Dual function of rare earth carboxylate compounds on the barrier properties and active corrosion inhibition of epoxy coatings on mild steel

In this work, two rare earth carboxylate compounds, lanthanum 4-hydroxycinnamate (La(4-OHcin)3) and yttrium 3-(4-methylbenzoyl)propanoate (Y(mbp)3), were incorporated into bisphenol-based epoxy resin to investigate their effectiveness in coating barrier properties and active corrosion inhibition. EIS results showed that the incorporation of rare earth carboxylate inhibitors significantly improved corrosion resistance compared to the inhibitor free coating, with the global impedance modulus remaining at a level higher than 1 GΩ cm2 after 219 days immersion. Following EIS experiments, cross-sectional views of the coatings exhibited a pore-plugging behavior by rare earth containing precipitates, which reinforced the coating barrier properties and delayed the electrolyte diffusion process. These effects were also reflected from the electrochemical parameters extracted from breakpoint frequency analysis and equivalent circuit modelling. Filiform corrosion experiments for artificially scratched coatings suggest that the addition of rare earth carboxylates effectively suppressed the initiation and growth of filaments as well as the development of the coating delamination front. The active corrosion inhibition is possibly related to the formation of a surface protective film consisting of bimetallic complexes and rare earth metal rich precipitates. The electrochemical measurements and surface analyses evidence the dual function of rare earth carboxylate species for enhancing coating barrier properties against electrolyte penetration and providing active corrosion inhibition for the underlying AS1020 mild steel.