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 Cr 3þ in the rust layer is coordinated with O 2� and is positioned in the double chains of vacant sites in the network of FeO3(OH)3 octahedra in the goethite crystal. This Cr 3þ 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.