N,N'-Dialkylcystine Gemini and Monomeric N-Alkyl Cysteine Surfactants as Corrosion Inhibitors on Mild Steel Corrosion in 1 M HCl Solution: A Comparative Study.

Gemini surfactant, N,N'-dialkylcystine 2(C(12)Cys), derived from cystine, and a monomeric N-alkyl cysteine counterpart, (C(12)Cys), were synthesized and purified. The characterization of surfactants 2(C(12)Cys) and (C(12)Cys) was made by Fourier transform infrared, (1)H NMR, and elemental analysis. The effect of 2(C(12)Cys) and (C(12)Cys) on mild steel (MS) corrosion in 1 M HCl solution was explored as a function of their concentration and electrolyte temperature by means of gravimetric and electrochemical experiments (potentiodynamic polarization and Electrochemical impedance spectroscopy), surface analytical techniques (scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDAX) and atomic force microscopy (AFM)) and theoretical study. The investigated compounds exhibited surface active properties and performed as good inhibitors for corrosion control of mild steel (MS) in acid solution. However, compared to monomeric (C(12)Cys), Gemini surfactant 2(C(12)Cys) showed high corrosion inhibiting ability at very low concentration. The EIS results revealed a greater charge transfer resistance in 2(C(12)Cys) solution compared to that in (C(12)Cys) solution. SEM/EDAX observations validate the development of an inhibitive film by the adsorbed molecules of surfactant on the MS surface. The AFM micrographs supported the SEM/EDAX results and exhibited a lowering in the roughness of the corroded MS surface in the presence of both (C(12)Cys) and 2(C(12)Cys) surfactants. Further, quantum chemical calculations and Monte Carlo simulations were used to study the dependence of corrosion inhibiting efficacy on the molecular structure and adsorption strength.