Scanning Electron Microscopy/Micro-Raman: A Very Useful Technique for Characterizing the Morphologies of Rust Phases Formed on Carbon Steel in Atmospheric Exposures

The morphological structures displayed by iron oxides present a very wide range of shapes and sizes, reflecting to a large extent the growth environment where the iron has been exposed. However, the structure of the rust phases formed on carbon steel in atmospheric exposure includes many imperfections, and real component structures appear to diverge from the ideal crystallographic structure of typical iron oxides. Rigorous information on the morphology of the rust phases formed in atmospheric exposure is very scarce, and there are even fewer studies of analytical information on the chemical compounds corresponding to these rust morphologies. The aim of this paper is to advance this field using scanning electron microscopy (SEM)/micro-Raman spectroscopy to perform a more direct and rigorous characterization of the main rust phases. The equipment used consists of a fully integrated confocal Raman microscopy module, which includes an optical video microscope and a high-resolution XY stage on top of the SEM main stage. Correlation between the SEM images and Raman images is achieved by overlaying the video and SEM images of the sample itself. The study consists of a micro-Raman characterization of the morphologies most commonly observed for the main rust phases: lepidocrocite (γ-FeOOH), goethite (α-FeOOH), akaganeite (β-FeOOH), and magnetite (Fe₃O₄).