Molecular structure effects on the mechanisms of corrosion protection of model epoxy coatings on metals

We seek to reveal the corrosion protection mechanisms of intact thermosetting epoxy coatings on metal substrates as a prerequisite for the future design of innocuous alternatives to bisphenol A-based epoxy resins. The structure-property relationships dictating the barrier protection of the model coatings are studied using epoxy resins of different molecular weights, applied onto metal substrates, and cured with a phenolic crosslinker. The effect of the metal surface passivation on the corrosion resistance of the coatings is also investigated using two commonly applied pretreatments. We combine confocal and scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis to monitor and visualize the morphological variations leading to the coating degradation in acidic electrolyte at high temperature. The corrosion protection of the coatings is quantified using electrochemical impedance spectroscopy (EIS) and correlated to the formation of osmotic "blisters" on the coating surface during immersion. Differences in the corrosion and delamination resistance are interpreted in terms of the coating molecular network structures using dynamic mechanical analysis, differential scanning calorimetry, and uniaxial tensile tests. The results reveal that the coating corrosion resistance is mainly dependent on the density of its crosslinked molecular network which delays diffusion of corrosive species. The densely crosslinked network is also found to have a higher toughness which endows the coatings with a better ability to deform in response to the increased pressure from the underlying corrosion products at the coating-metal interface. This understanding could be used as a fundamental basis for the development of safe, sustainable alternatives to BPA-based epoxy resins.