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This investigation focuses on sodium biphosphate (Na 2 HPO 4 ) as corrosion inhibitor for construction steel. All the tests are carried out in a solution that simulates the composition of the pores in chloride-contaminated carbonated concrete. The carbonated solution (CS) contained Na 2 CO 3 (0.0015 mol L −1 ), NaHCO 3 (0.03 mol L −1 ) and NaCl (0.1 mol L −1 ), resulting in [Cl − ]/[OH − ]=10000. Inhibited solutions (IS20, IS60 and IS100) incorporated 20, 60, and 100 mmol L −1 Na 2 HPO 4 respectively. These were labeled IS20, IS60 and IS100 respectively and result in [HPO 4 2− ]/[Cl − ] = 0.2, 0.6, and 1. Cyclic voltammograms and anodic polarization curves were complemented with micro-Raman spectroscopy and XPS, to evaluate the surface film composition. The results show that chloride contamination promotes active corrosion. When phosphate ions are incorporated, steel becomes passive with a more positive corrosion potential (Ecorr), and pitting presents as the predominant form of localized corrosion. Raman spectra show a broad band, centred in 982 cm −1 , suggesting that phosphates incorporate to the passive film. Phosphates are also present in the corrosion products. The surface film becomes more protective to pitting for the highest biphosphate content. However, after pitting no repassivation was detected. After over one month in immersion, steel remains passive in the condition IS100, with inhibition efficiency higher than 99%. In contrast, in the case of IS60 and IS20, pitting was detected. It can be concluded that phosphate ions are good candidates to be used as corrosion inhibitors for steel in chloride-contaminated concrete. [ABSTRACT FROM AUTHOR]

Abstract: Phosphate ions were evaluated as corrosion inhibitors in solutions that simulate the composition of the pores in concrete contaminated with chloride ions. Cyclic voltammograms and potentiodynamic polarization tests were complemented with micro-Raman spectroscopy and impedance spectroscopy to follow the performance of this inhibitor. Long term performance involved weight loss evaluation. Chloride contamination increases the accumulation of corrosion products on the metallic surface and promotes pitting corrosion. In contrast, pitting is inhibited when phosphate ions are incorporated in a 1:1 phosphate to chloride molar ratio, even after a 90 days exposure. Micro Raman spectra clearly show the incorporation of phosphates to the passive film. Impedance spectroscopy results can be interpreted assuming a duplex surface film formed in the presence of phosphates. In the conditions of this investigation, phosphate ions behave as mixed-type corrosion inhibitors, protecting steel against corrosion in chloride-contaminated environments. [Copyright &y& Elsevier]

Abstract: The corrosion behaviour of steel was studied in aerated near neutral citrate solutions without and with various concentrations of NaCl. The potentiodynamic anodic polarization curve in 0.1M citrate solution exhibits four anodic peaks A1, A2, A3 and A4 prior to the oxygen evolution reaction. Addition of Cl− ions to the solution enhances the four peaks currents, specially A3, which is followed by pitting corrosion. The negative going scans of the cyclic voltammograms show two anodic reactivation peaks A5 and A6 and one cathodic plateau P1. A diffusion controlled process in the potential range of A1, A2 and P1 was detected by RDE experiments. The potentiostatic current time transients, at different concentrations of NaCl and applied potentials E a >A3, were studied. The pit nucleation rate (t i −1) is found to increase with increasing the concentration of NaCl and the applied anodic potential. The impedance spectra exhibit four different behaviours depending on the potential range used. They were fitted with a single time constant circuit at E a <−700mV. However, at −700mV< E a <−480mV, they were fitted with a circuit with two time constants. At E a >−480mV, the second semicircle is replaced by negative polarization resistance which is disappeared at E a >−300mV. The electrode impedance was found to decrease with the applied potential. [Copyright &y& Elsevier]