Abstract: The relation between atmospheric corrosion resistance and the composition and microstructure of scales of weathering steel is discussed The scale of cold-rolled plates consists of lepidocrocites,iron hydroxides and dispersed iron particles. Comparing with that of carbon steel, the atmospheric resistance of weathering steel is very good, as lepidocrocites grow just adjacent to iron matrix. For the scales of carbon steel the iron matrix was not protected fron atmospheric corrosion for iron hydroxide was made as "harbour" between scale and metal. Magnetites and hematites were formed on the surface of hot-rolled plates.
The scale with developed magnetite and a sawtooth structure connected with matrix coule provide a good protection to the matrix.The rare earth added made the steel cleaner and inclusions more g ranular. It seems that rare earth in the steel inhibited the decomposition of magnetite under the reaction with water and air. The steel treated by rare earth had better atmospheric corrosion resistance.