Abstract: As an essential alloy element in the steel industry, Mn plays an important role in improving the mechanical properties of steel. However, the effect of Mn on the corrosion resistance of steel is still unclear. In this paper, the influence of solid solution Mn on the corrosion resistance of low carbon steel in brackish water was explored by combining experiments with theoretical calculations. Chemical analysis, surface element analysis, the first-principles calculation and activation energy calculation indicate that Mn dissolves preferentially than Fe, and it first enters the solution in the form of Mn²⁺ leaving vacancies in the surface layer of substrate, which reduces the energy barrier of surrounding iron dissolution, promotes the dissolution of Fe and accelerates the corrosion of substrate. Subsequently, continuously enriched Mn²⁺ in solution partially transforms into MnFe₂O₄, which exists in the rust layer and promotes the formation of α-FeOOH. Finally, a protective rust layer forms on the steel surface, which slows down the long-term corrosion of steel.

Key words: Solid solution Mn, Low carbon steel, Corrosion resistance, First-principles calculation, Protective rust layer