Infrared Spectroscopic Analysis of Phase Transformation Processes in Rust Layers Formed on Weathering Steels in Bridge Spans

Abstract Infrared spectroscopy (IRS) was used to study the formation and transformation of rust phases and the weathering characteristics of ASTM A588 weathering steels in bridge spans. The technique enables not only clear differentiation between various rust phases, but also detects various forms of amorphous and not well-crystallized rust phases with clarity. Starting from the amorphous mix (AM), a mixture of γ-, δ-FeOOH, and ferrihydrite (F) in their highly amorphous forms, the rusting proceeds through the formation first of γ-FeOOH or ferrihydrite, depending on atmospheric conditions, evolution of amorphous δ-FeOOH, which slowly crystallizes and final stabilization of δ-FeOOH containing traces of γ-FeOOH. γ-Fe2O3 ⋅ H2O forms in the intermediate stages of transformation of γ-FeOOH going over to γ-FeOOH. α-FeOOH might emerge ultimately as the most stable phase. However, its formation is found to be very slow and heavily dependent on certain prevalent conditions, including very high or very low pH values...