Synergistic effect of SAMs of S-containing amino acids and surfactant on corrosion inhibition of 316L stainless steel in 0.5 M NaCl solution

Abstract

The corrosion inhibition performance of S-containing amino acids L-cysteine (CYS) and L-methionine (MET), as environmental friendly self-assembled monolayers (SAMs), for 316L stainless steel in 0.5 M NaCl solution at room temperature was studied by potentiodynamic polarization curves as well as electrochemical impedance spectroscopy (EIS) measurements. The property of the inhibitive molecules adsorbed on 316L steel surface was studied by FTIR spectroscopy, while the morphology of corroded metal surface was analyzed by scanning electron microscopy (SEM). The results showed that the SAMs of individual CYS and MET significantly retarded the corrosion rates of 316L steel, and the inhibitive property could be enhanced further by combined use with sodium dodecyl benzene sulfonate (SDBS) which often served as a surfactant. It was also found in all measurements that CYS always performed better than MET in corrosion inhibition property, without consideration of the assistance of the SDBS. The conclusions of potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) measurements indicated that SAMs of CYS or MET separately and in collaboration with SDBS could be classified as mixed type inhibitor. The adsorption of CYS and MET separately on 316L steel surface obeyed Langmuir's adsorption isotherm. SEM photographs showed obviously less-corroded morphologies in the presence of the SAMs compared to untreated metal surface. Based on the electrochemical measurements, type of adsorption isotherm, FTIR analysis and morphological characterization as well as the support of quantum chemical calculations, a theoretical model was proposed to explain the mechanism of the synergistic inhibition effect of SAMs of CYS and the surfactant (SDBS) for corrosion of 316L steel.