Corrosion

What It Means

Corrosion is the electrochemical deterioration of metal caused by oxidation in the presence of water and electrolytes. In bridges, corrosion is the single most common deterioration mechanism and a primary driver of condition rating downgrades. Two forms dominate: atmospheric corrosion of structural steel girders, beams, and connections, and chloride-induced corrosion of reinforcing steel embedded in concrete decks and substructures. Atmospheric corrosion is accelerated by salt spray in coastal environments, deicing salts in northern states, industrial pollution, and poor drainage that traps water against steel surfaces. Painted steel bridges typically require recoating every 25-30 years to maintain the protective coating system; weathering steel (A588 or equivalent) is designed to form a protective oxide patina but fails in chronically wet or high-chloride environments. Chloride-induced corrosion of rebar is the dominant failure mode for concrete decks: deicing salt chlorides penetrate the concrete cover, reach the rebar, and initiate corrosion that expands the steel volume by 4-6 times, causing cracking, delamination, and spalling of the concrete cover. The typical deck life in a high-chloride northern state is 25-40 years before major rehabilitation or replacement is required. Prestressed concrete is particularly vulnerable because prestressing strands are under high tension and even localized corrosion can cause strand fracture with disproportionate strength loss. FHWA estimates that corrosion costs the U.S. bridge inventory approximately $8.3 billion annually in direct maintenance, rehabilitation, and premature replacement, with indirect user costs many times higher. Modern corrosion mitigation includes epoxy-coated and galvanized rebar, stainless clad rebar, silica fume and fly ash concrete mixes, sealers, cathodic protection systems, and sacrificial anode systems.

Frequently Asked Questions