Corrosion Behavior of Carbon Steel under Soil Drying Conditions

Abstract

Understanding the corrosion behavior of steel under realistic environmental conditions is essential for ensuring the durability of buried infrastructures. In this study, the corrosion behavior of carbon steel was investigated during a soil drying process under varying moisture content conditions. Electrochemical impedance spectroscopy combined with time-resolved analysis was used to monitor the temporal change in the corrosion rate, and the results were compared with the oxygen reduction behavior on Pt electrodes and the moisture dynamics evaluated using glassy carbon electrodes. The corrosion rate of carbon steel increased during drying, driven by enhanced oxygen transport through gas-phase pathways formed in the soil, which promoted the cathodic oxygen reduction reaction (ORR). However, the corrosion current remained substantially lower than the ORR-limited current on Pt electrodes. This was probably because of the suppression of anodic dissolution caused by the reduction in the liquid-phase volume near the steel surface, even as cathodic reactions became dominant. As drying further progressed, the NaCl concentration increased and the oxygen solubility decreased, leading to suppression of the ORR. In addition, the continued loss of the electrolyte phase further limited anodic processes, eventually resulting in the complete cessation of corrosion. These findings provide new insights into corrosion mechanisms under non-stationary soil moisture conditions, and they will contribute to more realistic assessment of the long-term performance of buried steel structures.