Ordinary carbon steels tend to corrode in air. The corrosion products from ordinary carbon steels in air are a cohesive mixture of fine particles of FeOOH[=1/2(Fe2O3・H2O)] and Fe3O4.

Corrosion occurs by the combined effect of water and oxygen and continues as long as water and oxygen are both supplied. Therefore, corrosion can be suppressed or prevented by avoiding water and oxygen from direct contact with the surface of a steel sheet. Painting and chemical treatment are carried out to achieve this.
The ultimate target for steel materials used in air, with regard to corrosion resistance, is to use them in the unpainted condition without excessive alloying of Ni and/or Cr. The materials developed for this purpose are referred to as atmospheric corrosion-resistant steels (weathering steels). The figure shows the thickness losses of steels caused by corrosion in air. The corrosion loss of atmospheric corrosion-resistant steel is only half that of ordinary carbon steel. The photograph shows a comparison of the cross section of rust formed in air between ordinary carbon steel and atmospheric corrosion-resistant steel. The black and gray parts on the base metal are amorphous layers of enriched chromium and copper, while the yellow parts are crystalline rust layers. The whole surface of atmospheric corrosion-resistant steel is covered with an amorphous layer that provides high corrosion resistance. It is believed that this amorphous layer suppresses the progress of corrosion by forming stabilized rust, and preventing oxygen and water penetrating through the rust. However, neither the detailed structure of stabilized rust nor its formation mechanism has yet been thoroughly clarified.

The use of unpainted atmospheric corrosion-resistant steels is spreading for buildings and bridges to be constructed in inland areas. It is expected that their area of application will be extended to coastal areas by further improvements in their characteristics through future research and development.