With the rapid development of deep-sea platforms, seawater desalination systems, and marine transport pipelines, metallic materials operating in chloride-containing high-velocity seawater environments are prone to erosion-corrosion and pitting damage, which critically affects their service safety and lifespan. Cast duplex stainless steels, which combine excellent strength, toughness, and corrosion resistance, are widely used in key components such as pumps, valves, and propulsion systems. However, systematic comparative studies on the corrosion behavior of different grades of stainless steels under dynamic seawater conditions remain limited. On this basis, three duplex stainless steels (CD3MN, CE3MN, and CD3MWN) and one austenitic stainless steel (CN7MS) were tested through 216 h of static immersion and 10 m/s erosion-corrosion experiments in natural seawater from the Qingdao area to elucidate the effects of microstructure and alloying elements on corrosion behavior. Surface morphology observations, corrosion rate measurements, 3D surface profiling, and energy-dispersive spectroscopy analyses were performed to comprehensively evaluate the corrosion resistance of the samples. The results show that under static conditions, all four steels exhibit excellent passivation stability, with corrosion rates of approximately 0.001 0 mm/a and maximum pitting depths ranging from 9.55 to 13.64 μm. Under erosion conditions, both the corrosion rate and pitting depth increase significantly, with CN7MS showing the highest corrosion rate (0.405 5 mm/a) and CD3MWN the lowest (0.146 1 mm/a), indicating superior erosion-corrosion resistance.