Lavender Essential Oil and the Amphoteric Surfactant Cocamidopropyl Betaine as Synergistic Inhibitors of Carbon Steel Corrosion: Experimental and DFT Studies

The synergistic corrosion inhibition effect of lavender essential oil (LEO) and cocamidopropyl betaine (CAPB) on carbon steel in 3.5% NaCl solution was investigated. The inhibitors were characterized by GC–MS and FTIR, revealing linalyl acetate as the major LEO component and the functional groups responsible for metal interaction. Corrosion inhibition efficiency was evaluated using potentiodynamic polarization and Tafel analysis. The highest efficiency, up to 95%, was achieved by the LEO/CAPB mixture, compared to 78 and 67% for LEO and CAPB, respectively. The inhibition efficiency of LEO/CAPB remained high (87%) at 328 K, confirming its good thermal stability. Adsorption followed the Flory–Huggins isotherm (R2 = 0.999), suggesting multilayer adsorption. Thermodynamic parameters showed negative $$\Delta G_{{{\text{ads}}}}^{0}$$ values, confirming spontaneous adsorption, with magnitude indicating a mixed physisorption–chemisorption mechanism for LEO/CAPB. Surface analyses by optical microscopy and profilometry showed that the mixture forms a uniform, protective film on the steel surface, in contrast to severe corrosion in the blank solution. DFT calculations on LEO components identified linalyl acetate as the most reactive, due to its high HOMO, low LUMO, and narrow energy gap, favoring donor–acceptor interactions. A synergistic mechanism is proposed where CAPB acts as a molecular bridge, enhancing LEO adsorption and reducing corrosion. The results confirm the LEO/CAPB system as a highly efficient, eco-friendly corrosion inhibitor for carbon steel in saline environments.