Rust Evolution and Electrochemical Properties of Field-exposed Carbon Steel in a Tropical Marine Environment

The corrosion behavior of carbon steel (CS) exposed in a tropical marine atmosphere for 2 years was studied by the weight loss, scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and electrochemical impedance spectroscopy techniques. The results show that the corrosion layer became more loose and thick with increasing exposure time, and after 2 years exposure, the rust layer developed into the dual-layer structure. The XRD and Raman analyses confirmed that the corrosion products contained gamma-FeOOH, alpha-FeOOH, gamma-Fe2O3, and Fe3O4. The polarization curves show that the corrosion potentials (E-corr) and corrosion current densities (I-corr) changed with increasing exposure duration. Furthermore, an equivalent circuit is proposed for modeling the impedance data, and the charge transfer resistance (R-2) decreased with exposure time during 6 months and increased from 6 months to 24 months, demonstrating that the corrosion rate increased first and then decreased with exposure time.