Characterization of rust layers on weathering steels air-exposed for a long period

Several weathering steels and a plain carbon steel exposed to atmosphere for 17 years at Yokkaichi (at a coastal and industrial region) and Mikuni Pass (at a rural region) in Japan were analyzed and compared to find effects of environments on the formed rusts. Specimens were examined with electron probe microanalysis (EPMA), X-ray diffraction (XRD) and transmission electron microscope (TEM). Their cross-sections were also analyzed by EPMA. XRD detected mainly alpha-FeOOH as major rust constituent and eta-FeOOH and gamma-FeOOH as minor ones in the surface region. The concentration of beta-FeOOH was especially high on the skyward surface of all the specimens irrespective of steel composition. EPMA results showed that Si, Na, S were abundant on the skyward surface. In the cross-section, alloying elements Cu and Cr were enriched in the inner layer of rust as well as Si, while P and Ni did not show any characteristic distribution. TEM examination of cross-section of rusts showed that alpha-FeOOH was also main constituent of the rust. The beta-FeOOH showed no special distribution feature in the main part of rust except in the very surface region. Amorphous rust judged from electron diffraction and Fe3O4 distributed mainly near the interface between steel and rust layer. The gamma-FeOOH was contained much more in rust on plain carbon steel than weathering steels. A quantity ratio alpha/gamma*, that is, the ratio of alpha-FeOOH to the sum of beta-FeOOH, gamma-FeOOH and magnetite, is > 1 for a weathering steel, and < 1 for a mild steel. The Si in the inner layer was suggested in the form of SiO44- state.