Sunflower Head Pectin with Different Molecular Weights as Promising Green Corrosion Inhibitors of Carbon Steel in Hydrochloric Acid Solution

[Image: see text] Three sunflower head pectin (SFHP) with different molecular weights (M(w) = 4.50, 97.23, and 254.64 kDa) were obtained by enzyme-assisted extraction and characterized by FTIR and (1)H NMR spectroscopy. The corrosion inhibition of mild steel in 1 M HCl solution was evaluated by the weight loss measurement. The inhibition efficiency (IE%) increased as its concentration increases and decreased as the temperature increases. The SFHP with the lowest M(w) of 4.50 kDa exhibited an IE(max) of 92.05% at the medium concentration (2.0 g L(–1)). The inhibition properties of SFHP (M(w) = 4.50 kDa) were investigated electrochemically and theoretically. The electrochemical impedance spectroscopy (EIS) revealed that the charge-transfer resistance increased as its concentration increases, the double-layer capacitance decreased as concentration increases, and the IE% also increased as concentration increases. The potentiodynamic polarization (PP) revealed that the SFHP acted as mixed-type inhibitor. The IE% reached 90.3% at the medium concentration (2.0 g L(–1)) of SHFP. The three-dimensional super depth digital microscopy and scanning electron microscopy tests confirmed the formation of inhibitor films on the surface of mild steel. The adsorption of SFHP on the mild steel surface was proved to obey the Langmuir adsorption isotherm. The theoretical studies via density functional theory and molecular dynamics simulation further revealed the mechanism of corrosion inhibition.