DC High Resistance Measurement as a Simple Technique for In Situ Corrosion-monitoring of Epoxy/Glass Flake Coated Carbon Steel

Abstract

This study aims to conduct in-situ corrosion monitoring of epoxy/glass-flake coated carbon steel under actual operating conditions using DC resistance measurements. The corrosion resistance of glass-flake/epoxy (GFE) coated carbon steel discs in a 3.5 wt.% NaCl solution at 60 °C was evaluated through DC high resistance measurements. The initial resistance of the dry, undamaged GFE coated carbon steel disc was 210 GΩ. Following exposure to the salt solution for two hours, the resistance decreased to 22.1 GΩ. Despite fluctuations, monitoring over 43 days revealed two general trends: resistance increased from 175 MΩ at 24 hours post-exposure to 557 MΩ by day 20, before returning to 210 MΩ by day 43. These results along with EIS and morphological analyses revealed that despite the coatings' substantial thickness, their porosity and non-uniformity compromised their effectiveness as barriers against the corrosive medium. The impedance diagrams at the open circuit potential also indicated coating failure and substrate corrosion with two-time constants observed upon initial immersion. During monitoring, the combined resistances (Rs, Rpore, and Rct) peaked initially, sharply decreased on the first day, steadily rose until the sixth day, and reached their lowest point on the ninth day.