Unlocking the potential of zinc phosphate nanoplatelets for alkyd coating formulation with advanced anti-corrosive protection at reduced pigment content

Abstract

Novel zinc phosphate 2D nanoplatelets (ZPnp) were prepared by direct precipitation followed by a nano-milling process, effectively utilizing the preferential fragmentation of the lamellar precursor into thin platelets. The XRD analysis identified hopeite (Zn3(PO4)2·4H2O) as the crystalline phase, while SEM/TEM revealed rectangular, plate-like particles with lateral dimensions below 100 nm. DLS enabled tracking of the milling process, and BET was used to confirm the nanoscale character of ZPnp. The effect of incorporating 1.6 wt.% nano-milled ZPnp slurry (40 wt.% solids) into a mixed urethane–alkyd (UreAlkyd) topcoat on barrier (passive) and active-healing performance was compared with that of a coating containing 4 wt.% commercial Al-treated zinc phosphate (ZPA) micropowder. The performance of the coatings was assessed using EIS and directly compared through standard salt-spray testing. Ion release was quantified by AAS in saline solutions at different pH values. It was found that the more compact/denser inhibitive layer formation observed in long-term exposures for ZPnp prevails over the relatively better active-site-blocking effect of the commercial ZPA at initial immersion times. The ZPnp-containing formulation exhibited higher corrosion resistance, along with lower capacitance, in both artificially scratched and intact coatings compared to the commercial ZPA-containing sample. Hence, the development of a superior active/barrier anti-corrosion protection system that possesses prolonged healing activity at more than six times lower anti-corrosion pigment content was demonstrated.