U.S. patent number 3,616,354 [Application Number 04/850,741] was granted by the patent office on 1971-10-26 for method for installing cathodic protection.
Invention is credited to Gordon Ian Russell.
United States Patent |
3,616,354 |
Russell |
October 26, 1971 |
METHOD FOR INSTALLING CATHODIC PROTECTION
Abstract
A method for providing cathodic protection for a metallic
structure in contact with the ground, for example, a pipeline,
comprises disposing a suitable anode at depth in a borehole so that
the anode is disposed generally opposite a preselected ground zone
or stratum of relatively low electrical resistivity. A liquid
electrolyte which is operatively compatible with both the
surrounding ground and with the anode is provided in the borehole
and may comprise naturally occurring ground water which may
optionally be modified, for example, by the addition of an
ionizable salt.
Inventors: |
Russell; Gordon Ian
(Burlington, Ontario, CA) |
Family
ID: |
31947228 |
Appl.
No.: |
04/850,741 |
Filed: |
August 18, 1969 |
Current U.S.
Class: |
205/724; 205/738;
205/739 |
Current CPC
Class: |
C23F
13/04 (20130101); C23F 13/12 (20130101); E21B
41/02 (20130101); C23F 13/06 (20130101); C23F
2213/22 (20130101) |
Current International
Class: |
C23F
13/02 (20060101); C23F 13/00 (20060101); E21B
41/02 (20060101); E21B 41/00 (20060101); C23f
013/00 () |
Field of
Search: |
;204/147,148,196,197 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Underground Corrosion" NBS Circular No. 579, 1957. pp.
186-188..
|
Primary Examiner: Tung; T.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of my copending
application Ser. No. 448,334, filed Apr. 15, 1965, now abandoned.
Claims
What I claim as new and desire to protect by Letters Patent of the
United States is:
1. A method for providing cathodic protection for a metallic
structure in contact with the ground, which method comprises
providing a borehole in the ground, which borehole extends
downwardly through a plurality of ground zones of different
electrical resistivities, identifying a ground zone of relatively
low electrical resistivity at a depth of at least 20 feet within
said borehole, ensuring the presence within said borehole of an
aqueous liquid electrolyte of a water-soluble chloride, positioning
an anode freely in said electrolyte in said borehole generally
opposite said ground zone of relatively low electrical resistivity,
and connecting said metallic structure and said anode to a source
of direct electrical current whereby a positive voltage is applied
to said anode with respect to said metallic structure.
2. A method as claimed in claim 1 wherein said electrolyte
comprises naturally occurring ground water.
3. A method as claimed in claim 2 which comprises the step of
sampling and analyzing said naturally occurring ground water.
4. A method as claimed in claim 3 which additionally comprises the
step of chemically modifying said naturally occurring ground water
by the addition of an ionizable salt of a water-soluble
chloride.
5. A method claimed in claim 1 which additionally comprises
periodically removing said electrolyte from said borehole for
modification thereof, and returning a resulting modified
electrolyte to said borehole.
6. A method as claimed in claim 1 in which said anode is disposed
within said borehole upwardly of a lower end thereof, whereby solid
materials formed during the flow of electrical current through said
electrolyte settle below said anode.
7. A method as claimed in claim 6 in which said solid materials are
periodically removed from said borehole.
8. A method as claimed in claim 1 which comprises disposing a
plurality of anodes in said electrolyte in said borehole opposite
respective ground zones of relatively low electrical
resistivities.
9. A method as claimed in claim 1 which comprises connecting said
anode to said source of direct electrical current by means of an
electrical conductor having an external covering of an electrically
insulting material which is selected so as to be substantially
chemically nonreactive with said electrolyte.
10. A method as claimed in claim 1 in which said anode is selected
from the group consisting of graphite, high silicon iron alloys,
lead-silver alloys, lead-platinum alloys, platinized-titanium,
platinized-tantalum and platinized-niobium.
11. A method as claimed in claim 1 which comprises disposing said
anode at a depth in said borehole of from 50 to 500 feet.
12. A method as claimed in claim 1 which comprises disposing a
porous liner in said borehole, which liner is formed of a
substantially chemically nonreactive material.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method for providing cathodic
protection for metallic structures in contact with the ground and
to installations comprising structures protected by such a
method.
The corrosion of underground structures such as pipelines and steel
substructures and of other metallic structures in contact with the
ground is a very serious and extensive problem and numerous
proposals have been made for the provision of cathodic protection
for such structures to prevent electrochemical corrosion
thereof.
Existing procedures for providing such cathodic protection can
conveniently be classified into two types. The first type involves
the use of a sacrificial anode buried in the ground some distance
from the structure to be protected. Such sacrificial anodes are
formed of metals such as magnesium and zinc which are more
electronegative than the structures being protected.
Such sacrificial anodes are consumed relatively rapidly by
electrochemical reaction while corrosion of the metallic structure
such as a pipeline is prevented. Since such sacrificial anodes must
be renewed relatively frequently, it is generally considered to be
totally uneconomical to dispose such anodes at depth in the ground
even though their disposition near the surface of the ground, for
example at depths down to 30 feet, can present some relatively
serious problems which are fully recognized by those conversant
with corrosion technology and engineering.
The relatively high operating and replacement costs involved in the
use of a necessarily large number of large sacrificial anodes to
provide adequate protection has led to the use of so-called
permanent or impressed voltage-type anodes. In this latter
procedure, a voltage is externally impressed from a suitable source
of direct electrical current between the structure to be protected
and an anode buried in the ground so as to ensure that the flow of
current between the anode and the structure through the ground is
always such that the structure forms the cathode and its corrosion
is consequently prevented.
Since, in this second procedure, reliance is place on the
externally impressed current, it is not necessary to use a highly
electronegative anode. Consequently, the anodes which are presently
used in this system are normally formed from electropositive metals
or from graphite presenting the best possible balance between such
factors as capital cost, maintenance cost, operating effectiveness
and anode life. Merely, by way of illustration there may be
mentioned the use in this second procedure of anodes formed of
graphite, high silicon cast iron, lead-silver alloys and
lead-platinum anodes as well as of anodes comprising relatively
thin platinum coatings on cores formed of metals such as titanium,
tantalum and niobium.
As already indicated, when corrosion-protection anodes are disposed
close to the surface of the ground, their operation is frequently
relatively unreliable and this is as true in the case of the
permanent-type anodes as it is in the case of the aforementioned
sacrificial anodes. For example, the anodes may be adversely
affected both directly and in their protection performance by such
factors as variations in the level and composition of surface water
ground pollution resulting from the presence of industrial
effluent, freezing and numerous other factors. Another important
problem which arises when a corrosion-protecting anode is disposed
near the surface of the ground is that the electrical current flow
path between such an anode and the structure being protected is
relatively direct and consequently a large number of relatively
closely spaced anodes is needed if a large structure such as a
pipeline is to be protected adequately.
It is already well known that the necessary interanode spacing for
the protection of large structures such as pipelines can be
considerably reduced by disposing the anodes at greater depths,
such as 30 feet or more, in the ground. With such presently known
deep anode systems, the current flows from the deeply buried
permanent anode into the surrounding stratum and then upwardly to
the structure to be protected. Generally such upward current flow
will involve the passage of the current through strata of higher
electrical resistivity than that immediately surrounding the anode,
and this interposition of a higher resistivity stratum is
beneficial in extending the distance or range over which cathodic
protection is provided. Consequently, in the case of pipelines, the
boreholes containing the anodes can be spaced apart along the line
at greater separations.
In order to provide effective electrical coupling between such a
deep anode and the surrounding strata, it is customary to provide a
solid backfill above the anode in the borehole. For example, the
use of coke breeze for this purpose is very widespread. Numerous
proposals have been made for improving the corrosion protection
provided by such systems, and for reducing the capital and
operating costs of such systems. The attainment of effective
protection on a desired economic basis is still, however, plagued
by numerous difficulties.
The current outputs of such permanent anodes with solid backfill
will in some cases progressively decrease, sometimes rapidly and
sometimes somewhat gradually and, since the protection afforded by
such a system is dependent on the anode-structure current flow, the
protection provided by such a system will inevitably decrease when
the current flow decreases in this manner. When such a situation
arises, as it might after even only a few weeks operation, there is
no alternative but to install additional or new anodes or to resort
to the use of higher and higher anode voltages. In order to reduce
the need for frequent replacement of such deep well anodes and in
order to obtain adequate flow of protective current to the
structure being protected, it has become customary to dispose a
relatively long string of such anodes in each such borehole. This
is, of course, very costly.
It is a principal object of the present invention to provide a
method for providing cathodic protection for a metallic structure
in contact with the ground which method is characterized by giving
improved protection as indicated by an economic analysis of its
operation.
Another object of this invention is to provide a novel method for
providing cathodic protection by the use of which method
operational utilization of a single anode or of a set of anodes in
a single borehole may be significantly prolonged.
Yet another object of this invention is to provide a novel method
for providing cathodic protection in which method steps may be
taken at any time after installation of the anode or anodes in the
borehole to inspect, rejuvenate or otherwise treat the installation
to avoid, counteract or otherwise modify detrimental conditions
that develop during its operation.
Another object of this invention is the provision of a method for
providing cathodic protection by which method improved
anode-structure current flow may be obtained without the use of
particularly long strings of anodes in each borehole.
A further object of this invention is to provide a method for
providing cathodic protection by means of which the anodes which
are utilized can readily be removed from the boreholes, and in
which method any undesirable materials formed in such a borehole
during operation of the system can be removed relatively facilely
therefrom.
Another important object of this invention is to provide an
installation comprising a metallic structure in contact with the
ground which structure is cathodically protected against corrosion
by the method of the invention.
Other objects will become apparent as the description herein
proceeds.
SUMMARY OF THE INVENTION
The present invention is based on the finding that, by the use of
the aforementioned permanent or impressed voltage-type of anode in
a deep well bore together with the presence of an aqueous liquid
electrolyte meeting certain requirements around such an anode in
the bore, it is possible to obtain exceptionally effective
corrosion protection for a metallic structure in contact with the
ground provided that the electrolyte and anode which are used and
the position of such an anode in the borehole comply with certain
critical parameters which will be explained in greater detail
hereinafter.
In its broadest scope, the present invention provides a method for
providing cathodic protection for a metallic structure in contact
with the ground, which method comprises providing a borehole in the
ground, which borehole extends downwardly through a plurality of
ground zones of different electrical resistivities, identifying a
ground zone of relatively low electrical resistivity at depth
within said borehole, ensuring the presence within said borehole of
an aqueous liquid electrolyte compatible with said ground,
positioning an anode in said electrolyte in said borehole generally
opposite said ground zone of relatively low electrical resistivity,
said anode being operatively compatible with said electrolyte, and
connecting said metallic structure and said anode to a source of
direct electrical current whereby a positive voltage is applied to
said anode with respect to said metallic structure.
In order to facilitate comprehension of the various factors which
determine the method of carrying out this invention, reference will
first be made to the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
The accompanying drawing is a somewhat schematic illustration of a
cathodic protection installation in accordance with the invention
and also shows a typical resistivity log as obtained during the
carrying out the method of the invention and as used for
determining the correct positions for the anode or anodes in the
borehole.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
There is shown somewhat schematically in the accompanying drawing a
metallic structure 10 such as a pipeline which is to be protected
in accordance with the invention against corrosion. Such corrosion
can be caused, for example, by stray currents passing through the
ground 11. For convenience, the description hereinafter will be
restricted to the protection of such a pipeline but it should be
understood that the invention is widely applicable to the
protection of other metallic structures which are in contact with
the ground and which may, for example, be partially or completely
buried in the ground.
The cathodic protection installation shown comprises a borehole
generally indicated at 12 which extends downwardly through various
ground strata or zones which are shown as comprising upper ground,
strata 13, rock strata 14 and substrata 15. Disposed generally
axially within the borehole 12 below the level of the rock strata
13, there are supported a pair of impressed voltage-type anodes
which are shown schematically and diametrically enlarge at 16 and
17.
The anodes 16 and 17 are interconnected by an insulated electrical
conductor or cable 18 while an insulated conductor or cable 19
connects the upper anode 16 to a suitable source 20 of direct
electrical current. Such a source 20 is conventional and may, for
example, comprise a number of storage batteries or more usually a
rectifier supply unit. Since such a source is conventional, it will
not be described herein in greater detail. It will be understood
that in the installation shown, the cable 19 also serves to support
the anodes 16 and 17 at the desired depths in the bore hole 12 and
that this cable 19 will be suitably insulated. Separate anode
suspension members may, of course, be used.
It will also be noted that in accordance with a preferred feature
of this invention, the borehole 12 is provided with a liner 22
which is formed of an electrically insulating material. An
important function of this liner 22 is to prevent plugging of the
bore hole by collapse of its walls. The lower portion of the liner
22 is perforated as shown at 23 to permit ion flow therethrough and
it will be noted that such perforations extend from the bottom of
the liner 22 to a level above the upper anode 16. The installation
shown is essentially completed by a cap 24 and a vent pipe 25, the
purpose of which will become apparent as the description herein
proceeds.
It should also be noted that an aqueous electrolyte 26 is provided
within the borehole 12. The nature of this electrolyte and its
purpose will be described hereinafter in greater detail but, before
proceeding with such description, it should perhaps be stressed
that the installation herein described with reference to the
accompanying drawing differs from known deep anode systems for
providing cathodic protection in that it involves the use of
permanent (or impressed voltage-type) anodes disposed in a deep
borehole, such anodes being surrounded by an aqueous electrolyte.
As already explained, this differs from the known use of deep well
anodes in which it has been standard practice to provide a solid
backfill such as coke breeze around and above the anodes in an
attempt to obtain effective electrical coupling and larger anode
area between the anodes and the surrounding strata.
In order to obtain optimum electrical coupling for the anode, or
each anode, the positioning of the anodes within the borehole is
carried out in accordance with a very important feature of this
invention so that each such anode is positioned generally opposite
a ground zone or stratum of relatively low electrical resistivity.
The second step, therefore, in the method of the invention after
having obtained or drilled a suitable borehole in the ground
involves the identification of one or more suitable low resistivity
ground zones. Any appropriate resistivity logging method may be
used for this purpose and a typical resistivity log obtained for
this purpose is illustrated in the accompanying drawing. From this
log, it will be seen that the strata resistivities for the
particular borehole increase gradually downwardly from the ground
surface through the ground strata 13 to a depth of about 80 feet.
Between about 80 and about 110 feet, there exists in the rock
strata 14 a very pronounced low resistivity ground zone while the
substrata 15 extending between the 100 and the 130 foot depths show
somewhat higher resistivities. Below a depth of about 135 feet, the
resistivity varies only slightly about a relatively low average
value.
In this particular instance in order to obtain optimum anode-ground
coupling, i.e. maximum current flow from the anode into the ground
for a given voltage applied to the anode, such an anode will be
placed in accordance with this invention at a depth of from about
90 to 105 feet. A 15-foot anode 16 (as shown) could conveniently be
used at this position. If desired, for example, to obtain
additional current flow through the ground to the pipeline 10,
further anodes, such as anode 17 having a length of 20 feet, may be
positioned at depths greater than about 135 feet.
In general, it is preferred to place the, or each, anode generally
opposite a relatively low resistivity ground zone which is itself
disposed below a ground zone of higher resistivity. As already
indicated, such anode disposition will lead to protection of the
metallic structure such as the pipeline 10 to a greater distance or
range from the borehole 12.
The length of each of the anodes and the number of anodes to be
used in each borehole such as borehole 12 in the method of this
invention, will be determined by several factors, amongst which
there may be mentioned the primary requirement of establishment of
an adequate anode-structure current flow to give the necessary
corrosion protection. By placing the anodes opposite preselected
ground zones of relatively low electrical resistivities and by
conforming with other important requirements of the invention still
to be explained, it is, however, possible to obtain effective
protective current flow without resorting to the use of an
excessive number of anodes or to the use of lengthy anode strings
as is generally the case with the existing method in which solid
backfills are used.
As hereinbefore indicated, the present invention is restricted to
cathodic protection methods and installations in which permanent
anodes are disposed "at depth" in a borehole. Although it is
somewhat difficult to provide a precise definition for the phrase
"at depth" when used herein and in the appended claims, this phrase
is intended to make it clear that the invention involves disposing
the anodes at depths similar to those used in the well-known deep
anode system. Such systems can perhaps best be distinguished from
the surface anode type by the use of a somewhat negative-type
definition. The use of sacrificial anode as the sole source of
protective anode-structure current is precluded from the scope of
this invention since the latter involves the positioning of anodes
at such depths that, if in fact only sacrificial anodes were used,
the total anode area, i.e. the anode size and/or the number of
anodes, required would be impractically high so that such use would
be economically infeasible.
In general, the anodes used in the method of the present invention
will be disposed at depths of at least 20 feet and more
particularly at depths of more than about 30 feet. Under the
majority of practical circumstances, it will be economically
infeasible to position anodes at depths greater than 500 feet since
at such depths the additional cost for drilling the borehole
becomes too high. In such a case, increased anode area would
frequently also be required to provide adequate anode-structure
current flow and the use of such great depths is, therefore,
generally contra-indicated. Nevertheless, it is possible that under
somewhat unusual circumstances, it could be desirable to use anodes
at such great depths. From an economic and practical point of view,
the anodes used in the method of the present invention will
normally be disposed at depths in the range of from about 50 to
about 150 feet.
Another important feature of the method of the invention involves
the step of ensuring the presence within said borehole of an
aqueous liquid electrolyte. Normally, an aqueous solution will be
present in the borehole due to the naturally occurring ground water
existing in the ground through which the borehole is drilled. This
naturally occurring ground water may itself constitute the aqueous
electrolyte required in accordance with the method of the
invention. Alternatively, such naturally occurring ground water may
be chemically modified to provide some effective operation as will
be explained hereinafter in greater detail. In the relatively rare
event that no naturally occurring ground water is present in the
borehole, a separately prepared liquid electrolyte may be
introduced thereinto to provide the required liquid environment for
the anode. Such electrolyte will be chosen so as to be operatively
compatible with the electrolyte naturally present in bound form in
the surrounding ground zones as identified by analyzing samples
thereof. In such a case, it will be necessary to seal any porous
strata below the anodes by using a suitable liner to prevent
drainage of the electrolyte from the borehole. In order to
determine the most suitable and practical liquid electrolyte to be
used in the method of the invention, attention must be given to
such factors as the chemical nature of the surrounding ground
zones, the chemical nature of any naturally occurring ground water
present in the borehole, the chemical nature of the anode or anodes
being used, and the chemical resistance of the electrical
insulating material of the cables 18 and 19.
As hereinbefore indicated, these factors are set down as essential
features of this invention by requiring that the aqueous
electrolyte is operatively compatible with the surrounding ground
and that the anode, or each anode, is operatively compatible with
the aqueous electrolyte. As was the case with the phrase "at depth"
hereinbefore discussed, it is difficult to set down a general
definition of the phrase "operatively compatible" which will apply
to all possible combinations of the several variables.
In general it can, however, be indicated that the would of a
suitable operatively compatible aqueous liquid electrolyte and of a
suitable anode material will be determined on the basis of the
chemical composition of any naturally occurring ground water
present in the borehole. The material of the anode will be such
that the anode will operate effectively in the presence of such
ground water. For example, if such ground water contains a
relatively large proportion of chloride ions, an anode will be used
which is sufficiently resistant to attack by chloride ions or by
chlorine under the actual operating conditions. In such a case, the
use of an unmodified high silicon cast iron anode would be
completely contra-indicated. Similarly, if a naturally occurring
ground water present in the borehole contained a significant
proportion of sulfate ions, anodes of materials which cannot be
used with such electrolytes, for example, graphite and cast iron
anodes, would not be the preferred materials from the point of view
of anode performance. When naturally occurring ground water is
present in the borehole, such water will be sampled and analyzed to
provide data on which the selection of a suitable anode material
can then be based. Another important factor affecting the choice of
a suitable anode material is the anode-electrolyte potential drop
to which the anode will be subjected in use. The permissible
maximum value for such potential drops for different anode
materials with which an oxide or other protective film must be
maintained on the anode or on its supporting core are known or are
readily determinable for the more common permanent anode materials
and these values will not be exceeded during operation of the
method in accordance with the invention using this particular type
of anode. The values of the actual anode-electrolyte potential
drops are, however, also dependent on the voltage applied to the
anode and this is in practice in turn dependent on the
anode-structure current which is required so as to give adequate
corrosion protection, a higher anode-structure current requiring a
higher anode voltage and consequently, with other conditions
unchanged, a higher anode-electrolyte potential drop. In view of
the operating limits on such potential drops, it is necessary with
the conventional solid back-filled systems to resort in such cases
to the use of larger and greater numbers of anodes to obtain
adequate anode-structure current.
An important feature of the present invention is that it permits
ready and continuous access to the electrolyte from the top of the
borehole so that this electrolyte can be chemically modified if and
when required to improve the anode-electrolyte interaction, for
example, to reduce the anode-electrolyte potential drop thereby
frequently avoiding the need for the use of larger or more anodes
as required in solid back-filled installations to ensure continuing
effective performance.
One particularly important manner in which the aqueous electrolyte
may be modified to improve the anode-electrolyte interaction is by
the addition thereto of an ionizable salt. Such addition can be
very effective in reducing the anode-electrolyte potential drop and
may incidentally improve the electrolyte conductivity thereby
permitting greater current flow without requiring the use of higher
anode voltages or areas. Any such addition of an ionizable salt to
the electrolyte in this manner will be governed by the chemical
nature of the existing "electrolyte." Any such salt which is added
must be chemically compatible with the surrounding ground, i.e.
there must be no undesirable chemical interaction therebetween, and
such added salt must have no adverse effect on anode behaviour, for
example, it must not prevent the formation of any protective oxide
films which are required on the metal anodes or their supporting
cores for effective noncorroding anode operation. In many cases and
particularly where chloride ions are present in naturally occurring
ground water, a water-soluble chloride such as sodium chloride or
potassium chloride can frequently be added with beneficial results
to such an electrolyte comprising a naturally occurring ground
water.
Other electrolyte additives which may be mentioned by way of
further example are inhibitors which will serve to reduce anode
corrosion, sequestering or other agents which will inhibit the
formation of undesirable precipitates on the anode and additives
which will facilitate the liberation of any gaseous products which
are released at the anode.
The expression "operatively compatible" when applied to the
electrolyte and to the anode will be understood by those conversant
with corrosion technology. An important distinction should,
however, be drawn between this statement that the meaning of the
expression will be obvious to those skilled in the art and the
claims that the specific use of electrolytes and anodes which are
operatively compatible with each other and that the electrolyte
will be compatible with the surrounding ground is both novel and
inventive. The distinction is fine but exceedingly important. The
specific selection of electrolyte/anode combinations with the
nature of the surrounding ground in mind is a completely novel step
in the installation of deep well permanent anode cathodic
protection systems and, in addition to such novelty, when
considered with the other important novel features of the
invention, leads to very significant operating advantages which
will be identified in greater detail hereinafter and which cannot
be obtained with the conventional solid back-filled systems.
Another important feature concerning the chemical nature of the
electrolyte is that is should be chemically nonreactive with
respect to both the liner member 22 and the insulation on the
cables 18 and 20. Again numerous factors come into play. There may,
for example, be mentioned the costs of the different materials
available for such linings and insulation, and the chemical nature
of any naturally occurring ground water electrolyte. The properties
of the selected materials will in turn apply certain restrictions
to what materials can be added to the electrolyte.
The more important advantages presented by the method of the
invention will now be summarized, reference also being made at the
same time to some other important features of the invention.
One very important advantage of this invention is that it permits
the same degree of corrosion protection to be obtained as with
conventional solid back-filled anodes but with the use of smaller
anodes or with the use of a smaller number of anodes. For example,
in one particular bore hole, generally equivalent initial
anode-structure currents were obtained by the use of two anodes,
one 20 -feet long and the other 5 -feet long, in a naturally
occurring ground water electrolyte with the small addition of
sodium chloride, on the one hand, and by the use of a 120 -foot
anode string using a solid coke breeze backfill. The advantage
results primarily from the improved anode-ground coupling resulting
partially from the disposition of the anodes in optimum positions
with reference to the ground zones and partially from the
considerably improved contact obtained between the ground and the
anode due to the interposition of a continuous liquid electrolyte
of good conductivity in distinction to that obtained with the solid
backfill in which interparticle contact can become highly resistive
due to the reactions taking place therein.
Consequently, the capital cost of the anodes is lower and the use
of such smaller anodes permits such anodes to be disposed in
optimum positions more readily. In turn, this leads to greater
utilization of the increased protection range obtainable by making
use of the spreading effect of an overlying high resistivity ground
zone as already considered herein.
Another important advantage of this invention is that the
progressive loss of anode-structure current during use which occurs
all too frequently with the conventional solid back-filled systems
can be avoided or, if it does occur, can be rectified in a
relatively simple manner. The illustrative example of the use of a
120 -foot anode string mentioned above illustrates this point. In
use, the current obtained from that string decreased progressively
to such an extent that the anodes had to be abandoned after about a
year of operation whereas the installations of this invention as
hereinbefore exemplified by the two-anode arrangement functioned
successfully for several years with insignificant loss of
anode-structure current. Loss of anode-structure current flow due
to the entrapment of gaseous product bubbles around the anode is
one cause of loss of protective current flow in the existing solid
back-filled systems. This problem does not arise with the method of
this invention since, due to the presence of a liquid environment
around the anode, such bubbles are free to escape upwardly for
venting to the atmosphere through the vent pipe 25. Similarly solid
precipitates formed during operation are free to fall to the bottom
of the borehole as shown at 27 in the drawing. In solid back-filled
systems, such precipitates frequently contribute to anode
deactivation. This advantageous difference is particularly
important under circumstances under which siliceous precipitates
are likely to be formed. In accordance with the invention, such
solid precipitates may readily be removed from the bottom of the
borehole 12 from time to time as required or the electrolyte may be
withdrawn, filtered and returned on a continuous or semicontinuous
basis for this same purpose. This is not possible, however, with a
solid backfilled system where all the backfill material would need
first to be removed at considerable expense.
Another feature made possible by the absence of solid backfill
material is that the liquid electrolyte may be periodically removed
from the borehole and treated outside the hole. For, example, with
many systems, the acidity of the electrolyte will progressively
increase during use of the system and such increased acidity will
in turn lead to increased anode corrosion and attack on the liner
22 and on the cable insulation. In accordance with a useful feature
of the invention, it is a simple matter to withdraw such acidified
electrolyte and to neutralize it before returning it to the
borehole so avoiding the need for the use of expensive highly
acid-resistant materials for the borehole liner and for the cable
insulation. Any precipitates formed during such neutralization can,
of course, be separated, for example, by filtration before such
electrolyte is returned to the borehole, if necessary. Yet another
possible use for such periodic treatment of the electrolyte is that
changes taking place in the electrolyte during use which might
subsequently lead to modification or breakdown of the surrounding
ground zone and the consequent drop in anode-structure current can
be rectified as required. Such modification, if not avoided, may
lead to the conversion of a relatively low resistivity ground zone
into an exceptionally high resistivity material; this effect, which
is not fully understood, has been referred to in certain literature
as "induration." This difficulty is readily avoided in the method
of the invention by treating the electrolyte as required.
Yet another advantage of the method of the invention is that the
anodes can be repositioned in boreholes at any time if so desired
and similarly can be removed for inspection and/or maintenance when
desired.
With the use of two or more anodes in a single borehole, such
anodes can obviously be operated independently by the provision of
separate feed conductors or they can be connected together as shown
in FIG. 1 and then operated as a single anode.
* * * * *