Atmospheric corrosion of ductile cast iron and the formation of protective inner rust layer

Uncoated ductile cast iron (DCI) is widely used in infrastructure, yet its atmospheric corrosion remains far less understood than that of carbon and weathering steel. Through a 4-year field exposure at two subtropical coastal sites differing mainly in temperature and precipitation, DCI was tracked alongside carbon steel (SM) and weathering steel (SMA). Mass-loss kinetics were combined with compositional, microstructural, and electrochemical analyses of the rust layers. Graphite-induced acceleration of early corrosion is well established. However, this study found that the corrosion rate of DCI remained below that of both steels and stabilized within about 1–2 years, once the protective rust layer had formed. By the fourth year it reached roughly one-third to one-half that of carbon steel. The maximum corrosion depths nevertheless remained comparable, and the pitting factor of DCI was comparable to that of SMA and higher than that of SM. This reversal did not stem from the outer rust, whose α/γ-FeOOH ratio was merely comparable to that of carbon steel. Protection instead originated in a dense inner layer, enriched in α-FeOOH and Fe3O4, showing the highest ion permeation resistance. From the oxidized Si enriched at the substrate interface, it is inferred that a Cu-refined, Si-based amorphous oxide network seals this layer and produces a pseudo-passive state. Finally, a phenomenological three-stage model describes the atmospheric corrosion progression of DCI. These findings allow preliminary corrosion and pitting allowances for uncoated DCI to be estimated from weathering-steel data, supporting durability-based design.