U.S. patent application number 13/625387 was filed with the patent office on 2013-01-24 for corrosion protection of steel in concrete.
The applicant listed for this patent is Nigel DAVISON, Gareth Kevin GLASS, Adrian Charles ROBERTS. Invention is credited to Nigel DAVISON, Gareth Kevin GLASS, Adrian Charles ROBERTS.
Application Number | 20130020191 13/625387 |
Document ID | / |
Family ID | 47555016 |
Filed Date | 2013-01-24 |
United States Patent
Application |
20130020191 |
Kind Code |
A1 |
GLASS; Gareth Kevin ; et
al. |
January 24, 2013 |
CORROSION PROTECTION OF STEEL IN CONCRETE
Abstract
An electric field modifier for boosting current output of a
sacrificial anode for reinforced concrete to enhance its protective
effect and direct the current output in a preferred direction to
improve current distribution in galvanic protection of steel
exposed to air. The combination comprises a sacrificial anode and
an electric field modifier and an ionically conductive filler
embedded in a cavity and the sacrificial anode is directly
connected to the steel. The modifier comprises an element with an
anode side supporting an oxidation reaction in electronic contact
with a cathode side supporting a reduction reaction. The cathode of
the modifier may form a cell with the sacrificial anode and is
separated therefrom by the filler. The filler contains an
electrolyte that connects the sacrificial anode to the cathode of
the modifier. The reduction reaction on the cathode of the modifier
may substantially comprise the reduction of oxygen from the
air.
Inventors: |
GLASS; Gareth Kevin;
(Lichfield, GB) ; ROBERTS; Adrian Charles;
(Chilwell, GB) ; DAVISON; Nigel;
(Coton-in-the-elms, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GLASS; Gareth Kevin
ROBERTS; Adrian Charles
DAVISON; Nigel |
Lichfield
Chilwell
Coton-in-the-elms |
|
GB
GB
GB |
|
|
Family ID: |
47555016 |
Appl. No.: |
13/625387 |
Filed: |
September 24, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12814120 |
Jun 11, 2010 |
8273239 |
|
|
13625387 |
|
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Current U.S.
Class: |
204/196.01 |
Current CPC
Class: |
C23F 13/02 20130101;
C23F 13/06 20130101; C23F 2201/02 20130101 |
Class at
Publication: |
204/196.01 |
International
Class: |
C23F 13/00 20060101
C23F013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2009 |
GB |
0910167.6 |
Claims
1. A sacrificial anode reinforced concrete protection assembly for
protecting steel in hardened reinforced concrete elements exposed
to the air, the assembly comprising: a sacrificial anode; a
connector connected to the sacrificial anode for electronic
connection of the sacrificial anode to steel in a reinforced
concrete element; and an electric field modifier; wherein the
sacrificial anode comprises: a sacrificial anode for reinforced
concrete elements; a metal less noble than steel; and is at least
in part surrounded by the electric field modifier; and the modifier
comprises an element having an anode side in electronic connection
with a cathode side, and the anode side comprises an anode adapted
to support an oxidation reaction, which anode faces away from the
sacrificial anode; and the cathode side comprises a cathode adapted
to support a reduction reaction, which cathode faces the
sacrificial anode; and the modifier is separated from the
sacrificial anode.
2. An assembly according to claim 1, wherein the sacrificial anode
has a charge capacity of at least 100 kC.
3. An assembly according to claim 2, wherein the sacrificial anode
has a charge capacity of at least 150 kC.
4. A sacrificial anode reinforced concrete protection assembly for
protecting steel in hardened reinforced concrete elements exposed
to the air, the assembly comprising: a sacrificial anode; a
connector connected to the sacrificial anode for electronic
connection of the sacrificial anode to steel in a reinforced
concrete element; and an electric field modifier; wherein the
sacrificial anode comprises: a sacrificial anode for reinforced
concrete elements; a metal less noble than steel; and is at least
in part surrounded by the electric field modifier; and the modifier
comprises an element having an anode side in electronic connection
with a cathode side, and the anode side comprises an anode adapted
to support an oxidation reaction, which anode faces away from the
cathode side; and the cathode side comprises a cathode adapted to
support a reduction reaction; and the modifier is separated from
the sacrificial anode.
5. An assembly according to claim 4, wherein the sacrificial anode
has a charge capacity of at least 100 kC.
6. An assembly according to claim 5, wherein the sacrificial anode
has a charge capacity of at least 150 kC.
7. A sacrificial anode reinforced concrete protection assembly for
protecting steel in hardened reinforced concrete elements exposed
to the air, the assembly comprising: a sacrificial anode; a
connector connected to the sacrificial anode for electronic
connection of the sacrificial anode to steel in a reinforced
concrete element; and an electric field modifier; wherein the
sacrificial anode has a charge capacity of at least 80 kC, and
comprises a metal less noble than steel; the modifier comprises an
element which has an anode side in electronic connection with a
cathode side, and the anode side comprises an anode adapted to
support an oxidation reaction, which anode faces away from the
sacrificial anode; the cathode side comprises a cathode adapted to
support a reduction reaction, which cathode faces the sacrificial
anode; and the cathode side of the modifier and the sacrificial
anode are arranged to form a cell.
8. An assembly according to claim 7, wherein the sacrificial anode
has a charge capacity of at least 100 kC.
9. An assembly according to claim 8, wherein the sacrificial anode
has a charge capacity of at least 150 kC.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to electrochemical protection
of steel in reinforced concrete construction using sacrificial
anodes and, in particular, to the use of sacrificial anode
assemblies in treating steel corrosion in corrosion damaged
concrete elements which are exposed to the air.
[0002] BACKGROUND OF THE INVENTION
[0003] Above ground steel reinforced concrete structures suffer
from corrosion induced damage mainly as the result of carbonation
or chloride contamination of the concrete. As the steel
reinforcement corrodes, it produces by-products that occupy a
larger volume than the steel from which the products are derived.
As a result, expansion occurs around reinforcing steel bars. This
causes cracking and delamination of the concrete cover of the
steel. Typical repairs involve removing this patch of corrosion
damaged concrete from the concrete structure(s). It is good
practice to expose corroding steel, in the area of damage, and to
remove the concrete behind the corroding steel. The concrete
profile is then restored with a compatible cementitious repair
concrete or mortar. The concrete then consists of the "parent"
concrete (i.e., the remaining original concrete) and the "new"
patch repair material.
[0004] The parent concrete, adjacent to the repair area, is
typically likely to suffer from some of the same chloride
contamination or carbonation that caused the original corrosion
damage. It is to be appreciated that steel corrosion still remains
a risk in the parent concrete. Corrosion in concrete is an
electrochemical process and electrochemical treatments have been
used to treat this corrosion risk. Examples are described in WO
94029496, U.S. Pat. No. 6,322,691, U.S. Pat. No. 6,258,236 and U.S.
Pat. No. 6,685,822.
[0005] Established electrochemical treatments include cathodic
protection, chloride extraction and re-alkalisation. These are
classed as either permanent or temporary treatments. Permanent
treatments are based on a protective effect that is only expected
to last while the treatment is applied. An example of a permanent
treatment is cathodic protection. The accepted performance
criterion can only be achieved while the treatment is applied (BS
EN 12696:2000). Chloride extraction and re-alkalisation are
examples of temporary treatments (CEN/TS 14038-1:2004). Temporary
treatments rely on a protective effect that persists after the
treatment has ended. In practice, this means that an applicator
treats the structure and thereafter hands a treated structure back
to a client or customer at the end of a treatment contract.
[0006] Electrochemical treatments may also be classed as either
impressed current or galvanic (sacrificial) treatments. In
impressed current electrochemical treatments, an anode is connected
to the positive terminal and the steel is connected to the negative
terminal of an external source of DC power. An impressed current
anode will often be an inert electrode. An anode is an electrode
supporting a substantial oxidation reaction and, in impressed
current treatments, an electrode is turned into an anode by an
applied voltage.
[0007] In galvanic electrochemical treatments, the protection
current is provided by one or more sacrificial anodes that are
directly connected to the steel. Sacrificial anodes are electrodes
comprising metals less noble than steel (more negative than) with
the main anodic reaction being the dissolution of a sacrificial
metal element. The natural potential difference between the
sacrificial anode and the steel drives a protection current when
the sacrificial anode is connected to the steel. Sacrificial anode
assemblies are self powered assemblies. The protection current
flows as ions from the sacrificial anode into the parent concrete
and to the steel, and returns as electrons through the steel and a
conductor to the sacrificial anode. The convention of expressing
the direction of current flow as the direction of movement of
positive charge is used in this specification.
[0008] Sacrificial anodes for concrete structures may be divided
into discrete or continuous anodes (U.S. Pat. No. 5,292,411).
Discrete anodes are individually distinct elements that contact a
concrete surface area that is substantially smaller than the
surface area of the concrete covering the protected steel. The
anode elements are normally connected to each other through a
conductor that is not intended to be a sacrificial anode and are
normally embedded within cavities in the concrete (ACI Repair
[0009] Application Procedure 8--Installation of Embedded Galvanic
Anodes (www.concrete.org/general/RAP-8.pdf). Discrete sacrificial
anode systems generally include an anode, a supporting electrolyte
and a backfill. An activating agent is often included to maintain
sacrificial anode activity. The backfill provides space to
accommodate the products of anodic dissolution and prevent
disruption of the surrounding hardened concrete. Discrete
sacrificial anodes have the advantage that it is relatively easy to
achieve a durable attachment between the anode and the concrete
structure by embedding the anodes within cavities formed in the
concrete.
[0010] Galvanic protection of steel in concrete using embedded
discrete anodes differs from sacrificial cathodic protection of
steel in soils and waters (BS EN 12954:2001). Anode assemblies that
are embedded within concrete must be dimensionally stable as
concrete is a rigid material that does not tolerate embedded
expanding assemblies. Anode activating agents are specific to
concrete or need to be arranged in a way that would present no
corrosion risk to the neighbouring steel (WO 94029496 or GB
2431167). Anodes are located relatively close to the steel in the
concrete and embedded anodes are small (a discrete anode assembly
diameter is typically less than 50 mm) when compared to anodes in
other environments. Galvanic protection criteria for
atmospherically exposed concrete differ from those for the cathodic
protection of steel in soil and water. Steel is normally passive in
uncontaminated, alkaline concrete. In atmospherically exposed
concrete, protection is usually achieved by restoring the passive
film on reinforcing steel. This effectively polarises the anodic
reactions on the steel. In soil and water, a passive film on steel
is not normally stable and the objective of the protection is to
polarise the cathodic reaction (usually the reduction of oxygen) to
prevent steel corrosion.
[0011] One problem with the use of sacrificial anodes in galvanic
treatments is that the power to arrest an active corrosion process
on the steel in concrete is limited by the voltage difference
between the sacrificial anode and the steel. This problem is
greatest for discrete sacrificial anode systems where large
currents are required from relatively small anodes to protect
relatively large surfaces of the steel. A compact discrete anode
will typically deliver current into an area of parent concrete
adjacent to the anode that is one tenth to one fiftieth of the area
of the steel that it is expected to protect.
[0012] A number of methods have recently been proposed to increase
the power of sacrificial anodes in concrete using a power supply
including integrating a cell with the anode assembly and applying
an impressed current off a sacrificial anode assembly using an
external power supply (WO 05106076, U.S. Pat. No. 7,264,708 and GB
2426008). Some early teaching also exists on increasing the power
of a sacrificial anode in sacrificial cathodic protection
applications applied to steel in soil and saline water where
different protection criteria apply (U.S. Pat. No. 4,861,449). Soil
and saline water assemblies are also not of a size suitable for use
in concrete.
[0013] In WO 05106076, a sacrificial anode assembly is formed by
connecting the cathode of a cell or battery to a sacrificial anode.
In one arrangement, the sacrificial anode forms the casing of a
cell where the cathode of the cell is adjacent to the cell casing.
An alkaline cell commonly has this property. The anode of the cell
is then connected to the steel. The problem with this arrangement
is that the sacrificial anode is not directly connected to the
steel. Furthermore the charge capacity of a cell is substantially
smaller than the charge capacity of a similarly sized sacrificial
anode. As a result, the useful life of the assembly is limited by
the charge capacity of the cell. For example, an AA sized cell has
a charge capacity of only about 10 kC (kilo coulombs). An objective
of this invention is to improve the useful life of such an
assembly.
[0014] The sacrificial anode in the arrangement in WO 05106076 is
not connected directly to the steel, and thus the sacrificial anode
cannot continue to deliver a protection current after the charge
capacity of the cell has expired. It is another objective of this
invention to provide an assembly in which the sacrificial anode is
connected to the steel so that the sacrificial anode can continue
to deliver protection current after other components of the
assembly have expired.
[0015] In U.S. Pat. No. 7,264,708, an automated means is provided
to connect a sacrificial anode to the steel after an external power
supply, or a battery driving current from the sacrificial anode to
the steel, has expired. In the example in this disclosure, diodes
are used to provide the sacrificial anode to steel connection. The
problem with this arrangement is that power is required to achieve
such a connection and this reduces the power of the protective
effect. A typical diode (a diode based on a doped silicon
semiconductor) will use a voltage of 0.6V to become a conductor and
there is not sufficient voltage within a typical sacrificial anode
system to drive a substantial current through such diodes. Another
problem with this arrangement is that the power supply is located
away from the anodes and is connected to the anodes by electric
cables that have to be maintained and protected from the
environment and from vandalism.
[0016] GB 2426008 (U.S. patent application Ser. No. 11/908,858)
discloses a new basis for corrosion initiation and arrest in
concrete that relies on an acidification-pit re-alkalisation
mechanism. A temporary electrochemical treatment is used to deliver
a pit re-alkalisation process from sacrificial anodes before the
anodes are manually connected to the steel. The pit re-alkalisation
process arrests active corrosion by restoring a high pH at the
corroding sites. The pit re-alkalisation process (applied as a
temporary impressed current treatment) typically lasts less than 3
weeks. The corrosion free condition is then maintained with the low
level galvanic generation of hydroxide at the steel. The switch
between the impressed current and galvanic treatments is achieved
manually and this is facilitated by the limited duration of
temporary impressed current treatments. The power supply and the
electric cables used for the temporary impressed current treatment
are removed from the site. The problem with this approach is that
the temporary impressed current treatment typically requires a
skilled operator.
[0017] Another problem with discrete sacrificial anode systems is
current distribution. This problem is greatest for anodes that are
tied to exposed steel in cavities formed within the concrete at
areas of the concrete repair. A number of solutions have been
proposed to improve the current distribution from an anode tied to
the steel (see, for example, GB 2451725, WO 05121760, WO 04057056).
However these solutions are all based on restricting the current
flow to the nearest steel by increasing the resistance for current
to flow to the nearest steel.
[0018] A problem to be solved by this invention is to increase the
initial power available from a sacrificial anode assembly to arrest
an active corrosion process while the sacrificial anode is
connected to the steel in the concrete, and to improve current
distribution from a sacrificial anode connected to the steel by
directing an increased current away from the nearest steel.
SUMMARY OF THE INVENTION
[0019] This invention discloses a method of controlling the current
output off discrete sacrificial anodes that are less noble than
steel using additional anode-cathode assemblies to modify the
electric field in the environment next to the anode while the
sacrificial anode is connected to with an electron conducting
conductor to the steel.
[0020] In one arrangement an electric field modifier with an air
cathode is used to sustain a high current output off a sacrificial
anode embedded in concrete. The use of an air cathode in the
modifier needs to be combined with an environment like concrete
exposed to the air because in this environment, cathodic protection
is achieved by changing the environment at the steel to induce
steel passivity or anodic polarisation (GB 2426008) and cathodic
reaction kinetics are weakly polarised. In environments like soil
and water, where cathodic protection is achieved by cathodically
polarising the steel, an air cathode will not work because the
steel to be protected represents an air cathode with a very large
surface area relative to the air cathode that might be assembled
within an anode assembly and the air cathode in the anode assembly
will not have the capacity to deliver the necessary protection
current to polarise the air cathode on the steel that is to be
protected.
[0021] In another alternative arrangement, an electric field
modifier is placed in the environment adjacent to the sacrificial
anode to provide an initial boost to the sacrificial anode current
output to arrest the corrosion process and the sacrificial anode
continues to function after the charge in the modifier has been
consumed because it is connected to the steel through an electron
conducting conductor and a path for ionic conduction is formed from
the sacrificial anode through an electrolyte to the protected
steel. A path for ionic conduction is formed at least after the
charge in the modifier has been consumed and the modifier no longer
functions. In this case, the life of the sacrificial anode,
determined in part by its charge capacity, is much greater than the
life of the modifier in the anode assembly.
[0022] In another alternative arrangement an electric field
modifier is arranged to boost the current from the sacrificial
anode that flows to steel further away from the anode relative to
the current that flows to the steel closer to the anode. In this
case the sacrificial anode is preferably tied to a section of steel
bar and the modifier is arranged to boost the current flowing from
the sacrificial anode away from this section of steel bar.
[0023] The electric field modifier contains at least one anode
electrode electronically connected by an electron conducting
connection to at least one cathode electrode. The anode and cathode
preferably face away from each other. The oxidation reaction on the
anode (anode reaction) and the reduction reaction on the cathode
(cathode reaction) can occur without any external driving
potential.
[0024] One type of electric field modifier is an element comprising
a side or face that is an anode supporting an oxidation reaction
that is in electronic contact with a side or face that is a cathode
supporting a reduction reaction, where the anode and the cathode
face away from each other (i.e., the anode and the cathode face
substantially different directions). A natural potential difference
is generated by the oxidation and reduction reactions on the anode
and the cathode respectively that tries to drive a current through
the modifier. If an electrolyte connects the anode of the modifier
to its cathode, an ionic current, stimulated by electrochemical
reactions, will flow from the anode to the cathode. Electrochemical
reactions consume reducing and oxidising agents at the anode and
cathode, respectively (i.e., reductants are oxidised and oxidants
are reduced at the anode and the cathode, respectively). It is
preferable that these reactions should be restricted prior to use
to enhance the shelf life of the modifier. This may be achieved by
keeping the modifier in a dry environment to limit the quantity of
electrolyte at the anode and the cathode, and/or by preventing the
electrolyte at the anode from making contact with the electrolyte
at the cathode.
[0025] The modifier is located in the electric field between a
sacrificial anode and the steel. The modifier increases the current
flowing through a path that intersects the modifier when the
cathode of the modifier faces the sacrificial anode and the anode
of the modifier faces away from the sacrificial anode. As a result,
the modifier also increases the total current delivered by the
sacrificial anode. The modifier effectively behaves as a current
pump that pumps electric current through the modifier.
[0026] The sacrificial anode is a sacrificial metal element from a
compact discrete sacrificial anode assembly for use in reinforced
concrete construction. Such assemblies will have a charge capacity
of at least 80 kC (kilo coulombs) It is preferable that the
sacrificial anode has a charge capacity of at least 100 kC and more
preferable that the sacrificial anode has a charge capacity of at
least 150 kC.
[0027] In another arrangement, a compact discrete sacrificial
anode, for reinforced concrete construction, is arranged with the
cathode of the modifier to form a cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] This invention will now be described further with reference,
by way of example, to the drawings in which:
[0029] FIG. 1 illustrates the effect of an electric field modifier
on the current flow between a sacrificial anode and the steel.
[0030] FIG. 2 shows an arrangement illustrating the use of a
sacrificial anode/modifier assembly located within a cavity formed
in the concrete for the purposes of installing the assembly.
[0031] FIG. 3 shows an arrangement illustrating the use of a
sacrificial anode/modifier assembly when installing the assembly in
an area of concrete patch repair.
[0032] FIG. 4 shows the sandbox arrangement that was used to test
the theory in Examples 1 and 2.
[0033] FIG. 5 shows the changes in galvanic current output when an
electric field modifier was inserted into and removed from the sand
in Example 1.
[0034] FIG. 6 shows the early galvanic current output of a control
test and two tests involving two different modifiers in Example
2.
[0035] FIG. 7 shows the medium term galvanic current output of a
control test and tests involving two different modifiers in Example
2.
[0036] FIG. 8 shows the experimental arrangement used in Example 3
to test the effect of a modifier on the protection current
delivered to steel in a cement mortar.
[0037] FIG. 9 shows a section of the steel cathode that was used in
Example 3.
[0038] FIG. 10 shows the early galvanic current output of a control
test and a test involving a modifier in Example 3.
[0039] FIG. 11 shows the galvanic current output from day 6 to day
21 of a control test and a test involving a modifier in Example
3.
[0040] FIG. 12 shows the galvanic current output from day 15 to day
60 of a control test and a test involving a modifier in Example
3.
DETAILED DESCRIPTION OF THE INVENTION
[0041] The effect of an electric field modifier on current flow is
illustrated in FIG. 1. In this example, a modifier 1 is placed, in
an electrolyte 4, between a sacrificial anode 2 and protected steel
3. The sacrificial anode 2 is connected to the steel 3 by a
connection 5. A galvanic protection current that flows from the
sacrificial anode 2 through the electrolyte 4 to the steel 3
returns to the sacrificial anode 2 via the connection 5. The
modifier 1 has a surface facing the sacrificial anode 2 that acts
as a cathode and a surface facing the steel 3 that acts as an anode
and a natural potential difference between the anode and the
cathode stimulates reactions on the anode and the cathode. The
anode and cathode electrodes of the modifier 1 are connected back
to back by an electron conducting connection and face in opposite
directions. Other electrode arrangements of the modifier 1 are also
envisaged.
[0042] In FIG. 1, lines in the electrolyte 4 with arrowheads show
the direction of positive ionic current flow through the
electrolyte. Current is drawn from the sacrificial anode 2 through
the modifier 1 to the steel 3 by the voltage between the anode and
the cathode of the modifier. When the anode and cathode reactions
on the modifier 1 increase, the current that would flow along a
path that intersects the modifier 1, the total current flowing from
the sacrificial anode 2 to the steel 3 is increased. Furthermore,
current that bypasses the modifier 1 is reduced or reversed. Thus
the current output of a sacrificial anode 2 may be directed through
specific regions of the electrolyte while the total current is
increased.
[0043] The modifier 1 acts like an electric current pump. The
electrochemical reactions on its electrode surfaces drive electrons
(i.e., current) on its inside from its cathode electrode to its
anode electrode. This may be used to change the ionic current in
the electrolyte outside the modifier 1. It is to be appreciated
that the modifier 1 may be used to increase the flow of external
current, change the direction of the external current or even
reverse the direction of the external current.
[0044] An electric field modifier 1 is preferably in the form of a
sheet shaped as a tube or a hollow container. In one embodiment,
the inner surface is the cathode and the outer surface is the
anode. A sacrificial anode is typically located within the modifier
1 comprising the tube or the hollow container. To increase the
current output of a sacrificial anode 2, the cathode of the
modifier 1 faces the sacrificial anode 2 and the anode of the
modifier 1 faces away from the sacrificial anode 2. The modifier 1
may comprise a single element or several discrete elements with
gaps between them or the modifier 1 may be a single element that is
perforated so as to have gaps or voids. Several modifiers 1 may be
used either in series or parallel with one another.
[0045] The anode of the modifier 1 is an electrode supporting an
oxidation reaction, while the cathode of the modifier 1 is an
electrode supporting a reduction reaction. Suitable oxidizable
materials (also termed reducing agents or reductants) for the anode
of the modifier 1 include, for example, zinc, aluminium, magnesium
or alloys thereof. For use in concrete a zinc or zinc alloy anode
is preferred. The oxidation reaction supported by a zinc anode is
zinc dissolution.
[0046] The cathode of the modifier 1 includes an electron
conducting surface on which reduction can take place, together with
a reducible material. Suitable reducible materials (also termed
oxidizing agents or oxidants) for the cathode include oxygen and
manganese dioxide. The electron conducting surface and reducible
material forms an electrode that is more noble than the anode of
the modifier 1 (i.e., for the modifier to be effective, the
potential of the cathode is more positive than the potential of the
anode). Suitable electron conducting surfaces on which reduction
can take place are carbon, silver and nickel. This surface
preferably resists oxidation.
[0047] Other examples of possible anode and cathode materials for
the modifier 1, can be found in the field of battery technology.
Cathode materials are usually oxygen from the air or solids that
may be porous. Solid cathode materials include metal oxides such as
manganese dioxide.
[0048] The modifier 1 differs from a cell or battery in that its
anode is connected to its cathode that faces away from its anode
before use with a connection that allows electrons to flow between
its anode and its cathode. The circuit is completed, during use, by
the introduction of an electrolyte 4. By contrast the anode and the
cathode of a cell or a battery are connected by an electrolyte
before use and the circuit is typically completed by electron
conducting components when the cell or the battery is used.
[0049] In use, an electrolyte connects the anode of the modifier 1
to the steel 3, located in concrete to be protected, and an
electrolyte 4 connects the sacrificial anode 2 to the cathode of
the modifier 1. An electrolyte connection between the anode and the
cathode of the modifier is not required for the modifier 1 to
function and is preferably omitted, prior to use, to preserve the
shelf life of the modifier. The electrolyte connection 5 between
the sacrificial anode and the cathode of the modifier 1, may be
formed in advance of using a sacrificial anode/modifier 1 assembly
and may be part of this assembly. Alternatively, the electrolyte
connection, between the sacrificial anode and the cathode of the
modifier, may be formed during installation of the assembly.
[0050] As the modifier 1 operates, its oxidizable and reducible
materials are consumed. Thus the modifier 1 has a limited useful
life that depends on the charge capacity of these materials. The
life of the modifier will end when either the available oxidizable
or reducible material is consumed. Anode materials like zinc tend
to have a relatively high charge density and occupy a small volume
compared to cathode materials like manganese dioxide. However the
volume of the cathode, and therefore the modifier, 1 may be
minimized if oxygen from the air is used as the main reducible
material. The cathode may then comprise a thin carbon or silver
coating that facilitates the reduction of oxygen from the air. Such
a cathode is referred to as an air cathode and effectively has an
unlimited life. The life of the modifier is then determined by its
anode.
[0051] Both oxygen and water are required to support an air
cathode, but oxygen is not available to support a relatively high
cathodic reduction reaction rate in all environments. Oxygen from
the air is readily available in concrete structures that are
exposed to the air and periodically allowed to dry. In air dried
concrete (which will not be completely dry), cathodic oxygen
reduction rates equivalent to a current density of more than 200
mA/m.sup.2 can occur. This is more than an order of magnitude
greater than typical cathodic protection current densities in
concrete and, under these conditions, an air cathode works well
since it can promote and support high current densities. A modifier
with an air cathode is suitable for use in concrete dried in the
air.
[0052] In other environments like sea-water and soil, cathodic
protection current densities tend to be of the same order as the
limiting current equivalent to the maximum rate of oxygen reduction
and in these environments an air cathode in a modifier cannot be
effective because oxygen access then limits the cathode current
output. A modifier with an air cathode will then block the current
output of a sacrificial anode. A modifier 1 with an air cathode is,
therefore, not generally suitable for use in soil and in
sea-water.
[0053] FIG. 1 also shows that the direction of current in the
electrolyte 4 that bypasses the modifier 1 may be reversed. Current
flows through the electrolyte 4 from the anode of the modifier 1 to
the cathode of the modifier 1. Reversing the current direction in
the electrolyte 4, that bypasses the modifier 1, represents
inefficient use of the charge in the modifier 1 in many
circumstances as this charge does not form part of the current
flowing to the steel. One method of minimizing the magnitude of the
reversed current is to use a modifier 1 with a smaller potential
difference between its anode and its cathode. A zinc-air modifier
will have a potential difference between its anode and its cathode
that is similar to the potential difference between a sacrificial
anode 2 and passive steel and will, therefore, tend to use its
charge more efficiently than a modifier 1 with an anode cathode
combination that has a higher potential difference.
[0054] The useful life of an electrode depends on the charge stored
in the oxidizable or reducible material and the efficiency of the
use of this charge. In some cases the useful life of the
sacrificial anode 2 (i.e., the period of time that the sacrificial
anode 2 has a capacity to deliver a galvanic protection current to
the steel 3) may be substantially greater than the useful life of a
modifier 1 (i.e., the period of time a modifier has a capacity to
increase the current that flows on a path that intersects the
modifier). For example, the useful life of the sacrificial anode 2
may be two or three or ten times the useful life of the modifier 1.
This is preferable when a high current is only required at the
start of a galvanic treatment to arrest a corrosion process in
concrete, as it results in the more efficient use of the charge in
the sacrificial anode 2. In this case, a path for ionic conduction
between the sacrificial anode 2 and the protected steel 3 is
required to continue to deliver the galvanic current once the
useful life of the modifier 1 expires. This may be achieved by
leaving gaps or voids within the modifier 1 that are filled with a
porous material containing the electrolyte 4, or by using a
modifier 1 that is transformed into a porous material containing an
electrolyte 4 as it is consumed, or by a combination of these
features.
[0055] A zinc-air modifier 1 may be transformed into a porous solid
by the corrosion of the zinc and the disruption of the electron
conducting surface of the air cathode. The electron conducting
surface may be disrupted by the corrosion of the zinc when it is a
thin zinc surface treatment or coating attached directly to a zinc
surface that supports oxygen reduction. Other modifiers, with a
cathode comprising an electron conducting surface and a porous
reducible material, may also be transformed into a porous solid
when the electron conducting surface of the cathode is disrupted by
the consumption of the anode.
[0056] The charge in the sacrificial anode 2 may also be consumed
more efficiently if the current output of the sacrificial anode 2
responds to the aggressive nature of the environment. It is
preferable for the protection current to respond positively to
factors affecting steel corrosion risk to improve the efficient use
of the charge in the sacrificial anode. Thus the sacrificial anode
current output, in a dry or cold environment, is preferably lower
than its current output in a hot or wet environment. The use of a
modifier 1 allows the current output of the sacrificial anode 2 to
be boosted without limiting the effects of wet/dry or hot/cold
cycles on the current output of the sacrificial anode 2.
[0057] In some cases, it is desirable to direct the current off the
sacrificial anode 2 to improve current distribution. This is
relevant when the sacrificial anode 2 is tied directly to a section
of steel in uncontaminated repair material at an area of corrosion
damaged concrete repair. In this case, the current needs to flow to
the steel in the adjacent parent concrete as opposed to the steel
in the repair material. To boost this current, the modifier 1 may
be positioned to the side of the sacrificial anode 2, facing away
from the closest portion of steel 3. The cathode of the modifier 1
faces the sacrificial anode 2.
[0058] One arrangement illustrating the use of a sacrificial
anode/modifier assembly is shown in FIG. 2. This arrangement is
suited to the embedment of the assembly into a cavity 8 formed in
the concrete for the purposes of installing the assembly. The
cavity 8 may be a drilled or a cored hole in the concrete 9 which
is typically no more than 50 mm in diameter. The cavity 8 is
preferably sized to accept the assembly.
[0059] The sacrificial anode 10 is in the form of a bar located at
the center of the hole or cavity 8 and will typically be no more
than about 200 mm in length and be cast around a conductor. The
sacrificial anode 10 is connected to the steel 11 via a conductor
12 (e.g., typically an electric cable or a wire). A preferred
conductor 12 substantially comprises titanium as this would also
allow the sacrificial anode 10 to be used with an impressed current
(e.g., an external power supply driving a high current off the
anode) which may be used to provides a facility to manage future
corrosion risk by providing facility to deliver a temporary
treatment to arrest future corrosion.
[0060] The modifier 13, comprising an anode 14 and a cathode 15 in
the form of a tube or hollow cylinder that is open at both ends,
substantially surrounds the sacrificial anode 10. The cathode may
be an air cathode and oxygen from the air may diffuse into the tube
through either of its openings (e.g., the top or the bottom in FIG.
2). Such openings also provide a path for ionic conduction between
the sacrificial anode 10 and the steel 11 at the end of the useful
life of the modifier 13.
[0061] A filler 16 provides an electrolyte that is an ionic
conductor to connect the sacrificial anode 10 to the cathode 15 of
the modifier 13. The filler 16 will preferably be in the form of a
porous solid or putty containing the electrolyte. A backfill 17
provides an electrolyte to connect the anode 14 of the modifier 13
to the parent concrete 9. The backfill 17 and the filler 16 may
conceivably be the same material or different materials and may be
installed at the same time or at different times. The filler 16 may
be separated from the backfill 17 by a porous layer in which the
pores are lined with a hydrophobic material. This provides a
breathable hydrophobic layer that allows oxygen to move to an air
cathode but limits the formation of a flow path through an
electrolyte between the anode 14 and the cathode 15 of the modifier
13 and therefore enhances the efficient use of the modifier. A
hydrophobic porous material may be produced by treating a porous
material, like hydrated cement paste, with a silane based water
repellent. Breathable hydrophobic material may extend from outside
the assembly to any part of the air cathode to promote oxygen
access to the air cathode.
[0062] The cavity 8 in the concrete may be partially filled with
the backfill 17 and the sacrificial anode 10 and the modifier 13
are installed in the cavity 8 such that the backfill 17 fills the
spaces between the sacrificial anode 10, the modifier 13 and the
parent concrete 9. This may be achieved by first installing the
backfill 17 and then pressing the sacrificial anode 10 and the
modifier 13 into the backfill 17. In this arrangement, the backfill
17 acts as both a filler and a backfill. The sacrificial anode 10
and the modifier 13 may be pre-assembled as a separate unit or
assembly with the modifier 13 being attached to and spaced from the
sacrificial anode. However, the sacrificial anode 10 must not be
attached to the modifier 13 with an electron conducting attachment.
Such an attachment is to be avoided as it consumes the sacrificial
anode and the cathode of the modifier. The assembly in the cavity
8, may then be finally covered with a cementitious repair mortar or
concrete 18, as illustrated in FIG. 2.
[0063] An activating agent, adapted to maintain the sacrificial
anode activity, may be applied as a coating on the sacrificial
anode 10, or it may be included within the filler 16 or within the
body of the sacrificial anode. The anode 14 of the modifier 13 may
also be coated with an activating agent, or aggressive ions in the
concrete may be drawn to the anode of the modifier by ionic current
induced in the adjacent concrete to maintain the activity of the
anode.
[0064] Another arrangement illustrating a method of using a
sacrificial anode/modifier assembly is shown in FIG. 3. This
arrangement is suited for attaching the assembly to a section of
steel bar exposed at an area of a concrete patch repair. The
sacrificial anode 21 is attached to the steel bar 22 with an
electron conducting tie 23. The sacrificial anode 21 may be spaced
off the steel bar 22 by a spacer 24 to improve current
distribution. The sacrificial anode 21 is substantially surrounded
by a modifier 25 having a "U" shaped cross section. The modifier 25
comprises a cathode 26 facing the sacrificial anode 21 and an anode
27 facing away from the sacrificial anode 21. The modifier 25 is
positioned so as to direct current away from a section of the steel
bar 22. The cathode 26 of the modifier 25 is connected to the
sacrificial anode 21 by the electrolyte in a filler 28. The filler
28 is generally in the form of a porous solid or a porous putty.
The pores of the filler 28 may be partially filled with air to
promote the function of an air cathode and may include a breathable
hydrophobic material. An electrolyte should also be present in the
pores of the filler 28 to facilitate ionic conduction and
electrochemical reactions (oxidation at the sacrificial anode 21
and reduction at the cathode 26 of the modifier 25.). The anode 27
of the modifier 25 may be connected to the concrete 29 by a
cementitious concrete repair material 30.
[0065] An activating agent, adapted to maintain activity of the
sacrificial anode 21, may be applied as a coating on the
sacrificial anode 21, or it may be included within the filler 28 or
within the body of the sacrificial anode 21. The anode 27 of the
modifier 25 may also be coated with, or contain within its body, an
activating agent. The cathode 26 of the modifier 25 may be an air
cathode and the ends of the "U" section modifier may be left open
to facilitate the diffusion of oxygen from the air through the
repair material 30 and the filler 28 to the cathode 26 of the
modifier 25. These openings also provide a path for ionic
conduction between the sacrificial anode 21 and the steel 22 in the
concrete 29 that bypasses the modifier 25 to facilitate the
continued function of the sacrificial anode 21 once the charge in
the modifier 25 is exhausted.
[0066] In the arrangement shown in FIG. 3, it is preferable to form
an assembly comprising the sacrificial anode 21, the modifier 25
and the filler 28 as a preformed unit or assembly. The preformed
unit or assembly also preferably includes the spacer 24, the
connector 23 or a connection point, and an activating agent adapted
to maintain the activity of the sacrificial anode 21. Openings
within the modifier 25, that are provided to facilitate the
transfer or movement of oxygen from the air to the cathode 26, may
be treated with a breathable hydrophobic (i.e., water repellent)
treatment to improve the diffusion of oxygen from the air into the
filler 28.
[0067] According to one aspect, this invention provides a method of
protecting steel in hardened reinforced concrete elements exposed
to the air using an ionically conductive filler and an assembly
comprising a sacrificial anode and an electric field modifier that
includes the steps of connecting the sacrificial anode to the steel
with an electron conducting conductor and connecting the modifier
to the concrete with an electrolyte wherein the sacrificial anode
is a metal less noble than steel and the sacrificial anode is
substantially surrounded by the modifier, and the modifier
comprises an element with a side that is an anode supporting an
oxidation reaction in electronic contact with a side that is a
cathode supporting a reduction reaction, and the cathode of the
modifier faces the sacrificial anode and is separated from the
sacrificial anode by the filler, and the filler is a porous
material containing an electrolyte that connects the sacrificial
anode to the cathode of the modifier, and the anode of the modifier
faces away from the sacrificial anode.
[0068] According to another aspect, this invention provides an
assembly to protect steel in hardened reinforced concrete elements
exposed to the air comprising a sacrificial anode and an electric
field modifier, wherein the sacrificial anode is a metal less noble
than steel and the sacrificial anode includes a connector to
electronically connect it to the protected steel, and the
sacrificial anode is substantially surrounded by the modifier, and
the modifier comprises an element with a side that is an anode
supporting an oxidation reaction in electronic contact with a side
that is a cathode supporting a reduction reaction, and the cathode
of the modifier faces the sacrificial anode and is separated from
the sacrificial anode, and the anode of the modifier faces away
from the sacrificial anode.
[0069] The cathode of the modifier may comprise an air cathode with
a reduction reaction that substantially comprises the reduction of
oxygen from the air. A breathable hydrophobic material may be
included with the sacrificial anode/modifier assembly.
[0070] The useful life of the sacrificial anode may be
substantially greater than the useful life of the modifier and a
path, for ionic conduction between the sacrificial anode and the
concrete, may be provided at least after the useful life of the
modifier has ended.
[0071] The sacrificial anode may be connected to a section of steel
in an area of concrete patch repair and the modifier may be
positioned relative to the sacrificial anode to enhance the flow of
current in a direction away from a section of steel. The assembly
may include a face that is tied to a section of steel within an
area of concrete patch repair and the modifier may be positioned
relative to the sacrificial anode to enhance the current flowing in
a direction away from the side of the assembly to be tied or
otherwise connected to the steel.
[0072] A cavity, which is sized to accept the assembly, may be
formed in the hardened concrete and thereafter the assembly may be
installed within the cavity. The assembly may be installed in a
backfill in the cavity wherein the backfill contains the
electrolyte that connects the anode of the modifier to the
concrete.
[0073] The assembly may include an activating agent specially
adapted for use in concrete in order to activate the sacrificial
anode. The anode of the modifier and the sacrificial anode may
comprise zinc, aluminium or magnesium or alloys thereof.
EXAMPLE 1
[0074] An electric field modifier was constructed using a zinc
casing of a standard zinc chloride D size cell (also referred to as
a zinc-carbon battery with the International Electrotechnical
Commission classification of R20). A sheet of zinc was cut from the
casing and flattened and sanded to clean any deposit(s) from the
zinc. It measured approximately 55 by 100 mm. One side of the zinc
sheet was coated with two coats of an electrically conductive
silver paint of the type used to make electrical connections on
circuit boards. The sheet was then baked at 240.degree. C. for 15
minutes to remove the coating solvent. Carbon, in the form of a
graphite rod, was then rubbed onto the silvered surface to produce
a loose thin grey coating. Any coating on the reverse side of the
zinc sheet was removed using a 220 grit sandpaper to leave a clean,
bright zinc surface. The silver and carbon surface is designed to
act as an air electrode (i.e., the cathode) to facilitate the
reduction of the oxidizing agent, oxygen, while the zinc surface is
designed to provide the reducing agent (zinc) to be oxidized (i.e.,
the anode). When an electrolyte is added, the reduction of oxygen
and the oxidation of zinc will provide an electric field to enhance
current flow from a sacrificial anode to steel.
[0075] The test arrangement is shown in FIG. 4. A high resistivity
sandbox was used in the place of a concrete or mortar in specimens
to facilitate accelerated testing of the theory. The sandbox 33 was
formed using fine damp sand to simulate a high resistivity porous
environment, similar to concrete, for testing purposes. The sand
was dampened with water, but it was not saturated, so as to provide
some electrolyte and some air in a resistive porous environment.
Approximately 1 kg of damp fine sand was mixed with a tablespoon of
table salt to produce an environment that contained an activating
agent for the zinc anodes. It was placed in a plastic container
measuring 100 by 150 by 50 mm to form the sandbox 33. A clean zinc
sheet, also taken from a D-cell, was inserted into the sand at one
end of the sandbox 33 to act as a sacrificial anode 34. A similarly
sized sheet of steel 35 was inserted into the sand at the other end
of the sandbox 33.
[0076] The zinc 34 was connected to the steel 35 by cables 36 and
an ammeter 37. After 10 minutes, the initial galvanic current
reduced to 0.55 mA. The rate of change at this point was
sufficiently slow that it could be regarded as being substantially
stable for a short term test.
[0077] The modifier 38 was then inserted into the sand, between the
zinc sacrificial anode 34 and the steel 35, with its silver surface
facing the zinc anode 34 and the zinc surface facing the steel 35.
As the modifier 38 was inserted, the current began to rise. The
current continued to rise, following insertion of the modifier 38,
and peaked at 0.82 mA between 5 and 20 minutes. After 20 minutes,
the current started to show signs of falling.
[0078] The galvanic couple was left connected overnight. After 10
hours, it was measured again at 0.68 mA. The air temperature was
approximately 15.degree. C.
[0079] The sandbox 33, with the modifier 38, was then placed in a
warmer environment. After 39 hours, the sandbox 33 had warmed up to
about 20 to 25.degree. C. The current was again measured and this
time it was 1.26 mA. The modifier 38 was removed and the current
then stabilised at 0.48 mA after 30 minutes. The modifier 38 was
again inserted into the sand, but this time it was rotated so the
silvered surface faced the steel 35. The current fell to -0.08 mA.
The electric field of the modifier 38 completely overcame the
electric field of the zinc steel couple and reversed the direction
of the current flow.
[0080] The above procedure was then repeated after water had been
added to the sand to replace water lost through evaporation. This
time the current between the zinc sacrificial anode 34 and the
steel 35 was recorded using a datalogger. The current-time
behaviour is shown in FIG. 5.
[0081] The starting galvanic current was measured without the
modifier 38 being present. The galvanic current stabilised at just
over 2 mA. The modifier 38 was then inserted (at time zero in FIG.
5) between the sacrificial anode 34 and the steel 35 with the
cathode of the modifier 38 facing the sacrificial anode 34. The
galvanic current increased to 3.3 mA over the next 45 minutes.
After 45 minutes the modifier 38 was removed and the galvanic
current fell back to 2 mA for 20 minutes. After 65 minutes the
modifier 38 was again inserted between the sacrificial anode 34 and
the steel 35, but this time the anode of the modifier 38 faced the
sacrificial anode 34. The galvanic current fell to 0.7 mA for 30
minutes. After 95 minutes the modifier 38 was again removed and the
galvanic current rose to 2 mA.
[0082] The above test has shown that a modifier 38 may be used to
substantially increase or decrease the current output of the
sacrificial anode.
EXAMPLE 2
[0083] Two electric field modifiers of approximately 55 by 50 mm in
size were constructed using the same zinc sheet as described in
Example 1. One side of each zinc sheet was first coated with two
coats of silver paint and then baked as described in Example 1.
Thus one side of each sheet was zinc and the other side was a
conductive silver coating. The silver coated surface was then
coated with a carbon rich paint. Two make the carbon paint, a
carbon bar from the center of a zinc-carbon battery was sanded down
to produce a fine carbon powder. The power was mixed with a drop of
clear outdoor varnish and approximately 10 times as much varnish
solvent thinner. A carbon to binder ratio in the dry paint film of
greater than 10:1 was targeted. The painted zinc sheet was then
baked further to remove the solvent. The conductivity of the
painted surface was checked using a resistance meter with two
probes which were lightly pressed onto the carbon coated surface.
The resistivity was less than 1 ohm. One of these sheets will be
referred to as the zinc-air modifier in this example.
[0084] A manganese dioxide-carbon mixture was then applied to the
carbon coated surface of the other zinc-carbon sheet. The manganese
dioxide-carbon mixture was sourced from the cathode side of a
standard zinc chloride D size cell. It was applied as a layer to
the carbon coated surface of one zinc-carbon sheet and then covered
with wall paper paste and then covered with a thin absorbent paper
tissue and then pressed firmly together under a weight of
approximately 60 kg. The manganese dioxide-carbon mixture and
absorbent tissue was then trimmed to the edge of the zinc sheet to
provide a zinc sheet with a 2 mm thick manganese dioxide-carbon
layer on one side and uncoated zinc on the other side. This
modifier is referred to as a zinc-manganese dioxide (MnO.sub.2)
modifier.
[0085] A batch of a damp fine sand-salt mixture, containing both
electrolyte and air was made as described in Example 1. The mixture
was used to fill three small sandboxes 33 measuring 90 by 65 by 35
mm. A bare zinc sheet measuring approximately 55 by 50 mm was
partially inserted into one end of each sandbox 33 and a similarly
sized steel sheet was partially inserted into the other end of each
sandbox 33. The zinc was connected to the steel through a 100 ohm
resistor in each sandbox to form a galvanic cell. A galvanic
current flowed through the resistor and produced a voltage that was
measured to monitor the galvanic current. The general layout was
similar to that shown in FIG. 4, with the ammeter being replaced by
a 100 ohm resistor.
[0086] The galvanic currents in the sandboxes 33 were first
measured without any modifiers being used. The sandbox 33 that
produced the highest galvanic current was chosen to be the control.
The zinc-air modifier was inserted between the zinc sacrificial
anode and the steel of the second sandbox The carbon surface of the
modifier faced the zinc sacrificial anode. The zinc-manganese
dioxide modifier was inserted between the zinc sacrificial anode
and the steel of the third sandbox. The manganese dioxide surface
of the modifier faced the zinc sacrificial anode. The galvanic
current was logged (recorded on a data logger) during this
process.
[0087] The galvanic currents from the three sandboxes 33 are shown
in FIGS. 6 and 7. The electric field modifiers were inserted into
the sand between the zinc anode and the steel at time zero in these
Figures. Immediately after the modifiers were inserted, the
galvanic cell with the zinc-manganese dioxide modifier produced the
highest galvanic current (FIG. 6). However this high initial
current decayed over 10 hours and then the galvanic cell with the
zinc-air modifier produced the highest galvanic current. The
currents from all three cells decayed at a slow rate, probably as
the result of the sand between the zinc and the steel drying out.
After 7 days, the sandboxes were inserted into a large plastic bag
to slow the rate of further drying of the sand and the galvanic
currents stabilised, to primarily show daily fluctuations that
would be associated with daily variations in temperature (FIG. 7).
Over time, the galvanic current produced by the cell with the
zinc-manganese dioxide modifier recovered to a value closer to that
of the zinc-air modifier.
[0088] These results again indicate that an electric field modifier
is capable of substantially boosting the short term current output
of a sacrificial anode. In addition, a modifier with a more
powerful manganese dioxide cathode, at the start, may become a
modifier with an air cathode after the manganese dioxide is spent
(consumed by reduction) as a cathode material.
EXAMPLE 3
[0089] The test arrangement for Example 3 is shown in FIG. 8. Two
cement mortar blocks 41 (measuring 270 mm long by 175 mm wide by
110 mm high) were cast using damp sand, Portland cement and water
in the weight ratio 4:1:0.8. The mortar was of a relatively poor
quality and some bleed water formed on top of the casting. A steel
cathode 42 with a surface area of 0.12 m.sup.2 was positioned in
the outer edge of each mortar block during the casting process. The
steel cathode was formed from two 300 mm by 100 mm steel shims that
were cut and folded to form a set of 20 mm wide by 90 mm long steel
strips connected by a 10 mm by 300 mm strip to allow both sides of
the steel to receive current during the testing process. A segment
of the cut and folded steel cathode 42 is shown in FIG. 9. An
electric cable 43 was connected to the steel cathode 42 and
extended beyond the cement mortar 41 to enable electrical
connections to be made to the steel cathode 42. A hole 44,
measuring 40 mm in diameter by 70 mm deep, was formed in the center
of the cement mortar block 41 to house a sacrificial anode
assembly. The cement mortar blocks were covered and left for 7 days
to cure.
[0090] An electric field modifier 45 was made from a zinc cylinder
from a standard zinc chloride D size cell described in Example 1
after removing the base, top and inside of the cell. The zinc
cylinder measured 32 mm in diameter by 55 mm long. It was lightly
sanded and washed with soap to remove any deposit(s). The inside of
the zinc cylinder was then coated with two coats of silver
conductive paint and one coat of carbon conductive paint and baked,
as described in Example 2, to form the cathode 46 of the modifier
45. The outer surface of the cylinder formed the anode 47 of the
modifier 45. A salt paste, consisting of a starch based wall paper
paste and table salt (primarily sodium chloride) in equal volumes
was mixed up and applied to the outer zinc surface of the modifier.
The modifier was then baked again in an oven at 240.degree. C. for
15 minutes to dry the salt paste and form a crusty layer of salt on
the outer zinc surface. The purpose of the salt-starch coating was
to provide an activating agent for the zinc anode. This modifier 45
is referred to as a zinc-air modifier as the anode reaction is the
dissolution of zinc and the cathodic reaction is the reduction of
oxygen from the air.
[0091] Two zinc sacrificial anodes were formed by casting a 15 mm
diameter, 35 mm long bar of zinc around a titanium wire. The charge
capacity of the anodes was about 150 kC. The surface of the zinc
bar was coated with the salt paste, described above, and baked to
form a crusty layer of salt on the zinc surface.
[0092] After the cement mortar specimens had cured for 7 days, the
40 mm diameter hole in the center of each specimen was partially
filled with lime putty 50 and the zinc sacrificial anode 49 was
inserted into the lime putty 50 such that the sacrificial anode 49
and the putty 50 filled approximately 85% of the space within the
hole. The sacrificial anode 49 was connected to the steel cathode
42 by an electric cable 51 and a 100 ohm resistor 52 and the
galvanic current was measured and recorded, as described in Example
2. The two specimens were left for 1.5 hrs to stabilize and the
specimen that produced the highest galvanic current was selected as
the control specimen while the second specimen was used to test the
zinc-air modifier.
[0093] After 1.5 hours, water was added to the lime putty 50, in
both specimens to soften the putty 50. The zinc-air modifier 45 was
then pressed into the lime putty 50 around the sacrificial anode 49
in one specimen to substantially surround the sacrificial anode 49.
The galvanic currents were recorded and are given in FIGS. 10, 11
and 12. In these figures, time zero is the time when the modifier
45 was installed. The control specimen has no modifier.
[0094] Initially no positive effect of the modifier 45 was seen
(FIG. 10). Indeed the effect appeared to be negative. The control
specimen, with the wet putty, appeared to deliver substantially
more current than the specimen with the wet putty and the modifier
45. However, as the putty 50 started to dry and harden, a
significant positive effect of the modifier 45 became evident.
[0095] To explain this observation, it is noted that a galvanic
current of 3 mA is a relatively high current for such a small
sacrificial anode assembly in a cement mortar. It equates to a
cathode current density on the modifier of 550 mA/m.sup.2. It is
postulated that it is difficult for the cathode 46 of the modifier
45 to support such a high current density in a very moist putty 50
as oxygen from the air must come into contact with the carbon on
the cathode 46 of the modifier 45 to sustain the cathodic reduction
reaction. In this case, the cathode of the modifier would block the
high current density. As the putty 50 eventually dries, oxygen has
easier access to the cathode 46 of the modifier 45 while the anode
reactions (the dissolution of zinc) become more restricted. Thus
the modifier 45 tends to sustain the current as the putty 50 dries
and hardens. This observation indicates that both electrolyte and
air are needed for the modifier with an air cathode to work.
[0096] After 2.6 days, the sacrificial anode assembly, in each
cement mortar specimen, was covered with cement mortar which filled
the remainder of the hole or cavity. The two specimens were placed
outside and exposed to the weather of the UK Midlands. The weather
was initially sunny and dry with direct sunlight falling on the
specimens in the late afternoon and the specimens were drying
fairly rapidly. This weather was sustained until day 11. The daily
maximum air temperature rose from 17.degree. C. on day 3 to
26.degree. C. on days 8 and 9. On day 12, the first of a series of
cold fronts passed over the region and the daily maximum
temperature dropped to a low of 13.degree. C. There were also more
clouds and less sunshine. On day 15, it began to rain with some
significant rain showers wetting the specimens. Intermittent
showers continued through to day 19. On day 17, the position of the
control and zinc-air modifier mortar blocks was switched to
minimize the effect of any changes in microclimate. The daily
maximum air temperature rose to 17.degree. C. by day 20.
[0097] The galvanic currents from the two specimens, between days 6
and 21, are provided in FIG. 11. The data suggests the modifier 45
has a substantial positive effect on the galvanic current output of
the anode assembly. The modifier 45 resulted in an average galvanic
current over any 24 hour period on day 6 onwards that was between
1.6 and 5.6 times higher than the control specimen. The effect of
the daily variations in air temperature and rain on day 15 are also
evident in the data and indicates that a beneficial responsive
behaviour of the protection current output to changes in the
aggressive nature of the cement mortar was retained and amplified
by the presence of the modifier 45. The most pronounced daily
variations occurred between days 7 and 12 when the specimens were
directly heated by the sun's radiation. These pronounced variations
disappeared when the weather clouded over. The effect of wetting
the specimen with rain water is a slower process that occurred
after day 15.
[0098] The galvanic currents from the two specimens between days 15
and 65 are shown in FIG. 12. The data suggests that the effect of
the modifier 45 lasted until day 45. After the modifier 45 expired,
the sacrificial anode 49 continued to deliver current at a similar
magnitude to the control specimen. Thus, it is possible to produce
an anode assembly with a modifier where the modifier delivers an
initial boost in the sacrificial anode current output without any
substantial adverse effect on the longer term galvanic current
output of the sacrificial anode.
* * * * *