US3373092A - Electrodeposition of platinum group metals on titanium - Google Patents

Description

United States Patent 3,373,092 ELECTRODEPOSITION 0F PLATINUM GROUP METALS 0N TTTANTUM Kenkichi Yoshimura, Yokohama-ski, Kanagawa-ken,

Kiyoshi Aoki, Tokyo, and Shiro Honda, Yokohamashi, Kanagawa-ken, Japan, assignors to Ajinomoto Co., Inc., Tokyo, Japan No Drawing. Filed Mar. 24, 1964, Ser. No. 354,825 Claims priority, application Japan, Mar. 29, 1963, 38/ 14,116 4 Claims. (Cl. 20432) ABSTRACT OF THE DISCLOSURE When titanium objects are immersed in fused alkali metal halide salts until the surface is roughened, platinum metal electrodeposits formed thereafter in aqueous electrolytes perfectly adhere to the titanium and are so continuous that the plated titanium, when employed as an anode in a mercury cell for caustic and chlorine production, significantly reduces the cell voltage as compared to similarly plated titanium electrodes pretreated in a conventional manner.

This invention relates to the electrodeposition of metals of the platinum group on titanium.

It has recently been proposed to replace graphite anodes in the electrolysis of alkali metal chloride solutions by platinum coated titanium anodes. Although the titanium electrodes have numerous advantages over the graphite anodes, it has been difficult heretofore to produce satisfactory platinum-coated titanium electrodes by electrodeposition, a method otherwise best suited for the purpose.

.It has been found that the peeling of ele'ctrodeposited coatings of platium on titanium in service can be prevented to a certain degree by removing the oxide film, which normally covers the titanium base metal, prior to platinum plating. That is to say, the removal of the oxide film in conventional processes is satisfactory, however, the platinum metal coatings produced by electrodeposition do not show consistently good adhesion to the titanium base.

When a titanium electrode is covered with a defective or incomplete coating of platinum, and is employed as the anode in the electrolysis of an alkali metal chloride solution, the cell voltage is higher than that required for an otherwise comparable graphite electrode, and the energy consumption is correspondingly higher.

The object of the invention is a method of preparing titanium for electroplating with metals of the platinum group, which permits the platinum metal to be deposited in a continuous strongly adhering layer. ...A more specific object is the provision of titanium anodes covered With platinum metal for use as electrodes in alkali metal chloride electrolysis, which require a low cell voltage.

Another object is a method of preparing titanium for electroplating with platinum group metals, which improves the cathode current efiiciency of the electroplating process.

We have found that platinum group metals can be deposited galvanically on titanium surfaces at significantly higher currnt efficiency when the titanium surface is prepared for plating by immersion in a fused salt bath consisting mainly of halides of the alkali metals. The chlorides and bromides of sodium and potassium are cheapest and most readily available. They may be employed singly or in mixtures with each other and with the costlier alkali metal halides, such as lithium chloride, sodium iodide or sodium fluoride.

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When a titanium object is immersed in a fused alkali metal halide bath, small particles of titanium are released from the surface and are dispersed in the molten salt. The titanium surface is roughened and assumes a characteristic etched appearance. When the surface treatment has progressed to an adequate extent, the object is withdrawn from the salt bath and rinsed with water to remove adhering salt. In order to further clean the surface, it is desirable to treat the etched object with a hot solution of a strong inorganic acid, such as phosphoric, sulfuric and hydrochloric acids, and to rinse again with water. Then the object is ready for electroplating in an aqueous electrolyte by methods which may be entirely conventional in themselves.

As a general rule, one hour immersion in a salt bath held at a temperature of at least about 25 C. above its melting point, but well below the melting point of titanium, gives good results in most instances, but time and temperature of immersion may be varied to suit specific requirements.

Typical compositions and operating temperatures of fused salt baths of the invention are listed in Table 1 together with reference numbers by which these baths will be identified hereinafter. Immersion time in these salt baths is one hour, unless specifically stated otherwise.

TABLE 1 N 0. Composition Temperature, C.

5 2 parts NaOl 1 part NaF- 750 6- 1 part NaCl 1 art KCl 860 7- 1 part NaGl 1 part NaBr 850 8- 1 part; NaCl 1 part NaL. 850 9. 1 part NaOl 1 part KBr 860 10 1 part NaCl 1 part LiC1 720 The following examples are further illustrative of the invention, but it will be understood that the invention is not limited thereto. 9

Example 1 Ten identical titanium disks having a diameter of 30 mm., a thickness of 0.3 mm., and five approximately uniformly spaced holes 3 mm. in diameter, were prepared for plating by respective immersion for one hour in the ten fused salt baths listed in Table 1. They were rinsed free of salt with water, dipped in hot 40% phosphoric acid until their surfaces appeared clean, rinsed again with water, weighed, and then plated in a bath of the following composition: v grams per liter Chloroplatinic acid, H PtCl -fi H O 5 Monoammonium phosphate 45 Diammonium phosphate 240 The platinum plating bath was operated at pH 5-7 at about 100 C. The cathode current density was 0.36 amp./dm. the plating time 90 minutes. 7

Weight loss of each titanium disk in the fused salt bath treatment (60 minutes), the amount of platinum de-' posited, and the current efficiency calculated from the deposit weight and the known current and plating time are listed in Table 2. Corresponding values are also listed for the following four conventional acid pretreatments:

(A) 10 minutes immersion in boiling phosphoric acid.

(B) 15 minutes immersion on an aqueous solution of 43% phosphoric acid, 9% hydrofluric acid, and 1.5% ammonium hydroxide at room temperature.

(C) 20 minutes immersion in boiling 35% hydrochloric acid.

(D). 10 minutes immersion in boiling 50% sulfuric acid.

Each' acid'pickling treatment was followed by a water rinse before electroplating.

TABLE 2 Pretreatment Platinum Cathode Current Deposited, Efficiency, N Weight Loss, mg./crn. Percent mgJcm.

l5. 8 4. 6 53. 8 10. 6 4. 4 51. 4 17. 7. 0 72. 2 l8. 4 7. 2 73. 8 20. 9 4. 7 48. 2 28. 6 7. 4 76. 0 33. 1 7. 2 4. 1 12. 9 6. 9 71. l 28. 9 7. 3 75. 2 15. 7 6. 6 68. 4 21. 3 4. 9 59. 4 17. 3 4. 7 56. 5

The adhesion of the platinum electrodeposits was tested by bending each disk until either the plated coating separated from the base metal, or the disk broke. One half of the disk was held in a vise, and the free half was bent first 90 in one direction and then 180 in alternating directions until the coating or the base metal failed.

In the disks that had received a salt bath pretreatment, the base metal failed after the ninth or tenth bending and before the plated coating in each instance. In the four disks pretreated with acid, theplatinum layer separated from the base metal after 1, 1, 2, and 3 bends.

Another set of fourteen disks identical with those listed in Table 1 was tested in an amalgam cell. Each disk was mounted as the anode in a cell whose'mercury cathode had an area of 27.3 cm. The gap between the electrodes was 5 mm. Brine having a concentration of 300 grams NaCl per liter, a temperature of 60 C., and a pH of 3 was electrolyzed between the electrodes at anode current densities of 20, 60, and 100 amp./dm. The cathode was replenished at a rate to hold its sodium content between 0.07 and 0.08%. The voltage required was measured, and is tabulated in Table 3 for each disk.

TABLE 3 Pret'rrgatment Voltage at Current Density of ampJdm. 60 ampJdm. 100 ampJdrn.

1 An additional set of disks, prepared by the pretreatments A, C and D described hereinabove was immersed in boiling hydrochloric acid to test for permeability 4 differences are evident from Table 3 at 60 and 100 amp./drn.

Example 2 Disks respectively pretreated one hour in bath No. 6 and 15 minutes at room temperature in acid pickle B were plated with rhodium from the following bath:

Grams per liter Rhodium chloride trihydrate -1 4.1 10

Sulfuric acid The acid pickled disks were plated two hours at pH 0.8, a cathode current density of 0.15 amp./dm. and about 80 C. The disks pretreated in the fused salt bath were plated six hours under otherwise unchanged conditions.

Tests of the pretreated and of the rhodium plated disks by the methods described in Example 1 had the following results:

Bath No. 6

1 After 1 bend.

While the advantages gained by the pretreatment of the invention prior to rhodium plating are smaller than in platinum plating, they are still significant.

Example 3 Two sets of disks respectively pretreated 60 minutes in salt bath No. 6 and 15 minutes in acid pickle B were plated with a platinum-rhodium alloy from a bath prepared from Grams per liter Chloroplatinic acid 3.5 Rhodium chloride trihydrate 1.2 Monoammonium phosphate 20 Diammonium phosphate 168 Sulfuric acid 3 All disks were plated two hours at pH 2.5, a cathode density of 0.15 amp./dm. and 80 C.

The alloy plates deposited on diiferently pretreate titanium disks performed in tests as follows: V

Salt Bath No. 6 Acid Pickle B Weight loss in pretreatment, mg./

Alloy deposited, mgJcm. 3.2 3.0 Failure of eleetrodeposit in bend None 3. 3.69 4.15 4.25 100 ampJdm. 4. 68 4. 77

of the platinum plate. Peeling of the electrodeposit started within one to four hours, and dissolution of the base metal resulted 'in weight losses ranging from 25 to 116 milligrams. Disks pretreated by salt bath 6 (equal parts of NaCl and KCI) had not started peeling when removed from the boiling hydrochloric acid after 6-7 hours, and the weight loss of each disk, if any, did not exceed 4 milligrams.

The imperviousness and continuity of the platinum plates produced on the disks pretreated according to this invention is believed to account not only for their greater corrosion resistance as. compared to conventionally acid pickled disks, but also for the lower voltage required for equal current density in the amalgam cell tests. At 20 amp./dm. the potential between the disks treated in a fused salt bath and the mercury cathode was between 3.40 v. and 3.49 v. Under otherwise identical conditions, the acid pickled anodes required 3.70 v. to 3.95 v. Similar 1 After one bend.

The superior adhesion of the salt bath treated platinum alloy coating is accompanied by a small, but consistent improvement in amalgam cell voltage.

Example 4 Chloroplatinic acid 3.5 Palladium chloride 1.1 Monoamrnonium phosphate 2O Diammonium phosphate 198 All disks were plated two hours at pH 5.5, a cathode current density of 0.15 amp./dm. and 80 C. The plated disks yielded the following test results:

Salt Bath N0. 6 Acid Pickle B Weight loss in pretreatment, mg./

23.6 15. 9 Alloy deposited, mgJcmfi 3. 4 3. 7 Failure of electrodeposit test None Voltage of amalgam cell at:

amp./ 3. 42 3. 50 60 amp./dm. 3. 90 4.00 100 amp./dm. 4. 43 4. 65

1 After one bend.

These results agree with those obtained in Examples 1 to 3 in showing the superior adhesion of platinum metal electrodeposits formed on titanium surfaces pretreated by immersion in the fused alkali metal halide baths of the invention and the lower energy consumption of amalgam cells in which plated anodes of the invention are employed.

We have also found that the nature of the container holding the fused salt bath has a surprising effect on the rate of titanium removal. A one hour treatment is generally applicable, and usually preferred when the fused salt bath is continued in a suitable metal crucible, such as a nickel crucible, but much shorter immersion times are adequate it the crucible is made of graphite. The rate of titanium removal is also significantly increased if graphite is dispersed in a fused alkali metal halide bath of the invention which is contained in a metal crucible, such as a nickel crucible. With adequate titanium removal, the nature of the container or the admixture of graphite powder to the alkali metal halides did not affect the properties of the plated object.

Example 5 Titanium disks of the dimensions described hereinabove were immersed for the time listed below in mixtures of equal parts of NaCl and KCl at closely similar temperatures. One salt bath was contained in a nickel crucible and had an average temperature of 860 C. during the immersion period (pretreatment No. 6). A second salt bath additionally contained 5% graphite powder and was contained in a nickel crucible at an average temperature of 850 C. (pretreatment No. 64:). The third salt bath was contained in a graphite crucible at 840 C. (pretreatment No. 6b). The processes of washing, electroplating and testing in an amalgam cell were carried out in the same manner as described in Example 1. The results were as follows.

While the invention has been described with reference to specific embodiments, it is to be understood that it is not limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

What we claim is:

1. A method of electroplating a titanium object with a metal of the platinum group, which comprises immersing said object in a fused alkali metal halide bath until the surface of said object is roughened, removing said object from said bath, rinsing the removed object with water until adhering alkali metal halide is substantially removed, and electrodepositing a metal of the platinum group on the roughened surface from an aqueous electrolyte.

2. A method as set forth in claim 1, wherein said bath is contained in a graphite vessel.

3. A method as set forth in claim 1, wherein said bath contains graphite powder dispersed therein.

4. A method as set forth in claim 1, wherein said metal of the platinum group is selected from the group consisting of platinum, palladium and rhodium.

References Cited UNITED STATES PATENTS 2,709,154 5/1955 Hansgirg 20439 2,746,915 5/1956 Giesker et al. 20456 3,024,174 3/ 1962 Stetson 204--39 3,293,167 12/ 1966 Greenspan 204-290 2,790,738 4/1957 Alexander et al. 156-18 X 2,873,233 2/1959 Schnable 20432 3,065,154 11/1962 Wiesner 204-32 FOREIGN PATENTS 877,901 9/ 1961 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

G. KAPLAN, Assistant Examiner.