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

## TL;DR

A new pH-sensitive epoxy coating with a smart nanocomposite improves steel corrosion resistance by controlled release of a Schiff base inhibitor.

## Contribution

A novel pH-responsive encapsulation method for a Schiff base-loaded MOF to enhance long-term steel corrosion protection.

## Key findings

- Encapsulated UiO-66-NH2 reduced coating surface roughness and eliminated micrometric defects.
- The coating achieved a polarization resistance 41 times higher than the neat epoxy after 6 weeks.
- Schiff base molecules showed potential for adsorption onto steel surfaces via DFT calculations.

## 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
(R
p) 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.