US3698050A

US3698050A - Method of producing a composite electrode

Info

Publication number: US3698050A
Application number: US146619A
Authority: US
Inventors: Leonard R Rubin
Original assignee: Engelhard Minerals and Chemicals Corp
Current assignee: BASF Catalysts LLC; Engelhard Minerals and Chemicals Corp
Priority date: 1971-05-25
Filing date: 1971-05-25
Publication date: 1972-10-17
Anticipated expiration: 1989-10-17

Abstract

A composite material comprising titanium or a titanium alloy substrate to which is bonded a platinum group metal coating is prepared by a method which includes the steps of wrapping a platinum group metal wire around the substrate, which is in the form of a cylinder, and then converting the wire-wrapped cylinder to a unitary body by annealing and swaging steps. The resultant composite material is particularly useful as an electrode material.

Description

United States Patent Ru in 1451 Oct. 17, 1972 METHOD OF PRODUCING A 2,652,621 9/1953 Nelson ..'..29/25.17 COMPOSITE ELECTRODE 2,867,032 1/1959 Gehrke et al. ..29/25.l 7 3,165,825 l/l965 Barney ..72/700 X [72] Invent 3,363,304 1/1968 Quinlan ..72/47 x [73] Assignee: Engelhard Minerals & Chemicals 3,426,420 2/1969 Grant et al ..29/474.l X

Corporation I Primary Examiner-John F. Campbell [22] Flled' May 1971 Assistant Examiner-Richard Bernard Lazarus [2]] Appl. No.: 146,619 Att0rneyMiriam W. Leff and Samuel Kahn 57 ABSTRACT [52] US. Cl. ..29/25.l8, 29/25.l 1, 29/4733, 1

29 4739 29 4741 29 475 72 47 72 7 A composite material comprising titanium or a titani- 51 int. (:1 .1101 9/16, 1101 9/44 alloy Substrate to which is bonded a Platinum 158 Field of Search....29/25.l 1, 25.17, 25. 1 8, 473.3, P metal mating is P P by which 29/4739 72/700 47 cludesthe steps of wrapping a platinum group metal wire around the substrate, which is in the form of a [56] References Cited cylinder, and then converting the wire-wrapped cylinder to a unitary body by annealing and swaging UNITED STATES PATENTS steps. Theresultant composite material is particularly f l l t d t 1. 1,700,454 1/1929 Schumacher .;,....29/2s.17 use as 6 W 6 ma em 2,375,068 5/1945 Bennett ..29/474.l X 7 Claims, 6 Drawing Figures ag lalialazlallalalalm PATENTEDBBTIT I912 3,698,050

SHEET 1 OF 2 FIG.

WIRE WRAP 2 ANNEAL FIG. 3 4

Ho. I00

SWAGE RE-ANNEAL //v l/EN TOP 81/ LEONARD R. RUB/N A T TORNE V METHOD or PRODUCING A COMPOSITE ELECTRODE,

BACKGROUND OF THE INVENTION This invention relates to a method of fabricating a composite material comprising titanium or a titanium alloy substrate and bonded thereto a platinum group metal coating.

Composite materials of this type are well known and find utility in many fields. They are particularly useful as electrodes in cathodic protection systems, in electrochemical processes such as the electrolysis of brine to produce chlorine, or for auxiliary non-consumable anodes for nickel or chrome plating. The electrodes havemany configurations, e.g. sheets, wires, rods or tubes, with the platinum group metal as a continuous layer or a partial or intermittent coating on the substrate.

Depending on the ultimate use, the physical and chemical properties required as well as the design of the composite material will vary. However, a requirement of the material which is of major importance in most applications is that the platinum groupmetal be strongly bonded to the substrate. It is also important that a minimum amount of the expensive platinum group metal is used.

Reference to the literature shows that a great many and varied methods have been utilized to produce the composite materials. Among the well-known techniques used to apply the platinum group metal are cladding, spraying, electroplating, chemical or thermal reduction, powder metallurgy, and inlay of metal strips. It is known also to follow the deposition of the platinum group metal by various heat treatments,'rolling and swaging, in order to increase the adherence of the platinum coating to the substrate. In U. S. Pat. No. 2,719,797, for example, a platinum coating is applied to a tantalum substrate by the known methods such as electroplating or chemical decomposition and the bonding is achieved by subjection the coated substrate to a temperature of 800 C to l400 C in an inert atmosphere. Thereafter the coated material is drawn out to the desired diameter. Partial coatings have been formed according to British Pat. No. 922,599 by rolling into sheet form a mixture of powders of titanium or an alloy of titanium and a noble metal. In another method striped platinum electrodes have been formed by an inlay technique or by tack or spot welding noble metal strips to one or more faces of a refractory metal sheet, encasing the sheet with a metal sheath, heating the encased assembly under pressure in an inert atmosphere, and subsequently removing the sheath.

Still other electrodes are composed essentially of loose platinum wire wrapped around a wire substrate, e.g., of titanium coated conductor metal. The platinum wire may be spot welded to the substrate at intervals. These loose wire type electrodes have both mechanical and electrical disadvantages. The wire is easily torn and the electrodes have a higher resistivity than electrodes with bonded coatings. If high currents are used, passage of the current tends to be mainly at the points where the wire is tack welded to the substrate. If gases collect behind the unbonded wire, the wire tends to lift and the electrodes fail.

The many and varied problems in fabricating suitable composite materials of platinum group metals on refractory metal substrates are very well known to those skilled in the art. However, because of the extensive applications in industry of materials of this type, there has been a long history of research directed'to finding improved methods of obtaining the composite materials The primary object of the present invention isto provide a simple and economical method for producing a composite material in which there is a strong bond between the platinum group metal coating and the refractory base metal substrate. Another object is to provide a composite material which exhibits structural integrity when used as an electrode. A particular object is to provide an improved method for preparing a composite material comprised of titanium or a titanium alloy substrate and a platinum group metal coating. And still a further object is to provide an improved method for finding a composite material consisting of a refractory base metal substrate with a platinum group metal alloy as the coating. These and other objects are accomplished by the invention described below.

The invention may be best understood by reference to the accompanying illustrative figures and examples taken in connection with the description.

THE DRAWINGS FIG. 1 is a simplified flow diagram of the method of this invention.

FIG. 2 is a drawing which illustrates the initial step of wrapping a wire around the substrate.

FIG. 3a and 3b are drawings which illustrate two embodiments of the composite material which may be formed by the method of this invention. FIG. 3a is a transverse sectional view of a composite material with a continuous coating and FIG. 3b shows a composite structure with an intermittent coating.

FIGS. 4 and 5 are photomicrographs of samples after the annealing and re-annealing steps, respectively, as described more particularly in Example 2.

The steps outlined in FIG. 1 are explained in detail in the description and examples. In FIG. 2 a platinum group metal wire 10, e.g. of Pt or a Pt alloy such as percent Pt-2O percent Ir, is wound around titanium rod 11 and secured at both

ends

12 and 13. Securing the wire may be accomplished by many known methods, e.g. by tack welding both ends of the wire to the rod or by simply tying the ends through holes drilled in the ends of the rod. By the annealing, swaging, and second annealing steps, the wire-wrapped bar is converted to a diffusion bonded unitary composite structure which may take various configurations. Two embodiments are represented in FIG. 3a and FIG. 3b, with the coatings greatly exaggerated for the purposes of illustration. FIG. 3a shows a continuous coating 10a of the platinum group metal on titanium substrate 11a, and FIG. 3b shows a composite structure with an intermittent coating 10b of the platinum group metal on titanium substrate Ilb.

THE INVENTION In accordance with this invention a composite metal material comprising titanium or a titanium alloy substrate to which is bonded a platinum group metal coating is produced by a method which comprises:

a. wrapping a wire comprised of a platinum group metal around the substrate, said substrate being in the form of a cylinder, to form a continuous taut helical configuration of the wire around the cylinder,

. annealing the wire-wrapped cylinder in an inert atmosphere at a temperature in the range of about 800 to 1200 C.,

c. swaging the annealed wire-wrapped cylinder at ambient temperature to convert the wire into' a coating, and

. re-annealing the resultant material in an inert atmosphere at a temperature of about 800 to 1200 C. i

The coating may be any platinum group metal or alloy thereof which can be made into a wire. Platinum, palladium, iridium, and rhodium, and alloys thereof can readily be drawn into wire. Ruthenium and osmium are too brittle to form as fine wire, however, there are alloys of ruthenium or osmium with other metals that are sufficiently malleable to be made into suitable wire. It is a particular advantage of this invention that it is simple to form an alloy coating using the method of this invention provided the alloy can be formed as a wire. Examples of particularly useful alloy coatings are 80Pt-20lr, 80Pt-20Rh, and 95.5Pt-0.5ThO

The substrate material is titanium or an alloy of titanium that has similar expansion characteristics and allotropic modification on heating as titanium. Titanium has thermal expansion characteristics that materially aid in the developing of a coherent coating by the method of this invention. The expansion coefficients of titanium and platinum, for example, are very close (with that of titanium being slightly greater than the thermal coefficient of expansion for platinum), so that there is no problem of the substrate shrinking away from the platinum group metal wire or coating when it is heated. In addition an allotropic modification takes place in titanium at 880 C with a 5 percent increase in volume. Thus when the wire-wrapped titanium is heated above this temperature there is a resultant tightening action on the wire that aids in developing contact for diffusion bonding. It is noteworthy that tantalum and niobium, which do not have comparable thermal coefficients of expansion as titanium and are not subject to the same allotropic modification do not.

form a cohesive bond with a platinum group metal when subjected to the same steps of preparation described herein.

In accordance with this invention there are two annealing steps, one in which the platinum group metal wire is diffusion bonded to the substrate and the second in which the platinum group metal, now converted to a coating, is diffusion bonded to the substrate. In both these steps the diffusion bonding is aided by the allotropic modification of the titanium when it is heated. Thus the annealing steps are preferably carried out at a temperature above 880 C. Annealing is performed in an inert atmosphere, i.e. an atmosphere which will not harm the titanium substrate. Oxygen or nitrogen, for example, has a harmful effect on titanium at elevated temperatures. Annealing can be performed, for example, in a vacuum furnace or in the presence of an inert gas such as helium or argon.

It was noted above that the platinum coating may be continuous or intermittent. Depending on the design requirements, the wire is chosen of suitable diameter and length to give the thickness and coating desired. The dimensions of wire may vary considerably and can be easily calculated. For example, when a 3 mil platinum wire, which has a cross section of 7.07 square mils, is wound on a 7.07 mil pitch on a inch diameter titanium rod, the length of wire required to form a 1 mil thick coating is about 1 l .74 feet of wire per square inch of rod surface. a

An alternative approach is to use a ribbon or metal foil having a thickness of about 1-3 mils. This approach entails handling difficulties and higher labor costs. In the preferred method of this invention, the substrate is wrapped with wire, which is, in essence, formed into a ribbon in the swaging step. In contrast to ribbon or foil, wire is easy to handle, and generally readily available commercially.

Before wrapping the wire around the substrate, the substrate material should be suitably cleaned. The usual methods of cleaning the titanium or titanium alloy surface may be used. For example, the surface may be cleaned with emery paper and acetone or it may be subjected to an acid etch. It is an advantage of the process of this invention that the cleaning of the substrate is not critical. A simple cleaning of the titanium surface with emery paper and acetone has proved sufficient.

The wire is wrapped around the cylindrical substrate at preferably a high degree tension to form a tight coil, preferably near the yield point of the wire, and it is secured at both ends of the cylindrical substrate. The substrate is in the form of a cylinder since the wire will make even contact at all points on this cylindrical configuration, thus enhancing diffusion bonding of the wire to the substrate. lt is possible for the substrate to be a hollow cylinder.

During the swaging, the wire is flattened on the substrate. This step is performed at ambient temperature to avoid oxidizing the titanium. It is still another advantage of this process that the coating can be formed at room temperature. During this swaging step, the substrate may be reduced in diameter.

After the second annealing step the composite material may be further reduced in diameter or converted to another configuration, e.g. round-cornered square rod, flat stock, channels, ells, tees, and the like.

EXAMPLE 1 A sample of inches diameter titanium rod was cut to an approximately 10 inch length to allow chucking in a lathe. An adaptor was built to hold a spool of 3 mil platinum wire in the tool holder of the lathe and to apply enough friction to the spool to keep the wire relatively taut when being paid out. The running tension on the wire was measured. On 3 mil wire, the drag is approximately grams, which generates a stress level of about 22,000 psi in the wire while it is being paid out.

The rod was rubbed with emery cloth to remove surface contamination and starting and finishing holes were drilled for the wire. it was wiped off with acetone soaked rags to remove handling contamination and then chucked into the lathe. The wire was passed through the starting hole and tied off, The feed on the lathe was set at 7.1 mils/revolution and the wire paid off on to the rod. At the end of the run, the wire was cut, passed through the finishing hole, and tied off.

The wire-wrapped rod was annealed for 1 hour in vacuum at 950 C. This temperature wasvchosen to take advantage of the allotropic modification which takes place in titanium at 880 C with a 5 percent increase in volume. The resultant tightening action on the wire aids indeveloping contact for diffusion bonding.

The wire-wrapped rod was then swaged without lubricants to a diameter of 0.365 inch including the wire wrapping. The turns of wire had flattened on the surface until they butted against each other, forming a relatively continuous sheet. After swaging the coated rod was re-annealed'for 1 hour at 950C in vacuum, completing the diffusion bonding.

In the resultant composite material, the platinum coating which was essentially continuous was firmly bonded to the titanium substrate.

A composite sample prepared in this manner was bent to an angle of approximately 90 on a 1.5 inch radius. The coating remained firmly adhered to the substrate. This demonstrates the tenacity of the bond.

EXAMPLE 2 Several samples of the titanium rod were coated with platinum using the procedure in Example 1. Someof the samples were used for metallographic examination at various stages of the process.

FIG. 4 is a photomicrograph at a magnification of about 500 times of a cross section through a single turn of platinum wire on a titanium rod after wire wrapping and an annealing treatment at 950 C for 1 hour in a vacuum, but prior to swaging. It shows that diffusion bonding had occurred between the Pt wire and the Ti rod.

FIG. 5 is a photomicrograph at a magnification of 200 times of a section through the surface of a platinum coated titanium rod after wire wrapping, annealing, .swaging, and re-annealing. Both annealing and re-annealing wire performed at 950 C for 1 hour in a vacuum. It shows that diffusion bonding had occurred between the Pt coating, which is essentially continuous, and the titanium substrate.

EXAMPLE 3 Several samples of titanium rod were coated with platinum using essentially the same procedure as Example l, but with the variation described below.

The titanium rods were cleaned as described in Example l, pre-coated'with a thin film of platinum by deposition from an organo-metallic compound of platinum, and then wire-wrapped as described in Example l. As in the samples prepared in Examples 1 and 2, all of them were wire-wrapped without difficulty or incident. I

As an alternative procedure, some Pt wire-wrapped titanium rods, prepared as otherwise described in Example 1, were annealed at 800 C. Because of the close thermal coefficients expansion of platinum and titanium, the bonding was adequate. However, it is preferred to anneal above 880 C to cinch the wire in case contact was marginal. In the second annealing step after being swaged these samples were subjected to temperatures of 950 C in a vacuum to insure diffusionbonding.

EXAMPLE 4 Niobium and tantalum rods were wire-wrapped with platinum wire and subjected to annealing temperatures of 950 C and swaged, in a procedure similar to that described in Example 1.

Theplatinum wire would not adhere to either materialaft'er wrapping and vacuum annealing for an hour at 950 C. Diffusion studies have shown that his extreme-.

ly difficult to make platinum and tantalum stick to each other, even when metallographically polished slugs are EXAMPLE 5 The surface of a composite platinum on titanium rod prepared as described in the preferred method of Example l was covered with Platers tape, leaving a V2 inch diameter circle of platinum exposed. The tape portion was immersed in a beaker of reagent grade HCl (37 percent l-lCl) as an anode at 5 amperes for 30 minutes (3670 amperes/ftF). (Normal current density in service is about 200 amperes/ft?) This half-hour test is enough to remove completely a l50 microinch platinum coating deposited by a plating technique. The test was repeated 7 times without change in overvoltage. The test was discontinued during the 8th cycle because of a rise in voltage above the 800 millivolts considered allowable in service. The coating was found to have failed by .delamination due to attack on the swaged wire at the tape-wire-electrolyte interface, which is the locus of a current discontinuity not found in actual service.

EXAMPLE 6 A 0.375 inch diameter titanium :rod was wire wound with 3 mil Pt wire, diffusion bonded at 950. C in vacuum, swaged to 0.365 inch diameter including wire, finish bonded, and rolled with two intermediate anneals at 950 C to 0.131 inch thickness without visible damage to the coat. A titanium pipe, 0.540 inch O.D. X 0.065 inch wall was wire wound, swaged to 0.500 inch diameter, finish bonded, and rolled to 0.128 inch thickness without incident, difficulty, or visible surface damage. The fabrication of these two flat pieces has demonstrated that the wire wrapping technique is not limited to designs requiring cylindrical symmetry. One rod was bent at right angles in the coated area without apparent damage.

Coated rods were also easily fabricated using the technique of the invention in such a manner as to supply only an intermittent surface cover to the titanium substrate. Such spacing at intervals has been shown to be desirable in the design of certain types of electrodes used for cathodic protection.

What is claimed is:

1. A process for producing a composite metal material comprising titanium or a titanium alloy substrate to which is bonded a platinum group metal coating which comprises the steps of:

a. wrapping a wire comprised of a platinum group metal around the substrate, said substrate being in the form of a cylinder, to form a continuous taut helical configuration of the wire around the above about 880 C.

3. A process according to claim 1 wherein annealing and re-annealing is performed at a temperature above 880 C.

4. A process according to claim 1 wherein the wire is composed of Pt, Pd, Rh, or Ir.

5. A process according to claim 1 wherein the wire is an alloy of a platinum group metal, said alloy being formable into a wire.

6. A process according to claim 1 wherein the resultant platinum group metal coating is a continuous layer on the substrate.

7. A process according to claim' 1 wherein the resultant platinum group metal coating is an intermittent layer on the substrate.

Claims

2. A process according to claim 1 wherein the resultant material is re-annealed at a temperature above about 880* C.
3. A process according to claim 1 wherein annealing and re-annealing is performed at a temperature above 880* C.
4. A process according to claim 1 wherein the wire is composed of Pt, Pd, Rh, or Ir.
5. A process according to claim 1 wherein the wire is an alloy of a platinum group metal, said alloy being formable into a wire.
6. A process according to claim 1 wherein the resultant platinum group metal coating is a continuous layer on the substrate.
7. A process according to claim 1 wherein the resultant platinum group metal coating is an intermittent layer on the substrate.