Corrosion of Embedded Metals
of reinforcing steel and other embedded metals is the leading cause
of deterioration in concrete. When steel corrodes, the resulting
rust occupies a greater volume than the steel. This expansion creates
tensile stresses in the concrete, which can eventually cause cracking,
delamination, and spalling.
Steel corrodes because it is not a naturally occurring material.
Rather, iron ore is smelted and refined to produce steel. The production
steps that transform iron ore into steel add energy to the metal.
Steel, like most metals except gold and platinum, is thermodynamically
unstable under normal atmospheric conditions and will release energy
and revert back to its natural state—iron oxide, or rust.
This process is called corrosion.
corrosion to occur, these elements must be present:
- There must be at least two metals (or two locations
on a single metal) at different energy levels
In reinforced concrete, the rebar may have many separate areas
at different energy levels. Concrete acts as the electrolyte, and
the metallic connection is provided by wire ties, chair supports,
or the rebar itself.
Corrosion is an electrochemical process involving the flow of charges
(electrons and ions). At active sites on the bar, called anodes,
iron atoms lose electrons and move into the surrounding concrete
as ferrous ions. This process is called a half-cell oxidation reaction,
or the anodic reaction, and is represented as:
The electrons remain in the bar and flow to sites called cathodes,
where they combine with water and oxygen in the concrete. The reaction
at the cathode is called a reduction reaction. A common reduction
To maintain electrical neutrality, the ferrous ions migrate through
the concrete pore water to these cathodic sites where they combine
to form iron hydroxides, or rust:
This initial precipitated hydroxide tends to react further with
oxygen to form higher oxides. The increases in volume as the reaction
products react further with dissolved oxygen leads to internal stress
within the concrete that may be sufficient to cause cracking and
spalling of the concrete cover.
Corrosion of embedded metals in concrete can be greatly reduced
by placing crack-free concrete with low permeability and sufficient
concrete cover. Low-permeability concrete can be attained by decreasing
the water to cementitious materials ratio of the concrete and the
use of pozzolans and slag. Pozzolans and slag also increase the
concrete resistivity thus reducing the corrosion rate even after
it initiates. ACI 318, Building Code Requirements for Structural
Concrete provides minimum concrete cover requirements that
will help protect the embedded metals from corrosive materials.
Additional measures to mitigate corrosion of steel reinforcement
in concrete include the use of corrosion inhibiting admixtures,
coating of reinforcement (for example, with an epoxy resin), and
use of sealers and membranes on the concrete surface. Sealers and
membranes, if used, have to be periodically reapplied.
Concrete and the Passive Layer
Although steel’s natural tendency is to undergo corrosion
reactions, the alkaline environment of concrete (pH of 12 to 13)
provides steel with corrosion protection. At the high pH, a thin
oxide layer forms on the steel and prevents metal atoms from dissolving.
This passive film does not actually stop corrosion; it reduces the
corrosion rate to an insignificant level. For steel in concrete,
the passive corrosion rate is typically 0.1 µm per year. Without
the passive film, the steel would corrode at rates at least 1,000
times higher (ACI222 2001).
of concrete’s inherent protection, reinforcing steel does
not corrode in the majority of concrete elements and structures.
However, corrosion can occur when the passive layer is destroyed.
The destruction of the passive layer occurs when the alkalinity
of the concrete is reduced or when the chloride concentration in
concrete is increased to a certain level.
The Role of Chloride Ions
of reinforced concrete to chloride ions is the primary cause of
premature corrosion of steel reinforcement . The intrusion of chloride
ions, present in deicing salts and seawater, into reinforced concrete
can cause steel corrosion if oxygen and moisture are also available
to sustain the reaction. Chlorides dissolved in water can permeate
through sound concrete or reach the steel through cracks. Chloride-containing
admixtures can also cause corrosion.
No other contaminant is documented as extensively in the literature
as a cause of corrosion of metals in concrete than chloride ions.
The mechanism by which chlorides promote corrosion is not entirely
understood, but the most popular theory is that chloride ions penetrate
the protective oxide film easier than do other ions, leaving the
steel vulnerable to corrosion.
The risk of corrosion increases as the chloride content of concrete
increases. When the chloride content at the surface of the steel
exceeds a certain limit, called the threshold value, corrosion will
occur if water and oxygen are also available. Federal Highway Administration
(FHWA) studies found that a threshold limit of 0.20% total (acid-soluble)
chloride by weight of cement could induce corrosion of reinforcing
steel in bridge decks (Clear 1976). However, only water-soluble
chlorides promote corrosion; some acid-soluble chlorides may be
bound within aggregates and, therefore, unavailable to promote corrosion.
Work at the FHWA (Clear 1973) found that the conversion factor from
acid-soluble to water-soluble chlorides could range from 0.35 to
0.90, depending on the constituents and history of the concrete.
Arbitrarily, 0.75 was chosen, resulting in a water-soluble chloride
limit of 0.15 % by weight of cement.
Although chlorides are directly responsible for the initiation
of corrosion, they appear to play only an indirect role in the rate
of corrosion after initiation. The primary rate-controlling factors
are the availability of oxygen, the electrical resistivity and relative
humidity of the concrete, and the pH and temperature.
Carbonation occurs when carbon dioxide from the air penetrates the
concrete and reacts with hydroxides, such as calcium hydroxide,
to form carbonates. In the reaction with calcium hydroxide, calcium
carbonate is formed:
|Ca(OH)2 + CO2 →
CaCO3 + H2O
This reaction reduces the pH of the pore solution to as low as
8.5, at which level the passive film on the steel is not stable.
Carbonation is generally a slow process. In high-quality concrete,
it has been estimated that carbonation will proceed at a rate up
to 1.0 mm (0.04 in.) per year. The amount of carbonation is significantly
increased in concrete with a high water-to-cement ratio, low cement
content, short curing period, low strength, and highly permeable
or porous paste.
Carbonation is highly dependent on the relative humidity of the
concrete. The highest rates of carbonation occur when the relative
humidity is maintained between 50% and 75%. Below 25% relative humidity,
the degree of carbonation that takes place is considered insignificant.
Above 75% relative humidity, moisture in the pores restricts CO2
penetration. Carbonation-induced corrosion often occurs on areas
of building facades that are exposed to rainfall, shaded from sunlight,
and have low concrete cover over the reinforcing steel.
Carbonation of concrete also lowers the amount of chloride ions
needed to promote corrosion. In new concrete with a pH of 12 to
13, about 7,000 to 8,000 ppm of chlorides are required to start
corrosion of embedded steel. If, however, the pH is lowered to a
range of 10 to 11, the chloride threshold for corrosion is significantly
lower—at or below 100 ppm. Like chloride ions, however, carbonation
destroys the passive film of the reinforcement, but does not influence
the rate of corrosion.
Dissimilar Metal Corrosion
When two different metals, such as aluminum and steel, are in contact
within concrete, corrosion can occur because each metal has a unique
electrochemical potential. A familiar type of dissimilar metal corrosion
occurs in an ordinary flashlight battery. The zinc case and carbon
rod are the two metals, and the moist paste acts as the electrolyte.
When the carbon and zinc are connected by a wire, current flows.
In reinforced concrete, dissimilar metal corrosion can occur in
balconies where embedded aluminum railings are in contact with the
reinforcing steel. Below is a list of metals in order of electrochemical
||11. Stainless Steel
When the metals are in contact in an active electrolyte, the less
active metal (lower number) in the series corrodes.
ACI Committee 222, Protection of Metals in Concrete Against
Corrosion, ACI 222R-01, American Concrete Institute, Farmington
Hills, Michigan, 2001, 41 pages.
ACI Committee 318, Building Code Requirements for Structural
Concrete, ACI 318-05, American Concrete Institute, Farmington
Hills, Michigan, 2005, 443 pages.
Clear, K.C., and Hay, R.E., “Time-to-Corrosion of Reinforcing
Steel in Concrete Slabe, V.1: Effect of Mix Design and Construction
Parameters,” Report No. FHWA-RD-73-32, Federal Highway Administration,
Washington, DC, April, 1973, 103 pages.
Clear K.C., “Time-to-Corrosion of Reinforcing Steel in Concrete
Slabs,” Federal Highway Administration, PB 258 446, Vol. 3,
and Causes of Concrete Deterioration, IS536, Portland Cement
Association, Skokie, Illinois, 2002, 16 pages.