Corrosion Management with a Homologous Series of Phosphonate-Functionalized Linear Amino Acids as Environmentally Acceptable Corrosion Inhibitors

The interaction of five systematically structured homologous phosphonated linear amino acids (PLAAs, HOOC-(CH₂)_n-N-(PO₃H₂)₂), glycine (PLAA-C2, n = 1), β-alanine (PLAA-C3, n = 2), γ-aminobutyric acid (PLAA-C4, n = 3), 5-aminovaleric acid (PLAA-C5, n = 4), and ε-aminocaproic acid (PLAA-C6, n = 5), with carbon steel in an aqueous solution is explored. The inhibition efficiency of the PLAAs was investigated through electrochemical impedance spectroscopy, potentiodynamic polarization, and gravimetric studies. Inhibitor adsorption onto the carbon steel surface and its impact on inhibitory performance were further studied through Density Functional Theory simulations. The inhibitory performance increased with the increase in the alkyl chain length due to enhanced orbital overlap and stronger binding interactions between the carbon steel surface and PLAAs. PLAA-C6 exhibited the best performance, significantly inhibiting corrosion at concentrations as low as 10 ppm, reaching the optimum performance at 50 ppm. The synergistic effects of two selected inhibitors (PLAA-C4 and PLAA-C6) with Zn²⁺ions were investigated, showing substantially enhanced corrosion protection. This was ascribed to the formation of Zn-PLAA protective films on the carbon steel surface.