Acid-Responsive Anticorrosion Microcapsules for Self-Protecting Coatings

Encapsulated anticorrosion agents provide a suitable alternative to dispersion of metal-based compounds in protective polymeric coatings on metal substrates. Stimuli-responsive microcapsules enhance protection abilities by autonomously responding to corrosion-induced environmental changes, rather than relying on damage-induced mechanical stimuli. This work reports pH-responsive microcapsules with triggered release over a wide range of acidic pH values (pH < 6) and robust enough to be incorporated in commercial solvent-based epoxy coatings. The pH responsiveness is achieved by integrating acid labile ketals that undergo rapid hydrolysis within the cross-linked polyamide shell and readily release acetylenic diol or jojoba oil as the anticorrosive agent. The microcapsules are stable up to a temperature of 150 °C and provide long-term room temperature stability up to 3 months. Degradation and release kinetics of the microcapsules are quantified at various pH levels (1 ≤ pH ≤ 9) using ¹H NMR and gas chromatography, respectively. Microcapsules exhibit complete release in under 5 min at pH 1 and ≈2 hours at pH 5, respectively. Coating performance is evaluated by electrochemical corrosion tests conducted in 5 wt% salt solutions with varying pH and concentration of the microcapsules in the coating. Inhibition efficiencies up to 70% are achieved in acidic saltwater solutions.