Corrosion mechanisms of ductile iron pipes in water distribution system: impacts of ionic strength and cement mortar lining coverage

The corrosion process of the ductile iron pipes with different cement mortar lining coverage (lined (100%), unlined (0%), 50%, and 90% lining coverage) in water containing chloride and sulfate were investigated. Electrochemical measurements, including the potentiodynamic polarization curves and electrochemical impedance spectra methods, were carried out to analyze the corrosion mechanisms. Results indicated that the maximum corrosion potential of ductile iron pipes decreased as the increment of ionic strength in a logarithm tendency, while the corrosion current density increased linearly with ionic strength. Additionally, the cement mortar lining coverage affects the inner ductile iron corrosion significantly. The corrosion rate of unlined ductile iron was about 2–5 times higher than that of lined ductile iron pipes. Different from the corrosion rate of lined ductile iron pipes, that of ductile iron pipes with minor defects (90% lining) was higher in the first 10 d, followed by a lower one. The cement mortar lining coverage with significant defects (50% of lining) showed poor protection ability on the iron surface. Moreover, a roughly linear correlation between average corrosion rate (mm a –1 ) and the relative proportion of bare ductile iron area was fitted.