Dual-Loaded MOF Nanocontainers Teach Epoxy Coatings to Heal Their Own Scratches

Corrosion is a slow-motion disaster that imposes significant economic losses and safety concerns across marine, automotive and construction industries, and the epoxy coatings widely deployed as protective barriers gradually lose the fight as water, dissolved salts and temperature swings infiltrate the polymer matrix. Now a research team has reported a dual-weapon strategy: they loaded a zeolitic imidazolate framework, ZIF-8, with two complementary corrosion inhibitors — the organic molecule 2-mercaptobenzothiazole (MBT) and zinc acetate (ZA) — and embedded the resulting pigment into epoxy coatings applied to carbon steel. Their measurements show that the dual-loaded coating not only blocks electrolyte penetration better than any of the single-component alternatives, but also actively repairs damage by releasing protective chemistry precisely where and when the coating is breached.

The design logic hinges on the peculiar chemistry of ZIF-8, a porous metal–organic framework built from zinc(II) nodes coordinated by 2-methylimidazolate linkers. Its nominal pore aperture is smaller than the dimensions of MBT molecules, yet framework flexibility and defect-mediated transport allow uptake beyond what a rigid-pore model predicts. Crucially, the Zn–imidazolate coordination bonds that hold the framework together are vulnerable to protonation under acidic conditions and to hydroxylation under strongly alkaline conditions — exactly the microenvironments that develop at anodic and cathodic corrosion sites. The framework therefore acts as a pH-responsive gate: stable at neutral pH, but dismantling itself at the two pH extremes where corrosion chemistry is most aggressive, spilling out its cargo only when protection is needed.

Characterization confirmed the loading while remaining appropriately cautious about its precise geometry. X-ray diffraction showed that the sodalite-type crystalline structure of ZIF-8 survived contact with both inhibitors, though additional reflections indicated secondary crystalline contributions the authors conservatively attributed to residual or recrystallized inhibitor species rather than claiming unambiguous intracrystalline encapsulation. Infrared spectroscopy revealed carboxylate stretches from acetate and thiazole-ring and C–S modes from MBT overlapping the framework bands, with the largest peak shifts appearing in the dual-loaded sample — a sign of significant inhibitor–framework interaction. Thermogravimetric analysis showed improved thermal stability for the dual-loaded material, and electron microscopy documented altered particle morphologies consistent with guest incorporation.

Perhaps the most revealing experiments involved gas adsorption. Pristine ZIF-8 boasted a Brunauer–Emmett–Teller surface area of 1360 m² g⁻¹ and a pore volume of 0.62 cm³ g⁻¹. Loading dropped these values to 429 m² g⁻¹ and 0.20 cm³ g⁻¹ for the dual-loaded pigment. The researchers then performed a sequential ethanol-washing experiment: the first wash restored the surface area to 715 m² g⁻¹, subsequent washes brought it to a stable plateau around 761 m² g⁻¹. Because the surface area never returned to its original value even after three extraction cycles, the results point to a weakly adsorbed external fraction that washing removes and a strongly retained, pore-associated fraction that resists extraction — strong evidence that the inhibitors are not merely sitting on particle surfaces.

Release tests in 3.5 wt.% sodium chloride solutions across pH 3 to 12 confirmed the stimuli-responsive behavior. ICP–OES measured the highest zinc release at pH 3 (230 ppm) and pH 12 (125 ppm), with substantially lower values at neutral and mildly alkaline pH. UV–visible spectroscopy showed MBT following the same trend, with maximum release at the pH extremes. At low pH, protonation of the imidazolate linkers destabilizes the metal–ligand bonds; at high pH, deprotonation and hydroxylation have a similar effect. In between, the intact framework releases its cargo only slowly, sustaining a gradual protective diffusion suitable for long-term immersion in environments such as seawater, where local pH fluctuates as electrochemical reactions proceed.

The payoff showed up in electrochemical testing. Intact coatings containing 1.08 wt.% of the dual-loaded pigment — roughly 78 micrometers thick on grit-blasted carbon steel — maintained a coating resistance near 47,870 MΩ cm² after 84 days of immersion, whereas neat epoxy had collapsed to 98 MΩ cm² by the same point, with pronounced delamination indices. All modified coatings began above 10¹⁰ Ω·cm² in impedance modulus after seven days, but the dual-loaded system degraded the least. Contact-angle measurements added a hydrophobic argument: the modified coating reached about 78.9°, versus roughly 52° for neat epoxy, making the surface less welcoming to water ingress.

Damaged coatings told the more dramatic story. When the researchers drilled 1-millimeter scratches through the films and immersed them in salt solution, the dual-loaded composite reached 97.6% active protection efficiency after 24 hours — ahead of 95% for ZA-only and 96% for MBT-only counterparts — and still delivered 96.9% after 168 hours. Scanning Kelvin probe mapping of the scratched regions showed the fitted Volta potential shifting progressively from −535 mV on bare steel to −22 mV for the dual-loaded coating, the highest and most uniform value among all samples, indicating suppressed electrochemical activity across the damaged zone.

Spectroscopy provided direct chemical evidence of the healing mechanism. On inhibitor-free scratches, X-ray photoelectron spectroscopy found about 68% of the iron present as ferric oxide and oxyhydroxide — the signature of unimpeded rusting — with oxygen dominating 70 atomic percent of the surface. On the inhibitor-loaded surface, the ferric fraction fell to about 45.8%, metallic iron rose from roughly 6.5% to 12%, and new components appeared: an Fe–inhibitor feature at 712.84 eV consistent with iron–mercaptobenzothiazolate coordination, a dominant nitrogen signal at 400.42 eV indicating metal-coordinated thiazole nitrogen, and zinc 2p signals showing ZnO-like and Zn–O–C environments. Raman spectra corroborated the picture, detecting benzothiazole vibrational bands from MBT alongside zinc-derived interfacial features in the dual-loaded sample, with band shifts between single- and dual-loaded systems suggesting cooperative interaction between the two inhibitors.

Accelerated exposure testing reinforced the electrochemical and spectroscopic findings. During 650 hours of salt-spray exposure per ASTM B117, neat epoxy developed five blisters rated No. 6 and the plain ZIF-8 coating twelve blisters, while the dual-loaded coating showed no visible blistering or surface defects until 312 hours and only two small No. 8 blisters at 650 hours. Pull-off adhesion tests in both dry and wet states showed the lowest adhesion loss for the dual-loaded system, and cathodic disbonding — in which strongly alkaline conditions generated by the protective current attack the coating–metal interface — produced a delamination diameter of only about 10 millimeters, compared with roughly 28 millimeters for neat epoxy.

Density functional theory calculations and ab initio molecular dynamics supplied an atomistic rationale. The computed adsorption energies were favorable for all loaded systems: −1.542 eV for ZA, −0.738 eV for MBT and −2.539 eV for the co-loaded configuration, the latter reinforced by short Zn···S contacts of 2.60 and 2.70 Å between the zinc acetate center and MBT’s sulfur atom — shorter than van der Waals separation, indicating weak secondary coordination that redistributes electron density between the guests. Bader analysis showed electrons flowing from framework to guests, and the projected density of states revealed a sharp hybridized Zn-3d/S-3p state just below the Fermi level, an electronic fingerprint of the synergy. Simulated protonation distorted the Zn–N coordination network locally, while hydroxylation broke two Zn–N bonds outright; in explicit water over two picoseconds, the hydroxylated framework degraded rapidly and mobilized its cargo, whereas protonation produced slower, localized stress redistribution. The study’s authors note that scanning electron microscopy with energy-dispersive spectroscopy alone cannot definitively identify specific zinc phases, and that the corrosion products are conservatively described as zinc-containing hydroxide or basic-salt deposits. Even with those caveats, the combined evidence portrays a coating in which a porous framework warehouses two complementary inhibitors, holds them until mechanical damage creates the pH extremes of active corrosion, and then deploys an organic chemisorbed film and an inorganic zinc-rich barrier in concert — a self-healing recipe the authors position as a promising platform for durable protection in aggressive saline environments.

Subject of Research: Dual-loaded ZIF-8 nanocontainers delivering organic and zinc-based corrosion inhibitors in self-healing epoxy coatings

Article Title: Dual-loaded MOF-based epoxy coatings incorporating organic and inorganic inhibitors for superior corrosion resistance: Experimental and atomistic insights

Article References: Esfandabadi, A. S., Aghaie-Khafri, M., Akbarinezhad, E., Soleimani, M., & Pourfath, M. (2026). Dual-loaded MOF-based epoxy coatings incorporating organic and inorganic inhibitors for superior corrosion resistance: Experimental and atomistic insights. Results in Engineering, 32, Article 113679. https://doi.org/10.1016/j.rineng.2026.113679

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113679

Keywords: corrosion, ZIF-8, metal-organic framework, epoxy coating, self-healing coatings, MBT, zinc acetate, pH-responsive release, electrochemical impedance spectroscopy, DFT calculations, salt spray test, carbon steel