Free fatty acid influenced corrosion inhibition mechanisms of some inedible plants seeds oils on low alloyed medium-carbon steel in H₂SO₄

Abstract

Purpose The purpose of this paper is to investigate the mechanisms of low alloyed medium-carbon steel (LAMCS) corrosion in 0.5 M H2SO4 inhibited by seeds oils of rubber (SOR), Neem (SON) and Jatropha (SOJ) containing varying degree of free fatty acid (FFA). Design/methodology/approach Specific gravity, acid values and FFA compositions of oils were determined. Potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) are techniques used to investigate the corrosion inhibition mechanisms with evaluated Gibbs free energy of adsorption. Findings Corrosion inhibition efficiencies of oils reached values >99% as obtained from PDP and EIS. Protective oxide layer was formed on LAMCS consequent on containment of carbonyl and hydroxyl groups in the FFA of SOR, SON and SOJ, respectively. The SOR and SOJ are found to be mixed inhibitors, whereas SON behaved as anodic inhibitor. Mechanism of adsorption of SOR was synergistic between physisorption and chemisorption, while SON and SOJ exhibited physisorption. SEM micrographs images showed that uninhibited sample exhibited thicker mass of corrosion products. Formation of protective oxide layer was confirmed by XRD diffractograms. Practical implications This study has shown that the need for modification of vegetable seed oils containing FFA is unnecessary as the hydroxyl and carbonyl groups of the FFA contained in the respective oil were found to be the center of adsorption of the oils on the steel surface. Hence, cost and by-products associated with modification of oils used as corrosion inhibitors are eliminated. Originality/value SOR, which has the highest percentage FFA, was found to be the most influential on the corrosion inhibition mechanism of LAMCS, specifically within 0.01–0.02 g/mL concentration. FFA contained in the respective seed oil aided formation of protective oxide layer at interface between H2SO4 and LAMCS, relative to amount composed.