Corrosion inhibition of Euphorbia hirta plant extract on mild steel in acid medium by electrochemical and computational study

Euphorbia hirta, a common weed, has been investigated for its anti-corrosion prospective on mild steel in 1M H2SO4 by impedance, polarization, and computational methods. The impedance and polarization results yielded a maximum efficiency of 89.74–92.92%, respectively, at room temperature (298 K) at the maximum extract dose of 2.5 g/L of the inhibitor. The broadening of Nyquist semi-circular loops at higher inhibitor concentrations confirmed the corrosion-diminishing effect on carbon steel. The shift in Ecorr values of Tafel curves between the blank and the highest dose of inhibitor was observed to be 32 mV, revealing the inhibitor’s mixed-type inhibition. The absence of any parallel complex reactions is indicated by the similarity of Tafel branches. The values of the energy gap, the global softness, and the fraction of electron transfer of the phytochemical molecules clearly indicated their bonding nature with the metal surface. Further, the DFT results and Fukui functions confirmed the behaviour of metal-inhibitor surface interactions by providing an insight into the reactive sites of the major phytochemical molecules of the extract. Monte Carlo studies provided the most stable geometrical orientations of the adsorbed organic molecules on the (1 1 0) plane of the metal specimen surface. The suppressed peak level observed from UV-Vis spectra indicated the depletion of inhibitor species in the solution, and further, a protective barrier was confirmed by the scrutiny of scanning electron microscopic images. The study concluded that the Euphorbia hirta, being a sustainable weed, might favourably adsorb on the steel surface and can provide a promising solution for the inhibition of corrosion on the metal surface, and offers novel insights into molecular-level profiles.