Comparison of the Corrosion Inhibition Ability of Different Coumarate-Based Compounds Incorporated into Waterborne Binders

One of the most widely applied methods for the mitigation of corrosion is to cover metallic surfaces with polymeric coatings; however, the barrier properties provided by the polymer may not be enough to prevent corrosion. To improve the performance of the coatings, the incorporation of corrosion inhibitors in their formulation can be done to provide active protection to the system. Among the organic corrosion inhibitors, p-coumaric acid (p-CA) is promising for corrosion protection. Therefore, in this work, the corrosion protection efficiency of four p-coumaric-based inhibitors, methyl (H1), butyl (H4), trifluoromethoxy (HCF3), and p-4-ethyloxymethacrylate p-coumaric acid (HMA), is investigated. The inhibitors are incorporated into environmentally friendly waterborne polymeric binders by batch miniemulsion polymerization. The barrier corrosion protection of the coatings produced from these hybrid latexes is analyzed by electrochemical impedance spectroscopy (EIS) of the intact and scratched coated steel substrates. Of the intact coatings, the one with H1 showed the best response, with impedances of 106.3–106.7 Ω and phase angles of 82–84°, considerably higher than the control without inhibitor (105–105.2 Ω and 60–66°, respectively). The better performance of H1 has been attributed to its higher solubility in the media. However, the best impedance results for the scratched films have been obtained for the coating with HMA, as the impedance was maintained at ∼104.3 Ω for 24 h, while the impedance drops from ∼104.6 to 104.1 Ω for the control scratched sample. As HMA is attached to the polymeric chains, it is likely able to protect the exposed metallic area without completely leaching from the coating, thus explaining why it is able to maintain its performance. The EIS analysis also strongly suggests that the inhibitors have an additional protective effect through increasing pore resistance and decreasing metal corrosion as indicated from the higher Rpore and lower Cdl data extracted from the fits.