<p>How alloying low-density steel with Cr and Ni enhances marine corrosion resistance is elucidated by modulating the nucleation and growth kinetics governing the development of protective rust layers. How these alloying elements affect rust-layer evolution and the overall corrosion process is systematically elucidated. Specifically, Cr/Ni additions refine the steel’s microstructure and elevate the Volta potential across both austenitic grains and their interfaces. Over extended exposure, Cr/Ni alloying facilitates the formation of protective spinel oxides such as FeCr₂O₄ and NiFe₂O₄, which markedly reduce corrosion rates and current densities while enhancing polarization resistance. This transformation supports a shift from localized pitting toward more uniform corrosion. Theoretical analysis using a dissolution–diffusion–deposition framework further reveals that Cr promotes heterogeneous nucleation of FeCr₂O₄ and dense Fe₃O₄ deposition, forming a compact, defect-suppressing oxide layer. In contrast, Ni slows matrix dissolution and favors homogeneous nucleation of corrosion products. The superior capacity of Cr to suppress local acidification and drive the formation of dense, protective scales accounts for its more pronounced effect compared to that of Ni.</p>