Electrochemical and microstructural study on depassivation of ductile iron surface in chloride-contained simulated cement-mortar pore solution

Abstract Depassivation on the surface of ductile iron (DI) influences the service life of cement-mortar-lined ductile iron (CMLDI) pipe in water engineering. In order to investigate the depassivated behavior and chloride threshold value (CTV) of the DI surface in chloride-contained simulated cement-mortar pore (SCMP) solution, electrochemical techniques (OCP, TP and EIS) and microstructural analyses (SEM, XPS and AFM) were used in this study. Chloride solution was gradually unleashed in six SCMP solutions with variant pHs ranging from 13.6 to 10.5, until the DI surface became depassivated. Results show that, when the chloride concentration increases to a critical value (CTV), the OCP, TP and EIS have simultaneously sudden changes, and the Fe2+/Fe3+ ratio produces a significant reduction, while there occur some defects and corrosion products on the DI surface. The depassivation of DI surface is primarily related to chloride (Cl−) and hydroxyl (OH−) concentration of SCMP solution, and the CTV at DI depassivation exponentially decreases with the hydroxyl concentration of SCMP solution. Finally, by fitting on the experimental data, a CTV equation of DI surface was proposed.