Abstract
Despite extensive research on the matter of corrosion inhibition efficiency, the interactions between the defect structure of the passive layer and the inhibitor molecules still remain poorly understood. In this study, the corrosion inhibition mechanism of ethylenediamine-tetraacetic acid as a carboxylate-based organic inhibitor on steel specimens in simulated concrete pore solution was studied. The point defect model was used to describe the response of the passive oxide film on the steel surface to the perturbation caused by the addition of the carboxylate compound. The electrochemical behavior of the steel specimens was evaluated through open circuit potential, electrochemical impedance spectroscopy and potentiodynamic analysis. The reduction in efficiency outside the optimal concentrations was discussed from an electrochemical point of view. We suggest that the performance of the inhibitor is highly dependent on the positively charged entities on the passive layer including anion vacancies and interstitial cations. To further investigate the physicochemical behavior of the organic molecules, density functional theory and adsorption isotherms were applied. The topography and morphology of the surface were analyzed through scanning electron microscopy. To confirm the inhibitive effect of EDTA, the elements and chemical bonds present on the surface were characterized via X-ray photoelectron spectroscopy. The surface analysis confirmed that the addition of EDTA formed a network of chemical bonds, which significantly hindered the corrosion phenomenon.
Highlights
Considering recent progress in building material selection and additive development, it is safe to say that concrete materials are well studied
The highly alkaline condition of concrete ensures a strong passive state, which is a protective anti-corrosion oxide/ hydroxide layer on the steel bar. When it becomes contaminated with aggressive agents such as chloride ions or the concrete undergoes carbonation processes, the stability of this layer starts to decrease and pitting corrosion starts when the chloride content exceeds a critical amount of 0.4–1% by cement weight.[1]
The Langmuir, generalized Langmuir–Freundlich, and Temkin adsorption isotherms were used in this study, which are represented by eqn (1)–(3), respectively,[14,15,16]
Summary
Considering recent progress in building material selection and additive development, it is safe to say that concrete materials are well studied. In most of the works, the immersion time for the metal is 1 hour or less, but it cannot be guaranteed that a passive layer develops within a period of less than 48 hours, especially when the chemical composition of the electrolyte is just Ca(OH)[2] and does not consist of NaOH or KOH, which are necessary for concrete pore simulation.[11,12,13] in some works, aggressive ions are added a er a certain exposure time to ensure a strong passive state before corrosion This is not consistent with reality given that agents such as atmospheric chloride are always present. Based on the results, to achieve a comprehensive understanding of the physicochemical processes and inhibition mechanism, the point defect model was exploited
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