Inhibitor Efficiency of Cocogem Surfactants Based on C₁₈ Carboxylic Acids on Corrosion of Carbon Mild Steel in CO₂ Medium

Corrosion protection of carbon steel in 1% NaCl salt solution saturated with carbon dioxide was studied by potentiodynamic polarization method in the presence of three newly synthesized dissymmetric cocogem surfactants—N,N′-bis(2-hydroxypropyl) hexamethylenediammonium octanoate stearate (NNBOS), N,N′-bis(2-hydroxypropyl) hexamethylenediammonium octanoate oleate (NNBOO) and N,N′-bis(2-hydroxypropyl) hexamethylenediammonium octanoate linolenate (NNBOL). Experimental results revealed that all three compounds—NNBOS, NNBOO and NNBOL—possess high inhibitor efficiency on the protection of carbon steel in 1% NaCl solution saturated with carbon dioxide. Increase in concentrations of NNBOS, NNBOO and NNBOL inhibitors up to 100 ppm in the medium causes increase in their protective effects up to 99.4%. The study of polarization curves confirmed belonging of all three inhibitors to mixed-type inhibitors. Adsorption of inhibitor on the surface of C1018 carbon mild steel corresponds to negative value of Langmuir isotherm model Δ \({G}_{{\text{ads}}}^{0}\) $G_{\text{ads}}^0$ . Thermodynamic data for adsorption process were calculated and discussed. Effect of molecular structure on the inhibitor protective effectiveness was studied. Experiments revealed that adsorption energy of NNBOL inhibitor at a concentration of 100 ppm is higher than adsorption energy of NNBOS and NNBOO inhibitors. It occurs due to the presence of three unsaturated double bonds in linolenic acid residue in NNBOL compound corresponding to experimentally determine high inhibitor efficiency.