Monitoring the development of rust layers on weathering steel using in situ Raman spectroscopy under wet-and-dry cyclic conditions

Abstract

The change of rust composition on weathering steel was consecutively monitored by in situ Raman spectroscopy during exposure to atmosphere under cyclic 4 h wet-4 h dry condition for 144 h in the presence of NaCl deposit. Raman spectra of the rust surface in the presence of NaCl deposit at 0.93 mg cm⁻² corresponded to those of lepidocrocite (γ–FeOOH) and magnetite (Fe₃O₄) in the initial 12 h exposure. After 12 h exposure, akaganeite (β–FeOOH) started to form, and its molar ratio on the rust surface increased to 90 % at 30 h exposure. The Raman spectra further changed after 30 h exposure in which γ–FeOOH again emerged. The reappearance of γ–FeOOH is assumed to be caused by capture of chloride ions in β–FeOOH resulting in the decrease of free chloride ions in the surface solution layer. When the amount of NaCl deposit was decreased to 0.11 mg cm⁻², the surface was covered with dark brown corrosion product which may be amorphous FeOOH and after repeated dry-wet cycles, yellow-brown spots emerged which consisted of β–FeOOH.