Evaluation of Active Components of Black Pine Essential Oil as Sustainable Corrosion Inhibitors–Part II

This study evaluates the corrosion inhibition performance of α-pinene, β-pinene, and caryophyllene, constituents of black pine (Pinus nigra) essential oil, on carbon steel in 1 M HCl. At concentrations reflecting their natural abundance in 100 ppm essential oil, α-pinene (66.5 ppm) showed the highest efficiency among individual compounds (up to 94.99%), while β-pinene and caryophyllene exhibited lower efficiencies due to their minor natural content. At an identical concentration (80 ppm), caryophyllene displayed the highest inhibition efficiency after 4 h immersion (96.16%), exceeding α-pinene (92.46%), β-pinene (89.75%), and slightly surpassing the essential oil (95.26%). Electrochemical measurements revealed time-dependent enhancement of protection for all inhibitors. Potentiodynamic polarization indicated mixed-type inhibition with predominant cathodic control and a decrease in corrosion current density from 173.33 μA cm−2 (blank) to 7.03 μA cm−2 (caryophyllene). SEM confirmed reduced surface degradation in inhibited systems, while contact angle measurements showed increased hydrophobicity after prolonged exposure to caryophyllene, indicating formation of a stable adsorbed film. DFT calculations corroborated experimental trends, identifying caryophyllene as the most efficient inhibitor due to favorable electronic properties. The results highlight individual phytochemicals as promising sustainable corrosion inhibitors.