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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]

The purpose of this investigation is to analyze the effectiveness of phosphates as inhibiting agents for steel bars embedded in mortars. Three mix designs were selected for this study: mix A had no admixed chlorides and was used as reference; mix designs B and C were contaminated with 1% in weight of chlorides/weight of cement and mix C also incorporated 7% Na 3 PO 4 .12 H 2 O. After setting for 48 h, the samples were cured for 7 days immersed in water. Three specimens of each group were then immersed in aerated saline solutions 0.5 mol L −1 NaCl during 720 days. The corrosion potential ( Ecorr ), the polarization resistance ( Rp ) and the electrochemical impedance spectra were recorded regularly. After 720 days of immersion one set of mortars was anodically polarized and another set cathodically polarized. After that, Raman spectra of corrosion products were registered. Additional specimens were used to evaluate porosity and chloride profiles. The presence of phosphate ions as inhibitors has no effect on Ecorr values and suggests mixed-type inhibition. Mix C remains passive until 180 days of exposure, with Rt> 100 kΩ cm 2 . At longer times, Rt decreases in time but the inhibition percentage is always higher than 95% when evaluated by impedance spectra. Using Rp , inhibition stays above 70%. Anodic and cathodic polarization curves, together with corrosion potential values, suggest that phosphates behave as a mixed-type inhibitor. [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]