Interphase corrosion inhibition mechanism of sodium borate on carbon steel rebars in simulated concrete pore solution

Abstract

Sodium borate (Na2B4O7) corrosion inhibition mechanism was studied on carbon steel rebars in 0.6 mol/L Cl– simulated concrete pore solution (SCPS) at different temperatures and concentrations. Electrochemical testing including cyclic potentiodynamic polarization (CPP), electrochemical impedance spectroscopy (EIS), and Mott–Schottky plots were utilized to understand the influence of inhibitor concentration and temperature on the thermodynamics of the adsorption and activation processes. It was found Na2B4O7 imparts corrosion protection due to the formation of a 3D Fe–B–O stable interphase passive film, achieving an inhibition efficiency up to 86%. This can be attributed to the film having a lower donor current density and less vacancies, according to the Mott–Schottky tests. The formed interphase passive film was more stable and compact with less point defects compared to the blank due to Na2B4O7 adsorption, this phenomenon was substantiated using the atomic packing factor, film thickness, and Mott–Schottky plots. It was revealed, using adsorption isotherms, that Na2B4O7 interacted with the surface via physiochemical adsorption, creating a thinner/compact passive film composed of a rhombohedral calcite-type structure. SEM, XRD, and XPS were used to illustrate that Na2B4O7 aides in the formation of a stable double-layered net structure Fe2O3/FeBO3 passive oxide film. Finally, an interphase corrosion inhibition mechanism of Na2B4O7 was proposed.