Bifunctional properties of Acacia concinna pod as a natural surfactant-based eco-friendly benign corrosion inhibitor towards carbon steel protection in saline medium: Experimental and theoretical research

A facile approach has been targeted in combating both the electrochemical and microbial-induced corrosion (MIC) using an eco-friendly surfactant-based plant extract, Acacia concinna pod extract (AcPE) as a corrosion inhibitor for carbon steel (CS) in saline medium (3.5% NaCl). The ethanolic extract from the pod of Acacia concinna was found to contain glycidyl oleate, a fatty acid-based phytochemical, analysed by Gas Chromatography-Mass Spectrometry (GC-MS) showed the dual inhibitory effect. The anti-corrosive effect of AcPE on CS with varying concentrations ranging from 50-400 ppm was investigated by Gravimetric, Potentiodynamic Polarisation (PDP), and Electrochemical Impedance Spectroscopy (EIS). The findings indicated that percentage inhibition efficiency (IE%) increased with an increase in concentration and showed an optimum IE of 93.8% at 200 ppm. However, with a further increase in concentration of AcPE, the IE % decreased. As a result, the efficiency of the corrosion inhibitor is said to be at its highest level when the inhibitor concentration approaches critical micelle concentration (CMC). Plots of Polarisation Resistance (Rp), Charge transfer resistance (Rct) vs inhibitor concentrations yielded a straight line with a slope, which drastically changed at the CMC of the inhibitor studied. PDP analysis revealed that AcPE acts as a mixed-type inhibitor. The adsorption of AcPE on CS followed Langmuir adsorption isotherm. The protective layer was analysed by Scanning electron microscopy with Energy Dispersive X-ray (SEM-EDX), Atomic Force Microscopy (AFM), and Fourier-Transform Infrared spectroscopy (FT-IR). Density Functional Theory (DFT) calculations were utilized to predict the behaviour of inhibitor and to rationalize the experimental results. AcPE was evaluated as a potential biocide to mitigate the effects of MIC using the Colony Forming Units (CFU) assay, and the results showed a percentage biocidal efficiency (BE %) of 96.8% at 250 ppm.