Effect of Filler Conductive Threshold on Cathodic Protection and Anticorrosion Mechanism of Zinc-rich Epoxy Coatings

In this study, reduced graphene oxide (rGO) fillers with electrical conductivities spanning nearly five orders of magnitude (4.2 × 10–4 – 20.58 S/cm) were prepared by hydrothermal reduction and incorporated at a fixed loading of 0.5 wt % into zinc-rich epoxy coatings containing 50 wt % zinc powder. The influence of the reduction-dependent rGO state on coating structure, interfacial properties, cathodic protection, and corrosion resistance was systematically evaluated. A non-monotonic conductivity–performance relationship was observed, with rGO160 (1.62 S/cm) showing the best overall protection under the present formulation. XPS and FTIR revealed that rGO160 retained an intermediate surface polarity, while cross-sectional SEM and mechanical tests indicated a dense coating structure, reduced interfacial defects, and the highest pull-off adhesion strength (∼6.4 MPa). KPFM further showed a comparatively uniform contact-potential-difference distribution. After 30 days of immersion, the MZrZE-rGO160 coating retained the highest low-frequency impedance modulus (|Z|0.01Hz ≈ 4 × 107 Ω·cm2) and exhibited the lowest relative corrosion propagation after 300 h of salt-spray exposure. DFT calculations showed comparatively less favorable H2O and O2 adsorption on the idealized rGO160 surface (+0.03 and +0.61 eV, respectively), serving as supplementary thermodynamic descriptors rather than direct kinetic evidence. The results demonstrate that the optimum protection arises from the coupled balance of conductivity, surface chemistry, sheet dispersion/restacking, conductive-network connectivity, and coating integrity. Accordingly, 1.62 S cm–1 should be regarded as a formulation-dependent optimum among the investigated rGO fillers rather than a universal conductivity threshold.