Tamarind shell tannin extracts as green corrosion inhibitors of mild steel in hydrochloric acid medium

Abstract

Tamarind shell tannin extract (TSTE) was examined for corrosion inhibition in a 0.5 M HCl solution. The results were examined using electrochemical impedance spectroscopy (EIS), potentiodynamic polarisation (PD), and electrochemical noise measurement (ENM) analysis. Increased concentration of TSTE increased the inhibition efficiency up to a maximum concentration of 1000 ppm. The inhibition efficiencies were 87.77% and 86.34% for the methanol (TME) extract and 70% for the acetone (TAE) extract, respectively. Potentiodynamic polarization analysis showed that TSTE was a mixed type corrosion inhibitor with major mitigation of corrosion at cathodic sites. Nyquist plots showed that there was an increase in the resistance of charge transfer values and decrease in the constant phase element (CPE) as the concentration of TSTE was increased. The effect of TSTE adsorption followed the Langmuir isotherm. The TSTE molecules were physically adsorbed on the mild steel surface with a free energy adsorption of ΔGads TME = −21.92 kJ mol−1 and ΔGads TAE = −21.62 kJ mol−1. Moreover, the surface morphology and elemental composition studies using SEM and EDX, verified the occurrence of TSTE adsorption on the surface of mild steel.