Hierarchical lamellar ZIF-8-based epoxy coatings with time-adaptive, pH-triggered stepwise inhibitor release for durable corrosion protection of magnesium alloys

Magnesium alloys are highly susceptible to rapid and non-uniform corrosion in chloride-containing environments. The corrosion process typically evolves from an initially aggressive stage to a relatively stabilized stage during long-term exposure. However, conventional epoxy coatings (EP) and single-stage inhibitor delivery systems cannot effectively adapt to these time-dependent and heterogeneous corrosion conditions, often resulting in premature inhibitor depletion and limited long-term protection. To address this mismatch between corrosion evolution and inhibitor release behavior, a hierarchical porous corrosion inhibitor carrier based on diatomite (DE)-supported sheet-like ZIF-8 loaded with sodium molybdate (Na₂MoO₄@SZIF-8-DE) was rationally designed and incorporated into an E51 epoxy matrix for the protection of AZ31B magnesium alloy. The in-situ growth of SZIF-8 on the DE surface effectively suppressed MOF agglomeration while constructing a stable hierarchical porous architecture, which enhanced the inhibitor loading efficiency (12.49%) and structural stability. Ultraviolet–visible (UV–Vis) spectroscopy confirmed the pH-responsive and stepwise release behavior of Na₂MoO₄ from Na₂MoO₄@SZIF-8-DE, enabling rapid inhibitor release during the early aggressive corrosion stage and sustained release during the subsequent stabilized stage. Electrochemical impedance spectroscopy (EIS) results showed that coatings containing Na₂MoO₄@SZIF-8-DE/EP exhibited significantly higher low-frequency impedance compared with pure EP coatings. Notably, the coating with 10 wt.% Na₂MoO₄@SZIF-8-DE/EP maintained the highest impedance after 60 days of immersion in 3.5 wt.% NaCl solution, indicating superior long-term corrosion protection. Moreover, simulation analysis demonstrated enhanced interfacial binding between Na₂MoO₄@SZIF-8-DE and the AZ31B substrate, which facilitated the formation of a dense and stable protective interface.