Corrosion inhibition of steel rebar in chloride-contaminated concrete pore solution: Ecofriendly glutamic acid inhibitor and its synergy with galvanized coating

Abstract

In this study, we explore the effectiveness of glutamic acid as corrosion inhibitor for protecting steel rebars within a concrete pore solution contaminated with chloride (CCCPS) along with its synergy with galvanized coating are investigated. The study is defined in two phases: (a) examining the inhibition efficiency (%IE) of glutamic acid at various concentrations of 0.25, 0.50, 0.75, and 1 g/L using a combination of experimental and theoretical methods, and (b) evaluating its synergistic effect with Zn present in the galvanized coating. In the first phase, various electrochemical tests and theoretical investigations are employed. The electrochemical examinations consistently demonstrated that at a concentration of 1 g/L, the maximum %IE reaches ∼ 80 %. Langmuir adsorption isotherm fitting effectively describes the relationship between inhibitor concentration and its coverage percentage. Thermodynamic data indicate the adsorption of these inhibitors on the steel surface involves both physical and chemical adsorptions with a ΔG°ads (standard free energy of adsorption) of around –23.5 kJ/mol. Moreover, the activation energy for corrosion of steel rebar increases from 55.8 kJ/mol for the blank CCCPS to 78.3 kJ/mol for the inhibited CCCPS. Molecular dynamics simulations validate the binding of glutamic acid molecules to the surface of the steel, primarily through nitrogen atoms orientation towards d-orbitals of steel substrate. In the second phase, the carbon steel rebar underwent hot-dip galvanization, resulting in a galvanized coating with a thickness of approximately 36 µm. The synergistic effect of glutamic acid at its optimum concentration (0.75 g/L) was investigated with comparison of the corrosion behavior of three samples: bare steel, galvanized steel, and scratched galvanized steel in simulated CCCPS with and without the inhibitor. Results indicate that the presence of the inhibitor significantly enhances the %IE, up to 99 %, for scratched galvanized steel. FESEM/EDS analysis further reveals a substantially lower volume of corrosion products in the scratched area of the latter sample compared to its counterpart in the inhibitor-free solution. Furthermore, as indicated by the findings from X-ray photoelectron spectroscopy (XPS), the interactions between the N and Fe atoms, acting as donors and acceptors, respectively, hold significant importance in the chemisorption process.