In-Situ Monitoring of Local Corrosion Process of Scratched Epoxy Coated Carbon Steel in Simulated Pore Solution Containing Varying percentage of Chloride ions by Localized Electrochemical Impedance Spectroscopy

Abstract

The present work focuses on the corrosion behaviour of scratched epoxy coated carbon steel in simulated pore solution (SPS) containing varying % of chloride ions (added as NaCl) by localized electrochemical impedance spectroscopy (LEIS). LEIS results indicate that the |Z| at the scratch is decreased significantly with an increase in % of Cl− ions in SPS, which is evidenced by the formation of a depressed semicircle in the LEIS Nyquist plots. The intermediate corrosion products involved in the local corrosion process appears to be a function of the % of Cl− ions present in SPS. The competition between the aggressive Cl− ions and the inhibitive OH− ions and the extent of decrease in pH at interface determines the rate of corrosion. The LEIS maps acquired at 10Hz provides a better understanding of the involvement of the anodic and cathodic areas on the local corrosion process of scratched epoxy coated carbon steel. The continuous decrease in |Z| at the scratch is due to the higher extent of dissolution of Fe at the scratch while the change in |Z| over the coating is determined by the extent of dissolution of Fe and accumulation of corrosion products over the coating due to the high bulk pH of SPS. No significant decrease in |Z| at the scratch is observed when the % of Cl− ions in SPS is 0.05. However, when the % of Cl− ions in SPS is increased beyond 0.05%, a significant increase in the corrosion rate of scratched epoxy coated carbon steel is observed. Hence, 0.05% NaCl can be considered as a threshold level of chloride ions in SPS. A pictorial model is proposed to explain the local corrosion process and mechanism of scratched epoxy coated carbon steel in chloride containing SPS.