CA1114036A - Composite conductor - Google Patents

Abstract

Abstract of the Disclosure Trimetallic electrical conductor comprising iron, copper and titanium or other valve metal has corrosion-resistant and heat-resistant characteristics enabling use in processes involving exposure to aqueous acidic solutions, such as electrolyte baths, and, at other times, exposure to elevated temperatures of 1100°F. or higher, such as needed for treatment of anodic coatings to serve as insoluble anodes in electrolytic processes.

Description

C);36 The present invention relates to electrically conductive materials and more particularly to corrosion-resist-ant electrical conductors.
There are needs for conducting electricity through corrosive environments that are destructive to well-known electrical con~uctor metals such as copper and aluminum. For -~
instance, some processes for electrowinning of metals or gases are performed in sulfuric acid media and others release chlorine gas. For some processes, the electrode, either anode or cathode, is in the form of a coating on a portion of an electrical conductor and in some particularly important processes the anode is a coating comprising an expensive metal such as platinum or a platinum-group oxide, e.g., ruthenium oxide. In view of costs of coating materials and need to pass substantial amounts of electricity between conductor and electrolyte, the coating is usually very thin and of relatively large area, and also considering possible porosity and permeability in the coating, the coat of electrode material on the conductor affords little or no protection .i against corrosion of the substrative conductor by corrosive media in the environment of the coating. Further difficulties in providing satisfactory electrical conductors include needs for heat resistance when the desired coating requires heat ~ treatment for preparation, repair or reconstruction while the `~ ~ coating is in situ and the conductor must endure thermal cy-cling between room temperature and elevated temperatures which, in some important instances, are up to 1500F. or higher-.
It has now been discovered that certain difficulties ~ 30~ stemming from thermal and corrosive effects on electric r~

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- ' ' . ' ' ` ~'14~3fi conductors are overcome, or at least ameliorated, in a special process using a special conductor for electricity.
An object of the invention is to provide an electrical conductor having both heat resistance and corrosion resistance.
Another object of the invention is providing an improvement in processes wherein an electric conductor is exposed, at different times, to aqueous corrosive media and to elevated temperatures.
Other objects and benefits of the invention will become apparent from the following description and the accompanying drawing which shows:
A print of a photomicrograph, at 10qX, of a polished, unetched, cross-sectional portion of a titanium-copper-steel composite cylindrical-shelled embodiment of the electrical conductor of the invention.
The present invention contemplates an electric conductor useful in processes wherein the conductor is exposed to corrosive aqueous media of the kind detrimental to ~ -conductor metals such as copper or aluminum and wherein, at times when the conductor is away from the corrosive media, the conductor is heated and cooled throughout temperature ranges extending from room temperature to elevated tempera- -tures of 1100F. Actually, in many instances, the invention is operable for processes requiring higher temperature heating of the conductor inasmuch as embodiments of the conductor provided by the invention have successfully endured thermal fatigue in repeated cyclic heating and cooling between room temperature and 1550F. and it is contemplated that the conductor satisfactorily endures higher temperatures, e.g., .'' . ~.

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a.~fi 1800F. or possibly even as high as 1900F.
The invention is particularly applicable in, among other things, electrochemical processes wherein an electric conductor carries an anode of platinum-group metal oxide, e.g., a ruthenium oxide coating prepared by plating and oxida-tion methods referred to in U.S. Patent No. 3,763,002, that is sometimes subjected to heating above 1100F. and cooling to room temperature and at other times is exposed to aqueous chloride or sulfate baths for electrochemical use. Other useful anode coatings are made (or replaced or rejuvenated, etc.) by powder metallurgical techniques requiring heating for sintering, oxidation or other thermal processing wherein the heat resistance of the conductor of the invention is beneficial.
The invention provides a heat-resistant and corrosion-resistant electric conductor having a trimetal composite structure comprising a longitudinally extending core of ductile steel (or other ductile metal such as iron nickel or cobalt and alloys containing at least 50% of such metals), a continuous, fluid-impervious sheath of titanium or malleable alloys of titanium~having valve metal character-istics surrounding the length of the core and an intermediate stratum of copper in substantially continuous contact with the interior surface of the sheath which extends between the sheath and the core along the length of the core and in suf-ficient proximity with the core to have support from the ; core. Advantageously, for good conductivity and durability, the conductor, especially a portion thereof for conducting directly to an electrode coating, has a metallurgical diffus-ion bond substantially continuously joining the copper and titanium. Clean metal-to-metal copper-titanium pressure junctions, such as can result from .:,. ~ . : .. . :

~14~;3fi high compressive stresses, with little or no apparent diffu-sion, can be satisfactory; nonetheless, the conductor with the copper-titanium diffusion bond is deemed most reliable for best operability. It is to be understood that durability and physical continuity are most important for the titanium-to-copper junction. The steel-to-copper junction may be a diffusion or pressure bond or some other close-fitting, possibly movable, junction providing support for the sheath.
The steel of the core is a mild steel, e.g., SAE
1010, or other stable ductile steel characterized by good metallurgical stability during the heating desired for the conductor and by ductility such as elongation of about 20% or more in room temperature tensile testing. Special ; alloyed ferritic steels that can maintain a ferritic structure while heated and cooled between room temperature(or below if desired for conductor operation) and high elevated tempera-tures of 1600F. ~r higher, e.g., 1900~F. are advantageous for conductors that are to be heated to such temperatures. The copper o the intermediate stratum, between the core and the sheath, is desirably high-purity copper of the ductile high-conductivity kinds frequently used for electrical wiring or bus bars. Oxygen-free copper is an advantageous metal for the intermediate stratum. The titanium of the sheath is commer-cially pure titanium or a malleable titanium alloy having the known valve metal characteristic of forming a non-conducting oxide when exposed to an oxidizing electrolyte.
Advantageously, for good conductivity characteris-; tics, the cross-sectional configuration of the conductor is circular, and thus has titanium and copper disposed in con-centric cylindrical shells around a cylindrical steel core.

The core diameter should be at least 50%, more advantageously : ~ . . , ' 3fi 70~ or greater, of the outer diameter of the conductor. The copper stratum is sufficiently thick to carry the required current without excessi~e heating or exceeding other current density limits. ~hickness of the titanium sheath is desirably 0.01-inch or greater, up to 0.5-inch, but not more unless for some special situation, such as an unusually severe need for shielding the copper. The circular cross-section with thin cylindrical shells, particularly at portions where trans-- versely outward electric conduction is needed, e.g., where the conductor has a platinum-metal oxide coating to serve as an anode, is advantageous for providing a large electrically conducting surface area in relation to the linear cross-section dimension, e.g., circular cross-section area proportions of about 20~ to 80% steel, 10~ to 50% copper and 10% to 30%
titanium with conductor diameters about 0.1 to 1 inch, or larger, e.g., to 3-inch. The conductor can be formed in cross-sections other than circular, e.g., rectangular, including rolled plates; yet, even so, it is reco~ended that the steel core form at least 50% of the greatest linear dimension of the cross-section, e.g., the diagonal of a rectangular cross section.
Although, in many instances, the trimetallic conductor is made in the form of a long rod, preparation for ; use will often require bending to special configurations, e.g., u-forms. The conductor has good ductility for bending and lS generally satisfactory for crack-free room-temperature bending of 90-degrees or 180-degrees around a .
~ radius of about 2 times the conductor diameter. When the ~ , ~

- conductor has been bent to a curvilinear configuration, the , ~
~ 30 length of the longitudinal path through the conductor, such ~
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~ ~4$3fi as the length before bending from a straight rod form, is still referred to herein as the conductor length. Beneficial characteristics of the conductor also include rigidity and dimensional stability, which aid, inter alia, maintainin~
desired spacing of eIectrodes.
For purposes of giving those skilled in the art a better understanding of the invention, the following examples are given.
Example I

A steel-copper-titanium concentrically shelled composite conductor (referred to as conductor C-l) was prepared using 36-inch lengths of cleaned (pickled and degreased) SAE1010 low-carbon steel rod, phosphorus-deoxidized copper tubing and unalloyed titanium tubing (commercially obtained as ASTM-338-73 Grade 2). The steel rod, 3/8-inch diameter and 36 inches long, was inserted into the copper tubing, which was of 1/2-inch OD (outside diameter) and 3/8-inch ID (inside diameter) and the steel-copper assembly was swaged at both ends, with the ends exposed, and cold drawn to 0.43-inch diameter. Subsequently, after again cleaning, the cold drawn steel-copper rod was assembled in the titanium tube (0.44-inch ID, 0.5-inch OD), -the ends were swaged, and the swaged assembly was heat treated !:
for stress-relief by heating 1 hour in air at 900F., followed by air cooling to room temperature. Then, with a series of drawing and stress-relief steps, the trimetal piece was cold-drawn down rom 0.5-inch to 0.43-inch diameter, thus resulting in a trimetallic steel-copper-titanium con-ductor about 36 inches in overall length. After drawing, the conductor was gi~en another heating at 900F. The conductor : ....... .

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~4~36 was bent 180 degrees around a radius of about one inch, without cracking, to a u-shape about 18 inches deep suitable for use as a conductor for carr~ing insoluble anodes or other electrodes in electrochemical plant operations and was again stress-relief treated at 900F. Then the conductor was given a further treatment of one hour at 1300F. for enhanced bonding of the copper and titanium. The exterior titanium surface was highly satisfactory for subsequent coating treat-ments. Electrical resistance measurements confirmed that the conductor had good electrical conductivity from the copper to the titanium and had good endurance of conductivity and satis-factorily stable resistance characteristics that endured through thermal cycling between room temperature and elevated temperatures up to 1300F. Visual examination after thermal cycling of the conductors confirmed that the conductors were dimensionally stable, had good retention of the u-shapes and satisfactorily resisted any tendencies for warping or other distortion. Micrographic examination of cross-sectional specimens showed satisfactory junctions of steel to copper ; 20 and of copper to titanium.
Results of electrical resistance measurements before and after cyclic heat treatments of conductor C-1 at 1300F. for one hour are set forth in the following Table I
along with results pertaining to further examples. Electrical measurement technlques for obtaining the resistance measure-ments set forth in the Table were to connect the two testing electrodes of a Xelvin bridge to exterior portions of the titanium sheath, with the electrodes spaced a known distance ,~ apart (usually about an inch fro~ each end of the conductor), then energize the bridge and take the resistance reading.

' 3fi When referring to the resistance measurement results, it is to be understood that the importance of the results is the stability and the close comparability, and the small variation, of the readings on an~ one conductor. The conductors of the examples differ from each other in various respects, e.g., dimensionsr and the averages of resistance readings also differ from o~e conductor to the other. The substantial constancy of the resistance readings after thermal cycling indicates that the titanium and copper remained in good electrically conductive contact and provided for the copper to carry most of the electric current, thus indicating success in overcoming delaminating tendencies, e.g., differential expansion, that might have resulted in higher resistance.
Successful durability of good electrical conductivity characteristics of conductor C-l was further demonstrated, .
following the six thermal cycles to 1300F., with voltage-drop profile readings set forth in Table II, which were taken at different distances along the titanium sheath while a 60 ampere current flow was being passed through the conductor from direct-current voltage source contacts attached on the titanium sheath, near each end of the conductor, at a current-flow distance of 32 inches from each other. The profile read-ings evidence that the conductor resisted delamination and provided a continuous path for titanium-to-copper-to-titanium conduction of the applied current.

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L4G~36 T~3~E I
, Overall Resistance, Milliohms Conductor C-l C~2 C-3 Number (1300F. (1300F. (1550F.
of Reheats Reheat) Reheat) Reheat) 0 _ 2.4 1 ` 3.5 2.9 1.4

2 3.0 2.8 1.2

3 3.1 2.8 1.5

4 4.0 2.9 1.5 3.6 3.~ 1.4 . 6 3.9 3.1 1.1 ~, .
7 - 3.1 1.1 , 8 - 3.7 1.
:~ 9 - 4.1 1.4 ..
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I TABLE II
'f. : Current : ~ Conducting Voltage Distance Drop (inches) (millivolts) ~ ~ :

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3fi Exam~le II

A tltanium-copper-steel composite conductor (C-2) was prepared by techniques comprising providing an annealed and cleaned 6-foot length of titanium tubing of 0.5-inch OD
and 0.032-inch wall thickness, inserting into the tube an equal length of cleaned copper-coated steel with 0.430-inch OD and 0.030-inch copper coating (commercially available under the name Copperweld/ trademark of Copperweld Steel Company) and cold-swaging both ends of the titanium tube on the copper-clad steel core. Then the cold swaged assembly was heated in air for one hour at 1400F., air cooled, cold drawn to 0.430-inch on a hydraulic drawbench at a speed of about one inch per second, and heated one hour at 1400F. Thereafter, the trimetallic conductor was bent to u-shape with bends of about l-inch radius and subjected to successive electrical resistance measurements and intermediate heat treatments of one hour each at 1300F., the electric resistance readings being taken with a conductor ~ -length of 72 inches. Resistance measurement results are set forth in ~able I.

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3fi Example III

In another example (C-3), a 24-inch length of steel-copper-titanium conductor was prepared by assembling, swa~in~, drawing and u-bending copper-coated steel and titanium tubing as referred to for Example II. Heat treatments were: 1550F/l hr. between swage and draw; 900F/l hr between draw and u-bend;
then 1550F/l hr before resistance measurements. Results of electric resistance measurements of the conductor, with bridge contacts on the titanlum spaced apart by a current flow distance of 24-inches,which confirm good stability of electri-cal conductivity characteristics, are set forth in Table I.
The accompanying drawing shows a print of a photomicrograph at lOOX magnification of a polished unetched cross-sectional specimen taken from conductor C-3 after ~,~ thermal cycling ten times to 1550F. Electron microprobe analysis on a cross-section specimen showed the copper-titanium diffusion zone comprised layers with varying copper/
titanium ratios.
Both the resistance measurements and the subsequent microexamination confirm that the conductor maintained a satisfactory copper-titanium diffusion bond sufficiently for -good electriaal conductivity, even though some partial de-lamination was found at a few other places in the copper-titanium junction.

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;. ,: . . -~4~36 Exam~le I~

In another example (C-4), 12-inch lengths of cleaned 0.25-inch diameter SAE 1010 steel rod, 1/4-inch ID/3/8-inch OD phosphorus deoxidized copper tubing and .43-ID/.50-inch OD commercial unalloyed titanium tubing were assembled, the steel in the copper and the titanium around the copper, and the three were cold swaged and drawn, with stress-relief at 1300F., to result in a 0.385-inch diameter trimetal composite rod of about one foot length. After drawing, the rod was cut -- 10 into two portions of about 6-inch length and a cross-section specimen was taken and set aside. One portion of the drawn bar was heated 1 hour at 1300F. and the second for 5 hours at 1300F. and both air-cooled. Cross-section (diametrically -transverse) specimens were taken from the center of each of the heat-treated portions and were polished for micrographic inspection, as was the specimen that had been taken prior to heat treatment. Examination of the specimens at 75 and 1000 X magnification showed: the as-drawn specimen(a) had clearly discernible boundaries and gaps between the different metals and no signs of intermetallic diffusion; the l-hour/1300F. heat-treated specimen(b) had an excellent diffusion bond joining the copper and titanium and about 70 i or 80 percent of the steel in contact with the copper had , an open gap between; the 5-hour/1300F. heat treated specimen (c) showed 100% of the copper-titanium junction continuously ~-diffused for about double the transverse extent of specimen-b ; and about the same as specimen-b at the steel-copper junction.
The heat-treated copper-titanium junctions were definitely good for electrical conductivity and for enduring thermal ~ 30 cycling, specimen-c being deemed better in this latter respect.
!~ Both specimens showed the steel disposed satisfactorily for ; 12-.

- , . ~ : . :.. : , 41Q~fi supporting the copper and titanium. And in both, the outer surface of the titanium was sufficiently remote from the diffusion zone to provide that the titanium retained its characteristic corrosion resistance.
In view of the results set forth in the Table, it is noted that range of variation of the resistance of each of the conductors when subjected to thermal cycling, 6 or 10 cycles, was restricted to a small proportion of about 10% or less of the mean value of the conductor resistance (after each cycle).
~;For carrying the invention into practice, it is `~contemplated that the metals of the trimetallic composite conductor can be varied within the concept that the core is a steel or other iron-group metal (iron, nickel, cobalt) characterized by phase stability and an average thermal expansion coefficient of about 3.7 to 10.4, advantageously

5.8 to 7.7, x 10 6 inch per inch per F in temperature ranges of room temperature to 1100F or higher desired Re (including heat treatment) of the conductor, the inter-mediate~metal~is ductile copper characterized by good electr~ical conductivity, e.g., 90%IACS or more at room temperature, and the sheath is ductile titanium. If the expense is not a prohibition, the core can be a special ron~or~iron-group alloy having a composition controlled to charactèrize the core with a specially desired expansion intermedlate between those of the copper stratum and the vaive~metal sheath. For instance, nickel-iron or nickel-oobalt-iron alloys may be utilized for the core metal.
Y t, along with the undorstanding that there .. : .,:. . . , : , can be alloy variations in the compositions for the iron .
core, copper stratum and titanium sheath~ it is emphasized as important that the conductor have the core of iron-group metal and that the copper stratum and the titanium sheath of the conductor meet together with a direct copper-to-titanium interface junction. Trials of different arrangements, contrary to the present invention, with a solid copper rod (without an iron core) in a titanium sheath, and with nickel and chromium plating ~etween a titanium sheath and a copper tube surrounding a steel rod core resulted in unsatisfactory results of poor bonding, delamination and excessive variation of electrical resistance characteristics.
In an illustrative example of a process wherein the ; conductor is used at relatively low temperatures such as about room temperature during some periods and is heated to elevated temperatures of 1100F. or higher during other periods of time, a steel-copper-titanium conductor made according to Example III is provided with an exterior coating of ruthenium oxide anode material adhering to a preselected length of the titanium sheath by techniques comprising heating the coating material in situ on the conductor to a tempera-ture o 1~00F. or higher, e.g., 1200F. or 1350F. A portion of the conductor including the oxide-coated length is assembled as an insoluble-anode carrier in an electrolytic cell with a copper cathode and an acidic sulfate electrolyte for electro-winning of copper. The conductor serves satisfactorily, resist-ing corrosion and conducting electricity, to transmit current -~ between a direct-current power source and the anodic coating while the cell functions in electrowinning operations for desired operating periods totaling 1000 or more hours. Sub-sequently, the used conductor is removed from the cell and undergoes heating to 1100F. or higher during a refurbishing .. , .. , . ~ . . , . . . . ~ . . . . . . .

of the anodic coating, and thereafter is cooled to roomtemperature. Then, the anode-carrying portion of the con-ductor is again assembled in an electrolytic ceIl and serves satisfactorily in electrowinning operations. The cycle of use at room temperature, heating to elevated temperatures and cooling to room temperature is repeated 5 or 10 times or more with good endurance experience of stability of electrical conductivity of the conductor.
The present invention is particularly applicable in providing heat- and corrosion-resistant electrical conductors for electrochemical processes, including electro-winning of metals and gases, and is specially applicable in providing conductors for transmitting electric current for insoluble (sometimes called inert) anodes in electrolyte baths for electrowinning of nickel or copper or zinc. More-over, the invention is generally applicable in providing electric conductors for electrowinning of other metals, e.g., zinc, or gas products, e.g., chlorine or chlorates, and -i for other power transmission, e.g., overhead power lines, power conduction through corrosive chemical plant environment, and furnace wiring. Furthermore, the inYention can be applied in providing anodlc protection, such as for ship hulls or other vessels, and is also applicable for cathodes in the production of electrorefining starter sheets.
The platinum-group coating materials may be metals, alloys, oxi~es or othex compounds of the group platinum, palladium, ruthenium, iridium and osmium.
Although the present invention has been described in conjunction with preferred embodiments, it is to bè under-stood that modifications and variations may be resorted towithout departing from the spirit and scope of the invention.

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A trimetallic heat-resistant and corrosion-resistant electric conductor, suitable for use in support-ing and conducting electric current to an electrode in an acidic electrolyte cell, comprising a continuous fluid-impervious sheath composed of a valve metal selected from the group titanium and malleable alloys of titanium having the known value metal characteristics of forming a non-conducting oxide when exposed to an oxidizing electrolyte, a hollow metal stratum consisting essentially of copper disposed in substantially continuous contact with the interior surface of the sheath, and a core of ductile metal selected from the group iron, nickel, cobalt and alloys composed at least 50% by weight thereof disposed in the interior of the hollow copper stratum and in proximity to the copper sufficiently for providing mechanical support to the copper stratum.

2. A conductor as set forth in claim 1 having on an exterior portion of the titanium sheath a coating of platinum-group material.

3. A conductor as set forth in claim 1 having a diffusion bond joining the sheath and the hollow stratum.

4. A conductor as set forth in claim 1 wherein the cross-sectional configuration is circular with the core formed as a solid cylinder and the hollow stratum and the sheath each formed as hollow cylinders and dis-posed concentrically around the core.

5. A conductor as set forth in claim 4 wherein the cross-sectional area proportions are 20% to 80% core, 10% to 50% stratum and 10% to 30% sheath.

6. A conductor as set forth in claim 1 wherein the cross-sectional configurations of the core, stratum and sheath are rectangular.

7. A conductor as set forth in claim 1 wherein the core metal is mild steel.

8. A conductor as set forth in claim 1 wherein the hollow stratum metal is copper characterized by elec-trical conductivity of at least 90% IACS (International Annealed Copper Standard) at room temperature.