The primary cause of corrosion (rust) in steel bridges and their supporting structures is exposure of unprotected steel to humid air. The corrosion rate is greatly accelerated if the air is contaminated (marine salts and industrial acids). These contaminants are picked up from the surrounding environment.
Experts have found that more than 95 percent of structural damage on bridges can be traced back to some form of corrosion. Bridges in dry locations, like deserts, are very unlikely to be damaged by corrosion.
Corrosion of mild steel components and high tensile support wires can soon compromise structural integrity. This results in safety issues, costly traffic restrictions and ultimately bridge closure. While corrosion of the reinforcing steel is not the sole cause, it is a significant contributor. It has therefore become a major concern.
According to a 1997 report, of the 581,000 bridges in USA, about 17% were rated as structurally deficient. The cost to rectify these deficiencies was estimated at £60 to £90 billion. The final cost is expected to be a lot more.