Gravimetric, Electrochemical, Surface Morphology, DFT, and Monte Carlo Simulation Studies on Three N-Substituted 2-Aminopyridine Derivatives as Corrosion Inhibitors of Mild Steel in Acidic Medium

Three N-substituted 2-aminopyridine derivatives, namely, 6-(2,4-dihydroxyphenyl)-4-phenyl-2-(phenylamino)nicotinonitrile (DPPN), 6-(2,4-dihydroxyphenyl)-2-((4-hydroxyphenyl)amino)-4-phenylnicotinonitrile (DHPN), and 6-(2,4-dihydroxyphenyl)-2-((4-methoxyphenyl)amino)-4-phenylnicotinonitrile (DMPN) were investigated for their inhibitive effects on mild steel corrosion in 1 M HCl solution using electrochemical, surface, chemical, and theoretical studies. Results showed that the protection capabilities of inhibitors used in the study increase with increase in their concentrations and attained the maximum numerical values of 95.81, 96.24, and 96.63% for DPPN, DHPN, and DMPN, respectively, at the 20.20 × 10^-5 mol L^-1 concentration. The results of the electrochemical impedance spectroscopy studies revealed that DPPN, DHPN, and DMPN molecules retard corrosion by adsorbing at the metal/electrolyte interface. Adsorption of the DPPN, DHPN, and DMPN molecules on the surface was found to obey the Langmuir adsorption isotherm model. Polarization measurement indicated that DPPN, DHPN, and DMPN molecules are mixed-type inhibitors with predominant cathodic inhibitive action. SEM and EDX analyses showed that the corrosion-induced surface roughness of mild steel is significantly reduced by the inhibitors because of the development of protective films by DPPN, DHPN, and DMPN molecules on the surface. The results of theoretical DFT and Monte Carlo simulation studies supported experimental studies and posited that the DPPN, DHPN, and DMPN molecules adsorbed on the mild steel surface as protonated species. Both the experimental and theoretical studies showed that the order of inhibition efficiencies of the studied compounds is DMPN > DHPN > DPPN.