Experimental and theoretical studies of azomethines derived from benzylamine as corrosion inhibitors of mild steel in 1 M HCl

Two azomethine ligands derived from 2,4-dihydroxybenzaldehyde and benzylamine derivatives were successfully synthesized through the conventional condensation method. The physicochemical properties were elucidated using elemental analysis (CHNS), infrared spectroscopy (IR) and nuclear magnetic resonance spectroscopy (NMR). The corrosion retardation outcomes of two azomethines derived from benzylamine namely as A1 and A2 for the low carbon steel in 1 M HCl at dissimilar concentrations were studied based on polarization measurement and electrochemical impedance spectroscopy (EIS). Based on the impedance results, it shows that an increase in the additives concentration will simultaneously increases the resistance of charge transfer while reducing double-layer capacitance. The polarization measurement parameters proved that in the existence of inhibitors, corrosion potential (E_corr) has been altered in the direction of the cathodic region as compared to uninhibited solution and obeys Langmuir adsorption isotherm. A1 and A2 additives are regarded as mixed-type inhibitors since the corrosion potential shifts are less than ±85 mV as comparison to the corrosion potential in the absence of inhibitors’ solution. Based on the electrochemical measurement, A2 shows up to 95% inhibition efficiency. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed improvement of metal substrate at the interface of metal/solution with the presence of A1 and A2 inhibitors. From the density-functional theory study (DFT), A2 inhibitor improved inhibition efficiency due to strong solubility in water as opposed to A1. The presence of fluorine in A2 causes the frontier orbitals of HOMO more delocalized than A1, thus increase the adsorption of whole molecule to the interface of metal/solution, which also explain improved inhibition efficiency as compared to A1. The findings were supported with further investigation based on the elemental composition analysis of interface of metal/solution using X-ray Photoelectron Spectroscopy (XPS). Based on the XPS analysis, the adsorption of A2 inhibitor on mild steel-electrolyte interface has been proved mostly via chemisorption from carbon as found in the benzene ring and physisorbed via nitrogen in C[dbnd]N⁺.