ASTM A-588 weathering steels, grades A and B, used in bridge structures in the United States and abroad, are not as immune to atmospheric corrosion as originally predicted. Under dry conditions and wet and dry cycles of nearly equal length, the steel forms a protective rust layer and corrodes the least. However, under wetter conditions and in the presence of stagnant water, more rusting and the formation of nonprotective rust layers are found. The presence of chloride as well as other acid-forming pollutants in the atmosphere enhances corrosion. The rust layer forms coarse flakes, which lose adherence and protectiveness. Excessive flaking and a greater-than-expected loss of section thickness result. The presence of salt and high humidity, favorable for water condensation, promotes pitting under the flakes and sheets of rust. The rust develops in the form of small mounds and grows stepwise in layers, with the iron for the growth of the rust mound obtained from the metal underneath these mounds. Because of this, wide pits develop under the mounds and corrosion proceeds by the differential aeration cell mechanism. In areas where water stagnates, rusting occurs over the entire surface and a continuous rust layer develops from a sedimentary deposit. The presence of chloride and soluble salts derived from atmospheric contaminants promotes corrosion by not only increasing the conductivity of the condensed water, but by reducing the availability of oxygen at the corroding anodic sites as well.