US2719797A - Platinizing tantalum - Google Patents

Description

United States PatentO $719,797 iPLAEUINIZING Edgar F. Rosenblatt, Montclair, and ilohann G. 1Cdhn, East 'Orange, 'N. J ,ass'ignors to "Baker & Co., Inc Newark, N. "J., 'a corporation of New 'Jersey No Drawing. :ApplicationMay '28, "1950.,

QSeriaLNo. 163,782

'13 Claims. (01. 117-65) "invention relates 'generallytoa'processfor coating tantalum and like metals with metals or. alloys of the platinum group and "is furthermore concerned withsimproved anodes manufactured according to this coating process. The inventionalso 'relates'to the application of 'theseimprovedanodes in certain electrochemical reactions anditrelates'p'articularlyto, a-process to produc'e"platinized tantalum for use as non-corrodingianodesin electrochemical'processes. t

In electrochemical processes, such as "the electrolysis of 'chlorides and the, production 'ofpercompounds, the proper selection of material from which to manufacture 'the elec'tr'odes is a'matter of prime'importance. In these processes, it is conventional to 'use electrodes of iron, steel, 'etc. in the cathode position; howeverfthere, are only 'a few ltnown materials "that may "constitute the'an'o'des, *becati'se' most materials while in the anode position are susceptible to intense 'corros'ion. If 1onlythefcheniical "characteristics of -'a materi'al were to be "considered in-'the 'selection 'of 'asuitableanodic material, the metals of "the platinum group would be the universal choice because "they arehighly'resistant to corrosion; however, the high cost ofthcse precious'metals prohibitstheir extended'commercial use. Substitute materials'have, :therefore, been resorted to, whenever possible, the primary anodic matei'ialemployed industrially beingthat-of graphite 1 Many disadvantages accompany 'the use o'f graphite as an anodic material. A graphite anode undergoes "continual disintegration and must 'be "replaced from time to time *therby causing interruption of the electrochemical process which is a'costly-operation. In the manufacture -of'chlor'ine bythe electrolysisofbrine, it 'is necessary to puiifyfltheproduct from traces of carbon dioxide which result frorn the'oitidation of the 'graphiteanodes. 'Partic'leswhich dust (SE from'the graphite become deposited in the 'dia'phragr'ns which surround the electrodes thereby necessitating replacement of'the diaphragms along withthe electrodes. Theseditficul'ties are not-encountered when anodes of'the platinum metals are used. -Alsothe 'overvoltage "characteristics "forclilorine discharge are more favorable with platinum metal anodes than with 'graphite anodes.

here-has -been a continualsearch for"a*m aterial which =hasthe desirable anodic-characteristics of'thepl'atinum metalsf'but-not their prohibitive costs. It-h a's 1ong-been considered that'tantalum would makeqan ideal anodic material because "of its remarkable chemical inertness. Tantalum is -not attacked by chlorine, hydrt'ic lilori'c acid, 'nitri'cacid,or aqua-regia. Then, too-its cost is veryf low as compared'withthe cost' of the platinum metals, How- -ever,'when atantalum electrode is used as {an anode, the flow of electric"current through the cell quicklyfstops. This phenomenon is'caused bythe formation of what'rnay be referred V to as an anodicifilm comprising "a layer of oXide which decreases "the material. 7

In order to prevent the formation of this film, attempts have been made 'to-"coat a tantalum base with platinum electric conductivity -'6f "the .metal. It. has 'beensuggested .that coating .with a platinum metal be accomplished by such .methods as electrolysis, hammering, welding, rolling, and-the like; however, none of these methods have been found .to lbe,.-satisfactory. They do not produce a coat of platinum metal .which adheres with sufficient tenacity to the tantalum baserthat the coated tantalum metal will'be commercially suiteddior .use as an anodein electrochemical,processes. Electroplating .of a platinum metal .onto ,a tantalumibase results in a coating that ,may easily be stripped fromtheibase. Attempts to cover the tantalum strip with -a platinum metal foil and to holdthemetals together asby sweating, rolling or hammering, have proved to be unsatisfactory because the'platinum metal foil :is held '10 the tantalum only by mechanical .contact whichis not,su'fiicient topermit of its use as an :anode. The coats of platinum metal that have beenma'de by any of these processes are not truly "bonded to the tantalum, 'i. e. the tantalum is not platinizetl in'the sense thattheplatinum metal is united Withthetantalumbyatornic attraction forces asis obtainedby thisinvention.

it is, therefore, 'one object of :this invention .to provide a thin, firmlyadherent, practically inseparable coat of .one or several of the platinum metals onto a base OfLtantalum or like metal. It is a further object of the invention 'to achieve =t'his result in a simple and-economical manner. Anotherobject 'of'this invention is'to'manu'facture a composite metal stoekhavingabodily newarrangement 'between tantalum and'platinummetals. It is a further object of this invention to improve. certain.electrochemical processes by using the improvedelectrodes of this'invention. Further objectsiandwariouszadvantages of this invention will become apparentifrom a considerationzofzthis specification and'the appended claims.

By the term platinumimetalf as herein used, is meant the precious, metals ,in group MIII. .of sthe ,periodic .chart, excepting psmium, ,i. e. this .term includes ,the following metals: ruthenium, Platinum. 7 v "Broadly, the-processeof this,invention,involvesa-combination of successive treatments whereby-a thin.deposit of a platinum metal is made upon the surface ofza tantalum body and thereafter the platinum metal deposit istbondeditwthe' tantalum 'body by heating the whole' toa high temperature and under inert conditions! The thin deposit of a platinum metal may be applied'accordin-g to thisinvention in one: of several :difierent ways, such as .byelectroplating or by .the chemicalidecomposition of ,.a platinum metal compound 'applied on:thesurface .of the tantalum. It .should ,be :clearly understood that the thin {deposits .madeaccording totany-of these processes are. not ,firrn deposits until they have been 5 subsequently bonded in. thermannertdescribedlater in: thisqspecification.

For the ,purpose ofthis invention, ;columbium,=may Lbemused instead ,of-.,tantalum. Also,ralloys, inCllldlng5Sil1- tered products of these metals, may'be;used,;i.-e'. alloys of the metals themselves as well as with the addition of other metals which do -not change the inertness of the :alloy. -At*the present -:time, :tan'talum is :preferre'd1over .use of =colurnbium :because 5 the :cost :of ;columbium is much-greater than'the cost of :tantalum' It;is intended also that the term "itantalunf as .used herein be mot limited to ,thechemically npure :metal 'but that vit include other marketable form 'which contains small and, there'- tfore, harmless \traces of impurities, such :as carbon, I iron, =etc. v i

. a Chemicaldecompasition As statedmbovegone"methodofapplyingathin coat =of a platinum metal is-by decomposing a metal compound on the suffaceofthe tantalum. 'To accomplishthis treat mentkthe tantalum Fbase is pre'ferablvwashed -as with -rhodiuin, palladium, iridium and n Q carbon tetrachloride and acetone. The surface of the tantalum metal is then cleaned and roughened to an extent depending upon the amount of coverage that is desired. This roughening may be accomplished by rubbing the metal with emory papers, by sand blasting, or by chemically etching with hydrofluoric acid. The etching may also be accomplished electrolytically in hydrofluoric acid using lead or silver as the cathode material. The cleaned and roughened tantalum is then dipped into or otherwise coated with a solution of, e. g. chloroplatinic acid (HzPtCls), which has been dissolved in a suitable volatile solvent. It has been found that one gram of chloroplatinic acid dissolved in a solution containing 15 ml. isopropyl alcohol and 15 ml. ethyl acetoacetate produces a uniform platinum coating. The more important constituent of this solvent is the ethyl acetoacetate since solutions having a higher concentration of this constituent give better results than do those having a greater proportion of the alcohol. After dipping the tantalum into this solution, the excess solution is removed and the solvent is evaporated otf at a relatively slow temperature. The coated tantalum is then heated above the decomposition temperature (approximately 250 C.) of the chloroplatinic acid whereby a thin coat of platinum is left upon the tantalum surface, all other products of the decomposition being volatile. The platinum coating resulting from the above treatment amounts to approximately 0.00034 gram/cm. In like manner, tantalum and columbium may be coated with other metals of the platinum metal group by using one of the following chemical compounds ruthenium nitroso bromide (RuNOBrsnHzO) palladium di-n-butylamine nitrite (Pd (C4H NH2) 2 (N02) 2) and iridium chloride acid H2IrCls6H20) Electroplating Another method for depositing the thin coat of a platinum metal upon the tantalum base is by electrolysis. After the tantalum base has been roughened as by etching in a manner as explained above, it is placed in the cathode position in a plating bath. A plating bath having the following compositon has been found to be suitable:

20 g. Pt(NH3)2(N02)2 commercially known as P-salt 100 g. ammonium nitrate 1 liter of ammonia Bonding Following the deposition of the thin platinum metal coating, the coated tantalum is subjected to the bonding treatment of this invention. An example of the manner in which this treatment is carried out follows:

Tantalum strips which have been platinized according to the above described chemical decomposition or electroplating procedures are placed into acold furnace. The furnace is evacuated to about l0- mm. of mercury and is then heated to a temperature below the lowest melting point of the metals involved and high enough to cause the metals to become bonded as required by this invention.

800 c. to 1400" 0. have been found suitable, the 0p- Temperatures ranging from approximately timum temperature being about 1000" C. After the bonding temperature has been maintained for approximately 15 minutes, the heating is discontinued, and when the temperature in the furnace has substantially dropped the evacuation is stopped and the coated tantalum is removed. Instead of treating the metal in a vacuum, it has been found that any inert atmosphere such as helium, neon, argon, and the like may be substituted. These gases do not chemically combine with the tantalumas does oxygen, for example, and by the term inert conditions 'a's used in this specification and in the claims is meant such surroundings which are inert with respect to tantalum, such as a vacuum or a noble-gas atmosphere.

A platinum layer on a tantalum strip which has not been subjected to the bonding treatment of this invention will upon being immersed in aqnal regia become completely dissolved. When, however, such a coated strip is bonded in the manner described above, aqua regia will not completely dissolve the platinum layer and the weight of the bonded strip after it has been immersed in aqua regia is greater than the weight of its original tantalum constituent, thus indicating that the bonding treatment causes the platinum and tantalum metals to interdiffuse and become alloyed in such a composition that the alloy is not attacked by hot aqua regia.

Tantalum strips which were platinized by the above described chemical decomposition procedure were heated in vacuum at 1200" C. and 1400 C. Platinizing according to this procedure produces layers of only up to 0.03 mm. in thickness per application. The layers thus deposited were not attacked by hot aqua regia, however, when these strips were tested as anodes in cells for. the production of chlorine and chlorates the current ceased to flow and the strips became corroded with a coherent tarnish layer. This corrosion of the anode indicates that the tantalum metal has diffused through the thin platinum layer to the outside surface of the platinum film where the tantalum becomes oxidized and stops the flow of current through the cell. If platinized tantalum strips are bonded at temperatures of less than 800 C., the platinum metal layer is easily dissolved by aqua regia and the platinum films flake off from the tantalum base when the stripsare' used as anodes, thus indicating that substantially no alloying of the platinum andtantalum metals occurs when there is insufficient heating.

It is desirable, therefore, to coat the tantalum base in such a manner that the outside layer of the platinum metal film will consist of substantially pure platinum whereas the inside layer of the film will be firmly bonded to the tantalum base by the formation of a thin layer of platinum-tantalum alloy. A tantalum strip which has been platinized in such a desired manner will behave like an ordinary platinum electrode, the apparently vulnerable zone of platinum-tantalum alloy being protected from contact with the electrolyte solution in thecell .by the outer layer of unalloyed platinum. To insure that the outer layer remains unalloyed during the heat treatment by which tantalum and platinum become bonded, a heavier platinum metal coat is required than in the alloy anodes described above. Also from a commercial standpoint, it is advantageous to apply a relatively thick platinum metal coat on a thick tantalum base and, thereafter, roll or otherwise draw out the thick coated bar to obtain the desired reduced strips having a top layer of unalloyed platinum metal. Such a procedure permits of more case in the handling of the metal composites and enables using smaller processing equipment having obvious economical advantages as well as reducingthe losses incurred during the etching treatment.

A heavy deposit of platinum metal may be applied to the tantalum base by electroplating in the manner described above; however, if such heavy deposit exceeds approximately 1 mu. in thickness, it cannot be successfully bonded. Apparently, during the electroplating process, stresses are generated in the platinum film cans ing blisters "to form which dofn'ot beco'me eoherently zb'onded to the tantalu'm base "when ith'e coated :metal is subsequently heated. I

-In practicing this "invention, it 'is, therefore, desirable to apply aiprimar'y coat which does "not exceed l'mu. in thickness. Following the bonding o'f the primary coat to the tantalum base in the manner as lreieina'liove described, a ise'cond coat of any desired thickness, such as for example 25 -'mu., may' be electroplated upon ithe "surface of 'the thin primary coat. The blistering =which oc- -'cu'rs"as stated -'abovewhen a thick platinum metal deposit is electroplated direetl-y upon a tantalum base, does not occurwhen the second ='coat-is deposited.

Following such a procedure, iit-ha s been foundpOssible, for example, to platinize a tantalum rod of 6.7 millimeters diameter with a platinum film o'f25 microns in thickness, and, "thereafter, .to fdrawioutthe coated and bonded rod .to a diameter of .67 millimeter'without any danger of splitting the platinum Tfilmifrom the tantalum base. Iliis-rod a'fter'being reduced lin-diameter behaves like a-.puret platinum ano'de thus indicating that-the platinum-tantalum alloy whichis formed at the interface-of the film and-thebase does not extend lto the outside-surface of the film, and that the outer surface layer of the film comprises substantially pure .platinum. tAlso,-there isno'ilakingtotf o'f the platinum filmeven after 'long and continued use of the rod as an .anodezin a .chlorineor chlorate production cell thus indicating that :platinum film-is-firmly bonded to the tantalum base. 7 The secondary coat may be applied in severalseparate layers andbetween the periods of the deposition of these several layers, the metals may be subjected to a bonding treatment. It is preferred that the primary coat be not more than 5% of the thickness of the secondary coat or the combined layers which may comprise the secondary coat It has been furthermore observed that the original surface film of the tantalum body that is not removed by the cleaning and roughening process, becomes diffused throughout the interior of the tantalum mass during the subsequent heating treatments whereby pure tantalum metal becomes joined by atomic attraction forces with the platinum deposit. When, however, no precautions are taken to prevent the formation of the surface film during the heating treatments, the film becomes considerable and is not completely dissolved into the tantalum mass, thus preventing the platinum deposit from joining directly to the pure tantalum. Also, it is noted that the diffusion of this surface film into the tantalum mass causes the development of permanent hardness of the tantalum, and when relatively large amounts of oxygen or nitrogen are absorbed in the metal, the tantalum becomes brittle. This brittleness is especially pronounced in the case where very thin strips of tantalum are used. By depositing platinum metals and bonding them to tantalum according to the process of this invention, these undesirable results are eliminated.

Examination with a microscope of the platinum metal films of this invention reveals that they have a rough or wavy profile, the indentations of which develop into pores when the coated sheets are reduced to small thicknesses. Although these pores expose the tantalum to the development of an anodic film, the coated tantalum spots are not affected when the composite metal is used as an anode. Also, the thin platinum outer layer prevents the alloy layer from being attacked.

The inner core of the tantalum base may consist of copper, silver, or other like metal. For example, a tantalum tube which has been platinized on its outer surface according to the method of this invention may be slipped over a tightly fitting copper rod and the composite rod may then be swaged and drawn to any desired thickness. When using such a composite rod as an anode in the electrolytic production of chlorine, chlorates, and percompounds, it is necessary that the tantalum covering ltrave iuobpenings tltrough Mitten the core may he attacked. aFro'm the a'bo've :Hescriptio'n, it=will be seen that we have disclosed :a process by which platinum :metals may be successfully a'nil firrnly bonded to taritalu'm metals, and that tantalum :whi'ch has' lteen platini'zed according =to :the manner itau'ght-fherein :m'ay be used as an an'ode in eleetr'ohemical processes, mud-especially in the fproduction of: chlorine; chlorates, andper-ompounds. The examples wh'ich ltav'e been-included in this spe'eification .have ibeen given by' way of illustration 'oril'yand t-tot by way of restriction or limitation. We -desire to have tit aundersto'od that wedo not' intend to "limit ourslves to ithe particular details tiesc'ribed i'e'xcept z'as #dfined -in the claims." I I 11. 'A rn'ethod of providinga'nadherenteoatifig of platinum metal on a base ot' material or the group consist- :ing of itantalum a'n'ti eolumbiu'm, eom'pris'ing the sequenftial .Imanufacturingsteps of dep'ositing a tliin cdntinuous llayer rof :platinum metal -"on' sai'd "b'ase "and-he'atin'g isaid filayer rand said base 'in' proximlty therto under inert conditions at an elevated temperature blowtliefmelt- -ing "pointiof said platinum metaban'd said base (material zandtwithinithei'ran'gemf about 800' cs'to about 1400 C. to inter-diffuse .L-said' ;metals :at '!the ibounda'ry thei'e'of, wherebyssaid platinum metal {layer becomes attended to :said material omits router surface and :is 'z'separate'd from the base by an interlayer of an alloy of lth'e platinum .withthe basmmetal A ,7

2. 'A method of coating a base formed of material selected from the group consisting of tantalum, columbium and alloys of these metals platinum metal comprising the sequential manufacturing steps of depositing a primary coat of platinum metal on said base heating the coated base under inert conditions at an elevated temperature within the range of about 800 C. to about 1400 C. the melting point of said metals to alloy the adjacent surfaces of said metals, electroplating a second coat of platinum metal on said primary coat, and heating the coated mass at an elevated temperature below the melting point of said metals and within the range of about 800 C. to about 1400 C. to form coatings of pure Pt interspersed with interlayers of an alloy of Pt with the base metal.

3. The method according to claim 2 wherein said second coat comprises a plurality of separate deposited layers and the composite metal is subjected to the step of heating-under inert conditions below the melting point of said metals and within the range of about 800 C. to about 1400" C. to alloy the adjacent surfaces of said metals between the steps of applying each separate layer deposit.

4. The method according to claim 2 wherein the thickness of the primary coat is not more than 5 percent of the thickness of the second coat.

5. A method of producing an anode for use in electrochemical processes by providing an adherent coating of platinum metal on a base of tantalum which comprises the sequential manufacturing steps of cleaning and roughening the surface of the tantalum, depositing a thin first coat of platinum metal on the tantalum base, bonding said coat to said base by heating said metals in an inert atmosphere at a temperature above about 800 C. but

below about 1400 C. whereby the remaining film migrates into the metals and the adjacent surfaces of said layer and said base are alloyed and said metals are joined together by atomic attraction forces, electroplating a second coat of platinum metal upon said first coat, heating the metals to an elevated temperature within the range of about 800 C. to about 1400 C., and reducing the thickness of the bonded metals.

6. The method according to claim 5 wherein the cleaning step is performed by etching the tantalum with hydrofluoric acid, the inert atmosphere being a vacuum, and the bonding temperature being about 1000" C.

, .7. The methodaccording to claim 5 wherein said second coat comprises a plurality of separate deposited layers and the base and deposited layers are subjected to the step of heating under inert conditions below the each separate layer deposit, the thickness of said first layer being less than about 5 percent of the thickness of said second coat.

8. The method according to claim 1 wherein the step of depositing the layer of platinum is accomplished by washing said base metal with a solvent, cleaning and roughening said base metal as by etching, coating said cleaned base metal with a solution comprising a soluble compound of the platinum metal and a volatile solvent, evaporating the solvent from the liquid, and heating the coated base metal above the decomposition temperature of the soluble compound to decompose said compound and to volatilize the products of decomposition excepting the platinum metal.

9. The method of claim 8 wherein the soluble compound is chloroplatinio acid and the solvent is a mixture of ,isopropyl alcohol and ethyl acetoacetate.

10. The method according to claim 1 wherein the step of depositing the layer of platinum is accomplished by electrolysis.

11. The method of claim 2 wherein the primary coat is deposited by washing said base metal with a solvent, cleaning and roughening said base metal by etching, coating said cleaned base metal with a solution comprising a soluble' compound of the platinum metal and a volatile solvent, evaporating the solvent from the liquid, and heating the coated base metal above the decompo sition temperature of the soluble compound to decompose said compound and to volatilize the products of decomposition excepting the platinum metal.

12. The method of claim 11 wherein the soluble compound is chloroplatinic acid and the solvent is a mixture of isopropyl alcohol and ethyl acetoacetate.

13. The method of claim 2 wherein the primary coat is deposited by electrolysis.

References Cited in the file of this patent UNITED STATES PATENTS 1,465,553 Kirk Aug. 21, 1923 2,114,161 Antisell Apr. 12,1938 2,226,720 Hansell Dec. 31, 1940 2,282,097 Taylor May 5, 1942 2,303,497 Reeve Dec. 1, 1942 2,370,242 Hensel Feb. 27, 1945 2,375,154 Volterra May 1, 1 945 2,401,040 Becker May 28, 1946 2,417,459 Eitel Mar. 18, 1947 2,418,460 Buehler Apr. 8', 1947 2,492,204 Van Gilder Dec. 27, 1949 2,497,109 Williams Feb. 14, 1950 2,539,096 Miller Jan. 23, 1951

Claims (1)

1. A METHOD OF PROVIDING AN ADHERENT COATING OF PLATINUM METAL ON A BASE OF MATERIAL OF THE GROUP CONSISTING OF TANTALUM AND COLUBIUM, COMPRISING THE SEQUENTIAL MANUFACTURING STEPS OF DEPOSITING A THIN CONTINUOUS LAYER OF PLATINUM METAL ON SAID BASE AND HEATING SAID LAYER AND SAID BASE IN PROXIMITY THERETO UNDER INERT CONDITIONS AT AN ELEVATED TEMPERATURE BELOW THE MELTING POINT OF SAID PLATINUM METAL AND SAID BASE MATERIAL AND WITHIN THE RANGE OF ABOUT 800* C. TO ABOUT 1400* C. TO INTER-DIFFUSE SAID METALS AT THE BOUNDARY THEROF, WHEREBY SAID PLATINUM METAL LAYER BECOMES BONDED TO SAID MATERIAL ON ITS OUTER SURFACE AND IS SEPARATED FROM THE BASE BY AN INTERLAYER OF AN ALLOY OF THE PLATINUM WITH THE BASE METAL.