<i>In-Situ</i> Corrosion Monitoring of Scratched Epoxy Coated Carbon Steel in Saturated Ca(OH)₂ with or without 3% NaCl by Scanning Electrochemical Microscopy and Electrochemical Impedance Spectroscopy

Abstract

The present work is investigated the in-situ monitoring of local corrosion process of scratched epoxy coated carbon steel in saturated Ca(OH) 2 with and without 3% NaCl using SECM and correlated with EIS. The results obtained from EIS analysis showed that the corrosion resistance of scratched epoxy coated carbon steel decreases in Cl - containing solution as the increase in wet/ dry corrosion cycles. This was indicated by decrease in film resistance ( R f ) and charge transfer resistance ( R ct ), while the coated steel maintain the resistance values in saturated Ca(OH) 2 , most of which recovered after drying. The corrosion process was monitored using SECM by setting the tip potential at -0.70 V vs Ag/AgCl, where the consumption of dissolved oxygen occurred at the surface of test sample. The consumption of dissolved oxygen current ( I ’ oxy-c ) values was increased during the immersion in a solution with 3% NaCl. However, in wet/dry corrosion cycles, I ’ oxy-c was decreased due to the coverage of hydroxides/oxides at scratch area which suppressed the consumption of dissolved O 2 . It was found that the continuous decrease in corrosion was mainly attributed to continuous formation of corrosion products at anodic spots.