Corrosion protection of Aluminium Alloy 2024 through an epoxy coating embedded with smart microcapsules: The responses of smart microcapsules to corrosive entities

This article employed a layer-by-layer approach to modify the interfacial boundary of halloysite clay nanotubes (HNTs) using (poly (diallyldimethylammonium chloride)) (PDDA) and poly (styrene sulfonate) (PSS) polyelectrolytes to form core (inhibitor-loaded HNTs)-shell (polyelectrolytes layers (PDDA/PSS)) structured smart microcapsules or micro-containers. Responses of the smart microcapsules to changes in pH (H⁺), Cl⁻, OH⁻ and H₂O during the release of the loaded inhibitor (2-mecaptobenzothiazole inhibitor (2-MBT)) coupled with the number of PDDA/PSS polyelectrolytes layers (n) required to obtain optimal response were characterized. The anticorrosion efficacies of several organic compounds were screened to identify the most suitable inhibitor for the protection of Al alloy 2024 in 3.5 wt.% NaCl. 2-MBT which exhibits the best inhibition efficiency was successfully loaded into the HNTs lumen as demonstrated by TGA and DSC results. The release profiles of 2-MBT reveal that the sensitivity of the smart microcapsules to the various external stimuli decreased in the following order: pH > Cl⁻ > H₂O > OH⁻; and three-layered polyelectrolyte (3n) displays the highest sensitivity amongst the studied layer numbers (n). The release profiles were fitted to the Pappas and Weibull's models which describe the release mechanisms. Compatibility of the smart microcapsules with epoxy coating coupled with the anticorrosion effect on Al alloy 2024 substrate were evaluated by means of long-term salt spray and immersion tests.