Synergistic corrosion inhibition effect of copolymer and an amphoteric surfactant on carbon steel in 3.5 NaCl solution: experimental and theoretical research

The copolymer poly (aniline-co-orthotoluidine) noted poly (ANI-co-OT) was chemically synthesized and characterized by FT-IR, UV–Vis, and XRD techniques. XRD results confirm the amorphous nature of the copolymer. FT-IR and UV–Vis results indicate that the spectrum of the copolymer includes all the bands relating to the functional groups of the homopolymers polyaniline (PANI) and poly-orthotoluidine (POT). It was revealed that, contrastingly to the copolymer’s solubility in dimethylformamide (DMF), the homopolymers have a low solubility. The potentiodynamic polarization technique has been employed in order to study this copolymer’s inhibition effects on the corrosion of carbon steel X52 in a 3.5% NaCl solution. The aforementioned study illuminated the following. The copolymer exhibits high inhibition activity towards the corrosive action of NaCl and its adsorption obeys the Langmuir adsorption isotherm model. The calculated Gibbs free energy (∆G0ads) revealed the chemisorption of this copolymer on the surface of the carbon steel. In addition, a synergistic effect was observed when the copolymer poly (ANI-co-OT) was mixed with the amphoteric surfactant cocamidopropyl betaine (CAPB) where the inhibition efficiency increased from 68 to 92%. Also, it was perceived that the adsorption of the copolymer/surfactant mixture adhered to the Langmuir adsorption isotherm. The calculated Gibbs free energy (∆G0ads) revealed both chemisorption and physisorption of the mixed copolymer and surfactant on the carbon steel surface. Increasing temperature slightly decreases the inhibition efficiency, indicating that the mixed copolymer and surfactant adsorb on the carbon steel surface via simultaneous chemisorption and physisorption. The good inhibition efficiency was related to the formation of inhibitor–adsorption film on the surface of the carbon steel, which is confirmed by the surface analysis. Quantum chemical results using the density functional theory (DFT) corroborated the experimental results.