Nutshell Extract as a Carbon Steel "Green" Inhibitor in Marine Environments.

Natural nutshell extract was obtained by mechanical compression. The extract was combined with ethanol and a nonionic surfactant, and it was labeled as EES. The EES inhibitor effect on S135 carbon steel, exposed to a simulated marine-coastal environment (SME), was deduced by mass loss measurement, adsorption isotherm, electrochemical measurements, and surface analysis. The Langmuir adsorption isotherm suggested that a monolayer of the extract was attached by physical-chemical interaction with the steel surface. The increase in the protective efficiency of the adsorbed EES inhibitor was ascribed to the gain of the surface coverage as a function of the inhibitor concentration. It was considered an antioxidant activity of the inhibitor, attributed mostly to the Fe-ion capture by anacardic acid and the posterior ion chelation. This fact was collaborated by the negative zeta potential of the nutshell extract, added to the SME. Electrochemical impedance spectroscopy (EIS) diagrams revealed that the steel polarization resistance ( ) increased as a function of the inhibitor concentration, while the thickness () of the Fe-oxide layer was reduced to ≈0.50 nm.