Note: Descriptions are shown in the official language in which they were submitted.
CA 02289887 1999-11-18
PATENT
IMPROVEMENT IN CATHODIC PROTECTION SYSTEM
Back round of the Invention
Technical Pield
This invention elates generally to the field of cathodic
protection systems for steel-reinforced concrete structures,
and is particularly concerned with the performance of
cathodic protection systems utilizing conductive paint
anodes.
Description of the Prior Art
The problems associated with corrosion-induced
deterioration of reinforced concrete structures are now well
understood. Steel reinforcement has generally performed
well over the years in concrete structures such as bridges,
buildings, parking structures, piers, and wharves, since the
alkaline environment of concrete causes the surface of the
steel to "passivate" such that it does not corrode.
Unfortunately, since concrete is inherently somewhat
porous, exposure to salt results in the concrete over a
number of years becoming contaminated with chloride ions.
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Salt is commonly introduced to the concrete in the form of
seawater, set accelerators or deicing salt.
When the chloride contamination reaches the level of
the reinforcing steel, it destroys the ability of the concrete
to keep the steel in a passive, or non-corrosive state. It
has been determined that a chloride concentration of 0.6
Kg per cubic meter of concrete is a critical value above
which corrosion of steel can occur. The products of
corrosion of the steel occupy 2.5 to 4 times the volume of
the original steel, and this expansion exerts a tremendous
tensile force on the surrounding concrete. When this
tensile force exceeds the tensile strength of the concrete,
cracking and delaminations develop. With continued
corrosion, freezing and thawing, and traffic pounding, the
utility or the integrity of the structure is finally
compromised and repair or replacement becomes
necessary. Reinforced concrete structures continue to
deteriorate at an alarming rate today. In a recent report to
Congress, the Federal Highway Administration reported that
of the nation's 577,000 bridges, 226,000 (39% of the total)
were classified as deficient, and that 134,000 (23% of the
total) were classified as structurally deficient. Structurally
deficient bridges are those that are closed, restricted to
light vehicles only, or that require immediate rehabilitation
to remain open. The damage on most of these bridges is
caused by corrosion of reinforcing steel. The United States
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Department of Transportation has estimated that $90.9
billion will be needed to replace or repair the damage on
these existing bridges.
Many solutions to this problem have been proposed,
including higher quality concrete, improved construction
practices, increased concrete cover over the reinforcing
steel, specialty concretes, corrosion inhibiting admixtures,
surface sealers, and electrochemical techniques such as
cathodic protection and chloride removal. Of these
techniques, only cathodic protection is capable of
controlling corrosion of reinforcing steel over an extended
period of time without complete removal of the salt
contaminated concrete.
Cathodic protection reduces or eliminates corrosion of
the steel by making it the cathode of an electrochemical
cell. This results in cathodic polarization of the steel,
which tends to suppress oxidation reactions (such as
corrosion) in favor of reduction reactions (such as oxygen
reduction). Cathodic protection was first applied to a
reinforced concrete bridge deck in 1973. Since then,
understanding and techniques have improved, and today
cathodic protection has been applied to over one million
square meters of concrete structure worldwide. Anodes, in
particular, have been the subject of much attention, and
several types of anodes have evolved for specific
circumstances and different types of structures.
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One type of anode that has been used since the late
1970s on concrete surfaces not subject to traffic is
conductive paints and mastics. These anodes typically
consisted of carbon dispersed in solvent or water based
paints, and typically covered the entire surface of the
concrete to be protected. The anodes were applied by
spray, brush, or roller coating the paint onto the concrete
surface after surface preparation, which typically consisted
of light sandblasting or medium to high pressure water
blasting. The black-colored paint was then typically
overcoated with a lighter colored topcoat to improve
appearance. The paint was typically applied to a thickness
of 0.20 to 0.25 mm (8 to 10 mils) dry film thickness.
Current was supplied to the paint by primary anode
conductors, typically 0.8 mm (0.031 inch) diameter
platinized wire or catalyzed ribbon. Since the conductive
paint had limited electrical conductivity, primary anode
conductors were placed relatively close together, typically 3
meters (10 feet), to avoid excessive voltage loss and
uneven current distribution.
Cathodic protection systems utilizing conductive paint
anodes were often successful, but problems were also
commonly encountered. Excessive anode current density
resulted in bond loss and deterioration of conductive paint
anodes, sometimes in a year or less. Failures of conductive
paint anodes were also recorded in wet, freeze-thaw and
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splash-zone environments. The durability of conductive
paints in such environments has improved somewhat with
the development of improved, longer lasting materials.
Conductive paint anodes have also experienced
difficulty when they were placed on the underside of
surfaces that were not chloride contaminated and not
subject to direct wetting. This problem was especially
common in very dry climates, such as the western United
States and Canada. In these cases, it appeared that the
thin layer of concrete immediately beneath the concrete
dried out and became very resistive. In some cases, the
resistance increased greatly and the desired protective
current could not be delivered even at rectifier voltages of
50 volts.
Summary of the Invention
The present invention relates to a method of cathodic
protection of reinforced concrete, and more particularly, to
a method of maintaining or improving current delivery from
an anode used in a cathodic protection system.
The method of the present invention comprises
applying a conductive paint onto an exposed surface of the
concrete in an amount effective to form a conductive paint
anode bonded to the surface. The anode and concrete
have an interface. The conductive paint anode is
permeable. The conductive paint anode is electrically
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connected to the reinforcement through a source of
impressed current.
A lithium salt solution selected from the group
consisting of a lithium nitrate (LiN03) solution, a lithium
bromide (Liar) solution, and combinations thereof, is
applied to the external surface of the anode after the
conductive paint anode has been applied to the concrete.
The lithium salt solution quickly and effectively migrates
through the pores of the permeable anode to the interface
between the anode and the concrete. The lithium salt at
the interface functions to provide improved current
delivery.
The lithium salt solution preferably comprises a
surface active agent which wets the exposed surface of the
conductive paint anode and facilitates migration of the
solution through the anode to the interface of the anode
with the concrete.
Preferably, enough lithium salt solution is applied to
the external surface of the conductive paint anode to
position at the interface of the anode and the concrete
structure at least 10 grams of lithium salt, dry basis, per
square meter of anode.
Preferably, the conductive paint anode has a thickness
which is less than about 20 mils (0.5 mm).
The present invention also resides in a liquid treating
agent applied to an impressed current permeable
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conductive paint anode to provide improved current
delivery, and in a reinforced concrete structure prepared by
the method of the present invention.
Description of
Preferred Embodiments
The present invention relates broadly to all reinforced
concrete structures with which cathodic protection systems
are useful.
Generally, the reinforcing metal in a reinforced
structure is steel. However, other ferrous based metals can
also be used.
The cathodic protection system of the present
invention comprises at least one conductive paint anode at
a surface of the concrete structure. Multiple conductive
paint anodes at spaced intervals are commonly used.
The cathodic protection system is of the impressed
current type. In an impressed current system, a power
supply is positioned in the connection between the anode
and the concrete reinforcement. The power supply
provides an impressed flow of electrical current between
the anode and the reinforcement. The impressed current
flow is opposite and essentially equal to that which
naturally occurs in a reinforced structure which has no
cathodic protection, thus "passivating" the reinforcement.
The net result is very little or no anodic action on the
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reinforcement, and little or no corrosion of the
reinforcement occurs.
Conductive paints comprise an organic polymeric
binder, which can be either water-based or solvent-based,
and electrically conductive particles dispersed in the binder.
Binders commonly used are acrylic copolymers or
homopolymers. A preferred electrically conductive particle
to make the paint conductive is carbon or graphite. These
materials have a relatively low density which aids in their
dispersion in a polymeric binder. Conductive paints are
typically 50-80% carbon or graphite by weight of dry film.
Examples of suitable conductive paint formulations useful
for cathodic protection of reinforced concrete are disclosed
in Patents Nos. 4,931,156 to Dowd et al., 5,364,511 to
Moreland et al, and 5,431,795 to Moreland et al.,
incorporated by reference herein.
The conductive paint may be applied to the surface of
the concrete by spray, brush, or roller coat. Other means
of application of the paint will be apparent to those skilled
in the art. Once applied to the concrete surface, the
conductive paint anode forms an interface with the
concrete. When properly applied and cured, a good bond
results between the anode and the concrete at the anode-
concrete interface. Preferably, the conductive paint is
applied so that it has an average thickness when dried of
less than about 20 mils and more than about eight (8) mils.
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In addition, the conductive paint anode is permeable
to permit the passage of both gasses, such as oxygen and
chlorine from the electrolysis of water and sodium chloride,
respectively, and moisture to and from the concrete
structure. Without some permeability, the conductive paint
would not adhere to the concrete. These criteria are
disclosed, by way of example, in the aforementioned Patent
No. 5,364,511. For the present invention, the conductive
paint anode preferably is sufficiently permeable to allow the
passage of at least 25m1 of solution per square meter of
anode surface in a single application, or the placement of
at least 10 grams, dry basis, of lithium salt (to be
described) at the interface of the anode and concrete
surface. Preferably, the anode will be sufficiently
permeable to allow the passage of 100m1 of solution per
square meter of anode surface in a singe application.
Customarily, the permeability of a conductive paint on
a concrete surface is achieved by controlling the pigment-
volume ratio in the paint, referred to as the PVC value.
This is the ratio of the volume of pigment in a unit volume
of a dried paint film. At a loading of 50-80% carbon, the
paint film is binder-starved, and the dried film is porous.
This technology is disclosed, by way of example, in Patent
No. 4,716,188, incorporated by reference herein. A high
ratio of carbon or graphite to polymer also enhances the
conductivity of the dried film.
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A lithium salt solution selected from the group
consisting of lithium nitrate (LiN03) solution, lithium
bromide (Liar) solution, and combinations thereof, is
applied to the exposed surface of the dried or cured
conductive paint anode. The application can be
immediately after drying or curing of the conductive paint
anode or much time after, for instance months or years.
For purposes of the present application, the term
~~solution" includes dispersions and suspensions. A
preferred liquid medium for the lithium salt is water. The
pores within the dried or aired anode are typically small,
but are of sufficient diameter to permit the passage of the
solutions, dispersions or suspensions of a lithium salt to the
anode-concrete interface, for instance by capillary
attraction.
Alternatively, the lithium nitrate or lithium bromide
may be dissolved in an organic solvent, such as alcohol, for
application to the surface of the anode, followed by
transport to or near the interface between the anode and
the concrete by capillary action.
The lithium salt solutions can be applied by spraying,
brushing, or roller coating. Other methods of application of
the solutions will be apparent to those skilled in the art.
If the anode coating is thick (e.g., greater than about
20 mils dried), it may be advantageous to produce thin
spots in the anode coating to facilitate penetration of the
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salt solution. This may be accomplished by drilling or
abrading the anode coating in selected locations. It may
also be accomplished by placing a template over the
concrete substrate during the application of the anode. A
template in the form of a wire mesh with wires placed on
four centimeter centerline spacing, for example, creates a
pattern of thin areas in the anode through which the salt
solution more easily penetrates. The thin areas of anode
should be preferably less than about 20% of the total
anode area.
The lithium salts of the present invention, once
delivered to or near the interface, remain at or near the
interface for a long period of time. The diffusion
coefficients for such materials in concrete are small making
further penetration of the lithium salts into the concrete
more difficult.
If the lithium salts are, over a long period of time,
eluded from the interface between the anode and the
concrete, for instance by rainfall, then the salt solutions
can be reapplied to the external surface of the anode to
again deposit the lithium salts at or near the interface
between the anode and the concrete. The lithium salt
solutions can be reapplied as often as is necessary
throughout the life of the cathodic protection system.
The principle advantage of the use of the lithium salts
as taught by the present invention is that the current flow
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from an impressed current anode will be maintained or
improved.
The term ~~improved" as used herein may be taken to
mean that either (1) the flow of protective current may be
increased without undue increase in system voltage, or (2)
the same protective current may be passed with a
significant reduction in system voltage.
It is theorized that the high operating voltage
sometimes observed when using conductive paint anodes in
dry environments is due, at least in part, to a dry and
highly resistive layer of concrete immediately beneath the
anode-concrete interface. The lithium salts in the present
invention are humectants and absorb water from the
atmosphere. Moisture is thus retained at or drawn into the
interface by the lithium salt positioned at the interface.
The moisture functions as an electrolyte, which helps
counteract the increase in electrical resistance at the
interface. However, the benefit is greater than would be
expected from the presence of moisture alone.
It is also theorized that the dry and resistive layer
immediately beneath the anode-concrete interface impedes
the diffusion of ions to and from this region. This, in turn,
results in the harmful build-up of acid adjacent to the
anode. Acid is generated at the anode interface as a by-
product of the dominant anodic electrochemical reaction,
namely oxygen evolution. As follows:
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2Hz0 -~ OZ + 4H+ + 4e-
The use of the lithium salts of the present invention,
by increasing the moisture content at the anode-concrete
interface, increases the rate of diffusion of acid and
diminishes the buildup of acid in this region. This, in turn,
results in better long-term maintenance of the bond
between the conductive paint anode and the concrete.
Although not to be held to any theory, it is also
believed that operation of the conductive paint anode at a
lower anodic potential, as results from the use of lithium
salts of the present invention, prolongs the effective life of
the carbon component of the conductive paint. Carbon is
slowly oxidized to carbon monoxide and carbon dioxide at
the operating potentials of the conductive paint anode on
concrete. Such oxidation will be more rapid at higher
operating potential, and lower at lower operating potential.
Lower operating potentials, as result from practice of the
present invention, will also discourage the generation of
chlorine, which can be deleterious to the binders in the
conductive paint.
The amount of lithium salt required at or near the
interface between the anode and the concrete varies
depending upon the type of reinforced concrete structure,
its location, its degree of salt contamination from such
sources as seawater and deicers, and other factors.
Broadly, the amount of lithium salt is that amount effective
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to maintain or improve the current flow at the anode-
concrete interface, and is relatively large compared for
instance, to the amount of contaminating salt which may be
present in the concrete from seawater and deicers.
Preferably, the lithium salt is applied in a range from about
grams per square meter of anode to about 400 grams
per square meter of anode, dry basis. The preferred range
of lithium salt is from about 40 to 200 grams per square
meter. If too little lithium salt is applied, the amount of
10 lithium salt retained at or near the interface will be
insufficient to maintain or improve the current flow from
the anode or reduce the resistivity at the interface between
the anode and concrete. If too much lithium salt is applied,
this will result in an additional expense for no benefit.
The concentration of lithium salt in an aqueous
solution for application to the surface of a conductive paint
anode may range from about 20 to about 900 grams per
liter. If a solution is too dilute, then a large number of
coats is required to deposit the required amount of lithium
salt at or near the interface between the anode and the
concrete. The upper end of the range of concentration of
lithium salt in the aqueous solution is limited by the
solubility of the salt in water. When using an aqueous
solution containing about 300 grams per liter of lithium
salt, for concrete with a typical degree of dryness, about
three coats of solution are required to deposit the preferred
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amount of salt. The application is best done using brief
drying periods between coats.
It may be advantageous to add certain agents to the
lithium salt solutions prior to applying the solutions to the
exposed surface of an applied anode.
For instance, it may be advantageous to include a surface
active agent in the lithium salt solution. The surface active
agent wets the surface of the thermally applied anode and
increases the rate of diffusion of the solution through the anode
to the interface of the anode with the concrete.
A large number of surface active agents are
commercially available. The surface active agent should be
one which has good wettability characteristics and
preferably is one which is soluble in water or other polar
solvent. A preferred surface active agent is a cationic
amine or ammonium compound. Surface active agents
generally have a hydrophobic portion, usually including a
long hydrocarbon chain, and a hydrophilic portion which
renders the compound soluble in water or other polar
solvent. In a cationic surface active agent, the hydrophilic
portion of the molecule carries a positive charge which is
responsible for the surface active properties. Examples of
cationic surface active agents are amine acetates, alkyl
trimethyl ammonium chlorides, dialkyl dimethyl ammonium
chlorides, alkyl pyridinium chlorides and lauryl dimethyl
benzyl ammonium chloride.
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A cationic surface active agent that has been found to
be particularly useful in the present invention includes the
following combination of ingredients:
n-alkyl (50% C14, 40% C12, 10% C16) dimethyl benzyl 80 ppm
ammonium chloride
Octyl decyl dimethyl ammonium chloride 12.5 ppm
Dioctyl dimethyl ammonium chloride 6.25 ppm
Didecyl dimethyl ammonium chloride 6.25 ppm
This cationic surface active agent is marketed by
Lysol~ as their deodorizing cleanser. It was found to be
effective when used in the amount of about 0.2 to about
2% by volume, preferably about 1% by volume, of
humectant solution. The Lysol cleanser is disclosed in
Patents Nos. 5,454,984 and 5,522,942. Another surface
active agent found to be effective is 'SPRAY AND WASH"
marketed by Dow Brands, Indianapolis, Indiana.
Preferably the surface active agent (active part)
should be present in the humectant solution in a
concentration of at least 50 parts per million (ppm). The
preferred range of concentration of the surface active agent
should be from about 100 ppm to about 1000 ppm.
In the practice of the present invention, the surface
active agent locates at the surface of the anode and
provides compatibility of the anode with the humectant
solution or dispersion applied to the anode.
It may also be advantageous to decrease the diffusion
of the lithium nitrate and lithium bromide away from the
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anode-concrete interface. This may be done by application
of the lithium nitrate or lithium bromide together with a
jelling agent capable of thickening the solution following
placement at the anode-concrete interface. This may be
accomplished by application of a hot solution, which
congeals upon cooling, or by using a material which can be
cross-linked following placement. Such materials are well
known .
It has been found that the lithium salts applied as
taught by the present invention have an additional benefit.
If a cathodic protection system utilizing an inert anode such
as conductive paint is selectively wetted on only a portion
of its surface, then current density is greatly enhanced in
those wetted areas. This may cause large currents to flow
in those select areas causing a high wear rate of the anode
in those locations. This uneven wear rate may eventually
cause the system to fail prematurely. By the use of the
lithium salts as taught by the present invention, a more
even distribution of current resulting in more uniform
protection of the reinforcing steel and in extended service
life of the cathodic protection system is achieved.
EXAMPLE I
Four newly constructed 12 x 9 x 2 inch (30.3 x 22.9 x
5.1 centimeter) concrete blocks were cast containing a mild
steel expanded mesh 0.1875 inch (0.475 centimeter) thick
having diamond dimensions of 1.0 inch LWD x 0.5 inch SWD
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(2.54 centimeter LWD x 1.27 centimeter SWD). The surface
area of the steel mesh was about 1 square foot per square
foot of top concrete surface. The mix proportions for the
concrete specimens were as follows:
Type 1A Portland Cement - 715 Ib/yd3 (425 kg/m3)
Lake Sand Fine Aggregate - 1010 Ib/yd3 (600 kg/m3)
No. 8 Marblehead Limestone - 1830 Ib/yd3 (1090 kg/m3)
Water - 285 Ib/yd3 (170 kg/m3)
Air - about 6%
Sodium chloride was added to the mixture for two of
the blocks to a concentration of 7.5 Ib/yd3 (0.195% by
weight) of chloride ion. Sodium chloride was added to the
mixture for the other two blocks to a concentration of 10
Ib/yd3 (0.260% by weight) of chloride ion.
Following a 24-hour mold-curing period, the blocks
were wrapped in plastic to retain moisture for 28 days.
After the 28-day curing period, the blocks were lightly
sandblasted to produce a rough, clean surface. The top
surface of all four blocks was then brush coated with about
30 mils (750 microns) of Duodac 85 WB Conductive Coating
marketed by Duochem Inc. The coating comprises a water-
based acrylic copolymer and is about 40% by volume solids.
Duochem Inc is the assignee of Patent No. 4,931,156.
After curing, the conductive coating was about 12 mils (300
microns) dry film thickness. The active surface area of the
Duodac 85 Conductive Coating was 0.677 ft2 (0.0629 mz)
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for each block. The paint was allowed to cure one week
before energizing.
One of the blocks with 7.5 Ib/yd3 (0.195% by weight)
of chloride, and one of the blocks with 10 Ib/yd3 (0.260%
by weight) of chloride were maintained as controls. The
other block with 7.5 Ib/yd3 (0.195% by weight) of chloride
was brush coated with 3 coats of 360 gram/liter lithium
nitrate solution. The solution also contained 1% by volume
Lysol~ deodorizing cleanser. A total of 22.8 milliliters
were used resulting in a treatment of 8.21 grams, or 12.1
grams/ft2 (130 grams/m2) lithium nitrate. The other block
with 10 Ib/yd3 (0.260 by weight) of chloride was brush
coated with 3 coats of 360 gram/liter lithium nitrate
solution. This solution also contained 1% by volume
Lysol~ deodorizing cleanser. A total of 24.1 milliliters
were used resulting in a treatment of 8.68 grams, or 12.8
grams/ft2 (137.7 grams/m2) dry basis lithium nitrate.
After drying for 3 days, each conductive paint anode
was connected to the anode of a power supply and the
steel was connected to the cathode of the power supply.
The blocks were then energized at a current density of 1.46
mA/ft2 (15.7 mA/mZ). The blocks with 7.5 Ib/yd3 (0.195%
by weight) of chloride were maintained at room
temperature and 55% relative humidity (RH), while the
blocks with 10 Ib/yd3 (0.260 by weight) of chloride were
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maintained at room temperature and 80% RH. Voltage for
each of the blocks was recorded as follows:
Anode-Cathode
Voltage
55% RH 80% RH
Time-on-line Control Treated Control Treated
1 hour ~ 2.73V 1.81V 1.89V 1.709V
1 days 3.79V 2.24V 2,16V 1.85V
2 days 4.46V 2.29V 2.17V 1.88V
3 days 4.46V 2.27V 2.24V 1.94V
days 5.49V 2.46V 2.11V 1.89V
17 days 6.86V 2.71V 2.04V 1.89V
24 days 9.01V 3.12V 2.01V 1.88V
In both environments, the voltage for the blocks
5 treated with lithium nitrate is seen to be much lower than
the voltage for the untreated blocks. This lower voltage is
expected to result in less stress to both the anode and the
cement paste at the anode-concrete interface and a longer
service life for the cathodic protection system.
10 From the above description of the invention, those
skilled in the art will perceive improvements, changes and
modifications. Such improvements, changes and
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modifications within the skill of the art are intended to be
covered by the appended claims.
