The primary cause of corrosion of steel bridges is the exposure of the steel to atmospheric conditions. This corrosion is greatly enhanced due to marine (salt spray) exposures and industrial environments. The only corrosion prevention method for these structures is to provide a barrier coating (paint).
Changes in environmental protection regulations have brought about transformation of the approach to corrosion protection for steel bridges. Until the mid- to late-1970s, virtually all steel bridges were protected from corrosion by multiple thin coats of lead- and chromate-containing alkyd paints applied directly over mill scale on the formed steel. Maintenance painting for prevention of corrosion was rare and primarily was practiced on larger bridge structures. Since the majority of the steel bridges in the interstate highway system were constructed between 1950 and 1980, most of these structures were originally painted in this manner; therefore, a large percentage of the steel bridges in the interstate system are protected from corrosion by a coating system that is now beyond its useful service life.
Moreover, the paint system commonly used for steel bridge members contains chromium and lead and can no longer be used because of the effects it has on humans and the environment. The bridge engineers have a choice of either replacing the lead-based paints with a different coating or painting over the deteriorating areas. Removal of lead-based paint incurs high costs associated with the requirements to contain all the hazardous waste and debris.
Developments include: (1) improved and environmentally safe coating systems and (2) methodologies to optimize the use of these systems, such as “zone” painting (adjusting coating types and maintenance schedules based on the aggressiveness of the environment within different zones on the bridge). Overpainting techniques to eliminate the cost of expensive paint removal also have been developed. (reference)