The present investigation adopted long-term in-situ electrochemical and spectroscopic approaches to study the combined active, self-healing and passive protective mechanisms of a new class of innovative anti-corrosive coatings based on epoxy doped with clay nanotubes impregnated with active species for the protection of carbon steel in 3.5% NaCl solution. The suitability of the as-received clay nanotubes to encapsulate the active agents was confirmed by different spectroscopic measurements. Tube end stopper with Ferric ion and polymer encapsulation with chitosan cross-linked with glutaraldehyde were adopted to tunnel the release of the active agents loaded into the nanotubes. The improved passive barrier performances of the various innovative coatings were revealed by the electrochemical impedance spectroscopic, while their active feedback and self-healing abilities were revealed by the optical and spectroscopic techniques. The optical/spectroscopic techniques revealed the degree of pit formation at the steel/coating interface and the iron rust formation around the artificially marked defects, including the ability of the marked defects to self-heal over exposure times. Adhesion and impacts tests were adopted to compare the physical/mechanical properties of the various coatings. The results afforded insights into the effects of exposure time on the protective and failure behaviours of both the reference and modified coatings.

We describe a combined electrochemical and first principle density functional study to probe the corrosion inhibiting and adsorption behavior of methionine (Met) and phenylalanine (Phe) on polycrystalline and nanocrystalline iron in acid media. Met functioned as a better inhibitor for both Fe microstructures, and was more favorably adsorbed on the nanocrystalline surface. The nanocrystalline surface however diminished adsorption of Phe. The comparable values of our computed physisorption energies (−94.2 kcal mol⁻¹ and −86.6 kcal mol⁻¹ for Phe and Met respectively) as well as the stable adsorption orientations of both molecules on Fe suggest a controlling influence of a soft epitaxial adsorption mechanism in which C, N, O, S atoms of the molecules align with epitaxial grooves on the Fe lattice. The significant contribution of physisorptive interactions also correlates with the similarity in experimental inhibition efficiencies on polycrystalline Fe (Phe = 73% and Met = 82%), though for Met the thiol group imparts an added ability for covalent interaction with Fe, which accounts for the higher efficiency. Furthermore, we have related the diminished inhibition efficiency of Phe on the nanocrystalline Fe surface to disruption of the epitaxial patterns on the lattice as the surface becomes increasingly defective, leading to weaker adsorption. The improved efficiency of Met on the nanocrystalline surface is related to scaling up of the covalent interactions around defect sites. Our theoretical conclusions are validated by the consistency with our experimental findings.