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Understanding Corrosion and Cathodic Protection of Reinforced Concrete Structures
Understanding Corrosion and Cathodic Protection
of Reinforced Concrete Structures
by
Steven F. Daily
Corrpro Companies, Inc.
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:
1.

Carbonation, and
2.

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 % 2
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/yd
3
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 3
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.
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