In order to understand the atmospheric corrosion behavior of structural steels at semi-closed space such as box girder of steel bridge, the temperature and relative humidity were measured with thermo-hygrometers, and corrosion rate of structural steel was estimated using steel coupons with various deposited salt amount on the surface. Besides, Fe-Ag galvanic type ACM (Atmospheric Corrosion Monitor) sensors were employed to evaluate the influences of both the deposited salt amount and relative humidity on the corrosion of steels.
It was found that the relative humidity inside box girders was lower than that outside, but it may be as high as 100% in some cases. Although the pollutant deposition is possible near the entrance of box girder, the airborne pollutant hardly invade into the interior of the box girder. The corrosion rate of structural steel without pollutant deposition on the surface is extremely low inside the box girder. The pollutant deposited on the surface before exposure promotes the corrosion of steel, but the effect of pollutant on corrosion rate of steel becomes weak with time. It can be suggested that low corrosion rate of structural steel inside box girder results mainly from no invasion of airborne pollutant.