Phosphate ions as effective inhibitors for construction steel in carbonated concrete exposed to marine environment

L. YOHAI; M.B. VALCARCE; M. VÁZQUEZ

Troia

Simposio; 11th Intenational Symposium on Electrochemical Methods in Corrosion Resarch; 2015

Reinforced concrete is one of the most widely used building materials because of distinctive features such as low cost, availability and formability, as well as stregth and durability. However, aggressive service environments and contaminated aggregates may accelerate deterioration and failure. Steel corrosion is one of the main risks. Inhibitors are frequently incorporated to minimize corrosion damage. As there are some environmental risks associated to the use of nitrite ions as corrosion inhibitors, this investigation focuses on sodium phosphate (Na3PO4) 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 Na3PO4 concentration being tested is 0.1 mol L-1 and the pH of the synthetic medium that simulates carbonated concrete is 9 (SCC), while the phosphate/chloride ratio is 1. Cyclic voltammograms, and anodic polarization curves complemented with micro-Raman spectroscopy to evaluate the surface film compostion in different regions of the samples, after completing the polarization curves. The electrodes were stabilized for 24 h at open circuit potential (OCP). The results show that chloride contamination promotes active corrosion. When phosphate ions are incorporated the steel passivates (OCP is more positive) and pitting is the predominant form of localized corrosion. The changes in the composition of the surface layer in the presence of the contaminant, and of both the contaminant and the inhibitor together are investigated by micro Raman spectroscopy. Raman spectra show a broad band, centered in 982 cm-1, suggestin that phosphates incorporate to the passive film. Phosphates are also present in the corrosion products. It can be concluded that phosphate ions are good candidates to be used as corrosion inhibitors for steel in chloride-contaminated concrete. The role of phosphates in the improved protectiveness of the passive layer in chloride-rich solutions is interpreted on the basis of a mechanism that takes into account solubility products of iron (II) phosphates, which is lower than that of the respective hydroxides. Relevant parameters associated to pitting corrosion are shown in the table below. OCP/mVOCP/mVEPIC-OCP/mViPAS/μA cm-2 SCC + Cl--498 ± 51--------------- SCC + [HPO4-2]/[Cl-]=1-17 ± 7268 ± 69290 ± 550.3 ± 0.12