Passivity of iron surface in curing cement paste environment investigated by electrochemical impedance spectroscopy and surface characterization techniques

The curing behaviour of cement paste and formation of passive film on iron were investigated by electrochemical techniques and X-ray photoelectron spectroscopy (XPS). Open circuit potential of iron was monitored with the newly developed solid-contact-chloride reference electrode in cement paste with pH ∼13 laid at ∼0 VSHE for 56 d, indicating iron was kept in passive state. Electrochemical impedance spectroscopy results showed significant increasing trend of film and cement paste resistance with curing time. XPS elucidated formation of ferric hydroxide and ferric oxide was efficient to passivate the iron rather than ferrous products. Iron surface receded from corrosion due to high-alkaline environment of cement paste and formation of passive film.