Corrosion behavior of long-term service-exposed ductile iron for water pipelines in chloride-containing media

Abstract

This study aims to investigate the corrosion behavior of ductile iron recovered from a municipal water pipeline after approximately 30 years of service, with particular emphasis on the effects of anion species and temperature on electrochemical response, rust-layer evolution and graphite–matrix-associated localized damage. Electrochemical measurements were conducted in NaCl, Na2SO4 and NaHCO3 solutions at 5, 20 and 35 °C, while immersion tests were performed in 3.5 wt.% NaCl solution at 20 and 35 °C for up to 168 h. The corrosion products, rust-layer morphology, and underlying substrate damage were characterized using X-ray diffraction, Raman spectroscopy, scanning electron microscopy coupled with energy-dispersive spectroscopy, and confocal laser scanning microscopy. Among the investigated electrolytes, NaCl produced the lowest polarization resistance, whereas Na2SO4 yielded the highest values. Increasing temperature reduced the polarization resistance and promoted interfacial corrosion processes. γ-FeOOH was identified as the dominant crystalline corrosion product; however, the rust layer formed at 35 °C was coarser, more porous and less protective. The overall corrosion damage was dominated by widespread dissolution of the ferritic matrix, accompanied by preferential attack at graphite–matrix interfaces. After prolonged exposure to NaCl solution, adjacent locally damaged regions became interconnected, forming a corrosion-induced crack-like trench. These findings clarify the chloride-assisted corrosion response of long-term service-exposed ductile iron and provide a practical basis for corrosion-risk assessment, maintenance planning and service-life management of aging water pipelines.