Electrochemical Monitoring of Steel Corrosion in NS4 Solution Under Cathodic Protection Environment: Insights of Electrochemical Impedance Spectroscopy

Effective techniques are crucial for enhancing the durability of underground steel pipelines. Cathodic protection (CP) is commonly used to prevent corrosion, but long-term monitoring is limited by intrusive methods. This study evaluates electrochemical impedance spectroscopy (EIS) to track corrosion product formation on steel exposed to a soil-mimicking solution under CP. The steel was immersed for 15 days, with CP applied for 11 days, and EIS measurements recorded periodically. Voltammetric measurements showed that CP reduced the corrosion rate from 410 to 7 µm yr⁻¹ between days 5 and 15. EIS results indicate that, under open circuit conditions, the electrochemical response is consistent with mixed activation–diffusion behavior, whereas under CP the impedance response becomes dominated by activation and interfacial film effects within the measured frequency range. This shift is associated with substantial modification of the interfacial environment and the development of a more resistive surface film. X-ray diffraction identified the formation of calcareous deposits, primarily aragonite and brucite, together with iron oxide corrosion products.