The protective ability of rust layers formed on weathering steels exposed in an industrial region for various periods has been studied by potential measurement, and rust composition and structure were also studied. The potential of weathering steel in 0.1 mol/L Na_2SO_4 solution tends to become noble with exposure time. The potential of weathering steels with rust layers, which were exposed for a long time and had superior atmospheric corrosion resistance, was over - 0.3 V (vs. SCE). The rust layer possesses a double-layer structure composed of an inner dark layer of #alpha#'-FeOOH and an outer bright layer under reflected polarized light, which is observed typically in a protective rust layer. On the other hand, the potential of mild steel exposed for a long time is less noble at about -0.45 V (vs. SCE). These results mean that the potential reflects the different characteristics of the rust layer. The potential becomes noble with increasing amount of #alpha# FeOOH in rust, which suggests that it depends on the composition of the rust layer. Furthermore, the potential correlates with the ratio of rust composition #alpha#/#gamma#~*: #alpha#-FeOOH/(#gamma#-FeOOH+, 8-FeOOH+ Fe_3O_4), and the potential of -0.3V (vs. SCE) corresponds to #alpha#/#gamma#~*=2.Therefore the potential as well as the ratio #alpha#/#gamma#~* can be used to evaluate the protective ability of the rust layers formed on weathering steels.
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