Inhibiting Steel Corrosion in Simulated Concrete with Low Phosphate to Chloride Ratios

Phosphate ions are studied as corrosion inhibitors in pore simulating solutions highly contaminated with chloride ions. The investigation aims at understanding the role of phosphates in the corrosion inhibition mechanism, employing potentiodynamic polarization tests, micro-Raman spectroscopy, impedance spectroscopy, X-ray photo electronic spectroscopy (XPS) and weight loss tests. Two inhibitor/chloride ratios were assessed, [PO43−]/[Cl−] = 0.2 and 0.6. When [PO43−]/[Cl−] = 0.6, pitting is inhibited, even after 90 days exposure. [PO43−]/[Cl−] = 0.2 only delayed the onset of localized attack. XPS showed that phosphates incorporate to the surface film. Phosphate ions behaved as mixed-type corrosion inhibitors. The results are interpreted by the participation of phosphates in the duplex passive film being formed on carbon steel.