Adsorption and Surface Analysis of Sodium Phosphate Corrosion Inhibitor on Carbon Steel in Simulated Concrete Pore Solution

Corrosion of steel-reinforced concrete exposed to marine environments could lead to structural catastrophic failure in service. Hence, the construction industry is seeking novel corrosion preventive methods that are effective, cheap, and non-toxic. In this regard, the inhibitive properties of sodium phosphate (Na₃PO₄) corrosion inhibitor have been investigated for carbon steel reinforcements in 0.6 M Cl⁻ contaminated simulated concrete pore solution (SCPS). Different electrochemical testing has been utilized including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and Mott-Schottky plots to test Na₃PO₄ at different concentrations: 0.05, 0.1, 0.3, and 0.6 M. It was found that Na₃PO₄ adsorbs on the surface through a combined physicochemical adsorption process, thus creating insoluble protective ferric phosphate film (FePO₄) and achieving an inhibition efficiency (IE) up to 91.7%. The formation of FePO₄ was elucidated by means of Fourier-transform infrared spectroscopy (FT–IR) and X-ray photoelectron spectroscopy (XPS). Quantum chemical parameters using density functional theory (DFT) were obtained to further understand the chemical interactions at the interface. It was found that PO₄³⁻ ions have a low energy gap (ΔE_gap), hence facilitating their adsorption. Additionally, Mulliken population analysis showed that the oxygen atoms present in PO4³⁻ are strong nucleophiles, thus acting as adsorption sites.