Appraising the corrosion inhibitory efficacy and adsorption mechanism of crude leaf extract of Datura discolor on low-carbon steel in low pH media via gravimetric experiments and AFM analysis

Abstract

Background The study investigated the inhibitory action of Datura discolor leaf extract against the corrosion of low-carbon steel in 0.5 M solutions of three acids. The purpose was to ascertain the effect of extract concentration, immersion time and temperature variation on corrosion inhibitory efficiency, and to relate to existing reports which show that organic molecules in most plant extracts inhibit corrosion. Extraction of the leaves was done by maceration using methanol and solvent was removed by evaporation to dryness. Classical gravimetric (mass loss) experiments were used and experimental data was fitted to adsorption isotherm models to ascertain the best approximation. Surface examination of the low-carbon steel substrates was carried out using the atomic force microscope. Results Inhibitory protective efficiency of extract was found to appreciate with increasing Datura discolor leaf extract concentration at a fixed temperature, with values ranging 77.6–88.8%, 91.35–98.08% and 19.64–44.64% in H₂SO₄, HCl and HNO₃ solutions respectively at 27⁰C. Elevation of temperature was found to depreciate the inhibitor efficiency at constant inhibitor concentration. Best isotherm model fitting was obtained with Langmuir model both at 27⁰C and 60⁰C and in all the hostile media, while Temkin model gave good approximation only at 60⁰C and in H₂SO₄ and HCl solutions only. The negative values of free energy of adsorption \({(\varDelta G}_{ads}^{0})\) suggested that the adsorptive interaction of the inhibitor with the substrate surface was very spontaneous. Values of \({\varDelta G}_{ads}^{0}\) were all consistent, fluctuating between − 16.35 kJ mol^− 1 and − 17.63 kJ mol^− 1 in both H₂SO₄ and HCl solutions, and between − 9.76 kJ mol^− 1 and − 10.25 kJ mol^− 1 in HNO₃ solution, and this suggests that adsorption of the inhibitor molecules occurred via physisorption. Values of the activation energy of the corrosion reaction \({(E}_{act})\) are all < 40 kJ mol^− 1, suggesting that the inhibition occurred by a physical adsorption mechanism. Conclusions The study concludes that Datura discolor crude leaf extract suppressed the corrosion reactions and the inhibition was found to arise from the physisorptive interaction of the organic molecules with the substrate/solution interface, forming a stabilize inhibitor film on the substrate surface as revealed by the atomic force micrographs.