<p>The corrosion protection mechanism of a Cr–Ni–Cu–Mo multi-alloyed weathering steel (Q500qENH) is systematically investigated by coupling experimental characterization with dissolution–diffusion–deposition modeling. Compared with conventional Q500q steel, Q500qENH steel exhibits one order of magnitude lower metal-ion concentration in the electrolyte, effectively alleviating acidification caused by hydrolysis and retarding substrate dissolution. The rust layer evolves through sequential deposition of Fe₃O₄, MoO₂, Cr₂O₃, and CuO, forming a dense and defect-minimized microstructure. Thermodynamic and kinetic analyses reveal that the nucleation rates of Fe₃O₄ and CuO in Q500qENH steel are two orders of magnitude higher than in Q500q steel, accelerating the establishment of a compact barrier film. The multi-alloy synergy enhances α-FeOOH and FeCr₂O₄ formation, increasing charge-transfer resistance (<i>R</i>_ct) and polarization resistance (<i>R</i>_p) over exposure time. These results demonstrate that Cr, Mo, and Cu collectively improve ion equilibrium and oxide nucleation behavior, offering a quantitative understanding of rust layer evolution and superior long-term corrosion protection in multi-alloyed weathering steels.</p>