Studies on corrosion inhibition of steel reinforcement by phosphate and nitrite

• Published in: Materials and structures Vol. 36; no. 262; pp. 530 - 540
• Main Author: Dhouibi, L.
• Format: Journal Article
• Language: English
• Published: 01.10.2003
• ISSN: 1359-5997
• DOI: 10.1617/13893

This work deals with the effectiveness of sodium phosphates and nitrites used as inhibitors against steel corrosion in concrete reinforcement. First, concrete pore water was simulated with several alkaline solutions. Then, reinforced concrete specimens with inhibitors added in fresh concrete mix are immersed in chloride solution. The effectiveness of inhibitors was assessed by applying electrochemical techniques, namely Electrochemical Impedance Spectroscopy (EIS), Polarisation Curves (PC), and Open Circuit Potential (OCP) measurements. The final concrete condition was analysed with Scanning Electronic Microscopy (SEM), X- Ray Diffraction (XRD) and Infrared Spectroscopy (FT-IR). In solutions simulating concrete contaminated with chloride, the influence of the inhibitors on the steel corrosion was assessed by (PC) and (EIS). The results obtained show that phosphate prevents pitting corrosion when its content is equal to chloride concentration, and that nitrite only contributes to increase the value of pitting potential. Corrosion rate is reduced with both inhibitors at the different contents tested. For reinforced concrete specimens immersed in chloride solution, their conditions were assessed by applying EIS. The results indicate that after 1 year of immersion with the two inhibitors the corrosion rate decreased. Then, after 3 years of immersion no influence of inhibitors on the corrosion rate was observed. However a significant increase in concrete electrical resistance was observed when inhibitors were present. Visual examinations showed that all steel bars were corroded in the presence of chlorides. Results from analytical tests done on the concrete after 3 years of immersion show that as far as the final concrete condition is concerned, the tested inhibitors did not change the type of compounds in concrete. But the final free chloride content remained higher than the critical chloride threshold. The results of FT-IR technique showed that nitrites are likely washed out of concrete during immersion in chloride solution and phosphates interfere with the equilibrium between CO32- and HCO3 in concrete. The main conclusion of this study is: the efficiency of the tested inhibitors decreases with time, after two years of immersion in chloride solution.