Coiled tubing (CT) is known to be susceptible to corrosion due to the complexity of wellbore environment and limited material selection. Since the weld seam (WS) and the base metal (BM) of CT are different in composition and microstructure, WS material usually shows very different (lower) corrosion resistance compared with BM. To better understand the corrosion-related behaviors of CT materials and the mechanism behind them, the WS and BM samples were directly processed from a finished product of CT, and a series of electrochemical experiments were conducted in laboratory conditions to study the corrosion behaviors of both WS and BM materials. The results showed that the corrosion potentials of BM and WS under the condition of 60 °C and saturated CO2 increased by 57.1% and 41.6%, respectively, comparing with the condition of no CO2 presence. Also the corrosion current of the WS was found to be 14 times of the BM’s. A corrosion model was established assuming the principles of galvanic couple corrosion applied on the working conditions of CT. The surface morphologies of the samples were analyzed by the multi-physical field coupling method. The results showed a very good consistence between the corrosion model and the experimental data. Meanwhile, this model was also used to evaluate the impacts of the welding reinforcement, the area ratio of WS and BM, and the material defects on their corrosion rates. The results indicated that the area ratio of WS and BM is in direct proportional to the corrosion rate, and the other two parameters showed very little effect.