Smart coating with dual-pH sensitive, inhibitor-loaded nanofibers for corrosion protection

The study addresses the challenge of protecting AA2024-T3 aluminum alloy, which is prone to localized corrosion due to microgalvanic coupling between intermetallic particles and the aluminum matrix. Conventional approaches using free inhibitor pigments in coatings can degrade barrier properties and suffer rapid, uncontrolled leaching. Encapsulation in micro/nanocontainers improves isolation and release control but can undermine coating integrity, create interfacial defects, lead to heterogeneous inhibitor distribution, and limit replenishment of inhibitors to damaged regions. Continuous electrospun nanofibers are proposed as an alternative delivery system: their interconnected network can interlock with the matrix, be incorporated in thin coatings, and enable transport and replenishment of inhibitors to damage sites for long-term protection. A key trigger for on-demand release is local pH shifts during corrosion (acidic at anodes, alkaline at cathodes). Existing pH-responsive fibers often respond only to one pH extreme. The hypothesis is that a chitosan/poly(acrylic acid) (PAA) polyelectrolyte coacervate shell—composed of weak polyelectrolytes—will be stable near neutral pH but open at both low and high pH to release an encapsulated inhibitor. The study aims to fabricate core-shell nanofibers by coaxial electrospinning with Ce(NO3)3 inhibitor in the core and chitosan/PAA coacervate as the shell, characterize their structure and dual-pH-responsive release, embed them in a PVB coating, and evaluate corrosion protection and self-healing on AA2024-T3 via EIS, including scribe tests.

• Inhibitive pigments (e.g., strontium chromate, lithium carbonate, cerium cinnamate) can protect Al alloys but may compromise coating barrier properties and leach rapidly.
• Encapsulation in micro/nanocapsules isolates inhibitors and controls release, but capsules can degrade coating integrity, create defects, distribute heterogeneously, and are discrete, limiting inhibitor replenishment and sustained protection.
• Electrospun nanofibers can enhance mechanical interlocking with coatings, be integrated into thin films, and form continuous networks that facilitate inhibitor transport to damage sites, enabling repeated healing. Prior works showed repeated suppression of corrosion with nanofibers versus single events with microspheres, though earlier systems often required mechanical triggers.
• pH is an intrinsic corrosion trigger on aluminum alloys (acidic at anodic, alkaline at cathodic sites). Few inhibitor-loaded fibers are pH-responsive; most respond to only one pH extreme. Polyaniline nanofibers were reported to be dual-pH sensitive but lacked continuous networks.
• Chitosan/PAA (weak/weak polyelectrolyte) systems can exhibit stability near neutral pH and degrade/swelling at both acidic and alkaline pH due to changes in charge density and polymer interactions, making them promising shells for dual-pH-responsive release.
• Cerium salts act as mixed inhibitors: Ce3+ suppresses cathodic reactions via formation of insoluble cerium hydroxides/oxides; nitrate can passivate anodic sites, particularly at low pH, and cerium is less toxic than chromates.

• Materials: Chitosan (medium MW, 75–85% deacetylation), PAA (Mw 450,000), Ce(NO3)3·6H2O, dyes (Hoechst 33258, rhodamine B), PVB, acids/solvents as specified. Substrates: AA2024-T3 abraded to 1200 grit.
• Chitosan/PAA coacervate preparation: 1.5 wt% chitosan in 60% aq. formic acid and 12 wt% PAA in 90% aq. acetic acid, stirred overnight at 50 °C; PAA added dropwise to chitosan at 2:1 (v/v) chitosan:PAA to form coacervate.
• Demonstration of dual-pH response: Constructed (BB)/(chitosan/PAA)5 coatings on glass and observed dye release and morphology after 24 h immersion in DI water at pH 2.5, 7, 10.
• Coaxial electrospinning of Ce(NO3)3-loaded nanofibers: Shell—chitosan/PAA coacervate; Core—0.5 M Ce(NO3)3 in acetone. Coaxial nozzle with inner/outer needle diameters 0.64/1.02 mm; applied potential 15 kV; nozzle-to-collector distance 20 cm; flow rates core 0.2 mL/h, shell 1.0 mL/h. Fibers collected on grounded plate; dried at 100 °C for 1 h. Optimization of parameters was not performed.
• Characterization: SEM (ThermoFisher Apreo FE-SEM, 5 kV) for morphology; EDS (EDAX Octane Elect Plus) probing Ce Mα at 0.88 keV for Ce presence and distribution; Confocal microscopy with fluorescent dyes (rhodamine B in core, Hoechst 33258 in shell) for core/shell localization; TEM (FEI Tecnai G2 Biotwin) to confirm core-shell (core ~85 nm, total fiber ~120 nm).
• Release study: Nanofiber-coated glass slides immersed in 10 mL DI water at pH 2.5, 7, 10 (adjusted by 0.1 M H2SO4/NaOH). UV–vis at 250 nm quantified Ce(III) release over time; sampling with replacement; triplicate measurements. Exposed nanofiber area ~7.5 cm². Post-release morphology by SEM.
• Coating fabrication for corrosion tests (dip-coating): For Fiber-PVB (no inhibitor) and Ce-Fiber-PVB (with inhibitor), AA2024-T3 first dip-coated in 1.25 wt% PVB ethanol solution (10 s); electrospun nanofibers collected for 1 h; dried 100 °C for 1 h; then three more PVB dip coats (10 s each). Typical thicknesses: PVB 14 µm; Fiber-PVB 17 µm; Ce-Fiber-PVB 16 µm. Dip-coating chosen over bar-coating to avoid defects and ensure PVB infiltration and pore sealing.
• Electrochemical testing: EIS in 5 mM Na2SO4 at pH 2.5, 7, 10 and in 100 mM NaCl (neutral pH). Three-electrode cell: SCE reference, Pt mesh counter, coated sample as working electrode (area commonly 1 cm² for EIS; for NaCl tests ~standard setup). OCP stabilization 1 h (Na2SO4) or 30 min (scribe tests); frequency 10^5 to 0.01 Hz; 10 mV AC amplitude. Data fitted with equivalent circuits: two-time-constant model (solution resistance Rs; pore resistance Rpore and coating CPE; polarization resistance Rp and double-layer CPE) or one-time-constant for highly capacitive coatings. Extracted Rpore, Rp, and derived capacitances.
• NaCl barrier assessment: Compared PVB, Fiber-PVB, and Ce-Fiber-PVB in 100 mM NaCl via EIS.
• Scribe self-healing tests: Nanofibers or Ce-loaded microspheres embedded beneath an epoxy topcoat (EpoThin 2) on PVB-primed AA2024-T3. Artificial 1 cm scratch with ~0.5 mm tip to expose metal. Immersion in 100 mM NaCl; repeated EIS over 18 h; re-scribed same location after 18 h and continued monitoring. Microspheres fabricated by coaxial electrospraying per prior work; electrospray/electrospinning durations matched to approximate similar inhibitor content.

• Dual-pH responsiveness confirmed: Chitosan/PAA coacervate released bromophenol blue faster at pH 2.5 and 10 than at pH 7; coatings dissolved markedly at pH 2.5 and developed large pores at pH 10, with minimal change at pH 7.
• Nanofiber structure: Electrospun fibers were bead-free with diameters <200 nm; confocal imaging showed core-shell localization without leakage; EDS detected Ce uniformly along fibers; TEM confirmed concentric core-shell with overall diameter ~120 nm and core ~85 nm.
• Ce(III) release: Two-stage kinetics with initial burst followed by sustained release. After 2 h, cumulative Ce(III): 0.16 mM at pH 2.5 and 0.14 mM at pH 10; both more than twice the amount at pH 7. Release continued (positive slope) beyond 2 h at pH 2.5 and 10, indicating non-depletion and sustained capability.
• pH-triggered shell behavior: Fibers dissolved/swollen at pH 2.5; swelled and formed pores at pH 10; negligible morphology change at pH 7, indicating an open state at extremes and closed state near neutral to limit leakage.
• EIS in 5 mM Na2SO4: • At pH 2.5: Fiber-PVB degraded rapidly with |Z|0.01Hz dropping more than a decade; Ce-Fiber-PVB exhibited increasing low-frequency impedance and maintained near-ideal capacitive behavior at high frequency, indicating improved protection via Ce(NO3)3 release. Rp of Ce-Fiber-PVB exceeded Fiber-PVB by >1 decade after 72 h, though slight decline after 72 h due to film instability in acid. Rpore of Ce-Fiber-PVB remained stable/slightly increased versus a strong decline for Fiber-PVB. • At pH 10: Fiber-PVB showed continuous degradation; Ce-Fiber-PVB’s |Z|0.01Hz increased over time, nearly two orders of magnitude higher than Fiber-PVB by the end. Rpore for Ce-Fiber-PVB increased to ~2.0 × 10^6 Ω·cm²; Rp increased from ~5.0 × 10^4 to ~2.1 × 10^6 Ω·cm², consistent with robust cerium hydroxide/oxide film formation. • At pH 7: Fiber-PVB had an initial drop then stabilization in impedance; Ce-Fiber-PVB maintained impedance without significant increase, consistent with limited release due to closed shell but some protection from slow diffusion and film formation.
• EIS in 100 mM NaCl: Initial barrier property of Fiber-PVB and Ce-Fiber-PVB similar to PVB due to effective PVB infiltration. Over time, Fiber-PVB Rpore decreased from ~3.4 × 10^5 to ~2.0 × 10^3 Ω·cm², indicating accelerated degradation; Ce-Fiber-PVB showed increasing Rpore and lower coating capacitance (~1×10^-9 F·cm^-2) than Fiber-PVB, evidencing active protection from released Ce species.
• Scribe self-healing: Microsphere-containing coatings provided transient protection (|Z|0.01Hz ~8.0×10^5 Ω initially, dropping to ~1.3×10^5 Ω at 1.5 h, partial recovery to ~6.0×10^5 Ω, then decline; after re-scribe at 18 h, no recovery), indicating limited inhibitor throwing power. Nanofiber-containing coatings showed progressive healing: |Z|0.01Hz increased from ~5.0×10^5 to ~2.4×10^6 Ω over 18 h, brief drop (~4.0×10^4 Ω) followed by recovery (>2.1×10^5 Ω), and after re-scribe continued increasing to ~1.6×10^6 Ω, demonstrating repeated, sustained inhibitor delivery and healing.

The results demonstrate that coaxially electrospun chitosan/PAA coacervate nanofibers can act as dual-pH-responsive carriers for Ce(NO3)3. The weak polyelectrolyte shell remains relatively closed near neutral pH to minimize leakage but opens under acidic and alkaline conditions representative of local anodic and cathodic environments during corrosion, enabling on-demand inhibitor release. Released Ce3+ promotes formation of insoluble cerium hydroxide/oxide deposits, mainly at cathodic sites, lowering charge transfer and enhancing barrier properties; NO3− can contribute to anodic passivation at low pH. Consequently, Ce-Fiber-PVB coatings show improved impedance, higher Rpore and Rp, and lower capacitances compared to Fiber-PVB, especially under alkaline and chloride-containing environments. The continuous nanofiber network provides transport pathways that replenish inhibitors to damaged regions, overcoming the limited throwing power of discrete capsules and enabling repeated self-healing in scribed coatings. Some drawbacks include enhanced coating hydrophilicity from the fiber network, which speeds water ingress, and reduced film stability/efficacy under bulk acidic conditions due to redissolution and buffered local pH rise.

This work introduces a smart, dual-pH-responsive nanofiber delivery system for corrosion inhibitors based on coaxially electrospun chitosan/PAA coacervate shells with Ce(NO3)3 cores. The fibers exhibit clear core-shell morphology, uniform inhibitor loading, and triggerable release at both low and high pH. When embedded in a PVB matrix on AA2024-T3, the system provides enhanced corrosion protection across pH 2.5–10 and in chloride media, significantly improving impedance and resistances compared to coatings with empty fibers. In scribe tests, the nanofiber-based system delivers sustained and replenishable inhibitor supply, enabling repeated self-healing that outperforms microsphere-based systems. Future work should optimize electrospinning parameters and fiber loading to increase inhibitor content and control release rates, tailor the coating matrix to mitigate hydrophilicity and water uptake, characterize in-depth coating structural evolution during service, and extend studies to long-term durability and other alloy/coating systems.

• The presence of nanofibers increases coating hydrophilicity, accelerating water ingress and potentially reducing barrier performance absent inhibitors.
• Under acidic bulk conditions, cerium films can redissolve and the local pH rise is buffered, limiting inhibition effectiveness and leading to modest performance gains at low pH.
• Electrospinning parameters and fiber properties were not optimized; higher inhibitor loadings or alternative parameters may further improve performance but were not explored.
• Post-EIS cross-sectional/thickness changes or internal structural evolution of coatings were not examined.
• Release quantification focused on Ce(III) via UV–vis; nitrate release and speciation of cerium (e.g., Ce(IV)) were not directly measured.
• The affiliation for one author (superscript ²) is not provided in the supplied text, and some experimental details (e.g., exact working electrode area in all tests) may vary by setup.
• Tests were conducted under laboratory conditions; long-term field performance and mechanical durability were not assessed.