Enhancing the corrosion resistance of low-density steels via Al-rich oxide film formation by grain size optimization

Fe-Mn-Al-C low-density steels have gained widely attention in recent years due to its low density and superior mechanical properties. This study investigated the effects of grain size on oxide film formation and corrosion behavior. Low-density steels with various grain sizes (8.7–32 μm) were prepared by adjusting the solution treatment temperature between 950 ℃ and 1150 ℃. The results indicate that as the solution treatment temperature increases, the grain size increases, and the grain boundary density decreases. The fine-grained sample (S950) rapidly forms an Al-rich oxide film. The film thickness reaches 25.2 ± 1.2 nm, with a low defect density (0.68 ×10²¹/cm³ and 0.26 ×10²¹/cm³) and the lowest corrosion rate (0.160 mm/y). Conversely, the coarse-grained sample (S1150), characterized by a grain size of 32 μm, forms a thinner oxide film (18 ± 0.9 nm) with a reduced Al content (90.17 at% to 69.91 at%) and a higher defect density, and the corrosion rate increases by 43 %. Further analysis reveals that smaller grain sizes and high-density grain boundaries promote the preferential dissolution of elements such as Al at the grain boundaries. This drives the formation of a uniform and stable oxide film. The coarse-grained structure results in discontinuous oxide film growth due to sparse grain boundaries, which increases the Fe/Mn content, and diminishes the corrosion resistance of steels.