The rust layers formed on the weathering steel bridges exposed for 2.5 to 18 years in both of mountain and seashore atmospheric environments in Japan have been characterized by the composition analyzed by X-ray diffraction (XRD) and evaluated on the protective ability by the potential (ERL/S) of steels with rust layers corresponding to the corrosion rates. The fraction of rust constituents is newly described in the ternary rust diagram which consists of γ*; γ-FeOOH(L)+β-FeOOH(B)+Fe3O4, X-ray amorphous substance (Am) and goethite(α-FeOOH)-type oxyhydroxide (G). Exposure term dependence of the rust composition leads to the schematic progress of long-term growth of the protective rust layer in mountain atmosphere, rust layer is mainly composed of Am and the remainder; G rich (Stage III, 2≤α/γ*) via L rich (Stage I, α/γ*<1) and G, L less(Stage II, 1≤α/γ*<2). ERL/S has been related to the change of rust composition and increased with decreasing atmosphere corrosion rate of the weathering steel. The presence of deicing salt increases B and reduces the ratio of α/γ* and ERL/S. Therefore, α/γ* is a good parameter for evaluating the protective ability of the rust layer in mountain area. However, evaluation by α/γ* is restricted to the application to the rust layer in seashore area. In conclusion, both of α/γ* and ERL/S are necessary to evaluate the protective ability of the rust layer against atmospheric corrosion in various environments.
