Testing phosphate ions as corrosion inhibitors for construction steel in mortars

Abstract

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% Na3PO4.12 H2O. 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 cm2. 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.