STEEL - Materials for Civil and Construction Engineers - page 128

structures can cause serious problems and embarrassing and/or dangerous
failures. For example, corrosion of steel bridges, if left unchecked, may re-
sult in lowering weight limits, costly steel replacement, or collapse of the
structure. Other examples include corrosion of steel pipes, trusses, frames,
and other structures. It is estimated that the cost of corrosion in the United
States alone is $8 billion each year (Frank and Smith 1990).
Corrosion is an electrochemical process; that is, it is a chemical reaction
in which there is transfer of electrons from one chemical species to another.
In the case of steel, the transfer is between iron and oxygen, a process called
oxidation reduction . Corrosion requires the following four elements (with-
out any of them corrosion will not occur):

1. an anode —the electrode where corrosion occurs
2. a cathode —the other electrode needed to form a corrosion cell
3. a conductor —a metallic pathway for electrons to flow
4. an electrolyte —a liquid that can support the flow of electrons

Steel, being a heterogeneous material, contains anodes and cathodes. Steel
is also an electrical conductor. Therefore, steel contains three of the four el-
ements needed for corrosion, while moisture is usually the fourth element
(electrolyte).
The actual electrochemical reactions that occur when steel corrodes are
very complex. However, the basic reactions for atmospherically exposed
steel in a chemically neutral environment are dissolution of the metal at the
anode and reduction of oxygen at the cathode.
Contaminants deposited on the steel surface affect the corrosion reac-
tions and the rate of corrosion. Salt, from deicing or a marine environment,
is a common contaminant that accelerates corrosion of steel bridges and re-
inforcing steel in concrete.
The environment plays an important role in determining corrosion
rates. Since an electrolyte is needed in the corrosion reaction, the amount of
time the steel stays wet will affect the rate of corrosion. Also, contaminants
in the air, such as oxides or sulfur, accelerate corrosion. Thus, areas with
acid rain, coal-burning power plants, and other chemical plants may accel-
erate corrosion.

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Methods for Corrosion Resistance
Since steel contains three of the four elements needed for corrosion, protec-
tive coatings can be used to isolate the steel from moisture, the fourth element.
There are three mechanisms by which coatings provide corrosion protection
(Hare 1987):

3.10.1

1. Barrier coatings work solely by isolating the steel from the moisture.
These coatings have low water and oxygen permeability.
2. Inhabitive primer coatings contain passivating pigments. They are low-
solubility pigments that migrate to the steel surface when moisture
passes through the film to passivate the steel surface.