A detailed electrochemical/theoretical exploration of the aqueous Chinese gooseberry fruit shell extract as a green and cheap corrosion inhibitor for mild steel in acidic solution

Abstract

The aqueous extract of Chinese gooseberry fruit shell is a green source of various biologically active compounds like sucrose, maltose and folate, which are highly soluble in water and act as inhibiting molecules for carbon steel. The other components like methyl hexanoate, phytosterol and octadecadienoic acid are insoluble or less soluble in water and cannot be extracted from the Chinese gooseberry fruit shell through water-based extraction process. With corrosion inhibitory action in acidic environments. In the present work, the Chinese gooseberry fruit shell was used as a cheap waste material for extraction of green corrosion inhibitors for mild steel corrosion mitigation in subjection to HCl solution. Detailed explorations based on electronic-level density functional theory (DFT) and atomic-level molecular simulations (molecular dynamics and Mote Carlo) combined with electrochemical methods were applied to assess the Chinese gooseberry fruit shell corrosion inhibition characteristics, electronic properties, and interactions over the steel substrate in HCl solution. AFM, SEM, and contact angle test were performed to assess the morphology of the unexposed and exposed metal surfaces. According to EIS results, 92% efficiency was derived using 1000 ppm extract after 2.5 h metal immersion in inhibited HCl solution. The polarization test results clarified a prominent cathodic inhibition activity for this inhibitor with slight impact on the mechanism of hydrogen evolution reaction. The inhibition efficiency of Chinese gooseberry fruit shell extract was additionally evaluated by weight loss experiments at different temperatures and the results disclosed that during 5 h steel immersion at 25 °C the efficiency raised to 94% for 1000 ppm sample. Furthermore, the molecular simulation results clarified that the major active organic compounds of Chinese gooseberry fruit shell extract have a propensity to adsorb on the metal substrate enabling them to construct a corrosion preventing film over the surface, which is in support of experimental findings. The electronic-structure DFT studies evidenced the donor-acceptor interactions of reactive sites of inhibitor molecules. It is noteworthy that among different water-soluble ingredients of the Chinese gooseberry fruit shell extract the N-containing compounds including folate structure can supply more inhibiting activity than sucrose and maltose due to the presence of different polar oxygenated centers (i.e., hydroxyl and carbonyl) and importantly nitrogenous moieties (e.g., imine -N and amine -NH and -NH2). The stronger folate inhibition potentiality was computationally affirmed by DFT-evidenced strongest protonation affinity leading to its possible strongest physisorption and strongest MC/MD-proposed adsorption energies.