The rust layer formed on the weathering steels coated with a surface treatment promoting protective rust formation exposed at a coastal region containing 0.4mg/dm2/day of air-borne salt particles for 9 years has been investigated by means of X-ray diffraction, Raman spectroscopy, Electron probe micro-analysis and Mössbauer spectroscopy. In the surface treatment, Cr2(SO4)3 is contained to form the protective rust layer. Corrosion loss vs. time curve of surface treated weathering steels follows Y = AXB type function, where X is the exposure duration, Y is the corrosion loss, A and B are the constants. The surface of the treated weathering steels were covered with triple layers ; outer rust layer, surface treatment film and inner rust layer. The outer rust layer consisted of mainly β-FeOOH, and the inner one mainly α-FeOOH. Cr enrichment was observed in the inner rust layer. It can be said that the surface treatment works well to promote the formation of Cr-containing α-FeOOH layer formed on weathering steel.
