L-Proline-promoted synthesis of 2-amino-4-arylquinoline-3-carbonitriles as sustainable corrosion inhibitors for mild steel in 1 M HCl: experimental and computational studies

The inhibition of mild steel corrosion in 1 M HCl by 2-amino-4-(4-nitrophenyl) quinoline-3-carbonitrile (AAC-1), 2-amino-4-phenylquinoline-3-carbonitrile (AAC-2) and 2-amino-4-(4-hydroxyphenyl) quinoline-3-carbonitrile (AAC-3) has been investigated using weight loss, electrochemical (potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)), surface (SEM, EDX and AFM) and quantum chemical calculation methods. The results indicated that the investigated inhibitors exhibited good inhibition efficiency against the corrosion of mild steel in acidic solution. The weight loss and electrochemical results suggested that inhibition efficiencies were enhanced with an increase in the concentration of 2-amino-4-arylquinoline-3-carbonitriles (AACs). The results showed that the inhibition efficiencies of the investigated AACs obeyed the order: AAC-3 (96.52%), AAC-2 (95.65%) and AAC-1 (94.78%). Potentiodynamic polarization study revealed that the investigated AACs act as cathodic type inhibitors. Adsorption of the AACs on a mild steel surface obeyed the Langmuir adsorption isotherm. Furthermore, SEM, EDX and AFM studies clearly revealed the film-forming ability of AACs on the mild steel surface. Quantum chemical calculations were undertaken to provide mechanistic insight into the mechanism of inhibition action of AACs. On the basis of experimental and theoretical studies it was concluded that the presence of electron releasing hydroxyl (-OH) groups in AAC-3 increases the inhibition efficiency whereas electron withdrawing nitro (-NO2) groups in AAC-1 decrease the inhibition efficiency.