Influence of phosphate ions on passive film formation in amino acid-containing concrete pore solutions with chloride ions

There is need for development of highly effective green corrosion inhibitors to protect steel rebar. In the present study, 2 wt % L-arginine (LA: C6H14N4O2) with different amounts (i.e., 0.1, 0.25 and 0.50 wt %) of trisodium phosphate dodecahydrate (SP: Na3PO4∙12H2O) was used as an eco-friendly corrosion inhibitor in a simulated concrete pore (SCP) solution with 3.5 wt % NaCl to mitigate the corrosion of steel rebar. Steel rebar immersed in 2 wt % LA with 0.25 wt % SP (i.e., L2P2 solution) exhibited excellent corrosion resistance by forming P-zwitterion-(Cl)–Fe, zwitterion-(Cl)–Fe, and FePO4 complexes as uniform and protective passive films. However, SP concentrations lesser or greater than 0.25 wt % with 2 wt % LA exhibited a reduction in total impedance. The small amount of SP could not form and stabilize the P-zwitterion-(Cl)–Fe complex, whereas a large amount caused competition between P-zwitterion-(Cl)–Fe and zwitterion-(Cl)–Fe complexes where phosphate ions interfere with the formation of P-zwitterion-(Cl)–Fe. Therefore, a proper concentration of phosphate ions is required to form and stabilize the P-zwitterion-(Cl)–Fe and FePO4 complexes. Density functional theory (DFT) and molecular dynamic (MD) simulation results suggest that the energy gap (ΔE) for formation of LA-P (C6H13O5N4P: N(ω)-phosphonato-L-Arginine) is high, and it tends to strongly adsorb from amino acid groups and carbon chains until N–P bonds are formed.