Crocin as a Natural, Eco-Friendly, and Sustainable Corrosion Inhibitor for Aluminum and Aluminum Alloys in Sodium Chloride Environment

Aluminum and aluminum alloys are resistant to corrosion, and their corrosion products are colorless and non-toxic. Aluminum is also a very good conductor of heat and electricity, is a non-magnetic material, has high diffuse reflectance and a low coefficient of secondary thermal emission. This chapter presents the role of crocin, a natural substance derived from the flowers of crocus, as a green inhibitor against corrosion in the aggressive environment of chloride ions of the AA1050 commercial pure aluminum alloy and the AA5083, AA5754 and AA6082 aluminum alloys, representative of the Al-Mg (5xxx) and Al-Si-Mg (6xxx) series. Research involves experimental methods, i.e. potentiodynamic electrochemical techniques, mass loss measurements, stereomicroscopy, surface glossiness measurements and scanning electron microscopy equipped with energy dispersive spectroscopy. Theoretical computational methods, i.e. density functional theory (DFT), semiempirical PM6, molecular mechanics (MM) simulations and multiscaling ONIOM (DFT/PM6) and ONIOM (DFT/MM) approaches, were also employed in order to confirm and clarify the experimental findings. The results prove that for the AA1050 commercially pure aluminum alloy crocin acts as a mixed-type green inhibitor against corrosion in the aggressive environment of NaCl with principally anodic inhibitive effect. Its protective action is enhanced with the increase of its concentration and the concentration of 1.25 mM exhibits the optimum inhibition efficiency, which is also equally effective for protection of the AA5083, AA5754 and AA6082 aluminum alloys. The molecular structure of crocin and the interactions between its molecules and the aluminum alloys surface through physisorption and chemisorption, in particular, account for the reactivity of crocin. The anchored groups onto the positively charged passivated aluminum oxide surface may be the strong electrostatic potential regions on the gentiobiose groups of crocin. Thus, crocin forms a stable protective layer, which covers the surface almost completely, and obstructs corrosive reactions by repelling the aggressive chloride ions towards the solution. Besides, crocin delays pitting corrosion by favoring the reconstruction of the aluminum oxide layer and repassivation of the surface.