Marine carbon steels are constantly subjected to active corrosion due to significant amounts of aggressive agents in seawater. Once sand particles are entrained in seawater, the relative movement between the seawater and marine structures could further lead to erosion-corrosion of marine carbon steels. The size of the sand particles would play an important role in the synergy of erosion and corrosion. In this work, the erosion-corrosion performances of the EH36 marine carbon steel at sand impacts of different particle sizes were studied in 3.5%NaCl solution using the EIS, gravimetric measurements, and surface morphology characterization. A computational fluid dynamics simulation is used to simulate the impact velocity and trajectory of the sand particles in the test cell. The simulation results reveal that at a relatively lower flow velocity of 2 m/s, the average impact velocity of the sand particles on the electrode surface is presented as a decreasing trend along with increasing size (100-850 μm). However, increment in the particle size could still lead to rise in the impact energy due to mass increase. The EIS and gravimetric measurement results show that at low flow rate conditions, corrosion is the main contributor to the steel degradation in the sand-containing electrolyte. Meanwhile, corrosion is the prerequisite for severe erosion in this case. The steel loss induced by erosion would rise with an increase in the particle size. The surface characterization results show that the erosion-corrosion pattern changed from the typical “flow mark” to pitting damage with increasing particle size. It suggests that the increase in the impact energy could lead to a pitting initiation, thereby accelerating localized corrosion. It was determined that the particle size increase would promote the synergy of erosion and corrosion compared to pure corrosion, pure erosion, and erosion-corrosion performances. The initiation and propagation of localized erosion-corrosion are determined by the coupled effect of local sand impacts, anodic dissolution, and flow-enhanced analyte transportation. When the diameter of the sand particle is 100 μm, the erosion-corrosion process is controlled by the analyte transportation, leading to the formation of a typical “flow mark”. When the diameter of the sand particle ranges from 430 μm to 850 μm, the synergy of the sand impact and local anodic dissolution would effectively retard the analyte transportation, resulting in the formation of stable pitting damage.