Biogenic amino acid methionine-based corrosion inhibitors of mild steel in acidic media

N, N-Diallyl methionine ethyl ester hydrochloride 5 underwent alternating copolymerization with SO₂ via the Butler cyclopolymerization protocol in dimethyl sulfoxide (DMSO) to give water-soluble cycloterpolymer 6 with a ∼1:1 molar ratio of sulfide and sulfoxide groups as a result of oxygen transfer from DMSO. Half of the sulfide groups in 6, upon oxidation with H₂O₂, afforded polymer sulfoxide 7 and polymer sulfone 8. The solution properties of these polymers were determined via a viscometric technique. The thermal stability of these polymers was determined by thermogravimetric analysis. The inhibition efficiency obtained from gravimetric mass loss, potentiodynamic polarization, and electrochemical impedance spectroscopy techniques agreed well with each other. The corrosion efficiencies increase with increasing concentration of the polymers. At a polymer concentration of 175 μM, the maximum inhibition efficiency of copolymer compounds 6–8 was determined to be 92%, 97%, and 95%, respectively. The synthesized polymer compounds acted as mixed-type inhibitors. Polymer compound 7 adsorbed onto the metal surface via chemisorption and physisorption and obeyed Langmuir, Temkin, and Freundlich adsorption isotherms. Analyses by X-ray photoelectron spectroscopy and scanning electron microscopy-energy-dispersive X-ray spectroscopy indicated that the adsorbed polymers formed a thin film on the metal surface and prevented further corrosive attack.