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Corrosion of Steel in Concrete
The
corrosion process that takes place in concrete is
electrochemical in nature, very similar to a battery.
Corrosion will result in the flow of electrons between
anodic and cathodic sites on the rebar. For corrosion to
occur four basic elements are required:
-
Anode - site where corrosion occurs and current
flows from.
-
Cathode - site where no corrosion occurs and current
flows to.
-
Electrolyte - a medium capable of conducting
electric current by ionic current flow (i.e. soil,
water or concrete).
-
Metallic Path - connection between the anode and
cathode, which allows current return and completes
the circuit.
Reinforcing steel in concrete normally does not corrode
because of the formation of a passive oxide film on the
surface of the steel due to the initial corrosion
reaction. The process of hydration of cement in freshly
placed concrete develops a high alkalinity, which in the
presence of oxygen stabilizes the film on the surface of
embedded steel, ensuring continued protection while the
alkalinity is retained. Normally, concrete exhibits a pH
above 12 because of the presence of calcium hydroxide,
potassium hydroxide, and sodium hydroxide - the term pH
is a measure of the alkalinity or acidity, ranging from
highly alkaline at 14 to highly acidic at zero, with
neutrality at 7. Although the precise nature of this
passive film is unknown, it isolates the steel from the
environment and slows further corrosion as long as the
film is intact. However, there are two major situations
in which corrosion of reinforcing steel can occur. These
include:
-
Carbonation,
-
Chloride contamination
Carbonation is a process in which carbon dioxide from
the atmosphere diffuses through the porous concrete and
neutralizes the alkalinity of concrete. The carbonation
process will reduce the pH to approximately 8 or 9 in
which the oxide film is no longer stable. With adequate
supply of oxygen and moisture, corrosion will start. The
penetration of concrete structures by carbonation is a
slow process, the rate of which is determined by the
rate at which carbon dioxide can penetrate into the
concrete. The rate of penetration primarily depends on
the porosity and permeability of the concrete. It is
rarely a problem on structures that are built with good
quality concrete with adequate depth of cover over the
reinforcing steel.
The roll of the chloride ion in inducing reinforcement
corrosion is well documented. Chloride ions can enter
into the concrete from de-icing salts that are applied
to the concrete surface or from seawater in marine
environments. Other sources include chloride containing
admixtures which are used to accelerate curing,
contaminated aggregates and/or mixing water, air born
salts, salts in ground water, and salts in chemicals
that are applied to the concrete surface. If chlorides
are present in sufficient quantity, they disrupt the
passive film and subject the reinforcing steel to
corrosion.
The levels of chloride required to initiate corrosion
are extremely low. There have been many recommendations,
both codes and publications, for maximum chloride
concentrations. The American Concrete Institute (ACI)
Publication 222R-96 "Corrosion of Metals in Concrete",
recommends the following chloride limits in concrete for
new construction, expressed as a percent by weight of
cement (acid-soluble test method):
-
Pre-stressed concrete 0.08 %
-
Reinforced concrete in wet conditions 0.10 %
-
Reinforced concrete in dry conditions 0.20 %
Field experience and research have shown that on
existing structures subjected to chloride ions, a
threshold concentration of about 0.026% (by weight of
concrete) is sufficient to break down the passive film
and subject the reinforcing steel to corrosion. This
equates to 260-ppm chloride or approximately 1.0 lb/yd3
of concrete.
The removal of the passive film from reinforcing steel
leads to the galvanic corrosion process. Chloride ions
within the concrete are usually not distributed
uniformly. The steel areas exposed to higher
concentrations of chlorides start to corrode, and
breakdown of the oxide film eventually occurs. In other
areas, the steel remains passive. A classic example of
this uneven exposure is the application of de-icing
salts to a bridge deck in which the top mat of steel
receives more chloride than the bottom mat. This uneven
distribution results in macro-cell corrosion, in which
large anodic sites on the top mat and large cathodic
sites on the bottom mat are encountered. The concrete
acts as the electrolyte and the metallic conductor is
provided by wire ties, chair supports, and the steel
bars. Figure 1 illustrates how a macro corrosion
cell can develop from differences in chloride ion
concentration.
Figure 1. Differences in chloride ion con-centration
establish differences in electrical potential.
Patching of delaminated and spalled concrete with
conventional concrete is yet another example of the
corrosion mechanism. Strong electrochemical macro-cells
are established near the interface between the old
chloride-contaminated concrete and the new chloride-free
concrete. The short distance between anode and cathode,
together with the large difference in chloride
concentration, result in strong potential gradients,
which accelerate corrosion. Such a macro-cell is shown
in Figure 2. In many cases, this kind of repair
will require rehabilitation again in only one or two
years.
Figure 2. Macro-cell corrosion through concrete
patching.
Differences within the grain structure of the metal or
different residual stress levels can also lead to
galvanic corrosion. When chlorides are uniformly
distributed around the steel, local action micro-cells
form and dominate the corrosion process. Anodic and
cathodic sites may be observed very close to each other
on the same bar under such circumstances. This
micro-cell effect generally leads to a type of localized
corrosion known as pitting corrosion. In this case,
metal loss from anodic sites creates a pit. As corrosion
proceeds, the condition inside the pit becomes
progressively more acidic and further loss occurs from
the bottom of the pit rather from the sides. The
cross-sectional area of the steel is progressively
reduced to a point in which the steel can no longer
carry the applied loading.
All of the corrosion processes described above require
oxygen. In the absence of oxygen, the corrosion rate is
appreciably reduced even with chloride concentrations
above the threshold level, except in acid solutions.
However, keeping oxygen from reinforcing steel in the
field is extremely difficult, if not impossible. When
corrosion of reinforcing steel occurs, the corrosion
products or rust can occupy several times the volume
than the original steel, causing tensile forces to
develop in the concrete. Since concrete is relatively
weak in tension, cracks can develop as shown in
Figure 3a, exposing the steel to even more
chlorides, oxygen and moisture - and the corrosion
process accelerates. As corrosion continues,
delaminations - separations within the concrete and
parallel to the concrete surface occur (Figure 3b).
Delaminations are usually located at, or near, the level
of the reinforcing steel. Eventually pieces of concrete
break away forming spalls in the concrete (Figure 3c),
which require repair to maintain structural integrity.
Figure 3. Corrosion-induced cracking of the concrete.
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Cathodic Protection Fundamentals
There are many ways to slow down the
corrosion process, however cathodic
protection (CP) is the only technology that
has proven to stop corrosion in existing
reinforced concrete structures, regardless
of the chloride content in the concrete.
What is CP? Quite simply CP is a widely used
and effective method of corrosion control.
In theory it is defined as the reduction or
elimination of corrosion by making the metal
a cathode via an impressed direct current
(DC), or by connecting it to a sacrificial
or galvanic anode. Cathodic areas in an
electrochemical cell do not corrode. By
definition, if all the anode sites were
forced to function as current-receiving
cathodes, then the entire metallic structure
would be a cathode and corrosion would be
eliminated.
For decades, CP has been successfully used
to protect underground pipelines, ship
hulls, offshore oil platforms, underground
storage tanks, and many other structures
exposed to corrosive environments. The first
application of CP to a concrete structure
was a bridge deck in 1973. This system
continues to function with no physical
delamination of the concrete. CP of steel in
concrete is quite simply a means of fighting
fire with fire, or in this case, electricity
with electricity. The corrosion process
generates electric currents. CP supplies a
source of external current to counteract the
corrosion current. Hence, corrosion can be
eliminated.
As indicated above, there are two types of
CP systems - impressed current and galvanic.
An impressed current CP system for concrete
structures may require the following basic
components:
-
DC power supply (rectifier).
-
Inert anode material, such as catalyzed
titanium anode mesh.
-
Wiring and conduit.
-
Instrumentation, such as embedded
silver/silver-chloride reference
electrodes.
A schematic of an impressed current CP
system using catalyzed titanium anode mesh
is shown in Figure 4.
Figure 4. Schematic of impressed current CP
system.
A rectifier is used to convert alternating
current (AC) to direct current. A rectifier
works on the same principle as an AC adapter
for a computer or a battery charger. In an
impressed current CP system, the rectifier
provides the power (i.e. low voltage direct
current) and controls the amount of power to
each zone. Rectifiers are available in many
types and operating outputs. Mainly, they
are designed to provide either constant
current or constant voltage to the anode
system.
The anode is one of the most critical
components for a cathodic protection system.
It is used to distribute protective current
to the reinforcing steel and provides
locations for anodic reactions to take place
in lieu of the reinforcing steel. By using
relatively inert materials, such as
catalyzed titanium, anode consumption is
minimized. One 4 of the main benefits of
catalyzed titanium is that its life
expectancy can be determined through
accelerated life testing. N.A.C.E. Standard
TM0294-94, "Testing of Embeddable Anodes for
Use in Cathodic Protection of
Atmospherically Exposed Steel-Reinforced
Concrete" gives procedures for accelerated
life testing of these anodes. Based on test
results using this method, it has been found
that the life of catalyzed titanium anodes
can readily exceed 40 years for existing
structures, and over 100 years for new
reinforced concrete structures (i.e.
cathodic prevention). Figure 5 shows
the application of ELGARDä titanium anode
mesh to a bridge deck. The mesh is
subsequently covered with a concrete
overlay.
Figure 5. ELGARD Anode Mesh installation on
a bridge deck
A sacrificial or galvanic anode system for
reinforced concrete uses a more reactive
metal (anode) such as zinc or
aluminum-zinc-indium (Al-Zn-In), to create a
current flow. Sacrificial anode systems are
based on the principle of dissimilar metal
corrosion and the relative position of
different metals in the galvanic series. The
direct current is generated by the potential
difference between the anode and reinforcing
steel when connected. The sacrificial anode
will corrode during the process and is
consumed. Current will flow from the anode,
through the concrete, to the corroding
reinforcing steel. Galvanic anodes may be
installed as cast anodes in soil or
thermally sprayed onto atmospherically
exposed concrete to form a sacrificial
coating. Figure 6 shows the arc-spray
application of an Al-Zn-In coating to a
reinforced concrete bridge pier.
Figure 6. Arc-spray application of
sacrificial Aluminum-Zinc-Indium
Galvanic CP systems have the benefit of no
auxiliary power supply and the advantage of
being used for pre-stressed or post
tensioned concrete without the risk of
elevated potential levels, which can lead to
hydrogen embrittlement of the steel. The
anode life, however, may be relatively short
as compared to the inert anodes, which are
used with impressed current systems. Also,
the current that is produced by a galvanic
anode is a function of its environment (i.e.
moisture and temperature conditions) and the
output cannot be easily adjusted or
controlled as with the impressed current
method.
Reference electrodes are used to evaluate
cathodic protection levels. They may be
portable devices or permanently embedded
probes in the concrete structure. The most
commonly used embedded reference electrodes
are silver/silver chloride (Ag/AgCl).
Reference electrodes should have a separate
ground connection to the reinforcing steel.
CP systems also require a negative
connection to the reinforcing steel (return
path for electric current).
With CP, chloride ions will slowly migrate
away from the reinforcing steel and toward
the anode. Furthermore, the production of
hydroxide ions at the steel surface will
cause the concrete to revert back to an
alkaline state. These factors when taken
together will quickly arrest the corrosion
process when current is applied, and allow
the passivating film to reform on the
surface of the reinforcing steel. It is
important to realize that with cathodic
protection corroded reinforcing steel cannot
be restored to its original native state,
but corrosion of steel in concrete can be
effectively stopped through the application
of cathodic protection.
When evaluating a structure as a candidate
for cathodic protection, several parameters
should be considered. These may include:
-
Remaining service life should be > 10
years.
-
Delaminations and spalls should be < 50%
of structure area.
-
Chloride content should be > 0.026% by
weight of concrete (1.0 lbs./yd 3 ).
-
Half-cell potentials should be > -200
mV, indicating a breakdown of the
passivating film.
-
The candidate structure should be
structurally sound.
-
The majority of reinforcing steel bars
should be electrically continuous.
-
AC power should be available.
The process of cathodic protection for
reinforced concrete structures surprisingly
takes little power. Data has shown that
typical CP operating current densities range
between 0.2 and 2.0 mA/m 2 for cathodic
prevention of new reinforced concrete
structures, as compared with 2 to 20 mA/m 2
for CP of existing salt contaminated
structures. This will result in power
consumption ranging from 1-3 watts per 1,000
m 2 of concrete for new construction, and
3-15 watts per 1,000 m 2 for existing
structures.
Once the CP system has been installed it is
necessary to provide routine monitoring and
maintenance. For impressed current systems,
this involves visual inspection of the
system and periodic checks at the power
supply to ensure proper operation. As a
minimum, the periodic checks should entail
measurement of the voltage and current for
each anode zone. Ensuring the supply of
direct current from the rectifier to the
structure in accordance with the operation
and maintenance manual is the most important
operating parameter. Remote monitoring
systems may also be incorporated to help
facilitate monitoring of the rectifier. As
indicated above, galvanic anode systems have
no power supply and therefore they require
minimal monitoring and maintenance.
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