Characterization of Rust Formed on Structural Carbon and Weathering Steels Exposed to Tropical Climate of Thailand

Structural carbon steel (CS) and weathering steel (WS) were exposed to various atmospheric climates of Thailand for one year. After the exposure, the samples were cleaned and corrosion losses were determined. In this study, polarized light microscopy, x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe micro analysis (EPMA), and spot Raman spectroscopy were used to characterize uncleaned samples in order to identify the nature of the corrosion products. The results show that a majority of corrosion product was composed of an amorphous phase while ledidocrocite (γ-FeOOH) was the second most abundant. Goethite (α-FeOOH), akaganeite (β-FeOOH), and magnetite (Fe₃O₄) were detected in all samples. Magnetite was only present in samples exposed to a marine environment. EPMA results indicated that the distribution of Cr and Cu was concentrated near metal/corrosion product interfaces of WS sample where there were S and Cl penetration. SEM images showed that the outer layer of the rust was porous while the inner layer was dense. Comparison with other previous studies revealed that the phases present in the corrosion product of CS and WS exposed to climates in Thailand were slightly different from those observed in samples exposed to a nontropical region. However, they were similar to those found in other studies in South East Asia. A mechanism of rust transformation was discussed.