l-Cysteine as corrosion inhibitor for mild steel in 1Â M HCl and synergistic effect of anionic, cationic and non-ionic surfactants

The corrosion inhibition performance of sulphur containing amino acid l-cysteine (CYS) as environmentally safe corrosion inhibitor for mild steel (MS) in aerated and unstirred 1 M HCl solution, in the temperature range of 30-60 °C, was studied by weight loss, potentiodynamic polarization and ac impedance measurements. The nature of the protective film formed on the MS surface is analysed by FTIR spectroscopy. The surface morphology of corroded MS specimens was studied by scanning electron microscopy (SEM). CYS, which contains a mercapto group in its molecular structure, significantly reduces the corrosion rates of MS; the maximum inhibition efficiency (IE) being 85.62% at 30 °C in the presence of CYS concentration of 500 ppm. The effect of the addition of very small concentrations of sodium dodecyl sulphate (SDS), cetyl pyridinium chloride (CPC), and triton X-100 (TX) respectively, on the corrosion inhibition behaviour of CYS was also studied. IE of CYS is significantly increased in the presence of all the three surfactants. The effect of surfactants on the corrosion inhibition behaviour of CYS appears to be synergistic in nature; the effect of non-ionic surfactant TX being more pronounced than CPC and SDS. CYS separately and in combination with surfactants acts as mixed type inhibitor. The change in impedance parameters indicated that CYS and CYS-surfactants molecules acted by accumulating at the metal/solution interface. SEM micrographs show a clearly different morphology in the presence of CYS and CYS-surfactant additives. The adsorption of CYS separately and in the presence of surfactants on MS surface obeyed Langmuir's adsorption isotherm. The calculated thermodynamic parameters for adsorption reveal a strong interaction between the CYS and the MS surface. The results obtained by electrochemical studies are consistent with the results of the weight loss measurements.