Sambucus javanica Extract as a Green Corrosion Inhibitor for Q420 Steel in HCl Medium: Experimental Assessment and Theoretical Simulations

As environmental protection regulations have become stricter, green corrosion inhibitors are in high demand for industrial acid pickling processes. In this work, Sambucus javanica extract (SJE) was developed using a green water‐based oscillatory extraction method, and its protective performance for Q420 steel in an acid medium was systematically elucidated. Electrochemical measurements demonstrated that SJE increases the double‐layer capacitance ( C dl ), enhances the charge transfer resistance ( R ct ), and suppresses both anodic and cathodic corrosion currents ( I corr ). The maximum inhibition efficiency reached 95.26% at 6.0 g/L and 293 K, and the steel's corrosion rate declined from 4.2121 mm/year to 0.1997 mm/year. Surface characterizations confirmed that SJE molecules adsorb onto the steel surface to form a dense protective film, significantly reducing surface roughness and inhibiting corrosive ion attack. The adsorption model analyses revealed that SJE exhibited spontaneous adsorption onto the steel surface by covering multiple reactive centers, and this process complied with the Langmuir model and involved intermolecular repulsive forces. Quantum chemical calculations revealed that SJE, which possesses abundant active sites and favorable electronic properties, facilitates the bonding process with the metal substrate. Molecular dynamics simulations further confirmed the stable adsorption of SJE on the Fe (110) surface with negative adsorption enthalpies.