Magnetically tunable orientation of Fe3O4/BNNSs nanosheets for enhanced barrier and corrosion protection in waterborne epoxy coatings

To overcome the challenges of agglomeration and the difficulty in achieving oriented alignment of hexagonal boron nitride nanosheets (BNNSs) in epoxy matrices, a magnetically responsive Fe 3 O 4 /BNNSs composite was successfully prepared. Fe 3 O 4 nanoparticles grown in situ on the BNNS surface impart magnetic responsiveness to the nanosheets, enabling their alignment under an external magnetic field, while the BNNSs themselves serve as the primary impermeable barrier filler. By applying a horizontal magnetic field during the curing stage, the nanofillers are driven to align parallel to the substrate, forming an ordered layered structure. Comprehensive characterization confirmed the in-situ growth of Fe 3 O 4 nanoparticles on the BNNS surface and demonstrated the composite’s strong superparamagnetic behavior. Performance evaluation revealed that the epoxy coating containing magnetically aligned Fe 3 O 4 /BNNS fillers (BFM/EP) exhibited the most superior overall properties. After 20 days of immersion in deionized water, the BFM/EP coating displayed a remarkably low water absorption of only 6.48%, compared with 19.28% for the pure epoxy coating, indicating significantly enhanced anti-permeability. Benefiting from the improved barrier performance, the BFM/EP coating also retained the highest adhesion after 14 days of salt-spray exposure. Long-term electrochemical impedance spectroscopy (EIS) over 40 days further demonstrated that BFM/EP exhibited the highest coating resistance (1.93 × 10 8 Ω·cm 2 ), the lowest coating capacitance (1.99 × 10 -10 F·cm 2 ), and the largest low-frequency impedance modulus (1.69 × 10 10 Ω·cm 2 ). These results indicate that the dominant enhancement in corrosion protection originates from the magnetic-field-induced parallel alignment of high-aspect-ratio BNNSs, while Fe 3 O 4 functionalization mainly enables magnetic manipulation and contributes to improved filler dispersion. The resulting ordered filler network effectively enhances the tortuous diffusion pathway for corrosive species, leading to superior impermeability and long-lasting corrosion protection.