Localized Electrochemical Impedance Spectroscopy Observation on Scratched Epoxy Coated Carbon Steel in Saturated Ca(OH)2 with Various Chloride Concentration

The in-situ local corrosion behavior of scratched epoxy coated carbon steel is investigated in sat. Ca(OH)2 with varying concentration of Cl- ions by localized electrochemical impedance spectroscopy (LEIS). The localized corrosion process and mechanism of coated steel (scratch area) is measured by LEIS plots and 3D topographic images. The LEIS responses measured at the defect are attributed to the pore impedance with defect in the highfrequency range and an interfacial corrosion reaction in the low-frequency range of corroding steel at the base of defect within 1-10 h immersion. The continuous decrease in |Z| at the scratch is due to the higher extent of dissolution of Fe with increase of Cl- ion concentration. However, the resistance values of coated steel in sat. Ca(OH)2 with each concentration of Cl- ions are not changed significantly with increase in immersion time from 1-10 h. On the other hand, LEIS Nyquist plots clearly showed that the measured impedance at high frequency is related to corrosion products formed at the defect which acts as anodic zones and the low frequency part are related to corroding of carbon steel with immersion of 1-5 days. 2D topographic images clearly showed that corrosion occurs at scratch and followed by coating degradation at scratch front as well as away from scratch due to cathodic reactions (reduction of O2) leads to coating delamination. No significant change in corrosion resistance is observed for 0 and 0.0085 M/L of Cl- ions containing solution for 5 days of immersion as well as 1-10 h immersion. This is due the formation of better passive film on the steel surface (defect) in which the competition between the aggressive Cl- ions and the inhibitive OH- ions determines the rate of corrosion. A significant decrease in corrosion resistance is observed with higher concentration of Cl- ions (0.17 and 0.51 M) due to the preferential adsorption of Cl- ions at the defect site.