Exploring the dual aspects of Sapindus mukorossi pod as a novel sustainable biosurfactant-based corrosion inhibitor for carbon steel in saline environment: An experimental and theoretical insights

Abstract

In this study, a novel application of the ethanolic extract of Sapindus mukorossi pod (SmPE) was investigated as a biosurfactant-based corrosion inhibitor for carbon steel (CS) against both electrochemical and microbiologically influenced corrosion (MIC) in a saline medium (3.5 % NaCl). Gas chromatography-mass spectrometry (GC–MS) identified cis-vaccenic acid as the primary active phytochemical in SmPE, which acts as a biosurfactant molecule. The corrosion inhibitive performance of SmPE on CS was evaluated with varying concentrations ranging from 50$-$400 ppm by gravimetric analysis and electrochemical techniques such as Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP). The results revealed that inhibition efficiency (IE %) increased with SmPE concentration, reaching a maximum of 96.9 % at the critical micelle concentration (CMC) of 250 ppm, accompanied by a minimal corrosion rate (CR) of 0.0062 mm/y. The CMC was confirmed by a distinct change in electrochemical parameters at this concentration. PDP analysis indicated that SmPE functions as a mixed-type inhibitor. Surface characterization via Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX), Fourier Transform Infrared Spectroscopy (FT-IR), Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS) confirmed the adsorption of SmPE molecules on the CS surface. Ultraviolet-visible (UV–vis) spectroscopy demonstrated complex formation between Fe²⁺ ions and the inhibitor. Density Functional Theory (DFT) calculations provided a theoretical understanding of the inhibitor's interaction with the CS surface. Additionally, SmPE exhibited notable biocidal activity against MIC, with a biocidal efficiency (BE %) of 94.40 ± 0.90 % at 250 ppm, as determined by Colony Forming Units (CFU) assay.