Mössbauer Spectroscopic Study on X-ray Amorphous Substance in Rust Layer of Weathering Steel Subjected to Long-Term Exposure in North America

Abstract

The corrosion product formed on weathering steel exposed in a rural environment in the United States for 16 years has been investigated using Mössbauer spectroscopy, X-ray diffraction and Raman spectrometry. Mössbauer spectroscopy was used to measure the fraction of each oxide in the rust layer and micro-Raman spectrometry was used to locate and map the oxides to 2 microns spatial resolution. The protective inner-layer closest to the steel substrate consisted of nano-sized goethite ranging in size from 5-30 nm. The outer-layer close to the rust layer surface, consisted of lepidocrocite and goethite with the former oxide being most abundant. Comparison of the goethite in the rust layer was made with synthetic chromium substituted goethite with nearly identical microstructural characteristics being recorded. It can be said that most of the so-called X-ray amorphous substance, generally considered as mixture of nano-phase oxides of crystal size less than about 15 nm and actual amorphous phase with very short-range ordered atomic arrangement, possesses goethite structure with particle size less than 15 nm. Considering this nano-phase goethite, new quantitative determination method of total goethite in rust layer is proposed. It is concluded that chromium-substitution in the goethite is important for formation of a nano-phase oxide layer which may help protect the weathering steel from further corrosion.