Nanostructure of Protective Rust Layer on Weathering Steel Examined Using Synchrotron Radiation X-rays

The X-ray absorption fine structure (XAFS) spectrum of pure goethite around the Fe K absorption edge and that of the protective rust layer formed on weathering steel exposed for 17 years in an atmospheric environment around the Cr K edge, have been examined using synchrotron radiation X-rays. It was found that the rust layer on the weathering steel mainly consisted of Cr-goethite. By examining the fine structure at the Cr K edge and the Fe K edge, we concluded that Cr3+ in the rust layer is coordinated with O2− and is positioned in the double chains of vacant sites in the network of FeO3(OH)3 octahedra in the goethite crystal. This Cr3+ site indicates that the protective effect of the rust layer is due to the dense aggregation of fine crystals of Cr-goethite with cation selectivity.