Inhibition of mild steel corrosion in hydrochloric acid medium by polymeric inhibitors containing residues of essential amino acid methionine

Ethyl ester hydrochloride of amino acid l-methionine 1 was converted to cationic monomers N,N-diallyl methionine ethyl ester hydrochloride 2 and hydrochloride salt of N,N-diallylmethionine 3. Cationic monomers 2 and 3 underwent alternate copolymerization with SO₂ in dimethyl sulfoxide to give terpolymers 4 and 5, respectively, both having ≈ 1:1 ratio of sulfide and sulfoxide motifs owing to O transfer from dimethyl sulfoxide to the S. The sulfide groups in 5 have been oxidized with H₂O₂ to give polymer sulfoxide 6. In the presence of a small concentration of 35.2 µM (~ 11 ± 1 ppm) of each of the polymers 4, 5 and 6, the inhibition efficiency against mild steel corrosion in 1.0 M HCl at 60 °C was determined to be 90.8, 98.7 and 93.0%, respectively. The inhibition efficiency obtained from gravimetric weight loss was corroborated by the findings of potentiodynamic polarization and electrochemical impedance spectroscopy methods. Adsorption of polymer compounds onto the mild steel surface followed a mixture of chemisorption and physisorption processes and obeyed Langmuir, Temkin and Freundlich adsorption isotherms. The X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques further confirmed that the synthesized compounds formed a protective film onto the metal surface and prove it against further corrosion attack.