Note: Descriptions are shown in the official language in which they were submitted.
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1
CORROSION PROTECTION USING A SACRIFICIAL ANODE
This invention relates to corrosion protection using a sacrificial
material.
BACKGROUND OF THE INVENTION
U.S. Patent 6,346,188 (Shuster) assigned to ENSER Corporation and
issued February 12, 2002 discloses a method for corrosion protection of marine
piles
in which an anode is located within a jacket surrounding the pile at water
level and a
battery is mounted on the pile above the water level for providing an
impressed
current between the anode of the jacket and the steel of the pile. The anode
is
preferably formed of titanium or other non-corroding materials which are high
on the
Noble scale. However the patent mentions that other materials such as zinc can
be
used but these are disadvantageous since they tend to corrode. The intention
is that
the battery have a long life and be maintained effectively so that the
impressed
current remains in place during the life of the marine pile bearing in mind
that the salt
water in the marine environment is particularly corrosive.
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 for charging batteries 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
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2
power supply does not fail leading to unexpected and unacceptable corrosion of
the
steel within the structure to be protected. While such maintenance can be
carried
out and the power supply thus ensured, this is a relatively expensive process.
Alternatively galvanic systems can be used which avoid 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 a corrosion protection. These
systems
have obtained considerable success and are widely used.
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
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 exposure conditions, age
of the
anode, and build up of corrosion products overtime.
SUMMARY OF THE INVENTION
It is one object of the invention to provide an improved method for
corrosion protection.
According to one aspect of the invention there is provided a method for
corrosion protection of a metal section in an ionically conductive covering
material
comprising:
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=
3
locating an impressed current anode in ionic contact with the ionically
conductive material;
locating a sacrificial anode of a material which is less noble than the
metal section in ionic contact with the ionically conductive material;
providing a DC power supply; and
providing a connection of the DC power supply so as to apply a
potential difference between the impressed current anode and the sacrificial
anode.
Preferably the potential difference causes ions of the sacrificial anode
material to move to the sacrificial anode.
Preferably there is provided an additive which acts to limit gassing from
the sacrificial anode.
Preferably the additive is a surfactant.
Preferably the additive comprises cellulose.
Preferably the additive comprises alloying zinc metal with suitable
elements such as nickel or indium which is arranged to reduce the hydrogen
over-
potential significantly and hence limit hydrogen gassing.
Preferably the method includes restricting dendritic growth of sacrificial
anode material on the sacrificial anode.
Preferably the method includes causing moisture movement towards
the sacrificial anode.
Preferably there is an ionically conductive membrane separator
between the sacrificial anode and the impressed current anode.
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Preferably the ionically conductive membrane separator is located
around or adjacent to the sacrificial anode.
Preferably the ionically conductive membrane separator acts to contain
sacrificial material at the sacrificial anode, to avoid dendritic growth of
sacrificial
anode material on the sacrificial anode beyond the membrane and to allow
moisture
movement to the sacrificial anode.
In one arrangement the sacrificial anode material or a portion thereof is
provided as particles or powder. This can be associated with or intermixed
with an
ionically conductive filler material or electrolyte. In one example, the whole
of the
sacrificial anode can be provided in the particulate form associated with a
connection member which provides the electrical connection to the steel or the
DC
power supply. That is the particles are in connection with the connector and
are
located at or adjacent the sacrificial anode connection. In this way when the
DC
voltage is applied the ions of the sacrificial anode material move toward and
deposit
on the particulate material to form additional sacrificial anode material. The
sacrificial anode can include a solid core of the material at the connector or
may not.
Where there is no core, the connector can be for example simply a pin of a
suitable
material such as brass or galvanized steel.
Preferably the sacrificial anode is connected to the steel while the DC
power supply is connected to the impressed current anode so that the cathodic
protection is carried out by the DC power supply while the recharging or
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regeneration of the sacrificial anode is occurring. However this is not
essential and
the recharging or regeneration can be carried out as a separate step.
Preferably the method includes increasing galvanic current generated
by the sacrificial anode subsequent to application of the potential difference
relative
5 to that before the application.
Preferably the method includes increasing alkalinity at the surface of
the sacrificial anode.
Preferably the increased alkalinity acts to dissolve zinc corrosion
products into soluble zincate ions.
Preferably the method includes limiting current flowing to the sacrificial
anode by the potential difference such that the sacrificial anode is
reactivated
without recharging the sacrificial anode with additional ions of the
sacrificial material.
Preferably the method includes causing water movement from the
negatively charged impressed current anode to the positively charged
sacrificial
anode.
Preferably the method includes increasing a total surface area of the
sacrificial anode material at the sacrificial anode.
Preferably the method includes increasing a quantity of hydroxyl ions
at the immediate vicinity of the sacrificial anode.
The sacrificial anode can be formed of solid anode material which can
be cast, rolled or formed, or it can be formed of powdered or finely divided
sacrificial
anode material or zinc oxide.
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According to a second aspect of the invention there is provided an
anode apparatus for cathodically protecting a metal section in an ionically
conductive
material, the anode apparatus comprising:
a sacrificial anode of a material which is less noble than the metal
section;
an impressed current anode;
the sacrificial anode and the impressed current anode comprising
components of the anode apparatus so that, when the components of the anode
apparatus are located in contact with the ionically conductive material, each
of the
sacrificial anode and the impressed current anode is in ionically conductive
communication with the other and with the metal section;
the impressed current anode and the sacrificial anode being electrically
separated to prevent electrical communication therebetween;
an electrical connection to the sacrificial anode;
an electrical connection to the impressed current anode
and an ionically conductive membrane separator between the
sacrificial anode and the impressed current anode.
Preferably the ionically conductive membrane separator is located
around or adjacent to the sacrificial anode.
Preferably the ionically conductive membrane separator is arranged to
contain sacrificial material at the sacrificial anode, to avoid dendritic
growth of
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sacrificial anode material on the sacrificial anode beyond the membrane and to
allow
moisture movement to the sacrificial anode.
Preferably there is provided a DC power supply.
Preferably the impressed current anode is perforated so to allow
passage of ionic current to pass through the impressed current anode.
Preferably the impressed current anode surrounds at least a portion of
the sacrificial anode.
Preferably the sacrificial anode and the membrane separator, and
optionally the impressed current anode, comprise common components of a
common anode assembly.
Alternatively the impressed current anode and the sacrificial anode
comprise side by side elements.
Preferably there is provided an ionically conductive filler material
adjacent to the sacrificial anode.
Preferably the sacrificial anode is re-charged by locating an impressed
current anode in ionic contact with the ionically conductive material and
connecting a
first terminal of a DC power supply to the impressed current anode so as to
cause
ionic current to flow through material to cause sacrificial anode ions to be
deposited
on the sacrificial anode.
Preferably sacrificial anode ions are present in the ionically conductive
material.
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Preferably the re-charging causes hydroxyl ions to be generated at the
surface of the sacrificial anode.
Preferably the re-charging causes an alkali environment to be re-
generated around the sacrificial anode.
Preferably, in a first step, the sacrificial anode is connected to the
metal section to provide corrosion protection of the metal section by the
corrosion of
the sacrificial anode which results in the formation of corrosion products of
the
sacrificial anode and, in a second step after corrosion of the sacrificial
anode has
occurred, the current supplied by a DC power supply causes the ions of the
sacrificial material, from the corrosion products of the sacrificial anode, to
be re-
deposited on the sacrificial anode.
Preferably the DC power supply is applied temporarily.
Preferably there are ions of the sacrificial material available to be
deposited.
Preferably the ions of the sacrificial material are soluble.
Preferably the ions of the sacrificial material are electrochemically
mobile.
Preferably the metal section is steel and the ionically conductive
material is concrete or mortar.
Preferably there is provided a connection between the sacrificial anode
and the metal section so that the sacrificial anode provides corrosion
protection.
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Preferably the connection between the sacrificial anode and the metal
section remains in place when the DC power supply is in operation.
In one arrangement, at least a portion of the sacrificial anode is
fabricated in the ionically conductive material by depositing ions of the
sacrificial
material,
In this case the method can include incorporating ions of the sacrificial
material in the ionically conductive material where the sacrificial anode is
generated
in the ionically conductive material by depositing the incorporated ions of
the
sacrificial material on the sacrificial anode.
Preferably the second terminal of the DC power supply is connected to
the sacrificial anode and to the metal section.
Preferably the connection of the DC power supply across the
impressed current anode and the metal section creates a current between the
metal
section and the impressed current anode which is used to passivate the metal
section.
Preferably the connection of the DC power supply across the
impressed current anode and the metal section creates a current between the
metal
section and the impressed current anode which is used to provide corrosion
protection to the metal section while the ions of the sacrificial material are
being
deposited on the sacrificial anode.
Preferably the sacrificial anode and the impressed current anode
comprise common components of an anode apparatus so that, when the common
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components of the anode apparatus are located in the ionically conductive
material,
each of the sacrificial anode and the impressed current anode is in ionically
conductive communication with the other and with the metal section.
Preferably the impressed current anode and the sacrificial anode are
5 electrically separated to prevent electrical communication therebetween.
Preferably the impressed current anode is perforated so to allow
passage of ionic current in the ionically conductive material to pass through
the
impressed current anode.
Preferably the sacrificial anode forms a rod (or tubular
10 element/component) and the impressed current anode forms a sleeve
surrounding
the rod.
Preferably the impressed current anode and the sacrificial anode
comprise side by side plates.
Preferably there is provided an ionically conductive filler material
adjacent to the sacrificial anode.
Preferably the ionically conductive filler material is different from the
ionically conductive material.
Preferably the ionically conductive filler material contains sacrificial
anode ions
Preferably the ionically conductive filler material is porous.
Preferably the ionically conductive filler material is deformable or a gel.
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Preferably the ionically conductive filler material contains at least one
activator to ensure continued corrosion of the sacrificial anode.
Preferably the ionically conductive filler material is hydroscopic.
Preferably the ionically conductive filler material 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.
Preferably there is provided a plurality of sacrificial anodes and
wherein the impressed current anode is separate from said sacrificial anodes.
Preferably there is provided a plurality of impressed current anodes
and wherein the sacrificial anode is separate from said impressed current
anodes.
Preferably the impressed current anode is arranged to be mounted
temporarily to provide current through a surface of the ionically conductive
material.
Preferably there are sacrificial anode ions available to be deposited.
Preferably the sacrificial anode ions are soluble.
Preferably the sacrificial anode ions are electrochemically mobile.
Although not essential, typically this arrangement is designed for use
where the metal section is steel and the ionically conductive material is
concrete or
mortar.
In some cases a portion of the structure or the sacrificial anode is
wetted.
In some cases the impressed current anode is mounted temporarily for
the purpose of causing the ions of the sacrificial material to be deposited.
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The above methods can be carried out using an anode apparatus for
cathodically protecting a metal section in an ionically conductive material,
the anode
apparatus comprising:
a sacrificial anode of a material which is less noble than the metal
section;
an impressed current anode;
the sacrificial anode and the impressed current anode comprising
components of the anode apparatus so that, when the components of the anode
apparatus are located in contact with the ionically conductive material, each
of the
sacrificial anode and the impressed current anode is in ionically conductive
communication with the other and with the metal section;
the impressed current anode and the sacrificial anode being electrically
separated to prevent electrical communication therebetween;
a first electrical connector arranged for connection to the sacrificial
anode;
and a second electrical connector arranged for connection to the
impressed current anode.
This anode apparatus can be used in a method for corrosion protection
of a metal section in an ionically conductive covering material where the
impressed
current anode and the sacrificial anode are both located in contact with the
ionically
conductive material and a DC power supply is connected between the impressed
current anode and the metal section so as to create a current between the
metal
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section and the impressed current anode which is used to passivate the metal
section and, while the first step is terminated, there is a connection between
the
sacrificial anode and the metal section so that the sacrificial anode
continues to
provide corrosion protection.
Preferably this is used where, in a first step, the sacrificial anode is
connected to the metal section to provide corrosion protection of the metal
section
by corrosion of the sacrificial anode which generates corrosion products of
the
sacrificial anode material in the ionically conductive material and wherein,
in a
second step after corrosion of the sacrificial anode has occurred, the current
applied
by the DC power supply through the ionically conductive material causes the
sacrificial anode ions, from the corrosion products of the sacrificial anode
material, to
be re-deposited on the sacrificial anode. In a similar manner, sacrificial
anode ions
may be deposited to create a sacrificial anode or to increase the size of an
existing
sacrificial anode.
In this method the recharging or deposition process can be used
repeatedly and periodically to ensure continued operation of the anode
apparatus
over a much longer period than would be possible with the given quantity of
the zinc
or other galvanic material such as aluminum, magnesium or other material
(which is
less noble than the metal section to be protected) in the anode. This can be
done,
for example, using a solar cell where the re-charging occurs each day.
Alternatively
and more typically, this is done by periodic maintenance where a worker visits
the
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site periodically and applies a power supply for a period of time necessary to
effect
the re-charging.
In one particular aspect of the invention, which is independently
patentable, there is provided an apparatus for cathodically protecting a metal
section
in an ionically conductive material comprising an impressed current anode and
a
conductor which can eventually form a sacrificial anode where the sacrificial
anode
and the impressed current anode comprising components of the anode apparatus
so
that, when the components of the anode apparatus are located in contact with
the
ionically conductive material, each of the sacrificial anode and the impressed
current
anode is in ionically conductive communication with the other and with the
metal
section. In this way the above described charging of the sacrificial anode can
take
place wholly in situ so that the ions are applied onto the conductor by
deposition
from the ionically conductive material. As set out above, the impressed
current
anode and the sacrificial anode should be electrically separated to prevent
electrical
communication therebetween
In one arrangement, to provide the ions, the impressed current anode
can be formed of the sacrificial anode material such as zinc so that
application of the
DC power causes corrosion of the impressed current anode and generates
sacrificial
anode ions which are then available to be deposited on the sacrificial anode.
However the ions of the sacrificial material can be provided in the ionically
conductive material itself or in an additional filler material at or adjacent
the sacrificial
anode.
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Preferably, simultaneously with the connecting of the second terminal
of the DC power supply to the sacrificial anode, the second terminal of the DC
power
supply is also connected to the metal section such that the first terminal of
the power
supply is connected to the impressed current anode and the second terminal of
the
5 power supply is connected to the sacrificial anode and the metal section.
This
arrangement can be used not only to cause the recharging action but also acts
to
provide enhanced protection of the metal section by generating a protective
current
which may be greater than the galvanic current alone to effect passivation of
the
steel (metal section) while providing the option of re-charging the
sacrificial anode at
10 the same time.
Connecting the sacrificial anode to the metal section can provide a
galvanic corrosion protection back up to provide corrosion protection to the
metal
section when the DC power supply or impressed current anode system is not
functional. Having the sacrificial anode connected to the metal section
provides a
15 simple, automatic corrosion protection back up system should the
impressed current
system become non-operational.
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 of zinc, which is less noble than the metal section so
that it
preferentially corrodes relative to the metal section to be protected. The
impressed
current anode is one which is used in conjunction with an external power
supply and
does not need to be less noble than the metal section. Typically such
impressed
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current anodes are formed of titanium, 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.
The sacrificial anode and the impressed current anode preferably form
common components of the anode apparatus. That is, the apparatus as supplied
for
use includes both components as a common system. However they may or may not
be assembled into a common attached construction which can be inserted into
the
material or applied onto the surface as a common assembly. A common assembly
is, of course, preferred for convenience but the components can be inserted
separately, for example, in one or in separate drilled holes or slots in a
concrete
construction, installed in new concrete or applied separately to the concrete
surface
or elsewhere. The impressed current anode for example can be applied
temporarily
to the outer surface of the ionically conductive material such as a plate
attached to
the exterior surface of the concrete for recharging sacrificial anodes within
the body
of the concrete.
The apparatus preferably includes as a part of the apparatus a DC
power supply with positive and negative terminals. This can be any form of
device
which can provide a DC output at a required voltage such as a battery, solar
cell or it
can be a rectifier. The power supply may also be supplied separately and/or
temporarily so that it is not itself an integral component of the apparatus.
However
in the method of use of the system a suitable source of DC power must be used
at
least during a part of the time.
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As a further component of the apparatus, there is preferably provided a
switchable junction box having connectors for connection to the positive and
negative terminals of the power supply, to the first and second electrical
connectors
and to the metal section. This can, however, be provided as separate
components,
again not an integral part of the system. Also connections can be made on site
without a specific switchable junction box.
Preferably the impressed current anode is perforated so to allow
passage of ionic current to pass through the impressed current anode. However
this
is not essential since the impressed current anode and the sacrificial anode
can
comprise separate elements merely located in adjacent relationship for
cooperation
in the material. The ionic current must pass from the sacrificial anode to the
metal
section but this can pass through or around the impressed current anode or
around
parts of the impressed current anode. However, where the sacrificial anode and
the
impressed current anode are formed as a common assembly, it is preferred that
the
ionic current passes through or around the impressed current anode. The
impressed
current anode may therefore be formed as separate pieces or spaced apart to
allow
current to pass to the metal section. Thus for example the impressed current
anode
can be perforated by macroscopic holes formed through or cut into the anode.
In another preferred example, the impressed current anode is formed
from electrically conductive components in a matrix and there are provided
spaces in
the matrix between the conductive components to allow the ionic current to
pass
through the matrix. This can be achieved, for example, by sintering the anode
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material and / or other materials or reducing oxides to form an electrically
conductive
matrix.
In order to obtain uniform, symmetrical deposition of the anode
material on the sacrificial anode during recharging, when that process is
being used,
it is preferred that the impressed current anode surrounds the sacrificial
anode, that
is the impressed current anode is arranged in a plane containing the
sacrificial
anode to fully, substantially fully, partially, or discretely surround the
sacrificial anode
so that ionic current passing to or from the sacrificial anode around 360
degrees in
the plane passes through the impressed current anode. If the impressed current
anode is arranged wholly or partly to one side, the deposition will occur
preferentially
to that side and hence may be less effectively deposited. Therefore
preferably, in a
coaxial arrangement, the sacrificial anode forms a rod or cylindrical element
and the
impressed current anode forms a sleeve surrounding the rod or element. The
cylindrical element may comprise solid sacrificial material and / or
sacrificial anode
material in the form of particles or powder which may be intermixed with
ionically
conductive filler material. The cylindrical element of sacrificial anode
material may
be contained within or encased in an ionically conductive sleeve or membrane.
Alternatively, the sacrificial anode may be in the form of a plate or rod or
ribbon or
other similar element and the impressed current anode may be placed on one
side
of the plate such that the deposition may occur primarily on the one side of
the
sacrificial anode to which the impressed current anode is placed.
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Preferably there is provided an ionically conductive filler material which
is not the ionically conductive material itself which is located between the
impressed
current anode and the sacrificial anode and thus preferably in the coaxial
arrangement, the filler material forms a cylinder surrounding the rod or
cylindrical
element. Preferably, the ionically conductive filler material is in ionic
contact with at
least part of the surface of the sacrificial anode.
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 alkaline 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.
Other 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.
Other additives may be included for specific purposes. One such
specific but important function is the prevention, as much as possible, of
gassing at
the anode either at discharging or during recharge. The addition of zinc oxide
within
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the activator achieves reduction of gassing of the active zinc during
discharge and
also permits overcharge of the anode without damage. Other additions that
limit
gassing during discharge or during anode inactivity are surfactants. Examples
of
these are perfluorosurfactants and polyethylene glycol (PEG) of intermediate
5 molecular weight (300-1000), more preferably 400, which also acts as
cathodic
inhibitor. Surfactants are also known to promote more uniform and efficient
dissolution of the metal.
More importantly, gassing during recharging of the anode should be
avoided so that a more uniform solid metal is formed. The use of additives,
such as
10 cellulose or alloying zinc metal with suitable elements (<10%) such as
nickel or
indium, can reduce the hydrogen over-potential significantly and hence limit
hydrogen gassing.
These additives reduce dendritic growth of the deposited metal
resulting in a more even and uniform deposit. Generally the additives are
specifically
15 adsorbed at rapid growth sites such as dendrites and restrict further
growth so new
growth sites are initiated.
In the case of zinc or zinc powder, additions of lead, bismuth, calcium
and aluminum improve uniformity of material consumption and utilisation.
For use during a sacrificial or galvanic phase of operation of the above
20 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
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contain 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 concreteor
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, but this is not essential and the anode may be only
partially buried or in 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
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 intention is therefore, in the arrangement described in more detail
hereinafter, to marry a galvanic anode with an impressed current anode for use
with
an impressed current and/or re-chargeable galvanic anode system. The
configuration allows the impressed current anode to deliver current either to
the steel
reinforcement or the galvanic anode separately, or to both the steel
reinforcement
and the galvanic anode concurrently. The anode assembly can be used in three
different ways, that is, a) as a normal galvanic anode, b) as an impressed
current
CA 02879225 2015-01-21
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anode, and c) importantly, as a rechargeable galvanic anode. The assembly
preferably includes an inner solid and / or granular zinc core acting as the
galvanic
anode, surrounded by or interspersed with a suitable activating electrolyte.
The zinc
and activator are preferably encased within a porous or mesh-type impressed
current electrode.
The galvanic anode provided herein can therefore be flexible in
operation so that continuous protection can be provided to a structure or
structural
component over periods compatible with impressed current corrosion protection
systems.
The configuration can allow the impressed current anode to deliver
cathodic current either to the steel reinforcement, to the galvanic anode or
to the
steel reinforcement and galvanic anode together. The anode assembly is to be
used
in three different ways, viz., as a normal galvanic anode, as an impressed
current
anode and most importantly, as a rechargeable galvanic anode. The latter
capability
allows multiple use of the same mass of zinc as it is recycled into the
activating
electrolyte and back from the electrolyte in the recharging process,
eliminating the
need for the use of larger volume anodes for long term protection.
In a preferred arrangement in an alkaline activator, the corrosion
product of zinc is ultimately believed to be primarily zinc oxide. It is
possible,
therefore, to reverse the corrosion process and redeposit zinc metal back into
the
anode assembly. The arrangement described herein provides a method of re-
depositing zinc metal without having to remove the anode assembly from the
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23
structure it is protecting. A counter or impressed current electrode which can
be
used as an anode for re-charging the zinc is provided. This counter electrode
is
preferably part of the anode assembly. The same electrode can then be utilised
if
there is a need to change the setup into an impressed current system.
The sacrificial anode may be any of the more electro-negative
materials such as zinc, aluminum, magnesium or alloys thereof.
The DC power supply can be a battery. The power supply may be a
rectifier generating DC power from an AC supply voltage. Preferably the DC
power
supply has a potential greater than 1.5V. Where the power supply is a battery
it can
be rechargeable. Where the power supply is a battery it can be replaceable in
the
assembly. This is a convenient way periodically to do the recharge and/or
provide
an additional step of the impressed current to the steel by inserting a new
battery
and just leaving it until it becomes depleted, whereupon and the system then
works
galvanically until a later time when the depleted battery is removed and
another one
is inserted. The battery can be mounted at any convenient location, such as in
the
junction box or monitoring unit or somewhere convenient. A single battery can
supply power to a group of anodes.
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.
According to another aspect of the invention there is provided a
method for corrosion protection of a metal section in an ionically conductive
covering
CA 02879225 2015-01-21
24
material comprising:
locating an impressed current anode in contact with the ionically
conductive material;
locating a sacrificial anode of a material which is less noble than the
metal section in contact with the ion ically conductive material;
providing a DC power supply;
providing a connection of the DC power supply between the impressed
current anode and the metal section so as to create a current between the
metal
section and the impressed current anode to provide corrosion protection of the
metal
section;
and providing a connection between the sacrificial anode and the metal
section so that the sacrificial anode can provide corrosion protection of the
metal
section.
The connection between the impressed current anode and the metal
section and the connection between the sacrificial anode and the metal section
can
be in place simultaneously or either can be connected when required. The
connection of either can be carried out manually using simple connectors or
using a
switch box or by an automatic control system.
In one arrangement of the above method, the connection of the DC
power supply provides an initial impressed current and, when the initial
impressed
current is terminated, the connection between the sacrificial anode and the
metal
section continues to provide corrosion protection.
CA 02879225 2015-01-21
In another arrangement of the above method, the connection between
the sacrificial anode and the metal section provides corrosion protection and,
subsequent to a period of the corrosion protection provided by the sacrificial
anode,
the DC power supply is connected between the impressed current anode and the
5 metal
section causing the metal section to be further protected. This can be carried
out periodically during the operation of the sacrificial anode. After initial
installation,
the first action in protection can be either the sacrificial anode or the
impressed
current anode as selected by the person skilled in the art in accordance with
the
status of the installation.
10 In both
cases, the connection of the sacrificial anode can be in place
while the impressed current is connected or can be connected when the
impressed
current is terminated.
Preferably the initial current provided by the impressed current anode
is sufficient to passivate the metal section. However the specific effect
obtained in
15 the
first step is not essential and other effects can be obtained advantageously
using
this method.
Preferably the sacrificial anode and the impressed current anode
comprise common components of the anode apparatus so that, when the common
components of the anode apparatus are located in the ionically conductive
material,
20 each of
the sacrificial anode and the impressed current anode is in ionically
conductive communication with the other and with the metal section. However
separate anode elements can be provided.
CA 02879225 2015-01-21
26
Preferably the impressed current anode and the sacrificial anode are
electrically separated to prevent electrical communication therebetween.
Preferably there are provided connectors for connection to the positive
and negative terminals of the power supply, with a first electrical connector
connected to the impressed current anode, with a second electrical connector
connected to the sacrificial anode and/or to the metal section.
Preferably the impressed current anode is perforated so to allow
passage of ionic current in the ionically conductive material to pass through
the
impressed current anode. Many different techniques can be provided to obtain
the
effect of the perforation so that the ionic current can pass through.
Preferably the sacrificial anode forms a rod or tubular or cylindrical
element and the impressed current anode forms a sleeve surrounding the rod or
element. However other arrangements can be provided such as parallel or side
by
side plates.
Preferably there is provided an ionically conductive filler material
adjacent to the sacrificial anode where the ionically conductive filler
material is
different from said ionically conductive material.
Preferably the ionically conductive filler material contains at least one
activator to ensure continued corrosion of the sacrificial anode. Many
different types
of activator are available and can be used.
CA 02879225 2015-01-21
27
Preferably the ionically conductive filler material 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.
In one example there is provided a plurality of separate sacrificial
anodes and the impressed current anode is separate from said sacrificial
anodes.
In another example there is provided a plurality of impressed current
anodes and wherein the sacrificial anode is separate from said impressed
current
anodes. However, typically the sacrificial anode and the impressed current
anodes
are parts of a common construction.
In another arrangement which can be used, the impressed current
anode is mounted temporarily to provide current through a surface of the
ionically
conductive material. That is the impressed current anode is arranged to be
mounted
(utilized/ installed and operated) temporarily during charging of the
sacrificial anode.
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 protection systems such as for pipes or other constructions in soil,
and in
many other systems where such anodes can be used.
Preferably there is provided a porous or deformable material to absorb
corrosion products from the sacrificial anode. This may be an encapsulating
component or may be in the sacrificial anode itself.
Preferably the assembly includes a reinforcing layer, such as disclosed
CA 02879225 2015-01-21
28
in US Patent 7,226,532 issued June 5 2007 to Whitmore, to which reference may
be
made, 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.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be described in conjunction
with the accompanying drawings in which:
Figure 1 is a schematic illustration of a corrosion protection method
according to the present invention using a first arrangement of anode
apparatus.
Figure 2 is the schematic illustration of Figure 1 showing the
connection of the components for operation in the sacrificial protection mode.
Figure 3 is the schematic illustration of Figure 1 showing the
connection of the components for operation in the impressed current protection
mode.
Figure 4 is the schematic illustration of Figure 1 showing the
connection of the components for operation in the re-charging mode.
Figure 5 is the schematic illustration of Figure 1 showing the
connection of the components for operation in the combined recharging and
impressed current modes.
Figure 6 is a schematic illustration of a corrosion protection method
CA 02879225 2015-01-21
29
according to the present invention using a second arrangement of anode
apparatus.
Figure 7 is a schematic illustration of a further corrosion protection
method according to the present invention using a further arrangement of anode
apparatus where an existing sacrificial anode is re-charged by a temporary
plate
electrode mounted on an exterior surface of the concrete ionically conductive
material.
Figure 8 is a cross-sectional view through a first example of an anode
apparatus according to the invention.
Figure 9 is a graph of current output of the anode of Figure 8 to steel,
a) with the anode as originally made, b) with the anode after a period of
charging via
the porous conductive impressed current anode.
Figure 10 is a graph of cumulative charge output of the anode of
Figure 8 to steel, a) with anode as originally made, b) after a period of
charging via
the porous conductive tube.
Figure 11 is a cross-sectional view of a second example of an
=
apparatus according to the present invention.
Figure 12 is a plan view of a test arrangement for the embodiment of
Figure 11.
Figure 13 is a schematic illustration of an experimental specimen used
for determining solution movement at an applied current between two electrodes
embedded in highly alkaline mortar.
Figure 14 is a graph showing an increase in volume of solution in the
CA 02879225 2015-01-21
=
tube adjacent to the negatively charged electrode in Figure 13.
In the drawings, like characters of reference indicate corresponding
parts in the different figures.
DETAILED DESCRIPTION
In Figure 1 is shown a covering material 10 within which is embedded
steel material 11 and an anode body 12.
The covering material 10 is a suitable material which allows
communication of ions through the covering material between the anode body 12
and the steel 11. The covering material is generally concrete but can also
include
10 mortar or masonry materials, or soil, water or other ionically
conductive material,
where there is a steel structure which requires corrosion protection to
prevent or
inhibit corrosion. The steel material 11 is illustrated as being a reinforcing
bar
arrangement but other steel elements can be protected in the manner of the
arrangement shown herein including steel structural members such as lintels,
steel
15 beams and columns, pipes, tanks or other elements in contact with the
concrete or
other covering material.
The anode member may include or be constructed in part as the
arrangement shown in US Patents 6,165,346 issued Dec. 26, 2000; 6,572,760
issued June 3, 2003 6,793,800 issued Sept. 21, 2004, 7,226, 532 issued Jun 5,
20 2007, 7,914,661 issued Mar 29, 2011, and 7,959,786 issued Jun 14, 2011
of the
present inventor, and in 6,022,469 (Page) issued Feb. 8, 2000 and 6,303,017
(Page
and Sergi) issued Oct 16, 2001 assigned to Vector Corrosion Technologies and
in
=
CA 02879225 2015-01-21
31
6,193,857 (Davison) issued Feb. 27, 2001 assigned to Vector Corrosion Tech.,
Bennett 6,217,742 issued April 17, 2001, 7,160,433 issued Jan 9, 2007,
8,157,983
issued Apr 17, 2012 and 6,471,851 issued Oct 29, 2002 assigned to Vector
Corrosion Technologies, Giorgini 7,998,321 issued Aug 16, 2011, Schwarz
7,851,022 issued Dec 14, 2010, Glass et al. 8,211,289 issued July 3, 2012,
8,002,964 issued Aug 23, 2011, 7,749, 362 issued Jul 6, 2010, 7,909,982 issued
Mar 22, 2011, and 7,704,372 issued Apr 27, 2010 assigned to Vector Corrosion
Technologies, to which reference should be made for further details as
required.
A DC power supply 14 is provided which generates a voltage at
terminals 15 and 16 of the power supply.
In the embodiment shown the power supply is formed by a battery
which may be a lead acid battery with an output of 6 or 12 volts and a
lifetime of 1 to
weeks, or may be a zinc air battery well known and commercially available
which
provides an output voltage of the order of 1.5 volts and has a lifetime of the
order of
15 3 to 5 years. The voltage may drop during current draw in operation from
the
nominal value of 1.5 volts to as low as 1.0 volts. Such batteries of this type
are
commercially available from ENSER Corporation or others. A suitable battery
may
have a capacity up to 1200 ampere hours.
Alternative power supplies may be used including solar panels and
20 conventional rectifiers which require an exterior AC supply voltage and
which
convert the AC supply into a DC voltage at the terminals 15 and 16.
CA 02879225 2015-01-21
32
The anode apparatus 12 includes a sacrificial anode 20 of zinc or other
material which is less noble than the metal section together with an impressed
current anode 21. The sacrificial anode 20 is in the form of a rod and the
impressed
current anode 21 is in the form of a sleeve surrounding the rod with an
ionically
conductive filler material 22 which is generally not the ionically conductive
material
located as a cylinder between the impressed current anode 21 and the
sacrificial
anode 20. In this coaxial and combined structure, the impressed current anode
is
arranged in a radial plane of a central axis of the rod to fully surround the
circumference of the sacrificial anode so that ionic current passing to or
from the
10 sacrificial anode around 360 degrees in the plane generally passes through
the
impressed current anode on its path to the steel 11.
Thus the sacrificial anode 20 and the impressed current anode 21 form
common components of the anode apparatus 12 so that each of the sacrificial
anode
and the impressed current anode 21 is in ionically conductive communication
with
15 the other and with the metal section. The filler material is not
electrically conductive
so that the impressed current anode and the sacrificial anode are electrically
" separated to prevent electrical communication therebetween.
A switchable junction box 23 is provided having connectors 231 and
232 for connection to the positive and negative terminals of the power supply.
The
20 box further includes a connector 233 to a lead 236 to the impressed
current anode
21, a connector 234 to a lead 237 to the sacrificial anode 20 and a connector
235 to
a lead 238 to the metal section 11. Leads 236, 237 and 238 are preferably
wires and
CA 02879225 2015-01-21
33
are preferably corrosion resistant. Lead 236 has the greatest need for
corrosion
resistance as it is connected to an impressed current anode during operation.
Examples of corrosion resistant materials for the impressed current connection
include titanium, niobium, nickel, platinized wires and insulated wires.
The impressed current anode is perforated either with macroscopic
holes 211 or a microscopic structure so to allow passage of ionic current from
the
anode 20 to pass through the impressed current anode. Macroscopic holes can be
provided by forming the impressed current anode in separate pieces.
In the arrangement where the anode 21 is perforated microscopically,
the impressed current anode has sufficient porosity and ionically conductive
material
within the spaces between the impressed current anode material to allow the
ionic
current to pass through the impressed current anode.
The ionically conductive filler material 22 preferably contains at least
one activator to ensure continued corrosion of the sacrificial anode. 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 or minimized. For zinc, this pH is typically greater than 12 and may
be
greater than 13, 13.3 or 13.4. It is preferable that the zinc corrosion
products remain
partially or substantially soluble. This can be achieved by suitable pH or by
- incorporating ions or other chemicals which are corrosive to the sacrificial
anode
material and/or prevent the surface of the sacrificial anode material from
passivating.
CA 02879225 2015-01-21
34
Examples of materials which help to produce soluble corrosion products and /
or
prevent passivation are disclosed in the patent documents referenced above.
The ionically conductive filler material 22 is also preferably highly
ionically conductive, hygroscopic, and will accommodate volume changes as the
sacrificial anode is charged and discharged. The ionically conductive filler
material
may also be porous or deformable to accommodate these changes.
In Figure 6 is shown a schematic illustration of the method using a
second arrangement of anode apparatus in which the sacrificial anode 20A and
the
impressed current anode 21A are formed as two parallel plates, mesh, ribbon or
wires with the filler material 22A therebetween. In this case the re-charging
of the
sacrificial anode may occur primarily on one side. In an alternative
construction, the
two parallel layers of plates or mesh may be applied to the surface of the
covering
material.
In Figure 7 is shown a schematic illustration of the method using a
further arrangement of where an existing sacrificial anode 40 is re-charged by
a
temporary surface applied electrode (impressed current anode) 41 on an
exterior
surface of the concrete 10 forming the ionically conductive material. In this
case a
conductor 42 connects the impressed current anode 41 to one terminal of the
power
supply 14 and a conductor 43 connects the buried sacrificial anode 40 to the
other
terminal of the DC power supply. At the same time the second terminal can be
connected to the steel if the protection of the steel is intended to continue
during the
re-charging process. Although the surface applied electrode is a preferred
CA 02879225 2015-01-21
embodiment for recharging an existing sacrificial anode, other impressed
current
anodes such as embedded impressed current anodes may be used.
The four separate functions provided by the junction box can be
performed simply as follows. These functions may also be performed manually by
5 direct connection of the appropriate connectors without the need for a
junction box.
a) Normal galvanic anode as shown in Figure 2: the zinc core is
connected to the steel via the junction box. The impressed current anode is
set at
the off position. This allows the anode to perform as a simple galvanic anode.
b) Impressed current anode as shown in Figure 3: the zinc anode
10 is set
to the off position and the impressed current anode is connected to the steel
via the DC power source. The current output can be regulated by controlling
the
applied voltage.
C)
Recharging of galvanic anode as shown in Figure 4: the
impressed current anode is connected via the DC power source to the zinc
anode.
15 The
steel is set to the off position. This allows the zinc ions or zinc corrosion
products present in the electrolyte to be deposited onto the zinc core as zinc
metal
building up the thickness of the zinc anode. Zinc oxide, zincates and zinc
hydroxide
are three common corrosion products produced while the zinc anode is in
operation.
d)
Recharging of galvanic anode and impressed current as shown
20 in
Figure 5: the impressed current anode is connected via the DC power source to
both the zinc anode and the steel. This allows the re-charging process
described at
C) and the impressed current described at b) to proceed concurrently.
CA 02879225 2015-01-21
36
The first two functions are well understood and need no further
description. However the arrangement, where both options are available (and
operable) concurrently is novel.
The third function is novel with respect to the use of galvanic anodes
for steel reinforcement protection and involves making the zinc anode cathodic
allowing deposition of zinc. Zinc may be deposited from a number of zinc
compounds and through various reactions and is likely to include Reactions 1,
2 and
3 if zinc is in an alkaline environment.
ZnO + 20H" + H20 Zn(OH).42" (1)
Zn(OH)42- Zn2+ + 40H- (2)
Zn2+ + 2e" Zn (3)
Theoretically, all the zinc oxide and other zinc ions and zinc corrosion
products can be re-deposited on the core as usable zinc for subsequent
consumption. In reality, as with rechargeable alkaline batteries, the level of
each
subsequent recharge is likely to be reduced.
A typical reaction at the impressed current electrode is likely to be:
20H- 1/202 + H20 + 2e- (4) or
H20 --> 1/202 + 2H+ + 2e- (5)
There is therefore a net balance of the hydroxyl ions which means
there is no overall loss in alkalinity within the assembly. There is a net
increase in
hydroxyl ions at the surface of the zinc anode which is beneficial in
accommodating
large amounts of the soluble zincate ions once the anode is used again, in
galvanic
CA 02879225 2015-01-21
37
mode, to protect the steel reinforcement. The reaction at the impressed
current
anode (Eq 4 or 5) involves the production of oxygen gas which needs to escape
from the assembly and into the concrete pore structure. The impressed current
anode, therefore, should be porous, be in the form of a net or be vented.
A preferred way to employ the anode arrangement herein is to initially
set it up as a normal galvanic anode, allowing it to run for a period of say
10-20
years according to exposure conditions. Occasional monitoring will determine
when
recharging of the anode is required. An external power supply is then used to
recharge the anode over a relatively short period, preferably no more than 14 -
60
days. The anode is then able to produce adequate current for a further period
of
time, say 5 ¨ 20 years. The process can be repeated several times until
recharging
becomes essentially ineffective. If required, the impressed current part of
the anode
can then be simply used as part of an impressed current corrosion protection
system. Protection of the steel reinforcement could therefore be achieved for
the
whole life of the structure.
The assembly has great flexibility which allows variable application
types. For example, a preliminary use of the impressed current part of the
anode can
deliver an initial high level of charge over a limited period in order to
passivate the
steel to virtually stop any ongoing corrosion. Alternatively, the impressed
current part
of the anode can be operated to deliver a cumulative charge to increase the
alkalinity of the concrete surrounding the steel and reduce future corrosion
and
current demand from the galvanic anode. Applied charge of 20,000 to 150,000
and
CA 02879225 2015-01-21
38
more typically, 70,000 to 120,000 Coulombs per square meter of steel has been
shown to be sufficient to passivate the steel. Applied charges of around
700,000
Coulombs/m2 have been effective at re-alkalizing (increasing the pH) of
carbonated
concrete. The charge required to increase the pH of concrete which is not
carbonated will be less than 700,000 Coulombs/m2. This can then be followed by
a
lower level of galvanic current to maintain passivity of the steel. Using the
impressed
current anode to deliver the high initial charge is beneficial as this
prevents
unnecessary consumption and degradation of the sacrificial anode, allows a
smaller
sacrificial anode to be used and allows the sacrificial anode to provide
higher current
to the steel after the high initial charge has been passed to the steel by the
impressed current anode. Recharging of the anodes can still be carried out if
required. Furthermore, additional externally applied current can be delivered
via the
impressed current anode of the assembly if steel passivity is lost, if the
current from
the sacrificial anode is not sufficient to polarize the steel or if either the
corrosion
potential or the corrosion rate of the steel increases above desired levels.
The sacrificial anode may be connected to the steel while the
impressed current anode is polarising the steel for the purpose of
reactivating or
increasing the activation of the sacrificial anode. This can be achieved by
increasing
the alkalinity at the anode surface which can dissolve zinc oxide corrosion
products
into soluble zincate ions, according to equation (1), and allow them to
dissipate away
from the anode surface and allowing better subsequent current flow and
improved
performance of the anode.
CA 02879225 2015-01-21
39
The current flowing to the sacrificial anode may be limited or controlled
such that the sacrificial anode is reactivated without necessarily recharging
the
sacrificial anode.
The assembly also has the capability to operate principally as an
impressed current anode with a rechargeable galvanic anode backup for periods
when the impressed current anode is off line or is otherwise non-functional.
Similarly, the impressed current anode can be available to operate as a backup
to
the sacrificial anode should the sacrificial anode become non-functional.
In a preferred arrangement, the inert anode may be capable of
delivering a high level of current, possibly as high as 1mA/cm2. The
resistance of
the electrolyte is preferably therefore as low as possible, so that a gel may
be more
suitable than a solid. Considerable levels of oxygen gas can be produced
during
charging which needs to disperse adequately through the anode walls and
surrounding concrete.
In order for the anode to be rechargeable, the electrolyte is preferably
highly alkaline. This allows high concentrations of Zn(OH)42" in solution
after the
dissolution of zinc which, with supersaturation, is believed to precipitate
out as ZnO.
These reactions are believed to be as set out in Equations 6 and 7 below,
which are
essentially the reverse of Reactions 1 and 2.
Zn + 401-I" Zn(OH)42" + 2e" (6)
Zn(OH)42" ZnO + 20H" + H20 (7)
Other electrolytes which are not highly alkaline are also suitable as
CA 02879225 2015-01-21
long as soluble or electrochemically mobile zinc ions are present.
Preferably the assembly includes sufficient moisture to be highly
ionically conductive and to allow sacrificial anode ions to be mobile during
charging
or recharging. Humectants, gels and other hydroscopic materials can be
beneficial
5 in this
regard. In an alternative arrangement, charging or recharging of sacrificial
anodes can be improved by applying water or another wetting solution to at
least a
portion of the structure and or specifically the sacrificial anode to keep it
sufficiently
conductive during the charging or recharging process.
Testing has shown that zinc can be deposited onto many substrates
10
including; zinc, titanium, copper, brass, 70/30 brass, steel, stainless steel
and alloys.
As such, partially discharged and fully consumed sacrificial anodes can be
regenerated.
Example 1
15 In one
example, a cast zinc anode, 8cm long with a minimum diameter
of 0.7cm, was located in ZnO/thixotropic paste packed inside a conductive
ceramic
impressed current anode tube. The zinc paste was made from a solution
saturated
with LiOH with 2M KOH and 20% ZnO along with carboxymethyl cellulose sodium
gelling agent. The paste was packed in the space between the zinc anode and
the
20 inner
side of the 28mm tube. Testing has shown that ions can pass through the
porous tube walls such that the zinc anode can pass current onto the external
steel
reinforcing bar even though it is located inside the impressed current anode.
CA 02879225 2015-01-21
41
Subsequently, charging of the zinc can be accomplished by reversing the flow
of
ions through the impressed current porous tubular anode by applying an
external
voltage between the impressed current anode and the sacrificial anode. An
applied
voltage of around 6-8 Volts resulted in a current of up to 1.6A to be
delivered to the
inner zinc anode achieving a total charge / recharge of just under 40,000
Coulombs.
Surprisingly, the zinc anode performed better after recharging than it did
originally.
After charging of the zinc anode, when the zinc anode was reconnected to the
steel,
the current output and cumulative charge output of the recharged zinc anode
through the porous tubular impressed current anode to the steel is increased
compared to the original zinc anode. The reasons for this improvement in
performance are not fully understood but may relate to an increased surface
area of
the zinc metal after deposition or to the relative increase of hydroxyl ions
at the
immediate vicinity of the zinc surface which encourages dissolution of zinc
and zinc
corrosion products such as zinc oxide and deposition of zinc from zinc
corrosion
products such as zincate ions (equations 1-3). It is evident, nonetheless,
that the
current output of the anode after charging is increased.
In Figure 8 shows an example of an anode apparatus 30 as previously
described where the apparatus includes a Cast Zinc Core 31 inside a 28mm
diameter porous conductive impressed current anode 32. An upper end is closed
by
an attached disk 33 forming a porous form and a lower end is closed by a
Porous
Fabric Cap 36. Between the core 31 and the cylindrical anode 32 is provided a
filler
material of LiOH + 2M KOH + 20% ZnO + carboxymethyl cellulose sodium 35. The
CA 02879225 2015-01-21
42
core is attached to a steel wire 34 for connection as described above.
Figure 9 is a graph of current output of the anode of Figure 8 to steel,
a) with the anode as originally made, b) with the anode after a period of
charging via
the porous conductive impressed current anode.
Figure 10 is a graph of cumulative charge output of the anode to steel,
a) with anode as originally made, b) after a period of charging via the porous
conductive tube.
Example 2
An assembly 49 to demonstrate the ability to charge/recharge an
anode in situ was constructed as shown in Figure 11. It consisted of a zinc
wire 50
partly immersed in a highly alkaline (7 molar OH-) gel 51. A copper wire
connector
53 for the sacrificial anode to be formed in situ was also immersed in the
same gel.
The gel was contained within a perforated plastic tube 54 lined both
internally and
externally by a layer of fibre fabric 55 and ionically conductive membrane
acting as a
separator of the anode and cathode 56. Between the external fabric and the
tube a
mixed metal oxide (MMO) coated titanium mesh 57 was fixed circumferentially
and
had a titanium connection wire 58 attached to one side. The whole assembly was
encased in a mortar 59 enriched with Li0H.
The anode assembly 49 was cast centrally in a cement mortar prism
approximately 80mm x 50mm x 40mm high ensuring that the whole assembly was
encased within the cement mortar 59. As shown in Figure 12, the prism was then
CA 02879225 2015-01-21
43
placed in a larger container 61 filled almost to the height of the prism with
an alkaline
solution 60. An external mesh 62 of MMO coated titanium was placed along the
periphery of the container to act as the metal section.
The zinc wire 50 was connected electrically to the external titanium
mesh 62. The assembly 49 was then seen to act as a galvanic anode passing
current to the external titanium mesh (metal section) and producing zinc
corrosion
products until all available zinc was consumed.
An external power supply (not shown) was then connected to the
internal MMO coated titanium mesh anode 57 within the anode assembly 49 and
the
copper wire 53 ensuring that the copper was cathodic. Zinc corrosion products
from
the consumed (corroded) zinc wire 50 were deposited on the copper wire 53 to
form
a sacrificial anode during this charging process. Subsequent connection of the
copper wire 53, now carrying the deposited zinc and the external MMO coated
titanium mesh (metal section) allowed current to pass between the charged
anode
53 and the metal section 62. The current produced by the charged anode (copper
wire with deposited zinc) was comparable to the current produced by the
original
zinc wire. Comparison of current produced by the original 'discharge' of the
zinc
wire and the zinc which was deposited on the copper wire is shown in Table 1.
Table 1: Current output of original zinc wire and deposited zinc on copper
wire
Current output (mA)
Maximum Minimum Mean
CA 02879225 2015-01-21
44
Original zinc wire 5.47 0.05 0.70
Deposited zinc on 5.20 0.05 0.51
Copper Wire
Turning now to Figures 13 and 14, a sacrificial anode 100 and an
impressed current anode 101 are provided in the ionically conductive covering
material 102 where a DC power supply 107 is connected across the sacrificial
anode
and impressed current anode. This provides a recharging phase which can be
carried out during cathodic protection where the sacrificial anode is
connected to the
steel 108 or as a separate step.
Movement of moisture towards the sacrificial anode 100 from the
impressed current anode 101 is obtained during the recharge phase. This is
believed to be by the process of electro-osmosis. A simple experiment, as
depicted
in Figure 13, demonstrated measurable water movement from the negatively
charged electrode 101 to the positively charged electrode 100 embedded in the
high
alkalinity mortar 102, the latter 100 representing the sacrificial anode
material during
recharge. Item 103 represents a perforated plastic tube at the anode 101.
Added
water 104 was detected at the second plastic tube 105.
Table 2 below and Figure 14 summarise the increase in volume of
water observed with time of the recharged anode at a current of 1.5mA. This
application of current thus results in water movement which can replenish the
electrolyte around the anode and facilitate better deposition of zinc metal
during
CA 02879225 2015-01-21
recharge.
Table 2, Moisture movement with time at an applied current of 1.5mA
between two electrodes embedded in a highly alkaline mortar.
Current output (1.5 mA)
Time (days) Required Drive Difference in Volume
of water
Voltage to height of water which migrates
maintain current level in two during
application
(V) compartments of recharge
(mm) current (ml)
0 1.94 0 0
1 2.23 0.5 0.03
6 2.19 3 0.19
22 2.04 7 0.44
28 1.98 7.5 0.47
5
Also shown in Figure 13 is a separator 106 in the form of a
microporous ionically conductive membrane, which is used primarily to limit or
avoid
dendritic growth of the sacrificial material (zinc metal) during recharge, to
restrict
zinc deposition to within the contained volume encased by the separator while
10 allowing moisture movement and ionic conductivity through its pores. It
is desirable
that the pore size and pore distribution of the separator are such that it
optimises
movement of moisture. It is preferable that it restricts movement of moisture
out of
the anode-encasing electrolyte into the bulk surrounding electrolyte but
allows the
beneficial electro-osmotic movement of moisture back into the anode-encasing
CA 02879225 2015-01-21
46
electrolyte during charging.
As explained previously, the potential difference across the anodes
100, 101 causes ions of the sacrificial anode material to move to the
sacrificial
anode 100.
Additives 109 are provided in the structure at or adjacent the anode
100 which acts to limit gassing from the sacrificial anode. The additives can
be a
surfactant, a form of cellulose or can comprise alloying zinc metal with
suitable
elements such as nickel or indium which is arranged to reduce the hydrogen
over-
potential significantly and hence limit hydrogen gassing.
The membrane 106 acts for restricting dendritic growth of sacrificial
anode material on the sacrificial anode. The membrane separator 106 is located
around or adjacent to the sacrificial anode and acts to contain sacrificial
material at
the sacrificial anode, to avoid dendritic growth of sacrificial anode material
on the
sacrificial anode beyond the membrane and to allow moisture movement to the
sacrificial anode.
In order to provide ions for communication to the anode 100, particles
or powder 110 of sacrificial anode material or sacrificial anode corrosion
products
are provided alone or intermixed with an ionically conductive filler material
111 at or
adjacent the sacrificial anode.
As explained previously the potential difference caused by the DC
power supply 107 causes an increase in the galvanic current generated by the
sacrificial anode subsequent to application of the potential difference
relative to that
CA 02879225 2015-01-21
47
before the application. Also this action acts to increase alkalinity at the
surface of
the sacrificial anode where the increased alkalinity acts to dissolve zinc
oxide
corrosion products into soluble zincate ions and allow them to dissipate away
from
the surface.
The DC power supply is in some cases arranged so as to limit current
flowing to the sacrificial anode by the potential difference such that the
sacrificial
anode is reactivated without recharging the sacrificial anode with additional
ions of
the sacrificial material.
The application of the DC power supply causes water movement from
the negatively charged impressed current anode to the positively charged
sacrificial
anode. This also can cause an increase in a total surface area of the
sacrificial
anode material at the sacrificial anode. This can also result in increased
quantity of
hydroxyl ions at the immediate vicinity of the sacrificial anode.
