Corrosion inhibition mechanism of aromatic amino acids for steel in alkaline pore solution simulating carbonated concrete environment

Abstract

Corrosion inhibition mechanism of two aromatic amino acids, phenylalanine (Phe) and tryptophan (Trp), was explored for steel in an alkaline pore solution simulating a carbonated concrete environment. The electrochemical impedance spectroscopy and potentiodynamic polarization test results revealed that both compounds can be used effectively as corrosion inhibitors for carbonated concrete environments with high inhibition efficiency. The dissolution of steel got controlled as Phe and Trp acted as anodic‐type inhibitors. Phe and Trp adsorbed on the steel surface by physiochemisorption process following the Langmuir isotherm. The inhibitive solution analysis through UV–visible technique confirmed the formation of inhibitor–Fe complexes. The surface of steel was analyzed after submergence in corrosive solutions with and without inhibitors by means of optical microscopy, scanning electron microscopy–energy‐dispersive X‐ray spectroscopy, Fourier transformed infrared, and X‐ray photoelectron spectroscopy, which highlights the formation of a compact, barrier layer comprised of metal–inhibitor complex. Subsequently, with the help of the aforementioned techniques, an inhibition mechanism for Phe and Trp was conjectured, which revealed that the presence of additional heterocycle in amino acids can raise the chances of chelation and formation of a more adherent and thermodynamically stable layer. Phe bound with Fe ions through the carboxylate functional group only, while Trp chelated the Fe ions via a nitrogen atom of the pyrrole ring and carboxylate functional group.