Abstract: By prefabricating pitting pits of different depths on 316L stainless steel+Q345R carbon steel bimetallic composite plates, the development of pitting pits under typical CO₂ corrosion conditions was discussed. The results show that when the prefabricated pit was in the stainless steel layer, the corrosion was slight and no new pit was generated. When the bottom of the pit just touched the carbon steel layer, the carbon steel preferentially corroded, forming a hole, and the pitting rate reached 3.911 mm/a, and a layer of uneven and thin corrosion products was attached to the bottom of the pit, and its composition was mainly FeCO₃. When the pit was completely penetrated to carbon steel layer, carbon steel layer preferentially corroded and then expanded to the periphery, forming an uneven metal loss morphology, with a pitting rate of 4.954 mm/a, and a thick and relatively tight corrosion product was also attached to the bottom of the pit, which was mainly composed of FeCO₃. It is shown that once the surface of the bimetallic composite plate was pitted and penetrated into the stainless steel layer, it will cause significant galvanic corrosion, and the carbon steel layer will preferentially corroded and propagate to the surrounding areas, which will directly endanger the pressure-bearing performance of the composite panel.