Microstructure Evolution and Corrosion Mechanisms of Q370qD Steel Following Different Heat Treatments

Bridge steel, frequently employed in cross-sea bridge construction, exhibits excellent weldability, superior strength, and toughness. The service life and stability of steel structures are influenced by the presence of corrosive ions within marine environments, necessitating an in-depth examination of the corrosion mechanisms affecting bridge steel. In this study, Q370qD bridge steel was subjected to heat treatment to evaluate the influence of microstructural variations on its corrosion behavior. The microstructure of untreated steel (alloy F) predominantly consists of granular ferrite. Subsequent high-temperature heat treatment induces a partial transformation in the steel microstructure (alloy A), yielding lath carbide-free bainite. Post-immersion tests show both alloy surfaces densely covered with γ-FeOOH, α-FeOOH, and a mixture of Fe₃O₄ and Fe₂O₃. Over time, γ-FeOOH undergoes partial conversion into the more stable α-FeOOH form, enhancing the protective barrier against the matrix for both alloys. Alloy F exhibits a significant reduction in corrosion rate compared to alloy A. The proportion of α-FeOOH in alloy A initially decreases then increases with prolonged exposure, while in alloy F, it consistently rises. The corrosion resistance of alloy A surpasses that of alloy F, which is attributed to the lath-shaped carbide-free bainite's effectiveness in obstructing Cl- penetration and thereby improving corrosion resistance.