Exploring the corrosion inhibition mechanism of Serine (Ser) and Cysteine (Cys) in alkaline concrete pore solution simulating carbonated environment

Abstract

The corrosion inhibition mechanism of two amino acids, Serine (Ser) and Cysteine (Cys) for steel in alkaline concrete pore solution simulating carbonated environment was explored by the means of electrochemical techniques, namely, open circuit potential, potentiodynamic polarization scan and electrochemical impedance spectroscopy. High inhibition efficiencies (>90 %) were recorded by both the inhibitors. Moreover, used amino acids prevented the corrosion of steel by acting as anodic-type inhibitors. The inhibitive solution analysis was performed through UV–vis technique which confirmed the formation of inhibitor-Fe(III) complexes signifying clear interaction between steel and corrosion inhibitors. Surface of steel was analysed after submergence in corrosive solutions, with and without the inhibitors, by the means of optical microscopy, SEM-EDS, FTIR and XPS. The results highlighted the formation of a compact, barrier layer comprising of metal-inhibitor complex. The type of adsorption occurred and the isotherm followed during the adsorption were explored by performing gravimetric analysis at six different concentrations of Ser and Cys. The outcomes revealed that both the amino acids adsorbed on the steel surface by physiochemisorption process following the Langmuir isotherm. Subsequently, with the help of the aforementioned techniques, an inhibition mechanism for Ser and Cys in carbonated concrete environment was conjectured. Ser bound with Fe ions through the carboxylate functional group only, while Cys formed chelate with Fe ions through thiolate as well as carboxylate functional groups. A more thermodynamically stable protective layer was formed in the case of Cys attributed to the strong interaction between Fe ions and thiolate functional group.