The Steel Structures Painting Council (SSPC) outlines corrosion mechanisms and maintenance suggestions for unpainted, low alloy, high strength weathering steel, used in highway bridges. These bridges were built from 1960 to 1980. By adding small amounts of copper, chromium and phosphorus to steel, the resulting low alloy had higher strength. It appeared that a stronger adhering oxide film gave a protective patina , resistant against atmospheric corrosion, and painting was not necessary. However, some of these bridges are undergoing unacceptable rates of corrosion and must be repaired. A protective patina depends on the stabilization and limitation of the growth of the amorphous layer. Alternating wet and dry cycles help form the oxides. With the presence of salts, acids, or prolonged wetness, the protective oxide layer does not form. A crystalline oxide often forms, which leads to loose rust scale. Metal is lost through scaling, pitting and crevice corrosion. The main causes of bridge corrosion are described. The successful results of painting weathering steel is noted. The Federal Highway Administration (FHWA) is studying the procedures for surface preparation on the steel, coating systems and accelerated testing of the results. Preliminary results are included. Maintenance recommendations are noted at the end.