Comparative study on corrosion resistance of carbon steel and ductile iron: Implications for the development of corrosion-resistant steels

This study provides a comparative investigation of the corrosion resistance of Q235 carbon steel and ductile iron (DI) by integrating long-term field corrosion monitoring with accelerated laboratory testing. Real-time, on-site monitoring revealed that DI consistently exhibits lower corrosion current density, highlighting its superior corrosion resistance. In 14-day dry/wet cyclic corrosion tests, DI exhibited a significantly lower corrosion rate (1.3 mm/year) than Q235 (2.8 mm/year). Results demonstrated that DI forms a denser inner rust layer, enriched with α-FeOOH, impeding Cl⁻ penetration. In contrast, Q235's rust layer is porous and cracked. Although both materials develop a dual-layer rust structure composed of Fe₃O₄, γ-FeOOH, and α-FeOOH, the higher α/γ* ratio and low-frequency impedance of DI (|Z|_0.01 Hz increasing from 252 Ω·cm² to 797 Ω·cm² compared to Q235 steel) underscore its superior long-term performance. Mechanistic studies indicate that the uniformly distributed spherical graphite promote uniform dissolution through a "large anode-small cathode" effect while catalyzing the transformation of Fe²⁺ to Fe³ ⁺, thereby stabilizing α-FeOOH formation. This work provides important insights for developing novel cost-effective corrosion-resistant steels, demonstrating that optimized microstructural design and rust layer densification strategies can enhance corrosion resistance without relying on expensive alloying elements.