Comprehensive effect of Mn content on marine atmospheric corrosion and mechanical properties of medium manganese steel for marine infrastructure applications

The role of Mn on the corrosion resistance in simulated marine atmosphere and mechanical property of medium manganese steel was investigated. The microstructure of the medium manganese steels was examined by electron backscatter diffraction (EBSD), and transmission electron microscope (TEM). The mechanical properties of test steels were evaluated through impact testing and tensile testing. The corrosion resistance of medium-manganese steels was investigated through dry-wet cyclic corrosion tests, electrochemical techniques, and compositional analysis. Reducing manganese content in medium-Mn steel improves corrosion resistance by lowering the initial corrosion rate. A galvanic couple forms between the manganese-rich austenite and the tempered martensite matrix, accelerating the preferential dissolution of the Mn-enriched austenite. A reduction in manganese content leads to an increased corrosion potential in the experimental steel. It also decreases both the volume fraction of retained austenite and its manganese concentration, thereby suppressing the galvanic corrosion effect during corrosion. In later stages, 3Mn and 4Mn steels develop a dense, Cr-Ni-Cu-enriched inner rust layer that effectively hinders penetration of corrosive agents, significantly reducing corrosion rates. However, reducing Mn content notably decreases impact toughness—4Mn steel exhibits 102 % higher impact energy (75 ± 12 J) at –40°C than 3Mn steel (37 ± 8 J), mainly due to reduced retained austenite stability, while minimally affecting strength-ductility synergy.