Note: Descriptions are shown in the official language in which they were submitted.
1
CATHODIC CORROSION PROTECTION WITH CURRENT LIMITER
This invention relates to a method and for cathodically protecting and/or
passivating a metal section in an ionically conductive material particularly
to an
arrangement which limits a current supply by the anode assembly.
BACKGROUND OF THE INVENTION
Impressed current systems using a battery are known. Such impressed
current systems can use other types of power supply including common
rectifiers
which rectify an AC voltage from a suitable source into a required DC voltage
for the
impressed current between the anode and the steel. It is also known to provide
solar
panels to be used in a system of this type.
In all cases such impressed current systems require regular
maintenance and checking of the status of the power supply to ensure that the
power
supply does not fail leading to unexpected and unacceptable corrosion or
overprotection of the steel within the structure to be protected. While such
maintenance can be carried out, this is a relatively expensive process.
Alternatively, galvanic systems can be used which avoid the necessity
for any power supply since the voltage between the steel and the anode is
provided
by selecting a suitable material for the anode which is sufficiently electro-
negative to
ensure that a current is generated to provide corrosion protection.
There are two primary limitations of ordinary galvanic anodes as used in
steel reinforced concrete. The first relates to the mass of zinc per anode
which,
depending on the required current output, limits the useful life of the anode.
The
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second is the actual current output of the anode which may or may not be
sufficient to
halt corrosion of the steel. The current output is limited by the driving
voltage, which
is essentially a fixed property and varies with the circuit resistance which
is a function
of the exposure conditions, age of the anode, and build-up of corrosion
products over
time.
Reference is also made to US patents 8961746 (Sergi) issued February
24th 2015, 8968549 March 3 2015 (Sergi) and 7264708 (Whitmore) issued
September
4 2007 the disclosures of which are incorporated herein by reference or may be
referenced for more relevant information.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a method
for cathodically protecting and/or passivating a steel member in an ionically
conductive
concrete or mortar material, comprising:
providing an anode construction for communication of an electrical
current to the steel member in the ionically conductive concrete or mortar
material;
generating a voltage difference between the anode construction and the
steel member so as to cause a current to flow through the ionically conductive
concrete or mortar material between the anode and the steel member so as to
provide
cathodic protection of the steel member;
providing at least one electrically conductive circuit between the anode
construction and the steel member;
providing a field effect transistor (FET) in the electrically conductive
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circuit which acts as a current regulating device to limit the current between
the steel
member and the anode construction to a maximum value.
The current regulating device which is often called a current regulating
diode typically uses a field effect transistor but other arrangements can be
used.
In accordance with a key feature which can be used independently
herein there is provided a resistor in parallel with the current regulating
diode.
According to a further aspect of the invention there is provided a method
for cathodically protecting and/or passivating steel components in an
ionically
conductive concrete or mortar material, comprising:
providing a plurality of anode constructions at spaced positions in the
ionically conductive concrete or mortar material for communication of an
electrical
current to the steel components in the ionically conductive concrete or mortar
material;
providing a DC power supply;
connecting the DC power supply to each of the anode constructions in
parallel so as to generate a voltage difference between each anode
construction and
the steel components so as to cause a current to flow through the ionically
conductive
concrete or mortar material between each anode construction and the steel
components so as to provide cathodic protection of the steel components;
providing between each anode construction and the DC power supply a
respective electrically conductive circuit which acts to limit the current
through the
respective anode construction to a maximum value.
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In one preferred embodiment, each electrically conductive circuit
comprises a field effect transistor (FET) in which acts as a current
regulating device
to limit the current between the steel components and the anode construction
to a
maximum value.
In accordance with a key feature which can be used independently
herein there is provided a resistor in parallel with the current regulating
device.
In constructions where a plurality of individual anodes are provided
within a body of ionically conductive concrete or mortar material and the
anodes are
connected in parallel to a DC power supply, a number of known problems can
arise
as follows:
-a- Uneven distribution of the current from the anodes dependent on
the installation environment surrounding each anode;
-b- Difficulty to design required current levels for individual anodes
requiring that current output be designed for a zone of anodes, which can
result in an
uneven current distribution to the protected steel within each zone.
-c- Regions where individual anodes experience high voltage and
therefore deliver high anode current density which can result in acid
generation at the
anode mortar/interface and/or anode breakdown;
-d- In cases where individual anodes are accidentally placed close
to or in contact with steel shorting will occur which will result in dumping
of most
available current within the zone of anodes to this single point of contact.
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-e- In an effort to avoid shorts, strict quality control is necessary
during installation to ensure anodes are positioned as far away from steel as
possible,
a process that incurs high costs;
The arrangement of the present invention provides a number of features:
-a- Where the current regulating device works as a "constant current"
control for the anode or anodes;
-b- Where the Current regulating device is a diode or p-n junction
device and the regulated current is the leakage current through the device.
-c- Where the current regulating device is a current regulating diode
(CRD).
-d- Where the CRD comprises a FET which controls the current.
-e- Where a resistor in parallel with the CRD controls current output
with varying voltage;
-f- Where the CRD can be selected for a specified maximum
operating current density (e.g 110mA/m2 of anode surface as recommended in
IS012696-2016 and adding the resistor in parallel can achieve short-term
increase up
to 220 mA/m2 of anode surface when necessary by changing the voltage;
-g- Where the controlled current limits the effects of short-circuiting if
an anode is accidentally installed close to or in contact with steel;
-h- Where the controlled current provides a reduction of acid
generation of point anodes;
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-i-
Where the controlled current ensures that if any point anode in a
zone or chain malfunctions or dies, the rest of the system is not compromised,
with
each other anode still producing the same amount of current as before.
The anode current control system disclosed herein can be used with
individual anodes, with a plurality of discrete anodes or with mesh or ribbon
anodes.
For example a 1 mA CRD can be used in parallel with a resistor which
can provide an increased current when additional voltage is applied.
This
arrangement can be used to apply current for a short-term limit of 220 mA/m2
of anode
surface, as allowed in ISO 12696-2016, or other current as desired.
In one arrangement using four steel bars in a parallel array and four
ribbon strips anodes in four corners of the array, each anode had attached
between
the DC supply and the anode a 0.1 mA CRD connected to each ribbon. The fourth
of
the ribbons had a 50k0 resistor in parallel to the CRD. Up to 10 V was applied
between the anodes and the steel.
Anodes 1 to 3 controlled current at a nominal 0.1mA (Anode-1 0.1 mA
to 0.13 mA, Anode-2 exactly 0.07, Anode-3 0.11 mA to 0.12 mA) even if the
voltage
was increased from 1 V to 10 V. In Anode 4 the current increased
proportionally to
the increased voltage beyond that range and up to 0.26 mA at 10 V (a near 3
times
increase from 0.09 mA at 1V) as current was also able to pass through the
resistor. It
was found that irrespective of which bar any of the anodes was shorted to, the
anodes
continued to deliver the same amount of current.
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The arrangement using the CRD only without the additional resistor has
the following advantages:
-a-
The current from each anode is regulated exactly to the required
current over a wide range of applied voltage;
-b- Shorting of
any anode to steel does not increase the current
delivered by the shorted anode or influence the current delivered by any other
anodes
within the same zone/chain;
-c-
Current distribution to the steel is improved as each anode
delivers the same controlled level of current;
-d- If any single
anode fails, it has no detrimental affect on the
remaining anodes within the same zone;
By using the additional built-in resistor in parallel to the current
regulating
device an adjustable and optimal current distribution can be established by
adjusting
the applied voltage.
In cases where a string of point anodes in a zone has no CRD attached
to them, each individual anode delivers variable current to the surrounding
steel
depending on the local conductivity of the concrete resulting in an uneven
current
distribution. This is often dealt with by choosing smaller zones depending on
the local
concrete environment. Controlling the current output of each anode to a pre-
set
desired level by the introduction of CRDs ensures that the steel receives much
better
consistently distributed current where local concrete conductivity has minimal
influence. As a consequence, the zone size can be increased, particularly as
anodes
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in a low conductivity area can be chosen to locally deliver higher current by
selecting
the appropriate CRD.
In one arrangement, a resistance in the circuit of the CRD is used to
generate the control current or voltage from the voltage difference between
the anode
and the steel member.
In one arrangement, the anode construction and the CRD form
components of a common body which is at least partly buried in or attached to,
as a
single unit, the concrete or mortar material.
In one arrangement, a voltage difference between the anode
construction and the steel member is used to generate a reference voltage or
current
for the CRD.
In one arrangement, the common body is buried in the concrete or mortar
material while in an unset condition and the concrete or mortar material is
caused to
set with the common body therein and wherein said current limiting components
which
limit the current to said maximum value act to restrict formation of gas
bubbles in the
concrete or mortar material at the steel member and/or at the anode while the
concrete
or mortar material sets.
In one arrangement, the anode construction comprises a sacrificial
anode. In this arrangement the CRD can be provided as a common construction
with
the anode as explained above. In this case the CRD is mounted in the connector
wire
extending from the anode body for connection to the steel. Thus the anode has
buried
in its body a connecting wire portion which attaches to one terminal of the
CRD and
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the other terminal of the CRD is attached to another wire portion which
extends from
the CRD to an end for connection to the steel to be protected. The CRD can be
buried
in the mortar surrounding the anode body with the connector wire extending
outward
from the mortar layer. Other connecting arrangements including threaded rods
and
two wire couplings can be used.
In one arrangement, said anode construction comprises a sacrificial
anode and an impressed current anode, generating a voltage difference between
the
sacrificial anode and the steel member so as to cause a first current to flow
through
the ionically conductive concrete or mortar material between the first
sacrificial anode
and the steel member so as to provide cathodic protection of the steel member
wherein a voltage difference between the impressed current anode and the steel
member is generated by a storage component of electrical energy with two poles
for
communicating a second current generated by release of the electrical energy
and by
electrically connecting one pole to the steel member and by electrically
connecting the
other pole to the second anode.
In this arrangement, the storage component can be contained within a
sleeve or canister defining the anode on an exterior surface. In this
arrangement, the
impressed current anode can comprise stainless steel.
Preferably the transistor is a normally closed transistor so that, if the
control voltage or current falls below a threshold, the transistor allows
continued
passage of current between the anode and the steel member.
Preferably the transistor is an FET with a source and drain with the
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current through the FET controlled by a voltage at a gate.
In this arrangement, the voltage at the gate can be generated by a
resistance in the electrical circuit.
In this arrangement, the voltage at the gate can be generated by a
resistance across the transistor.
In this arrangement, the voltage at the gate can be generated across a
cell connected between the anode and the transistor.
In this arrangement, the voltage at the gate can be generated by a
sacrificial anode separate from said anode construction.
A further aspect of the present invention which can be used
independently of the above features relates to a method for cathodically
protecting
and/or passivating a metal section in an ionically conductive material,
comprising:
providing an anode for communication of an electrical current to the
metal section in the ionically conductive material;
generating a voltage difference between the anode and the metal
section so as to cause a current to flow through the ionically conductive
material
between the anode and the metal section so as to provide cathodic protection
of the
metal section;
and providing electrical components which limit the current to a
maximum value. The arrangement for limiting the current is provided by
connecting a
semi-conductor device in an electrically conductive path between the anode and
the
metal, the semi-conductive device being arranged to restrict current to a
leakage
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current and thus limit the current to a maximum value defined by the leakage
current.
It is known that, when a semiconductor device including a P-N junction
is reverse biased it should not conduct any current. However, due to an
increased
barrier potential, the free electrons on the P side are dragged to the
positive terminal,
while holes on the N side are dragged to the negative terminal. This produces
a current
of minority charge carriers and hence its magnitude is small. Within a typical
temperature range, the reverse current is almost constant.
Reverse leakage current in a semiconductor device is the current from
that semiconductor device when the device is reverse biased. The term is
particularly
.. applicable to most semiconductor junctions, especially diodes and
thyristors. In
general, such leakage currents can be used in devices such as diodes of the
types,
Silicon diodes, Shottky diodes, Zener diodes and Constant Current diodes. The
same
arrangement can be used in transistors such as those of the types: FET; IFET;
MOSFET. The same arrangement can also be used in other devices such as an
Analog Switch, Capacitor or Other PN devices.
In electronics, leakage is the gradual transfer of electrical energy/
electrons across a boundary normally viewed as insulating such as:
Spontaneous discharge of a charged capacitor;
Flow of current across a transistor in the "off" state;
Reverse-polarized diode.
Reverse leakage current is also known as "zero gate voltage-drain
current" with MOSFETs. The leakage current increases with temperature. As an
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example, the Fairchild Semiconductor FDV303N has a reverse leakage of up to 1
microamp at room temperature rising to 10 microamps with a junction
temperature of
50 degrees Celsius. For all basic purposes, leakage currents are very small,
and, thus,
are normally negligible.
However up to now persons in the field of cathodic protection have not
realized that the reverse or leakage current, of a diode or P-N junction
device or similar
devices such as those above which perform the similar function, provides the
required
level of current for use in cathodic protection at the required voltages and
over the
required time period.
The arrangement herein is preferably provided as a common component
with the anode so that both can be attached to, buried in or engaged with the
ionically
conductive material. However, the components may be separate with the anode in
contact with the material and the semi-conductor device located at a different
position
for example outside the material for servicing or other actions.
In a typical system described herein, the voltage difference in the
reverse direction across the semi-conductive device is typically in the range
0.2 to 6
volts. This range is acceptable for cathodic protection systems using either a
galvanic
voltage using a sacrificial anode or using a low voltage power supply and an
impressed current anode. The present inventor has realized that this level of
voltage
is suitable and matches the range of action of the semi-conductor device
available.
Preferably the current for a single anode is in the range 0.1 to 5 milliamps
and can be of the order of 100 A or less. Typically using a conventional
system
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without the current limiting arrangement, the current, especially initially
may be over
times higher. The present inventor has realized that the current may be too
high
at the outset and can be reduced by the use of the system herein so that a
longer life
of the cathodic protection system can be obtained before the current falls
below an
5 acceptable level. The leakage current from electronic devices, including
semi-
conductors which are available have been found by the inventor herein to be
suitable
for the requirements.
In this way, a long-life system can be designed with high charge capacity
using this simple inexpensive arrangement of utilizing the leakage current
through a
10 diode or other similar device to limit the current at the outset and in
some cases from
the outset over many years during periods when the current would otherwise be
higher. This allows the anode to remain active and providing the desired
current for a
longer time period.
The system also allows a sacrificial anode to provide a specifiable
current for many years. This capability has not been possible with sacrificial
anodes
previously since the current from sacrificial anodes installed in outdoor
environments
exposed to the weather will increase and decrease significantly due to changes
in
temperature, humidity and the resistance of the ionically conductive material
such as
concrete. Preferably in one arrangement, the semi-conductor device forms part
of a
combined unit inserted in or in ionic contact with the ionically conductive
material
which includes the anode and a connector. In this arrangement, preferably the
semi-
conductor device is associated with and operates only in respect of a single
anode.
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The current limiting system can be used when the anode is installed and
connected to the metal section while the ionically conductive material is
unset where
the limitation of the current by the semi-conductor device prevents gas
generation
during curing of the ionically conductive material.
In an impressed current system preferably the voltage difference is
generated by a storage component of electrical energy with two poles for
communicating electrical current generated by release of the electrical energy
and by
electrically connecting one pole to the metal section and by electrically
connecting the
other pole to the anode.
Preferably the diode is of the type having two connecting wires where
one wire is connected directly or indirectly to the anode and the other wire
is connected
directly or indirectly to a mounting component for attachment to the metal
section or
to the metal section itself. The metal section is commonly reinforcing steel.
Typically,
the diode and the diode connecting wire cannot tolerate the high forces
necessary to
mount the anode to the metal section so that a fixed component is attached
with the
anode to provide the mounting forces. In one arrangement this may be a simple
wire
wrapping system well known in the art.
Preferably the current limiter described above is associated with and
operates only in respect of a single anode and is not part of a larger system
limiting
or regulating current to a plurality of anodes.
In one particularly preferred method, the anode is installed and
connected to the metal section while the ionically conductive material is
unset and the
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limitation of the current by the electrical components prevents gas generation
during
curing of the ionically conductive material. The generation of gases during
setting is
a severe problem in that it forms bubbles in the concrete.
The arrangement described herein can be used in a system where the
voltage difference is generated by a storage component of electrical energy
with two
poles for communicating electrical current generated by release of the
electrical
energy and by electrically connecting one pole to the metal section and by
electrically
connecting the other pole to the anode. However the same current limiting
system
and the same mechanical connection can be used with sacrificial or galvanic
anodes
and also with combined systems where there is both an impressed current anode
driven by a power supply and a separate sacrificial anode.
In this arrangement, preferably the anode and the storage component
are both at least partly contained in or buried in the ionically conductive
material,
typically concrete.
In this arrangement preferably the storage component is connected as
a single unit with an impressed current or non-sacrificial anode and/or with a
sacrificial
anode.
In this arrangement preferably the storage component is contained
within a closed or sealed canister defining the anode on an exterior surface.
In this
.. case the anode can be formed of stainless steel.
In this arrangement in some cases in order to provide a longer life
replacement electrical energy can be introduced by re-charging the storage
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component or by replacing the storage component.
The storage component can be a cell or battery of cells or can be a
capacitor.
The arrangement therefore described above provides an arrangement
which acts to limit the current between the anode and the reinforcing bar.
This
arrangement can provide one or more of the following features:
It acts to regulate current from a battery, capacitor or galvanic anode.
The circuit may reduce the available voltage when the current is being
limited but does not reduce the total current available to protect the steel.
This is ideal
for battery or galvanic anode systems as these have limited capacity (limited
stored
charge) and do not function after the limited capacity is consumed.
The current can be limited over a wide range of circuit resistances from
short circuit to resistance where the available voltage is sufficient to
result in the full
set, desired current value.
The current limiter can be part of a combined unit which includes battery
or capacitor or anode and connector.
The current limiter allows batteries or high output anodes to be installed
and connected to the steel in fresh concrete/mortar without detrimental
effects of high
current densities discharging through the low resistance fresh material which
can
cause gas generation (oxygen and hydrogen) during curing which will create gas
bubbles, voids, reduce bond to the steel and leave pores/capillaries in the
concrete/mortar. Pores/cavities allow direct path to steel for water and salts
to
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penetrate and CO2 to carbonate the concrete. All of which lead to premature
corrosion of the steel.
The current limiter also extends the service life of high voltage anodes
such as batteries and high surface area (high initial current output)
sacrificial or
impressed current anodes. Using the current limiter saves capacity of the
battery
and/or anode such that improved performance and higher current output from the
anode(s) may be achieved in the future. The desired current output as allowed
by the
current limiter can be provided for a much longer period of time.
Where, as stated above the anode is not sacrificial to the metal section,
typically the material is therefore electropositive relative to the metal
section. However,
some part of the anode may be sacrificial or the anode may be fully
sacrificial.
The arrangement herein can be used where the anode is in the form of
a plurality of associated anodes all connected to the cell or battery of
cells.
The storage component as defined above can be a cell or battery or
battery of cells / batteries or it can be a capacitor or a supercapacitor or
ultracapacitor
which provides a system for storing charge different from conventional
electrolytic
cells or batteries. A supercapacitor is a high-capacity electrochemical
capacitor with
capacitance values much higher than other capacitors. These capacitors
typically
have lower voltage limits than standard or conventional capacitors. They
typically
store 10 to 100 times more energy per unit volume or mass than standard
capacitors,
can accept and deliver charge much faster than batteries, and tolerate many
more
charge and discharge cycles than rechargeable batteries. Supercapacitors do
not use
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the conventional solid dielectric of standard capacitors. They use
electrostatic double-
layer capacitance or electrochemical pseudo-capacitance or a combination of
both
instead. Electrostatic double-layer capacitors use carbon electrodes or
derivatives
with much higher electrostatic double-layer capacitance than electrochemical
pseudo-
capacitance, achieving separation of charge in a Helmholtz double layer at the
interface between the surface of a conductive electrode and an electrolyte.
The
separation of charge is of the order of a few angstroms (0.3-0.8 nm), much
smaller
than in a conventional capacitor.
Supercapacitors are a great advancement on normal capacitors being
capable of storing a high charge once fully charged. The capacity of a 2.7V
200F
supercapacitor is capable of holding a charge of the order of over 500C (A x
seconds).
Typical cathodic protection systems require around 170 to 400C/m2 of steel per
day
so such a capacitor is able to provide, when fully charged, enough charge to
protect
1m2 or more of steel for a day. This represents 2-5mA/m2 current density. In
order for
example to double this figure then we need to double the capacitance to around
400
F. If the capacitor is recharged on a daily basis, then logistically, a system
utilising
supercapacitors of this size spaced at intervals to provide current for 1m2 or
more of
steel can be an effective cathodic protection system. Daily recharging can
easily be
provided by solar panels, for example, but other means of producing reasonably
regular bursts of current could be used as charging components for the
supercapacitors. An example of such could be piezoelectric materials which can
be
incorporated in roads, parking garages, bridges, runways etc. enabling current
to be
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generated by loading and / or movement of the structure or vehicles passing
over
them.
That is, piezoelectric materials could be used to generate electricity to
power an impressed current system directly, or to charge / recharge batteries
or
capacitors / supercapacitors.
In some embodiments the anode is a sacrificial anode formed of a
material which is less noble than the metal section to be protected. However
in other
cases the anode is not less noble than the metal sections to be protected so
that it is
the same as the metal, typically steel or is more noble than the steel; so
that it is
partially or fully inert during the process. If the anode is formed of a
sufficiently inert
material, the anode does not corrode significantly during the flow of the
electrons.
High current output is required from the storage component such as a
battery. As described above, one pole is connected to the metal section to be
protected. Electrons flow from the storage component to the metal section such
that
corrosion of the metal section is reduced. The other pole is connected to an
anode or
if suitable, the casing of the storage component itself can be used as the
anode. In
the case of a zinc-alkaline battery the polarity of the battery is such that
the case of
the battery, if it is made of a suitable material will act as the anode and
will be able to
distribute the necessary current through the ionically conductive material
such as
mortar or concrete. Other batteries, such as most lithium batteries, typically
have only
a small pole which has the proper polarity which may not be large enough to
deliver
the required current into the ionically conductive material. A separate anode
can be
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provided for connection to the appropriate pole. The anode may encase or coat
the
whole storage component such as a battery or capacitor. Anodes can be made of
any
inert conductive material such as MMO coated titanium or other noble metal or
sub-
metal, conductive coating, conductive ceramic material etc. and can be
embedded in
an alkaline mortar or an inert material such as sand which may be dosed with
an alkali
solution. Stainless steel can also be a suitable current carrier when embedded
in
mortar or compacted sand dosed with alkali such as a saturated solution of
lithium
hydroxide. Anodes may also comprise sacrificial materials such as zinc which
are less
noble than the metal section to be protected.
Typically the single unit comprising the storage component and the
anode or anodes is at least partly buried in the ionically conductive
material. However
application to the surface or other modes of mounting where the anode is in
ionic
contact with the material can be used.
In one particularly preferred arrangement the storage component
comprises a cell with an outer case wherein the case is fully or partially
formed of the
anode material so that the anode is formed by the outer case either by an
outer surface
of the same material or as a coating or layer on the exterior of the case. In
this case
the outer case or at least the outer layer can be formed of a material which
is more
noble than steel. In this arrangement the anode forms directly the outer case
of the
cell where the case contains and houses the cathode material of the cell, the
electrolyte, the anode material and other components of the cell. That is, in
this
embodiment, the anode is defined by a layer or coating on the outer surface of
the
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storage component itself or actually as the outer surface of the storage
component
and not as an additional element which is separate from the storage component.
Where the storage component is a cell, the outer case of the cell can directly
carry the
material of the anode or even the outer case of the cell is the anode. The
anode
material may cover the whole surface or may be a partial covering leaving
other areas
exposed.
In another arrangement the case and the anode are formed
independently and the anode forms a separate body which conforms in shape to
the
outer case of the cell. Typically such cells are cylindrical but other shapes
can be
used. This arrangement is particularly applicable where the cell is
replaceable rather
than rechargeable to introduce the additional energy after the original cell
is sufficiently
depleted to be no longer effective.
In another arrangement the anode is a separate body which is
electrically connected to one terminal of the storage component.
The above features can be preferably used for protection of steel
reinforcing or structural members in concrete or mortar material where it is
well known
that corrosion can cause breakdown of the concrete due to the expansive forces
of
the corrosion products and due to the reduction to the steel strength. However
uses
in other situations can arise.
The term impressed current anode used herein is intended to distinguish
from the sacrificial anode where the sacrificial anode is formed of a
material, typically
zinc, which is less noble than the metal section so that it preferentially
corrodes relative
Date Recue/Date Received 2021-04-07
22
to the metal section to be protected. The impressed current anode is one which
is
used in conjunction with a power supply and does not need to be less noble
than the
metal section. Typically such impressed current anodes are formed of titanium,
platinum, niobium, carbon and other noble metals and oxides which do not
corrode
readily, or they can be formed of iron or less noble materials such as zinc.
For use during a sacrificial or galvanic phase of operation of the above
method, the ionically conductive filler material preferably contains at least
one
activator to ensure continued corrosion of the sacrificial anode. However the
activator
can also be located at other positions in the system. Suitable filler
materials can be
in the form of solids, gels or liquids.
Gels can include carbomethyl cellulose, starches and their derivatives,
fumed silica or polymer gel electrolytes, e.g. acrylic acid in a potassium
hydroxide
solution or polyvinyl chloride/acetate-KOH composites with additions of
bentonite,
propylene carbonate and or alumina. The alkali hydroxide in these gels acts as
a
suitable activator.
Suitable activators include alkali hydroxides, humectants, catalytic
materials and other materials which are corrosive to the sacrificial anode
metal.
Activators may be used alone or in combination.
For use during a sacrificial or galvanic phase of operation of the above
method, the ionically conductive filler material preferably has a pH
sufficiently high for
corrosion of the sacrificial anode to occur and for passive film formation on
the
sacrificial anode to be avoided. Alternatively, the filler may have a lower pH
and / or
Date Recue/Date Received 2021-04-07
23
contain other activators for corrosion of the sacrificial anode to occur and
for passive
film formation on the sacrificial anode to be avoided.
The anode and methods herein are preferably designed for use where
the metal section is steel and the ionically conductive material is concrete
or mortar.
The anode apparatus including the impressed current and sacrificial
components is typically buried in the concrete or other solid material so that
it is fully
encased by the concrete or a filler material, but this is not essential and
the anode
may be only partially buried or in direct or indirect physical or ionic
contact with the
concrete.
The anode apparatus including the impressed current and sacrificial
components may be surrounded by an encapsulating material or ionically
conducting
filler material which may be a porous material or porous mortar material.
Suitable
encapsulating materials can be inorganic or organic and may be any ionically
conductive cementitious, polymer or non-cementitious material or mortar
including
geopolymers or modified Portland cements. The encapsulating material may be
solid,
gel or liquid and may be deformable.
The power supply may include a solar panel which drives the impressed
current anode and rechargeable galvanic anode so as to provide long term
protection
when the solar power is on and off.
The construction and methods proposed herein are designed
particularly where the metal section is steel and the ionically conductive
material is
concrete or mortar. However the same arrangements may be used in other
corrosion
Date Recue/Date Received 2021-04-07
24
protection systems such as for pipes or other constructions in soil, and in
many other
systems where such anodes can be used.
Preferably the assembly includes a reinforcing layer, such as disclosed
in US Patent 7,226,532 issued June 5 2007 to Whitmore, the disclosure of which
is
incorporated by reference or to which reference may be made for further
details not
disclosed herein, to restrain and resist forces such as expansion, contraction
and
deformation forces which may be caused by corrosion of the anodes, deposition
of
sacrificial anode ions and other physical / environmental forces such as
freezing,
thawing, wetting, drying and thermal expansion / contraction.
The invention as defined and described herein can also be provided as
an assembly, as opposed to a method for cathodically protecting and/or
passivating a
metal section in an ionically conductive material. Thus the following
definitions of the
invention presented herein are included herein. Each of these independent
definitions
can be used in conjunction with any one of or all of the subsidiary features
as defined
above.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described in conjunction with
the accompanying drawings in which:
Figure 1 is a cross-sectional view of an anode assembly using a
sacrificial anode for use in a corrosion protection method according to the
present
invention.
Date Recue/Date Received 2021-04-07
25
Figure 2 is a side elevational view of the anode assembly of Figure 1.
Figure 3 is a cross-sectional view of an anode assembly similar to that
of Figure 1 but using a conventional wire wrapping attachment method to the
metal
section.
Figure 4 is an enlarged cross-sectional view of an anode assembly of
the type using a cell to provide current though an impressed current anode and
using
the current limiting device and mounting arrangement of Figures 1 and 2.
Figure 5 is an enlarged cross-sectional view of an anode assembly of
the type using a cell to provide current though an impressed current anode and
a
bipolar type transistor to limit the current from the cell to the steel.
Figures 6 to 9 show schematic illustrations of four embodiments of
current limiting system which uses a gate-controlled FET otherwise know as a
current
regulating diode (CRD) to limit the current in the electrically conductive
circuit
connecting an anode to the steel member.
Figure 10 is a schematic illustration of a protection system using a
plurality of anodes powered by a single power source where each of anodes is
associated with a current control circuit having a CRD.
Figure 11 is a schematic illustration of a protection system using a
plurality of anodes powered by a single power source where each of anodes is
associated with a current control circuit and the current control circuit
includes a
resistor in parallel to the CRD.
In the drawings like characters of reference indicate corresponding parts
Date Recue/Date Received 2021-04-07
26
in the different figures.
DETAILED DESCRIPTION
In the example shown in Figures 4 and 5 there is provided a cell which
may be rechargeable, as shown in prior PCT Application WO 2017/075699 filed
November 2 2016 and published 11 May 2017, the disclosure of which may be
referenced or is incorporated herein by reference, or may be a simple non-
rechargeable cell. The cell may form part of the anode structure or the anode
and the
cell may be physically separated. The anode body 10 is defined by a typical
alkaline
manganese dioxide-zinc rechargeable cell comprising the following main units:
a steel
can 12 defining a cylindrical inner space, a manganese dioxide cathode 14
formed by
a plurality of hollow cylindrical pellets 16 pressed in the can, a zinc anode
18 made of
an anode gel and arranged in the hollow interior of the cathode 14, and a
cylindrical
separator 20 separating the anode 18 from the cathode 14. The ionic
conductivity
(electrolyte) between the anode and the cathode is provided by the presence of
potassium hydroxide, KOH, electrolyte added into the cell in a predetermined
quantity.
Other types of rechargeable cells comprise similar main components (can,
cathode,
anode, separator and electrolyte) but the composition of the components may
differ.
Some of the types of cell may however be of a different construction such as
lead/acid
cells or lithium cells.
The can 12 is closed at the bottom, and it has a central circular pip
serving as the positive terminal. The upper end of the can 12 is hermetically
sealed
by a cell closure assembly which comprises a negative cap 24 formed by a thin
metal
Date Recue/Date Received 2021-04-07
27
sheet, a current collector nail 26 attached to the negative cap 24 and
penetrating
deeply into the anode gel to provide electrical contact with the anode, and a
plastic
top 28 electrically insulating the negative cap 24 from the can 12 and
separating gas
spaces formed beyond the cathode and anode structures, respectively.
Other types of rechargeable cells may be used. In the present
arrangement, the type described above is used in a method for cathodically
protecting
and / or passivating a metal section such as steel reinforcing bar 40 in an
ionically
conductive material such as concrete 41. The cell therefore includes a first
terminal
42 and a second terminal 43 defined by the outer casing 12. The first terminal
42 is
connected to the pin or nail 26 which is engaged into the anode material 18.
The
terminal 42 connects to a connecting wire 42A which extends from the terminal
42 to
threaded connector 53 for eventual connection to the steel reinforcing bar 40
as shown
in figure 4 through the mounting assembly generally indicated at 50 which
mechanically and electrically attaches the anode body to the bar 40.
In figure 4, an anode 44 is applied as a coating onto the casing 12 of the
cell. In this embodiment the anode 44 is of an inert material so that it is
more noble
than steel. Examples of such materials are well known. Thus the anode material
44
does not corrode or significantly corrode during the cathodic protection
process.
In this arrangement the application of the anode 44 onto the outside
surface of the casing 12 provides the structure as a common single unit where
the
anode is directly connected to the cell and forms an integral element with the
cell.
Anode 44 may comprise one or more layers and may include a mixed metal oxide
Date Recue/Date Received 2021-04-07
28
(MMO), catalytic or sub-oxide layer.
In this embodiment, as the anode 44 is formed of an inert material which
does not corrode in the protection process, the anode and the cell contained
therein
can be directly incorporated or buried in the concrete or other ionically
conductive
material without the necessity for an intervening encapsulating material such
as a
porous mortar matrix. As there are no corrosion products there is no
requirement to
absorb such products or the expansive forces generated thereby. As the process
does
not depend upon 7 continued corrosion of a sacrificial anode, there is no
necessity for
activators at the surface of the anode. As the chemical reaction at the
surface of any
inert anode during operation generates acid (or consumes alkali) it is
beneficial for the
anode to be buried in an alkaline material such as concrete or high alkalinity
mortar to
prevent material near the anode from becoming acidic. If desired, additional
alkali may
be added to the concrete or other material the anode is in contact with.
The apparatus shown herein includes an anode body generally
indicated at 10 which is connected to the reinforcing bar 40 by the mounting
assembly
generally indicated at 50. In addition, the anode body includes a current
limiting
system generally indicated at 51 which limits the flow of current from the
anode body
to the bar 40.
As previously described, the anode body can be defined by a power
supply typically in the form of a cell with the anode 44 on the outside
surface of the
cell and with the other terminal of the cell provided at the end of the cell
for connection
to the bar 40.
Date Recue/Date Received 2021-04-07
29
In other embodiments shown in Figures 1, 2, 3 and 6 to 8, the cell can
be omitted in which case the anode body comprises a sacrificial material which
is less
noble than the steel rebar, such as zinc where a voltage between the anode and
the
bar comprises the galvanic voltage between the two metal components.
In yet another embodiment, the anode body can comprise a combination
of both an impressed current anode and a sacrificial anode.
In this way the anode body is constructed and arranged so that when
the anode is ionically connected to the concrete, a voltage difference is
generated
between the anode 44 and/or 74 and the bar 40 so as to cause a current to flow
through the concrete between the anode and the bar 40 so to provide cathodic
protection and/or passivation of the reinforcing bar in the concrete.
In the embodiment shown in figure 1, 2, 4 and 5, the mounting assembly
is of the type shown in Published PCT application WO 2019/006540 filed 15 May
2018
and published 10 January 2019, the disclosure of which may be referenced or is
incorporated herein by reference.
The mounting 50 comprises a first abutment in the form of a threaded
rod 53 which is attached at one end to the anode body 10 and a second abutment
57
for engaging generally the opposed the face of the bar 40. In general the
second
abutment forms a hook member with two legs 68 and 69 which contact the
opposite
or rear surface of the bar 40 to provide a stable engagement.
In this embodiment the female threaded portion is provided by a
threaded hole through the flange 67. A screw action pulling the second
abutment
Date Recue/Date Received 2021-04-07
30
member toward the anode body is therefore provided by rotating the rod 53.
This can
most effectively be done by grasping manually the anode body and using it as a
handle
to turn the rod 53.
Of course, this requires a strong connection between the bottom end of
the rod 53 and the anode body. This connection is provided by a base plate 71
attached onto the bottom end of the rod 53 and engaged firmly into the upper
end of
the anode body. The solid anode body 74 includes a conventional covering of a
mortar
material 75 for purposes of retaining corrosion products and of carrying
conventional
activating materials described herein before.
Turning now to figure 4, there is shown in more detail the connection
between the terminal 42 of the cell and the rod 53 which is electrically
connected to
the bar 40 as described above.
The terminal 42 is connected to a wire 42A which in turn is connected to
a diode 51. An output wire 79 of the diode 51 is connected to the base plate
71
connected to the rod 53.
The diode 51 can be a conventional diode connected in reverse polarity
so as to prevent flow of current between the anode and the bar 40. In this
arrangement, the reverse or leakage current acts to limit the flow of current
from the
anode to the bar 40 to a value of the order of 0.1 to 1 milliamp. This maximum
value
is retained regardless of the conductivity between the anode 44 and the bar 40
through
the concrete. If the conductivity through the concrete is very high, for
example during
an initial installation when the concrete is fresh, the current is maintained
at the
Date Recue/Date Received 2021-04-07
31
maximum value. As the conductivity through the concrete falls to a lower level
(resistivity increases), the current is maintained at the desired level until
the voltage
drop through the concrete and the circuit (V=IR) at !max reaches the voltage
of the cell.
If the conductivity falls to a yet lower level, the current through the diode
or transistor
also falls dependent upon the conductivity and is not maintained by the action
of the
diode or transistor 51. The simple circuit therefore provided by the diode 51
does not
act as a regulator but instead merely acts as a current limiter.
Figures 1 and 2 show applications of the current limiting device in use
with a galvanic anode. In this arrangement, the diode or transistor 51 is
connected by
wires 51A and 51B connected between the anode 74 and the support plate 71
which
is connected to the rod 53.
Limitation of the current to a maximum value set during manufacture by
the selection of the diode 51 can ensure that the current remains during the
life of the
system at a relatively low level so as to dramatically increase the lifetime
of the cell
.. from a typical value in the absence of the current limiter which could be
of the order
of one year up to a more suitable lifetime of 10 years for example. The life
of a galvanic
anode may be extended from 5 to 10 years to over 50 years for example. In this
way
the current is maintained at a value which is suitable for cathodic protection
but at no
time is there any excess current over and beyond this desirable value which
may
damage the concrete or deplete the cell prematurely or degrade and shorten the
life
of a galvanic anode such that corrosion protection is not provided for the
desired
timeframe.
Date Recue/Date Received 2021-04-07
32
This arrangement is valuable in relation to an arrangement which uses
a non-sacrificial impressed current anode and a cell as the power supply for
generating the required voltage. In such an arrangement the current generated
between the anode 44 and the bar 40 can in some circumstances significantly
exceed
the desirable value.
In order to connect the terminal 42 to the rod 53, there is provided an
insulating or protective collar 83 surrounding the diode 51. The bottom end of
the collar
is attached to the top end of the cell and the top end of the collar receives
the base
plate 71 in a suitable receptacle portion. The collar 83 is attached to the
cell 44 by a
surrounding insulating layer 84 of a suitable plastic material. Inside the
collar 83 is
provided a conventional potting material 85 which surrounds the diode 51 and
wires
to maintain connection and to prevent damage from moisture penetration. The
structure is thus sufficiently strong to ensure that the base plate 71 is
attached to the
cell in a manner which allows the cell to be grasped manually and rotated as
an
operating handle to rotate the rod 53.
In the present method for cathodically protecting and/or passivating a
metal section in an ionically conductive material, as shown in Figures 1 and
2, a
sacrificial anode 74 is provided for communication of an ionic current to the
metal
section 40 in the ionically conductive material 91. The anode acts for
generating a
voltage difference between the anode 74 and the metal section 40 so as to
cause a
current to flow through the ionically conductive material 91 between the anode
and
the metal section so as to provide cathodic protection of the metal section in
the
Date Recue/Date Received 2021-04-07
33
conventional manner.
The current flowing between the anode the metal section is limited to a
low selected value by connecting the semi-conductor diode device 51 in an
electrically
conductive path between the anode and the metal. The semi-conductive device 51
maybe of the type which is arranged to pass current in a first direction and
to restrict
current in a second direction to a leakage current and is connected so that
current
between the anode and the metal passes in the second direction and thus limits
the
current to a maximum value defined by the leakage current.
The semi-conductor device diode 51 forms part of a combined unit
including the anode and the mounting arrangement or electrical connector to be
inserted in or attached to the ionically conductive concrete or mortar
material.
Where there is provided a coating 75 on the anode of a porous
absorption material the diode 51 can be located in the coating or in a potting
material
to provide suitable protection.
The wire or electrical connection 51A must be electrically connected to
the anode. The wire or electrical connection preferably will be cast into the
anode as
indicated at 51C or connected to a connector which is cast into the anode.
Less
durable connections such as mechanical connections or soldering directly to
the
exterior of the anode can be made. Wire or connector 51B must be electrically
connected to the bar 40. This wire or connector can be soldered or otherwise
connected to the support plate 71 which is connected to the attachment
mechanism.
As the diode is typically supplied with wires which are unsuitable for direct
connection
Date Recue/Date Received 2021-04-07
34
to the bar 40, typically the diode needs to be attached to the mounting 71
which
provides structural support for the attachment mechanism.
Many types of attachment can be used including the hook and rod
system described above and the traditional flexible wire arrangement which is
used to
wrap around the bar 40 as shown in Figure 3 where two wires 71A and 716 are
connected to the mounting 71 or directly connected to at least one wire or
other
electrical connector to connect to the bar 40. The sacrificial anode 74 is
attached
structurally to the mounting plate 71 by an insulating member 78 to form a
common
unit which can be easily handled and inserted into the material.
In the embodiments shown therefore the anode 74 includes an
electrically conductive connector for electrically connecting the anode to the
metal
section 40 and the diode 51 is located in the electrical connection between
the anode
and the connector.
Turning now to the arrangements shown in Figures 5 to 9 there is
method for cathodically protecting and/or passivating a steel member 101
buried in or
in contact with an ionically conductive concrete or mortar material 99. Using
the
constructions shown in Figures 1 to 4, there is provided an anode construction
100 for
communication of an electrical current to the steel member 101 in the
ionically
conductive material 99.
By using the sacrificial anodes of Figures 6 to 8 or the impressed current
anode of Figure 9, a voltage difference is generated between the anode
construction
Date Recue/Date Received 2021-04-07
35
100 or 104 and the steel member 101 so as to cause a current to flow through
the
ionically conductive material 99 between the anode 100, 104 and the steel
member
so as to provide cathodic protection of the steel member. The anode 104 of
Figure 9
is powered by a power supply 105 such as a simple cell connected between the
anode
and the steel 101.
In accordance with the invention described herein there are provided
electrical components 106 which limit the current to a maximum value with the
electrical components 106 including at least one electrical conductor 107
connected
to the anode construction. As shown schematically in these figures and in more
detail
in Figures 1 to 4, the electrical components including the electrical
conductor and the
anode construction form components of a common body which is attached to or
buried
in the concrete or mortar material as a single unit.
Turning now to figure 5, there is shown in more detail the connection
between the terminal 42 of the cell and the rod 53 which is electrically
connected to
the bar 40 as described above.
The terminal 42 is connected to a wire 42A which in turn is connected to
a transistor 78. An output wire 79 of the transistor 78 is connected to the
base plate
71 connected to the rod 53.
The transistor 78 in this embodiment is a conventional or bipolar
transistor in which case a base of the transistor 78 has a control current
provided by
a wire 80 connected through a resistor 81 in turn connected through a wire 82
to the
positive terminal of the battery connected to the anode 44.
Date Recue/Date Received 2021-04-07
36
As the transistor 78 is connected to the steel bar 40 and the wire 82 is
connected to the anode 44, the control current to the transistor 78 is
determined by
the voltage across the cell and the resistance of resistor 81. As this voltage
is typically
relatively constant at least until the cell is in its later stages of life,
this constant control
current controls the amount of current flowing through the transistor from the
cell to
the bar 40. As is well known the resistor 81 can be selected to provide a
control base
current to the transistor which sets the current flow through the transistor
to a
maximum value. This maximum value is retained regardless of the conductivity
between the anode 44 and the bar 40 through the concrete. As the conductivity
through the concrete is very high, for example during an initial installation,
the current
is maintained at the maximum value. As the conductivity through the concrete
falls to
a lower level, the current is maintained at the desired level until the
maximum voltage
of the cell is reached. If the conductivity falls to a yet lower level, the
current through
the transistor also falls dependent upon the conductivity and is not
maintained by the
action of the transistor. The simple circuit therefore provided by the
resistor and the
transistor does not act as a regulator but instead merely acts as a current
limiter.
As shown in Figures 6 to 9 a current limiting circuit between the anode
100 and the steel member 101 uses a field effect transistor 102 in the
electrically
conductive circuit 107 which acts to limit the current between the steel
member and
the anode construction to a maximum value. The current through the transistor
is
limited by a control voltage applied to a gate of the transistor. The
transistor is typically
Date Recue/Date Received 2021-04-07
37
a suitable form of Field effect transistor so that the control terminal acts
as a gate. An
arrangement is provided in the electrically conductive circuit for generating
control
voltage from the voltage difference between the anode and the steel member. In
Figures 6 to 8 this voltage difference is galvanic. In Figure 8 it is
generated in
response to the power supply 105.
The anode construction and the transistor form, as shown in Figures 1
to 4, components of a common body which is at least partly buried as a single
unit in
the concrete or mortar material. The transistor uses the voltage difference
between
anode construction and the steel member and in some cases a resistor to
generate a
reference voltage or current for the transistor.
In Figure 6, a resistor R1 is located between the source S and the anode
100. This creates a voltage drop between the gate and the source and acts to
enable
the voltage at the gate to control the flow of current through the transistor
to limit the
current to a required value. This is achieved by selection of a suitable
transistor having
current and control characteristics along with the value of the resistor so as
to provide
a substantially constant current as described above.
In Figure 7, the voltage at the gate is set by a voltage generated by a
small sacrificial anode 110 also located in the concrete. This anode is
separate from
the anode 100 and is not provided to directly or significantly assist in the
corrosion
protection but instead to provide the reference voltage at the gate. The
voltage is
generated galvanically relative to the steel 101 and remains consistent over
time so
as to set the current through the transistor at a required restricted value.
Date Recue/Date Received 2021-04-07
38
In this arrangement typically the anode 110 can be located in the
conventional mortar covering around the anode 100.
In Figure 8, the gate G control line is connected to a location between
-
the drain and the steel. In this location the voltage drop across the
transisitor provides
a gate voltage which is suitable to set the current flow at a required limited
level.
In each of these arrangements, the circuit operates to generate the
required gate voltage to maintain the gate voltage above or below a threshold
value
and to thus control the current passing through the transistor between source
S and
drain D at the required limited value described herein.
In each of these arrangements of Figures 7 and 8 there is no additional
resistor in the line from the anode to the steel which, if present, would act
to reduce
current flow when the system has reached an age and condition when the
transistor
is no longer acting to limit the current. At that stage the system provides
the maximum
available current due to the limited voltage drop between the anode and the
steel.
The arrangement used n Figure 9 uses a cell 105 to generate the voltage
between an impressed current anode 104 and the steel 101. It will be noted
that the
cell is located in the line from the anode to the transistor and the gate
voltage is set
by the voltage drop across the cell.
As a further alternative, not shown, the gate voltage can be provided by
a cell provided in the circuit. This arrangement has the advantage that the
voltage
can be more easily determined and maintained but of course increases cost and
complexity.
Date Recue/Date Received 2021-04-07
39
Typically the transistor 102 is a normally closed transistor so that, if the
control voltage or current falls below a threshold, the transistor defaults to
a closed
position and allows continued passage of current between the anode and the
steel
member.
The transistor is a normally closed MOSFET transistor with a gate to
source voltage of less than 0.7V.
Turning now to Figures 10 and 11, these include a field effect transistor
(FET) 102 in the electrically conductive circuit 106 which acts as a current
regulating
device or diode to limit the current between the steel member and the anode
construction to a maximum value. The arrangement used in Figure 10 is of the
same
construction as that shown in Figure 6 and operates in the manner of the
current
regulating diode where there is a connection from source to drain with a
resistor R1
which in known manner operates to limit the current passing through the
current
regulating diode to a maximum value as set forth above.
In Figure 10 there is shown a schematic illustration of a protection
system using a plurality of anodes Al, A2 and A3 etc powered by a single DC
power
source 110 connected to a bus line 111 individually connected to each of the
anodes
by a separate drop 116. Each of anodes is associated with a respective current
control
circuit defining a current regulating diode 106. The anodes cooperate with the
steel
components 112 in the concrete 113 where the steel components are connected
together and to the positive side of the power supply by a lead line 114.
In Figure 10, the current in each drop 116 is controlled by the FET 102
Date Recue/Date Received 2021-04-07
40
only.
In Figure 11, each of the diodes 106 is associated with a respective
resistor 115 arranged in parallel to the respective diode 106.
This arrangement of anodes when using the individual current control
circuits provides the advantages and operating conditions as explained above.
Since various modifications can be made in my invention as herein
above described, and many apparently widely different embodiments of same may
be
made within the spirit and scope of the claims without department from such
spirit and
scope, it is intended that all matter contained in the accompanying
specification shall
be interpreted as illustrative only and not in a limiting sense.
Date Recue/Date Received 2021-04-07
