3-Amino alkylated indoles as corrosion inhibitors for mild steel in 1M HCl: Experimental and theoretical studies

The present study describes the influence of ring and ring size of three 3-amino alkylated indoles (AAIs) namely, N-((1H-indol-3-yl)(phenyl)methyl)-N-ethylethanamine (AAI-1), 3-(phenyl(pyrrolidin-1-yl)methyl)-1H-indole (AAI-2) and 3-(phenyl(piperidin-1-yl)methyl)-1H-indole (AAI-3) on mild steel corrosion in 1M HCl solution using gravimetric, electrochemical, surface morphology (SEM, AFM), quantum chemical calculations and molecular dynamics simulations methods. Both experimental and theoretical results showed that the 3-amino alkylated indoles with cyclic amino groups exhibit higher inhibition efficiency compared to the one with opened-chain amino group. The results further suggested that the inhibition efficiency increases with increasing ring size of the amino group such that the piperidine-containing (6-membered ring) 3-amino alkylated indole showed higher inhibition performance than the pyrrolidine-containing (five membered) 3-amino alkylated indole. Experimental results revealed that the inhibition efficiency increases with increasing concentration of the inhibitors. Maximum inhibition efficiencies of 94.34% for AAI-1, 96.08% for AAI-2 and 96.95% for AAI-3 were obtained at 0.862 mM concentration. EIS measurements showed that the studied compounds inhibit mild steel corrosion by adsorbing on the steel surface. Polarization studies revealed that the compounds are cathodic type inhibitors. The adsorption of the studied compounds obeyed the Langmuir adsorption isotherm. SEM and AFM surface morphology analyses also provided evidence of formation of adsorbed film of the AAIs on the steel surface. Theoretical parameters such as EHOMO and electronegativity derived from quantum chemical calculations as well as binding energy derived from molecular dynamics simulations studies adequately corroborate the trend of experimental inhibition efficiencies of the studied inhibitors.