the earths crust contains about 0.001 gram. The high cost of iridium, in addition to its difiiculty in working, has largely prevented the use of this material for manufacturing or research applications. At present, the use of this hard, dense metal is confined to the hardening of platinum and other metals. Pure iridium, because of its stability at high temperature, has been used for crucibles in the study of very high temperature reactions.

However, the properties of iridium make it most at.- tractive for use in missle and aircraft development. As higher speeds are required for aircraft and missles, seri ous needs are arising for lightweightmetals capable of withstanding high temperatures. Some metals will meet all the requirements but oxidizerapidly at high temperatures. molybdenum is well-known. This metal will, however, form a volatile oxide at approximately 600 C. and will actually disappear under high temperature exposure if not protected from contact with oxygen. Similar difiiculties are encountered in stainless steels used for high temperature applications in that they flake, crust and scale at very high temperatures.

Therefore, if iridium is applied to the surfaces of these metals, their stability at high temperatures is improved. It is also considered that the best method for applying the protective coating of iridium is by electrodeposition.

A survey of the prior art provided no suitable plating technique for iridium plating. There was some indication that iridium would be deposited from a sulfate solution. These claims could not be substantiated.

Referring now to Fig. l, the steps of the method are broadly set forth in blocks 1 to 4, A, 5B 6 and 7. The first step consists of making an iridium cyanide complex and then plating from a melt of the complexed material. Sodium cyanide, potassium cyanide and/or combinations of these salts can be used to form the complex'which in turn is the plating melt.

In steps 2 and 3, when sodium cyanide is in a molten state (564 C.) pure iridium electrodes are installed in the melt and electrolyzed at amp/ft. with 60 cycle alternating current. This electrolysis forms (step 4) an iridium cyanide complex (which is deemed to be Na Ir(CN) which is reduced at the cathode to iridium metal using direct current. Mixtures of sodium andpotassium cyanide, which will have lowered melting points than either of the pure salts, can be used to form the complex of iridium which is used as the plating bath. As an example, 70 parts by weight of sodium cyanide and 30 parts by weight of potassium cyanide which has a melting point of about 490 C. can be electrolyzed with iridium electrodes at 10 a.s.f. with 60 cycle alternating current for 2 hours. The iridium metal can then be plated out of the same melt using direct current at 10-120 a.s.f.

After the iridium cyanide complex is formed from the above steps, it may be cooled and stored at step 5A for future use or it may be placed in a new melt to supply the complex instead of forming it by electrolysis as before.

The melt may also be used as outlined in step SE to 7, to plate iridium on other objects. Figs. 3 and 4 illustrate the plated article or object as a solid and hollow rod member, respectively. ,This is accomplished by mm For example, the high temperature strength of a the object to be platedthe cathode in the melt and passing direct current between the object to be plated and an iridium anode. The cathode current may range from circa l a.s.f. to 200 a.s.f. The plating time depends upon the thickness of iridium to be plated upon the object. In a melt with the iridium cyanide complex concentrations at circa 6 g./l., one mil can be deposited in approximately 50 minutes.

If the object to be plated is molybdenum or other metals which requires special preparation before plating, the metal must be given its particular preparatory steps for plating before it is introduced in the iridium plating bath. In the case of molybdenum, it must be anodically cleaned in sulfuric acid, rinsed, dipped in a warm alkali solution, rinsed,"dipped in diluted sulfuric acid, rinsed, given a chromium strike in a chromium plating bath, rinsed and given a nickel strike from a Watts nickel bath. After the nickel strike, the molybdenum should also be given a gold strike from a gold cyanide plating solution. This will keep the molybdenum from ditfusing through the iridium protective coating. Then the object is electroplated with iridium. If the molybdenum is to be subsequently used at high temperatures after iridium plating, it should be heat treated above 1100 C. for at least 30 minutes in a hydrogen atmosphere to drive out the entrapped gases.

The iridium metal concentration of a 202 g. melt of "salt can range from 10 mg. to an excess of 1 g. Oxidation at these high temperatures is prevented by introducing argon gas just above the surface of the melt. The temperature of the plating melt can range from its melting point about 490 C. to about 700 C., however, the greatest cathode sufficient is obtained with high iridium concentrations and a temperature of approximately 507 C.

The experiments performed by applicants were limited somewhat in their scope by the high cost of iridium.

The experimentalapparatus used in the work consisted of a resistance tube furnace 2% inches in diameter. The furnace was made of McDanel high temperature mullite tubing wound with kanthal resistance wire. The anodes for the molten salt bath were high purity 0.05 inch diameter iridium wire and the cathodes were 0.05 inch diameter copper wire. The molten salt reaction was carried out in crucibles placed within the furnace. These crucibles were 2 inches in diameterand 3 inches deep. They were made of high temperature, gas-tight mullite. Some difiiculty was experienced in finding a crucible that would withstand temperatures in excess of 700 C. and would not crack or leak when exposed to molten salts at these temperatures. After experimenting with a variety of crucibles, the gas-tight mullite crucible was found to be most satisfactory.

To prevent excessive oxidation of salts and electrodes at high temperature, argon was introduced just above the surface of the melt. The flow was measured by a flow meter at 2.5 to 3.0 l./min. The temperature of the bath was determined by an iron-constant thermocouple and a potentiometer. The heating was regulated with a variac. The temperatures used in the investigation were such that the meltviscosity was sufficiently low in spite of the cooling effect of argon (500 C.). The minimum temperature of the salt selected was 500 C, and further experiments were carried out at 600C. and 700 C. The power supply for plating'was a 1% volt battery, and adjustments were 'made with a variable resistor. A direct current milliammeter was used to measure the current. V

Several typesof salts were investigated as possibilities "for a plating medium. The requirements for the plating melt were that it should have a low melting point, a low viscosity, and react with the anode to form an iridium complex which could be reduced to the metal at the Qathode. Among the salts. that were. investigated were sodium hydroxide, sodium cyanide, and potassium cyanide. Sodium hydroxide alone did not form an iridium complex from which metal could be plated. Potassium or sodium cyanidcs alone had high melting points of 634 C. and 564 C., respectively. Mixtures of these salts were investigated to find a molten salt bath of lower melting point that would serve as a plating medium. Some of these mixtures had melting points as low as 400 C., but these melts proved unsatisfactory for a plating bath because they frothed, gassed, and spattered excessively. The bath finally selected consisted of 70 parts by weight of sodium cyanide and 30 parts by weight of potassium cyanide. This bath did not froth and gas excessively, had a relatively low melting point of approximately 490 CL, and alow viscosity; In this bath, an iridium complex was formed in the melt by the passage of alternating current; metalwas then plated on the cathode by electrolysis with a direct current.

The iridium complex was formed by inserting two iridium electrodes in the molten salt (500) and electrolyzing with 60-cycl'ealternatingcurrent at 10 amp/ft. The current was measured with an alternating current milliammeter and controlled with a variac. The electroly-zing time will vary depending upon the amount of complex to be formed, i.e-., the concentration of iridium desired. When 200 g. of 70 parts of sodium cyanide and '30 parts of potassium cyanide were in amolten state at 500 C., 20 minutes electrolyzing time with 60- 'cyclealternating current at 10 amp/sq. it. resulted in the'ele'ctrod'es losing 11 mg.

Throughout the experimentation, the anode efiiciency at all concentrations tended to be much higher than the cathode efficiency; Chemical dissolution of the iridium anode in the molten salt occurred to the extent of 1:0;3 mg./ 150 ml. of melt for each 20-minute plating period. These low values are insufiicient to explain the high anode efficiency; At low current densities, two to three times as much iridium dissolved at the anode as would be expected, based on the irid um being ionized in the trivalent state. All calculations of efficiency were based on the value of 1.80 g,/amp./hr. deposited at the cathode. Reference. was made to Perry, John HL, Chemical Engineering Handbook, 3rd edition, McGraw-Hill. The amount dissolved at the anode reached a maximum between 60-80 amp/sq. ft. and dropped off as a current density of 100 amp;/sq. ft. was approached. At current densities abovelOO amp/sq. ft. and at low temperatures, iridium ceases to dissolve at the anode and a black scaly film formed. The reason. for. the high. anode efliciency at low current density is. notv known. It can, however, be postulated that the anode did not always dissolve in the trivalent state. Some of it may have dissolved in the divalent or monovalent state, but no investigation was made to determine the valence state of the iridium dissolved at the anode.

The concentration of iridium in the bath. was determined by accurately weighing the anodes and cathodes before and after each electrolysis or plating period. During the experimentation, if the iridium concentration falls below limits, the time of alternating current treatment was increased, Since no suitable method was available to remove excess iridium, in cases where the concentration exceeded the ran e, the bath was diluted by removing some of the molten salt and replacing it j-with the mixture of sodium and potassium cyanide.

.Erom this work, the optimum current density, concen- .tration and temperature conditions for the iridium plating bath were determined.

An attempt was made to identify the iridium complex donned in the melt after the passage of the alternating current. It-is known that a potassium iridium cyanide .com'plex -(K' lr.(CN')' exists. Reference was made to .Rimbacher, E., and Korten, R, Z. lnor'g Chem. 52, 345 {1907). This complex reacts with a solution of silver "nitrate to yield a white precipitate of silver iridium assures? cyanide (A-g l'r(CN) This latter complex is soluble in ammonia and opaque crystals of (C s a s) s elficiency was maintained over a very narrowrange of current density. When the temperature was raised to 600 C., a level. of. cathode, efiiciency of 14-17 percent was obtained at cathode current densities between 10 and 60 amp/sq, ft. The deposition obtained under all conditions at this concentration had areas of incomplete coverage. In addition, microscopic examination (x) indicated small pits in the plated metal.

The effect of the temperature, and current" density on the cathodeefiiciency of a bath with an iridium concentration of 1 to 1.33 g./l. was. studied. The most efiicient operating temperature was 600 C. and the optimum current density was 10-60. amp/sq. ft. These conditions are similar to that obtained at lower iridium concentration. At the lower temperatures and high current densities, the anode noIon-ger lost weight and a black scaly film was formed on the surface of the anode.-

An increase in the iridium concentration to 2, to 3.33 g./l. indicated that a temperature of 600 C. and a cathodecurrent density of; 10-60 amp/sq. ft. results in the highest efiiciency'. At an iridium. concentration of 2 to 3.33 g./l., a cathode efliciency of 100 percent was obtained at 10 amp/sq. ft. and atemperature of 600 C. The iridium plating obtained. at. this concentration with temperatures of 500 C. and 600 C., and a. current density: of 10-60 amp/sq. ft. was uniform and adherent. The plating, however, was slightly coarse. It was decided to increase the iridium. concentration of. the bath.

The concentration was increased to 5.33 to. 6.66 g./l. The results obtained under these conditions was studied. Withthis concentration, a 100 percent. cathode efficiency was: obtained at 600 C. and at current. densities of 10 and 20 amp/sq. ft. This concentration also resulted in 100 percent cathode efiiciency at. 700 C. and at current densities between 10 and 40 amp/sq. ft. It was observed throughout the work that finer. grain deposits were obtained as the iridium concentration was increased. This was further verified by the deposits obtained, which were fine grain and adherent, at concentrations of 5.33 and 6.66 g./l.

Since a. high: iridium. concentration produces fine grain adherent deposits and a high cathode efiieiency, these conditions were selected for plating samples for the study of oxidation resistance. The temperature: chosen was 600 C. since the higher temperature caused the melt to creep and-in general to become more difficult to work. The current density had to be kept between 10 and 20 amp/sq. ft. in plating coatings thicker than about 0.1 mil, as a. higher current density caused dark, rough, and porous deposits. With the current density at the lower limit, a thickness of five mils could be built up with no difficulty. The conditions used for further studies were:

Iridium concentration g./l 5.33 to 6.66 Temperature C 600 Current density amp./sq, ft... 10

Under these conditions,'one' mil could be deposited in circa fifty minutes. A- copper wire, iridium plated by this technique, was repeatedly bent through an angle of 180. The iridium coating showed little or no brittleness and had perfect adhesion. It is believed that this was due to the type of crystalline structure obtained in the deposited coating.

The conditions which provided the most satisfactory deposit of iridum were applied to the plating of iridium on 410 stainless steel and molybdenum to observe the oxidation resistance that the iridium plate offered at high temperatures. The plating of the basis metals presented some difliculties, and only after a considerable amount of experimentation was a successful procedure evolved.

The following examples illustrate the manner of carrying out the present invention. However, it is understood the invention is not limited to the particular details or numerical values set out therein.

EXAMPLE 1 The procedure for plating iridium on 410 stainless was as follows:

(1) Degrease sample to remove grease and oil.

(2) Cold water rinse.

(3) Anodic clean in any proprietry alkaline metal cleaner.

(4) Cold water rinse.

(5) Dip in a 35 percent by volume sulfuric acid solution at 180 F. for one minute after the initiation of action.

(6) Transfer, without delay, to a Watts type nickel bath and plate at 50 amp/sq. ft. for five minutes.

(7) Cold water rinse.

(8) Dip in 25 percent by volume sulfuric acid solution at room temperature for one to two minutes to activate the surface of the nickel.

' (9) Air-blast dry and transfer to the iridium plating bath with iridium concentration of 5.33 to 6.66 g./ l. and plate at 10 am./sq. ft. until the desired thickness is obtained.

(10) Cold water rinse.

EXAMPLE 2 The process for plating iridium on molybdenum was as follows: 7

(l) Degrease to remove organics contaminants.

(2) Treat in a hydrogen atmosphere a minimum of 30 minutes at a temperature above 1100 C.

(3) Clean anodically in a 67 percent by volume sulfuric acid solution at 200 amp/sq. ft. for 40 seconds.

(4) Rinse in running cold water.

(5) Dip in a warm alkaline solution of any proprietary cleaner.

(6) Rinse in running cold water.

(7 Dip in a 10 percent by volume sulfuric acid solution at room temperature for 30 seconds.

(8) Rinse in running cold water.

(9) Chrome strike in a conventional chromium plating bath at 250 amp/sq. ft. for 2 to 5 minutes.

Composition of bath:

.250 g./l. chromic acid 2.5 g./l. sulfuric acid 32 oz./gal. nickel chloride 11 t1. oz./gal. hydrochloric acid (12) Rinse in running cold water. (13) Plate at 8 amp/sq. ft. for 15 minutes in the following gold bath at 55 C:

40 g./l. gold potassium cyanide '30 g./l. potassium cyanide 35 g./1. potassium tartratc 3 g./l. potassium hydroxide l0 g./l. potassium carbonate (14) Rinse in running cold water.

(15) Air-blast dry and transfer to the iridium bath and plate at 10 amp/sq. it. at 600 C. in a bath with iridium concentration of 5.33 to 6.66 g./l. until desired thickness is obtained.

An oxidation test was conducted on the iridium plated molybdenum and stainless steel. The testing consisted of accurately weighing plated and unplated samples and exposing them to 600 C. and 1000 C. for periods of 10, 20 and 30 minutes. After each exposure period, the samples were weighed and the amount of weight change recorded. Any change in weight was considered as evidence of deterioration. The samples were exposed in a 5 x 6 x 6 inch Ternco electric furnace.

Results for the stainless steel specimens are shown in Table. 1.

The plated and unplated molybdenum were tested in the same manner. The results after testing are shown in Table 2.

The weight changes caused by exposure of iridium plated and unplated stainless steel to high temperature oxidation are shown in Table l. The weight loss increased rapidly betwen 10 and 20 minutes exposure when exposed at 600 C. This loss was probably due to the removal of water and any occiuded gases. Theloss appeared to stop after a 20 minutes exposure. Examination of the unplated sample indicated that there was some attack on the sample after 30 minutes exposure to 600 C. It was also found that some discoloration of the iridium plated sample had also occurred after it had been exposed for 30 minutes. It is shown in Table 1 that a very small weight change had occurred in the iridium plated sample at all exposure times. The changes in weight at 600 C. were, in all cases, less for plated than unplated stainless steel. From the consideration of weight change alone, it probably would not be profitable to plateiridium on stainless steel for exposure to air at 600 C. for 30 minutes. r

1 Table 1 EFFECT OF IRIDIUM PLATING 0N WEIGHT CHANGE OF STAINLESS STEEL AFTER EXPOSURE TO 600 C. AND 1000 C. IN AIR Indicates weight gain. Indicates weight loss.

At 1000 C., the stainless steel gains weight progressively with time as shown in Table 1. The weight change of iridium plated stainless steel is much greater (30-1) than unplated stainless steel. It was noted that after a 30 minutes exposure at 1000 C. the 410 had oxidized and had scaled considerably. The iridium plated stainless steel under the same condition had gained three times more weight than the unplated samples. After the 30 minutes exposure, the iridium plate did not scale.

The results of the oxidation test for molybdenum are shown in Table 2. At 600 C., the molybdenum showed an increase in weight with time. The weight gained was approximately i0 times that of the iridium plated sample. The iridium plated molybdenum gains 0.3 mg./sq. in. over a 10-minute period and then the weight gain re- Table 2 EFFECT OF IRIDIUM PLATING ON WEIGHT CHANGE OF MOLYBDENUM AFTER EXPOSURE TO 600 C. AND 1000' C. IN AIR.

Time of Exposure (Minutes) A. Change In Weight In Air At 600 C.

(mg/sq. ln.):

Molybdenum Unplated +3.2 +3. 7 +4.0 Molybdenum Iridium P 0.0005

in. Th1 +0. 3 +0.3 +0. 3 B. OhangeIn Weight In Air At 1,000" O.

(mg/sq. m.):

Molybdenum Unplated 3l7. 3 596. 5 Molybdenum Iridium Plated 00005 in. 'Ihlck +0.31 +0.09 O.42

+ Indicates weight gain. Indicates weight loss. Sample completely disintegrated.

mained constant over the next 20 minutes. A dense black film appeared on the unplated molybdenum. The plated molybdenum showed only a slight amount of darkcolored oxide on the surface. This dark oxide was probably due to a difiusion through the iridium coating and possibly caused the weight gain.

At 1000 C., the unplated molybdenum lost a considerable amount of weight after exposure for minutes. Approximately half of the unplated sample had disintegrated after 20 minutes exposure to air at 1000 C. The unplated sample had completely disintegrated when exposed for 30 minutes. As indicated by the weight change in Table 2, iridium otters a considerable amount of protection to molybdenum, even in thicknesses of only 0.0005 inch of iridium. There is a considerable amount of discoloration of the plated sample after 30 minutes exposure. Since the thin coat of iridium offers so much protection to the molybdenum, it is believed that a thicker coat would probably offer more protection and would prevent such discoloration. However, the scope of this investigation was only to obtain a short time protection with as thin a coating as feasible.

The results of this investigation indicate that good electrodeposits of iridium were obtained using a concentration of 8001000 mg. of iridium in 200 g. of a fused salt (5.33 to 6.66 g./l) consisting of 70 parts by weight of sodium cyanide and 30 parts by weight of potassium cyanide in a molten condition at 600 C. The most satisfactory deposits were obtained at 10-20 amp/sq. ft. When iridium applied by this technique was plated 0.0005 inch thick on 410 stainless steel, the unplated sample lost 5 times as much weight as did the plated sample at 600 C. The iridium plating keeps the stainless steel from scaling when exposed for 30 minutes at 1000 C. When 0.0005 inch of iridium is plated on molybdenum, a considerable amount of protection at 1000 C. was provided and led to the conclusion that thicker plates of iridium would offer complete protection of molybdenum at high temperature.

In summary, the invention above described relates to a method of plating iridium on a metal such as molybdenum or stainless steel. Iridium has a very high melting point and does not readily oxidize when heated in air at high temperatures. The method included plating iridium for a melt of sodium and potassium cyanide. An iridium cyanide complex Was formed by inserting iridium electrodes in the melt and electrolyzing with alternating current. With this complex, a systematic investigation of the effect of iridium concentration, current density, and temperature on the deposition of iridium was carried out. At low temperatures and high current densities, the iridium anode ceased to dissolve and a scaly black film formed on its surface. At low iridium concentrai0 tlons, the cathode current efiicien'cy was low. As the iridium concentration was increased, the cathode efiiciency was improved and a finer grain deposit was obtained. A concentration of 0.8 to 1.0 g. of iridium in ml. of melt at 600 to 610C. gives a fine grain: adherent deposit at a high cathode efiiciency.

Methods for plating iridium on stainless steel and molybdenum were developed in the experimental work. After iridium had been deposited on the metals, oxidation tests were made. An iridium plate, 0.0005 inch thick, on 410 stainless steel protected it from rapid oxidation and scaling at 1000 C. When iridium (0.0005 inch thick) was plated on molybdenum and was exposed in air at 600 C. and 1000 C., only a slight amount of weight was lost. Unplated molybdenum exposed in air at 600 C. for 30 minutes gained 3.7 times as much weight as did the plated sample at this same temperature. At 1000 C., the unplated sample had completely disappeared in 30 minutes while the plated sample had only a small weight change.

The advantages of this invention are so material that they can be recognized instantly by inspection by persons skilled in the art who are familiar with the type of plating which is achieved by prior art processes. These material advantages may also be demonstrated by the usual tests for durability, plating and the like.

These advantages are not obtained by the use of any one of the individual steps which are included in the method, but, on the contrary, the advantages are the result of the method as a whole, as defined in the claim. However, certain phases of the method are themselves believed to be new and they are claimed alone as Well as in combination.

As many apparently widely different embodiments of the present invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments.

What is claimed is:

The method of plating an object having molybdenum as its major and essential ingredient, with a plating having a layer of 0.0005 inch of iridium as the essential and final protective coating for the object that comprises degreasing the object, treating the object in a hydrogen atmosphere fora minimum of 30 minutes at a temperature above 1100" C., anodically cleaning the object in a 67 percent by volume sulfuric acid solution at 200 amperes per square foot for 40 seconds, rinsing the object, cleaning the object by immersion in a warm alkali cleaner, rinsing the object, cleaning the object by immersion in 10 percent by volume sulfuric acid solution at room temperature for 30 seconds, rinsing the object, subjecting the object to a chrome strike in a chromium plating bath consisting of 250 grams per liter chromic acid and 2.5 grams per liter sulfuric acid at 250 amperes per square foot for 2 to 5 minutes, rinsing the object, subjecting the object to a nickel strike at 30 amperes per square foot for 5 minutes in a nickel strike bath consisting of 32 ounces per gallon nickel chloride and 11 fluid ounces per gallon hy drochloric acid, rinsing the object, electrolytically plating the object at 8 amperes per square foot for 15 minutes at 55 C. in a gold bath containing 40 grams per liter gold potassium cyanide, 30 grams per liter potassium cyanide, 35 grams per liter potassium tartrate, 3 grams per liter potassium hydroxide and 10 grams per liter p0- tassium carbonate, rinsing the object, drying the object with an air-blast, and electrolytically plating the object with 0.0005 inch layer of iridium from a bath comprising a molten alkali metal cyanide of the group consisting of sodium cyanide and potassium cyanide at a temperature of 600 C. and a current density of 10 amperes per square foot, said bath having an iridium concentration of 5.33 to 6.66 grams per liter.

(References on following page) 11 '12 References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS Plating," October 1953, pp. 11261133.

1,947,130 Bart Feb 13, 1934 Electrometallurgy Supplement to Metal Industry,

2,093,406 Atkinson Sept. 21, 1937 5' December 11, 1936, page 592.

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