Electrochemical and microscopic investigation on passive behavior of ductile iron in simulated cement-mortar pore solution

The passive behavior of ductile iron in simulated cement-mortar pore (SCMP) solution with different alkalinity was investigated by electrochemical measurements, such as open circuit potential (OCP), Tafel polarization curve (TP), electrochemical impedance spectroscopy (EIS), and microscopic tests, including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results show that, with increase in immersion time, the SCMP solution with the high pH value can cause the increase of OCP and capacitive loop of ductile iron surface, a morphology of growing surface film, and the high Fe2+/Fe3+ ratios in its surface Iron-oxide compounds, presenting the formation of passive film on the surface of ductile iron. But in the SCMP solution with low pH value, its OCP, TP curve and EIS show no obvious change, and also produce the corrosion morphology of rust with low Fe2+/Fe3+ ratios, indicating loss of surface protection on the ductile iron.