Effect of Cr on the corrosion behavior of low-C medium-Mn steel in simulated tropical seawater

This paper systematically investigates the effect of Cr content on the corrosion behavior of the rust layer and the nanoscale dense microstructure of low-carbon medium-Mn (LCMM) steel. The results revealed that increasing Cr addition effectively reduces the austenite phase fraction while enriching Cr and depleting Mn within the retained austenite. The increase in Cr content reduces the Volta potential difference between austenite and ferrite, thereby decreasing the susceptibility to micro-galvanic corrosion. Moreover, Cr modifies the corrosion product films (CPFs) by promoting the formation of protective α‑FeOOH and thermodynamically stable phases such as Cr(OH)₃ and FeCr₂O₄, while suppressing Mn‑oxide generation. FIB‑TEM analysis further reveals that the dense CPFs formed on the 1.5Cr steel consists of an inner amorphous Al-Cr-Mn‑containing oxide layer embedded with (Fe,Mn,Cr)₃O₄ nanoparticles, and an outer layer where polycrystalline α‑FeOOH fills the network of nano‑needle‑like Fe₃O₄. This dual‑layer structure synergistically inhibits Mn dissolution and outward diffusion, suppresses charge transfer, and enhances film compactness, ultimately leading to superior corrosion resistance in Cr‑modified LCMM steels.