Hollow tannic acid-etched zeolitic imidazolate frameworks with proposed ångström-scale ion trapping for anti-corrosive coatings.

Zeolitic imidazolate framework-8 (ZIF-8) holds great promise as a corrosion inhibitor carrier owing to its high specific surface area; however, its 3.4 Å aperture restricts the diffusion of inhibitors into the 11.6 Å cavities. Herein, ZIF-8 was etched into a hollow structure (hZIF-8) using tannic acid (TA), leading to phenolate-rich, hydrophilic functional shell and enlarged pore sizes. When incorporated into epoxy (EP) coatings, hZIF-8 proposed to introduce an ångström-scale ion-trapping mechanism. The hydrated radii of Cl- and Na+ exceed the pore window, necessitating partial dehydration, which is followed by rehydration inside the confined cavity, and a second dehydration step to exit. This dehydration-rehydration loop, combined with electrostatic Cl- repulsion, partial Na+ trapping, and ultra-tortuous diffusion pathways for ions within the cavities, enhances the corrosion resistance of EP coatings. Further grafting of benzotriazole onto hZIF-8 (BTA@hZIF-8) provides active inhibition properties while providing chloride trapping and/or barrier ability. Incorporation of the synthesized pigments into EP coatings resulted in high impedance at low frequency (|Z|0.01 Hz) of ∼8 × 1010 and ∼ 1011 Ω.cm2 for hZIF-8/EP and BTA@hZIF-8/EP coatings after 245 days of immersion in salt media, which is two and three orders of magnitude higher than the blank EP coatings.