Structure of Protective Rust Layers Formed on Weathering Steels by Long-term Exposure in the Industrial Atmospheres of Japan and North America

A protective and adherent rust layer develops on weathering steels during outdoor exposure. This layer acts as a barrier which slows further corrosion, thus enabling weathering steel to be used in structural applications without painting. To aid in understanding the mechanism of protective rust formation, studies have been made on the composition and microstructure of these layers. The present study was jointly conducted on Japanese weathering steel exposed in the industrial environment of Amagasaki, and US weathering steel exposed in the industrial environment of Bethlehem. The rust layers were studied by use of optical microscopy, electron microscopy with X-ray fluorescence, X-ray diffraction, and micro-Raman spectroscopy. The results of this collaboration show that, in general, the rust layers formed during long-term exposure are composed of one or more alternating layers of goethite and lepidocrocite, with scattered patches of maghemite and/or magnetite.