Passiflora edulis Sims peel extract as a renewable corrosion inhibitor for mild steel in phosphoric acid solution

Passiflora edulis Sims peel extract (PESPE) was explored as a green corrosion inhibitor for mild steel in 1 mol/L H3PO4 solution. First, components of PESPE were analyzed by UPLC-QTFMS, and their functional groups were investigated based on FTIR and UV–visible spectra. A total of 21 active components were identified, including amino acids, sugars, vitamins, flavonoids, flavonoids, and organic acids. The electrochemical corrosion inhibition effect of PESPE at 298, 308, and 318 K was studied through potentiodynamic polarization and EIS measurements. The electrochemical results show that PESPE acts as a mixed inhibitor and effectively retards the corrosion of mild steel by geometric coverage effect. The adsorption of PESPE on mild steel was found to follow the Langmuir isotherm. Elemental composition of the adsorption film was analyzed using XPS technique to confirm the adsorption of organic substances. Temperature affects the relative ratio of physical/chemical adsorption in mixed adsorption, as well as the inhibition efficiency and optimum concentration of PESPE. The monolayer adsorption film improves order on the steel surface. Five typical components in PESPE were theoretically investigated through density functional theory calculation and molecular dynamics simulation, further confirming their spontaneous adsorption in parallel orientation on the surface. Vitamin B2 and isovitexin 2″-O-glucoside display better adsorption stability and inhibition performance among the five considered molecules due to their numerous aromatic rings and heteroatoms.