Self-Assembly and Anticorrosive Property of <i>N</i>‑Alkyl-4-[2-(methoxycarbonyl)vinyl]pyridinium Bromides on X70 Steel in an Acid Medium: an Experimental and Theoretical Probe

Abstract

A new type of self-assembly inhibitors, <i>N</i>-alkyl-4-[2-(methoxycarbonyl)­vinyl]­pyridinium bromide surfactants (designated as PPA-<i>n</i>, <i>n</i> = 8, 10, 12, and 14), has been synthesized and characterized by various spectrum methodologies. The anticorrosive performance of PPA-<i>n</i> on X70 steel in 5 M HCl was evaluated via weight loss and electrochemical methods as well as theoretical calculation. Scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy evidenced that PPA-<i>n</i> molecules self-assembled and formed a compact monolayer on the X70 surface, which blocked the active sites and elevated the energy barrier of the corrosion reaction of X70 steel. The inhibitory efficiency of PPA-<i>n</i> was up to 98% at 75 μM PPA-14. The inhibition capacity as well as resistance to corrosion at higher temperature increased with prolonged alkyl chain in PPA-<i>n</i>. Density functional theory calculation suggested that the ester group and pyridinium ring might be the most active sites for PPA-<i>n</i> adsorbing on the X70 steel surface via the π* orbital of the pyridinium accepting the 4s electrons of iron and empty 3d orbital of iron taking up the nonpair electrons of the O atom in PPA-<i>n</i>. The energy gap of the frontier orbitals of PPA-<i>n</i> are in the order of PPA-8 > PPA-10 > PPA-12 > PPA-14, inferring a contrary order in the inhibition efficiency