The role of ferrite in the LBE corrosion resistance of 12 wt% Cr F/M steels at 550℃

Abstract

This study investigates the distinct corrosion behavior of 12 wt% Cr F/M steels in LBE at 550℃, with a focus on the influence of the initial matrix microstructure. Two steels of same composition but different initial microstructures were examined, one predominantly martensitic and the other predominantly ferritic. Under high-oxygen conditions (HOC, 1.01 ×10⁻³ wt% to 1.84 ×10⁻³ wt%), both steels formed duplex oxide scales of similar total thickness. However, the predominantly ferritic steel exhibited a thinner and discontinuous IOZ. In contrast, under low-oxygen conditions (LOC, 1.82 ×10⁻¹³ wt% to 6.25 ×10⁻⁸ wt%), the predominantly ferritic steel underwent deep LBE penetration (maximum ∼40 µm after 5000 h), markedly greater than that observed in the predominantly martensitic steel (maximum ∼10 µm after 5000 h). The key mechanistic insight reveals that the high density of microstructural interfaces (e.g., martensitic lath boundaries) within the martensite provides abundant pathways for rapid elemental diffusion (e.g., O and Cr), thereby promoting the nucleation and growth of the IOZ. Furthermore, the abundant Cr-rich M₂₃C₆ carbides precipitated at these microstructural interfaces accelerate the oxidation of interfaces due to the lower oxidation free energy of Cr. The resulting oxides facilitate the rapid formation of a continuous scale composed of Fe-Cr spinel and Cr₂O₃. Importantly, this oxide layer acts as an effective barrier against dissolution and LBE attack under low‑oxygen conditions. This work demonstrates the detrimental effect of ferrite on the corrosion resistance of 12 wt% Cr F/M steels in LBE and underlines the importance of microstructural design to maximize martensite content for enhanced performance.