pH-Responsive Epoxy Coating Incorporating a Novel Schiff Base-Loaded UiO-66-NH2 with an Encapsulated Shell for Long-Term Steel Corrosion Protection

Abstract

To address the low corrosion resistance of conventional epoxy coatings, a smart nanocomposite was designed by using UiO-66-NH2, a metal–organic framework (MOF), loaded with a new Schiff base as an inhibitor. The MOFs were encapsulated with a pH-responsive shell, allowing for the controlled release of the inhibitor. The crystallinity of UiO-66-NH2 remained preserved after encapsulation, and spherical particles were observed with an apparent size increase following encapsulation. The specific surface area of UiO-66-NH2 decreased from 860.18 to 131.06 m2 g–1 and pore volume from 1.9745 to 0.2871 cm3 g–1, indicating efficient pore occupation by the inhibitor. This change resulted in a loading efficiency of 30.5% and an encapsulation efficiency of 43.9%, which contribute to the enhanced corrosion resistance of the coating. The encapsulated UiO-66-NH2 exhibited smart behavior, releasing the highest amount of inhibitor at pH 12. The addition of UiO-66-NH2 and its encapsulated form eliminated micrometric defects in the coating, and the MOFs were uniformly distributed throughout the epoxy matrix. The incorporation of UiO-66-NH2 reduced the surface roughness of the epoxy coating from 3.909 to 3.260 μm, with a further decrease to 1.851 μm upon addition of its encapsulated form. Electrochemical impedance spectroscopy (EIS) studies in 0.2 M HCl revealed enhanced corrosion resistance after UiO-66-NH2 addition, mainly due to MOF-coating interactions, eliminating structural pores. After 6 weeks of immersion, the epoxy coating with encapsulated UiO-66-NH2 achieved a polarization resistance (Rp) of 327.33 MΩ cm2, surpassing the neat epoxy coating by ∼41 times and the UiO-66-NH2-containing coating by ∼10 times. Potentiodynamic polarization (PDP) and postcorrosion morphology aligned well with EIS results. Density functional theory calculations confirmed the potential for chemical and physical adsorption of Schiff base molecules onto the steel. The coating designed in this study shows potential for protecting steel in industrial environments, including chemical processing plants and water treatment facilities.