Investigation of intelligent protection and corrosion detection of epoxy-coated St-12 by redox-responsive microcapsules containing dual-functional 8-hydroxyquinoline

Smart coatings with self-healing capabilities have frequently been made using microcapsules. Incorporating bifunctional materials as a self-healing agent into microcapsules allows for the early detection of corrosion before substantial damage to the metal surface occurs. In this study, the oxidative polymerization of aniline in the direct miniemulsion method was used to synthesize polyaniline (PANI) microcapsules. The corrosion inhibitor 8-hydroxyquinoline (8-HQ) was used in the core of microcapsules, which can also act as a fluorescent indicator of iron ions. The synthesized microcapsules were characterized by Optical Microscopic (OM), Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared (FTIR), Ultraviolet-Visible (UV–Vis), and Dynamic Light Scattering (DLS) techniques. The corrosion resistance behavior and self-healing performance of 8-hydroxyquinoline loaded polyaniline microcapsules into epoxy coating were investigated by Electrochemical Impedance Spectroscopy (EIS), and standard salt spray test. It was found that the epoxy coating containing 5 wt% of the polyaniline microcapsules with 1000 ppm of encapsulated 8-hydroxyquinoline exhibited better corrosion protection performance with an impedance value of 1.54 × 108 Ω.cm2 at low frequency (0.01 Hz) out of all the samples after 10 weeks immersion in electrolyte with a concentration of 3.5 wt% NaCl. Additionally, the evaluation of the release of 8-hydroxyquinoline from microcapsules in a damaged area is characterized by Inductively Coupled Plasma (ICP) spectrometer and fluorescence microscope. The synergistic effects of polyaniline microcapsules and the 8-hydroxyquinoline healing agent are thus responsible for the coatings' self-healing abilities. The study's findings revealed that the dual-functional 8-hydroxyquinoline is successfully released from the microcapsules and acts as a fluorescent indicator and corrosion inhibitor by detecting corrosion at the early stages.