Abstract: Ductile iron pipes, a staple in global urban water supply systems, face a significant challenge in the form of corrosion, which threatens water quality and pipeline integrity. The deterioration mechanism of shrink-age defects on the passivation behavior of ductile iron in simulated concrete pore solution was investi-gated. The results indicated that shrinkage defects increase donor density and reduce the threshold value of chlorine concentration for rupture of ductile iron passion film (CTV) of ductile iron. Defects reduce CTV from 1-1.1 wt.% to 0.36-0.4 wt.%. Because the matrix/graphite around the defect has a higher Volta potential difference, the shrinkage defect preferentially corrodes and induces local corrosion of the sur-rounding matrix, while no significant corrosion was observed in the region away from the defect. High lattice distortion and Si segregation around the shrinkage defect improve the driving force of corrosion in thermodynamics. Furthermore, shrinkage defects elevate the content of Fe( Ⅲ ) compounds in the passive film, without compromising its duplex structure. These defects accelerate the nucleation and growth of the passive film, but generate more cation interstitials. This variation of chemical composition of passive film compromises the film’s integrity and protective properties, attributable to a potential mechanism of micro-environmental acidification and the synergistic effects of shrinkage defect-graphite-matrix multiple micro-galvanic couples.

Key words: Ductile iron, Shrinkage defect, Passive film, Chloride threshold value, Localized corrosion