Note: Descriptions are shown in the official language in which they were submitted.
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
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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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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
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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
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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
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~ 30 length of the longitudinal path through the conductor, such
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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.
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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
..
. - 3.6 1.1
. .
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I TABLE II
'f. : Current
: ~ Conducting Voltage
Distance Drop
(inches) (millivolts)
18 16
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17
22 18
24 19
26 20
28 21
23
32 26
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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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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-
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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
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36
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
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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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