Corrosion

Corrosion typically takes place in reinforced concrete where there is an anode where oxidation occurs (an uncoated reinforcing bar, for example), a cathode where reduction occurs (the same reinforcing bar or one in close proximity), an electrolyte (chloride ions from deicing salts or seawater), and an aqueous medium (water that seeps in through cracks in the concrete).

Both oxygen and moisture must be present in order for corrosion to occur. Non-uniformities in moisture, oxygen, and chloride levels can create electrical potentials that drive the corrosion reaction. Other factors that influence the rate of corrosion include concrete resistance and permeability, temperature, and depth of cover over the reinforcement.

When steel corrodes, it forms rust, which occupies a volume much greater than the steel itself. This exerts expansive stresses on the surrounding concrete. Because the concrete is low in tensile strength, these stresses can cause cracking and spalling, which, in turn, permit faster ingress of water, oxygen, and chlorides, accelerating corrosion further.

Corrosion can also reduce the cross-sectional area of the steel and can create local discontinuities in the steel surface, thus reducing the steel's tensile strength and its resistance to fatigue stresses.
Fusion-bonded epoxy coating protects against corrosion by serving as a barrier that isolates the steel from the oxygen and chloride that causes corrosion. Epoxy coating has high electrical resistance, which blocks the flow of electrons that make up the electrochemical process of corrosion.

From Yunovich, Thompson, Balvanyos, and Lave, Cost of Corrosion - Highway Bridges (Appendix D).

Powered by Google.