Corrosion Resistance and Mechanistic Insights of Pipeline Steels with Solvent-Free Epoxy Glass Fiber Reinforced Coatings: Comparative Roles of Fiber Cloth and Chopped Fibers

Protective epoxy coatings on oil and gas pipelines can effectively mitigate corrosion. However, their performance deteriorates under the impact of externally applied potentials or in the presence of physical defects such as scratches or pores. To better understand the impact and associated mechanism, this work studied the cathodic delamination, anodic corrosion resistance, and associated mechanisms of two coatings on pipeline steel in a weakly alkaline environment: a solvent-free epoxy glass fiber–reinforced plastic coating (FRP) and a modified variant coating (MFRP). Their performances were evaluated in the presence of pre-existing defects of drilled holes and crevices at the coating–steel interface. Under cathodic conditions the FRP coating exhibited a near-parabolic power-law increase in delamination length with time, indicating an ion-migration-controlled delamination process, whereas the MFRP coating showed negligible delamination. When the MFRP coating with a drilled hole defect acted as the anode, substantial iron substrate dissolved to primarily form Fe3+ and lead to a weak acidic pH. In contrast, both oxidation and mass transport within the crevice defect were substantially inhibited, leading to a reduced iron concentration (primarily Fe2+) and an elevated electrolyte pH. Multiphysics simulations revealed the spatial distribution of current densities across the exposed defects and reproduced the similar qualitative trend observed experimentally in terms of dissolved iron concentration. These findings elucidate the mechanisms by which the coatings mitigate corrosion at different defective coating–metal interfaces under potentiostatic conditions.