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Chitosan microspheres enable smart corrosion warning coatings
Researchers have developed a smart corrosion-warning coating based on chitosan microspheres encapsulating phenolphthalein. The pH-responsive system visibly signals localised corrosion through a striking red colour change within 30 seconds, offering a powerful tool for predictive maintenance of metallic substrates.
Detecting corrosion of metallic substrates at an early stage is crucial for planning maintenance strategies and preventing engineering failures. In this study, chitosan, a natural cationic polysaccharide, was used as a wall material to fabricate chitosan microspheres (CSM) via reverse emulsion crosslinking. The colour indicator phenolphthalein (PhPh) was encapsulated within the microspheres, producing colour-developing microspheres (CSM-PhPh) that were then incorporated into an epoxy coating to create a smart corrosion-warning system (EP/CSM-PhPh).
Unlike inorganic microcontainers, the cross-linked chitosan microspheres can adjust their pore size through chain movement, providing controlled release of the encapsulated indicator. Release studies confirmed enhanced release rates in both alkaline and acidic environments, demonstrating clear pH-responsive behaviour. Kinetics analysis showed that the initial release phase followed the Korsmeyer-Peppas model, while the second phase was governed by first-order kinetics.
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Rapid colour response and stable coating performance
Colorimetric tests revealed that the scratched area of the smart coating quickly developed a high-intensity red colour within 30 s when exposed to alkaline solutions. Immersion and salt spray tests in NaCl solution confirmed that the coating reliably identifies localised corrosion areas through visible red zones. The barrier properties of the coatings were assessed at varying CSM-PhPh loadings, with a 12.5 wt% addition delivering significant impedance of 1.17 × 10⁶ Ω·cm².
As the CSM-PhPh content increased, the hardness and hydrophobicity of the epoxy coatings decreased slightly, while adhesion remained strong, indicating overall favourable physical properties. The findings suggest that the system offers a promising approach for advanced smart coatings, combining reliable corrosion protection with intuitive visual warning capabilities for predictive maintenance applications.
Source: He, D. et al., Chitosan microspheres encapsulating phenolphthalein for epoxy coatings: pH-responsive release and smart corrosion warning. Progress in Organic Coatings 214, 110003 (2026).
