Efficiency of phosphate ion as a corrosion inhibitor of reinforcement steel in alkaline solutions in presence of chloride ions

L.YOHAI; M. VAZQUEZ; M. B. VALCARCE

Forianopolis

Simposio; 10th Intl. Symposium on Passivity of Metals and Semiconductors.; 2011

Corrosion of steel in concrete structures is a worldwide problem that costs millions of dollars per year. Steel rebar has a passive behaviour in concrete because the pH inside allows the formation of a protective layer on this material. However, in marine environments chloride ions can diffuse inside the concrete; reach the surface of steel and breakdown its passivity. A common treatment to prevent damage is to add a corrosion inhibitor in the concrete mixture. The aim of this study is to investigate phosphate ion as a corrosion inhibitor of reinforcement steel in concrete pore simulating solution (SSP), pH=13, contaminated with chloride ions. To evaluate the effect of chloride ions, 0.3 and 0.5 mol l-1 chloride ions concentration are used, corresponding to [Cl-]/[OH-]=3 and [Cl-]/[OH-]=5 respectively. The inhibitor concentration is 0.3 mol l-1 PO43- being the [PO43-]/[Cl-] ratios tested 1 and 0.6. Cyclic voltammograms are carried out in the various conditions. Anodic polarization curves are measured after 24 hs at the corrosion potential. A Raman spectrum is performed after carrying out the anodic polarization on the steel surface in order to investigate the composition of the corrosion products. In SSP, the steel remains passive stabilized by the formation of an oxide film mainly composed of Fe3O4, Fe(OH)2 and -FeOOH. This passive layer protects steel against corrosion and low (passivity current) jpas are measured. When chloride ions are added, the composition of this layer is similar to that found in the absence of Cl- but, the presence of chloride ions resulted in initiation and propagation of pitting corrosion on the steel rebar in the two concentrations evaluated. The pitting potential (Epit) decrease while jpas increase when the [Cl-]/[OH-] ratio increase. The addition of phosphate to the SSP + Cl- restored the surface passivity. Pitting is clearly inhibited and again low jpas are measured. Raman spectra shows that PO43- anions could be participating in the protective layer formed on the steel rebar.