Assessing Passivation and Corrosion of Post-tensioning Strand in Grouts Under Chloride Salt Exposure

This study examines the impact of different commercial grout pore solutions and varying concentrations of salts with different cations on the chemical composition of the passive layer and its breakdown process. X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization were used to study the passivation and corrosion behaviour of post-tension strands. The strands were passivated for 10 days in ex-situ leached pore solution from pastes made of ordinary portland cement (OPC) and other commercially available grouts, followed by exposure to 0.01 mol of chlorides per day in the form of NaCl, CaCl2, MgCl2, and KCl for the total duration of ten days. The study found that the passivated strands’ Fe 2p3/2 spectra generated from XPS testing included metallic iron (Fe), Fe cementite (Fe3C), a mixture of magnetite and wustite (Fe3O4/FeO), hematite, maghemite, and ferrihydrite (a-Fe2O3/c-Fe2O3/FeOOH), and the satellite structure of Fe2O3/FeOOH. The Nyquist and Bode plots of the EIS spectra showed a higher passivation capability of strands in commercially available grouts compared to OPC. However, potentiodynamic test results revealed that after adding chloride, samples that passivated in OPC pore solution had a higher critical chloride threshold, partially due to a higher pH of the pore solution in this system. The analysis of critical chloride threshold of strands exposed to various salts demonstrated that the cation of chloride salt can significantly affect the passive film breakdown, and the critical chloride threshold of strands changes in the increasing order of MgCl2 < NaCl < CaCl2 < KCl.