Influence of Phosphate on Corrosion Products of Iron in Chloride-Polluted-Concrete-Simulating Solutions: Ferrihydrite vs Green Rust

The corrosion of steel-reinforcing bar (rebar) in concrete induced by the diffusion of chloride ions was simulated with α-iron foils dipped in various aqueous electrolytes. Sodium chloride (NaCl) was added to saturated calcium hydroxide (Ca[OH]₂) solutions to simulate concrete polluted with chloride ions whereas solutions containing sodium bicarbonate (NaHCO₃), sodium carbonate (Na₂CO₃), and NaCl were used as carbonate- and chloride-containing concrete. The corrosion process was electrochemically induced by cyclic voltammetry and galvanostatic methods. Similar experiments were performed with solutions where various amounts of trisodium phosphate (Na₃PO₄) were added. The galvanostatic potential dropped to ~ –0.15 V vs standard hydrogen electrode (SHE) in the absence of phosphate, indicating that active corrosion took place, while it reached values of ~1 V_SHE with large phosphate concentrations. The corrosion products were analyzed by Mössbauer spectroscopy. They mainly consisted of iron(II) compounds—ferrous carbonate siderite (FeCO₃) or ferrous hydroxide (Fe[OH]₂), depending on the pH—and iron(II)-iron(III) compounds (i.e., green rusts) when phosphate ions were not present. In contrast, the corrosion product was mainly a poorly crystallized iron(III) oxyhydroxide, ferrihydrite, in the presence of phosphate. Similarly, the cyclic voltammetry curves were drastically different for the absence and presence of phosphate and showed that the pitting potential became more noble in the presence of phosphate. Finally, while immersed for long periods in the phosphate-containing solutions, the α-iron foils did not present a single trace of corrosion. The x-ray photoelectron spectroscopy analyses of their surfaces revealed the presence of an iron oxide film, likely γ-Fe₂O₃. The electrochemical impedance spectra obtained at the corrosion potential were composed of one unique capacitive loop, the size of which increased as the concentration of phosphate rose. The phosphate ions act as anodic inhibitors and, if used at low concentrations, can promote pitting corrosion.