A new occluded corrosion cavity (OCC) simulation cell was designed to study the underscale corrosion behavior of carbon steel (N80) in 0.2 mol L-1 NaCl solution. The chemical components of the solution in the OCC were measured and the electrochemical behavior of the occluded anode and the bulk cathode were studied by electrochemical impedance spectroscopy (EIS). The newly designed OCC cell can easily simulate the auto-catalyzing acidification process and may be used to study the mechanism of underscale corrosion. The corrosion scale exacerbates the underscale corrosion and the area ratio of the bulk cathode to the occluded anode (R = S-c/S-a) determines the development of simulated localized corrosion in the OCC cell. When R was within a certain range, the corrosion rate in the OCC could be kept at a persistently high level. The pH of the solution in the OCC decreased and the chloride ions (Cl-) concentrated as the local corrosion developed. The anodic process on the occluded anode was controlled by irreversible charge transfer and the cathodic process on the bulk cathode was controlled mainly by oxygen diffusion.