Applicability of winged bean extracts as organic corrosion inhibitors for reinforced steel in 0.5 ​M HCl electrolyte

Abstract

Corrosion inhibition properties of winged bean (WB) extracts on reinforced steel in 0.5 ​M HCl solution was studied through experimental and theoretical calculation methods. The electrochemical studies suggested that inhibition efficiency increased with increasing concentration of WB extracts up to 95%. Nyquist diagrams revealed an increase in the charge transfer resistance values and a decrease in the constant phase element as the concentration of WB extracts were increased. The potentiodynamic polarization results revealed that WB extracts behave as mixed-type inhibitors, which physically adsorbed onto the reinforced steel surface. Effect of temperature study demonstrated that the corrosion resistance behaviour of WB extracts decreased with an increase in temperature, yielding a corrosion rate of 3.39 mmpy and 4.02 mmpy at 333 ​K with the incorporation of 1000 ​ppm WBW and WBE extracts, respectively. The thermodynamic study implied that the adsorption process follows the Langmuir isotherm with free energy adsorption of ΔGads WBW ​= ​-17.29 ​kJ ​mol-1 and ΔGads WBE ​= ​-16.81 ​kJ ​mol-1. Corresponding to the molecular modelling study, the semi-empirical method and molecular dynamics (MD) simulation investigated the correlation between the inhibitor compounds and the metal surface. One of the phenolic molecule constituents (gallic acid) was chosen to establish the structural and electronic parameters responsible for the high inhibition efficiency. A greater Ebinding of 0.181 (a.u.) indicates that gallic acid in WB extracts can easily bind with the Fe surface, thus projecting a higher inhibitory performance. Surface morphology study affirmed the effective adsorption of WB extracts onto the surface of reinforced steel.